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NOVEL TUMOR ANTIGEN BINDING AGENTS AND USES THEREOF

20220024882 · 2022-01-27

    Inventors

    Cpc classification

    International classification

    Abstract

    The present invention provides compounds according to General Formula (1)(i) or (1)(ii): wherein A is a diagnostic or therapeutic agent comprising a binding site for a tumor antigen, and the spacer comprises at least one C—N bond.

    ##STR00001##

    Claims

    1. A compound according to General Formula (1)(i) or (1)(ii): ##STR00066## wherein A is a diagnostic or therapeutic agent comprising a binding site for a tumor antigen, and the spacer comprises at least one C—N bond.

    2. The compound according to claim 1, wherein the tumor antigen is prostate-specific membrane antigen (PSMA).

    3. The compound according to claim 1 or 2, wherein the diagnostic or therapeutic agent A comprises a radiolabel.

    4. The compound according to claim 3, wherein the radiolabel is a non-metallic radionuclide or a radiometal.

    5. The compound according to any one of claims 1-4, wherein the diagnostic or therapeutic agent A comprises a chelator.

    6. The compound according to claim 5, wherein the diagnostic or therapeutic agent A comprises a radiometal coordinated via the chelator.

    7. The compound according to any one of claims 1-6, wherein the compound is characterized by the following General Formula (1a): ##STR00067## wherein D is a chelator; Tbm is a tumor-antigen binding moiety; linker is a linker, preferably comprising a cyclic group or an aromatic group; spacer is a spacer comprising a C—N bond; and a is an integer selected from 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10.

    8. A compound characterized by the following General Formula (1a): ##STR00068## wherein D is a chelator; Tbm is a tumor-antigen binding moiety; linker is a linker, preferably comprising a cyclic group or an aromatic group; spacer is a spacer comprising a C—N bond; and a is an integer selected from 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10; or a pharmaceutically acceptable salt, ester, solvate or radiolabeled complex thereof.

    9. The compound according to claim 7 or 8, wherein the tumor-antigen binding moiety (Tbm) is a PSMA-binding moiety (Pbm).

    10. The compound according to claim 9, wherein the PSMA-binding moiety is characterized by General Formula (3): ##STR00069## wherein X and Y are each independently selected from O, N or NH or NH.sub.2, S or P, Z is selected from CH.sub.2 or substituted CH.sub.2, wherein one or both of the hydrogen atoms may be substituted, R.sup.1, R.sup.2 and R.sup.3 are each independently selected from —COH, —CO.sub.2H, —SO.sub.2H, —SO.sub.3H, —SO.sub.4H, —PO.sub.2H, —PO.sub.3H, —PO.sub.4H.sub.2, —C(O)—(C.sub.1-C.sub.10)alkyl, —C(O)—O(C.sub.1-C.sub.10)alkyl, —C(O)—NHR.sup.4, or —C(O)—NR.sup.4R.sup.5, wherein R.sup.4 and R.sup.5 are each independently selected from H, bond, (C1-C10)alkylene, F, Cl, Br, I, C(O) or —CH(O), C(S) or —CH(S), —C(S)—NH-benzyl-, —C(O)—NH-benzyl, —C(O)—(C.sub.1-C.sub.10)alkylene, —(CH.sub.2).sub.p—NH, —(CH.sub.2).sub.p—(C.sub.1-C.sub.10)alkyene, —(CH.sub.2).sub.p—NH—C(O)—(CH.sub.2).sub.q, —(CH.sub.rCH.sub.2).sub.t—NH—C(O)—(CH.sub.2).sub.p, —(CH.sub.2).sub.p—CO—COH, —(CH.sub.2).sub.p—CO—CO.sub.2H, —(CH.sub.2).sub.p—C(O)NH—C[(CH.sub.2).sub.q—COH].sub.3, —C[(CH.sub.2).sub.p—COH].sub.3, —(CH.sub.2).sub.p—C(O)NH—C[(CH.sub.2).sub.q—CO.sub.2H].sub.3, —C[(CH.sub.2).sub.p—CO.sub.2H].sub.3 or —(CH.sub.2).sub.p—(C.sub.5-C.sub.14)heteroaryl, and f, p, q, r and t are each independently an integer selected from 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10; preferably X O or S, and Y NH or O or S.

    11. The compound according to claim 10, wherein f is an integer selected from 1, 2, 3, 4, or 5; preferably f is 2 or 3.

    12. The compound according to claim 10 or 11, wherein Y is O or NH.

    13. The compound according to any one of claims 10-12, wherein Z is CH.sub.2 or C═O.

    14. The compound according to any one of claims 10-13, wherein the PSMA-binding moiety is characterized by General Formula (3)(ii): ##STR00070## wherein X is selected from O, N or NH or NH.sub.2, S or P, R.sup.1, R.sup.2 and R.sup.3 are each independently selected from —COH, —CO.sub.2H, —SO.sub.2H, —SO.sub.3H, —SO.sub.4H, —PO.sub.2H, —PO.sub.3H, —PO.sub.4H.sub.2, —C(O)—(C.sub.1-C.sub.10)alkyl, —C(O)—O(C.sub.1-C.sub.10)alkyl, —C(O)—NHR.sup.4, or —C(O)—NR.sup.4R.sup.5, wherein R.sup.4 and R.sup.5 are each independently selected from H, bond, (C1-C10)alkylene, F, Cl, Br, I, C(O) or —CH(O), C(S) or —CH(S), —C(S)—NH-benzyl-, —C(O)—NH-benzyl, —C(O)—(C.sub.1-C.sub.10)alkylene, —(CH.sub.2).sub.p—NH, —(CH.sub.2).sub.p—(C.sub.1-C.sub.10)alkyene, —(CH.sub.2).sub.p—NH—C(O)—(CH.sub.2).sub.q, —(CH.sub.rCH.sub.2).sub.t—NH—C(O)—(CH.sub.2).sub.p, —(CH.sub.2).sub.p—CO—COH, —(CH.sub.2).sub.p—CO—CO.sub.2H, —(CH.sub.2).sub.p—C(O)NH—C[(CH.sub.2).sub.q—COH].sub.3, —C[(CH.sub.2).sub.p—COH].sub.3, —(CH.sub.2).sub.p—C(O)NH—C[(CH.sub.2).sub.q—CO.sub.2H].sub.3, —C[(CH.sub.2).sub.p—CO.sub.2H].sub.3 or —(CH.sub.2).sub.p—(C.sub.5-C.sub.14)heteroaryl, and b, p, q, r and t are each independently an integer selected from 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10; preferably X O or S, and Y NH or O or S.

    15. The compound according to any one of claims 10-14, wherein X is O.

    16. The compound according to any one of claims 10-14, wherein R.sup.1, R.sup.2 and R.sup.3 are each independently selected from —COH, —CO.sub.2H, —SO.sub.2H, —SO.sub.3H, —SO.sub.4H, —PO.sub.2H, —PO.sub.3H, —PO.sub.4H.sub.2.

    17. The compound according to claim 16, wherein each of R.sup.1, R.sup.2 and R.sup.3 is —COOH.

    18. The compound according to any one of claims 14-17, wherein b is an integer selected from 1, 2, 3, 4 or 5, preferably b is 2, 3 or 4, more preferably b is 3.

    19. The compound according to any one of claims 14-18, wherein R.sup.1, R.sup.2 and R.sup.3 are each COOH, X is O, and b is 3.

    20. The compound according to any one of claims 9-19, wherein the PSMA-binding moiety is characterized by Formula (3)(a): ##STR00071##

    21. The compound according to any one of claims 9-13, wherein the PSMA-binding moiety is characterized by Formula (3)(b): ##STR00072##

    22. The compound according to any one of claims 7-21, wherein the linker is characterized by the Structural Formula (4): ##STR00073## wherein X is each independently selected from O, N, S or P, Q is selected from substituted or unsubstituted alkyl, alkylaryl and cycloalkyl, preferably from substituted or unsubstituted C.sub.5-C.sub.14 aryl, C.sub.5-C.sub.14 alkylaryl or C.sub.5-C.sub.14 cycloalkyl, and W is selected from —(CH.sub.2).sub.c-aryl or —(CH.sub.2).sub.c-heteroaryl, wherein c is an integer selected from 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10.

    23. The compound according to claim 22, wherein each X is O.

    24. The compound according to claim 22 or 23, wherein Q is selected from substituted or unsubstituted C.sub.5-C.sub.7 cycloalkyl.

    25. The compound according to claim 24, wherein Q is cyclohexyl.

    26. The compound according to any one of claims 22-25, wherein W is selected from —(CH.sub.2).sub.c-naphthyl, —(CH.sub.2).sub.c-phenyl, —(CH.sub.2).sub.c-biphenyl, —(CH.sub.2).sub.c-indolyl, —(CH.sub.2).sub.c-benzothiazolyl, wherein c is an integer selected from 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10.

    27. The compound according to claim 26, wherein W is selected from —(CH.sub.2)-naphthyl, —(CH.sub.2)-phenyl, —(CH.sub.2)-biphenyl, —(CH.sub.2)-indolyl or —(CH.sub.2)-benzothiazolyl.

    28. The compound according to claim 26 or 27, wherein W is —(CH.sub.2)-naphthyl.

    29. The compound according to any one of claims 22-28, wherein the linker is characterized by the following Structural Formula (4a): ##STR00074##

    30. The compound according to any one of claims 1-29, wherein said compound is characterized by General Formula (1)(b) or (1)(c): ##STR00075## wherein D is a chelator; spacer is a spacer comprising a C—N bond; and a is an integer selected from 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10, preferably 0 or 1; or a pharmaceutically acceptable salt, ester, solvate or radiolabeled complex thereof.

    31. The compound according to any one of claims 1-30, wherein the spacer comprises a linear or branched, optionally substituted C.sub.1-C.sub.20 hydrocarbyl, more preferably C.sub.1-C.sub.12 hydrocarbyl, even more preferably C.sub.2-C.sub.6 hydrocarbyl, even more C.sub.2-C.sub.4 hydrocarbyl, the hydrocarbyl comprising at least one, optionally up to 4 heteroatoms preferably selected from N.

    32. The compound according to claim 30 or 31, wherein the spacer comprises —[CHR.sup.6].sub.u—NR.sup.7—, wherein R.sup.6 and R.sup.7 are each be independently selected from H and branched, unbranched or cyclic C.sub.1-C.sub.12 hydrocarbyl, and u is an integer selected from 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10, wherein u is preferably 2, 3, or 4, more preferably 2 or 4.

    33. The compound according to any one of claims 1-32, wherein the spacer is —[CH.sub.2].sub.2—NH— or —[CH.sub.2].sub.4—NH—.

    34. The compound according to any of claims 1 to 33, wherein the spacer comprises at least one amino acid residue or an amino acid residue side chain, wherein the amino acid is preferably selected from lysine, aspartate, asparagine, diaminobutyric acid, phenylalanine, tyrosine, threonine, serine, proline, leucine, isoleucine, valine, arginine, histidine, glutamate, glutamine, and alanine.

    35. The compound according to claim 33 or 34, wherein the spacer comprises or consists of a lysine residue or a lysine residue side chain.

    36. The compound according to claim 35, wherein the spacer further comprises a further amino acid residue or a side chain thereof.

    37. The compound according to claim 36, wherein the further amino acid residue or the side chain thereof is selected from aspartate, asparagine and diaminobutyric acid.

    38. The compound according to any one of claims 1-37, wherein the spacer comprises or consists of Formula (2)(a) or Formula (2)(a)′ or Formula (2)(a)″: ##STR00076## wherein k is an integer selected from 0, 1, 2, 3, 4, 5, 6, 7 and 8, preferably 2, 3 or 4.

    39. The compound according to any one of claims 1-38, wherein the spacer comprises or consists of Formula (2)(b): ##STR00077## wherein m is an integer selected from 1 or 2, and n is an integer selected from 1, 2, 3, 4 or 5, preferably from 1, 2 or 3.

    40. The compound according to any one of claims 1-39, wherein the spacer comprises or consists of Formula (2)(c) or (2)(c)′: ##STR00078## wherein o is an integer selected from 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10 and k is as defined above.

    41. The compound according to any one of claims 1-38, wherein the spacer comprises or consists of Formula (2)(d) or (2)(d)′: ##STR00079## wherein A is an amino acid residue or -[A].sub.n is absent and n is an integer selected from 0, 1, 2, 3, 4, or 5, preferably from 0 or 1, and k is as defined above.

    42. The compound according to claim 41, wherein the spacer comprises or consists of Formula (2)(d)(i) or (2)(d)(i)′: ##STR00080## wherein k is as defined above.

    43. The compound according to claim 41, wherein the spacer comprises or consists of Formula (2)(d)(ii) or (2)(d)(ii)′: ##STR00081## wherein k is as defined above.

    44. The compound according to claim 41, wherein the spacer comprises or consists of Formula (2)(d)(iii) or (2)(d)(iii)′: ##STR00082## wherein k is as defined above.

    45. The compound according to claim 41, wherein the spacer comprises or consists of Formula (2)(d)(iv) or (2)(d)(iv)′: ##STR00083## wherein k is as defined above.

    46. The compound according to any one of claims 1-45, wherein said compound is characterized by General Formula (1)(n) or (1)(o): ##STR00084## wherein D is a chelator; A is an amino acid residue, a side chain thereof or —[CHR.sup.6].sub.uNR.sup.7—, wherein R.sup.6 and R.sup.7 are each be independently selected from H and branched, unbranched or cyclic C.sub.1-C.sub.12 hydrocarbyl, and u is an integer selected from 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10, wherein u is preferably 2, 3, or 4, more preferably 2 or 4; V is absent or selected from a single bond, N or NH, or an optionally substituted C.sub.1-C.sub.12 hydrocarbyl comprising up to 3 heteroatoms, wherein said heteroatom is preferably selected from N, wherein V more preferably contains 1 or 2 C—N bond(s); a is an integer selected from 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10; and n is an integer selected from 0, 1, 2, 3, 4, or 5, preferably from 0 or 1; or a pharmaceutically acceptable salt, ester, solvate or radiolabeled complex thereof.

    47. The compound according to any one of claims 1-46, wherein said compound is characterized by Formula (7)(a) or (7)(a)′: ##STR00085## wherein D is a chelator; or a pharmaceutically acceptable salt, ester, solvate or radiolabeled complex thereof.

    48. The compound according to any one of claims 1-46, wherein said compound is characterized by Formula (7)(b) or (7)(b)′: ##STR00086## wherein D is a chelator; or a pharmaceutically acceptable salt, ester, solvate or radiolabeled complex thereof.

    49. The compound according to any one of claims 1-46, wherein said compound is characterized by Formula (7)(c) or (7)(c)′: ##STR00087## wherein D is a chelator; or a pharmaceutically acceptable salt, ester, solvate or radiolabeled complex thereof.

    50. The compound according to any one of claims 1-46, wherein said compound is characterized by Formula (7)(d) or (7)(d)′: ##STR00088## wherein D is a chelator; or a pharmaceutically acceptable salt, ester, solvate or radiolabeled complex thereof.

    51. The compound according to any one of claims 1-46, wherein said compound is characterized by Formula (7)(e) or (7)(e)′: ##STR00089## wherein D is a chelator; or a pharmaceutically acceptable salt, ester, solvate or radiolabeled complex thereof.

    52. The compound according to any one of claims 5-51, wherein the chelator (D) is selected from 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA), N,N″-bis[2-hydroxy-5-(carboxyethyl)-benzyl]ethylenediamine-N,N″-diacetic acid (HBED-CC), 1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA), 2-(4,7-bis(carboxymethyl)-1,4,7-triazonan-1-yl)pentanedioic acid (NODAGA), 2-(4,7,10-tris(carboxymethyl)-1,4,7,10-tetraazacyclododecan-1-yl)-pentanedioic acid (DOTAGA), 1,4,7-triazacyclononane phosphinic acid (TRAP), 1,4,7-triazacydononane-1-[methyl(2-carboxyethyl)-phosphinic acid]-4,7-bis[methyl(2-hydroxymethyl)phosphinic acid] (NOPO), 3,6,9, 15-tetraazabicyclo[9,3,1]pentadeca-1(15),11,13-triene-3,6,9-triacetic acid (PCTA), N′-{5-[Acetyl(hydroxy)amino]pentyl}-N-[5-({4-[(5-aminopentyl)(hydroxy)amino]-4-oxobutanoyl}amino)pentyl]-N-hydroxysuccinamide (DFO), and Diethylenetriaminepentaacetic acid (DTPA), or derivatives thereof.

    53. The compound according to any one of claims 5-52, wherein the chelator is selected from DOTA, DOTAGA, NODAGA, DO3AP, DO3AP.sup.PrA or DO3AP.sup.ABn.

    54. The compound according to claim 52 or 53, wherein the chelator is DOTA.

    55. The compound according to any one of claims 1-54, wherein said compound is characterized by Structural Formula (8)(a) or (8)(a)′: ##STR00090## or a pharmaceutically acceptable salt, ester, solvate or radiolabeled complex thereof.

    56. The compound according to any one of claims 1-54, wherein said compound is characterized by Structural Formula (8)(b) or (8)(b)′: ##STR00091## or a pharmaceutically acceptable salt, ester, solvate or radiolabeled complex thereof.

    57. The compound according to any one of claims 1-54, wherein said compound is characterized by Structural Formula (8)(c) or (8)(c)′: ##STR00092## or a pharmaceutically acceptable salt, ester, solvate or radiolabeled complex thereof.

    58. The compound according to any one of claims 1-54, wherein said compound is characterized by Structural Formula (8)(d) or (8)(d)′: ##STR00093## or a pharmaceutically acceptable salt, ester, solvate or radiolabeled complex thereof.

    59. The compound according to any one of claims 1-54, wherein said compound is characterized by Structural Formula (8)(e) or (8)(e)′: ##STR00094## or a pharmaceutically acceptable salt, ester, solvate or radiolabeled complex thereof.

    60. Use of a compound according to any one of claims 1 to 59 for the preparation of a radiolabeled complex.

    61. A compound according to any of claims 1 to 59 for use as a medicament or as a precursor of a medicament.

    62. A radiolabeled complex comprising a radionuclide and a compound according to any one of the preceding claims.

    63. The radiolabeled complex according to claim 62, wherein the radiolabel is selected from the group consisting of .sup.94Tc, .sup.99mTc, .sup.90In, .sup.111In, .sup.67Ga, .sup.68Ga, .sup.86Y, .sup.90Y, .sup.177Lu, .sup.151Tb, .sup.186Re, .sup.188Re, .sup.64Cu, .sup.67Cu, .sup.55Co, .sup.57Co, .sup.43Sc, .sup.44Sc, .sup.47Sc, .sup.225Ac, .sup.213Bi, .sup.212Bi, .sup.212Pb, .sup.227Th, .sup.153Sm, .sup.166Ho, .sup.152Gd, .sup.153Gd, .sup.157Gd, or .sup.166Dy.

    64. The radiolabeled complex according to claim 62 or 63, wherein the radiolabel is .sup.177Lu.

    65. A pharmaceutical composition comprising the compound according to any one of claims 1 to 60, or a radiolabeled complex according to any one of claims 62-64, and, optionally, a pharmaceutically acceptable carrier, diluent and/or excipient.

    66. A kit comprising a compound according to any one of claims 1 to 60 or a pharmaceutically acceptable salt, ester, solvate or radiolabeled complex thereof, a radiolabeled complex according to any one of claims 62-64 or a pharmaceutical composition according to claim 64.

    67. The compound according to any one of claims 1 to 60, the radiolabeled complex according to any one of claims 62-64, the pharmaceutical composition according to claim 65 or the kit according to claim 66 for use in medicine and/or diagnostics.

    68. The compound according to any one of claims 2 to 60, the radiolabeled complex according to any one of claims 62-64, the pharmaceutical composition according to claim 65 or the kit according to claim 66 for use in a method of detecting the presence of (isolated) cells and/or tissues expressing prostate-specific membrane antigen (PSMA).

    69. The compound according to any one of claims 2 to 60, the radiolabeled complex according to any one of claims 62-64, the pharmaceutical composition according to claim 65 or the kit according to claim 66 for use in a method of diagnosing, treating and/or preventing cancer, preferably prostate cancer, pancreatic cancer, renal cancer or bladder cancer.

    70. The compound, radiolabeled complex, pharmaceutical composition or kit for use according to any one of claims 67-69, wherein said method or use comprises (a) administering said compound, radiolabeled complex or pharmaceutical composition to a patient, and (b) obtaining a radiographic image from said patient.

    71. An in vitro method of detecting the presence of cells and/or tissues expressing prostate-specific membrane antigen (PSMA) comprising (a) contacting said PSMA-expressing cells and/or tissues with a compound, radiolabeled complex, pharmaceutical composition or kit according to any one of the preceding claims; (b) applying detection means, optionally radiographic imaging, to detect of said cells and/or tissues.

    72. The compound, the radiolabeled complex, the pharmaceutical composition or the kit for use according to any one of claims 67-70, or the method according to claim 71, wherein radiographic imaging comprises positron emission tomography (PET) or single-photon emission computed tomography (SPECT).

    73. The compound, the radiolabeled complex, the pharmaceutical composition or the kit for use according to any one of claims 67-70 or 72, or the method according to claim 71 or 72, wherein said one or more cells or tissues comprise (optionally cancerous) prostate cells or tissues, (optionally cancerous) spleen cells or tissues, or (optionally cancerous) kidney cells or tissues.

    74. The compound, the radiolabeled complex, the pharmaceutical composition or the kit for use according to any one of claims 67-70 or 72-73, or the method according to any one of claims 71-73, wherein the presence of PSMA-expressing cells or tissues is indicative of a prostate tumor (cell), a metastasized prostate tumor (cell), a renal tumor (cell), a pancreatic tumor (cell), a bladder tumor (cell), and combinations thereof.

    Description

    BRIEF DESCRIPTION OF THE FIGURES

    [0315] In the following a brief description of the appended figures will be given. The figures are intended to illustrate the present invention in more detail. However, they are not intended to limit the subject matter of the invention in any way.

    [0316] FIG. 1 shows in Scheme 1 the synthesis of the Glutamate-Urea-Lysine Binding Motif for Ibu-DAB-PSMA.

    [0317] FIG. 2 shows in Scheme 2 the synthesis of the Linker Area, Precursor for Ibu-Dab-PSMA.

    [0318] FIG. 3 shows in Scheme 3 the synthesis of the DOTA-conjugated Precursor for Ibu-Dab-PSMA.

    [0319] FIG. 4 shows in Scheme 4 the coupling of the additional linker moiety and albumin-binding entity for Ibu-DAB-PSMA.

    [0320] FIG. 5 shows for Example 4 representative HPLC chromatograms of the ibuprofen-derivatized .sup.177Lu-PSMA-ligands. (A) Chromatogram of .sup.177Lu-Ibu-PSMA; (B) Chromatogram of .sup.177Lu-Ibu-Dβ-PSMA; (C) Chromatogram of .sup.177Lu-Ibu-Dα-PSMA; (D) Chromatogram of .sup.177Lu-Ibu-N-PSMA; (E) Chromatogram of .sup.177Lu-Ibu-DAB-PSMA. Retention times t.sub.R are indicated in the figures.

    [0321] FIG. 6 shows for Example 5 the n-Octanol/PBS distribution coefficients of .sup.177Lu-Ibu-PSMA, .sup.177Lu-Ibu-Dβ-PSMA, .sup.177Lu-Ibu-Dα-PSMA, .sup.177Lu-Ibu-N-PSMA and .sup.177Lu-Ibu-DAB-PSMA in comparison to the reference compound .sup.177Lu-PSMA-617. The experiments were performed three times (n=3) in quintuplicate.

    [0322] FIG. 7 shows for Example 6 the data from ultrafiltration assays of .sup.177Lu-Ibu-PSMA, .sup.177Lu-Ibu-Dβ-PSMA, .sup.177Lu-Ibu-Dα-PSMA, .sup.177Lu-Ibu-N-PSMA and .sup.177Lu-Ibu-DAB-PSMA in comparison to .sup.177Lu-PSMA-617. (n=3)

    [0323] FIG. 8 shows for Example 7 the uptake and internalization of .sup.177Lu-Ibu-PSMA, .sup.177Lu-Ibu-Dβ-PSMA, .sup.177Lu-Ibu-Dα-PSMA, .sup.177Lu-Ibu-N-PSMA and .sup.177Lu-Ibu-DAB-PSMA in comparison to .sup.177Lu-PSMA-617. (A) Data obtained in PSMA-positive PC-3 PIP cells (n=3). (B) Data obtained in PSMA-positive PC-3 flu cells (n=1).

    [0324] FIG. 9 shows for Example 8 the biodistribution data of the five ibuprofen-derivatized radioligands and .sup.177Lu-PSMA-617 obtained in PC-3 PIP/flu tumor-bearing mice. (A) Biodistribution data obtained 4 h after injection of the radioligands; (B) Biodistribution data obtained 24 h after injection of the radioligands.

    [0325] FIG. 10 shows for Example 8 tumor-to-background ratios at 4 h and 24 h after injection of the .sup.177Lu-PSMA-ligands. (A) Tumor-to-blood ratios, (B) tumor-to-liver ratios and (C) tumor-to-kidney ratios for all .sup.177Lu-Ibu-PSMA-ligands at 4 h and 24 h p.i.

    [0326] FIG. 11 shows for Example 9 the whole-body activity measured in a dose calibrator at 0 h, 4 h, 24 h, 48 h and 72 h after injection of the respective radioligands. The activity measured right after injection was set as 100%. Data for comparative radioligands .sup.177Lu-PSMA-ALB-53/56 and .sup.177Lu-PSMA-617 are included in this graph for comparison. The data points present the average of two mice which were injected with the same radioligand (n=2).

    [0327] FIG. 12 shows for Example 10 SPECT/CT images obtained 4 h after injection of the .sup.177Lu-PSMA-ligands shown as maximum intensity projections (MIP). (A).sup.177Lu-Ibu-PSMA; (B) .sup.177Lu-Ibu-Dβ-PSMA; (C).sup.177Lu-Ibu-Dβ-PSMA; (D) .sup.177Lu-Ibu-N-PSMA; (E) .sup.177Lu-Ibu-DAB-PSMA. PSMA+=PSMA-positive PC-3 PIP tumor xenograft; PSMA−=PSMA-negative PC-3 flu tumor xenograft; Ki=Kidney; Bl=urinary bladder.

    [0328] FIG. 13 shows a scheme presenting the coupling of the ibuprofen moiety to Precursor 1 (including the PSMA binding entity and a DOTA chelator) for synthesizing Ibu-sPSMA.

    [0329] FIG. 14 Representative HPLC chromatogram of .sup.177Lu-Ibu-sPSMA. The retention time tr is indicated in the figure.

    [0330] FIG. 15 Radiolytic stability presented as percentage of intact .sup.177Lu-Ibu-sPSMA up to 24 h. (A).sup.177Lu-Ibu-sPSMA incubated without L-ascorbic acid; (B) .sup.177Lu-Ibu-sPSMA incubated with L-ascorbic acid (average±SD, n=3). .sup.177Lu-Ibu-sPSMA was significantly more stable than .sup.177Lu-PSMA-617 and all other ibuprofen-derivatized PSMA radioligands. The stability of .sup.177Lu-Ibu-sPSMA was comparable to the stability of .sup.177Lu-PSMA-ALB-56.

    [0331] FIG. 16 Data from ultrafiltration assays of .sup.177Lu-Ibu-sPSMA in comparison to .sup.177Lu-PSMA-617. (n=3)

    [0332] FIG. 17 Uptake and internalization of .sup.177Lu-Ibu-sPSMA in comparison to .sup.177Lu-PSMA-617. (A) Data obtained in PSMA-positive PC-3 PIP cells (n=3). (B) Data obtained in PSMA-negative PC-3 flu cells (n=3).

    [0333] FIG. 18 Graph showing biodistribution data of .sup.177Lu-Ibu-PSMA, .sup.177Lu-Ibu-DAB-PSMA, .sup.177Lu-Ibu-sPSMA and .sup.177Lu-Ibu-PSMA-617 obtained in PC-3 PIP/flu tumor-bearing mice. (A) Biodistribution data obtained 1 h after injection of the radioligands; (B) Biodistribution data obtained 4 h after injection of the radioligands; (C) Biodistribution data obtained 24 h after injection of the radioligands and (D) biodistribution data obtained 96 h after injection of the radioligands.

    [0334] FIG. 19 The graphs show tumor-to-background ratios at 1 h, 4 h, 24 h and 96 h after injection of .sup.177Lu-Ibu-sPSMA in comparison to .sup.177Lu-Ibu-PSMA and .sup.177Lu-Ibu-DAB-PSMA. (A) Tumor-to-blood ratios, (B) tumor-to-kidney ratios and (C) tumor-to-liver ratios.

    [0335] FIG. 20 Whole-body activity measured in a dose calibrator at various time-points after injection. The activity measured right after injection was set as 100%. Published data of .sup.177Lu-PSMA-617 is included in the graphs for comparison. The data points present the average of two mice, which were injected with the same radioligand (n=2-3). (A) The graph shows data of all radioligands; (B) the graph shows data of .sup.177Lu-Ibu-PSMA, .sup.177Lu-Ibu-DAB-PSMA, .sup.177Lu-PSMA-617 and .sup.177Lu-PSMA-ALB-56 for better visualization of the single excretion curves.

    [0336] FIG. 21 SPECT/CT images obtained after injection of the .sup.177Lu-Ibu-sPSMA shown as maximum intensity projections (MIP). (A) SPECT/CT image acquired 4 h p.i.; (B) SPECT/CT image acquired 24 h p.i. PSMA+=PSMA-positive PC-3 PIP tumor xenograft; PSMA−=PSMA-negative PC-3 flu tumor xenograft; Ki=Kidney; Bl=urinary bladder.

    [0337] FIG. 22 Relative tumor growth of control mice and mice treated with (a) lower quantity of activity (2 MBq, 1 nmol per mouse) or (b) higher quantity of activity (5 MBq, 1 nmol per mouse). Each group of mice was injected with only vehicle (saline) (.circle-solid.), .sup.177Lu-Ibu-DAB-PSMA (.square-solid.), .sup.177Lu-PSMA-617 (.box-tangle-solidup.) and .sup.177Lu-PSMA-ALB-56 (.Math.), respectively, six days after tumor cell inoculation (average±SD, n=6-12). Average relative tumor volumes of each group are shown until the first mouse reached an endpoint.

    [0338] FIG. 23 Kaplan-Meier plot with survival curves of mice of each group (n=6-12). Control mice and mice treated with (a) lower quantity of injected activity (2 MBq, 1 nmol per mouse) and (b) higher quantity of injected activity (5 MBq, 1 nmol per mouse). Untreated control mice (-), .sup.177Lu-Ibu-DAB-PSMA (---); .sup.177Lu-PSMA-617 (-••-••) and .sup.177Lu-PSMA-ALB-56 (•••).

    [0339] FIG. 24 Relative body weight (RBW) of control mice and mice treated with (a) lower quantity of injected activity (2 MBq, 1 nmol per mouse) and (b) higher quantity of injected activity (5 MBq, 1 nmol per mouse). Average RBW of mice injected with only vehicle (saline) (.circle-solid.), .sup.177Lu-Ibu-DAB-PSMA (.square-solid.), .sup.177Lu-PSMA-617 (.box-tangle-solidup.) and .sup.177Lu-PSMA-ALB-56 (.Math.), respectively. Average RBW of each group shown until the first mouse reached an endpoint.

    EXAMPLES

    [0340] In the following, particular examples illustrating various embodiments and aspects of the invention are presented. However, the present invention shall not to be limited in scope by the specific embodiments described herein. The following preparations and examples are given to enable those skilled in the art to more clearly understand and to practice the present invention. The present invention, however, is not limited in scope by the exemplified embodiments, which are intended as illustrations of single aspects of the invention only, and methods which are functionally equivalent are within the scope of the invention. Indeed, various modifications of the invention in addition to those described herein will become readily apparent to those skilled in the art from the foregoing description, accompanying figures and the examples below. All such modifications fall within the scope of the appended claims.

    Example 1: Structural Design of Exemplified PSMA-Ligands

    [0341] In order to identify PSMA-ligands, which provide a balance between (i) the binding of the radioligand to albumin in order to achieve an optimal tissue distribution profile with high tumor uptake and (ii) blood activity levels that are not extensively high, which would result in a risk for undesired side effects to healthy tissue, the following five ibuprofen-derivatized PSMA-ligands were designed (Ibu-PSMA, Ibu-Dα-PSMA, Ibu-Dβ-PSMA, Ibu-N-PSMA and Ibu-DAB-PSMA):

    ##STR00060## ##STR00061## ##STR00062##

    [0342] The simplest design of an ibuprofen-derivatized PSMA-ligand is Ibu-PSMA. It was designed by introducing the albumin binder ibuprofen without any additional spacer entity by conjugating ibuprofen directly to the lysine residue. In Ibu-Dα-PSMA and Ibu-Dβ-PSMA an additional spacer based on D-aspartic acid (D-Asp, D) was used (in addition to the L-Lys residue) to introduce an additional negative charge to the construct. D-Asp was conjugated either via the α-carboxyl group to obtain Ibu-Dα-PSMA or via the β-carboxyl group to obtain Ibu-Dβ-PSMA. In Ibu-N-PSMA a different additional spacer entity based on D-asparagine (D-Asn, N) was employed acting as neutral entity (in addition to the L-Lys residue). Finally, the design of Ibu-DAB-PSMA was based on the use of D-diaminobutyric acid (DAB) as additional spacer entity (in addition to the L-Lys residue) to introduce an additional positive charge to the construct.

    1.6. Ibu-sPSMA:

    [0343] Ibu-sPSMA was designed in analogy to Ibu-PSMA. In contrast to Ibu-PSMA, in which the ibuprofen moiety was connected via a lysine side chain, the shorter L-2,4-diaminobutyric acid (L-DAB) was used as connecting unit.

    ##STR00063##

    Example 2: Chemical Synthesis of the Exemplified PSMA-Ligands

    2.1. Synthetic Strategy and Analysis of the PSMA-Ligands

    [0344] All five suggested PSMA-ligands with an albumin-binding moiety were synthesized via a solid-phase platform as previously reported for the synthesis of other PSMA-ligands (Umbricht, C. A.; Benesova, M.; Schibli, R.; Müller, C. Preclinical development of novel PSMA-targeting radioligands: modulation of albumin-binding properties to improve prostate cancer therapy. Mol Pharm 2018, Mol Pharm 2018, 15, (6), 2297-2306). This technique revealed to be useful for the development of the described ibuprofen-derivatized PSMA-ligands. A multistep synthesis (17 steps for Ibu-PSMA and 19 steps Ibu-Dα-PSMA, Ibu-Dβ-PSMA, Ibu-N-PSMA and Ibu-DAB-PSMA) provided these ligands in isolated overall yields of ≥2.8% after HPLC purification. The ligands were characterized by analytical RP-HPLC and MALDI-MS, respectively. The chemical purity of the compounds was ≥99.2%. Analytical data are presented in Table 1.

    TABLE-US-00001 TABLE 1 Analytical data of the PSMA-ligands: Ibu-PSMA, Ibu-Dα-PSMA, Ibu-Dβ-PSMA, Ibu-N-PSMA and Ibu-DAB-PSMA. Chemical Chemical MW Amount Yield purity.sup.b Compound formula [g/mol] m/z.sup.a [mg] [%] [%] Ibu-PSMA C.sub.68H.sub.99N.sub.11O.sub.18 1358.72 1358.60 3.8 2.8 >99.2 Ibu-Dα-PSMA C.sub.72H.sub.104N.sub.12O.sub.21 1473.75 1473.69 8.4 5.7 >99.5 Ibu-Dβ-PSMA C.sub.72H.sub.104N.sub.12O.sub.21 1473.75 1473.69 4.7 3.2 >99.6 Ibu-N-PSMA C.sub.72H.sub.105N.sub.13O.sub.20 1472.76 1472.70 12.0 8.2 >99.7 Ibu-DAB-PSMA C.sub.72H.sub.107N.sub.13O.sub.19 1458.78 1458.72 21.3 14.6 >99.5 .sup.am/z-peak of the unlabeled ligand obtained by mass spectrometry; .sup.bDetermined by analytical HPLC, λ = 254 nm;

    2.2. Synthesis of Precursor 1

    [0345] The PSMA-targeting urea-based PSMA-binding entity—L-Glu-NH—CO—NH-L-Lys—was prepared on a 2-chlotrotrityl chloride (2-CT) resin in analogy to the method described by Eder eta/. (Eder, M.; Schäfer, M.; Bauder-Wust, U.; Hull, W. E.; Wängler, C.; Mier, W.; Haberkorn, U.; Eisenhut, M. .sup.68Ga-complex lipophilicity and the targeting property of a urea-based PSMA inhibitor for PET imaging. Bioconjug Chem 2012, 23, (4), 688-97). The linker area consisting of a 2-naphthyl-L-Ala and a trans-cyclohexyl moiety was synthesized as previously reported by Benes̆ová et al. (Benesova, M.; Schäfer, M.; Bauder-Wüst, U.; Afshar-Oromieh, A.; Kratochwil, C.; Mier, W.; Haberkorn, U.; Kopka, K.; Eder, M. Preclinical evaluation of a tailor-made DOTA-conjugated PSMA inhibitor with optimized linker moiety for imaging and endoradiotherapy of prostate cancer. J Nucl Med 2015, 56, (6), 914-20). The conjugation of the DOTA-chelator conjugated via a Nα-amino-L-Lys to above described construct was previously reported by Umbricht et al. (Umbricht, C. A.; Benesova, M.; Schibli, R.; Müller, C. Preclinical development of novel PSMA-targeting radioligands: modulation of albumin-binding properties to improve prostate cancer therapy. Mol Pharm 2018, Mol Pharm 2018, 15, (6), 2297-2306).

    [0346] The following resin-immobilized precursor was used as the basis for the synthesis of the PSMA-ligands (“precursor 1”):

    ##STR00064##

    [0347] Precursor 1 is based on the PSMA-binding entity and a DOTA-chelator. This precursor was employed for the synthesis of the five exemplified ligands Ibu-PSMA, Ibu-Dα-PSMA, Ibu-Dβ-PSMA, Ibu-N-PSMA and Ibu-DAB-PSMA. The free amino group of the lysine side chain was used for conjugation of ibuprofen which was connected directly or via an amino acid entity.

    2.3. Synthesis of Ibu-PSMA

    [0348] The synthesis of Ibu-PSMA was performed by coupling the albumin-binding ibuprofen to the resin-immobilized precursor 1. The resin was swelled in anhydrous dichloromethane (DCM, Acros Organics) for 45 min and subsequently conditioned in N,N-dimethylformamide (DMF, Acros Organics). Relative to the resin-immobilized precursor 1 (0.10 mmol), 4.0-6.0 equiv 2-(4-(2-methylpropyl)phenyl)propanoic acid (ibuprofen; Sigma Aldrich; 0.400-0.600 mmol) were activated using 3.96 equiv N,N,N′,N′-tetramethyl-O-(1H-benzotriazol-1-yl)-uronium hexafluoro-phosphate (HBTU; Sigma Aldrich, 0.396-0.594 mmol) in the presence of 4.0-6.0 equiv DIPEA (N,N-diisopropylethylamine, Sigma Aldrich, 0.400-0.600 mmol) in anhydrous DMF. Two minutes after the addition of DIPEA, the activated solution was added to the precursor 1 and agitated up to 2 h. The resin was washed with DMF, DCM and diethyl ether, respectively, and dried under reduced pressure. The product was cleaved from the resin and subsequently deprotected within 3-6 h using a mixture consisting of trifluoroacetic acid (TFA, Sigma Aldrich), triisopropylsilane (TIPS, Sigma Aldrich) and Milli-Q water in a ratio of 95:2.5:2.5 (v/v). TFA was evaporated, the crude compound dissolved in acetonitrile (ACN, VWR Chemicals) and Milli-Q water in a ratio of 1:2 (v/v) and purified by RP-HPLC to yield Ibu-PSMA.

    2.4. Synthesis of Ibu-Dα-PSMA

    [0349] The additional spacer entity consisting of D-aspartic acid (D-Asp) was conjugated to NE-L-lysine of the precursor 1 before coupling the ibuprofen. The resin-immobilized precursor 1 was pre-swollen in DCM and conditioned in DMF as described above. Relative to precursor 1 (0.100 mmol), 4.0 equiv Fmoc and t-Bu protected D-Asp (Fmoc-D-Asp(O-t-Bu)-OH, Sigma Aldrich, 0.400 mmol) were activated using 3.96 equiv HBTU (0.396 mmol) in the presence of 4.0 equiv DIPEA (0.400 mmol) in anhydrous DMF. Two minutes after the addition of DIPEA, the activated solution was added to precursor 1 and agitated up to 2 h. The resin was washed with DMF. The Na-Fmoc-protecting group was cleaved by agitating with a mixture of DMF and piperidine (Fluka) in a ratio of 1:1 (v/v) twice for 5 min. The resin was again washed with DMF. Ibuprofen (4.0-6.0 equiv; 0.400-0.600 mmol) was activated using 3.96 equiv HBTU (0.396-0.594 mmol) in the presence of 4.0-6.0 equiv DIPEA (0.400-0.600 mmol) in anhydrous DMF. Two minutes after the addition of DIPEA, the activated solution was added to the resin and agitated up to 2 h. Subsequently, resin was washed with DMF, DCM and diethyl ether, respectively, and dried under reduced pressure. The product was cleaved from the resin and simultaneously deprotected with a mixture consisting of TFA, TIPS and water in a ratio of 95:2.5:2.5 (v/v) within 3-6 h. TFA was evaporated, The crude compound dissolved in acetonitrile (ACN, VWR Chemicals) and Milli-Q water in a ratio of 1:2 (v/v) and purified by RP-HPLC to yield Ibu-Dα-PSMA.

    2.5. Synthesis of Ibu-Dβ-PSMA

    [0350] The additional spacer entity consisting of D-aspartic acid (D-Asp) was conjugated to Nε-L-lysine of the precursor 1 before coupling the ibuprofen. The resin-immobilized precursor 1 was pre-swollen in DCM and conditioned in DMF as described above. Relative to precursor 1 (0.100 mmol), 4.0 equiv of Fmoc and t-Bu protected D-Asp (Fmoc-D-Asp-O-t-Bu, Merck group, 0.400 mmol) were activated using 3.96 equiv HBTU (0.396 mmol) in the presence of 4.0 equiv DIPEA (0.400 mmol) in anhydrous DMF. Two minutes after the addition of DIPEA, the activated solution was added to the precursor 1 and agitated up to 2 h. The resin was washed with DMF and the Na-Fmoc-protecting group was cleaved by agitating with a mixture of DMF and piperidine (Fluka) in a ratio of 1:1 (v/v) twice for 5 min. The resin was again washed with DMF. Ibuprofen (4.0-6.0 equiv; 0.400-0.600 mmol) was activated using 3.96 equiv HBTU (0.396-0.594 mmol) in the presence of 4.0-6.0 equiv DIPEA (0.400-0.600 mmol) in anhydrous DMF. Two minutes after the addition of DIPEA, the activated solution was added to the resin and agitated up to 2 h. Subsequently, resin was washed with DMF, DCM and diethyl ether, respectively, and dried under reduced pressure. The product was cleaved from the resin and simultaneously deprotected with a mixture consisting of TFA, TIPS and water in a ratio of 95:2.5:2.5 (v/v) within 3-6 h. TFA was evaporated, the crude compound dissolved in acetonitrile (ACN, VWR Chemicals) and Milli-Q water in a ratio of 1:2 (v/v,) and purified by RP-HPLC to yield Ibu-Dβ-PSMA.

    2.6. Synthesis of Ibu-N-PSMA

    [0351] The additional spacer entity consisting of D-asparagine (D-Asn) was conjugated to Nε-L-lysine of the precursor 1 before coupling the ibuprofen. The resin-immobilized precursor 1 was pre-swollen in DCM and conditioned in DMF as described above. Relative to precursor 1 (0.100 mmol), 4.0 equiv of Fmoc and Trt (trityl) protected D-asparagine (Fmoc-D-Asn(Trt)-OH, Sigma Aldrich, 0.400 mmol) were activated using 3.96 equiv HBTU (0.396 mmol) in the presence of 4.0 equiv DIPEA (0.400 mmol) in anhydrous DMF. Two minutes after the addition of DIPEA, the activated solution was added to the precursor 1 and agitated up to 3 h. The resin was washed with DMF and and the Nα-Fmoc-protecting group was cleaved by agitating with a mixture of DMF and piperidine (Fluka) in a ratio of 1:1 (v/v) twice for 5 min. The resin was again washed with DMF. Ibuprofen (4.0-6.0 equiv; 0.400-0.600 mmol) were activated using 3.96 equiv HBTU (0.396-0.594 mmol) in the presence of 4.0-6.0 equiv DIPEA (0.400-0.600 mmol) in anhydrous DMF. Two minutes after the addition of DIPEA, the activated solution was added to the resin and agitated up to 2 h. Subsequently, resin was washed with DMF, DCM and diethyl ether, respectively, and dried under reduced pressure. The product was cleaved from the resin with a mixture consisting of TFA, TIPS and water in a ratio of 95:2.5:2.5 (v/v) within 3-6 h. The t-Bu-protecting groups and the additional Trt-protecting group were cleaved simultaneously. TFA was evaporated, the crude compound dissolved in acetonitrile (ACN, VWR Chemicals) and Milli-Q water in a ratio of 1:2 (v/v) and purified by RP-HPLC to yield Ibu-N-PSMA.

    2.7. Synthesis of Ibu-DAB-PSMA

    [0352] The additional spacer entity consisting of D-diaminobutyric acid was conjugated to Nε-L-lysine of the precursor 1 before coupling the ibuprofen. The resin-immobilized precursor 1 was pre-swollen in DCM and conditioned in DMF as described above. Relative to precursor 1 (0.100 mmol), 4.0 equiv of Fmoc and Boc (tert-Butyloxycarbonyl) protected D-diaminobutyric acid (DAB; Fmoc-D-Dab(Boc)-OH, Iris Biotech, 0.400 mmol) were activated using 3.96 equiv HBTU (0.396 mmol) in the presence of 4.0 equiv DIPEA (0.400 mmol) in anhydrous DMF. Two minutes after the addition of DIPEA, the activated solution was added to the precursor 1 and agitated up to 3.5 h. The resin was washed with DMF and the Nα-Fmoc-protecting group was cleaved by agitating with a mixture of DMF and piperidine (Fluka) in a ratio of 1:1 (v/v) twice for 5 min. The resin was again washed with DMF. Ibuprofen (4.0-6.0 equiv; 0.400-0.600 mmol) were activated using 3.96 equiv HBTU (0.396-0.594 mmol) in the presence of 4.0-6.0 equiv DIPEA (0.400-0.600 mmol) in anhydrous DMF. Two minutes after the addition of DIPEA, the activated solution was added to the resin and agitated up to 2 h. Subsequently, the resin was washed with DMF, DCM and diethyl ether, respectively, and dried under reduced pressure. The product was cleaved from the resin with a mixture consisting of TFA, TIPS and water in a ratio of 95:2.5:2.5 (v/v) within 3-6 h. The t-Bu-protecting groups and the additional Boc-protecting group were cleaved simultaneously. TFA was evaporated, the crude compound dissolved in acetonitrile (ACN, VWR Chemicals) and Milli-Q water in a ratio of 1:2 (v/v) and purified by RP-HPLC to yield Ibu-DAB-PSMA.

    2.8. Synthesis of Ibu-sPSMA

    [0353] In analogy to the other ibuprofen-bearing PSMA ligands, Ibu-sPSMA was synthesized via a solid-phase platform as previously reported (see also section 2 above) for the synthesis of other PSMA-ligands (Umbricht, C. A.; Mol Pharm 2018, 15, (6):2297-2306). A multistep synthesis (17 steps) provided this ligand in an isolated overall yield of ≥14% after HPLC purification.

    2.8.1. Synthesis of Precursor 1

    [0354] The PSMA-targeting urea-based PSMA-binding entity—L-Glu-NH—CO—NH-L-Lys—was prepared on a 2-chlotrotrityl chloride (2-CT) resin in analogy to the method described by Eder et al. (Bioconjug Chem 2012, 23, (4), 688-97), see also section 2. above. The linker area consisting of a 2-naphthyl-L-Ala and a trans-cyclohexyl moiety was synthesized as previously reported by Benes̆ová et al. (J Nucl Med 2015, 56, (6), 914-20). In this case, however, a different precursor than for the other Ibu-PSMA ligands was used. The linker entity L-diaminobutyric acid was by two carbon atoms shorter as compared to L-lysine, which was used as linker for the synthesis of Ibu-PSMA. The conjugation of the DOTA-chelator to above described construct was previously reported by Umbricht et al. (Mol Pharm 2018, 15, (6):2297-2306).

    [0355] The following resin-immobilized precursor (precursor 1)

    ##STR00065##

    was used as the basis for the synthesis of the Ibu-sPSMA. Precursor 1 is based on the PSMA-binding entity and a DOTA-chelator. This precursor incorporated a shorter connecting entity than employed for other Ibu-PSMA ligands, e.g. Ibu-PSMA.

    2.8.2. Synthesis of Ibu-sPSMA

    [0356] The synthesis of Ibu-sPSMA was performed by coupling the albumin-binding ibuprofen to the resin-immobilized precursor 1 (FIG. 13). The free γ-amino group of the diaminobutyric acid side chain was used for conjugation of ibuprofen. The resin was swelled in anhydrous dichloromethane (DCM, Acros Organics) for 45 min and subsequently conditioned in N,N-dimethylformamide (DMF, Acros Organics). Relative to the resin-immobilized precursor 1 (0.10 mmol), 6.0 equiv 2-(4-(2-methylpropyl)phenyl)propanoic acid (ibuprofen; Sigma Aldrich; 0.60 mmol) were activated using 5.94 equiv N,N,N′,N′-tetramethyl-O-(1H-benzotriazol-1-yl)-uronium hexafluoro-phosphate (HBTU; Sigma Aldrich, 0.59 mmol) in the presence of 8.0 equiv DIPEA (N,N-diisopropylethylamine, Sigma Aldrich, 0.80 mmol) in anhydrous DMF. Two minutes after the addition of DIPEA, the activated solution was added to the precursor 1 and agitated up to 2 h to yield resin-immobilized compound 2 (FIG. 13). The resin was washed with DMF, DCM and diethyl ether, respectively, and dried under reduced pressure. The product was cleaved from the resin and subsequently deprotected within 2 h using a mixture consisting of trifluoroacetic acid (TFA, Sigma Aldrich), triisopropylsilane (TIPS, Sigma Aldrich) and Milli-Q water in a ratio of 95:2.5:2.5 (v/v) to give the crude product (FIG. 13). TFA was evaporated, the crude compound dissolved in acetonitrile (ACN, VWR Chemicals) and Milli-Q water in a ratio of 1:2 (v/v) and purified by HPLC to yield pure Ibu-sPSMA.

    [0357] The ligand was characterized by analytical HPLC and MALDI-MS, respectively. The chemical purity of the compound was ≥99%. Analytical data are presented in Table 2.

    TABLE-US-00002 TABLE 2 Analytical data of Ibu-sPSMA. Chemical Chemical MW Amount Yield purity.sup.b Compound formula [g/mol] m/z.sup.a [mg] [%] [%] Ibu-sPSMA C.sub.66H.sub.95N.sub.11O.sub.18 1330.55 1330.69 28 15 >99 .sup.am/z-peak of the unlabeled ligand obtained by mass spectrometry; .sup.bDetermined by analytical HPLC, λ = 254 nm;

    2.9. Synthesis of the Compound Lbu-DAB-PSMA as an Example

    [0358] The synthesis schemes 1-4, which are shown in FIGS. 1-4, respectively, show the details of the synthesis of the compound Ibu-DAB-PSMA as an example. Synthesis of the other exemplified compounds was performed in a similar manner.

    Example 4: Radiolabeling and Stability

    [0359] The stock solution of prior art PSMA-ligand PSMA-617 (ABX GmbH, Radeberg, Germany) was prepared by dilution of the ligand in MilliQ water to a final concentration of 1 mM. Ibu-PSMA, Ibu-Dα-PSMA, Ibu-Dβ-PSMA, Ibu-N-PSMA and Ibu-DAB-PSMA were diluted in Milli-Q water/sodium acetate (0.5 M, pH 8) to obtain a final concentration of 1 mM. All PSMA-ligands were labeled with .sup.177Lu (no-carrier added .sup.177Lu in 0.05 M HCl; Isotope Technologies Garching ITG GmbH, Germany) in a 1:5 (v/v) mixture of sodium acetate (0.5 M, pH 8) and HCl (0.05 M, pH ˜1) at pH ˜4.5. The PSMA-ligands were labeled with .sup.177Lu at specific activities between 5-50 MBq/nmol, depending on the experiment to be performed. The reaction mixture was incubated for 10 min at 95° C., followed by a quality control using RP-HPLC with a C-18 reversed-phase column (Xterra™ MS, C18, 5 μm, 150×4.6 mm; Waters). The mobile phase consisted of MilliQ water containing 0.1% trifluoracetic acid (A) and acetonitrile (B) with a gradient of 95% A and 5% B to 20% A and 80% B over a period of 15 min at a flow rate of 1.0 mL/min. The radioligands were diluted in Milli-Q water containing Nα-DTPA (50 μM) prior to injection into HPLC. FIG. 5 shows representative HPLC chromatograms.

    Example 5: n-Octanol/PBS Distribution Coefficient

    [0360] The n-octanol/PBS distribution coefficient of the five exemplified PSMA-binding agents .sup.177Lu-Ibu-PSMA, .sup.177Lu-Ibu-Dα-PSMA, .sup.177Lu-Ibu-Dβ-PSMA, .sup.177Lu-Ibu-N-PSMA and .sup.177Lu-Ibu-DAB-PSMA in a n-octanol/PBS system was performed in a similar manner as previously reported (Benesova, M.; Umbricht, C. A.; Schibli, R.; Müller, C. Albumin-binding PSMA ligands: optimization of the tissue distribution profile. Mol Pharm 2018, 15, (3), 934-946).

    [0361] Results are shown in FIG. 6. All radioligands showed hydrophilic properties with log D values <2.2. .sup.177Lu-Ibu-PSMA, .sup.177Lu-Ibu-N-PSMA and .sup.177Lu-Ibu-DAB-PSMA showed similar values, while the coefficients of .sup.177Lu-Ibu-Dβ-PSMA and .sup.177Lu-Ibu-Dα-PSMA were slightly lower, indicating more hydrophilic properties. The modification of the PSMA ligands with ibuprofen had an effect towards more hydrophobic properties of the radioligands as compared to prior art PSMA-ligand .sup.177Lu-PSMA-617, which does not contain an albumin-binding entity.

    Example 6: In Vitro Albumin-Binding Properties

    [0362] Plasma protein-binding properties of the five exemplified PSMA-binding agents .sup.177Lu-Ibu-PSMA, .sup.177Lu-Ibu-Dα-PSMA, .sup.177Lu-Ibu-Dβ-PSMA, .sup.177Lu-Ibu-N-PSMA and .sup.177Lu-Ibu-DAB-PSMA as well as of prior art PSMA-binding agent .sup.177Lu-PSMA-617 (which does not contain an albumin-binding entity) was determined using an ultrafiltration assay in a similar manner as previously reported (Benesova, M.; Umbricht, C. A.; Schibli, R.; Müller, C. Albumin-binding PSMA ligands: optimization of the tissue distribution profile. Mol Pharm 2018, 15, (3), 934-946). In short, the PSMA-ligands were labeled with .sup.177Lu at a specific activity of 50 MBq/nmol and incubated in human plasma samples or PBS at room temperature. The free and plasma-bound fractions were separated using a centrifree ultrafiltration device (4104 centrifugal filter units; Millipore, 30000 Da nominal molecular weight limit, methylcellulose micropartition membranes). The incubated solution was loaded to the ultrafiltration device and centrifuged at 2500 rpm for 40 min at 20° C. Samples from the filtrate were taken and analyzed for radioactivity in a γ-counter. The amount of plasma-bound radioligand was calculated as the fraction of radioactivity measured in the filtrate relative to the corresponding loading solution (set to 100%). The experiments were performed at least 3 times for each radioligand.

    [0363] Results are shown in FIG. 7. The ultrafiltration experiments of .sup.177Lu-Ibu-PSMA, .sup.177Lu-Ibu-Dα-PSMA, .sup.177Lu-Ibu-Dβ-PSMA, .sup.177Lu-Ibu-N-PSMA and .sup.177Lu-Ibu-DAB-PSMA revealed high serum protein binding, visible by the fact that <11% of the radioligands penetrated the filter membrane when incubated in human plasma. The radioligands did not show any retention by the filter membrane when incubated in PBS (which does not contain proteins). .sup.177Lu-Ibu-N-PSMA and .sup.177Lu-Ibu-DAB-PSMA showed slightly reduced plasma protein-binding properties as compared to the other ibuprofen-derivatized radioligands. All five exemplified PSMA-binding agents .sup.177Lu-PSMA-ligands showed increased binding to plasma proteins when compared to .sup.177Lu-PSMA-617 which showed an albumin-bound fraction of only about 59%.

    Example 7: In Vitro Cell Internalization Study

    [0364] Cell uptake and internalization of .sup.177Lu-Ibu-PSMA, .sup.177Lu-Ibu-Dα-PSMA, .sup.177Lu-Ibu-Dβ-PSMA, .sup.177Lu-Ibu-N-PSMA and .sup.177Lu-Ibu-DAB-PSMA were investigated using PSMA-positive PC-3 PIP and PSMA-negative PC-3 flu tumor cells kindly provided by Prof. Dr. Martin Pomper (John Hopkins Institutions, Baltimore, U.S.; Eiber, M.; Fendler, W. P.; Rowe, S. P.; Calais, J.; Hofman, M. S.; Maurer, T.; Schwarzenboeck, S. M.; Kratowchil, C.; Herrmann, K.; Giesel, F. L. Prostate-specific membrane antigen ligands for imaging and therapy. J Nucl Med 2017, 58, (Suppl 2), 67S-76S). Each radioligand was investigated by the performance of experiments performed 3 times in triplicate with PC-3 PIP tumor cell and once in triplicate with PC-3 flu tumor cells.

    [0365] Results are shown in FIG. 8. The uptake of all radioligands into PC-3 PIP tumor cells was comparable to .sup.177Lu-PSMA-617 after incubation of 2 h or 4 h, respectively (FIG. 8A). The internalized fraction of .sup.177Lu-Ibu-PSMA and .sup.177Lu-Dβ-PSMA was slightly higher than for .sup.177Lu-Ibu-Dα-PSMA, .sup.177Lu-Ibu-N-PSMA, .sup.177Lu-Ibu-DAB-PSMA and .sup.177Lu-PSMA-617, which were all in the same range (FIG. 8A). The uptake of all radioligands in PC-3 flu tumor cells was <2% after 4 h, which indicated a highly PSMA-specific cell uptake (FIG. 8B).

    Example 8: In Vivo Biodistribution Study

    [0366] In vivo experiments were approved by the local veterinarian department and conducted in accordance with the Swiss law of animal protection. Mice were obtained from Charles River Laboratories, Sulzfeld, Germany, at the age of 5-6 weeks. Female, athymic nude Balb/c mice were subcutaneously inoculated with PSMA-positive PC-3 PIP cells (6×10.sup.6 cells in 100 μL Hank's balanced salt solution (HBSS) with Ca.sup.2+/Mg.sup.2+) on the right shoulder and with PSMA-negative PC-3 flu cells (5×10.sup.6 cells in 100 μL HBSS Ca.sup.2+/Mg.sup.2+) on the left shoulder. Two weeks later, the tumors reached a size of about 80-300 mm.sup.3 suitable for the performance of the biodistribution studies.

    [0367] Biodistribution studies were performed 12-15 days after PC-3 PIP/flu tumor cell inoculation. The radioligands .sup.177Lu-Ibu-PSMA, .sup.177Lu-Ibu-Dβ-PSMA, .sup.177Lu-Ibu-Dα-PSMA, .sup.177Lu-Ibu-N-PSMA, .sup.177Lu-Ibu-DAB-PSMA and .sup.177Lu-PSMA-617 were diluted in 0.9% NaCl containing 0.05% bovine serum albumin (BSA) to prevent adhesion to the vial and syringe material. The radioligands were injected in a lateral tail vein in a volume of 100-200 μL. Mice were euthanized at different time points after injection (p.i.) of the radioligands. Selected tissues and organs were collected, weighed and measured using a γ-counter. The results were decay-corrected and listed as a percentage of the injected activity per gram of tissue mass (% IA/g) (Table 3 and 4).

    TABLE-US-00003 TABLE 3 Biodistribution data of .sup.177Lu-Ibu-PSMA, .sup.177Lu-Ibu-Dβ-PSMA and .sup.177Lu-Ibu-Dα-PSMA in PC-3 PIP/flu tumor-bearing mice. Average value ± SD obtained from each group of mice (n = 3-6). .sup.177Lu-Ibu-PSMA .sup.177Lu-Ibu-Dβ-PSMA .sup.177Lu-Ibu-Dα-PSMA 4 h.p.i. 24 h.p.i. 4 h.p.i. 24 h.p.i. 4 h.p.i. 24 h.p.i. Blood 5.96 ± 1.53 0.58 ± 0.09 13.2 ± 1.15 1.28 ± 0.05 2.33 ± 0.71 0.33 ± 0.06 Heart 2.28 ± 0.68 0.33 ± 0.06 4.19 ± 0.23 0.57 ± 0.02 0.89 ± 0.27 0.18 ± 0.03 Lung 3.72 ± 0.73 0.62 ± 0.12 8.55 ± 3.30 1.17 ± 0.06 1.54 ± 0.35 0.39 ± 0.09 Spleen 1.76 ± 0.23 0.68 ± 0.15 2.60 ± 0.23 1.03 ± 0.08 0.90 ± 0.04 0.46 ± 0.10 Kidneys 32.5 ± 0.86 16.5 ± 1.48 32.2 ± 2.46 21.3 ± 1.53 27.2 ± 3.31 18.2 ± 3.06 Stomach 0.79 ± 0.26 0.23 ± 0.02 1.51 ± 0.18 0.48 ± 0.13 0.25 ± 0.08 0.12 ± 0.03 Intestines 1.00 ± 0.27 0.23 ± 0.04 1.81 ± 0.23 0.40 ± 0.07 0.49 ± 0.08 0.11 ± 0.02 Liver 2.77 ± 0.43 0.91 ± 0.12 3.87 ± 0.16 0.69 ± 0.07 0.84 ± 0.22 0.37 ± 0.04 Salivary 1.66 ± 0.36 0.34 ± 0.07 3.33 ± 0.15 0.61 ± 0.02 0.74 ± 0.20 0.21 ± 0.04 glands Muscle 0.97 ± 0.37 0.12 ± 0.04 1.79 ± 0.48 0.22 ± 0.04 0.35 ± 0.08 0.07 ± 0.02 Bone 1.01 ± 0.18 0.20 ± 0.04 1.87 ± 0.16 0.26 ± 0.03 0.37 ± 0.10 0.11 ± 0.02 PC-3 PIP 81.3 ± 6.28 86.9 ± 18.0 65.7 ± 7.31  106 ± 9.70 49.4 ± 5.33 84.2 ± 14.9 Tumor PC-3 flu 2.19 ± 0.52 0.58 ± 0.19 3.86 ± 0.18 0.79 ± 0.12 1.00 ± 0.21 0.38 ± 0.05 Tumor Tumor-to- 14.1 ± 2.25  149 ± 16.7 5.03 ± 0.73 83.6 ± 9.06 22.5 ± 5.28  198 ± 32.6 blood Tumor-to- 29.7 ± 3.11 95.4 ± 10.6 17.0 ± 1.73  151 ± 3.37 60.8 ± 11.1  196 ± 44.4 liver Tumor-to- 2.60 ± 0.08 5.36 ± 0.73 2.06 ± 0.23 4.90 ± 0.29 1.82 ± 0.03 3.56 ± 0.34 kidney

    TABLE-US-00004 TABLE 4 Biodistribution of .sup.177Lu-Ibu-N-PSMA, .sup.177Lu-Ibu-DAB-PSMA and .sup.177Lu-PSMA-617 in PC-3 PIP/flu tumor-bearing mice. Average value ± SD obtained from each group of mice (n = 3-6). .sup.177Lu-Ibu-N-PSMA .sup.177Lu-Ibu-DAB-PSMA .sup.177Lu-PSMA-617 4 h.p.i. 24 h.p.i. 4 h.p.i. 24 h.p.i. 4 h.p.i. 24 h.p.i. Blood 3.58 ± 1.39 0.25 ± 0.07 3.66 ± 0.45 0.16 ± 0.02 0.02 ± 0.00 0.01 ± 0.00 Heart 1.23 ± 0.61 0.12 ± 0.03 1.32 ± 0.10 0.10 ± 0.01 0.03 ± 0.00 0.01 ± 0.00 Lung 2.20 ± 0.95 0.24 ± 0.06 2.40 ± 0.31 0.21 ± 0.03 0.07 ± 0.01 0.03 ± 0.00 Spleen 1.40 ± 0.75 0.24 ± 0.06 1.14 ± 0.11 0.32 ± 0.03 0.15 ± 0.04 0.05 ± 0.01 Kidneys 27.1 ± 8.37 8.02 ± 1.13 19.4 ± 1.84 6.00 ± 0.68 3.68 ± 1.05 0.76 ± 0.15 Stomach 0.51 ± 0.23 0.17 ± 0.09 0.62 ± 0.12 0.18 ± 0.13 0.08 ± 0.03 0.03 ± 0.01 Intestines 0.50 ± 0.18 0.09 ± 0.02 0.71 ± 0.05 0.11 ± 0.03 0.07 ± 0.05 0.04 ± 0.01 Liver 1.27 ± 0.55 0.32 ± 0.05 1.50 ± 0.12 0.56 ± 0.10 0.09 ± 0.01 0.07 ± 0.02 Salivary 0.94 ± 0.38 0.57 ± 0.35 0.86 ± 0.38 0.56 ± 0.37 0.04 ± 0.01 0.02 ± 0.00 glands Muscle 0.47 ± 0.19 0.06 ± 0.02 0.54 ± 0.09 0.05 ± 0.01 0.02 ± 0.00 0.01 ± 0.00 Bone 0.56 ± 0.22 0.08 ± 0.02 0.62 ± 0.08 0.09 ± 0.02 0.06 ± 0.02 0.03 ± 0.01 PC-3 PIP 65.4 ± 15.6 58.0 ± 21.0 66.2 ± 11.0 52.4 ± 2.35 56.0 ± 7.95 37.3 ± 5.80 Tumor PC-3 flu 1.15 ± 0.52 0.23 ± 0.01 1.19 ± 0.30 0.17 ± 0.01 0.08 ± 0.01 0.05 ± 0.01 Tumor Tumor-to- 20.1 ± 6.22  227 ± 41.7 18.2 ± 2.69  337 ± 40.4 2315 ± 132  2730 ± 239  blood Tumor-to- 18.2 ± 2.70  182 ± 52.6 44.6 ± 8.41 98.3 ± 21.1  598 ± 33.2  528 ± 62.4 liver Tumor-to- 2.48 ± 0.11 6.90 ± 2.43 3.01 ± 0.49 8.88 ± 0.99 15.7 ± 2.79 49.5 ± 4.48 kidney

    [0368] The biodistribution data and the tumor-to-background ratios are also shown in FIGS. 9 and 10, respectively.

    [0369] Uptake into the PC-3 PIP tumors was fastest for .sup.177Lu-Ibu-PSMA, which was designed without an additional amino acid-based spacer entity. The tumor accumulation reached 81.3±6.28% IA/g already at 4 h p.i. and was even slightly higher at 24 h p.i. (86.8±18.0% IA/g). .sup.177Lu-Ibu-Dβ-PSMA, .sup.177Lu-Ibu-N-PSMA and .sup.177Lu-Ibu-DAB-PSMA demonstrated similar accumulation in PC-3 PIP tumors at 4 h p.i., respectively (65-66% IA/g), but different retention in the tumor tissue. At 24 h p.i., a strongly increased tumor uptake was found for .sup.177Lu-Ibu-Dβ-PSMA (106±9.70% IA/g), while radioactivity levels decreased in the case of .sup.177Lu-Ibu-N-PSMA and .sup.177Lu-Ibu-DAB-PSMA (52-58% IA/g). In the case of .sup.177Lu-Ibu-Dα-PSMA, high accumulation in the tumor was only found after 24 h (84.2±14.9% IA/g). The tumor uptake of all radioligands containing ibuprofen was higher than after injection of prior art radioligand .sup.177Lu-PSMA-617 (37.3±5.80% IA/g) at 24 h p.i. Uptake in PC-3 flu tumors (PSMA-negative) was clearly below blood levels after injection of all radioligands confirming the PSMA-mediated uptake.

    [0370] Highest blood activity levels (13.2±1.15% IA/g) were detected for .sup.177Lu-Ibu-Dβ-PSMA at 4 h p.i., while all other compounds showed lower radioactivity accumulation in the blood pool at this time point (2.33-5.96% IA/g). Mice injected with .sup.177Lu-Ibu-PSMA, .sup.177Lu-Lbu-Dα-PSMA, .sup.177Lu-Ibu-N-PSMA and .sup.177Lu-Ibu-DAB-PSMA showed fast clearance of radioactivity from the blood resulting in <0.6% IA/g after 24 h whereas clearance of .sup.177Lu-Ibu-Dβ-PSMA was slower resulting in still ˜1.3% IA/g at this same time point.

    [0371] Kidney uptake was lowest for .sup.177Lu-Ibu-DAB-PSMA at both 4 h and 24 h p.i. (19.4±1.84% and 6.00±0.68% IA/g, respectively) whereas the other radioligands showed a kidney uptake of 27-33% IA/g at 4 h p.i. .sup.177Lu-Ibu-N-PSMA demonstrated fastest renal clearance resulting in 8.02±1.13% IA/g at 24 h p.i. reaching a similar level as .sup.177Lu-Ibu-DAB-PSMA. Radioactivity levels in all other tissues were below the blood levels and decreased overtime.

    [0372] Tumor-to-blood ratios of accumulated radioactivity were similar after injection of .sup.177Lu-Ibu-PSMA, .sup.177Lu-Ibu-Dα-PSMA, .sup.177Lu-Ibu-N-PSMA and .sup.177Lu-Ibu-DAB-PSMA (14-23), but lower after injection of .sup.177Lu-Ibu-Dβ-PSMA (5.03±0.73) at 4 h p.i. At 24 h p.i., the tumor-to-blood ratio of .sup.177Lu-Ibu-DAB-PSMA (˜337) was highest, followed by .sup.177Lu-Ibu-N-PSMA (˜227), .sup.177Lu-Ibu-Dα-PSMA (˜198), .sup.177Lu-Ibu-PSMA (˜149) and .sup.177Lu-Ibu-Dβ-PSMA (˜84). Tumor-to-kidney ratios were similar for all radioligands at 4 h p.i., but differed by a factor of ˜2 at 24 h p.i. with the highest ratios obtained after injection of .sup.177Lu-Ibu-DAB-PSMA and .sup.177Lu-Ibu-N-PSMA. The tumor-to-liver ratio at 24 h p.i. was highest for .sup.177Lu-Ibu-Dα-PSMA (196) and .sup.177Lu-Ibu-N-PSMA (182).

    Example 9: In Vivo Whole-Body-Activity Measurements

    [0373] In vivo experiments were approved by the local veterinarian department and conducted in accordance with the Swiss law of animal protection. Mice were obtained from Charles River Laboratories, Sulzfeld, Germany, at the age of 5-6 weeks. Female, athymic nude Balb/c mice were subcutaneously inoculated with PSMA-positive PC-3 PIP cells (6×10.sup.6 cells in 100 μL Hank's balanced salt solution (HBSS) with Ca.sup.2+/Mg.sup.2+) on the right shoulder and with PSMA-negative PC-3 flu cells (5×10.sup.6 cells in 100 μL HBSS Ca.sup.2+/Mg.sup.2+) on the left shoulder. Two weeks later, the tumors reached a size of about 80-300 mm.sup.3 suitable for the performance of the imaging studies.

    [0374] The single radioligands (specific activity: 30 MBq/nmol) were diluted in 0.9% NaCl containing 0.05% bovine serum albumin (BSA) and i.v. injected into PC-3 PIP/flu tumor bearing mice (30 MBq, 1 nmol, 100 μL) for SPECT/CT imaging purposes. The mice were measured in a dose calibrator at 4 h, 24 h, 48 h and 72 h p.i., respectively.

    [0375] Results are shown in FIG. 11. The whole-body measurements revealed different excretion patterns for the single radioligands which was manifest most prominently at the 4 h p.i-time point. The body retention at 4 h p.i. was highest for .sup.177Lu-Ibu-Dβ-PSMA (49%) and lower for .sup.177Lu-Ibu-PSMA (33%), .sup.177Lu-Ibu-Dα-PSMA (29%) and .sup.177Lu-Ibu-DAB-PSMA (17%) with .sup.177Lu-Ibu-N-PSMA (12%) showing the lowest body retention of radioactivity. All radioligands showed higher retention of radioactivity compared to .sup.177Lu-PSMA-617 (6.5%) with limited albumin-binding properties. Activity retention was, however, reduced in comparison to comparative radioligands, .sup.177Lu-PSMA-ALB-53 (93%) and .sup.177Lu-PSMA-ALB-56 (66%), which are equipped with albumin binders based on a p-iodophenyl- and p-tolyl-entity instead of ibuprofen, respectively (Umbricht, C. A.; Benesova, M.; Schibli, R.; Müller, C. Preclinical development of novel PSMA-targeting radioligands: modulation of albumin-binding properties to improve prostate cancer therapy. Mol Pharm 2018, Mol Pharm 2018, 15, (6), 2297-2306). In all cases of ibuprofen-derivatized radioligands, retention of radioactivity in the body decreased over time and reached similar retention fractions as .sup.177Lu-PSMA-617 at 72 h p.i. At this time point, .sup.177Lu-PSMA-ALB-53 and .sup.177Lu-PSMA-ALB-56 showed still 42% and 10% of the radioactivity, respectively, retained in the body.

    Example 10: In Vivo SPECT/CT Imaging

    [0376] SPECT/CT images were obtained using a dedicated small-animal SPECT/CT scanner (NanoSPECT/CT™, Mediso Medical Imaging Systems, Budapest, Hungary). SPECT/CT scans of 45 min duration were performed followed by a CT of 7.5 min. During the in vivo scans, the mice were anesthetized with a mixture of isoflurane and oxygen. Reconstruction of the acquired data was performed using HiSPECT software (version 1.4.3049, Scivis GmbH, Göttingen, Germany). All images were prepared using VivoQuant post-processing software (version 2.10, inviCRO Imaging Services and Software, Boston U.S.). A Gauss post-reconstruction filter (FWHM=1.0 mm) was applied to the images, which were presented with the scale adjusted to allow visualization of the most important organs and tissues, by cutting 5% of the lower scale.

    [0377] The SPECT images are shown in FIG. 12. The SPECT images visualize the PC-3 PIP tumor xenograft (right side) in which the radioligands accumulated to a high extent whereas in the PC-3 flu tumor (left side), accumulation of radioactivity was not observed. At the 4-h time point, some activity was also seen in the kidneys as well as in the urinary bladder as a consequence of renal clearance.

    Example 11: In Vitro Evaluation of .SUP.177.Lu-Ibu-sPSMA

    [0378] In this Example, a shorter methylene linker ((CH.sub.2).sub.2) than a lysine side chain ((CH.sub.2).sub.4) for the spacer connection of ibuprofen was evaluated in order to examine potential effects of the spacer length on the biodistribution profile of the radioligand. For this purpose, Ibu-sPSMA (“s” for “short” spacer) was designed and synthesized (Example 8.2). Ibu-sPSMA was radiolabeled with .sup.177Lu and preclinically evaluated. The stability of .sup.177Lu-Ibu-sPSMA as well as the albumin-binding properties and the capability to bind to PSMA-positive PC-3 PIP cells were investigated. Biodistribution studies and SPECT/CT imaging studies were performed with PC-3 PIP/flu tumor bearing mice. The new data was compared with those obtained with .sup.177Lu-PSMA-617 and with ibuprofen-functionalized PSMA radioligands or .sup.177Lu-PSMA-ALB-56.

    [0379] In vitro studies were conducted with .sup.177Lu-Ibu-sPSMA and compared to the results previously obtained with .sup.177Lu-PSMA-617 and, if appropriate, with .sup.177Lu-PSMA-ALB-56 (Umbricht et al, Mol Pharm 2018, 15, (6):2297-2306). Labeling efficiencies, n-octanol/PBS distribution coefficients (log D values) and albumin-binding studies were carried out. Uptake and internalization experiments were performed using the PSMA-transfected PSMA-positive PC-3 PIP tumor cell line and the mock-transfected PSMA-negative PC-3 flu tumor cell line.

    11.1. Radiolabeling

    [0380] Ibu-sPSMA was diluted in Milli-Q water/DMSO in a 3:1 (v/v) mixture to obtain a final concentration of 1 mM. The Ibu-sPSMA was labeled with .sup.177Lu (no-carrier added .sup.177Lu in 0.05 M HCl; Isotope Technologies Garching ITG GmbH, Germany) in a 1:5 (v/v) mixture of sodium acetate (0.5 M, pH 8) and HCl (0.05 M, pH ˜1) at pH ˜4.5. Ibu-sPSMA was labeled with .sup.177Lu at molar activities between 5-50 MBq/nmol, depending on the experiment to be performed. The reaction mixture was incubated for 10 min at 95° C., followed by a quality control using HPLC with a C-18 reversed-phase column (Xterra™ MS, C18, 5 μm, 150×4.6 mm; Waters). The mobile phase consisted of MilliQ water containing 0.1% trifluoroacetic acid (A) and acetonitrile (B) with a gradient of 95% A and 5% B to 20% A and 80% B over a period of 15 min at a flow rate of 1.0 mL/min. The radioligands were diluted in Milli-Q water containing Nα-DTPA (50 μM) prior to injection into HPLC (FIG. 14).

    11.2. Radiolytic Stability

    [0381] Radiolytic stability over time was assessed for Ibu-sPSMA in three independent experiments. For this purpose, Ibu-sPSMA was labeled with .sup.177Lu in a volume of 120 μL at a specific activity of 50 MBq/nmol with or without the addition of L-ascorbic acid (3 mg). After quality control using HPLC (t=0, radiochemical purity ≥98%), the labeling solutions were diluted with saline to 250 MBq/500 μL and incubated at room temperature. The radioligand's integrity was determined by HPLC after 1 h, 4 h and 24 h incubation time as previously reported (Siwowska et al., Mol. Pharmaceutical 2017, 14, (2), 523-532). The HPLC chromatograms were analyzed by integration of the peaks representing the radiolabeled product, the released .sup.177Lu as well as degradation products of unknown structure (FIG. 15). A quantitative assessment was performed by expressing the peak area of the intact product as percentage of the sum of integrated peak areas of the entire chromatogram.

    11.3. n-Octanol/PBS Distribution Coefficient

    [0382] The n-octanol/PBS distribution coefficient of .sup.177Lu-Ibu-sPSMA was performed according to the publication by Benesova, M. et al. Mol Pharm 2018, 15, (3), 934-946). .sup.177Lu-Ibu-sPSMA revealed a value of −2.43±0.01. The modification of the PSMA ligand with ibuprofen had an effect towards more hydrophobic properties of the radioligands as compared to .sup.177Lu-PSMA-617 (−4.38±0.01). The hydrophilicity of .sup.177Lu-Ibu-sPSMA was in the same range as the other ibuprofen-derivatized ligands and .sup.177Lu-PSMA-ALB-56 (−2.9±0.2).

    11.3. Albumin-Binding Properties

    [0383] Plasma protein-binding properties of .sup.177Lu-Ibu-sPSMA were determined using an ultrafiltration assay according to Benesova, M. et al. (Mol Pharm 2018, 15, (3), 934-946). In short, the Ibu-sPSMA-ligand was labeled with .sup.177Lu at a molar activity of 50 MBq/nmol and incubated in human plasma samples or PBS at 37° C. The free and plasma-bound fraction were separated using a centrifree ultrafiltration device (4104 centrifugal filter units; Millipore, 30000 Da nominal molecular weight limit, methylcellulose micropartition membranes). The incubated solution was loaded to the ultrafiltration device and centrifuged at 2000 rpm for 40 min at 20° C. Samples from the filtrate were taken and analyzed for radioactivity in a γ-counter. The amount of plasma-bound radioligand was calculated as the fraction of radioactivity measured in the filtrate relative to the corresponding loading solution (set to 100%). The experiments were performed in triplicates.

    [0384] The ultrafiltration experiments of .sup.177Lu-Ibu-sPSMA revealed high serum protein binding, demonstrated by the fact that ˜97% of the radioligand were retained in the filter membrane after incubation in human plasma. The radioligand did not show any retention by the filter membrane when incubated in PBS (which does not contain proteins). .sup.177Lu-Ibu-sPSMA showed increased binding to plasma proteins when compared to .sup.177Lu-PSMA-617, which showed an albumin-bound fraction of only about 59% (FIG. 16).

    11.4. Cell Internalization Study

    [0385] Cell uptake and internalization of .sup.177Lu-Ibu-sPSMA were investigated using PSMA-positive PC-3 PIP and PSMA-negative PC-3 flu tumor cells kindly provided by Prof. Dr. Martin Pomper (Johns Hopkins University School of Medicine, Baltimore, Md., U.S.A.) (Eiber, et al.; J Nucl Med 2017, 58, (Suppl 2), 67S-76S). .sup.177Lu-Ibu-sPSMA was investigated by performing experiments 3 times in 6 replicates with PC-3 PIP tumor cells and 3 times in 6 replicates with PC-3 flu tumor cells.

    [0386] The uptake and internalization of .sup.177Lu-Ibu-sPSMA into PC-3 PIP tumor cells was slightly higher than for .sup.177Lu-PSMA-617 (FIG. 17). The internalized fraction of .sup.177Lu-Ibu-sPSMA was 18% and 22% after incubation of 2 h or 4 h, respectively (FIG. 17A). The uptake of .sup.177Lu-Ibu-sPSMA in PC-3 flu tumor cells was <0.1% after 4 h, which indicated the highly PSMA-specific cell uptake in PC-3 PIP cells (FIG. 17B).

    11.5. Determination of K.SUB.D .Values

    [0387] The K.sub.D values, indicating the PSMA-binding affinity of the novel radioligand, were determined. The K.sub.D value of .sup.177Lu-Ibu-sPSMA was in the same range as the other ibuprofen-derivatized PSMA radioligands and also not substantially different from K.sub.D values of .sup.177Lu-PSMA-ALB-56 and .sup.177Lu-PSMA-617, determined under the same experimental conditions (Table 5).

    TABLE-US-00005 TABLE 5 K.sub.D data of the PSMA radioligands. .sup.177Lu-Ibu-sPSMA .sup.177Lu-PSMA-ALB-56 .sup.177Lu-PSMA-617 K.sub.D [nM] 40 ± 5 30 ± 6 13 ± 1

    Example 12: In Vivo Evaluation

    [0388] .sup.177Lu-Ibu-sPSMA was characterized in vivo and the data were compared to those obtained with .sup.177Lu-PSMA-617 and .sup.177Lu-PSMA-ALB-56.

    12.1. Tumor Mouse Model

    [0389] Mice were obtained from Charles River Laboratories, Sulzfeld, Germany, at the age of 5-6 weeks. Female, athymic nude BALB/c mice were subcutaneously inoculated with PSMA-positive PC-3 PIP cells (6×10.sup.6 cells in 100 μL Hank's balanced salt solution (HBSS)) on the right shoulder and with PSMA-negative PC-3 flu cells (5×10.sup.6 cells in 100 μL HBSS) on the left shoulder. Two weeks later, the tumors reached a size of about 80-300 mm.sup.3 suitable for the performance of the biodistribution and imaging studies.

    12.2. Biodistribution Study

    [0390] Biodistribution studies were performed 12-15 days after PC-3 PIP/flu tumor cell inoculation. .sup.177Lu-Ibu-sPSMA was diluted in 0.9% NaCl containing 0.05% bovine serum albumin (BSA) to prevent adhesion to the vial and syringe material. The radioligand was injected in a lateral tail vein in a volume of 100 μL. Mice were euthanized at different time points after injection (p.i.) of the radioligand. Selected tissues and organs were collected, weighed and measured using a γ-counter. The results were decay-corrected and listed as a percentage of the injected activity per gram of tissue mass (% IA/g) (Table 6, FIG. 18).

    [0391] .sup.177Lu-Ibu-sPSMA showed high accumulation in PC-3 PIP tumors already 1 h after injection (63±8% IA/g) which further increased until 24 h p.i. (132±15% IA/g) to the highest tumor uptake observed amongst all ibuprofen-bearing radioligands. Clearance of activity from tumor tissue was slow, which resulted in 57±9% IA/g retained activity in the tumor at 4 days after injection, compared to 20-34% IA/g for the other radioligands at the same time-point. Uptake into PSMA-negative PC-3 flu cells was clearly below blood levels, confirming the specific PSMA-mediated uptake in PC-3 PIP tumors.

    [0392] .sup.177Lu-Ibu-sPSMA showed the highest blood activity levels at 1 h p.i. (29±4% IA/g compared to 13-18% IA/g for the other ibuprofen-containing radioligands) which continuously decreased over time to similar blood activity levels as .sup.177Lu-Ibu-PSMA and .sup.177Lu-Ibu-Dα-PSMA 4 days after injection. In comparison to the blood activity levels of the fast cleared .sup.177Lu-Ibu-N-PSMA and .sup.177Lu-Ibu-DAB-PSMA, .sup.177Lu-Ibu-sPSMA exhibited approximately three times higher values at 24 h and 96 h after injection.

    [0393] The uptake in the kidney was very high (114±15% IA/g) at 1 h p.i. compared to only 30-33% IA/g for .sup.177Lu-Ibu-PSMA, .sup.177Lu-Ibu-N-PSMA and .sup.177Lu-Ibu-DAB-PSMA and 73±2% IA/g for .sup.177Lu-Ibu-Dα-PSMA). Renal clearance was, however fast so that activity levels similar to the other radioligands were reached already 4 h p.i. In a similar way, the liver showed high accumulation of activity at early time-points (17±4% IA/g at 1 h p.i. and 7.2±0.5% IA/g at 4 h p.i.), but the fast clearance resulted in a similar activity retention in the liver at 24 h p.i. and 96 h p.i. compared to the other ibuprofen-bearing radioligands.

    [0394] Table 6 shows biodistribution data of .sup.177Lu-Ibu-sPSMA in PC-3 PIP/flu tumor-bearing mice. The values represent the average value±SD of the percentage injected activity per gram tissue [% IA/g] obtained from each group of mice (n=4). Comparison of the features of .sup.177Lu-Ibu-sPSMA with the other ibuprofen-derivatized radioligands revealed exceptionally high accumulation and retention of activity in PSMA-positive PC-3 PIP tumors resulting in high tumor-to-kidney and tumor-to-liver ratios, in particular at late time points.

    TABLE-US-00006 TABLE 6 .sup.177Lu-Ibu-sPSMA 1 h.p.i. 4 h.p.i. 24 h.p.i. 96 h.p.i. Blood 29 ± 4  7.2 ± 0.7 0.76 ± 0.07 0.27 ± 0.02 Heart 9.3 ± 1.5 2.4 ± 0.2 0.39 ± 0.04 0.12 ± 0.01 Lung 17 ± 3  4.9 ± 0.7 0.79 ± 0.07 0.25 ± 0.03 Spleen 5.8 ± 1.2 2.1 ± 0.3 0.72 ± 0.15 0.22 ± 0.03 Kidneys 114 ± 15  28 ± 2  10 ± 2  2.3 ± 0.3 Stomach 2.6 ± 0.5 1.3 ± 0.9 0.50 ± 0.19 0.73 ± 0.82 Intestines 2.9 ± 0.3 1.1 ± 0.2 0.31 ± 0.07 0.13 ± 0.03 Liver 17 ± 4  7.2 ± 0.5 0.81 ± 0.09 0.22 ± 0.02 Salivary glands 6.6 ± 0.5 1.9 ± 0.2 0.37 ± 0.05 0.11 ± 0.02 Muscle 3.1 ± 0.7 0.84 ± 0.14 0.14 ± 0.02 0.04 ± 0.02 Bone 3.3 ± 0.4 1.1 ± 0.2 0.27 ± 0.04 0.08 ± 0.01 PC-3 PIP 63 ± 8  99 ± 7  132 ± 15  57 ± 9  Tumor PC-3 flu 5.1 ± 0.7 2.0 ± 0.3 0.66 ± 0.07 0.15 ± 0.02 Tumor Tumor-to- 2.2 ± 0.1 14 ± 3  173 ± 18  210 ± 35  blood Tumor-to- 3.7 ± 0.5 14 ± 2  163 ± 16  263 ± 42  liver Tumor-to- 0.56 ± 0.09 3.6 ± 0.2 13 ± 2  25 ± 2  kidney

    [0395] Due to the high blood activity levels, in particular at early time-points after injection, tumor-to-blood ratios of accumulated radioactivity of .sup.177Lu-Ibu-sPSMA were consistently lower as compared to .sup.177Lu-Ibu-DAB-PSMA, but reached similar values as .sup.177Lu-Ibu-PSMA, .sup.177Lu-Ibu-Dα-PSMA and .sup.177Lu-Ibu-N-PSMA at later time-points (FIG. 19A). The tumor-to-kidney ratio of .sup.177Lu-Ibu-sPSMA showed an equally low value as compared to .sup.177Lu-Ibu-Dα-PSMA (0.56±0.09 and 0.59±0.08, respectively) 1 h p.i., but increased significantly with time to give the highest ratios amongst the radioliogands at all other time-points (FIG. 19B). In a similar manner, tumor-to-liver ratios were low at 1 h and 4 h p.i., but outperformed the other radioligands at 24 h and 96 h after injection (FIG. 19C).

    12.3. Whole-Body-Activity Measurements

    [0396] All albumin-binding radioligands (molar activity: 25 MBq/nmol) were diluted in 0.9% NaCl containing 0.05% BSA and i.v. injected into non-tumor bearing mice (25 MBq, 1 nmol, 100 μL). The mice were measured in a dose calibrator at various time-points up to 56 h p.i. The radioligands were compared with previously obtained data from .sup.177Lu-PSMA-617.

    [0397] The whole-body measurements revealed different excretion patterns for the single radioligands, which was manifest most prominently at early time-points up to 8 h after injection (FIG. 20). Amongst all radioligands the body retention was highest for .sup.177Lu-Ibu-Dβ-PSMA with the only exception at late time-points (48 h and 56 h p.i), where the retention of .sup.177Lu-PSMA-ALB-56, containing a p-iodophenyl entity as stronger albumin binder, was higher. The other ibuprofen-bearing radioligands showed less retention in the body as compared to .sup.177Lu-PSMA-ALB-56. Amongst the albumin-binding radioligands, .sup.177Lu-Ibu-DAB-PSMA was characterized with the fastest excretion pattern with a retained activity of only 18% already 4 h after injection in comparison to the other albumin-binding radioligands (35-73%). All radioligands showed higher retention of radioactivity compared to .sup.177Lu-PSMA-617 with limited albumin-binding properties. In all cases, retention of radioactivity in the body decreased over time and reached similar retention fractions 32 h p.i.

    12.4. In Vivo SPECT/CT Imaging

    [0398] SPECT/CT images were obtained using a dedicated small-animal SPECT/CT scanner (NanoSPECT/CT™, Mediso Medical Imaging Systems, Budapest, Hungary). SPECT/CT scans of 45 min duration were performed followed by a CT of 7.5 min. During the in vivo scans, the mice were anesthetized with a mixture of isoflurane and oxygen. Reconstruction of the acquired data was performed using HiSPECT software (version 1.4.3049, Scivis GmbH, Göttingen, Germany). All images were prepared using VivoQuant post-processing software (version 2.10, inviCRO Imaging Services and Software, Boston U.S.). A Gauss post-reconstruction filter (FWHM=1.0 mm) was applied to the images, which were presented with the scale adjusted to allow visualization of the most important organs and tissues, by cutting 5% of the lower scale.

    [0399] The SPECT/CT images visualized the PC-3 PIP tumor xenograft (right side of FIG. 21) in which .sup.177Lu-Ibu-sPSMA accumulated to a high extent whereas in the PC-3 flu tumor (left side of FIG. 21), accumulation of radioactivity was not observed. At the 4-h time point, some activity was also seen in the kidneys as well as in the urinary bladder as a consequence of renal clearance. At the 24-h time point, activity was only visualized in the PC-3 PIP tumor (FIG. 21).

    Example 13. In Vivo Therapy Study

    [0400] The therapeutic efficacy of .sup.177Lu-Ibu-DAB-PSMA was assessed in vivo in a tumor mouse model (PSMA-positive PC-3 PIP tumor-bearing mice) and the data were compared to those obtained with .sup.177Lu-PSMA-617 and .sup.177Lu-PSMA-ALB-56 (Eiber et al., J. Nucl. Med., 2017, 58 (Suppl. 2) 67S-76S).

    13.1. Tumor Mouse Model

    [0401] Mice were obtained from Charles River Laboratories, Sulzfeld, Germany, at the age of 5-6 weeks. Female, athymic nude BALB/c mice were subcutaneously inoculated with PSMA-positive PC-3 PIP cells (4×10.sup.6 cells in 100 μL Hank's balanced salt solution (HBSS)) on the right shoulder. Six days later, the tumors reached a size of about 30-160 mm.sup.3 suitable for the performance of the in vivo therapy study. Mice were euthanized when a predefined endpoint criterion was reached or when the study was finalized at Day 84. Endpoint criteria were defined as (i) body weight loss of >15%, (ii) a tumor volume of >800 mm.sup.3 (iii) a combination of body weight loss of >10% and a tumor volume of >700 mm.sup.3 or (iv) signs of unease and pain or a combination thereof.

    13.2. Methods

    [0402] Six days after subcutaneous inoculation of 4×10.sup.6 PC-3 PIP tumor cells inoculation, three groups with statistically similar body weight and tumor volumes were intravenously injected. One group was injected with only the vehicle (saline containing 0.05% bovine serum albumin (BSA); Group A; n=6), and another two groups with .sup.177Lu-Ibu-DAB-PSMA (Group B: 2 MBq, 1 nmol (n=6) and Group C: 5 MBq, 1 nmol (n=6)) at Day 0 of the therapy study (Table 7). The monitoring of mice and the assessment of the therapy study was conducted as described by Eiber et al., J. Nucl. Med., 2017, 58 (Suppl. 2) 67S-76S). In short, the mice were monitored by measuring body weight and tumor size every second day over a period of 12 weeks. The relative body weight (RBW) was defined as [BW.sub.x/BW.sub.0], where BW.sub.x is the body weight in grams at a given Day x and BW.sub.0 is the body weight in grams at Day 0. The tumor dimensions were determined by measuring the longest tumor axis (L) and its perpendicular axis (W) with a digital caliper. The tumor volume (V) was calculated according to the equation [V=0.5×(LW.sup.2)]. The relative tumor volume (RTV) was defined as [TV.sub.x/TV.sub.0], where TV.sub.x is the tumor volume in mm.sup.3 at a given day x, and TV.sub.0 is the tumor volume in mm.sup.3 at Day 0.

    TABLE-US-00007 TABLE 7 Design of the therapy study. Tumor volume.sup.a Body weight.sup.a Injected [mm.sup.3] [g] radioactivity (average ± SD) (average ± SD) Group Treatment n [MBq] Day 0 Day 0 A Saline 12 — 66 ± 30 17 ± 1.5 B .sup.177Lu-Ibu-DAB-PSMA 6 2 58 ± 24 17 ± 1.3 C .sup.177Lu-Ibu-DAB-PSMA 6 5 65 ± 14 18 ± 1.5 D .sup.177Lu-PSMA-617 6 2 103 ± 24  16 ± 1.2 E .sup.177Lu-PSMA-617 6 5 104 ± 25  17 ± 0.9 F .sup.177Lu-PSMA-ALB-56 6 2 81 ± 25 15 ± 1.3 G .sup.177Lu-PSMA-ALB-56 6 5 92 ± 34 15 ± 1.3 .sup.aNo significant differences determined between the values measured for each group (p > 0.05).

    [0403] The efficacy of the radionuclide therapy was expressed as the tumor growth delay (TGD.sub.x), which was calculated as the time required for the tumor volume to increase x-fold over the initial volume at Day 0. The tumor growth delay index [TGDI.sub.x=TGD.sub.x(T)/TGD.sub.x(C)] was calculated as the TGD.sub.x ratio of treated mice (T) over the TGD.sub.x average of control mice (C) for a 5-fold (x=5, TGD.sub.5) increase of the initial tumor volume. The median survival was calculated using GraphPad Prism software (version 7). Survival of mice was assessed using Kaplan-Meier curves to determine median survival of mice of each group using Graph Pad Prism software (version 7).

    13.3. Results of the Therapy Study

    [0404] The results of the therapy study were combined with the results obtained in a therapy study including the group injected with only the vehicle (saline containing 0.05% BSA; Group A; n=6), .sup.177Lu-PSMA-617 (2 MBq and 5 MBq; Group D and E; n=6) and .sup.177Lu-PSMA-ALB-56 (2 MBq and 5 MBq; Group F and G; n=6) (Eiber et al., J. Nucl. Med., 2017, 58 (Suppl. 2) 67S-76S). The tumor growth of treated mice was more delayed than the tumor growth of untreated control mice (combined; n=12) (FIG. 22).

    [0405] The tumor growth delay index five (TGDI.sub.5) values of groups injected with 2 MBq .sup.177Lu-Ibu-DAB-PSMA (1.6) and .sup.177Lu-PSMA-ALB-56 (1.8), respectively, were clearly increased as compared to the one of the control animals (1.0 defined for controls). Only the TGDI.sub.5 of the mice injected with 2 MBq .sup.177Lu-PSMA-617 (1.1) was comparable to the value of the control animals (Table 8).

    TABLE-US-00008 TABLE 8 Tumor growth delay index with 5-fold increase of tumor size. first mouse of group median euthanized survival Group Treatment [d] [d] TGDI.sub.5 A Saline 16 26 1.0 ± 0.5 B .sup.177Lu-Ibu-DAB-PSMA 26 34 1.6 ± 0.4 C .sup.177Lu-Ibu-DAB-PSMA 70 n.d..sup.a n.d..sup.a D .sup.177Lu-PSMA-617 12 19 1.1 ± 0.1 E .sup.177Lu-PSMA-617 26 32 2.0 ± 0.3 F .sup.177Lu-PSMA-ALB-56 28 36 1.8 ± 0.5 G .sup.177Lu-PSMA-ALB-56 58 n.d..sup.a n.d..sup.a .sup.an.d. = not defined since majority of mice were still alive at the end of the study.

    [0406] The TGDI.sub.5 values of the groups injected with 5 MBq .sup.177Lu-PSMA-617 (2.0) was in the same range as for the albumin-binding radioligands applied at 2 MBq per mouse. The TGDI.sub.5 values of mice injected with 5 MBq .sup.177Lu-Ibu-DAB-PSMA or .sup.177Lu-PSMA-ALB-56, respectively, were not defined as in four mice of each group the tumors disappeared entirely. Regrowth of tumors in animals with total remission was not observed until the end of the study at Day 84. In each group regrowth of the tumor was observed in two mice from about 5 weeks after therapy on so that they reached the endpoint at Day 70 and Day 82 (.sup.177Lu-Ibu-DAB-PSMA) and at Day 58 and Day 68 (.sup.177Lu-PSMA-ALB-56), respectively. The median survival time remained, therefore, undefined for these groups of mice which received 5 MBq .sup.177Lu-Ibu-DAB-PSMA or 5 MBq .sup.177Lu-PSMA-ALB-56, respectively (FIG. 23). At the end of the study at Day 84, four mice were still alive in each of these groups. The median survival of mice treated with 2 MBq .sup.177Lu-Ibu-DAB-PSMA and .sup.177Lu-PSMA-ALB-56, respectively, was 34 and 36 days, hence, clearly increased compared to the median survival of control mice (26 days). On the other hand, the median survival of the group injected with 2 MBq .sup.177Lu-PSMA-617 (19 days) was shorter than for all other groups including untreated control mice (FIG. 23).

    [0407] At Day 16, when the first control mouse reached the endpoint, the average relative body weight (0.93-1.10) was comparable in all groups (FIG. 24). At the time of euthanasia, the average relative body weight of the groups injected with 5 MBq .sup.177Lu-Ibu-DAB-PSMA (1.06±0.10) and .sup.177Lu-PSMA-ALB-56 (1.20±0.14), respectively, was increased as compared to the average relative body weight of control mice (0.88±0.05) and mice treated with .sup.177Lu-PSMA-617 (0.86±0.05). These findings can be ascribed to the faster tumor growth in control mice and mice, treated with .sup.177Lu-PSMA-617 and, hence, the fact that they reached the endpoint sooner than mice treated with .sup.177Lu-Ibu-DAB-PSMA or .sup.177Lu-PSMA-ALB-56 (FIG. 24).

    [0408] As a result, .sup.177Lu-Ibu-DAB-PSMA performed significantly better than .sup.177Lu-PSMA-617 for both quantities of injected activity (2 MBq/mouse and 5 MBq/mouse, respectively). While .sup.177Lu-Ibu-DAB-PSMA was only slightly inferior compared to .sup.177Lu-PSMA-ALB-56 at the lower injected activity (2 MBq/mouse), it was even slightly superior when applied at the higher quantity of activity (5 MBq/mouse). The improved tumor-to-blood ratios of .sup.177Lu-Ibu-DAB-PSMA as compared to the results obtained with .sup.177Lu-PSMA-ALB-56 and the outcome of this therapy study, confirmed the superiority of .sup.177Lu-Ibu-DAB-PSMA over the existing .sup.177Lu-PSMA-617.