An implantable device for delivery of a macromolecular drug compound is provided. The device comprises a core having an outer surface and a membrane layer positioned adjacent to the outer surface of the core. The core comprises a core polymer matrix within which is dispersed a drug compound having a molecular weight of about 0.5 kDa or more, the polymer matrix containing a hydrophobic polymer. Further, the membrane layer comprises a membrane polymer matrix within which the macromolecular drug compound is optionally dispersed. The membrane polymer matrix contains a hydrophobic polymer in combination with a hydrophilic compound, and the weight ratio of the hydrophobic polymer to the hydrophilic compound within the membrane polymer matrix ranges from about 0.25 to about 200.