The invention provides methods of determining a therapeutically effective dose of an agent that targets a metabolic pathway based on measured levels of a metabolite in the pathway. The methods, which may include providing the agent in a therapeutically effective dose, are useful for treating disorders, such as cancer, in a subject. The invention also provides methods for assessing the impact of a therapeutic agent on a tumor in a subject by monitoring, in real time, metabolism of a molecule in the tumor, oxygenation of the tumor, or both. The invention further provides devices that determine a therapeutically effective dose of an agent that targets a metabolic pathway based on measured levels of a metabolite in the pathway and notify a subject to administer the dose.

Provided is a method for synthesizing a flavonoid compound. The method comprises providing a recombinant prokaryotic cell, wherein, in the prokaryotic cell, the transmembrane protein rhodanese Ygap of Escherichia coli is up-regulated or a target gene or target gene combination selected from the following groups is down-regulated: pyrB, accC, accB, purC, glyA, tktA, fabB, leuD, leuC, glpC, folK and leuA. Also provided are a prokaryotic cell for synthesizing a flavonoid compound and the use thereof, and the use of a kit and a regulation and control reagent. The present disclosure achieves significant improvement in the yield of the flavonoid compound.