Screening a combinatorial peptide library to develop a human glandular kallikrein 2-activated prodrug as targeted therapy for prostate cancer

Objective: Prostate cancer cells secrete the unique protease human glandular kallikrein 2 (hK2) that represents a target for proteolytic activation of cytotoxic prodrugs. The objective of this study was to identify hK2-selective peptide substrates that could be coupled to a cytotoxic analogue of thapsigargin, a potent inhibitor of the sarcoplasmic/endoplasmic reticulum calcium ATPase pump that induces cell proliferation-independent apoptosis through dysregulation of intracellular calcium levels. Methods: To identify peptide sequence requirements for hK2, a combination of membrane-bound peptides (SPOT analysis) and combinatorial chemistry using fluorescence-quenched peptide substrates was used. Peptide substrates were then coupled to 8-O-(12[L-leucinoylaminoldodecanoyl)-8-O-debutanoylthapsigargin (L12ADT), a potent analogue of thapsigargin, to produce a prodrug that was then characterized for hK2 hydrolysis, plasma stability, and in vitro cytotoxicity. Results: Both techniques indicated that a peptide with two arginines NH₂-terminal of the scissile bond produced the highest rates of hydrolysis. A lead peptide substrate with the sequence Gly-Lys-Ala-Phe-Arg-Arg (GKAFRR) was hydrolyzed by hK2 with a K_m of 26.5 μmol/L, k_cat of 1.09 s^-1, and a k_cat/K_m ratio of 41,132 s^-1 mol/L^-1. The GKAFRR-L12ADT prodrug was rapidly hydrolyzed by hK2 and was stable in plasma, whereas the GKAFRR-L peptide substrate was unstable in human plasma. The hK2-activated thapsigargin prodrug was not activated by cathepsin B, cathepsin D, and urokinase but was an excellent substrate for plasmin. The GKAFRR-L12ADT was selectively cytotoxic in vitro to cancer cells in the presence of enzymatically active hK2. Conclusion: The hK2-activated thapsigargin prodrug represents potential novel targeted therapy for prostate cancer.