Efficacy studies of an antibody-drug conjugate PSMA-ADC in patient-derived prostate cancer xenografts

It is timely and important to develop new treatment modalities for advanced prostate cancer, because even the newly FDA approved treatments, despite providing significant survival benefits, do not constitute cure of this disease. Antibody drug conjugates (ADCs) represent a promising approach to cancer therapy. Prostate-specific membrane antigen (PSMA) is expressed in advanced prostate cancer and targeting this protein is used for imaging of advanced prostate cancer as well as development of targeting strategies. The objective of our studies was to evaluate the efficacy of PSMA ADC against a series of patient-derived prostate cancer xenografts (LuCaP 58, LuCaP 77, LuCaP 96CR, and LuCaP 105) with different characteristics, including varying levels of PSMA expression and responses to androgen suppression.Mice bearing subcutaneous LuCaP prostate cancer-derived xenografts received PSMA antibody monomethyl auristatin E (MMAE) drug conjugate (PSMA ADC) in which the antibody and MMAE are linked via a protease-cleavable linker. PSMA ADC dose ranged from 1 to 6 mg/kg. Unmodified PSMA mAb + free MMAE at the amount equivalent to those contained in 6 mg/kg PSMA ADC was used as control. All treatments were administered once a week via tail-vein injections and repeated four times once a week and tumor responses were monitored for 10 weeks. IHC analyses were performed to determine PSMA and AR expression and effects on proliferation.Treatment responses varied widely across the tumor models, from complete tumor regressions in LuCaP 96CR to largely unimpeded tumor progression of LuCaP 58, which had the lowest baseline level of PSMA expression. Intermediate antitumor effects were seen for LuCaP 77 and LuCaP 105 tumors, despite their having similar basal expression of PSMA as LuCaP 96CR. Interestingly, we detected substantial differences in responses even within the same model, indicating that PSMA expression is not the only factor involved in treatment outcomes.Our results show high efficacy of PSMA ADC in advanced prostate cancer but also considerable variability in effects despite PSMA expression. Further studies to identify tumor characteristics that are predictive of treatment response are ongoing.