Effects of MTX-23, a Novel PROTAC of Androgen Receptor Splice Variant-7 and Androgen Receptor, on CRPC resistant to Second-Line Antiandrogen Therapy.

Although second-line anti-androgen therapy (SAT) is the standard of care in men with castration-resistant prostate cancer (CRPC), resistance inevitably occurs. One major proposed mechanism of resistance to SAT involves the emergence of androgen receptor (AR) splice variant-7, AR-V7. Recently, we developed MTX-23 using the principle of proteolysis targeting chimera (PROTAC) to target both AR-V7 and AR-full length (AR-FL). MTX-23 has been designed to simultaneously bind AR9s DNA binding domain (DBD) and the Von Hippel-Lindau (VHL) E3 ubiquitin ligase. Immunoblots demonstrated that MTX-239s degradation concentration 50% (DC50) for AR-V7 and AR-FL was 0.37 and 2 µM, respectively. Further studies revealed that MTX-23 inhibited prostate cancer (CaP) cellular proliferation and increased apoptosis only in androgen-responsive CaP cells. The anti-proliferative effect of MTX-23 was partially reversed when either AR-V7 or AR-FL was overexpressed, and was completely abrogated when both were overexpressed. To assess the potential therapeutic value of MTX-23, we next generated twelve human CaP cell lines that are resistant to the four FDA-approved SAT agents - abiraterone, enzalutamide, apalutamide, and darolutamide. When resistant cells were treated with MTX-23, decreased cellular proliferation and reduced tumor growth were observed both in vitro and in mice. These results collectively suggest that MTX-23 is a novel PROTAC small molecule that may be effective against SAT-resistant CRPC by degrading both AR-V7 and AR-FL.