Phase 1 study of HP518, a PROTAC AR degrader in patients with mCRPC: results on safety, pharmacokinetics, and anti-tumor activity

HP518 is an oral PROteolysis TArgeting Chimera (PROTAC) protein degrader targeting the wild-type androgen receptor (WT-AR) and mutant AR ligand-binding domain (AR-LBD). A multicenter, first-in-human, open-label Phase 1 dose escalation study was conducted in patients with metastatic castration-resistant prostate cancer (mCRPC) to evaluate the safety, pharmacokinetics, and anti-tumor activity of HP518. Twenty-two patients with mCRPC with disease progression on at least 1 novel androgen receptor pathway inhibitor (ARPI) and ≤ 1 line of chemotherapy received HP518 once daily orally in sequential cohorts. Patients were not selected for AR-LBD mutations. Objectives were to assess safety, tolerability, maximum tolerated dose, pharmacokinetics (PK), as well as efficacy by PSA₅₀ response and radiographic response per Response Evaluation Criteria in Solid Tumors (RECIST) v1.1 and Prostate Cancer Working Group 3 (PCWG3) criteria. Exploratory objectives included genomic profiling using cell-free DNA. The majority of treatment-emergent adverse events (TEAEs) were Grade 1 or 2. The most common AEs were nausea and vomiting, fatigue, constipation, diarrhea, and decreased appetite. Only one (vomiting) out of 10 serious adverse events (SAE) was considered drug-related. No patient experienced a dose-limiting toxicity (DLT), and no AEs led to dose reduction or study discontinuation. Following multiple dosing of HP518, the PK appeared to plateau showing a less than dose-proportional relationship between exposure and dosage. Two patients demonstrated a partial response, and three patients showed a PSA50 response. In this initial Phase 1 study, HP518 demonstrated an acceptable safety profile and responses in a limited subset of mCRPC patients with progression after ARPI warranting further investigation. ClinicalTrials.gov Identifier: NCT05252364.