A phase l and randomized phase ll trial of radium-223 dichloride, peposertib, and avelumab in advanced metastatic castrate-resistant prostate cancer (mCRPC): Phase l results.

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5069 Background: Ra-223 improves the overall survival in patients with mCRPC to the bone. We assessed the safety of standard-of-care Ra-223 in combination with peposertib, a DNA-dependent protein kinase inhibitor, with or without avelumab, an anti-PD-L1 antibody. Methods: Patients with mCRPC and two or more skeletal metastases identified by bone scintigraphy with or without lymph node metastases up to 3 cm in size, but no visceral metastases, were eligible. Progression after progression after at least one androgen receptor pathway inhibitor or taxane was required. Phase 1 consisted of a two-step sequential safety lead-in using a 3+3 design. Step 1 combined 6 standard cycles of Ra-223 (55 kBq/kg), with increasing doses of peposertib: 50, 100, and 200 mg PO bid on days 3-26 of each cycle. Step 2 combined Ra-223 with the maximum tolerated dose of peposertib and added standard avelumab 800 mg IV every 14 days, starting at cycle 2 of Ra-223. Results: A total of 9 patients participated in Step 1, where the maximum tolerated peposertib dose of 200 mg PO bid with Ra-223 was deemed tolerable. A total of 6 patients participated in Step 2 and the combination of Ra-223, peposertib at 200 mg PO bid, and standard avelumab was deemed tolerable. Of the 15 patients in Phase 1, none experienced grade 4 or higher treatment-related toxicities, while 7 experienced grade 3 toxicities, including neutropenia (1), anemia (1), cardiac chest pain (1), fall (1), maculopapular rash (2), and lymphopenia (2). The median (Q1, Q3) PSA velocity from the beginning to the end of treatment was lower for Step 2 versus Step 1: 2.1 (-0.1, 4.6) versus 23.5 (18.6, 77.1) ng/mL/month (p = 0.0067). OS and PFS were calculated with the limitation that the Phase 1 was non-randomized. Median OS was not reached for Step 2, as only 1 of 6 patients had died, versus 15.2 months for Step 1, where 8 out of 9 patients had died (p = 0.0537). The patient who died in Step 2 discontinued therapy prematurely after 3 cycles due to progression and survived 15.2 months (the 5 alive completed all 6 cycles). The median radiographic PFS was 7 months for Step 2 versus 12.7 months for Step 1 (p = 0.8910). Conclusions: The combination of Ra-223 and peposertib, with or without avelumab was well tolerated. The triplet combination showed the most promising results at this early evaluation. We continue to accrue in the randomized Phase II portion of the trial. Clinical trial information: NCT04071236 .