Phase Ib study assessing different sequencing regimens of atezolizumab (anti-PD-L1) and sipuleucel-T (SipT)in patients who have asymptomatic or minimally symptomatic metastatic castrate resistant prostate cancer.

Abstract

e17564 Background: Combining an immune checkpoint inhibitor with a tumor vaccine may leverage complementary mechanisms of action for treatment of mCRPC. Atezolizumab, a fully human anti–PD-L1 IgG1 antibody, is an approved treatment option in multiple indications; while SipT, an autologous cellular immunotherapeutic, is approved for treatment of metastatic castrate resistant prostate cancer (mCRPC). We present a phase Ib study evaluating safety, tolerability and objective response rate of atezolizumab + SipT in patients with mCRPC (NCT03024216). Methods: Eligible patients who had asymptomatic or minimally symptomatic progressive mCRPC and met standard criteria for sipuleucel-T were randomized to receive either atezolizumab 1200 mg IV every 3 weeks for 2 doses then SipT IV every 2 weeks for a total of three infusions (Arm 1) or SipT IV every 2 weeks for a total of three infusions followed by atezolizumab 1200 mg IV every 3 weeks for 2 doses (Arm 2). The primary endpoint was safety, while secondary endpoints included objective tumor response (PCWG2 and modified RECISTv1.1 criteria) and comparing systemic immune responses after induction between the arms. Results: As of February 6, 2020, 37 pts (median follow-up 7.4 months, median age 75 yrs [range 53.0–86.0]; median number of previous treatments 4 [range 1-8], 75.7% ECOG PS = 0) were enrolled. Three patients did not complete induction therapy (1 – withdrew consent, 1 developed toxicity, and 1 progressed). There were no grade 5 toxicities attributed to study drugs and grade 4 toxicities were noted in 2 patients, 1 in each arm (Arm 1 bronchitis and Arm 2 hypotension). Eight grade 3 toxicities were noted in arm 1 (hyponatremia, pulmonary embolus x3, bone pain, hypophosphatemia, shock, tooth fracture from a fall), while 4 grade 3 toxicities were noted in arm 2 (anemia, hypertension x 2, pneumonia). None of the grade 3 or 4 SAEs were noted to be irAEs. At 6 months, there were 11 SD (7 in Arm 1 and 4 in Arm 2), 18 PD and 7 unevaluable (3 withdrew from study and 4 have yet to reach 6 month evaluation). At this time, PFS was noted to be 8.2 months in Arm 1 and 5.8 months in Arm 2 (p = 0.054). Conclusions: The safety profile of the combination of atezolizumab with sipuleucel-T appears manageable in a heavily pre-treated population and consistent with those agents administered as monotherapy. Responses were seen but no CR. Immune activation studies may shed light on which sequence is optimal. Clinical trial information: NCT03024216 .