Safety and preliminary clinical activity of JNJ-63898081 (JNJ-081), a PSMA and CD3 bispecific antibody, for the treatment of metastatic castrate-resistant prostate cancer (mCRPC)

279 Background: PSMA expression is increased in response to anti-androgen therapies and is a promising therapeutic target for the treatment of prostate cancer (PCa). JNJ-081 is a bispecific antibody with one arm binding PSMA on cancer cells and the other binding CD3 on T-cells to promote anti-tumor activity. Methods: This Phase 1 Dose Escalation Study evaluated JNJ-081 in mCRPC participants (pts) who progressed after novel androgen targeting therapy (eg, abiraterone, enzalutamide or apalutamide). Prior chemotherapy was permitted. JNJ-081 was administered initially by intravenous (IV) then by subcutaneous (SC) route. Dose escalation followed a continuous reassessment method based on a Bayesian regression model. The primary endpoint to determine the recommended phase 2 dose (RP2D) was based on safety. Pharmacokinetics (PK), pharmacodynamics (PD), immunogenicity, and preliminary anti-tumor activity were also evaluated. Cytokine release syndrome (CRS) was graded by CTCAE V5. Results: As of 10 May 2021, 39 pts were dosed in 10 cohorts ranging from 0.1 µg/kg to 3 µg/kg IV and from 3 µg/kg to 60 µg/kg SC. Premedications included high dose corticosteroids. Step-up priming was implemented at higher SC doses. Most common treatment-emergent AEs were CRS (65%), fatigue (49%) and nausea (43%). 2 pts developed DLTs of Grade (G) 3 or G4 transaminases increased, 1 each at 30 µg/kg SC and priming with 10 µg/kg SC then 55 µg/kg SC, respectively. Both DLTs were in conjunction with or followed an episode of G2 CRS. SC route as well as step-up priming helped mitigate CRS and infusion-related reaction (IRR) during escalation to higher doses: at 3 µg/kg IV, 4 of 5 pts had G2 CRS or IRR; at 30 µg/kg SC, 3 of 4 pts had G2 CRS; priming with 10 µg/kg then 55 µg/kg SC, 4 of 5 pts had G2 CRS. Injection site reactions (G1 or G2) occurred in 24 of 26 pts treated via SC JNJ-081. No treatment related death was reported. Transient PSA decreases were observed at treatment doses greater than 30 µg/kg SC. Two subjects treated with 55 µg/kg had PSA decreases > 50%. No radiographic responses were observed. PK of JNJ-081 was linear over the dose range of 3-60 µg/kg following SC administration. SC bioavailability was approximately 25%. Anti-drug antibodies (ADA) were detected in 2 of 12 subjects treated by IV administration and 14 of 23 pts treated by SC administration. ADA resulted in loss of exposure in some SC pts. Intrapatient dose escalation in 3 pts did not overcome the reduced exposure after ADA seropositivity. Conclusions: JNJ-081 demonstrated transient decreases in PSA in mCRPC patients. Grade 2 CRS was observed at higher doses and was partially mitigated by SC and step-up dosing. ADA resulting in decreased exposure occurred in the majority of pts treated SC. PSMA remains a potential therapeutic target for T-cell redirection for the treatment of PCa. Clinical trial information: NCT03926013.