Results of an ongoing phase 1/2a dose escalation study of HPN424, a tri-specific half-life extended PSMA-targeting T-cell engager, in patients with metastatic castration-resistant prostate cancer (mCRPC).
Authors: De Bono Johann S. , Fong Lawrence , Beer Tomasz M. , Gao Xin , Geynisman Daniel M. , Burris III Howard A. , Strauss James Fredric , Courtney Kevin Dale , Quinn David I. , VanderWeele David James , Yaron Yifah , Law Che-Leung , Stein Mark N. 
Published: June 2021

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Background: HPN424 is a prostate-specific membrane antigen (PSMA)-targeting T cell engager designed to redirect T cells to kill PSMA-expressing prostate cancer cells; engineered with three binding domains: anti-PSMA for tumor cell engagement, anti-albumin for half-life extension and anti-CD3 for T cell engagement. HPN424 is optimized for small size and increased stability compared to other bispecific platforms. Methods: This Ph1/2a study is evaluating HPN424 in mCRPC patients (pts) who have received > 2 prior systemic therapies. Primary endpoints are safety, tolerability and determination of MTD/RP2D. Secondary objectives are pharmacokinetics (PK), pharmacodynamics, immunogenicity and preliminary anti-tumor activity. HPN424 is administered IV once weekly. Tumor assessments include PSA, CT and bone scans every 9-weeks. Results: As of 2/8/21, 80 pts were dosed in 15 cohorts with target doses ranging from 1.3 to 160ng/kg fixed dose, and up to 300ng/kg with step dosing to the target dose after initial priming dose. Pts had received a median of 6 prior systemic regimens, 75% received prior chemotherapy for mCRPC. Median age was 70 (43 – 91). Most common grade > 3 TEAEs were AST increase (18%), anemia (11%) and ALT increase (11%). DLTs include CRS G3 (n = 3), elevated lipase G3 (n = 1) and seizure G3 (n = 1). These events did not limit escalation, MTD has not been reached and escalation continues. All grade CRS occurred in 63% of pts, 4% grade 3 per ASTCT and no Grade 4/5 CRS. CRS G3 events occurred after first administration of target dose (n = 2 fixed dose, n = 1 step dose). Transaminase elevation occurred predominantly during Cycle 1, was transient and had no clinical sequelae. Disease progression was the primary reason for drug discontinuation; 2 pts (3%) discontinued due to TRAE. Reduction in circulating tumor cells (CTC) was seen in 32 of 56 pts (57%) with measurable CTC at baseline. Fifteen of 62 pts (24%) with > 24 weeks follow-up remained on treatment ≥ 24 weeks. Thirteen of 63 pts (21%) with post-baseline levels had PSA declines from baseline, including 3 PSA50, 2 PSA30 responses. In chemo-naïve pts, 5 of 15 (33%) showed PSA declines post-baseline. In the highest fixed dose cohort (160ng/kg) tested to-date, 3 of 7 evaluable pts had PSA declines from baseline and 1 had a confirmed partial response per RECIST. Conclusions: HPN424, a novel half-life extended PSMA-targeting T cell engager, was well tolerated when administered once weekly. AEs were transient, manageable and consistent with class of agent. Grade 3 CRS was observed in 4% of patients, occurring with first administration of target dose. Evidence of antitumor activity included PSA and CTC reductions and treatment duration > 24 weeks in 15/62 pts. Encouraging signals were seen at the highest fixed dose cohort including a confirmed RECIST partial response. NCT03577028 Clinical trial information: NCT03577028.