Harpoon Therapeutics, Inc.Harpoon Therapeutics, Inc. (NASDAQ:HARP), a clinical-stage immunotherapy company developing novel T cell engagers, today presented a poster with interim data from the ongoing dose-escalation portion of the Phase 1/2 trial for HPN217 in patients with relapsed/refractory multiple myeloma (R/R MM) at the 63rd American Society for Hematology (ASH) Annual Meeting and Exposition. HPN217 targets B-cell maturation antigen (BCMA) and is based on Harpoon’s proprietary Tri-specific T cell Activating Construct (TriTAC®) platform designed to recruit a patient’s own immune cells to kill tumor cells.

As of November 10, 2021, the data cutoff date for the interim clinical data presentation, 37 patients have been dosed across 10 cohorts at fixed doses of 5 to 2860 µg/week and in step dosing cohorts up to 3240 µg/week administered as an intravenous infusion. These interim data demonstrated:

HPN217 is generally well tolerated with one dose limiting toxicity (DLT) reported of Grade 4 AST elevation that resolved, MTD has not been reached
HPN217 is clinically active at higher dose levels with clinical benefit, disease control rate (DCR) of 88%, demonstrated in 7 of 8 disease evaluable patients in the 2150 µg/week cohort
2 stringent complete responses (SCRs) have been observed, one in each of the higher dose 2150 and 2860 µg/week cohorts
Transient and manageable cytokine release syndrome (CRS) reported in 9 of 37 patients (24%) were all Grade 1 or 2
Introduction of step dose regimens has allowed for the administration of higher target doses, currently at 3240 µg/week
“These encouraging data for HPN217 demonstrate robust clinical activity at higher doses, strong target engagement, and a manageable safety profile in this heavily refractory patient population,” said Natalie Sacks, M.D., Chief Medical Officer of Harpoon Therapeutics. “Dose escalation is ongoing to determine the RP2D for advancement into the expansion phase of the trial.”

Interim Results from the Ongoing HPN217 Phase 1/2 Trial Presented at ASH

This Phase 1/2 trial is a multicenter, open-label study designed to evaluate safety, tolerability, pharmacokinetics and clinical activity in patients with R/R MM who have had at least three prior systemic treatments including a proteasome inhibitor, an immunomodulatory drug and an anti-CD38 antibody. The initial ongoing phase of the trial is dose escalation, with the goal of determining a recommended dose for the expansion phase. The escalation phase began with single patient cohorts and transitioned to a 3+3 design when Grade 2 toxicity was observed. HPN217 is being administered to patients once weekly by intravenous infusion and the primary outcome measures are an assessment of safety and tolerability, pharmacokinetics and pharmacodynamics. Secondary endpoints include duration of response, progression free and overall survival. Tumor assessment is based on International Myeloma Working Group (IMWG) Response Criteria.

As of the November 10, 2021 data cut-off date, 37 patients have been treated in 10 cohorts with fixed doses ranging from 5 to 2860 µg/week or a step dosing regimen of 1620 µg priming dose followed by a 3240 µg/week target dose. Premedication to minimize CRS includes dexamethasone and other standard therapies. Enrolled patients had a median of 7 prior therapies. The most frequent treatment-emergent adverse events (TEAEs) occurring in greater than 20% were anemia, 17 patients (46%), fatigue, 12 patients (32%), and transient CRS, 9 patients (24%), No grade 3 or higher CRS was reported and one dose limiting toxicity (DLT) was reported, grade 4 AST, which resolved. Maximum tolerated dose has not been reached.

Clinical benefit was observed in the patients receiving higher doses. In 8 disease evaluable patients enrolled at 2150 µg/week an ORR of 63% was reported (5/8 patients) consisting of 1 stringent CR, 1 VGPR, and 3 PRs. including 1 patient with prior BCMA-targeting therapy exposure. The disease control rate, (DCR), was 88% based on 7/8 patients. For the 2860 µg/week cohort consisting of 5 evaluable patients, the ORR was 2/5 (40%) including a second stringent CR, with a DCR of 60%. As of the data cutoff, all responders remained on study treatment.

HPN217 demonstrated a dose proportional increase in Cmax and AUC with a median serum half-life of 74 hours (range of 38 – 197 hours), confirming half-life extension. Half-life, clearance rate, and volume of distribution were dose-independent, suggesting linear PK kinetics. Pharmacodynamic analysis shows a dose-dependent, transient increase in serum cytokines and chemokines (IL-6, IL-8, IL-10, TNFα).

Patients continue to be enrolled in the escalation phase of the trial, with a goal to identify a recommended Phase 2 dose for an expansion phase. The expansion phase of the trial will further evaluate the safety and activity of HPN217 in patients with R/R MM. This trial is titled, “A Phase 1/2 Open-label, Multicenter, Dose Escalation and Dose Expansion Study of the Safety, Tolerability, and PK of HPN217 in Patients With R/R MM”. For additional information about the trial, please visit www.clinicaltrials.gov using the identifier NCT04184050.

Preclinical Data for HPN217 Presented at ASH

The poster titled “The Effects of BCMA Expression, Soluble BCMA, and Combination Therapeutics on the Anti-Tumor Activity of HPN217, a BCMA-Targeting Tri-Specific T Cell Engager Against Multiple Myeloma” showcased translational studies to examine factors that may impact the therapeutic efficacy of HPN217. These factors include the target BCMA, in membrane-bound or soluble form, and concomitant or combination therapeutics such as gamma secretase inhibitor (GSI) and dexamethasone.

Preclinical data from this presentation for HPN217 demonstrated:

In a patient derived cell culture system, HPN217 was able to mediate multiple myeloma cell killing by autologous T cells in 80% of the cultures
Presence of dexamethasone appeared to have limited effect on the anti-tumor activity of HPN217-redirected T cells
GSI increased the expression of BCMA on multiple myeloma cells and enhanced the effect of HPN217
Preclinical evaluation of HPN217 in combination with approved and experimental multiple myeloma therapeutics is ongoing