Harpoon Presents Interim Data from Ongoing Dose Escalation Portion of T Cell Engager HPN328 Clinical Trial at 2022 ASCO Annual Meeting

• Clinically active and well tolerated in patients with solid tumors
• Favorable safety profile emerging: 22% of patients experienced Grade 1-2 cytokine release syndrome (CRS); no Grade 3 or higher CRS
• One confirmed partial response; 27% of small cell lung cancer (SCLC) patients with target lesion reductions of 30% or more
• Treatment duration now greater than 20 weeks in one-third of patients
• Dose escalation is ongoing; maximum tolerated dose not yet reached

SOUTH SAN FRANCISCO, Calif., May 26, 2022 (GLOBE NEWSWIRE) — Harpoon Therapeutics, Inc. (Nasdaq: HARP), a clinical-stage immunotherapy company developing novel T cell engagers, today announced interim safety and efficacy data from the ongoing dose escalation and expansion study evaluating HPN328, Harpoon’s half-life extended TriTAC^® targeting delta-like canonical Notch ligand 3 (DLL3), for the treatment of SCLC and other neuroendocrine cancers. The first scientific presentation of these interim data will be featured in a poster session at the American Society of Clinical Oncology (ASCO) Annual Meeting 2022, taking place in Chicago from June 3-7.

The encouraging interim results, as of the data cut-off date of April 21, 2022, showed that HPN328 demonstrated anti-tumor activity and a favorable safety profile in patients with SCLC, neuroendocrine prostate cancer and other neuroendocrine cancers. Seven of 18 patients (39%) had a decrease in sum of target lesion diameters, with 3 of 11 patients (27%) with SCLC across all dose cohorts experiencing a greater than 30% decrease in sum of target lesion diameters. Additionally, 4 of 6 patients (67%) with SCLC treated at greater than or equal to 1.215mg/week experienced a decrease in sum of target lesion diameters. To date, there have been no dose-limiting toxicities observed and no discontinuations due to adverse events. Grade 1-2 CRS occurred in 22% of patients. No grade 3 or higher CRS or any immune effector cell associated neurotoxicity syndrome (ICANS) events have been observed.

“DLL3 is expressed on the surface of tumor cells in more than 70% of small cell carcinomas, including small cell lung cancer, neuroendocrine prostate cancer, and other small cell neuroendocrine cancers, and HPN328 is specifically engineered to hit this target,” said Himisha Beltran, M.D., of Dana-Farber Cancer Institute, Boston, a Principal Investigator in this study. “The encouraging single-agent clinical activity observed to date in patients that have received multiple prior lines of therapy, combined with the favorable safety profile, suggest the investigational T cell engager HPN328 may offer meaningful clinical benefits as a monotherapy for patients expressing DLL3. I look forward to the clinical results from further investigations with this promising drug candidate.”

To date, study investigators have observed 1 confirmed partial response with a 53% decrease in sum of target lesion diameters at week 10 in a patient with SCLC who previously achieved a best overall response of stable disease on platinum-based chemo-immunotherapy. Another SCLC patient treated with 3 prior lines of therapy achieved a 65% decrease in sum of target lesion diameters with deepening of target lesion response, with treatment ongoing beyond six months. There were 6 instances of patients with best overall response of stable disease (4 SCLC, 1 neuroendocrine prostate cancer, and 1 thymic atypical carcinoid).

“We are pleased to share our continued progress with the HPN328 anti-DLL3 T cell engager clinical program and these interim data in a peer-reviewed setting, providing further clinical validation for our TriTAC technology in solid tumors,” said Julie Eastland, President and CEO of Harpoon Therapeutics. “Given the clinical activity and acceptable tolerability profile observed to date, we look forward to continuing dose escalation, with the goal of identifying a dose for expansion studies by the end of the year, as we further explore the full potential of HPN328 as both a single agent and in future combination studies with atezolizumab to help patients with cancer.”

Details of the ASCO poster presentation are as follows:

Title: Interim results of an ongoing phase 1/2a study of HPN328, a tri-specific, half-life extended, DLL3-targeting, T cell engager, in patients with small cell lung cancer and other neuroendocrine cancers
Abstract/Poster: 8566/193
Session Title: Lung Cancer—Non-Small Cell Local-Regional/Small Cell/Other Thoracic Cancers
Session Date and Time: Monday, June 6, 8:00 a.m. to 11:00 a.m. CT

About HPN328

HPN328, a Tri-specific T cell Activating Construct (TriTAC^®), is being evaluated as monotherapy in an ongoing open-label, multicenter two-part study to assess the safety, tolerability and pharmacokinetics in patients with advanced cancers associated with expression of DLL3. Part 1 of the study is designed to determine dosage(s) for further evaluation in expansion cohorts during Part 2.

In March 2022, the U.S. Food and Drug Administration (FDA) granted Orphan Drug Designation to HPN328 for the treatment of SCLC.

For additional information on the HPN328 clinical study, please go to ClinicalTrials.gov and use Identifier NCT04471727.

About Harpoon Therapeutics

Harpoon Therapeutics is a clinical-stage immunotherapy company developing a novel class of T cell engagers that harness the power of the body’s immune system to treat patients suffering from cancer and other diseases. T cell engagers are engineered proteins that direct a patient’s own T cells to kill target cells that express specific proteins, or antigens, carried by the target cells. Using its proprietary Tri-specific T cell Activating Construct (TriTAC^®) platform, Harpoon is developing a pipeline of novel TriTACs initially focused on the treatment of solid tumors and hematologic malignancies. Harpoon has also developed a proprietary ProTriTAC™ platform, which applies a prodrug concept to its TriTAC platform to create a therapeutic T cell engager that remains inactive until it reaches the tumor. Harpoon’s third proprietary technology platform, extended release TriTAC-XR, is designed to mitigate cytokine release syndrome. For additional information about Harpoon Therapeutics, please visit www.harpoontx.com.

Cautionary Note on Forward-looking Statements

This press release contains forward-looking statements within the meaning of the Private Securities Litigation Reform Act of 1995. Words such as “look forward,” “may,” “target,” “will,” and similar expressions (as well as other words or expressions referencing future events, conditions or circumstances) are intended to identify forward-looking statements. These forward-looking statements are based on Harpoon Therapeutics’ expectations and assumptions as of the date of this press release. Each of these forward-looking statements involves risks and uncertainties that could cause Harpoon Therapeutics’ clinical development programs, future results or performance to differ significantly from those expressed or implied by the forward-looking statements. Forward-looking statements contained in this press release include, but are not limited to, statements about the development and advancement of Harpoon Therapeutics’ platforms and product candidates, including progress, timing, scope, design and interim results of ongoing dose escalation portion of T cell engager HPN328 clinical trial, ability of TriTAC-XR T cell engager platform to mitigate toxicities, such as cytokine release syndrome, the candidate’s safety and tolerability profile, clinical benefits of HPN328, and other statements that are not historical fact. Many factors may cause differences between current expectations and actual results, including unexpected safety or efficacy data observed during clinical studies, preliminary data and trends may not be predictive of future data or results, may not demonstrate safety or efficacy or lead to regulatory approval by the FDA or other regulatory agencies, clinical trial site activation or enrollment rates that are lower than expected, unanticipated or greater than anticipated impacts or delays due to COVID-19, changes in expected or existing competition, changes in the regulatory environment, the uncertainties and timing of the regulatory approval process, the timing and results of unexpected litigation or other disputes, and the sufficiency of Harpoon Therapeutics’ cash resources. These and other factors that may cause Harpoon Therapeutics’ actual results to differ from those expressed or implied in the forward-looking statements in this press release are discussed in Harpoon Therapeutics’ filings with the U.S. Securities and Exchange Commission, including under “Risk Factors” in Harpoon Therapeutics’ quarterly report on Form 10-Q for the quarter ended March 31, 2022 and future filings by Harpoon Therapeutics. Except as required by law, Harpoon Therapeutics assumes no obligation to update any forward-looking statements contained herein to reflect any change in expectations, even as new information becomes available.

Contacts:

Investors:
Robert Uhl
ICR Westwicke
858-356-5932
robert.uhl@westwicke.com

Media:
uncapped communications
862-368-4464
media@harpoontx.com

Company:
Georgia Erbez
Chief Financial Officer
650-443-7400