Harpoon Therapeutics, Inc. 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 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. 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).