Heidelberg Pharma Advances to Cohort 9 in Phase I/IIa Trial of Lead ATAC Candidate HDP-101 in Multiple Myeloma

• HDP-101 continues to demonstrate a favorable safety profile with no dose-limiting toxicities observed
• Early evidence of clinical activity observed in Cohort 8, highlighted by a very good partial response observed in one patient

Ladenburg, Germany, 25 September 2025 – Heidelberg Pharma AG (FSE: HPHA), a clinical-stage biotech company developing innovative Antibody Drug Conjugates (ADCs), today announced the initiation of Cohort 9 in its ongoing Phase I/IIa dose escalation trial of HDP-101, the Company’s lead ATAC candidate for the treatment of relapsed or refractory multiple myeloma.

The Safety Review Committee (SRC) has confirmed that the 140 µg/kg dose level administered in Cohort 8 was safe and well tolerated. Based on these findings, the study is now progressing into Cohort 9 with an escalated dose of 175 µg/kg, administered in one dosing arm. Cohort 9 has already opened.

Eight patients were dosed in Cohort 8 and all patients completed the observation period, demonstrated a favorable safety and tolerability profile throughout. Encouraging signs of clinical activity have also emerged. Patients showed biological activity of HDP-101, and a very good partial response has already been observed in one patient. This adds to earlier positive outcomes, including one patient from Cohort 5 who is still on treatment and achieved complete remission, with no detectable tumor cells after ongoing HDP-101 monotherapy following multiple prior treatments. In addition, several patients across different cohorts have shown objective responses and promising anti-tumor activity, further supporting the therapeutic potential of HDP-101 in heavily pretreated patients with relapsed or refractory multiple myeloma.

Dr. András Strassz, Chief Medical Officer at Heidelberg Pharma, said: “Our lead ATAC candidate HDP-101 continues to demonstrate a strong safety and tolerability profile across all treated patients. The results from Cohort 8 demonstrate encouraging signs of clinical activity of HDP-101, including a very good partial response in one patient. These early efficacy signals are promising as we advance HDP-101 through the ongoing dose escalation study.”

Heidelberg Pharma’s Phase I/IIa clinical study is a non-randomized, open-label trial actively enrolling patients with relapsed or refractory multiple myeloma or other BCMA-expressing plasma cell disorders. The study is designed to evaluate the safety, tolerability, pharmacokinetics, and preliminary efficacy of HDP-101 in this patient population.