Heidelberg Pharma Presented New Clinical Data for Lead ATAC Candidate Pamlectabart Tismanitin at IMS Annual Meeting 2026

PRESS RELEASE

Heidelberg Pharma Presented New Clinical Data for Lead ATAC Candidate Pamlectabart Tismanitin at IMS Annual Meeting 2026

• Recommended Phase 2 Dose (RP2D) of 175 µg/kg for pamlectabart tismanitin established based on Phase I data
• Favorable safety profile observed across dose escalation, with the maximum tolerated dose (MTD) not reached
• Phase IIa dose expansion in patients with relapsed or refractory multiple myeloma (RRMM) progressing according to plan

Ladenburg, Germany, 24 September 2026 – Heidelberg Pharma AG (FSE: HPHA), a clinical-stage biotech company developing innovative Antibody Drug Conjugates (ADCs), presented new data from its Phase I/IIa clinical study with its lead Amanitin-based ADC candidate, pamlectabart tismanitin (HDP-101), at the 23^rd International Myeloma Society (IMS) Annual Meeting in Glasgow, UK.

The Phase I dose escalation part of the clinical study was completed in April 2026, after treating ten patient cohorts across a dose range of 20 to 218 µg/kg. Heidelberg Pharma’s Benefit and Risk Assessment team agreed with the recommendation of the study’s Safety Review Committee (SRC), which had conducted a comprehensive review of safety, tolerability and pharmacokinetic (PK) data from the ongoing trial, and selected the dose level from Cohort 9 – 175 µg/kg – as Recommended Phase 2 Dose (RP2D).

In the Phase I part of the study, 53 patients with relapsed or refractory multiple myeloma (RRMM) were dosed and included in the data analysis. All patients were heavily pretreated with a median of five prior lines of treatment. Pamlectabart tismanitin was well-tolerated and a maximum tolerated dose was not reached. All adverse events that occurred were reversible, and no treatment related grade 5 events were observed.

Clinical responses were observed from ≥90 µg/kg, with increasing efficacy at higher doses. Across Cohorts 5 – 9, the Overall Response Rate (ORR) was 42% (13 out of 31 evaluable patients), including seven partial responses (PR), three very good partial responses (VGPR), and three stringent complete responses (sCR). In Cohort 9 (175 µg/kg), two out of five evaluable patients achieved a response: one PR and one VGPR. Additionally, one patient achieved minimal response (MR) and one patient with non-measurable disease achieved complete response (CR). Two of these patients are still under treatment.

Dr. Dongzhou Jeffery Liu, Chief Executive Officer of Heidelberg Pharma: “The data presented at IMS further strengthen our confidence in pamlectabart tismanitin as a promising new BCMA-targeted ADC. The observed clinical activity and favorable safety profile in heavily pretreated RRMM patients highlight the potential of our Amanitin-based ADC product to address unmet needs in multiple myeloma patients. With the RP2D now established, we look forward to advancing into the Phase IIa study further evaluating the therapeutic potential of pamlectabart tismanitin.”

Heidelberg Pharma’s Phase I/IIa clinical study is a non-randomized, open-label, dose escalation 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 pamlectabart tismanitin in this patient population.

The poster can be found on the Company’s website: Poster IMS 2026

About Heidelberg Pharma

Heidelberg Pharma is the first company to develop cancer therapies using Amanitin, a compound derived from the green death cap mushroom. The biological mechanism of action of the toxin represents a new therapeutic modality and is used as a compound in the Amanitin-based ADC technology, the so-called ATAC technology.

Lead candidate HDP-101 (INN: pamlectabart tismanitin) is a BCMA ATAC in clinical development for multiple myeloma. The candidate has been granted Orphan Drug Designation and Fast Track Designation from the FDA. A second ATAC candidate, HDP-102 is in clinical development stage in Non-Hodgkin Lymphoma. HDP-103 against metastatic castration-resistant prostate cancer and HDP-104 targeting gastrointestinal tumors such as colorectal cancer have completed preclinical development. These programs are available for partnering.

The company is based in Ladenburg, Germany, and is listed on the Frankfurt Stock Exchange: ISIN DE000A11QVV0 / WKN A11QVV / Symbol HPHA. More information is available at http://www.heidelberg-pharma.com/.

ATAC^® is a registered trademark of Heidelberg Pharma Research GmbH.

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