Daiichi Sankyo Company Limited has announced the initiation of a three-part open-label phase 1/2 study in Japan with U3-1402, a novel Her3-targeting antibody drug conjugate, in patients with Her3-positive metastatic or unresectable breast cancer who are refractory or intolerant to standard treatment, or for whom no standard treatment is available.

U3-1402, the second clinical-stage investigational antibody drug conjugate in the Daiichi Sankyo Cancer Enterprise pipeline, comprises a humanized anti-Her3 antibody attached by a peptide linker to a novel topoisomerase I inhibitor (DXd) payload, utilizing Daiichi Sankyo’s proprietary payload and linker-payload technologies. U3-1402 achieves a Drug-Antibody Ratio (DAR) nearly eight molecules of DXd are attached per antibody.

“While treatment for patients with metastatic HER2-negative breast cancer has gradually improved over the past several years, there is still need for improvement, particularly by developing targeted therapies,” said Antoine Yver, MD, MSc, executive vice president and global head, Oncology research and development, Daiichi Sankyo. “The purpose of this study is to determine whether delivering a cytotoxic agent via a HER3 monoclonal antibody could be an effective and safe approach to treating patients with HER3-overexpressing breast cancer.”

In this three-part open-label phase 1/2 study, U3-1402 will be given as an intravenous infusion every three weeks. The first part of the study (dose escalation) will assess the safety, tolerability and maximum tolerated dose of U3-1402 in HER3-positive metastatic breast cancer patients who are refractory or intolerant to standard treatment, or for whom no standard treatment is available.

The second part of the study (dose-finding) will assess the safety and efficacy of U3-1402 and determine the recommended phase 2 dose in HER3-positive metastatic breast cancer patients who have received six or fewer prior chemotherapy regimens. The third part of the study (phase 2) will assess the safety and efficacy of the recommended dose of U3-1402 in HER3-positive metastatic breast cancer patients who have received six or fewer prior chemotherapy regimens.