Molecular Templates, Inc. Provides Update on MT-5111 Phase 1 Study - Seite 2

Fifteen of the 16 subjects have discontinued for disease progression. One study subject with metastatic breast cancer in cohort 2 (1 µg/kg) remained on treatment for 10 cycles with stable disease; although she had unmeasurable disease by RECIST criteria, she had three sub-centimeter hepatic lesions that disappeared at the end of cycle 8 before she discontinued for disease progression (new breast lesion) at cycle 10.  This subject had received three prior HER-2 targeting regimens which initially included pertuzumab plus trastuzumab followed by trastuzumab and T‑DM1 as monotherapies. One subject with metastatic pancreatic cancer in cohort 5 (4.5 µg/kg/dose) remains on study in cycle 2.

Cohort 6 (6.75 μg/kg) is now open for enrollment with cohort 7 (10 μg/kg) expected to open in 1Q21. The HER2positive breast cancer expansion cohort is planned to be initiated in 1H21 at a dose of 10 μg/kg (anticipated to be a therapeutic dose level), pending adequate safety. Dose escalation will continue to determine the Recommended Phase 2 Dose while the breast cancer expansion cohort collects efficacy and safety data.

“We are encouraged by the safety profile of MT-5111 to date in these heavily pretreated study subjects. Based on preclinical data, we believe that the study has reached clinically active dose levels, said Eric Poma, Ph.D., Molecular Templates’ Chief Executive and Chief Scientific Officer. “Given that HER2 positive breast cancer patients have generally had the highest response rates to other HER2-targeted therapies, we look forward to generating data from both the HER2-positive breast cancer cohort as well as the broader HER2-positive cohort enrolling all tumor types. We expect to provide an update on results from the subject currently on treatment as well as higher dose cohorts from the dose escalation portion of the Phase 1 study in 1H21.”

About MT-5111

MT5111 is an ETB consisting of a single chain variable fragment (scFv) with affinity for HER2, fused to the enzymatically active de-immunized Shiga-like toxin-A subunit (SLTA). In preclinical studies MT5111 specifically binds and kills HER2 expressing cells in a manner consistent with SLTA mediated cellular cytotoxicity. MT5111 has been specifically designed to avoid competition with and to overcome the primary mechanisms of tumor resistance to current HER2 targeted therapies. To accomplish this, first, MT5111 binds a HER2 domain that is distinct from the trastuzumab and pertuzumab binding sites, which results in MT5111 HER2-mediated binding and cell kill even in the presence of these monoclonal antibodies. As such, MT5111 may have the potential to be combined with other HER2 targeted therapies. Second, SLTA is a large molecule protein and is not a substrate of drug efflux transporters such as MDR1 which has been demonstrated to be one of the primary mechanisms of resistance to the antibody drug conjugate, TDM1. Third, MT5111 mediated ribosomal inhibition and cell death take place intracellularly so changes in the tumor microenvironment that inhibit immune-mediated mechanisms such as antibody-dependent cell-mediated cytotoxicity (ADCC) are not expected to inhibit MT5111 activity. Finally, mutations to the HER2 kinase domain that can induce constitutive downstream signaling to drive tumor proliferation are not expected to interfere with MT5111 activity, given that its mechanism of action is not dependent upon kinase domain binding and MT5111 works directly on ribosomes to mediate ribosomal inhibition and cell death. Based on these mechanisms of action, MT5111 represents a novel HER2 targeted therapy which could provide benefit in patients with HER2-positive cancers and potentially overcome mechanisms of tumor resistance to existing HER2-targeted therapies.