Background: XMT-1522 is an ADC consisting of a novel human IgG1 anti-HER2 monoclonal antibody conjugated to an auristatin-based cytotoxic payload (AF-HPA). An average of 12 AF-HPA molecules is conjugated to each antibody via a biodegradable polymer. In pre-clinical xenograft experiments XMT-1522 achieved complete, durable tumor regressions in models of HER2-positive and HER2 1+/2+ breast cancer, HER2 2+/3+ NSCLC, and HER2-positive and HER2 1+ gastric cancer. Methods: This study (NCT02952729) is comprised of two parts: a dose escalation segment (DES) and an expansion segment (EXP). The primary objectives of the DES are determination of the maximum tolerated dose and recommended Phase 2 dose (RP2D) and assessment of safety and tolerability. The DES will enroll patients with advanced or metastatic breast cancer who have progressed following standard therapies and have HER2 protein at least 1+ by IHC. XMT-1522 will be administered intravenously every 3 weeks. DES uses a 3+3 design. Post-dose assessments include LVEF measurement at the end of cycles 1, 3, then every 3 cycles, ophthalmologic exams at the end of cycles 1, 2, then every 2 cycles, and re-staging CT scans every 2 cycles. Pharmacokinetics of antibody, AF-HPA payload and an AF-HPA metabolite will be measured. Two patients have completed dose level 1 without DLT. The EXP segment will open at the RP2D and will further assess safety and tolerability of XMT-1522 and assess efficacy in selected patient populations. EXP will enroll 4 cohorts (N = 20 each).

Cohort 1: HER2 1+/2+ advanced breast cancer with 2-3 prior chemotherapy regimens

Cohort 2: HER2-positive advanced breast cancer with prior pertuzumab and ado-trastuzumab emtansine (T-DM1)

Cohort 3: HER2-positive advanced gastric cancer with prior trastuzumab

Cohort 4: HER2 2+/3+ NSCLC with at least 1 prior platinum regimen

The protocol requires archival tumor tissue for central confirmation of HER2 status, alternative HER2 measurements, and targeted gene expression and sequencing studies. Tumor biopsies will be requested at the time of progression from patients who responded to XMT-1522. Clinical trial information: NCT02952729