PRS-343 in HER2-Positive Solid Tumors

The study will evaluate PRS-343 administered by intravenous (IV) infusion every 3 weeks (Schedule 1) initially. If safety, PK, and PD data suggest a different dosing schedule should be evaluated, Schedule 2 and/or 3 (dosing every 2 weeks in a 28-day cycle or once a week in a 21-day cycle, respectively) may be conducted. Separate MTDs may be determined for each schedule evaluated. Dose-limiting toxicities (DLTs) will be reported during the first cycle of each schedule (e.g., 21 days after the first dose in Cycle 1 for Schedule 1). Patients will be monitored for safety throughout the study. Dosing will continue until criteria for study drug discontinuation are met (disease progression or withdrawal from the study). Patients with unknown HER2 status will be consented separately in a pre-screening visit in order to undergo HER2 testing prior to screening. All patients will be evaluated at screening (Day 28 to 1) and baseline (Day 1 predose). Once the MTD has been established, up to 30 additional patients with locally advanced or metastatic HER2+ solid tumors considered likely to respond to a HER2 targeted CD137 agonist (e.g. gastric/gastroesophageal/esophageal, breast, bladder) may be enrolled in individual expansion cohorts. The expansion cohort will be enrolled at the MTD and/or at a lower dose level if safety/PD/PK/efficacy data support further evaluation of a lower dose level in order to determine the RP2D. The RP2D may be equivalent to or lower than the MTD. An End-of-Treatment Visit will be performed at the time of treatment discontinuation. Patients will be evaluated 30 days after the End-of-Treatment Visit or prior to starting subsequent therapy, if sooner, at the Safety Follow-up Visit to assess any ongoing AEs as outlined in the protocol. Obinutuzumab pre-treatment cohorts: The potential of obinutuzumab pre-treatment to reduce formation of ADA will be studied in a cohort of up to ten patients receiving PRS-343 at a dose of 8 mg/kg Q2 weeks (corresponding to Cohort 11b). Patients will be assessed for tumor response/progression per RECIST v1.1