This is a dose-escalation and expansion study of ASP1951. The study consists of 3 periods for monotherapy and combination therapy: screening, treatment and follow up, followed by an optional Re-treatment period for participants that qualify.~The monotherapy escalation cohorts will evaluate escalating dose levels of ASP1951 in participants with locally advanced (unresectable) or metastatic solid tumor malignancies including but not limited to squamous cell carcinoma of the head and neck (SCCHN), colorectal cancer, metastatic castration-resistant prostate cancer (mCRPC) and cervical cancer.~The combination escalation cohorts will evaluate escalating dose levels of ASP1951 in combination with a fixed dose of pembrolizumab.~For dose expansion, the tumor-specific cohorts will include participants with squamous cell carcinoma of the head and neck (SCCHN), non-small cell lung cancer (NSCLC) (all PD-L1 status), NSCLC PDL1 high, and cervical cancer, as well as participants with any tumor types that respond to study drug treatment during dose escalation.~Participants may reinitiate study drug treatment in the optional Re-treatment period after confirmation that the participant meets all the re-treatment eligibility criteria.~After discontinuation of study drug, all participants will complete an end-of-treatment visit, along with 30-day and 90 day safety follow-up visits from the last dose of study drug.
The primary purpose of this study is to evaluate the tolerability and safety profile of ASP1951 when administered as a single agent and in combination with pembrolizumab in participants with locally advanced (unresectable) or metastatic solid tumors; characterize the pharmacokinetic profile of ASP1951 when administered as a single agent and in combination with pembrolizumab; and determine the recommended phase 2 dose (RP2D) of ASP1951 and/or maximum tolerated dose (MTD) when administered as a single agent and in combination with pembrolizumab. This study will also evaluate the anti-tumor effect of ASP1951 when administered as a single agent and in combination with pembrolizumab.