The purpose of this first-in-human study, CTMX-801-101, is to characterize the safety, tolerability, and antitumor activity of CX-801 as monotherapy and in combination with pembrolizumab in adult participants with advanced solid tumors.

The study is comprised of 2 parts. Part 1 involves CX-801 dose escalation to identify the maximum tolerated dose (MTD) of CX-801 as monotherapy and as combination therapy (CX-801 combined with pembrolizumab). Part 2 (dose expansion) will further assess safety and tolerability as well as preliminarily assess antitumor activity of CX-801 combination therapy in indication-specific expansion cohorts.

Inclusion Criteria:

• Metastatic or locally advanced unresectable solid tumor that has progressed after standard therapy
• Eastern Cooperative Oncology Group (ECOG) performance status of 0-1
• Measurable disease per RECIST v1.1
• Consent to fresh biopsy or if medically contraindicated, recent (within 6 months) archival tumor tissue
• Adequate organ function
• Additional inclusion criteria may apply

Exclusion Criteria:

• Recent history (within last 2 years) of localized cancers that are not related to the current cancer being treated
• Known active central nervous system (CNS) involvement by malignancy
• Prior PD-1/ PD-(L)1 inhibitor treatment discontinued due to grade 3 or higher immune related adverse event
• Systemic anticancer treatment within 4 weeks or 5 half lives prior to first dose of study treatment
• Investigational drug or device within 4 weeks prior to first dose of study treatment
• Radiation within 2 weeks prior to first dose of study treatment
• Serious concurrent illness
• Pregnant or breast feeding
• Additional exclusion criteria may apply