Vilastobart (XTX101) Monotherapy and Vilastobart and Atezolizumab Combination Therapy in Advanced Solid Tumors

Inclusion Criteria: Disease Criteria - * Part 1A and 1C: Any histologically or cytologically confirmed solid tumor malignancy that is locally advanced or metastatic and has failed standard therapy, or standard therapy is not curative or available; * Part 1B: * Any histologically or cytologically confirmed solid tumor malignancy for which anti-PD-1 or anti-PD-L1 treatment is approved and has progressed on or after prior anti-PD-1 or anti-PD-L1 therapy. * Patients with metastatic castrate-resistant prostate cancer if they have progressed on at least 2 lines of systemic therapy * Patients with extensive stage small cell lung cancer (SCLC) after at least 1 line of prior therapy * Patients with microsatellite stable colorectal cancer after at least 2 lines of prior therapy * Phase 2: Patients with histologically confirmed metastatic MSS CRC are eligible to enroll in Phase 2 as follows: * Patients must have had at least 1 prior chemotherapy regimen for metastatic CRC including all of the following agents: a fluoropyrimidine, irinotecan, oxaliplatin, bevacizumab or biosimilars, an anti epidermal growth factor receptor antibody (cetuximab or panitumumab), and v-raf murine sarcoma viral oncogene homolog B1 inhibitor/BRAF (encorafenib), if applicable * Patients with MSI-H/dMMR are excluded * ECOG performance status of 0 or 1 * Adequate organ function * Part 1B, Part 1C, and Phase 2 only: measurable disease per iRECIST Exclusion Criteria: * Received prior treatment wit…