This is a first-in-human, Phase 1/2, multicenter, open-label study designed to evaluate the safety and tolerability of vilastobart (XTX101) as monotherapy and vilastobart (XTX101) and atezolizumab combination therapy in patients with advanced solid tumors.

This is a first-in-human, Phase 1/2, multicenter, open-label study designed to evaluate the safety and tolerability of vilastobart (XTX101), a tumor-selective anti-CTLA-4 antibody, as monotherapy and vilastobart (XTX101) and atezolizumab combination therapy in patients with advanced solid tumors.

Part 1A will examine vilastobart (XTX101) monotherapy in an accelerated and standard 3+3 dose escalation design. Based on the results of Part 1A, patients with select advanced solid tumors will be enrolled in Part 1B, which will evaluate vilastobart (XTX101) monotherapy in relation to specific PD biomarkers.

Part 1C will examine vilastobart (XTX101) in combination with atezolizumab in a standard 3+3 dose escalation/dose de-escalation design. Part 1C may include a dose expansion cohort to further evaluate the safety, PK, and PD of dose levels that were previously cleared.

Phase 2 will examine vilastobart (XTX101) in combination with atezolizumab in patients with metastatic microsatellite stable colorectal cancer (MSS CRC) at the RP2D(s) defined in Part 1C.

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 with anti-CTLA-4 therapy
• Received prior immune-checkpoint therapy and experienced Grade 3 or greater toxicity lasting greater than 6 weeks
• Received prior approved systemic anticancer therapy within 4 weeks prior to study treatment
• Received prior radiotherapy within 2 weeks prior to study treatment
• Phase 2 only: Received prior anti-PD-1/L1 therapy or any investigational checkpoint inhibitory therapy
• Has a diagnosis of immunodeficiency
• Has known malignancy (other than disease under study) that is progressing or has required active treatment within the past 3 years
• Has an active autoimmune disease that has required systemic treatment in past 2 years, including the use of disease modifying agents, corticosteroids or immunosuppressive drugs
• Has an active infection requiring systemic intravenous therapy within 4 weeks prior to study treatment, or oral therapy within 2 weeks prior to study treatment
• Has a history of severe hypersensitivity reaction (≥ Grade 3) to any study intervention and/or any of its excipients
• Has a known psychiatric or substance abuse disorder that would interfere with the participant's ability to cooperate with the requirements of the study
• Phase 2 only: symptomatic bowel obstruction