This is a Phase 1, open-label, dose-escalation and dose-expansion study to evaluate the safety, tolerability, pharmacokinetic (PK), pharmacodynamic (PD) and clinical activity of etrumadenant (AB928) in combination with zimberelimab (AB122) (an anti-PD-1 antibody) in participants with advanced malignancies.

In the dose-escalation phase, escalating doses of etrumadenant in combination with zimberelimab will be assessed in participants with advanced malignancies. Eligible participants will receive oral administration of etrumadenant as well as IV infusion of zimberelimab. The recommended Phase 2 dose (RP2D) of etrumadenant will be determined upon completion of the dose-escalation phase.

In the dose-expansion phase, etrumadenant at RP2D in combination with zimberelimab may be assessed in participants with advanced clear-cell renal cell carcinoma (RCC) or metastatic castrate-resistant adenocarcinoma of the prostate (mCRPC).

Overall duration of treatment will depend on how well the treatment is tolerated. Treatment may continue until unacceptable toxicity or progressive disease or other reasons specified in the protocol.

Inclusion Criteria:

1. Male or female participants ≥ 18 years

2. Must have at least 1 measurable lesion per RECIST v1.1.

3. Eastern Cooperative Oncology Group (ECOG) performance status score of 0 or 1.

4. Must have received standard of care, including potentially curative available therapies or interventions.

5. Confirm that an archival tissue sample is available and ≤ 6 months old; if not, a new biopsy of a tumor lesion must be obtained. Biopsy must not put participant at undue risk and the procedure must not be more invasive than a core biopsy.

6. Adequate organ and marrow function

   Dose escalation only:

7. Pathologically confirmed non-small cell lung cancer, squamous cell carcinoma of the head and neck, renal cell carcinoma, breast cancer, colorectal cancer, melanoma, bladder cancer, ovarian cancer, endometrial cancer, Merkel cell carcinoma, or gastroesophageal cancer that is metastatic, advanced or recurrent with progression for which no alternative or curative therapy exists or standard therapy is not considered appropriate by the participant and treating physician (reason must be documented in medical records).

   Dose expansion only:

8. Participants with advanced clear-cell RCC or mCRPC.9. Clear-cell RCC participants may have received up to 2 prior lines of therapy, one of which must have included an anti-PD-(L)1 based therapy and must not have progressed within 16 weeks during an anti-PD-(L)1 therapy.

9. mCRPC participants must have progressed during or following treatment with an androgen synthesis inhibitor, and have also had one prior line of a taxane-containing regimen or the physician and participant consider the taxane-containing regimen to be inappropriate.

10. mCRPC participants must be naive to any immunotherapy (including but not limited to anti-PD-(L)1 or anti-CTLA-4 antagonists, sipuleucel-T, etc.).

Exclusion Criteria:

1. Use of any live vaccines against infectious diseases (eg, influenza, varicella) within 4 weeks (28 days) of initiation of investigational product.
2. Underlying medical conditions that, in the Investigator's or Sponsor's opinion, will make the administration of investigational product hazardous (eg, interstitial lung disease, active infections requiring antibiotics, recent hospitalization with unresolved symptoms) or obscure the interpretation of toxicity determination or AEs, or concurrent medical condition requiring the use of immunosuppressive medications or immunosuppressive doses of systemic or absorbable topical corticosteroids.
3. Has known psychiatric or substance abuse disorders that would interfere with cooperation with the requirements of the trial.
4. Is pregnant or breastfeeding or expecting to conceive or father children within the projected duration of the study, starting with the pre-screening or screening visit through 90 days after the last dose of etrumadenant in combination with zimberelimab.
5. Any active or documented history of autoimmune disease, or history of a syndrome that required systemic steroids or immunosuppressive medications, except for vitiligo or resolved childhood asthma/atopy. Participants with asthma who require intermittent use of bronchodilators (such as albuterol) will not be excluded from this study.
6. Prior malignancy active within the previous year except for locally curable cancers that have been apparently cured, such as basal or squamous cell skin cancer, superficial bladder cancer or carcinoma in situ of the cervix, breast, or prostate cancer.
7. Dose escalation: Prior treatment with an anti-PD-L1, anti-PD-1, anti-CTLA-4, or other immune checkpoint inhibitor or agonist as a monotherapy or in combination;
8. Use of other investigational drugs (drugs not marketed for any indication) within 28 days or at least 5 half-lives (whichever is longer) before investigational product administration.

Arcus will provide access to individual de-identified participant data and related study documents (e.g., protocol, Statistical Analysis Plan [SAP], Clinical Study Report [CSR]) upon request from qualified researchers, and subject to certain criteria, conditions, and exceptions.

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