Study of GS-2121 Given Alone or in Combination in Adults With Advanced Solid Tumors

Key Inclusion Criteria:

• Participants diagnosed with histologically or cytologically confirmed advanced solidtumors who have progressed despite standard therapy, are intolerant to standardtherapy, or are ineligible for standard therapy.

• Measurable disease per Response Evaluation Criteria in Solid Tumors (RECIST) v1.1criteria.

• Eastern Cooperative Oncology Group (ECOG) performance status 0 to 1.

• Tissue requirements:

1. Parts A-D: Pretreatment tumor tissue is required.

2. Parts A and C backfill cohorts: Participants must agree to fresh pre- andon-treatment biopsies.

• Adequate organ function.

Key Exclusion Criteria:

• Positive serum pregnancy test or participant who is breastfeeding.

• Requirement for ongoing therapy with any prohibited medications.

• Any anti-cancer therapy, whether investigational or approved within protocolspecified time prior to initiation of study including: major surgery (<4 weeks),experimental therapy (<21 days or <5 half-lives whichever is longer), approvedimmunotherapy or biologic therapy (<28 days), approved chemotherapy (<21 days or <42days for mitomycin or nitrosoureas), approved targeted small molecule therapy (<14days or <5 half-lives whichever is longer), hormonal therapy or other adjunctivetherapy for cancers other than cancer under evaluation in this study (<14 days) orradiation therapy (<21 days).

• Any prior allogeneic tissue/solid organ transplantation, including allogeneic stemcell transplantation.

• Have not recovered (ie, returned to Grade 1 or baseline) from AEs due to apreviously administered agent.

• Have known active central nervous system (CNS) metastases and/or leptomeningealdisease (LMD).

• Diagnosis of immunodeficiency, either primary or acquired.

• History of autoimmune disease or active autoimmune disease that has requiredsystemic treatment within 2 years prior to the start of study treatment.

• Have an active second malignancy.

• Active and clinically relevant bacterial, fungal, or viral infection that is notcontrolled or requires systemic antibiotics, antifungals, or antivirals,respectively.

• History of pneumonitis requiring treatment with corticosteroids, interstitial lungdisease, or severe radiation pneumonitis (excluding localized radiationpneumonitis).

• Ascites or pleural effusion that is symptomatic and/or requiring medicalintervention.

• Have active hepatitis B virus (HBV) or hepatitis C virus (HCV), or HIV.

• Meet any of the following criteria for cardiac disease: Myocardial infarction orunstable angina pectoris within 6 months of enrollment. History of seriousventricular arrhythmia (ie, ventricular tachycardia or ventricular fibrillation),high-grade atrioventricular block, or other cardiac arrhythmias requiringantiarrhythmic medications (except for atrial fibrillation that is well controlledwith antiarrhythmic medication). Mean QT interval corrected for heart rate using theFridericia's formula (QTcF) ≥ 470 msec. New York Heart Association Class > IIIcongestive heart failure or known left ventricular ejection fraction < 40%.

• Live vaccines within 28 days of initiation of study drug(s).

Note: Other protocol defined Inclusion/Exclusion criteria may apply.