A Study of NPX887 for Participants With Solid Tumors Known to Express B7-H7/HHLA2

Inclusion Criteria: * Histologically or cytologically confirmed recurrent, metastatic solid tumor refractory to, or intolerant of, standard of care therapy in one of the following indications: * Phase 1a (Dose Escalation): Non-small cell lung carcinoma (NSCLC), small cell lung carcinoma (SCLC), renal cell carcinoma (RCC), colorectal carcinoma (CRC), gastric and gastro-esophageal carcinoma, esophageal adenocarcinoma, biliary tract cancers, ovarian carcinoma, and other solid tumor types known to express B7-H7/HHLA2. * Phase 1b including Part 1b (Dose Expansion) and Part 1c (Randomized Dose Comparison): participants who have clear cell RCC, EGFR mutant lung adenocarcinoma, or gastric/GEJ adenocarcinoma. * In Phase 1b, participants must have confirmed B7-H7/HHLA2 expression in their tumor determined via archival tissue IHC testing through a central lab (pre-screening). * Phase 1a: Evaluable disease (measurable or non-measurable) by RECIST v.1.1 criteria; Phase 1b: Measurable disease by RECIST v1.1 criteria with additional disease-specific enrollment criteria applied to clear cell RCC, EGFR mutant lung adenocarcinoma, or gastric/GEJ adenocarcinoma. * Eastern Cooperative Oncology Group (ECOG) performance status 0, 1, or 2. * Ability to understand and the willingness to sign a written informed consent document * Willing to use highly effective contraceptive measures throughout the trial. Exclusion Criteria: * Treatment with any of the following: * Systemic anticancer treatment ≤14 days or within 5 half-lives prior to the first dose of study drug, whichever is shorter. * Limited-field radiotherapy ≤7 days or extended-field thoracic radiotherapy ≤8 weeks of the first dose of study drug. * Have any unresolved toxicity of ≥Grade 2 from previous anti-cancer treatment, except for alopecia, chronic stable neuropathy for \>4 months, changes in skin pigmentation, or requiring replacement therapy for endocrine abnormalities. * Participants with known brain metastases are excluded unless they are clinically stable, with no new or enlarging brain metastases as evidenced on MRI during screening. * History of Grade 3 immune-related pneumonitis or colitis. * Participants who discontinued prior immunotherapy due to immune-related toxicities, or history of unresolved prior immune-related toxicity except for endocrine abnormalities requiring replacement therapy or vitiligo. * Known autoimmune disease requiring immunosuppressive treatment requiring the equivalent of more than 10 mg prednisone daily.