The primary purpose of this study is to assess the safety, tolerability, pharmacokinetics (PK) and anti-tumor activity of GTAEXS617 (REC-617) in participants with advanced solid tumors.

Key Inclusion Criteria:

• Eastern Cooperative Oncology Group (ECOG) performance status 0-1.
• Life expectancy > 3 months.
• One of the following histologically or cytologically confirmed advanced solid tumors: head and neck squamous cell carcinoma (HNSCC), pancreatic adenocarcinoma, non-small cell lung cancer (NSCLC), breast carcinoma (hormone receptor-positive [HR+] and Human Epidermal Growth Receptor 2 negative [HER2-] that has progressed to a prior treatment with Cyclin-Dependent Kinase 4 (CDK4)/ Cyclin-Dependent Kinase 6 [CDK6] inhibitor), or platinum-resistant high-grade epithelial ovarian, primary peritoneal, or fallopian tube cancers (HGSOC), or triple negative breast cancer (TNBC).
• Must have disease that is advanced (ie, surgery or radiotherapy are not considered to be potentially curative), recurrent, or metastatic following SoC treatments.
• Adequate hematological, liver, and renal function.
• Must have tumor lesion(s) or metastases amenable to biopsy, excluding bone metastases.

Key Exclusion Criteria:

• Active and clinically significant (CS) infection.
• Refractory nausea and/or vomiting, chronic gastrointestinal disease, or previous significant bowel resection, with CS sequelae that would preclude adequate absorption of GTAEXS617.
• Symptomatic central nervous system (CNS) malignancy or metastases.
• Concurrent active or previous malignancy.
• Prior organ or allogeneic stem-cell transplantation.
• Moderate or severe cardiovascular disease.
• Received anticancer therapy within 28 days or 5 half-lives (whichever is shorter) before the first dose of the study treatment.
• Received treatment with known strong/moderate inhibitors and/or strong inducers of cytochrome P450 3A isoform subfamily (CYP3A) within 14 days or 5 half-lives before the first dose of study treatment.
• Received treatment with known inhibitors or inducers of P-glycoprotein (P-gp) or breast cancer resistance protein (BCRP) within 14 days or 5 half-lives before the first dose of study treatment.
• Received treatment with known substrates of organic anion transporting peptide or BCRP within 14 days or 5 half-lives before the first dose of study treatment.
• Unresolved or unstable serious toxic side-effects of prior chemotherapy or radiotherapy
• Has had or is scheduled to have major surgery <28 days prior to the first dose of study treatment.

Note: Other protocol Inclusion/Exclusion criteria may apply.