‣ \- Must have a pathologically confirmed advanced and unresectable or metastatic solid tumor listed below with documented disease progression on last standard treatment. Part 1 only: subjects must be refractory to, or intolerant of existing therapy(ies) known to provide clinical benefit for their condition.

‣ Dose Escalation:

⁃ Ovarian cancer with CCNE1 amplification

⁃ Endometrial cancer with CCNE1 amplification

⁃ Gastric, gastroesophageal junction (GEJ) or esophageal adenocarcinoma with CCNE1 amplification

⁃ Small cell lung cancer (SCLC)

⁃ Triple-negative breast cancer (TNBC; HER2, estrogen receptor and progesterone receptor negative)

⁃ HR+ (includes estrogen-receptor or progesterone-receptor) and HER2- breast cancer (must have progressed following treatment with a CDK4/6 inhibitor, and is not suitable for endocrine therapy \[ET\])

⁃ Other solid tumors with CCNE1 amplification

‣ Dose Expansion:

‣ Part 2A: HR+ and HER2- breast cancer that is locally advanced and unresectable (Stage III) or metastatic (Stage IV); previously treated with ≥1 line of standard of care (SOC) including CDK4/6 inhibitor plus ET and not suitable for further ET. Subjects must have progressed after receiving therapy for ≥3 months in the metastatic setting or for ≥6 months in the adjuvant setting. Subjects must have received ≤2 lines of systemic cytotoxic therapy (chemotherapy or cytotoxic antibody drug conjugate \[ADC\]) in the metastatic setting..

‣ Part 2B: Advanced platinum-based-chemotherapy resistant or refractory epithelial ovarian/fallopian/primary peritoneal carcinoma or clear cell ovarian cancer (defined as recurrence ≤6 months after completing platinum-based regimen) with progression on at least one platinum containing therapy and previously treated with ≤4 prior lines of systemic therapy administered for advanced/metastatic disease with CCNE1 amplification as determined by NGS by local liquid or tissue test.

‣ Part 2C: Advanced unresectable or metastatic gastric, GEJ or esophageal adenocarcinoma with progression on at least one systemic therapy and previously treated with ≤3 prior lines of systemic therapy administered for advanced/metastatic disease, with CCNE1 amplification as determined by NGS by local liquid or tissue test.

‣ Part 2D: Advanced endometrial adenocarcinoma or uterine papillary serous carcinoma previously treated with ≤4 prior lines of systemic therapy administered for advanced/metastatic disease, with CCNE1 amplification as determined by NGS by local liquid or tissue test.

‣ Part 2E: Advanced/recurrent uterine carcinosarcoma previously treated with 1 prior platinum-based chemotherapy regimen and ≤3 prior lines of systemic therapy, with CCNE1 amplification as determined by NGS by local liquid or tissue test. Prior bevacizumab or PARP inhibitors are allowed and must be at least 3 weeks prior to the start of study drug.

• Have adequate organ function

• Subjects with female reproductive organs must be surgically sterile, post-menopausal, or must be willing to use highly effective method(s) of contraception

• Ability to swallow oral medications.

• Consent to provide archived tumor tissues and paired tumor biopsy at pretreatment