A Study With NKT3447 for Adults With Advanced/Metastatic Solid Tumors

Inclusion Criteria: * Must have confirmed unresectable advanced/metastatic solid tumors (as outlined below) with disease progression on prior standard treatment, intolerance to or ineligibility for standard treatment, or no available standard treatment likely to improve the disease outcome in the judgment of the Investigator. * Measurable disease per the RECIST v1.1 * An Eastern Cooperative Oncology Group (ECOG) performance status of 0 to 1 * Able to swallow oral medications. Dose Escalation(Part 1): 1. Ovarian cancer 2. Endometrial cancer 3. Gastric cancer or gastroesophageal junction cancer 4. Small cell lung cancer (SCLC) 5. Triple-negative breast cancer (human epidermal growth factor receptor 2 \[HER2\], estrogen receptor \[ER\], progesterone receptor negative) 6. ER/progesterone-receptor positive, HER2 negative breast cancer (must have progressed following treatment with a CDK4/6 inhibitor, and not suitable for endocrine therapy) 7. Other solid tumors with CCNE1 amplification as determined by NGS by local liquid or tissue biopsy. Dose Expansion (Part 2): a. Platinum resistant or refractory ovarian cancer (defined as recurrence ≤6 months after completing platinum-based regimen) with progression on at least 1 platinum containing therapy with CCNE1 amplification as determined by NGS by local liquid or tissue biopsy. * Measurable disease per the RECIST v1.1 * An Eastern Cooperative Oncology Group (ECOG) performance status of 0 to 1 * Able to swallow oral medications. Exclusion Criteria: * Locally advanced solid tumor that is a candidate for curative treatment through radical surgery and/or radiotherapy, or chemotherapy. * History of another malignancy with exceptions * Visceral crisis with life-threatening complications, lymphangitic spread, CNS metastasis and/or carcinomatous meningitis * Failed to recover from effects of prior anticancer treatment therapy to baseline or Grade ≤ 1 severity (per CTCAE) * Clinically active interstitial lung disease * History of uveitis, retinopathy or other clinically significant retinal disease * Has known human immunodeficiency virus (HIV), active hepatitis B or C infection * Prior CDK2 inhibitor * Major surgery within 2 months or minor surgery within 10 days before the first dose of NKT3447