Objectives: Primary: Dose Escalation: To identify the recommended doses for expansion (RDEs) of NKT3447 in adult subjects with advanced/metastatic solid tumors Dose Expansion: To evaluate the preliminary antitumor activity of NKT3447 at selected dose levels and to determine preliminary recommended Phase 2 dose (RP2D) Secondary: To evaluate overall safety and tolerability of NKT3447 (Dose Escalation and Dose Expansion) To characterize the pharmacokinetic (PK) properties of NKT3447 (Dose Escalation and Dose Expansion) To evaluate progression-free survival (PFS), overall survival (OS), duration of response (DOR), time to response (TTR), and disease control rate of NKT3447 (Dose Escalation and Dose Expansion) To evaluate objective response rate (ORR) of NKT3447 (Dose Escalation only) Exploratory: To evaluate the effect of NKT3447 on the QT/QTc interval. To explore biomarkers potentially predictive of response to NKT3447. To evaluate the effects of NKT3447 on cyclin-dependent kinase 2 (CDK2)-related pharmacodynamic (PD) markers