The goal of this open-label dose escalation and expansion study is to evaluate the safety and tolerability of NKT5097 in adults with advanced/metastatic tumors (emphasis on breast cancer and solid tumors with CCNE1 amplification). Main questions to answer include:

• What is the recommended dose for expansion and/or Phase 2, for both monotherapy and in combination with ET
• What medical issues/symptoms do participants experience when taking NKT5097 as monotherapy as well as in combination with ET

This First-in-Human, Open-Label Study to Evaluate the Safety, Tolerability, PK, and Preliminary Anti-tumor Activity of NKT5097, a novel dual protein degrader of CDK2 and CDK4, is split into 3 Parts:

Part 1: Monotherapy Dose Escalation in selected advanced/metastatic non-CNS primary solid tumors will be enrolled based on a projected total of 5 dose levels

Part 2: Food Effect Analysis: Subjects with solid tumors (as noted in Part 1) will be enrolled (by backfilling selected dose cohorts) to evaluate the effect of dosing with food on NKT5097.

Part 3: Monotherapy Tumor-specific Expansion: Subjects may be enrolled (by backfilling selected dose cohorts) into each selected tumor-specific cohort. One or more of these cohorts may be opened at the discretion of the Sponsor in consultation with the DEC

Part 4: Combination Dose Escalation with ET in selected HR+/HER2- breast cancer will be enrolled based on a projected 2 dose levels

Part 5: Combination Dose Expansion with ET in HR+/HER2- breast cancer in one or more various cohorts

In addition to the above, the study will explore pharmacokinetics, various pharmacodynamic biomarkers, gene mutations, and tumor responses such as PFS and DOR.

Inclusion Criteria:

• Able to provide written informed consent
• Advanced unresectable or metastatic solid tumor (Part 1, 2 & 3 only)
• Advanced unresectable or metastatic HR+/HER2- breast cancer (Part 4 & 5 only)
• Refractory to or unable to tolerate existing therapies (Part 1, 2 & 4 only)
• Measurable or evaluable disease (Part 1, 2, & 4 only).
• Measurable disease (Part 3 & 5 only)
• Eighteen years of age or older
• ECOG status of 0 or 1
• Adequate organ function
• Patients with female reproductive organs must be surgically sterile, post- menopausal or willing to use effective contraception per protocol
• Patients who are capable of insemination must be willing to use highly effective contraception and to refrain from sperm donation during treatment and for 28 days after the last dose
• Able to swallow oral meds
• Willing to provide tumor tissue

Exclusion Criteria:

• Advanced solid tumor that is a candidate for curative treatment
• History of another malignancy except for the following: adequately treated local basal cell or squamous carcinoma of the skin, in situ cervical cancer, adequately treated papillary noninvasive bladder cancer, other adequately treated Stage I or Stage II cancers currently in complete remission
• Not recovered from the effects of prior anticancer therapy
• Clinically significant cardiovascular event, including myocardial infarction, arterial thromboembolism, or cerebrovascular thromboembolism, within 6 months
• Known active CNS metastases and/or carcinomatous meningitis
• Active interstitial lung disease requiring treatment
• History of uveitis, retinopathy, or other clinically significant retinal disease
• Major surgery within 30 days of administration of first dose
• Active uncontrolled infectious disease
• Significant liver disease (Child Pugh class B or C)
• Should not have received any prior selective investigational inhibitors or degraders (Part 5 only)