A Study to Investigate the Safety, Pharmacokinetics, and Preliminary Efficacy of IDE574 Therapy in Adult Participants With Advanced Solid Tumors

Part 1 - Monotherapy Dose Escalation and Expansion:

Part 1A - Monotherapy Dose Escalation Part 1A will evaluate increasing doses of IDE574 to assess safety, tolerability and to determine dose-limiting toxicities (DLTs), the maximum tolerated dose (MTD) or the recommended dose for expansion (RDE) in subjects with advanced or metastatic ER+, HER2- breast cancer, non-small cell lung cancer, castration-resistant prostate cancer and microsatellite-stable colorectal cancer.

Part 1B - Monotherapy Dose Expansion Part 1B will evaluate in ER+ HER2- advanced or metastatic breast cancer at the potential dose level(s) determined to be safe and tolerable during monotherapy dose escalation Part 1A. In parallel, a basket cohort may be enrolled at or below the highest safe dose level(s) determined to be safe and tolerable in Part 1A.

Part 2 - Combination Dose Escalation and Expansion

Part 2A - IDE574 Combination Therapy with Fulvestrant Dose Escalation Part 2A will evaluate participants with ER+ HER2- advanced or metastatic breast cancer with escalating doses of IDE574 in combination with fulvestrant to assess safety, tolerability and to determine DLTs, MTD or RDE.

Part 2B - IDE574 Combination Therapy with Fulvestrant Dose Expansion Part 2B will be evaluated in ER+ HER2- advanced or metastatic breast cancer at the potential dose level(s) determined to be safe and tolerable during combination dose escalation Part 2A.