This is a first-in-human, open-label, multicenter phase 1 study to evaluate safety, tolerability, pharmacokinetics (PK), pharmacodynamics (PD), and preliminary efficacy of OP-3136, a lysine acetyltransferases 6A and 6B (KAT6A/B) inhibitor, as monotherapy and in combination with other anticancer agents in participants with advanced solid tumors.

This study consists of 2 parts: a dose escalation part (Part 1) and dose expansion part (Part 2).

Part 1A (Dose Escalation for OP-3136 Monotherapy): This part of the study will evaluate the safety, tolerability, and PK in a range of doses of OP-3136, a lysine acetyltransferases 6A and 6B (KAT6A/B) inhibitor, administered orally once daily to participants with ER+ HER2- advanced or metastatic breast cancer (mBC), advanced or metastatic castration resistant prostate cancer (mCRPC), or advanced or metastatic non-small cell lung cancer (mNSCLC), and determine the maximum tolerated dose (MTD) and the recommended dose/regimen for expansion (RDE).

Part 1B (Dose Escalation for OP-3136 in Combination with Fulvestrant): This part of the study will evaluate the safety and PK of OP-3136 administered in combination with fulvestrant in participants with ER+ HER2- mBC, and determine MTD and RDE for this combination.

Part 1C (Dose Escalation for OP-3136 in Combination with Palazestrant): This part of the study will evaluate the safety and PK of OP-3136 administered in combination with palazestrant in participants with ER+ HER2- mBC, and determine MTD and RDE for this combination.

Part 2A (Dose Expansion for OP-3136 Monotherapy): This part will evaluate two expansion cohorts at the monotherapy RDE from part 1 in participants with ER+ HER2- mBC and participants with mCRPC.

Part 2B (Dose Expansion for OP-3136 in Combination with Fulvestrant OR Palazestrant): This part will evaluate the RDEs for OP-3136 in combination with fulvestrant from Part 1B OR the RDEs of OP-3136 in combination with palazestrant in an expansion cohort in participants with ER+ HER2- mBC.

Key Inclusion Criteria:

• Participants with advanced or metastatic ER+HER2- breast cancer, mCRPC, or NSCLC (Part 1) or advanced or metastatic ER+HER2- BC or mCRPC (Part 2).
• Part 1A (Dose escalation for OP-3136 monotherapy): Participants must have a tumor that is unresectable or metastatic and for which life prolonging measures do not exist or available therapies are intolerable or no longer effective.
• Part 1B (Dose escalation for OP-3136 in combination with fulvestrant): Participants with advanced or metastatic ER+ HER2- breast cancer that have progressed on or after at least 1 prior line of treatment that included endocrine therapy and CDK 4/6 inhibitor in advanced or metastatic setting and must have received no more than 2 prior lines of endocrine therapy (one of which must be in combination with CDK4/6 inhibitor) and no more than 1 prior line of chemotherapy or an antibody-drug conjugate in the advanced or metastatic setting.
• Part 1C (Dose escalation for OP-3136 in combination with palazestrant): Participants with advanced or metastatic ER+ HER2- breast cancer that have progressed on or after at least 1 prior line of treatment that included endocrine therapy and CDK 4/6 inhibitor in advanced or metastatic setting and must have received no more than 2 prior lines of endocrine therapy (one of which must be in combination with CDK4/6 inhibitor) and no more than 1 prior line of chemotherapy or an antibody-drug conjugate in the advanced or metastatic setting.
• Part 2A (Dose Expansion in ER+ HER2- mBC for OP-3136 monotherapy): Participants must have received up to 3 prior lines of endocrine therapy (one of which must be in combination with CDK4/6 inhibitor) and up to 1 prior line of chemotherapy or an antibody-drug conjugate.
• Part 2A (Dose Expansion in mCRPC for OP-3136 monotherapy): Participants must have received up to 4 lines of prior systemic therapy for prostate cancer. Prior therapy must include treatment with an androgen receptor pathway inhibitor(s).
• Part 2B (Dose Expansion in ER+ HER2- mBC for OP-3136 in combination with fulvestrant OR Dose Expansion in ER+ HER2- mBC for OP-3136 in combination with palazestrant): Participants must have progressed on or after at least 1 prior line of treatment that included endocrine therapy and CDK 4/6 inhibitor in advanced or metastatic setting. Participants must have received no more than 2 prior lines of endocrine therapy in the advanced or metastatic setting and no more than 1 prior line of chemotherapy or an antibody-drug conjugate in the advanced or metastatic setting.

Key Exclusion Criteria:

• Prior therapy with KAT6A/B inhibitor in any treatment setting.
• Participants with advanced/metastatic, symptomatic, visceral spread, that are at risk of life-threatening complications in the short term.
• Known active or symptomatic central nervous system (CNS) metastases, carcinomatous meningitis, leptomeningeal disease, or a spinal cord compression that require CNS-specific treatment, or participants who did not demonstrate clinical and radiologic stability during the last 2 months prior to the first dose of study treatment or require or are currently on steroid therapy for CNS metastases.
• History of cerebral vascular disease, including transient ischemic attack, within 6 months prior to the first dose of study treatment.
• History of or ongoing impaired cardiac function or clinically significant cardiac disease within 6 months prior to the first dose of study treatment.

Note: Additional inclusion/exclusion criteria may apply.