Phase 1 Study of OP-3136 in Advanced or Metastatic Solid Tumors

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.