A Study of Selpercatinib (LOXO-292) in Participants With Advanced Solid Tumors, RET Fusion-Positive Solid Tumors, and Medullary Thyroid Cancer (LIBRETTO-001)

Key Inclusion Criteria: For Phase 1: * Participants with a locally advanced or metastatic solid tumor that: * Has progressed on or is intolerant to standard therapy, or * For which no standard therapy exists, or in the opinion of the Investigator, are not candidates for or would be unlikely to tolerate or derive significant clinical benefit from standard therapy, or * Decline standard therapy * Prior multikinase inhibitors (MKIs) with anti-RET activity are allowed * A RET gene alteration is not required initially. Once adequate PK exposure is achieved, evidence of RET gene alteration in tumor and/or blood is required as identified through molecular assays, as performed for clinical evaluation * Measurable or non-measurable disease as determined by RECIST 1.1 or RANO as appropriate to tumor type * Eastern Cooperative Oncology Group (ECOG) score of 0, 1, or 2 or Lansky Performance Score (LPS) greater than or equal to (≥) 40 percent (%) (age less than \[\<\] 16 years) with no sudden deterioration 2 weeks prior to the first dose of study treatment * Adequate hematologic, hepatic and renal function * Life expectancy of at least 3 months For Phase 2: As for phase 1 with the following modifications: * For Cohort 1: Participants must have received prior standard therapy appropriate for their tumor type and stage of disease, or in the opinion of the Investigator, would be unlikely to tolerate or derive clinical benefit from appropriate standard of care therapy * Cohorts 1 and 2: …