A Study of Selpercatinib (LY3527723) in Participants With Advanced Solid Tumors Including RET Fusion-positive Solid Tumors, Medullary Thyroid Cancer and Other Tumors With RET Activation

Inclusion Criteria: * Participants with a locally advanced or metastatic solid tumor. * Evidence of a RET gene alteration in tumor and/or blood. * Measurable or non-measurable disease as determined by Response Evaluation Criteria in Solid Tumors (RECIST) v1.1. * Eastern Cooperative Oncology Group (ECOG) performance status score of 0, 1, or 2, with no sudden deterioration 2 weeks prior to the first dose of study treatment. * Archived tumor tissue sample available for cohort 1 and 2. * Cohorts 1 and 2: failed or intolerant to standard of care. * Cohorts 1-2: enrollment will be restricted to participants with evidence of a RET gene alteration in tumor (i.e., not just blood). However, a positive germline DNA test for a RET gene mutation as defined in the protocol is acceptable in the absence of tumor tissue testing for participants with MTC. * Cohorts 1-2: at least one measurable lesion as defined by RECIST v1.1 and not previously irradiated (unless progressive disease for the irradiated lesion\[s\] has been radiographically documented). Exclusion Criteria: * Cohorts 1-2, an additional validated oncogenic driver that could cause resistance to selpercatinib treatment if known. * Prior treatment with a selective RET inhibitor(s) (including investigational selective RET inhibitor(s), such as BLU-667, RXDX-105, etc). * Are currently enrolled in any other clinical study involving an investigational product or any other type of medical research judged not to be scientifically or med…