Safety, Efficacy, and Tolerability of BOS172738 in Patients With Advanced Rearranged During Transfection (RET) Gene-Altered Tumors

Inclusion Criteria: Participants are eligible to be included in the study only if all of the following criteria apply: * Male or female participants must be ≥ 18 years, at the time of signing the informed consent * Diagnosis of advanced solid tumor with a documented rearranged during transfection (RET) gene altered malignancy as determined locally by a DNA based assay of tumor tissue and/or blood * Participants must have no alternative approved therapy. * For participants in Part B expansion cohort only: documented local diagnosis of either 1) advanced RET gene fusion non-small cell lung cancer (NSCLC); 2) advanced RET gene mutant medullary thyroid cancer (MTC); or 3) other RET gene altered advanced tumors or NSCLC/MTC with prior specific RET gene targeted therapy * Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1 * Negative pregnancy test for females of child bearing potential; must be surgically sterile, postmenopausal, or willing and able to be compliant with a contraceptive regimen * Contraceptive use by men or women should be consistent with local regulations. * Capable of giving signed informed consent Exclusion Criteria: Participants are excluded from the study if any of the following criteria apply: * Inability to take oral medications or gastrointestinal (GI) conditions that can interfere with the swallowing or absorption of study medication * Uncontrolled or severe concurrent medical condition * History of upper GI bleeding, ulceration, or perforation within 12 months prior to the first dose of study drug * Known history of stroke or cerebrovascular accident within 6 months prior to the first dose of study drug * Participants with known infection with human immunodeficiency virus (HIV) or Acquired Immune Deficiency Syndrome (AIDS) (testing not required) * Participants who are hepatitis B surface antigen positive or participants who are hepatitis C antibody positive (Participants who have been successfully treated for hepatitis C virus \[HCV\] are eligible if an undetectable HCV viral load at least 6 months after completion of treatment can be demonstrated.) * Any evidence of serious active infections * Uncontrolled or severe cardiovascular disease * Uncontrolled intercurrent illness or psychiatric illness/social situations that would limit compliance with study requirements * Participants with a prior or concurrent malignancy other than the malignancies under study * Ongoing cancer directed therapy