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