Inclusion Criteria:

          1. Age ≥ 18 (or age ≥ 20 as required by local regulation).

          2. Histological or cytological confirmation of advanced/metastatic solid tumors harboring
             oncogenic RET fusions or mutations, who either have disease progression on, or are
             intolerant to standard therapy; OR are ineligible for standard therapy or for whom no
             standard therapy exists; OR are unlikely to tolerate or derive clinical benefit from
             standard therapy in the opinion of the Investigator OR have declined standard therapy.

          3. ECOG performance status ≤ 1.

          4. Existence of measurable or evaluable disease (according to Response evaluation
             criteria in solid tumors [RECIST v1.1] criteria).

          5. Subjects with asymptomatic primary CNS tumors or brain metastases are eligible for the
             study if they meet protocol specified criteria.

          6. Adequate organ function.

          7. Life expectancy ≥ 12 weeks.

        Exclusion Criteria:

          1. Locally advanced solid tumor that is a candidate for curative treatment through
             radical surgery and/or radiotherapy, or chemotherapy.

          2. Presence or history of any other primary malignancy within 3 years other than a
             history of adequately treated basal or squamous cell carcinoma of the skin, or any
             adequately treated in situ carcinoma.

          3. Major surgery within four weeks of the start of therapy.

          4. Clinically significant cardiovascular disease (either active or within six months
             before enrollment): myocardial infarction, unstable angina, coronary/peripheral artery
             bypass graft, symptomatic congestive heart failure (New York Heart Association
             Classification Class ≥ II), cerebrovascular accident or transient ischemic attack,
             symptomatic bradycardia, requirement for anti-arrhythmic medication. Ongoing cardiac
             dysrhythmias of CTCAE version 5.0 grade ≥ 2.

          5. Any of the following cardiac criteria:

               -  Mean resting corrected QT interval (ECG interval measured from the onset of the
                  QRS complex to the end of the T wave) for heart rate (QTc) > 470 msec obtained
                  from three ECGs, using the screening clinic ECG machine-derived QTc value

               -  Any clinically important abnormalities in rhythm, conduction, or morphology of
                  resting ECG (e.g., complete left bundle branch block, third degree heart block,
                  second degree heart block, PR interval > 250 msec)

               -  Any factors that increase the risk of QTc prolongation or risk of arrhythmic
                  events such as heart failure, congenital long QT syndrome, family history of long
                  QT syndrome, or any concomitant medication known to prolong the QT interval

          6. Known clinically significant active infections not controlled with systemic treatment
             (bacterial, fungal, viral including HIV positivity).

          7. Gastrointestinal disease (e.g., Crohn's disease, ulcerative colitis, or short gut
             syndrome) or other malabsorption syndromes that would impact drug absorption.

          8. Subjects being treated with or anticipating the need for treatment with strong CYP3A4
             inhibitors or inducers.

          9. Subjects with current or anticipated need for drugs that are sensitive CYP2C9
             substrates with narrow therapeutic indices.