∙ Phase I - Common inclusion criteria for Dose-Escalation / Dose-Expansion:

• Eastern Cooperative Oncology Group (ECOG) performance score of 0 or 1

• Available RET-gene abnormalities determined on tissue biopsy or liquid biopsy. If deemed appropriate by the investigator, determination on a pleural cell block is also acceptable.

• Adequate hematopoietic, hepatic and renal function

∙ Phase I Dose-Escalation - Specific inclusion criteria:

• Advanced solid tumors

• Measurable and/or non-measurable disease as determined by RECIST 1.1

• If patient has brain and/or leptomeningeal metastases, (s)he should be asymptomatic.

∙ Phase I Dose-Expansion - Specific inclusion criteria:

• Patient with RET gene fusion :

‣ Cohort 1, 3: locally advanced or metastatic NSCLC patients naïve to RET selective inhibitors and no prior systemic anti-cancer treatment. Patients who have been treated with neo-adjuvant or adjuvant chemotherapy may be included if it has been completed at least 6 months prior to the first dose of the study.

⁃ Cohort 2, 4: locally advanced or metastatic NSCLC patients with RET gene fusion and prior exposure to RET selective inhibitors.

• Measurable disease as determined by RECIST 1.1

• If patient has brain and/or leptomeningeal metastases,(s)he should have:

‣ asymptomatic untreated brain/leptomeningeal metastases off steroids and anticonvulsant for at least 7 days or

⁃ asymptomatic brain metastases already treated with local therapy and be clinically stable on steroids and anticonvulsant for at least 7 days before study drug administration.

∙ Phase II :

• Available RET-gene abnormalities determined on tissue or liquid biopsy

• Locally advanced or metastatic:

‣ NSCLC patients with primary RET gene fusion and prior exposure to RET selective inhibitors;

⁃ NSCLC patients with RET gene fusion and without prior exposure to RET selective inhibitors

⁃ patients with advanced solid tumors that harbour RET gene abnormalities (other than NSCLC patients with primary RET gene fusions) and has failed all the available therapeutic options

• Eastern Cooperative Oncology Group (ECOG) performance score of 0-2

• Measurable disease as determined by RECIST 1.1

• If patient has brain and/or leptomeningeal metastases,(s)he should have:

‣ asymptomatic untreated brain/leptomeningeal metastases off steroids and anticonvulsant for at least 7 days or

⁃ asymptomatic brain metastases already treated with local therapy and be clinically stable on steroids and anticonvulsant for at least 7 days before study drug administration.

• Adequate hematopoietic, hepatic and renal function