This is a Phase 1/1b, open-label, first-in-human (FIH) study designed to evaluate the safety, tolerability, pharmacokinetics (PK) and preliminary antineoplastic activity of APS03118 administered orally in participants with RET-altered solid tumors.

After being informed about the study procedure and potential risks and benefits, all patients giving written informed consent will undergo a 4-week screening period to determine eligibility for study entry. The eligible participants will be enrolled using a rolling 6 dose escalation scheme to identify MTD and/or RP2D.

Inclusion Criteria:

1. Male or female patients aged 18 years or older
2. Have histological or cytological confirmation of unresectable locally advanced or metastatic solid tumors harboring oncogenic RET mutations or fusions confirmed by NGS. Site may identify patients with a RET-mutated or RET-fusion solid tumor before the formal screening.
3. Eastern Cooperative Oncology Group (ECOG) performance status score of 0 or 1.
4. Resolution of all toxic effects of prior therapies to Grade 1 or below according to NCI CTCAE version 5.0 (except alopecia or Grade 2 neuropathy);
5. Patients with asymptomatic brain metastases if they have been previously treated, demonstrated no progression at least for 1 months before Screening, are asymptomatic and have had no requirement for steroids or enzyme inducing anticonvulsants in the last 14 days before Screening
6. Adequate organ function

Exclusion Criteria:

1. Have a locally advanced solid tumor that is a candidate for curative surgery treatment through radical surgery and/or radiotherapy.
2. Have an active fungal, bacterial, and/or active untreated viral infection
3. The patient has a serious pre-existing medical condition(s)
4. Have received or will receive a COVID-19 vaccine within 1 month before the first dose of APS03118.