This is a first-in-human, Phase I, open-label, multi-center, dose-escalation and dose expansion study to evaluate the safety, tolerability, pharmacokinetics, and preliminary efficacy of JMKX005425 in subjects with microsatellite instability-high (MSI) and/or mismatch repair deficiency (dMMR) advanced solid tumors.

Inclusion Criteria:

1. Have a microsatellite instability-high (MSI) and/or mismatch repair deficiency (dMMR), histologically or cytologically documented advanced solid tumor.
2. Has at least one measurable lesion per RECIST v1.1.
3. Has a life expectancy of ≥ 12 weeks.
4. Adequate organ function.

Exclusion Criteria:

1. Toxicities from prior anti-tumor therapy has not recovered to Grade 1 or below per CTCAE v5.0.
2. History of prior malignancy in the past 3 years, with the exception of curatively treated malignancies.
3. Has the dysphagia, or impairment of gastrointestinal function or gastrointestinal disease that may significantly alter the absorption of study drug.
4. Has the severe chronic or active infection
5. Has a history of severe cardiovascular disease.
6. Subject is pregnant or breastfeeding.