This is a Phase I clinical trial testing the safety and effectiveness of AK138D1in patients with advanced cancer. The study will enroll up to 200 patients with various types of advanced solid tumors who haven't responded to standard treatments. Patients will receive AK138D1 to determine the safest dose and evaluate if the drug can help treat their cancer.

This open-label, dose-escalation and expansion Phase I clinical trial aims to evaluate the safety, tolerability, pharmacokinetics, immunogenicity, and preliminary anti-tumor activity of AK138D1 in subjects with advanced malignant tumors. The study will test different doses of AK138D1 to find the recommended dose for future studies and assess whether the drug shows signs of effectiveness against cancer. Participants will receive AK138D1 through intravenous infusion and will be closely monitored for side effects and treatment response. The final number of participants enrolled will depend on the safety and effectiveness results observed during the study.

Inclusion Criteria:

1. The subject must sign the written informed consent form (ICF) voluntarily;
2. At enrollment, aged ≥ 18 to ≤ 75 years, both males and females are eligible;
3. ECOG performance status score of 0 or 1;
4. Has a life expectancy of ≥ 3 months;
5. At least 1 measurable lesion as per RECIST v1.1 that is suitable for repeated accurate measurement.
6. Adequate organ function.

Exclusion Criteria:

1. Prior human epidermal growth factor receptor 3 (HER3) -targeted therapies, including antibodies, antibody-drug conjugates (ADCs), chimeric antigen receptor T-cell immunotherapy (CAR-T), and others;
2. Concomitant participation in another clinical study, unless it is a non-interventional clinical study or the follow-up period of an interventional study;
3. Presence of active central nervous system (CNS) metastases.
4. Live vaccines or attenuated live vaccines administered within 4 weeks prior to the first dose, or planned to be administered during the study; use of inactivated vaccines is allowed;
5. Untreated subjects with active hepatitis B or active hepatitis C;
6. Known active pulmonary tuberculosis (TB); subjects with suspected active TB must undergo appropriate clinical assessment to rule out the presence of active disease;
7. Known active syphilis infection;
8. Subjects with known allergy to any component of any study drug; and with a history of known severe hypersensitivity reactions to other monoclonal antibodies;
9. Other reasons for ineligibility as evaluated by the investigator.