This study is an open-label phase I study to evaluate the safety, pharmacokinetics, and antitumor activity of SSGJ-612 in patients with advanced malignant solid tumors expressing HER2.

The clinical trial consists of two phases. In the dose escalation stage, the accelerated titration method combined with the traditional "3+3" design will be adopted, to evaluate the safety and dose-limiting toxicity (DLT), and to determine the maximum tolerated dose (MTD) or the maximum dose of administration (MAD). In the dose expansion phase, several safe and effective dose levels of SSGJ-612 will be expanded and further evaluated in larger groups of participants.

Inclusion Criteria:

1. Voluntarily participate in this study, be willing to follow and complete all trial procedures, and sign the informed consent form;
2. Aged ≥18 and ≤75 years old at the time of signing the ICF, regardless of gender;
3. Expected survival ≥3 months;
4. Performance status (PS) score of 0-1 according to the Eastern Cooperative Oncology Group (ECOG) scale;
5. Patients with pathologically or cytologically confirmed locally advanced or metastatic malignant tumors who have failed standard treatment, are intolerant to standard treatment, or have no standard treatment available, and cannot undergo complete surgical resection or receive radical concurrent/sequential chemoradiotherapy;
6. Tumor tissue with HER2 expression;
7. At least one measurable tumor lesion assessed as the target lesion according to RECIST v1.1 criteria, and the lesion is suitable for repeated and accurate measurement.

Exclusion Criteria:

1. Presence of brainstem, meninges, or spinal cord metastasis, or spinal cord compression;
2. Presence of active central nervous system (CNS) metastatic lesions;
3. Individuals with clinical symptoms or requiring repeated drainage (once a month or more frequently) of pleural effusion, pericardial effusion, or ascites;
4. Primary or secondary immunodeficiency, including positive human immunodeficiency virus (HIV) test;
5. Known active tuberculosis; known active syphilis infection;
6. Known history of allogeneic organ transplantation or allogeneic hematopoietic stem cell transplantation;
7. Use of any live vaccine or attenuated live vaccine within 4 weeks before the first dose, or plan to receive any live vaccine or attenuated live vaccine during the study;
8. Known severe allergic history to any component of the investigational drug, or history of severe allergic reaction to antibodies;
9. Pregnant or lactating women.