Inclusion Criteria:
1. Male or female participants aged 18 to 75 years, inclusive.
2. Histologically and/or pathologically confirmed advanced, metastatic, or recurrent malignant solid tumor with peritoneal metastasis, with or without distant metastases, with complete pathological documentation available. Eligible participants include those who have failed second-line or later standard therapy; have no available standard therapy; have failed available therapy; or, based on a comprehensive benefit-risk assessment by the investigator considering prior treatment-related toxicities, current performance status, comorbidities, and organ function, are considered unsuitable for further available standard therapy. Tumor types may include, but are not limited to, gastric cancer, colorectal cancer, fallopian tube cancer, and mesothelioma. Relevant toxicities from prior therapy must have recovered to Grade ≤1 during screening or returned to baseline before the first administration, unless otherwise judged by the investigator to pose no safety risk.
3. At least one measurable lesion according to RECIST Version 1.1, defined as a non-lymph-node lesion with a longest diameter ≥10 mm or a lymph node with a short-axis diameter ≥15 mm by CT or MRI, with a scan slice thickness ≤5 mm.
4. Mesothelin-positive tumor as confirmed by immunohistochemistry (IHC) or immunocytochemistry (ICC), defined as Mesothelin expression in ≥10% of tumor cells.
5. Eastern Cooperative Oncology Group (ECOG) performance status of 0 to 2.
6. Adequate hematologic, hepatic, renal, and coagulation function meeting all of the following criteria:
* White blood cell count (WBC) ≥2.0 × 10\^9/L;
* Absolute neutrophil count (ANC) ≥1.2 × 10\^9/L;
* Platelet count ≥70 × 10\^9/L;
* Hemoglobin ≥80 g/L;
* Total bilirubin ≤1.5 × the upper limit of normal (ULN);
* Alanine aminotransferase (ALT) and aspartate aminotransferase (AST) ≤3.0 × ULN, or ≤5.0 × ULN in participants with liver metastases;
* Serum creatinine ≤1.5 × ULN or creatinine clearance ≥50 mL/min;
* International normalized ratio (INR) and activated partial thromboplastin time (APTT) ≤1.5 × ULN in participants not receiving anticoagulant therapy.
7. No bleeding disorder or coagulation disorder.
8. Toxicities from prior treatment must have recovered to Grade ≤1 according to NCI CTCAE Version 5.0 or returned to baseline, except for toxicities considered by the investigator not to pose a safety risk.
9. Estimated life expectancy of at least 3 months.
10. From screening until at least 12 months after intraperitoneal infusion of SY001, participants and their partners must have no plans for conception and must agree to use effective non-pharmacological contraceptive measures during the study, such as condoms or a non-medicated intrauterine device, unless permanent contraception has already been performed (e.g., bilateral tubal ligation or vasectomy).
11. Ability and willingness to provide written informed consent and comply with all study procedures and requirements.
Exclusion Criteria:
1. Severe cardiovascular or cerebrovascular disease, including, but not limited to, any of the following:
* Acute myocardial infarction, unstable angina, stroke, coronary angioplasty, or coronary stent placement within 6 months before screening;
* New York Heart Association (NYHA) Class III or IV congestive heart failure or left ventricular ejection fraction (LVEF) \<50%;
* Uncontrolled hypertension despite optimal medical therapy, defined as systolic blood pressure ≥160 mmHg and/or diastolic blood pressure ≥100 mmHg;
* Arrhythmia of greater than Grade 2 according to NCI CTCAE Version 5.0;
* Corrected QT (QTc) interval \>480 ms on screening electrocardiogram.
2. Forced expiratory volume in 1 second/forced vital capacity (FEV1/FVC) ≤60% before enrollment or requirement for supplemental oxygen.
3. Uncontrolled active infection. Prophylactic antibacterial, antiviral, and antifungal therapy is permitted.
4. Positive syphilis TRUST test; positive anti-HIV antibody; positive anti-HCV antibody with HCV RNA above the lower limit of detection; or positive hepatitis B core antibody (HBcAb) with HBV DNA above the lower limit of detection.
5. Receipt of a live attenuated vaccine within 4 weeks before screening.
6. Any contraindication to intraperitoneal administration, including extensive intra-abdominal adhesions, complete intestinal obstruction, or severe intra-abdominal infection; or intestinal obstruction, active gastrointestinal bleeding, a history of major gastrointestinal bleeding within 3 months, severe gastroduodenal ulcer, severe ulcerative colitis, or other severe gastrointestinal inflammatory disease.
7. Known history of significant drug allergy or hypersensitivity to heterologous proteins, biological products, or any component of the study product.
8. Symptomatic central nervous system metastases or leptomeningeal metastases, or uncontrolled systemic or leptomeningeal metastatic disease considered by the investigator to make the participant unsuitable for enrollment.
9. History of autoimmune disease, including but not limited to epilepsy, systemic lupus erythematosus, nephritis, psoriasis, rheumatoid arthritis, inflammatory bowel disease, or autoimmune hepatitis.
10. History of substance abuse that cannot be discontinued or a history of psychiatric disorders.
11. Receipt of any immune cell therapy within 6 months before screening.
12. Prior Mesothelin-targeted therapy, including investigational Mesothelin-targeted treatment.
13. Current use of, or use within 14 days before the first administration of the study product, systemic immunomodulatory agents or systemic corticosteroids at a dose equivalent to prednisone \>10 mg/day.
14. Receipt of chemotherapy, targeted therapy, or another investigational anticancer agent within 14 days or within at least 5 half-lives before screening, whichever period is shorter.
15. History of Grade ≥2 immune-related adverse events involving major organs induced by prior or current immune checkpoint inhibitor therapy, including but not limited to myocarditis or central nervous system toxicity.
16. Participation in another drug clinical trial within 4 weeks before screening.
17. Surgery within 2 weeks before mononuclear cell collection that, in the investigator's judgment, may affect participant safety.
18. Pregnant or breastfeeding women.
19. Any other disease or medical condition that may increase the participant's risk or interfere with interpretation of the study results.
20. Any other condition that, in the investigator's judgment, makes the participant unsuitable for participation in the study.
