Inclusion Criteria:
* Able to understand and voluntarily sign the written informed consent form.
* Histologically or pathologically confirmed epithelial malignancies, including advanced gastric cancer or colorectal cancer that has failed at least two lines of treatment, or platinum-resistant advanced ovarian cancer, primary peritoneal carcinoma, or fallopian tube carcinoma.
* Clinical diagnosis of malignant ascites with a moderate or higher amount of ascites. Moderate or higher is defined as having a volume of ascites ≥1L based on CT assessment or actual drainage of ≥1L.
* The time interval between the most recent anti-tumor treatment and the first dose of M701 must meet the following criteria:
Intraperitoneal treatment: ≥2 weeks since the most recent intraperitoneal treatment.
* Adverse events (AEs) from previous treatments have recovered to grade ≤1 (excluding other AEs deemed by the investigator not to affect the safety of the study drug, such as hair loss).
* Eastern Cooperative Oncology Group (ECOG) performance status (PS) of 0-2. Estimated survival time ≥8 weeks.
* Organ function levels must meet the following requirements:
Hematology: Absolute neutrophil count (ANC) ≥1.5 × 10\^9/L, platelets ≥80 × 10\^9/L, hemoglobin ≥8.5 g/dL, lymphocyte ratio (lymphocyte count/leukocyte count) ≥10% (without transfusion within 14 days).
Liver function: Total bilirubin ≤1.5 times the upper limit of normal (ULN), aspartate aminotransferase (AST) and alanine aminotransferase (ALT) ≤3 times ULN (AST and ALT ≤5 times ULN allowed in the presence of liver metastasis).
Serum albumin ≥28 g/L. Renal function: Serum creatinine ≤1.5 times ULN.
Exclusion Criteria:
* Patients who have previously received M701 or any antibody-based drugs targeting EpCAM and/or CD3 within the 4 months prior to the first dose.
* Patients with microsatellite instability-high (MSI-H)/deficient mismatch repair (dMMR) colorectal cancer who have not previously received immunotherapy.
* Patients who have undergone major surgery within the 4 weeks prior to the first dose.
* Patients with extensive liver metastases (tumor volume occupying approximately \>70% of total liver volume).
* Active infections requiring intravenous antibiotics within 14 days before the first dose.
* Severe diarrhea (CTCAE grade ≥2).
* Severe respiratory distress requiring oxygen therapy.
* Active autoimmune diseases, except for the following conditions that are allowed for screening: type 1 diabetes, controlled hypothyroidism with replacement therapy only, skin diseases that do not require systemic treatment。
* Other severe medical conditions that may limit the patient's participation in the trial。
* Impaired cardiac function with New York Heart Association (NYHA) class 3 or 4.
* Occurrence of complete intestinal obstruction within 30 days before the first dose, or diagnosis of incomplete intestinal obstruction deemed unsuitable for participation in the trial based on symptoms and signs as determined by the investigator.
* Inability to adequately drain ascites due to objective reasons (including loculated ascites).
* Confirmed portal vein obstruction.
* History of immunodeficiency, including positive HIV test.
* Active hepatitis B virus infection, active hepatitis C virus infection, active syphilis, or positive HIV antibody.
* Pregnant or lactating women.
* Patients with fertility requirements during or within 6 months after treatment.
* Known history of neurological or psychiatric disorders deemed by the investigator to affect cognitive function or compliance, including unstable epilepsy, dementia, schizophrenia, etc.
