This is an open, prospective, single-arm, multi-cohort clinical study to evaluate the efficacy and safety of high-dose vitamin C combined with metformin in the treatment of malignant tumors.

Inclusion Criteria:

1. 18 years to 75 years.

2. Had a disease status that was measurable or evaluable as defined by Response Evaluation Criteria in Solid Tumors (RECIST, version1.1)： Cohort A: patients with advanced hepatocellular carcinoma who had failed previous standard first-line therapy and could not tolerate or reject existing therapies.

   Cohort B: patients with advanced pancreatic cancer who had previously failed standard first-line therapy, could not tolerate or reject existing therapies.

   Cohort C: patients with advanced gastric cancer who had failed previous standard second-line or above treatment, who could not tolerate or reject existing therapies.

   Cohort D: patients with advanced colorectal cancer who had failed previous standard second-line or above treatment, who could not tolerate or reject existing therapies.

3. Adequate hepatic, renal, heart, and hematologic functions (hemoglobin ≥80g/L, platelets ≥ 80×10^9/L, neutrophils ≥ 1.5×10^9/L, total bilirubin within 1.5×the upper limit of normal(ULN), ALT and AST≤2.5×the ULN (If liver metastases, serum transaminase≤5×the ULN), serum creatine ≤ 1.5 x ULN, creatinine clearance rate > 50ml/min).

4. At least 4 weeks after the last anti-tumor treatment (surgery, chemotherapy, radiotherapy, biotherapy or endocrine therapy) before enrollment.

5. Had a life expectancy of at least 3 months.

6. Had an Eastern Cooperative Oncology Group (ECOG) performance status (PS) of 0, 1, or 2.

7. Signed informed consent.

Exclusion Criteria:

1. In the past or at the same time with other malignant tumors (already cure period of IB or cervical, lower levels of noninvasive basal cell or squamous cell cancer, obtain complete remission (CR) > 10 years of breast cancer, obtain complete remission (CR) > 10 years of malignant melanoma, obtain complete remission (CR) > 5 years except of other malignant tumors).
2. Pregnant or lactating female patients.
3. Those who have applied excessive dose of vitamin C or (and) metformin in recent 1 month.
4. Patients with glucose-6-phosphate dehydrogenase deficiency.
5. Patients with hydronephrosis.
6. Had a history of clinically significant or uncontrolled heart disease, including but not limited to: (1)Myocardial infarction. (2)Angina.(3)Congestive heart failure above grade 2 of the New York heart association (NYHA).(4)Ventricular arrhythmias requiring continuous treatment.(5)Supraventricular arrhythmias, including uncontrolled atrial fibrillation.
7. The patients had mental disorders, and the researchers believed that the patients could not fully or fully understand the possible complications in this study.
8. Have a history of immunodeficiency, including: HIV positive, or other acquired or congenital immunodeficiency diseases, or a history of organ transplantation.
9. Those who cannot tolerate or may be allergic to the drugs used in this study.
10. Participated in clinical trials of other drugs within the past 1 month.
11. Other factors considered unsuitable for the study.