This study is divided into two parts: ① Part I was a retrospective observational study. Kidney transplant recipients infected with BK virus (BKV) in the First Affiliated Hospital of Sun Yat-sen University from 2015 to 2021 were retrospectively collected and divided into case group and control group whether to convert MPA drug to mizoribine. ② The second part was a prospective, open and interventional clinical trial. Thirty patients with positive urinary BK virus after kidney transplantation using Mycophenolic acid (MPA) were enrolled in the First Affiliated Hospital of Sun Yat-sen University. All patients who met the inclusion criteria were treated with mizoribine to place MPA (Mycophenolate or mycophenolate sodium enteric coated tablets) for 12 months as directed. At the baseline of follow-up (before enrollment) and at each follow-up point, clinical indicators of patients were recorded, and renal biopsy was performed to evaluate the occurrence of BKV nephropathy in patients with persistent elevated BK viruria or persistent BK viremia after conversion and patients with BK viruria after 12 months, and to judge the progress after early activation of BKV and the safety of mizoribine.

This study is divided into two parts: ① Part I was a retrospective observational study. Kidney transplant recipients infected with BK virus (BKV) in the First Affiliated Hospital of Sun Yat-sen University from 2015 to 2021 were retrospectively collected and divided into case group and control group whether to convert MPA drug to mizoribine. The basic data of the two groups, the concentration of mizoribine, the clinical benefit after conversion and the outcome of BKV infection were analyzed, and the clinical improvement rate and preliminary safety of mizoribine conversion on BKV infection were determined. ② The second part was a prospective, open and interventional clinical trial. Thirty patients with positive urinary BK virus after kidney transplantation using Mycophenolic acid (MPA) were enrolled in the First Affiliated Hospital of Sun Yat-sen University. The patients and their families voluntarily participated in the experiment and signed the Informed Consent on the premise of fully understanding the treatment plan. The treatment was approved by the hospital ethics Committee. All patients who met the inclusion criteria were treated with MPA (Mycophenolate or mycophenolate sodium enteric coated tablets) replaced with mizoribine for 12 months. At the baseline of follow-up (before enrollment) and at each follow-up point, clinical indicators of patients were recorded, and renal biopsy was performed to evaluate the occurrence of BKV nephropathy in patients with persistent elevated BK viruria or persistent BK viremia after conversion and patients with BK viruria after 12 months, and to judge the progress after early activation of BKV and the safety of mizoribine.

Inclusion Criteria:

• Kidney transplant recipients;
• Postoperative maintenance therapy with glucocorticoid + tacrolimus +MPA;
• Urinary BKV-DNA load ≥10^7/L;
• No gender limitation for recipients > 18 years of age;
• Voluntarily sign written informed consent.

Exclusion Criteria:

• Multiple organ transplantation;
• Acute rejection occurred within 1 week before enrollment;
• Recipients with signs of active infection;
• Recipients with white blood cell count below 3,000 /cubic metre;
• Women who are pregnant, breast-feeding, or unwilling to use appropriate contraceptive methods during the study period;
• Patients with serious gastrointestinal diseases and active peptic ulcer disease;
• Suffering from any mental illness;
• Patients with severe heart disease and abnormal cardiac function;
• Subjects with known allergy to the test drug;
• Other competent physicians judged that the recipients were not suitable for inclusion.