Leflunomide to Prevent Cytomegalovirus Reactivation in Stem Cell Transplant Patients

Inclusion Criteria: 1. Males or females undergoing Haplo-identical haematopoetic stem cell transplant. 2. Age ≥18yrs on the day of signing informed consent 3. Undetectable CMV from plasma sample in preceding 7 days 4. Patient who has WBC engraftment (ANC\>=500/cumm for 3 or more consecutive days). 5. Within 7 days of WBC engraftment 6. Adequate liver functions (ALT / AST less than 5 times upper limit of normal and Bilirubin \<=3 times upper limit of normal) at time of starting leflunomide 7. Understands the study procedures, alternative treatment available, and risks involved with the study, and voluntarily agree to participate by giving written informed consent. Exclusion Criteria: 1. Known hypersensitivity to Leflunomide 2. History of clinically significant CMV reactivation in past 1 year 3. Patient is receiving/has received drug known to have anti CMV activity within in last 7 days \[Ganciclovir, valganciclovir, foscarnet, letermovir, acyclovir (at doses \>=3200 mg PO per day or \>=25 mg/kg IV per day), valacyclovir (at doses \&\>=3000 mg PO per day)\] 4. Inadequate renal function (creatinine clearance \<=30 ml/min) 5. Chronic active hepatitis B (HBsAg+ or any HBV DNA+), hepatitis C, or/and HIV 6. Any psychiatric condition that might limit the ability of the patient to comply with the protocol