Immunology · Year 2 · from Immunology
Case 4: Paxlovid Drug Interaction with Tacrolimus in Transplant Recipient
Clinical Image
Source: Wikipedia - Nirmatrelvir/ritonavir - CC BY-SA 4.0
Case Presentation
A 62-year-old female with a history of renal transplantation 3 years ago, maintained on tacrolimus 4 mg twice daily (most recent trough level 7.2 ng/mL), mycophenolate mofetil 1000 mg twice daily, and prednisone 5 mg daily, develops mild COVID-19 symptoms including sore throat, rhinorrhea, and low-grade fever. She tests positive for SARS-CoV-2 by rapid antigen within 3 days of symptom onset. Her primary care physician prescribes nirmatrelvir/ritonavir (Paxlovid) 300 mg/300 mg twice daily for 5 days without consulting her transplant team. She continues her usual tacrolimus dose. By day 3 of Paxlovid therapy, she presents to the emergency department with tremor, headache, nausea, and decreased urine output. Vital signs show BP 162/98 mmHg, HR 88 bpm, and she is afebrile. Laboratory studies reveal serum creatinine 3.4 mg/dL (baseline 1.2), BUN 48 mg/dL, potassium 5.8 mEq/L, magnesium 1.2 mg/dL, glucose 198 mg/dL, and a stat tacrolimus trough level of 68 ng/mL (toxic; therapeutic range 5-15 ng/mL). The tacrolimus and mycophenolate are immediately held. She is admitted for continuous cardiac monitoring given hyperkalemia, treated with calcium gluconate, insulin/dextrose, and sodium polystyrene sulfonate. Tacrolimus levels are monitored every 12 hours and gradually decline over 5 days due to the drug's long half-life when CYP3A4 is inhibited. Her creatinine returns to 1.5 mg/dL by hospital day 7, and immunosuppression is carefully reintroduced at reduced doses.
Key Learning Points
- Ritonavir, the pharmacokinetic booster in Paxlovid, is a potent irreversible inhibitor of CYP3A4 and P-glycoprotein; tacrolimus is almost exclusively metabolized by CYP3A4, and co-administration can increase tacrolimus levels 5- to 70-fold within 2-3 days
- In transplant recipients requiring Paxlovid, the recommended approach is to hold tacrolimus entirely for the 5-day treatment course, reduce cyclosporine doses by 80%, and monitor drug levels daily with plans for dose re-titration after ritonavir clearance (half-life approximately 3-5 hours, but CYP3A4 recovery takes 3-5 days due to mechanism-based inhibition)
- Calcineurin inhibitor toxicity manifests as acute nephrotoxicity (afferent arteriolar vasoconstriction), neurotoxicity (tremor, posterior reversible encephalopathy syndrome), electrolyte derangements (hyperkalemia, hypomagnesemia), and new-onset diabetes
- Immunocompromised patients are at higher risk for severe COVID-19 and prolonged viral shedding, yet drug interaction management must be carefully coordinated between infectious disease and transplant teams before initiating Paxlovid