What happens when you take grapefruit with sirolimus?
Sirolimus (brand name Rapamune; also known as rapamycin) is an mTOR-inhibiting immunosuppressant used to prevent rejection in kidney transplant patients and to treat lymphangioleiomyomatosis. It is also being studied for tuberous sclerosis complex, vascular anomalies, and longevity research. Sirolimus is a substrate for two metabolic systems: the CYP3A4 enzyme (in the gut wall and liver) and P-glycoprotein (P-gp), a transporter that pumps drugs back out of intestinal cells.
Grapefruit juice contains furanocoumarins such as bergamottin and 6,7-dihydroxybergamottin that irreversibly inactivate intestinal CYP3A4 and partially inhibit P-glycoprotein. The combined effect on a drug like sirolimus is therefore larger than for a pure CYP3A4 substrate: more of the dose survives the gut wall and more of it stays in enterocytes long enough to be absorbed.
The FDA-approved prescribing information for Rapamune states explicitly: 'Grapefruit juice inhibits the CYP3A4-mediated metabolism of sirolimus. This juice therefore must not be taken with or be used for dilution of Rapamune.' The label does not assign a numeric AUC ratio because the magnitude is large, variable, and clinically unacceptable.
Why is this important?
Sirolimus is a paradigm of a narrow therapeutic window drug. Target trough whole-blood concentrations are usually 4 to 12 ng/mL after kidney transplant, depending on protocol. Below that range, the patient risks acute rejection; above it, the patient risks nephrotoxicity, infection, hyperlipidemia, thrombocytopenia, anemia, hyperglycemia, mouth ulcers, pneumonitis, and impaired wound healing.
Baseline interpatient variability in sirolimus exposure is already five- to tenfold. Layering grapefruit-induced CYP3A4 inhibition on top of that variability produces blood levels that no clinician can predict or dose around. A single glass of juice can move a patient from therapeutic to toxic, or destabilize a previously well-controlled regimen.
For transplant recipients, the cost of an interaction is enormous. Loss of graft function from sirolimus toxicity, or rejection from misjudged dose adjustments, can require dialysis or retransplantation. For lymphangioleiomyomatosis patients, supratherapeutic sirolimus levels can produce pneumonitis that is mistaken for disease progression.
What should you do?
Avoid grapefruit, grapefruit juice, pomelo (also called Chinese grapefruit), Seville (sour) oranges, tangelos, and minneolas completely while taking sirolimus. Read marmalade and citrus blend juice labels carefully, since Seville oranges are common ingredients.
Do not use grapefruit juice to dilute or mask the taste of Rapamune oral solution; the official label permits only water or orange juice for dilution.
Sweet oranges, mandarins, clementines, lemons, and limes are safe.
If you have already consumed grapefruit, contact your transplant coordinator or prescriber the same day. Do not skip or self-adjust sirolimus doses. Your team will likely arrange an early trough level and adjust the regimen if needed. Watch for new onset mouth sores, swelling in the legs or face, shortness of breath, fever, easy bruising, or rising creatinine on routine labs.
Coordinate carefully with every prescriber and pharmacist. Many drugs that interact with sirolimus through CYP3A4 (ketoconazole, itraconazole, voriconazole, clarithromycin, erythromycin, diltiazem, verapamil, rifampin, St. John's wort) need formal dose changes or are outright contraindicated; grapefruit belongs on that list.
Which specific products are affected?
The Rapamune brand offers tablets in 0.5 mg, 1 mg, and 2 mg strengths, and an oral solution of 1 mg/mL. Generic sirolimus tablets are available in the same strengths. All oral formulations are affected; the interaction occurs at the intestinal CYP3A4 enzyme and P-glycoprotein transporter, so any swallowed dose is at risk.
The same advice applies to everolimus (Afinitor, Zortress), a closely related mTOR inhibitor used in transplantation and oncology, which has nearly identical CYP3A4 and P-gp dependence. Avoid grapefruit while on everolimus.
Tacrolimus (Prograf) and cyclosporine (Neoral, Sandimmune, Gengraf) are also strongly affected by grapefruit and are commonly co-prescribed with sirolimus in transplant regimens. The cumulative interaction risk is therefore especially high in patients on multidrug immunosuppression.
The bottom line
Grapefruit is explicitly prohibited by the FDA Rapamune label because it inhibits the CYP3A4 and P-glycoprotein systems that clear sirolimus, producing unpredictable, potentially toxic blood levels. Transplant and lymphangioleiomyomatosis patients should treat grapefruit, pomelo, and Seville oranges as forbidden foods and contact their care team after any accidental exposure.