Rapamycin (Sirolimus) and Warfarin Interaction: What Clinicians and Patients Need to Know

What each label says about this pair

The Jantoven (warfarin sodium) label's interaction tables list CYP2C9, CYP1A2 and CYP3A4 inhibitors and inducers that can move the INR, and the label notes those tables are examples rather than a complete list [2]. The Rapamune interaction section is built around drugs that change sirolimus concentrations: strong CYP3A4 and P-glycoprotein inhibitors to avoid (ketoconazole, voriconazole, itraconazole, erythromycin, telithromycin, clarithromycin), strong inducers to avoid (rifampin, rifabutin), weaker modulators that may require a dose adjustment, plus cyclosporine, cannabidiol and grapefruit juice [1].

The practical instruction comes from the warfarin side: the label calls for more frequent INR checks when another drug, including botanicals, is started or stopped or its dose changes [2]. Bleeding is more likely in the first month, and keeping the INR in the therapeutic range does not eliminate the risk [2].

What the sirolimus label says about its own metabolism

Sirolimus is a substrate of both CYP3A4 and P-glycoprotein, and many Rapamune interactions involve other drugs changing sirolimus levels [1]. Strong inhibitors raise sirolimus exposure and strong inducers lower it, and the label says to avoid both [1]. The label says grapefruit juice inhibits CYP3A4-mediated metabolism of sirolimus and must not be taken with Rapamune or used to dilute it [1]. Cannabidiol can raise sirolimus levels [1]. The label says cyclosporine raises sirolimus concentrations, Rapamune is taken 4 hours after cyclosporine, and higher sirolimus doses are needed if cyclosporine is withdrawn [1].

Mean terminal half-life is about 62 hours (SD 16) in stable renal transplant patients [1]. Hepatic impairment raises sirolimus AUC by 43% (mild), 94% (moderate) and 189% (severe), with the maintenance dose reduced by about one third in mild-to-moderate and one half in severe impairment [1].

What the warfarin label says

Warfarin carries a boxed warning for major or fatal bleeding, and the label calls for regular INR monitoring in every treated patient [2].

The target is INR 2.5 (range 2.0 to 3.0) for venous thromboembolism and non-valvular atrial fibrillation; an INR above 4.0 appears to give no added benefit in most patients and is linked to higher bleeding risk [2]. For tilting disk or bileaflet mechanical valves in the mitral position the target is 3.0 (range 2.5 to 3.5) [2]. The more potent S-enantiomer is cleared mainly by CYP2C9, while the R-enantiomer is handled by CYP1A2 and CYP3A4, and inhibitors of these enzymes have the potential to increase the INR [2].

Where the two drugs overlap on paper

R-warfarin is metabolized partly by CYP3A4, while sirolimus is a substrate of CYP3A4 and P-glycoprotein [1][2]. The warfarin label's CYP3A4 entry is about inhibitors and inducers of that enzyme [2]. The Rapamune interaction section covers drugs that change sirolimus concentrations [1].

Shared interacting drugs

Cyclosporine appears in the warfarin label as a CYP3A4 inhibitor, and that label says inhibitors of CYP3A4 have the potential to increase the INR [2]. The Rapamune label says cyclosporine also raises sirolimus concentrations, Rapamune is taken 4 hours after it, and withdrawing cyclosporine means higher sirolimus doses are needed [1]. The warfarin label's advice about more frequent INR monitoring when another drug is started or stopped applies to a change in cyclosporine [2].

Voriconazole appears in Rapamune's avoid list and among warfarin's CYP2C9 inhibitors [1][2]. Fluconazole could increase sirolimus concentrations and is also listed as a warfarin CYP2C9 inhibitor [1][2]. The warfarin label reports INR changes with antibiotics and antifungals and asks for close INR monitoring when any of them is started or stopped [2].

Drugs that raise bleeding risk with warfarin

The warfarin label lists other anticoagulants (heparin, dabigatran and others), antiplatelets (aspirin, clopidogrel, dipyridamole, prasugrel, cilostazol), NSAIDs (ibuprofen, naproxen, diclofenac, celecoxib and others) and SSRIs or SNRIs (citalopram, escitalopram, fluoxetine, paroxetine, sertraline, venlafaxine, duloxetine and others) as drugs that increase bleeding risk, and it asks for close monitoring when they are used together [2]. Separately, the Rapamune label lists thrombocytopenia among the adverse reactions reported in 30% or more of renal transplant patients in the clinical studies [1].

Grapefruit, green vegetables and St John's wort

Two dietary rules come straight from the labels. The Rapamune label says grapefruit juice must not be taken with Rapamune or used to dilute it [1]. On the warfarin side, the amount of vitamin K in food affects the INR; the label asks for a normal, balanced diet with a consistent amount of vitamin K and no drastic changes such as eating large amounts of green leafy vegetables [2].

St John's wort is the botanical with a clear direction and a number behind it. In 12 healthy men given a single 25 mg warfarin dose, 14 days of St John's wort raised the apparent clearance of S-warfarin by 29% (ratio 1.29, 90% CI 1.16 to 1.46) and of R-warfarin by 23%, and the INR area under the curve fell to 0.79 of baseline [3]. The warfarin label lists St John's wort, ginseng and co-enzyme Q10 as botanicals that may decrease warfarin's effect, and garlic and ginkgo as botanicals that can cause bleeding on their own [2]. The Rapamune label lists St John's wort among agents that could decrease sirolimus concentrations [1]. The same supplement appears on both labels, in the same direction [1][2].

Low-dose rapamycin for longevity

Rapamycin is used off-label for longevity. The Rapamune label's approved uses are prevention of organ rejection in patients aged 13 and older receiving renal transplants and treatment of lymphangioleiomyomatosis, with no approved anti-aging or longevity indication [1]. Compounded rapamycin products are not FDA-approved, and FDA does not verify their safety, effectiveness or quality before they are marketed [4].

The 48-week PEARL trial (NCT04488601) gave healthy, normative-aging adults placebo or 5 mg or 10 mg of compounded rapamycin once weekly; adverse and serious adverse events were similar across groups, visceral adiposity did not change, and the authors concluded the drug was relatively safe over 48 weeks with efficacy not established [5]. The trial abstract does not report any warfarin interaction data.

Hepatic impairment

Both labels flag the liver. Sirolimus exposure climbs steeply with worsening hepatic impairment, from a 43% AUC increase in mild impairment to 189% in severe, and the maintenance dose is reduced accordingly [1]. The warfarin label lists moderate to severe hepatic impairment among the settings in which the risks of therapy are increased [2].