Drug Interactions between ivacaftor / lumacaftor and toremifene

GENERALLY AVOID: Coadministration with grapefruit juice may theoretically increase the plasma concentrations of toremifene. The proposed mechanism is inhibition of CYP450 3A4-mediated metabolism by certain compounds present in grapefruit. Because toremifene is associated with dose- and concentration-dependent prolongation of the QT interval, increased levels may potentiate the risk of ventricular arrhythmias such as torsade de pointes and sudden death.

GENERALLY AVOID: Due to their estrogenic effect, isoflavones present in soy such as genistein and daidzein may stimulate breast tumor growth and antagonize the antiproliferative action of toremifene. Supportive data are derived primarily from in vitro and animal studies. In vitro, low concentrations of these phytoestrogens have been found to promote DNA synthesis and reverse the inhibitory effect of tamoxifen on oestrogen-dependent breast cancer cell proliferation. In contrast, high concentrations of genistein greater than 10 microM/L have been found to enhance tamoxifen effects by inhibiting breast cancer cell growth. It is not known if these high concentrations are normally achieved in humans. Plasma concentrations below 4 microM/L have been observed in healthy volunteers given a soy diet for one month or large single doses of genistein. These concentrations are comparable to the low plasma concentrations associated with tumor stimulation reported in animals. In a study of 155 female breast cancer survivors with substantially bothersome hot flashes, a product containing 50 mg of soy isoflavones (40% to 45% genistein; 40% to 45% daidzein; 10% to 20% glycitein) taken three times a day was found to be no more effective than placebo in reducing hot flashes. No toxicity or recurrence of breast cancer was reported during the 9-week study period.

MANAGEMENT: Until more information is available, patients treated with toremifene should consider avoiding the consumption of grapefruit juice and soy-containing products. Patients should be advised to contact their physician if they experience vaginal bleeding or potential signs of blood clots such as chest pain, shortness of breath, sudden loss of vision, and pain, redness or swelling in an extremity. Patients should seek immediate medical attention if they experience symptoms that could indicate the occurrence of torsade de pointes such as dizziness, palpitations, or syncope.

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GENERALLY AVOID: Grapefruit juice may increase the plasma concentrations of ivacaftor. The proposed mechanism is inhibition of CYP450 3A4-mediated first-pass metabolism in the gut wall by certain compounds present in grapefruit. Elexacaftor and tezacaftor are also CYP450 3A4 substrates in vitro and may interact similarly with grapefruit juice, whereas lumacaftor is not expected to interact.

ADJUST DOSING INTERVAL: According to prescribing information, systemic exposure to ivacaftor increased approximately 2.5- to 4-fold, systemic exposure to elexacaftor increased approximately 1.9- to 2.5-fold, and systemic exposure to lumacaftor increased approximately 2-fold following administration with fat-containing foods relative to administration in a fasting state. Tezacaftor exposure is not significantly affected by administration of fat-containing foods.

MANAGEMENT: Patients treated with ivacaftor-containing medications should avoid consumption of grapefruit juice and any food that contains grapefruit or Seville oranges. All ivacaftor-containing medications should be administered with fat-containing foods such as eggs, avocados, nuts, meat, butter, peanut butter, cheese pizza, and whole-milk dairy products. A typical cystic fibrosis diet will satisfy this requirement.

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