Drug Interactions between letrozole / ribociclib and Prevymis

MONITOR: Coadministration with ribociclib may increase the plasma concentrations and pharmacologic effects of drugs that are substrates of CYP450 3A4. The proposed mechanism is decreased clearance due to ribociclib-mediated inhibition of CYP450 3A4 metabolism. In healthy study subjects, administration of midazolam, a sensitive CYP450 3A4 substrate, with multiple 400 mg daily doses of ribociclib increased the midazolam peak plasma concentration (Cmax) and systemic exposure (AUC) by 2.1-fold and 3.8-fold, respectively, compared to midazolam administered alone. When given at a clinically relevant dose of 600 mg daily, ribociclib is predicted to increase midazolam Cmax and AUC by 2.4-fold and 5.2-fold, respectively.

MANAGEMENT: Caution is advised when ribociclib is used concomitantly with drugs that undergo metabolism by CYP450 3A4, particularly those with a narrow therapeutic range. Dosage adjustments as well as clinical and laboratory monitoring may be appropriate for some drugs whenever ribociclib is added to or withdrawn from therapy.

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MONITOR: Coadministration with letermovir may increase the plasma concentrations of drugs that are substrates of CYP450 2C8, CYP450 3A4, and/or organic anion transporting polypeptide protein (OATP) 1B1 and 1B3. Letermovir has been shown to be a reversible inhibitor of CYP450 2C8 in vitro, although its effect on CYP450 2C8 substrates has not been evaluated clinically. Letermovir is also a time-dependent inhibitor and inducer of CYP450 3A4 in vitro. According to the product labeling, midazolam peak plasma concentration (Cmax) and systemic exposure (AUC) increased by an average of 1.7- and 2.3-fold, respectively, when a single 2 mg oral dose of midazolam was coadministered with letermovir 480 mg orally once daily. The Cmax did not change when midazolam 1 mg was administered intravenously with letermovir 240 mg orally once daily, but AUC increased by 1.5-fold and concentration at 24 hours postdose (C24hr) increased by 2.7-fold. The increased AUC of midazolam, a CYP450 3A4 probe substrate, indicates that net effect of letermovir on the isoenzyme is moderate inhibition. In addition, letermovir is an inhibitor of the hepatic uptake transporters, OATP 1B1 and 1B3. When a single 20 mg dose of atorvastatin, a CYP450 3A4 and OATP1B1/1B3 substrate, was coadministered with letermovir 480 mg orally once daily, atorvastatin Cmax, AUC and C24hr increased by an average of 2.2-, 3.3- and 3.6-fold, respectively. Additional use of cyclosporine is likely to further increase the magnitude of these interactions, since it is an inhibitor of CYP450 3A4 and a strong inhibitor of OATP 1B1 and 1B3.

MANAGEMENT: Caution is advised when letermovir is used concurrently with drugs that are substrates of CYP450 2C8, CYP450 3A4, and/or OATP 1B1 and 1B3, particularly those with a narrow therapeutic range. Dosage adjustments as well as clinical and laboratory monitoring may be appropriate for some drugs whenever letermovir is added to or withdrawn from therapy. Moreover, clinicians should be aware that the magnitude of CYP450 3A- and OATP1B1/3-mediated drug interactions with coadministered drugs may be different when letermovir is used with cyclosporine. The combined effect of the two drugs on CYP450 3A4 may be similar to that of a strong CYP450 3A4 inhibitor, hence clinicians should refer to the prescribing information for dosing recommendations of the CYP450 3A4 substrate with a strong CYP450 3A4 inhibitor. Similarly, letermovir and cyclosporine may demonstrate some additive effects on OATP1B1 inhibition, although cyclosporine by itself is already a strong OATP1B1/3 inhibitor.

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MONITOR: Coadministration with inhibitors of CYP450 3A4 may increase the plasma concentrations and risk of adverse effects of ribociclib, which is a substrate of the isoenzyme. Administration of erythromycin, a moderate CYP450 3A4 inhibitor, is predicted to increase ribociclib Cmax and AUC by 1.3- and 1.9-fold, respectively. Data with other, less potent inhibitors, are limited.

MANAGEMENT: Caution is advised when ribociclib is used with CYP450 3A4 inhibitors. Dosage adjustment for ribociclib is not generally considered necessary for suspected interactions with mild to moderate inhibitors. However, some manufacturers of ribociclib recommend that concomitant use with moderate CYP450 3A4 inhibitor verapamil should be avoided (AU, UK). Patients should be monitored for the development of ribociclib-related adverse effects such as infections, neutropenia, leucopenia, anemia, thrombocytopenia, nausea, vomiting, diarrhea, stomatitis, anorexia, alopecia, fatigue, headache, and abnormal liver function.

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