Coadministration has not been studied. Ribociclib is metabolised by CYP3A4. Ritonavir is metabolized by CYP3A4 and is a strong inhibitor of CYP3A4. Coadministration with strong CYP3A4 inhibitors is expected to increase ribociclib concentrations and is not recommended. Coadministration of ritonavir (100 mg twice daily) and ribociclib (400 mg single dose) increased ribociclib AUC and Cmax by 221% and 67% in healthy subjects. If coadministration is necessary, the ribociclib dose should be reduced to 400 mg once daily (in patients on 600 mg ribociclib) or to 200 mg (in patients who had their dose reduced to 400 mg) with close monitoring for toxicity. Ribociclib treatment should be interrupted in patients who have already had their dose reduced to 200 mg. If the strong inhibitor is discontinued, the ribociclib dose used prior to the initiation of the strong CYP3A4 inhibitor should be resumed after at least 5 half-lives of the strong CYP3A4 inhibitor.