pH-dependent apparent permeability of the three CDK4/6 inhibitors across the MDCKII, MDCKII-ABCB1, and MDCKII-ABCG2 cell monolayers. Bi-directional permeability experiments were performed at a fixed apical pH (7.4) and varying basolateral pH (7.4, 7.0, 6.5, and 6.0). The apparent permeability in the apical-to-basolateral (Papp,A-B) and basolateral-to-apical (Papp,B-A) directions and efflux ratio (ER) are expressed as the mean fold change relative to those determined in the control (at pH 7.4 in both apical and basolateral chambers).