Influx and efflux transporters contribute to the increased dermal exposure to active metabolite of regorafenib after repeated oral administration in mice.

The multikinase inhibitor regorafenib, which is a standard treatment for certain cancer patients after disease progression following other approved therapies, exhibits delayed-onset dermal toxicity. Here, we aimed to clarify the mechanisms that contribute to the increased dermal exposure to active metabolite M-5 of regorafenib after repeated oral administration. The dermal concentration of M-5 at 24 h after the last 5 oral administrations of regorafenib in mdr1a/1b/bcrp-/- mice was more than 190 times that in wild-type mice. The skin-to-plasma concentration ratio of M-5 in mdr1a/1b/bcrp-/- was also higher than in wild-type mice, suggesting possible involvement of P-glycoprotein and breast cancer resistance protein in regulating the dermal distribution. The area under the plasma concentration-time curve values of M-5 and its precursor M-2 in plasma of mdr1a/1b/bcrp-/- were at most 26 and 3 times those in wild-type mice, respectively. Interestingly, repeated administration of regorafenib markedly increased the area under the plasma concentration-time curve of M-5 in plasma, but not liver, compared with a single dose. Intravenous administration of M-5 dose-dependently reduced the liver-to-plasma concentration ratio. Our results indicate that hepatic uptake of M-5 may partially explain the accumulation of M-5 in the systemic circulation, but multiple factors, including influx and efflux transporters, are involved in determining dermal exposure to M-5.