Inhibition of 7-hydroxymethotrexate formation by amsacrine

Summary The inhibition of methotrexate (MTX) biotransformation to 7-hydroxymethotrexate (7-OH-MTX) by 4-(9-acridinylamino)-methanesulfon-m-anisidide (mAMSA) was studied in bile-drained rats in vivo and in incubates of isolated rat hepatocytes and rat-liver homogenate in vitro. In vivo, i.v. administration of 10 mg/kg mAMSA prior to [³H]-MTX infusion (50 mg/kg) led to a significant alteration in 7-OH-MTX kinetics. 7-OH-MTX peak concentrations and AUC in bile and serum were reduced by 75% and the recovery of MTX as 7-OH-MTX in bile and urine decreased by 70%, whereas MTX pharmacokinetics remained unaltered. In suspensions of isolated hepatocytes. 10 m mAMSA led to a 54% decrease in 7-OH-MTX formation. However, the hepatocellular influx and efflux of MTX was not perturbed by mAMSA. Preincubation of rat-liver homogenates with 1.25–10 m mAMSA reduced the formation of 7-OH-MTX by up to 73%. mAMSA appeared to inhibit MTX hydroxylation competitively, exhibiting aK_iof 3 m. Due to its inhibition of the MTX-oxidizing system, mAMSA may be beneficial in combination chemotherapy with MTX by reducing 7-OH-MTX-associated toxicity and, possibly, enhancing the cytotoxic effects of MTX.This study was financially supported by the Norwegian Cancer Society and the Erna and Olav Aakre Foundation for Cancer Research