PERTURBATION OF METHOTREXATE TRANSPORT BY VERAPAMIL, VINBLASTINE, TAUROCHOLATE AND BROMOSULFOPHTHALEIN IN RAT HEPATOCYTES

Background: Methotrexate (MTX) is extensively used in different combin ation chemotherapy regimens. More knowledge about interactions and the ir mechanisms in target cancer cells and normal cells is needed to imp rove therapeutic efficacy and reduce toxicity. Materials and methods: The effect of verapamil (VRP), vinblastine (VBL), taurocholate (TAURO) and bromosulfophthalein (BSP) on MTX transport were studied in freshl y isolated rat hepatocytes. Results: During 60 min, 50 mu M VRP decrea sed the hepatocellular MTX accumulation with 29%; whereas 100 mu M BSP reduced MTX entrance with 15%. 100 mu M VBL and 100 mu M TAURO reduce d the intracellular accumulation of MTX with 36% and 23%, respectively . VRP and BSP appeared to be selective MTX influx blockers, whereas VB L and TAURO inhibited both MTX influx and efflux however; with major i nhibition on MTX inflict. Dixon plot analyses for TAURO and BSP were s uggestive of competitive inhibition giving inhibition constants (K-i) values of 105 mu M for TAURO, and 800 mu M for BSP. Conclusion: The da ta demonstrate for the first time a selective inhibitory effect of VRP upon MTX influx in isolated rat hepatocytes, whereas BSP, a potent MT X efflux inhibitor in malignant cells fails to achieve this effect in the normal cell type here investigated.