Methotrexate resistance occurs by -
Ans. is 'b' i.e., Overproduction of DHFR \Ref: Goodman Gilman 11/ep. 1336; Katzung 10/ep. 887\Mechanism of action of methotrexateMethotrexate enter the cell by reduced folate carrierOnce inside the cell methotrexate is poly glutamylated.The poly glutamylated forms of methotrexate are potent inhibitors of dihydrofolate reductaseThe poly glutamased forms of methotrexate are converted back to mono glutamate form in lysosomes by the glutamyl hydrolase and is subsequently effluxed from the cellMethotrexate resistance can occur through any of the following five mechanismDecreased entry into the cell due to impaired transportDecreased retention as a consequence of lack of polyglutamate formationAn increase in DHFRAn altered mutated DHFR that binds less avidly than the normal enzyme to the drug.Increased level of lysosomal enzyme l glutamyl hydrolase that hydrolyse methotrexate poly glutamatesDecreased transportA common mechanism of methotrexate resistance is reduced folate transport carrierThe decreased transport is common mechanism to methotrexate resistance in relapsed ALL.Impaired PolvslutammationThe poly glutamated forms of methotrexate inhibits dihydrofolate reductase.The intracellular content of polyglutamate derivatives of methotrexate is controlled by a balance between fol polyglutamate synthetase and glutamyl hydrolase activity.Fol polyglutamate synthetase produces polyglutamate while l glutamyl hydrolase converts its bact to mono glutamate formA lower content of methotrexate polyglutamate could be due to decreased synthesis or an increased catabolism of methotrexate poly glutamates by l glutamate hydrolase.Increased DHFRThe main mechanism of action of methotrexate is inhibition of the enzyme DHFR.Methotrexate is a tight binding inhibitor of DHFR and the concentration of methotrexate required to achieve enzyme activity increases in direct proportion to the amount of enzyme in target cells.If is now well established that an important mechanism of resistance of cells to methotrexate is an increase in DHFR production due to amplification of the DHFR geneMutation in DHFRMutation is DHFR genes may be responsible for methotrexate resistance. Mutation in DHFR causes production of altered forms of DHFR that have decreased affinity for the inhibitor.
Depletion of folate
Overproduction of DHFR
Deficiency of thymidylate synthase
Deficiency of thiamine kinase
Anti-Neoplastic Agents