Methotrexate resistance occurs by
Overproduction of DHFR Mechanism of action of methotrexate Methotrexate enter the cell by reduced folate carrier Once inside the cell methotrexate is polyglutamylated. The polyglutamylated.forms of methotrexate are potent inhibitors of dihydrofolate reductase The polyglutamased.forms of methotrexate are conveed back to monoglutamate .form in lysosomes by the glytamyl hydrolase and is subsequently effluxed from the cell Methotrexate resistance can occur through any of the following five mechanism Decreased entry into the cell due to impaired transpo Decreased retention as a consequence of lack of pohyglutammate formation An increase in DHFR An altered mutated DHFR that binds less avidly than the normal enzyme to the drug. Increased level of lysosomal enzyme 2 glutamyl hydrolase that hydrolyse methotrexate polyglutamates Decreased transpo A common mechanism of methotrexate resistance is reduced.folale transpo carrier The decreased transpo is common mechanism to methotrexate resistance in relapsed ALL. Impaired Polyglutammation The polyglutammated.forms of methotrexate inhibits dihydrofolate reductase. The intracellular content of polyglutammate derivatives of methotrexate is controlled by a balance between folypolyglutamate synthetase and glutamyl hydrolase activity. Folypolyglutamate synthetase produces polyglutammate while A glutamyl hydrolase conves its bact to monoglutamate form A lower content of methotrexate polyglutamate could he due to decreased synthesis or an increased catabolism of methotrexate polyglutamates by A glutamate hydrolase. Increased DHFR The 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 propoion to the amount of enzyme in target cells. - If is now well established that an impoant mechanism of resistance of cells to methotrexate is an increase in DHFR production due to amphification of the DHFR gene Mutation in DHFR Mutation 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