Methotrexate drug resistance

Ryser, H.P., Shen, W.C. (1978). Conjugation of methotrexate to poly(L-lysine) increases drug transport and overcomes drug resistance in cultured cells. Proc. Natl. Acad. Sci. U.S.A., 75, 3867-3870. 

Such methotrexate-resistant cells are found in cancer patients who had been given methotrexate as a single agent, where a remission is followed by relapse with drug-resistant cancer. 

Ross, D. D., Schneider, E. (2000). Methotrexate cross-resistance in a mitoxantrone-seiected muitidrug-resistant MGF7 breast cancer ceii iine is attributabie to enhanced energy-dependent drug efflux. Cancer Res. 60, 3514-3521. 

In addition to the ABC transporters, the MFS family transporters play an important role in clinical drug resistance [161]. The most prominent ones are Mdrl ]162, 163] and Flul ]164]. CaMDRl, formerly known as BEN confers resistance to a variety of different compounds. Cells lacking CaMDRl are susceptible to 4-nitro-quinoline-N-oxide, methotrexate, and cycloheximide ]165, 166]. The closest yeast homologue of CaMDRl is PLRl. As mentioned above, Flrl is known to be involved in drug transport and it mediates resistance to the same spectrum of drugs as CaMDRl. 

The rescue of normal, but not tumor, cells from methotrexate toxicity by folinic acid is partly explained by differences in membrane transport. For example, osteogenic sarcoma cells (which do not respond to conventional doses of methotrexate treatment) are not rescued by folinic acid administered after methotrexate, presumably owing to the absence of transport sites for folinic acid in the neoplastic cells. The therapeutic effects of administration of methotrexate and rescue with folinic acid are superior to those of methotrexate alone. Resistance to methotrexate can develop from increased activity of dihydrofolate reductase, synthesis of an enzyme having a lower affinity for the inhibitor, decreased transport of the drug into tumor cells, decreased degradation of the reductase, and genetic amplification of the gene for dihydrofolate reductase. 

Figure 5. Overexpression of PTRl, a known methotrexate resistance protein, in a drug resistant strain of L. major (strain MTX 60.2). A. Comparison of 2D gel images revealed a highly overexpressed spot in the resistant mutant versus the wildtype. MALDI-TOF analysis identified the spot as PTRl. B. DNA amplification (visible in the ethidium bromide-stained panel to the left) is a common mechanism of protein overexpression in Leishmania. Southern hybridisation using a PTRl probe (right panel) confirmed a substantial amplification of the PTRl gene, providing a mechanism for the overexpression seen by 2D gels.

Much work has been done to elucidate the molecular mechanisms of drug resistance [756]. Comparative QSAR equations were derived for the inhibition of bacterial DHFR, bacterial cell cultures [213, 214, 288, 432, 677, 685], and different tumor cell lines [288, 432, 671, 678, 679, 681, 684, 687], all being sensitive and resistant to methotrexate (MTX), e.g. eqs. 156—161. 

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