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Mechanisms of sensitivity and natural resistance to antifolates in a methylcholanthrene-induced rat sarcoma
W W Li1, J T Lin, B I Schweitzer
1Laboratory of Molecular Pharmacology, Memorial Sloan-Kettering Cancer Center, New York, New York 10021.
Abstract:
A methylcholanthrene-induced rat sarcoma that can be propagated in vitro or in vivo was evaluated for resistance to antifolates and was found to be relatively resistant to methotrexate and 10-ethyl-10-deazaaminopterin but sensitive to trimetrexate. Rat sarcoma cell extracts contained low levels of dihydrofolate reductase activity, the target enzyme of methotrexate, and inhibition of this enzyme by these three antifolates was similar. Transport studies showed poor uptake of both methotrexate and 10-ethyl-10-deazaaminopterin. In contrast, trimetrexate achieved high intracellular levels. The poor uptake of methotrexate was not due to lack of polyglutamylation. Thus, the basis for natural resistance to methotrexate and 10-ethyl-10-deazaaminopterin, compared with trimetrexate, in this rat sarcoma cell line was due to decreased transport of these drugs.
Insights
This study found that a specific rat sarcoma cell line exhibits resistance to methotrexate and 10-ethyl-10-deazaaminopterin due to poor drug uptake. This resistance is overcome by trimetrexate, which achieves higher intracellular levels, highlighting the importance of drug transport in antifolate efficacy.
Area of Science:
- Oncology
- Pharmacology
- Biochemistry
Background:
- Antifolates are crucial in cancer chemotherapy, targeting dihydrofolate reductase (DHFR).
- Drug resistance mechanisms, including altered drug transport, can limit antifolate efficacy.
- Understanding resistance in specific cancer models is vital for optimizing treatment strategies.
Purpose of the Study:
- To investigate the mechanisms of antifolate resistance in a methylcholanthrene-induced rat sarcoma cell line.
- To compare the sensitivity and cellular uptake of methotrexate, 10-ethyl-10-deazaaminopterin, and trimetrexate in this cell line.
Main Methods:
- Evaluation of antifolate resistance through in vitro and in vivo propagation of rat sarcoma cells.
- Assay of dihydrofolate reductase (DHFR) activity in cell extracts.
- Drug transport studies to determine intracellular drug levels and uptake kinetics.
- Assessment of polyglutamylation status for methotrexate.
Main Results:
- The rat sarcoma cell line showed relative resistance to methotrexate and 10-ethyl-10-deazaaminopterin but sensitivity to trimetrexate.
- Dihydrofolate reductase (DHFR) activity was low, and enzyme inhibition by the antifolates was similar across all three drugs.
- Poor cellular uptake was observed for methotrexate and 10-ethyl-10-deazaaminopterin, while trimetrexate achieved high intracellular concentrations.
- Methotrexate uptake was not limited by polyglutamylation.
Conclusions:
- The primary mechanism for natural resistance to methotrexate and 10-ethyl-10-deazaaminopterin in this rat sarcoma model is decreased drug transport.
- Trimetrexate circumvents this resistance mechanism due to its efficient cellular uptake.
- These findings underscore the critical role of drug transport in determining antifolate efficacy and resistance in cancer cells.