Dihydrofolate reductase pathway bypass via reduced folate transporters and folate-dependent enzymes (DHFR bypass)

The Dihydrofolate reductase (DHFR) pathway bypass refers to a metabolic strategy where cells circumvent the inhibition of the DHFR enzyme by utilizing alternative sources of reduced folates or alternative enzymatic routes. This process is primarily mediated by reduced folate transporters, such as the Solute Carrier Family 19 Member 1 (SLC19A1), which import exogenous reduced folates like 5-formyltetrahydrofolate (leucovorin) directly into the cytoplasm (UniProt: P41440). Once inside, these folates are converted into active cofactors for folate-dependent enzymes, including Thymidylate Synthase and various transformylases, without requiring the DHFR-catalyzed reduction of dihydrofolate (UniProt: P00374). This bypass is a critical mechanism of resistance to antifolate drugs like methotrexate, as it allows cells to maintain DNA synthesis despite drug presence (PubMed: 12130515). However, it is also exploited therapeutically in leucovorin rescue protocols to protect healthy tissues from the lethal effects of high-dose methotrexate. In the context of oncology, the upregulation of these transporters or the increased availability of extracellular folates can lead to treatment failure. Understanding this pathway is essential for managing drug resistance and optimizing the therapeutic index of antifolate medications.

Other names

Folate metabolic bypassMethotrexate resistance pathwayReduced folate transport-mediated bypassLeucovorin rescue pathwayExogenous folate salvage

The bypass occurs when exogenous reduced folates are transported into the cell via SLC19A1 (RFC1), providing the necessary tetrahydrofolate derivatives for nucleotide synthesis even when DHFR is inhibited by antifolates (StatPearls: NBK548826).