Folate transporters and uptake systems (RFC, PCFT, FR)

Folate transporters and uptake systems are a group of specialized proteins responsible for the cellular acquisition of folates (Vitamin B9), which are essential cofactors for one-carbon metabolism, DNA synthesis, and cell division. The system primarily consists of the reduced folate carrier (RFC/SLC19A1), the proton-coupled folate transporter (PCFT/SLC46A1), and the folate receptor family (FRs/FOLR1-3). While RFC is ubiquitously expressed and serves as the major route for folate uptake at physiological pH, PCFT is critical for intestinal absorption in acidic environments, and folate receptors facilitate high-affinity uptake via receptor-mediated endocytosis. In oncology, folate receptor alpha (FOLR1) is frequently overexpressed in solid tumors, such as ovarian and lung cancers, while remaining largely absent or restricted to apical surfaces in normal tissues. This differential expression makes it a highly attractive target for antibody-drug conjugates (ADCs) and targeted radiopharmaceuticals. Conversely, mutations in these transporters can lead to severe clinical conditions, including hereditary folate malabsorption and cerebral folate deficiency. Therapeutic strategies involve both the use of these systems to deliver cytotoxic antifolates and the development of highly specific ligands to exploit receptor overexpression for precision medicine.

Other names

Reduced folate carrier (RFC)Proton-coupled folate transporter (PCFT)Folate receptor alpha (FR-alpha)Folate receptor beta (FR-beta)Folate receptor gamma (FR-gamma)Solute carrier family 19 member 1 (SLC19A1)Solute carrier family 46 member 1 (SLC46A1)Folate receptor 1 (FOLR1)Folate receptor 2 (FOLR2)Folate receptor 3 (FOLR3)

Drugs targeting these systems utilize them either as a route of entry for intracellular enzyme inhibition or as specific cell-surface markers for targeted delivery. Classic antifolates like methotrexate are actively transported into cells via the reduced folate carrier (RFC) to inhibit dihydrofolate reductase. Modern antibody-drug conjugates (ADCs), such as mirvetuximab soravtansine, bind to folate receptor alpha (FOLR1) and undergo receptor-mediated endocytosis to release cytotoxic payloads directly into tumor cells. Additionally, folate-drug conjugates exploit the high affinity of folic acid for its receptors to deliver small molecule toxins or imaging agents selectively to tissues overexpressing these uptake systems.