title: Enhanced receptor-mediated endocytosis and cytotoxicity of a folic acid-desacetylvinblastine monohydrazide conjugate in a pemetrexed-resistant cell line lacking folate-specific facilitative carriers but with increased folate receptor expression.
abstract: The reduced folate carrier (RFC), proton-coupled folate transporter (PCFT), and folate receptors (FR) are folate-specific transporters. Antifolates currently in the clinic, such as pemetrexed, methotrexate, and pralatrexate, are transported into tumor cells primarily via RFC. Folic acid conjugated to cytotoxics, a new class of antineoplastics, are transported into cells via FR-mediated endocytosis. To better define the role of PCFT in antifolate resistance, a methotrexate-resistant cell line, M160-8, was selected from a HeLa subline in which the RFC gene was deleted and PCFT was highly overexpressed. These cells were cross-resistant to pemetrexed. PCFT function and the PCFT mRNA level in M160-8 cells were barely detectable, and FR-α function and mRNA level were increased as compared with the parent cells. While pemetrexed rapidly associated with FR and was internalized within endosomes in M160-8 cells, consistent with FR-mediated transport, subsequent pemetrexed and (6S)-5-formyltetrahydrofolate export into the cytosol was markedly impaired. In contrast, M160-8 cells were collaterally sensitive to EC0905, a folic acid-desacetylvinblastine monohydrazide conjugate also transported by FR-mediated endocytosis. However, in this case a sulfhydryl bond is cleaved to release the lipophilic cytotoxic moiety into the endosome, which passively diffuses out of the endosome into the cytosol. Hence, resistance to pemetrexed in M160-8 cells was due to entrapment of the drug within the endosome due to the absence of PCFT under conditions in which the FR cycling function was intact.
authors.affiliation: Departments of Medicine (R.Z., I.D.G.) and Molecular Pharmacology (R.Z., N.D.-P., I.D.G.), Albert Einstein College of Medicine, Bronx, New York.
authors.forename: Rongbao
authors.initials: R
authors.lastname: Zhao
authors.forename: Ndeye
authors.initials: N
authors.lastname: Diop-Bove
authors.forename: I David
authors.initials: ID
authors.lastname: Goldman
mesh_terms: D000970:Antineoplastic Agents / Q000494:pharmacology*; D002478:Cells, Cultured; D019008:Drug Resistance, Neoplasm; D004705:Endocytosis; D005492:Folic Acid / Q000494:pharmacology*; D005493:Folic Acid Antagonists / Q000494:pharmacology*; D058972:Folic Acid Transporters / Q000032:analysis / Q000502:physiology*; D005971:Glutamates / Q000494:pharmacology*; D006147:Guanine / Q000031:analogs & derivatives* / Q000494:pharmacology; D006801:Humans; D000068437:Pemetrexed; D058979:Proton-Coupled Folate Transporter / Q000235:genetics / Q000502:physiology; D014747:Vinblastine / Q000031:analogs & derivatives* / Q000494:pharmacology
chemical_list: D000970:Antineoplastic Agents; D005493:Folic Acid Antagonists; D058972:Folic Acid Transporters; D005971:Glutamates; D058979:Proton-Coupled Folate Transporter; D000068437:Pemetrexed; C055170:4-desacetylvinblastine hydrazide; D014747:Vinblastine; D006147:Guanine; D005492:Folic Acid