The Antifolates
Section snippets
Antifolates achieve their pharmacologic effects by their perturbations of folate metabolism
The structures of the B9 vitamins in their oxidized and reduced forms are illustrated in Fig. 1. Folic acid is not a physiologic folate but is an important source of folates ingested because it is added as a supplement to foods and it is the predominant form of folate in vitamin supplements. The major dietary folate in nature, 5-methyltetrahydrofolate (5-methylTHF), is absorbed in the proximal small intestine by a highly specific transport mechanism, the proton-coupled folate transporter (PCFT,
Pharmacologic consequences of the formation of methotrexate polyglutamate derivatives within cells
Physiologic folates form polyglutamate derivatives that are retained and build to high levels within cells; these are usually preferred substrates for tetrahydrofolate cofactor-requiring enzymes.17, 18 In a series of reactions, mediated by folylpolyglutamate synthetase (FPGS), glutamate molecules are added successively at the γ-carboxyl moiety to form a peptide chain of up to 6 to 8 glutamate residues. Antifolates undergo the same reaction and this has profound pharmacologic ramifications. The
The membrane transport of antifolates mediated by facilitative carriers and export pumps
The physiologic folates and most antifolates are bivalent anions that diffuse poorly across cell membranes, and only low levels of folates are present in the diet and in the blood. Hence, specific processes are required to achieve their efficient transport across the epithelia and into systemic cells (Fig. 6). Transport across the apical-brush border membrane of the duodenum and proximal jejunum is mediated by the PCFT, a process that functions optimally at the low pH found at the
High-dose MTX with leucovorin rescue
With the recognition that MTX resistance in experimental tumor systems was often associated with impaired RFC-mediated transport across the cell membrane and with the assumption that delivery to cells within solid tumors is limited by a compromised vasculature, an approach was developed to circumvent these limitations that would permit the safe administration of high doses of MTX.32, 33, 34, 35
The rationale for high-dose MTX is that high blood and extracellular drug levels facilitate diffusion
Current clinical applications of methotrexate
MTX remains an important agent for the treatment of acute leukemia, lymphoma, osteosarcoma, and leptomeningeal metastases.44, 45, 46 MTX continues to be used for the treatment of gestational trophoblast tumors47 and in the cytoxan-MTX-fluorouracil regimen for the treatment of breast cancer.48, 49 This drug is also widely used for the treatment of rheumatoid arthritis, inflammatory bowel disease, psoriasis, and other inflammatory diseases.50, 51, 52, 53, 54, 55 Although introduced in the 1960s,33
The emergence of a new-generation 4-amino-antifolate: pralatrexate
As an understanding of the pharmacologic properties of MTX emerged and its interaction with intact cells that were key determinants of its cytotoxicity were identified, a search was undertaken to create a structural analogue that would enhance its activity and its selectivity. The key parameters identified were membrane transport and polyglutamation. This finding led to the development of pralatrexate.59, 60 This antifolate differs from MTX in the substitution of a carbon for nitrogen at the N10
Raltitrexed
The recognition that the polyglutamate derivatives of MTX have targets downstream of DHFR as direct inhibitors of thymidylate synthase and AICAR transformylase led to a drug development effort focused on the identification of antifolates, which, in their polyglutamate forms, are direct inhibitors of one or both of these enzymes. The first of these agents to be established in the clinic was raltitrexed (see Fig. 3).64, 65 In its polyglutamate forms, this drug has a high affinity for thymidylate
The impact of physiologic folates on the antitumor activities and toxicity of antifolates
Drugs that require polyglutamation to achieve activity, in particular raltitrexed and pemetrexed, are generally sensitive to the level of physiologic folates with cells that are substrates for FPGS and compete with these antifolates for this enzyme.79 Hence, the higher the folate levels in tumor cells, the poorer the activity of these agents.80, 81 On the other hand, folate sufficiency is also an important element in toxicity to patients receiving these drugs; the most sensitive indicator of
Mechanisms of resistance to antifolates
Mechanisms of resistance to MTX have been established by studies in tumor systems in vitro under conditions in which selective pressure has been applied by gradual exposure to increasing concentrations, or pulse exposures to high concentrations, of the drug. Chemical mutagenesis has also been used to accelerate and amplify mutational events contributing to resistance. All elements of the interaction of this drug with its cellular targets have been implicated in resistance. Membrane transport is
Targeting drugs to tumor cells via folate transporters
A major emphasis of current drug-development efforts is generally directed to targeting pathways or regulatory elements that drive the proliferation of malignant cells. Another approach is the development of cytotoxics and other agents that are selectively delivered to malignant cells via transporters that are selectively expressed or selectively active in malignant cells. These efforts have focused on 2 transporters: PCFT and folate receptors.
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