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Antimetabolites are cytotoxic agents, which have been developed for more than 50  years. Which cancer patient did not receive or will not receive 5-fluorouracil or methotrexate during the evolution his or her disease? Antimetabolites are defined as interfering with the synthesis of the DNA constituents; they are structural analogues, either of purine and pyrimidine bases (or the corresponding nucleosides), or of folate cofactors, which are involved at several steps of purine and pyrimidine biosynthesis. Their first mechanism of action is, therefore, to induce depletion in nucleotides inducing in turn an inhibition of DNA replication. However, some of them are able to get inserted fraudulently into nucleic acids, inducing structural abnormalities leading to cell death by other mechanisms, including DNA breaks. We present in this paper, for the three classes of antimetabolites, both ancient and recent molecules as well as molecules still in clinical trials, without exhaustivity.

Thymidylate synthase (TS) is an enzyme, which catalyzes the methylation of deoxyuridylate to deoxythymidylate using 5.10-methylenetetrahydrofolate as a cofactor. For this reason, TS has been widely investigated and is one of the best-known drug targets in the anticancer area. Antimetabolites have been developed to target TS and among them, pemetrexed is now considered as part of the standard treatment for lung cancer and mesothelioma. Intratumoral expression of TS mRNA has been shown to be associated with prognosis and with the response to 5-FU therapy in patients with breast, colorectal, head and neck cancer types. Recent findings suggest that TS might be a biomarker for NSCLC treated with pemetrexed, as lower response rates in squamous cell carcinoma and small cell carcinoma may be due to a higher expression of TS. Specific validation for this use as a biomarker is awaited. All these recent findings suggest that TS could be a useful predictive marker of the treatment efficacy of antifolate drugs and indicate that both Real-Time PCR and immuno-histochemistry might be used to assess TS expression levels. This may help in defining the best therapeutic strategy., (Copyright © 2011 SPLF. Published by Elsevier Masson SAS. All rights reserved.)

The pemetrexed (Alimta) is a new generation antifolate prescribed in the treatment of mesothelioma in association with cisplatin and in the 2nd line treatment of locally advanced or metastatic non-small lung cancer. Pemetrexed is an original molecule, different from the other antifolates. On the opposite methotrexate, pemetrexed inhibits several enzymes involved in the synthesis of purines and pyrimidines, in particular thymidylate synthase, dihydrofolate reductase, glycinamide ribonucleotide formyltransferase and 5-aminoimidazole-4-carboxamide ribonucleotide formyltransferase. Pemetrexed is transported in cells by three receptors, which make easier its cellular penetration. On the other hand, the polyglutamation of the product by the folylpolyglutamate synthetase increases considerably its activity notably towards the thymidylate synthase. Finally, unlike methotrexate, pemetrexed presents an atypical effect on cellular synchronisation. The wide spectre of activity of pemetrexed confers it a therapeutic advantage with regard to the other antifolates specific of one or other one enzymes. The clinical results show an anti-tumoral activity against non-small lung cancer and in mesothelioma and recently towards other solid tumours, in particular in head an neck, colon and mammary cancers.

Gemcitabine and pemetrexed are two novel antimetabolites with broad spectrum of antitumor activity and a safe toxicity profile. In addition, recent advances in cellular and molecular biology led to a better understanding of their mechanisms of action and resistance. Based on these latter advantages, several studies have been conducted using pemetrexed or gemcitabine either alone or with novel targeted therapies. Taken together, these recent data should result in a better and more specific use of these two very promising drugs.

Gemcitabine is pyrimidine analog which has demonstrated antitumoral activity in a variety of solid tumors. Laboratory studies demonstrating that gemcitabine is a potent radiosensitizer led to a variety of trials combining radiation and gemcitabine. In the clinic, phase I-II studies are still trying to determine the optimal dose and schedule which could be use in daily clinical practice. This review summarizes the mechanisms of interaction between radiotherapy and gemcitabine and presents several therapeutic schemes for each tumor location.

The drugs concerned by this review are cytarabine (ara-C), gemcitabine and fludarabine. Seventy-eighty per cent of a dose of ara-C are excreted under the form of ara-U (main metabolite). Plasma concentrations of ara-C are not related to drug pharmacodynamics (response to treatment) in contrast to intracellular levels of ara-CTP (active metabolite) which are associated with cytotoxic activity. Gemcitabine is able to autoactivate its own mechanism of action. Gemcitabine is characterized by a short half-life of elimination (15-20 min) and plasma pharmacokinetics of the drug are not linked to pharmacodynamics. Prolonged administration of gemcitabine is pharmacokinetically and pharmacologically justified and should deserve more intense clinical investigations. Total body clearance of F-ara-A (main circulating metabolite of fludarabine) is linked to creatinine clearance and drug-related neutropenia are more frequent in patients with creatinine clearance below 50 mL/min. So far there are no relationships between intracellular levels of F-ara-CTP and response to treatment.

Gemcitabine is a molecule presenting with major radiosensitizing or radio-potentiator capacities. This property has been extensively studied in vitro, with clinical therapeutic implications. Chemotherapy and radiation are very often used together, most of the time concurrently. This review analyses the results obtained with combined gemcitabine and radiation, for non small cell lung cancer, pancreatic adenocarcinoma, head and neck and uterine cervix carcinomas. Several therapeutic schemes are presented, for each tumor location. A description of the toxicities observed is presented, including the recall phenomenon description. Promising results justify the numerous current trials, as well as enthusiasm initiated by this therapeutic association.

High dosage methotrexate is currently used in the treatment of malignancies. When used at low- or moderate doses, methotrexate has antiproliferative and antiinflammatory effects and is a useful drug in skin diseases. The aim of this review is to describe pharmacology, indications, adverse effects and practical use in Dermatology. Pharmacodynamics of methotrexate is especially related to antifolic activity. Methotrexate is officially approved in the treatment of severe psoriasis, but many other proliferative or inflammatory diseases with cutaneous manifestations may benefit from this drug, usually in association with corticosteroids. The use of methotrexate needs some precautions and a precise follow-up to minimise the risk of severe adverse effects. However, efficacy of methotrexate was reported in open and retrospective small size studies. Prospective and comparative trials are required to confirm the indications, advantages and tolerance of methotrexate in dermatology.

1) The in vitro effect of virus-inhibiting factor (IF) or interferon on Ehrlich ascites tumor and sarcoma 180 was found to be cytostatic, but not cytocidal. 2) Mouse peritoneal macrophages or splenic lymphoid cells, in the presence of IF, did not affect multiplication of the tumor cells examined.