Experimental and clinical studies on nucleoside analogs as antitumor agents

Identification of Structural and Molecular Features Involved in the Transport of 3'-Deoxy-Nucleoside Analogs by Human Equilibrative Nucleoside Transporter 3

Combination antiretroviral drug treatments depend on 3′-deoxy-nucleoside analogs such as 3′-azido-3′-deoxythymidine (AZT) and 2′3′-dideoxyinosine (DDI). Despite being effective in inhibiting human immunodeficiency virus replication, these drugs produce a range of toxicities, including myopathy, pancreatitis, neuropathy, and lactic acidosis, that are generally considered as sequelae to mitochondrial damage. Although cell surface–localized nucleoside transporters, such as human equilibrative nucleoside transporter 2 (hENT2) and human concentrative nucleoside transporter 1 (hCNT1), are known to increase the carrier-mediated uptake of 3′-deoxy-nucleoside analogs into cells, another ubiquitously expressed intracellular nucleoside transporter (namely, hENT3) has been implicated in the mitochondrial transport of 3′-deoxy-nucleoside analogs. Using site-directed mutagenesis, generation of chimeric hENTs, and ³H-permeant flux measurements in mutant/chimeric RNA–injected Xenopus oocytes, here we identified the molecular determinants of hENT3 that dictate membrane translocation of 3′-deoxy-nucleoside analogs. Our findings demonstrated that whereas hENT1 had no significant transport activity toward 3′-deoxy-nucleoside analogs, hENT3 was capable of transporting 3′-deoxy-nucleoside analogs similar to hENT2. Transport analyses of hENT3-hENT1 chimeric constructs demonstrated that the N-terminal half of hENT3 is primarily responsible for the hENT3–3′-deoxy-nucleoside analog interaction. In addition, mutagenic studies identified that 225D and 231L in the N-terminal half of hENT3 partially contribute to the ability of hENT3 to transport AZT and DDI. The identification of the transporter segment and amino acid residues that are important in hENT3 transport of 3′-deoxy-nucleoside analogs may present a possible mechanism for overcoming the adverse toxicities associated with 3′-deoxy-nucleoside analog treatment and may guide rational development of novel nucleoside analogs.

The effect of 2'-fluoro-2'-deoxycytidine on herpes virus growth

The effect of 2'-fluoro-2'-deoxycytidine (dCfl) on the growth of certain viruses of the herpes type was investigated. It is shown that the compound has considerable anti-viral activity against HSV-I, HSV-II, pseudorabies virus and equine abortion virus. It has an effect comparable to that of araC and is more efficient than br5dC, but less so than acyclovir. Experiments with thymidine kinase-negative strains of HSV-I indicated that dCfl was phosphorylated by the viral kinase, and its Km appears to be low and close to that of thymidine. Density gradient centrifugation enabled us to show that dCfl was incorporated into cellular and viral DNA and RNA. The cytotoxic activity of dCfl appears to be about 10-times smaller than that of araC. Removal of the nucleoside analog, washing and replacement with deoxycytidine reversed this effect, indicating rather a cytostatic than cytotoxic effect.

Modulation of fluoropyrimidine metabolism in L1210 cells by L-alanosine

L-Alanosine, an analogue of aspartic acid which inhibits the conversion of inosine monophosphate to adenosine monophosphate (AMP), was evaluated in L1210 cells as a modulator of 5-fluorouracil (FUra) and 5-fluorouridine (FUrd) metabolism. L-Alanosine resulted in increased intra-cellular levels of 5-phosphoribosyl-1-pyrophosphate (PRPP), enhanced FUra metabolism to ribonucleotide derivatives, and resulted in more FUra residues incorporated into RNA. Sequential addition of L-alanosine and FUra also resulted in synergistic cytotoxicity as determined by soft agar cloning. Adenine antagonized these biochemical and biological effects of L-alanosine. L-Alanosine also augmented the rate at which FUrd was metabolized and was also associated with a greater incorporation of the FUra residues into RNA. Cytotoxicity after sequential L-alanosine and FUrd was also synergistic. The mechanism by which L-alanosine altered the metabolism of FUrd, however, was different from the way in which it enhanced FUra metabolism in that aspartic acid and not adenine was able to reverse the effects of L-alanosine on FUrd metabolism and cytotoxicity. L-Alanosine appeared to augment the RNA-directed activity of FUra and FUrd in that there was no correlation between the enhanced metabolism and cytotoxicity of these two fluoropyrimidines and either levels of fluorodeoxyuridylate (FdUMP) which inhibits thymidylate (TMP) synthetase or inhibition of the ability of cells to incorporate deoxyuridylate into acid-precipitable material.

Pharmacokinetics of 5-fluorouracil: inter-relationship with biochemical kinetics in monitoring therapy

The major difficulty in using pharmacokinetic concepts for monitoring individual therapy with 5-fluorouracil is based on the mostly intracellular location of the active nucleotide metabolites of 5-fluorouracil with no clear correlation of plasma levels of the drug. In addition, the basic biochemical mechanism of 'thymine-less cell death' following inhibition of de novo thymidylate synthesis by 2'-deoxy-5-flurouridine 5'-monophosphate (FdUMP) is poorly understood, and only some of the biochemical determinants of therapeutic response to 5-fluorouracil are known. Individualised therapy with 5-fluorouracil requires an intergrated approach which should include methods of pharmacokinetics and biochemical kinetics. 5-Fluorouracil stands as an example for most of the pyrimidine and purine metabolites to which similar consideration apply in monitoring of cancer chemotherapy.