Nucleoside substrate drugs

The recent determination of the crystal structure of a ternary catalytic complex of HW-1 RT with a substrate (dTTP) and the DNA-primer and template [121] (Fig. 8) has provided the structured basis of resistance it has been found that most mutations causing resistemce to nucleoside-analog drugs are located closely to the nucleoside binding site. 

As a salutary post script to the discussion that has been presented above, a final example of innovation is presented, an example which has caused a profound change in direction in the search of anti-AIDS drugs. Until a little over a year ago, all research into inhibitors of reverse transcriptase (RT, see above) relied upon mimicry of the nucleoside substrates in the polymerase reaction. Here is an enzyme for which the reaction pathway is known only in bald outline and for which there is no structural information. Janssen s approach was masterly [84]. A library of 600 molecules, each prototypes of different chemical series and without activity in standard pharmacological assays, were screened for anti-HIV activity in vitro. It was discovered that (45) had modest but specific anti-HIV activity and lead optimisation eventually uncovered (46) and (47) as representatives of the TIBO series. It was subsequently shown that these compounds act as non-competitive inhibitors of RT, an indication that even if structural data were available, they would be useless in developing the series towards a drug candidate. As if by coincidence, several... 

Both influx and efflux transporters are located in intestinal epithelial cells and can either increase or decrease oral absorption. Influx transporters such as human peptide transporter 1 (hPEPTl), apical sodium bile acid transporter (ASBT), and nucleoside transporters actively transport drugs that mimic their native substrates across the epithelial cell, whereas efflux transporters such as P-glycoprotein (P-gp), multidrug resistance-associated protein (MRP), and breast cancer resistance protein (BCRP) actively pump absorbed drugs back into the intestinal lumen. 

Didanosine is a synthetic purine nucleoside analog that inhibits the activity of reverse transcriptase in HIV-1, HIV-2, other retroviruses and zidovudine-resistant strains. A nucleobase carrier helps transport it into the cell where it needs to be phosphorylated by 5 -nucleoiidase and inosine 5 -monophosphate phosphotransferase to didanosine S -monophosphate. Adenylosuccinate synthetase and adenylosuccinate lyase then convert didanosine 5 -monophosphate to dideoxyadenosine S -monophosphate, followed by its conversion to diphosphate by adenylate kinase and phosphoribosyl pyrophosphate synthetase, which is then phosphorylated by creatine kinase and phosphoribosyl pyrophosphate synthetase to dideoxyadenosine S -triphosphate, the active reverse transcriptase inhibitor. Dideoxyadenosine triphosphate inhibits the activity of HIV reverse transcriptase by competing with the natural substrate, deoxyadenosine triphosphate, and its incorporation into viral DNA causes termination of viral DNA chain elongation. It is 10-100-fold less potent than zidovudine in its antiviral activity, but is more active than zidovudine in nondividing and quiescent cells. At clinically relevant doses, it is not toxic to hematopoietic precursor cells or lymphocytes, and the resistance to the drug results from site-directed mutagenesis at codons 65 and 74 of viral reverse transcriptase. 

NRTIs, which act as a substrate for RT, are not the only means of inhibiting reverse transcriptase. Like any enzyme, inhibitors can also bind allosteric positions away from the active site. An allosteric site on RT has been successfully targeted by drugs, and these drugs are called non-nucleoside reverse transcriptase inhibitors (NNRTIs). The three most frequently prescribed NNRTIs are shown in Figure A.46. 

Finally, transport can also be driven by the conversion of intracellular substrate to another chemical form. For example, in the case of nucleoside drugs, conversion to the corresponding nucleotides by appropriate kinases may be the limiting factor in cellular uptake and activation. The same principle applies to sulfation, glu euro nidation, prodrug activations, or other metabolic processes that provide a removal of the transported species from the transportable (free) internal pool. In some cases, transport is directly coupled to substrate modification, as in the uptake of sugars into bacterial cells by phosphoenolpyruvate (PEP)-coupled phosphorylation systems. 

These examples are unusual in that vaiacyciovir is an amino acid ester of a nucleoside that does not closely resemble the normal dipeptide substrates of the PEPT-1 transporter. A number of other drugs (such as methotrexate) are probably transported by proteins that normally transport the metabolites that they resemble and antagonize (e.g., folates). However, these cases represent fortuitous examples of drug transportability Tiatural selection" during the drug discovery and development process. With increased understanding of the specificity determinants of... 

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