Nucleoside drug sensitivity

Cells differ in their reliance on nucleoside uptake and salvage versus de novo biosynthetic pathways for normal growth, and, hence, they differ in their sensitivity to nucleoside drugs. Table 14.5 [adapted from Tables 1-4 in Cass (61)] lists some nucleoside drugs, diseases for which they have been used, and the transporters that recognize them. In addition to the es, ei, and N1-N5 nucleoside transporters, some nucleoside drugs also utilize nucleobase (NB) transporters. 

Evidence obtained from in vitro studies (cultured cell models) clearly suggests that NTs contribute to nucleoside-derived drug cytotoxicity. Nevertheless, key issues are to determine whether these findings can be transferred to clinical settings and whether they will help to understand what roles NT proteins actually play in tumor responsiveness to nucleoside-based therapies. However, the lack, until recently, of suitable molecular tools such as isoform-specific anti-NT antibodies and the difficulty to obtain and analyze tumor samples have delayed this approach. Clinical studies suggesting a link between NT expression and drug sensitivity are summarized in Table 2.2. 

Adenosine is a nucleoside that occurs naturally throughout the body. Its half-life in the blood is less than 10 seconds. Its mechanism of action involves activation of an inward rectifier K+ current and inhibition of calcium current. The results of these actions are marked hyperpolarization and suppression of calcium-dependent action potentials. When given as a bolus dose, adenosine directly inhibits atrioventricular nodal conduction and increases the atrioventricular nodal refractory period but has lesser effects on the sinoatrial node. Adenosine is currently the drug of choice for prompt conversion of paroxysmal supraventricular tachycardia to sinus rhythm because of its high efficacy (90-95%) and very short duration of action. It is usually given in a bolus dose of 6 mg followed, if necessary, by a dose of 12 mg. An uncommon variant of ventricular tachycardia is adenosine-sensitive. The drug is less effective in the presence of adenosine receptor blockers such as theophylline or caffeine, and its effects are potentiated by adenosine uptake inhibitors such as dipyridamole. 

For HSV at least three mechanisms have been described that generate resistance to AC V deficiency or loss of viral TK activity, alteration in substrate specificity of the virus-encoded TK, and alteration in the substrate specificity of the viral DNA polymerase (1,8). Most of the ACVr mutants that have been isolated in vitro and recovered from clinical specimens are TK-deficient (TK). However, resistant clinical mutants that have an altered TK or altered DNA polymerase activity have occasionally been described too. Although TK mutants are crossresistant with drugs that also depend on viral TK for their activation (i.e., GCV, penciclovir and brivudin (BVDU), they remain sensitive to agents, such as PFA, vidarabine (Ara-A), and the acyclic nucleoside phosphonate (ANP) analogs. PFA, a pyrophosphate analog, is a direct inhibitor of the viral DNA poly-merase in which it binds to the site involved in releasing the pyrophosphate product of DNA synthesis. Phosphorylation of Ara-A to Ara-A triphosphate is carried out by cellular enzymes phosphorylation of ANP derivatives to their mono- and diphosphoryl derivatives is also carried out by cellular enzymes. 

A sensitive method to measure toxic effects of drugs is to monitor cellular metabolism by incorporation of 3H-thymidine or 14C-protein hydrolysate. 14C-protein is used in preference to 3H-thymidine when it is necessary to avoid competition in uptake between the labeled thymidine and an unlabeled nucleoside reverse-transcriptase inhibitor. These methods are used mainly to monitor sublethal toxicity after initial screening or when comparing structure-activity relationships. The assays described here are carried out using 6-mL culture tubes (Note 1). These assays are carried out in parallel with the antiviral assay. 

Analysis of the Role of NTs in Sensitivity to Nucleoside Anticancer Drugs in Cultured Cell Models... 

The pharmacological blockade of ENT-type transport activities might increase sensitivity to nucleoside derived presumably by inhibiting efflux path ways [114—116]. Actually, sensitivity is promoted when drugs reach cells prior to the inhibition ofthe transporter function, whereas treatment of cells after exposure to NBTI results in resistance [117]. Consistent with this, in human tumoral cell lines, sequential treatment with the ENT blocker dipyridamole 2 h after their initial exposure to cytarabine increased the cytotoxicity of this nucleoside analogue and resulted in an increase in the cellular pools of cytarabine and its metabolites [118]. 

Lang, T.T., Seiner, M., Young, J.D., and Cass, C.E. (2001) Acquisition of human concentrative nucleoside transporter 2 (hcnt2) activity by gene transfer confers sensitivity to fluoropyrimidine nucleosides in drug-resistant leukemia cells. Molecular Pharmacology, 60 (5), 1143-1152. 

Zidovudine is FDA-approved for the treatment of adults and children with HIV infection and for preventing mother-to-child transmission it also is recommended for postexposure prophylaxis in HIV-exposed healthcare workers, also in combination with other antiretroviral agents. The standard of care for treatment-naive patients (Table 50-lB) is to combine zidovudine with a potent protease inhibitor and another nucleoside analog or with an NNRTl and another nucleoside analog. The Met-to-Val substitution at reverse transcriptase codon 184 associated with use of lamivudine greatly restores sensitivity to zidovudine, and these drugs are often used in combination. 

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