Medical Pharmacology Chapter 36:  Antiviral Drugs

Nucleoside/Nucleotide Reverse Transcriptase Inhibitors

• Antiretroviral Drugs Used in Treating HIV Infection

  • →Nucleoside/Nucleotide Reverse Transcriptase Inhibitors

    • Sites of Antiretroviral Drug Actions¹⁶

      a: The lifecycle of HIV-1, showing the step at which enfuvirtide acts, and also the steps targeted by other drugs.16  b:  "Simplified schematic of the mechanism by which enfuvirtide inhibits viral entry into host cells.16    The envelope glycoprotein of HIV-1 consists of two non-covalently associated subunits, gp120 and gp41. After attachment of HIV-1 to its target cells carrying the CD4 receptor, gp120 interacts with the CD4 receptor, which initiates a series of conformational changes in gp41 and gp120 that lead to the insertion of a region of gp41 into the membrane of the host cell, and formation of a ' pre-hairpin intermediate' (top). Further changes in the confirmation of gp41 bring the viral and cellular membranes into close enough proximity for membrane fusion (bottom left). Abbreviations: NNRTI, non-nucleoside reverse transcriptase inhibitor; NRTI, nucleoside reverse transcriptase inhibitor; PI, protease inhibitor" Attribution:  Slight modification of Figure 1 from:  LaBonte J Lebbos J Kirkpatrick nature Reviews Drug Discovery 2, 345-346 (May 2003).16

       

    • Nucleoside Reverse Transcriptase Resistance¹⁷

      Attribution: Wood BR Nucleoside Reverse Transcriptase Resistance (NRTI Resistance) https://www.youtube.com/watch?v=UHNcx6zrZwc (5/2014) MWAETC:  Project ECHO https://www.youtube.com/channel/UCqgJ2z1oIAhA-ZB_WocHibw

       

    • Zidovudine (AZT)

      • Zidovudine (AZT) is a thymidine analogue suitable for oral administration. Usually, zidovudine is formulated along with lamivudine for daily dosing (2X).⁶ 

        • Structural Comparison Between Thymidine and Zidovudine⁶

          Thymidine

          Zidovudine (AZT)

           

        • Dosage adjustment for zidovudine may be required for those patients with sufficient renal impairment requiring hemodialysis, peritoneal dialysis or continuous venovenous hemofiltration.

          • Zidovudine has an oral bioavailability of about 65% as a result of first pass (phase II) hepatic glucuronidation.

            • Phase II characteristics: Parent drug participates in conjugation reactions that: Form covalent linkage between a parent compound functional group and: Glucuronic acid Sulfate Glutathione Amino acids Acetate

          • Zidovudine does not induce the microsomal cytochrome P450 drug metabolizing system and is not a substrate for that system either.

        • Zidovudine should not be co-administered with stavudine a.k.a. d4T as a result of in vivo (and in vitro) antagonism..⁶  

          • The assessment of this antagonism was based on a randomized clinical study which involved adding stavudine (d4T) or didanosine (DDI) to HIV-infected individuals receiving zidovudine.⁹ 

            • Since both zidovudine and stavudine require thymidine kinase-mediated phosphorylation for activation, the parent compound are "prodrugs".

            • Furthermore zidovudine exhibits a >100-fold affinity for thymidine kinase compared to stavudine.

            • This characteristic was reflected in measurements of intracellular stavudine-triphosphate.

            • In absence of zidovudine, intracellular stavudine triphosphate levels were about 65 fmol/10⁶ cells (stavudine alone).

              • By contrast, levels of stavudine triphosphate levels in patients also receiving zidovudine were about 10 fmol/10⁶ cells or about 6-fold less.

            • These results, albeit in a small clinical study, were consistent with findings in the in vitro setting.⁹ 

      • Combination of zidovudine and stavudine represented a theoretical risk, since both drugs depend on intracellular phosphorylation by cellular thymidine kinase.⁴ 

        • The phosphorylation step is important because zidovudine is sequentially converted to 5'-mono-, di-, and triphosphates by thymidine kinase.

          • Zidovudine so phosphorylated is incorporated into proviral DNA, important because viral reverse transcriptase uses zidovudine-triphosphate as a substrate.

          • The tri-phosphorylated form prevents normal 5', 3'-phosphodiester bonding, thus resulting in DNA chain elongation termination due to the presence of the azido group in zidovudine (AZT).⁴ 

            • Chain termination occurs because zidovudine 5'-triphosphate which is incorporated into nascent DNA lacks a 3'-hydroxyl group.¹ 

              • The first phosphorylation step of zidovudine, yielding the monophosphate, results in a form that competitively inhibits cellular thymidylate kinase.¹ 

                • This competitive inhibition reduces quantities of intracellular thymidine triphosphate.

              • Zidovudine 5'-triphosphate slightly inhibits cellular DNA polymerase-α; however, this compound is a stronger inhibitor of mitochondrial polymerase-γ.¹ 

                • Conversion of zidovudine 5' monophosphate to diphosphate is relatively slow, resulting in monophosphate accumulation in cells.

                • Elevated concentrations of intracellular monophosphate acts as a reservoir and as a result, elevation in plasma zidovudine may not proportionally increase intracellular zidovudine triphosphate.¹

            •  

              Zidovudine (note the azido group)

            •  

              Structural Comparison Between Thymidine and Zidovudine⁶

              Thymidine

              Zidovudine (AZT)

        • Mutations and Zidovudine Resistance

          • Clinical resistance to zidovudine anti-retroviral activity is due to mutations in the viral reverse transcriptase enzyme.¹ 

             

            • Development of Antiretroviral Resistance (nucleoside analogue class)¹⁴

              Attribution:  Wainberg M Mechanisms of Nucleoside Analogue Antiretroviral Resistance:  NRTI Resistance, https://www.youtube.com/watch?v=Hrd1kNmqEOs (5/2014) MWAETC:  Project ECHO https://www.youtube.com/channel/UCqgJ2z1oIAhA-ZB_WocHibw

            • These mutations have been localized to specific codons.¹¹ 

              • At codon 41 the amino acid leucine is substituted for the normally occurring wild-type amino acid methionine.

              • At codon 67 the amino acid asparagine is substituted for the normally occurring wild-type amino acid aspartate.

              • At codon 70 the amino acid arginine is substituted for the normally occurring wild-type amino acid lysine.

              • At codon 210 the amino acid tryptophan is substituted for the normally occurring wild-type amino acid leucine

              • At codon 215 either the amino acid tyrosine or phenylalanine is substituted for the normally occurring wild-type amino acid threonine .

              • At codon 219 either the amino acid glutamine or glutamate is substituted for the normally occurring wild-type amino acid lysine.

                • These mutations have been described as "thymidine analogue mutations" or TAMs since they cause cross-resistance to other thymidine analogues including stavudine.¹ 

                • Two groups of resistance mutations frequently occur.¹  

                  • For example, mutations at codons 41, 210, and 215 is correlated with significant zidovudine resistance and cross-resistance to tenofovir and abacavir.

                    • Reduced levels of resistance and cross-resistance are associated with mutations at codons 67, 70 and 219.

                • The consequence of thymidine analogue mutations to zidovudine and stavudine is one that enhances excision of incorporated nucleotide anabolites by means of pyrophosphorolysis.¹ 

                  • Pyrophosphorolysis is the reverse reaction of nucleotide polymerization.¹²  

                    • Effectiveness of zidovudine and related compounds is based on DNA chain termination due to the absence of the 3'-hydroxyl group.

                    • In pyrophosphorolysis the viral reverse transcriptase enzyme complexed with the dideoxynucleoside monophosphate (ddNMP)-terminated primer binds pyrophosphate or ATP, results in removal of chain-terminating ddNMP, causing an unblocked DNA chain.

                    • Binding pyrophosphate or ATP drives the reverse reaction. (http://www.ncbi.nlm.nih.gov/pubmed/12069972 )¹²

            • When used as a single antiretroviral drug, zidovudine is associated with clinical resistance has mutations gradually develop.¹ 

              • About 30% of patients experience zidovudine resistance after 1 year of zidovudine monotherapy.

                • Prolonged therapy, however, results in cross-resistance to various nucleoside analogues.

                  • A mutation at codon 69, a threonine to serine mutation, induces cross-resistance to "all available nucleoside and nucleotide analogues".¹  

            • Administration of lamivudine or emtricitabine results in a particular mutation at codon 184 (a methionine to valine substitution) in the viral reverse transcriptase gene that restores zidovudine sensitivity.¹⁰ 

            • Combination treatment of zidovudine and lamivudine has been shown to extend long-term plasma HIV RNA suppression compared to zidovudine by itself.¹³ 

              • Consequently, zidovudine may be combined with lamivudine in the clinical setting.¹

        • Pharmacokinetics:¹

          • Following oral administration, zidovudine is rapidly absorbed, reaching peak plasma levels in about an hour.

            • The elimination t_½ for the parent compound (prodrug) is about an hour, which is shorter than that of the intracellular triphosphate active compound by about a factor of four (i.e. about 3-4 hours).

            • Zidovudine exhibits first-pass hepatic metabolism (phase II glucuronidation), reducing bioavailability to about 65%.

            • The pharmacokinetic characteristics of zidovudine is not notably affected in pregnancy with the drug concentration in the newborn similar to that in the mother.

            • The parent agent, zidovudine, crosses the blood brain barrier fairly well with a CSF to plasma ratio (cerebrospinal fluid: plasma ratio) of about 0.6.

              • Zidovudine can be detected in breast milk, fetal tissue and semen.¹

        • Adverse Effects:⁷

          • The most common zidovudine adverse effect is myelosuppression associated with macrocytic anemia (about 2%) or neutropenia (about 5%).⁷ 

          • Some adverse effects develop initially and then resolve during continued treatment.

            • Examples of these effects include headaches, insomnia, and gastrointestinal issues.

            • Lipoatrophy appears more frequently in patients receiving zidovudine or other thymidine analogue agents.

              • "Lipoatrophy:  Facial Fat Loss"¹⁵

                Chow DC Day LJ Shikuma CM Lipoatrophy: facial fat loss

                 

          • Less frequently observed toxicities include thrombocytopenia, nail hyperpigmentation and myopathy.

          • CNS effects may be noted at high doses, presenting as anxiety, confusion and tremor.

          • Co-administration of zidovudine with certain other agents result in increased zidovudine serum concentrations.

            • Elevated zidovudine in this setting can occur by:

              • Reduction of first-pass metabolism or by

              • Decreased drug clearance.

            •  Drugs which can, upon concurrent zidovudine administration, result in elevated zidovudine serum levels

              Probenecid	Methadone		Phenytoin
              Fluconazole	Atovaquone	Valproic acid	Lamivudine

            • By contrast, zidovudine administration may reduce phenytoin levels.

            • Increased hematological toxicity may be observed upon co-administration of other myelosuppressive agents such as:

              • Ganciclovir

              • Ribavirin

              • Cytotoxic drugs

            • As discussed earlier, combination protocols containing zidovudine and stavudine should be avoided because of drug-drug antagonism (competition for intracellular phosphorylation).⁷

        • Therapeutics:¹

          • Zidovudine (Retrovir, AZT) is approved (FDA) both for treating adults and children infected with HIV and for preventing mother-to-child HIV transmission.¹ 

            • Given the long time and broad experience with zidovudine, zidovudine remains recommended for post-exposure prophylaxis in HIV-exposed health-care workers.

              • This conclusion has been described in a 2005 Centers for Disease Control and Prevention (CDC) report (here) which has been slightly modified in 2013 (here).

                • Zidovudine is available as oral tablets, capsules, syrup and solution, including a solution suitable for intravenous injection.¹

                •  

                  Examples of Zidovudine Formulations

              • Zidovudine is also available in coformulation tablets with other antiretroviral drugs.¹  

                • For example, zidovudine may be compounded with lamivudine (Combivir) or with lamivudine and abacavir (Trizivir).

                  • As monotherapy, zidovudine decreases perinatal HIV transmission risk by about 67%.

                    • In this application use of zidovudine with other antiretroviral drugs is even more effective in preventing HIV perinatal transmission.¹