Zidovudine

The Discovery of Zidovudine

            Zidovudine (ZDV), also known as azidothymidine (AZT), is an antiretroviral medication typically used to treat HIV infection in adults and children +4 weeks old and to prevent mother-to-child transmission of HIV¹. ZDV was first described by Jerome P. Horowitz at the Barbara Ann Karmanos Cancer Institute as a part of an NIH funded anticancer program². Unfortunately, the compound showed no anticancer activity when tested on leukemic mice³. ZDV was then shelved. It was not until May 20, 1983, when researchers at the Institut Pasteur identified the retrovirus now known as HIV, that ZDV was revisited as a possible therapeutic⁴. This led research efforts to focus on a key enzyme, reverse transcriptase, as a target. A paper was published in October 1985 that demonstrated ZDV; (1) produced nearly no toxicity even at high doses to mice and dogs, (2) essentially completely inhibited viral replication without effecting T-cell immune reactivity, and (3) could be absorbed through oral administration⁵. On March 20, 1987, ZDV was FDA approved for use against HIV and AIDS. Only a short period of time had passed between demonstrated efficacy and FDA approval.

The mechanism of action is rather elegant and simple. ZDV belongs to the nucleoside analog reverse-transcriptase inhibitor (NARTI) class of reverse-transcriptase inhibitors and works because it is an analogue of the endogenous thymidine nucleoside. ZDV, in its inactive form, enters the cell and must be successively phosphorylated by cellular kinase enzymes, eventually yielding Zidovudine-triphosphate (ZDV-TP). ZDV-TP can then be incorporated into the growing DNA strand by the virus’ reverse-transcriptase enzyme. It has been shown that “… a relationship between the observed inhibitory effects of both AZT and nevirapine and the length of target template makes it unlikely that these drugs have any direct effect on [reverse transcriptase] through mechanisms other than direct inhibition of viral DNA chain elongation⁶.” This chain termination occurs because the deoxynucleotide that follows ZDV cannot form the 5’à3’ phosphodiester bond thus halting DNA elongation. While the safety of inhibiting DNA elongation is certainly a cause for concern, ZDV has been shown to inhibit HIV replication between 50-500 nM whereas inhibition of human fibroblasts and lymphocytes occurs at concentrations > 1 mM⁷. Furthermore, it has been theorized that cellular repair enzymes present in human cells are able to remove ZDV from the growing DNA strand. HIV has no such mechanism, which helps explain the noted low toxicity for human cells and high toxicity for the virus⁸. However, there are some adverse effects including flu-like symptoms, kidney and liver problems, and lipodystrophy⁹.

Regarding this assignment, it was somewhat surprising to discover that the newer generation of HIV medication, Nevirapine, is less effective than Zidovudine. This is because Nevirapine binds at the polymerase active site which can easily mutate to prevent binding. This is the case with HIV-2, against which Nevirapine is ineffective¹⁰. Resistance to ZDV can occur, though can be overcome when used in conjunction with foscarnet¹¹. Future development for ZDV may be limited. ZDV has been used for over three decades and long-term use is associated with resistance, though the mechanism of directly interrupting DNA elongation may continue to be utilized within HIV research.

While the science behind how the drug works is certainly interesting, a large portion of the drug’s history concerns both its commercial aspects and intellectual property. Shortly after the Institut Pasteur identified reverse transcriptase, the NCI and NIH quickly began research for compounds that were efficacious against HIV/AIDS. In 1984, Burroughs-Wellcome (BW) worked in conjunction with these government agencies in an attempt to find a treatment. During this time, Janet Rideout, the lead researcher for BW sent eleven compounds to the NCI for testing, one of which was AZT¹². NCI in vitro trials found AZT to be effective against HIV reverse transcriptase¹³. Following successful clinical trials, the FDA approved AZT in March 1987¹⁴ and BW filed for a patent in February 1988¹⁵. A legal battle ensued between BW and both Barr Laboratories and Novopharm. Barr Laboratories and Novopharm argued, unsuccessfully, that their respective work deserved to be recognized in the patent¹⁶. BW’s (later GSK) patent expired in 2005 and the FDA approved three generic versions¹⁷.

Since the expiration of the patent, ViiV Healthcare has taken over the proprietary name, Retrovir and dozens of companies have begun producing the generic version of AZT¹⁸. Considering AZT was one the first retrovirals drugs, it is also one of the most studied. There is a plethora of both pre-clinical and clinical trials from which data can be derived. Generally, it is a well-tolerated and efficacious therapeutic drug, especially when used in combination with various other retrovirals. Each manufacturer has their own Material Safety Data Sheet (MSDS) but the commonalities are that AZT is stable, non-reactive, non-flammable, and non-hazardous. However, the Globally Harmonized System of Classification and Labeling of Chemicals (GHS) indicate that AZT may have the following characteristics; germ cell mutagenicity, carcinogenicity, reproductive toxicity (effects on or via lactation), and specific target organ toxicity¹⁹.

References

1.     “Zidovudine Dosage, Side Effects.” National Institutes of Health, U.S. Department of Health and Human Services, 23 Feb. 2018, aidsinfo.nih.gov/drugs/4/zidovudine/0/patient.

2.     Horwitz, Jerome P. et al. “Nucleosides. V. The Monomesylates of l-(2'-Deoxy-β-D-Lyxofuranosyl)Thymine.” Journal of Organic Chemistry, vol. 29, no. 7, 1964, pp. 2076–2078.

3.     Special to the New York Times. “A FAILURE LED TO DRUG AGAINST AIDS.” The New York Times, The New York Times, 20 Sept. 1986, nytimes.com/1986/09/20/us/a-failure-led-to-drug-against-aids.html?sec=health.

4.     Barré-Sinoussi, F. et al. “Isolation of a T-Lymphotropic Retrovirus from a Patient at Risk for Acquired Immune Deficiency Syndrome (AIDS).” Science (Washington), vol. 220, no. 4599, 1983, pp. 868–870.

5.     Mitsuya, H. et al. “3'-Azido-3'-Deoxythymidine (BW A509U): An Antiviral Agent That Inhibits the Infectivity and Cytopathic Effect of Human T-Lymphotropic Virus Type III/Lymphadenopathy-Associated Virus in Vitro.” Proceedings of the National Academy of Sciences of the United States of America, vol. 82, no. 20, 1985, pp. 7096–7100.

6.     Quan, Y. et al. “Reverse Transcriptase Inhibitors Can Selectively Block the Synthesis of Differently Sized Viral DNA Transcripts in Cells Acutely Infected with Human Immunodeficiency Virus Type 1.” The Journal of Virology, vol. 73, no. 8, 1999, pp. 6700–6707.

7.     Furman, P. et al. “Phosphorylation of 3'-Azido-3'-Deoxythymidine and Selective Interaction of the 5'-Triphosphate with Human Immunodeficiency Virus Reverse Transcriptase.” Proceedings of the National Academy of Sciences of the United States of America, vol. 83, no. 21, 1986, pp. 8333–8337.

8.     Ostertag, W. et al. “Induction of Endogenous Virus and of Thymidine Kinase by Bromodeoxyuridine in Cell Cultures Transformed by Friend Virus.” Proceedings of the National Academy of Sciences of the United States of America, vol. 71, no. 12, 1974, pp. 4980–4985.

9.     “HIV Medicines and Side Effects Understanding HIV/AIDS.” National Institutes of Health, U.S. Department of Health and Human Services, 29 Aug. 2018, aidsinfo.nih.gov/understanding-hiv-aids/fact-sheets/22/63/hiv-medicines-and-side-effects.

10.  Ren, J. et al. “Structure of HIV-2 Reverse Transcriptase at 2.35-Å Resolution and the Mechanism of Resistance to Non-Nucleoside Inhibitors.” Proceedings of the National Academy of Sciences of the United States of America, vol. 99, no. 22, 2002, pp. 14410–14415.

11.  Sneader, Walter. Drug Discovery: A History. Wiley, 2006.

12.  Tachedjian, G. et al. “Zidovudine Resistance Is Suppressed by Mutations Conferring Resistance of Human Immunodeficiency Virus Type 1 to Foscarnet.” The Journal of Virology, vol. 70, no. 10, 1996, pp. 7171–7181.

13.  Mitsuya, H et al. “3'-Azido-3'-deoxythymidine (BW A509U): an antiviral agent that inhibits the infectivity and cytopathic effect of human T-lymphotropic virus type III/lymphadenopathy-associated virus in vitro” Proceedings of the National Academy of Sciences of the United States of America vol. 82,20 (1985): 7096-100.

14.  Brook, Itzhak. “Approval of Zidovudine (AZT) for Acquired Immunodeficiency Syndrome.” JAMA, American Medical Association, 18 Sept. 1987, jamanetwork.com/journals/jama/article-abstract/368218.

15.  Cochrane, James MT. “Zidovudine's Patent History.” The Lancet, vol. 356, no. 9241, 2000, pp. 1611–1612., doi:10.1016/s0140-6736(05)74463-9.

16.  “Burroughs Wellcome Co. v. Barr Laboratories, Inc., 828 F. Supp. 1208 (E.D.N.C. 1993).” Justia Law, law.justia.com/cases/federal/district-courts/FSupp/828/1208/2352375/.

17.  Office of the Commissioner. “HIV/AIDS History of Approvals - HIV/AIDS Historical Time Line 2000 - 2010.” U S Food and Drug Administration Home Page, Office of the Commissioner, www.fda.gov/ForPatients/Illness/HIVAIDS/History/ucm151081.htm.

18.  Approved Drug Products with Therapeutic Equivalence Evaluations, FDA, 2018.

19.  “Zidovudine.” National Center for Biotechnology Information. PubChem Compound Database, U.S. National Library of Medicine, pubchem.ncbi.nlm.nih.gov/compound/zidovudine#section=GHS-Classification&fullscreen=true.