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.