Zalcitabine

Zalcitabine (2'-3'-dideoxycytidine, ddC), also called dideoxycytidine, is a nucleoside analog reverse transcriptase inhibitor (NARTI) sold under the trade name Hivid®.

The recommended dosage is 0.750 mg (one tablet) every 8 hours, as part of a combination regimen.

Zalcitabine appears less potent than some other nucleoside RTIs, has an inconvenient three-times daily frequency and is associated with serious adverse events. For these reasons it is now rarely used to treat human immunodeficiency virus (HIV).

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History

Zalcitabine was developed in the National Cancer Institute (NCI) by Samuel Broder, Hiroaki Mitsuya, and Robert Yarchoan at the National Cancer Institute (NCI). Like didanosine, it was then licensed because the NCI may not market or sell drugs. The National Institutes of Health (HIH) thus licensed it to Hoffman LaRoche.

Zalcitabine was the third antiretroviral to be approved by the Food and Drug Administration (FDA) for the treatment of HIV infection and AIDS. It was approved on Jun 19, 1992 as a monotherapy and again in 1996 for use in combination with Zidovudine (AZT). Using combinations of NRTIs was in practice prior to the second FDA approval and the triple drug combinations with dual NRTIs and a protease inhibitor (PI) were not far off by this time.

The sale and distribution of zalcitabine has been discontinued since December 31, 2006. ^[1]

Mechanism of action

Zalcitabine is an analog of pyrimidine. It is a derivative of the naturally existing deoxycytidine, made by replacing the hydroxyl group in position 3' with a hydrogen.

It is phosphorylated in T cells and other HIV target cells into its active triphosphate form, ddCTP. This active metabolite works as a substrate for HIV reverse transcriptase, and also by incorporation into the viral DNA, hence terminating the chain elongation due to the missing hydroxyl group. Since zalcitabine is a reverse transcriptase inhibitor it possess activity only against retroviruses.

Pharmacokinetics

Zalcitabine has a very high oral absorption rate of over 80%. It is predominantly eliminated by the renal route, with a half-life of 2 hours.^[2]

Drug interactions

Lamivudine (3TC) significantly inhibits the intracellular phosphorylation of zalcitabine to the active form, and accordingly the drugs should not be administered together.^[2]

Additionally, zalcitabine should not be used with other drugs that can cause peripheral neuropathy, such as didanosine and stavudine.^[2]

Adverse events

The most common adverse events at the beginning of treatment are nausea and headache. More serious adverse events are peripheral neuropathy, which can occur in up to 33% of patients with advanced disease, oral ulcers, oesophageal ulcers and, rarely, pancreatitis.^[2]

Resistance

Resistance to zalcitabine develops infrequently compared with other nRTIs, and generally only occurs at a low level.^[3] The most common mutation observed in vivo is T69D, which does not appear to give rise to cross-resistance to other nRTIs; mutations at positions 65, 74, 75, 184 and 215 in the pol gene are observed more rarely.^[2][3]

Sources

1. ^ HIVID (zalcitabine) tablets. Dear Health Care Professional Letter. (June 2006)
2. ^ ^{a b c d e} HIVID® (zalcitabine) tablets. Product information. (September 2002)
3. ^ ^{a b} Moyle GJ. Use of viral resistance patterns to antiretroviral drugs in optimising selection of drug combinations and sequences. Drugs 1996;52:168-185

Further reading

• Yarchoan R, Mitsuya H, Broder S. AIDS therapies. Scientific American 1988;259(4):110-9.
• Harvey Stewart C. in Remington's Pharmaceutical Sciences 18^th edition: (ed. Gennard, Alfonso R.) Mack Publishing Company, 1990
• Rang H.P., Dale M.M., Ritter J.M.: Pharmacology, 3^rd edition. Pearson Professional Ltd, 1995.
• Mitsuya H, Broder S. Inhibition of the in vitro infectivity and cytopathic effect of human T-lymphotropic virus type III/lymphadenopathy virus-associated virus (HTLV-III/LAV) by 2',3'-dideoxynucleosides. Proc Natl Acad Sci USA 1986;83:1911-5.
• Yarchoan R, Perno CF, Thomas RV, et al. Phase I studies of 2',3'-dideoxycytidine in severe human immunodeficiency virus infection as a single agent and alternating with zidovudine (AZT). Lancet 1988;1:76-81.
• Mitsuya H, Yarchoan R, Broder S. Molecular targets for AIDS therapy. Science 1990;249:1533-44.
• NIH Bio and Oral History of Samuel Broder describing the development of anti-HIV drugs: http://aidshistory.nih.gov/transcripts/bios/Samuel_Broder.html
• NIH Bio and Oral History of Robert Yarchoan describing the development of anti-HIV drugs: http://aidshistory.nih.gov/transcripts/bios/Robert_Yarchoan.html
• Moyle G. A re-evaluation of zalcitabine. Expert Opin Investig Drugs 1998;7:451-62