Chemical structure of lamivudine.
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Lamivudine

Lamivudine (2',3'-dideoxy-3'-thiacytidine, 3TC) has the trade name EpivirĀ®. It is a potent reverse transcriptase inhibitor of the class nucleoside analog reverse transcriptase inhibitor (NARTI). more...

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Lamivudine has been used for treatment of chronic hepatitis B at a lower dose than for treatment of HIV. It improves the seroconversion of e-antigen positive hepatitis B and also improves histology staging of the liver. Long term use of lamivudine unfortunately leads to emergence of a resistant hepatitis B virus (YMDD) mutant. Despite this, lamivudine is still used widely as it is well tolerated.

History

Lamivudine was invented by Bernard Belleau and Nghe Nguyen-Ba at the Montreal-based IAF BioChem International, Inc. laboratories in 1989. The drug was later licensed to the British pharmaceutical company Glaxo for a 14 percent royalty.

Lamivudine was approved by the Food and Drug Administration (FDA) on Nov 17, 1995 for use with Zidovudine (AZT) and again in 2002 as a once-a-day dosed medication. The fifth antiretroviral drug on the market, it was the last NRTI for three years while the approval process switched to protease inhibitors. Its patent will expire in the United States on 2016-05-18.

Mechanism of action

Lamivudine is an analogue of cytidine. It can inhibit both types (1 and 2) of HIV reverse transcriptase and also the reverse transcriptase of hepatitis B. It needs to be phosphorylated to its triphosphate form before it is active. 3TC-triphosphate also inhibits cellular DNA polymerase.

Lamivudine is administered orally, and it is rapidly absorbed with a bio-availability of over 80%. Some research suggests that lamivudine can cross the blood-brain barrier. Lamivudine is often given in combination with zidovudine, with which it is highly synergistic. Lamivudine treatment has been shown to restore zidovudine sensitivity of previously resistant HIV. Several mutagenicity tests show that lamivudine should not show mutagenic activity in therapeutical doses.


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Failure of tenofovir + abacavir + 3TC combination; full report published, more insight
From AIDS Treatment News, 10/1/05 by John S. James

In 2003 researchers stopped a clinical trial abruptly when they found that a combination of three drugs that seemed likely to work well together failed to control the virus in many patients, and led to a very high rate of viral resistance; a comparison regimen that used efavirenz instead of the tenofovir worked well. A similar combination (tenofovir plus abacavir plus ddI) also failed. However, regimens that include AZT do not fail in this way. The problem was completely unexpected, and since it occurred, several different theories were proposed.

No one knows the answer for sure--and we will probably never know, since it would be unethical to test the failed triple combinations in patients with HIV. But many researchers now believe the most likely explanation is that the drug combinations failed to control the virus because of a low genetic barrier to HIV resistance. Two mutations that developed in this trial protect the virus against the three drugs.

The December 1, 2005 Journal of Infectious Diseases published the full report of the trial [1] and a commentary [2]; both are available free online.

"The most likely explanation is the low genetic barrier to resistance produced by synergistic selection pressure from all 3 drugs for 2 point mutation, M184V and K65R. Both abacavir and tenofovir DF select for the K65R mutation, which reduces susceptibility to both drugs, as well as to lamivudine [3TC]. M184V is selected for by lamivudine and abacavir, and it decreases susceptibility to both. Thus, the selection of 2 mutations, each of which may preexist as minority species, leads to virologic failure with this regimen." [1]

The authors conclude that physicians should not try new combinations on their own as the first antiretroviral treatment for patients. "Given the growing number of potent, well-studied combinations now available, there is no longer a rationale for the use of untested regimens outside of clinical trials in the treatment of therapy-naive HIV-infected patients." [1]

References

Note: Both articles below were made available free online by the journal; you do not need to be a subscriber.

They are available through the Table of Contents of the December 1 issue, http://www.journals.uchicago.edulJID/journal/contents/v192n11.html

(1.) Gallant JE, Rodriguez AE, Weinberg WG, and others. for the ESS30009 study. Early virologic nonresponse to tenofovir, abacavir, and lamivudine in HIV-infected antiretroviral-naive subjects. Journal of Infectious Diseases. December 1,2005; volume 192, pages 1921-1930.

(2.) Kuritzkes DR. Less than the sum of its parts: Failure of a tenofovir-abacavir-lamivudine triple-nucleoside regimen. Editorial commentary. Journal of Infectious Diseases. December 1,2005; volume 192, pages 1867-1868.

COPYRIGHT 2005 John S. James
COPYRIGHT 2005 Gale Group

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