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Antiretroviral drugs are medications for the treatment of infection by retroviruses, primarily HIV. Different classes of antiretroviral drugs act at different stages of the HIV life cycle. Combination of several (typically three or four) antiretroviral drugs is known as Highly Active Anti-Retroviral Therapy (HAART). more...

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Organizations such as the National Institutes of Health (Bethesda, Maryland, USA) recommend offering antiretroviral treatment to all patients with HIV-related symptoms. However, because of the complexity of selecting and following a regimen, the severity of the side effects, and the importance of compliance to prevent viral resistance, such organizations emphasize the importance of involving patients in therapy choices and recommend analyzing the risks and the potential benefits to patients without symptoms.

Combination therapy

The life cycle of HIV can be as short as about 1.5 days: from viral entry into a cell; through replication, assembly, and release of additional viruses; to infection of other cells. HIV lacks proofreading enzymes to correct errors made when it converts its RNA into DNA via reverse transcription. Its short life cycle and high error rate cause the virus to mutate very rapidly, resulting in a high genetic variability of HIV. Most of the mutations either are inferior to the parent virus (often lacking the ability to reproduce at all) or convey no advantage, but some of them have a natural selection superiority to their parent and can enable them to slip past defenses such as the human immune system and antiretroviral drugs. The more active copies of the virus, the greater the possibility that one resistant to antiretroviral drugs will be made, so antiretroviral combination therapy defends against resistance by suppressing HIV replication as much as possible.

Combinations of antiretrovirals create multiple obstacles to HIV replication to keep the number of offspring low and reduce the possibility of a superior mutation. If a mutation arises that conveys resistance to one of the drugs being taken, the other drugs continue to suppress reproduction of that mutation. With rare exceptions, no individual antiretroviral drug has been demonstrated to suppress an HIV infection for long; these agents must be taken in combinations in order to have a lasting effect. As a result the standard of care is to use combinations of antiretroviral drugs.

Combinations of antiretrovirals are subject to positive and negative synergies, which limits the number of useful combinations. For example, ddI and AZT inhibit each other, so taking them together is less effective than taking either one separately. Other issues further limit some people's treatment options from antiretroviral drug combinations, including their complicated dosing schedules and often severe side effects.

Current treatment guidelines

Antiretroviral drug treatment guidelines have changed many times. Early recommendations attempted a "hit hard, hit early" approach. A more conservative approach followed, with a starting point somewhere between 350 and 500 CD4+ T cells/mm³. The current guidelines use new criteria to consider starting HAART, as described below. However, there remain a range of views on this subject and the decision of whether to commence treatment ultimately rests with the patient and their doctor.

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Lexiva: blood levels not lowered when taken simultaneously with Nexium
From AIDS Treatment News, 5/27/05 by John S. James

On April 29, 2005 GlaxoSmithKline announced that a drug-interaction trial of Lexiva (fosamprenavir) with Nexium (esomeprazole, a proton-pump inhibitor, often used to reduce stomach acidity for relief of reflux, the unwanted flow of stomach acid into the esophagus), had found no reduction in the in the blood level of amprenavir. The data were presented at the 6th International Workshop on Clinical Pharmacology of HIV Therapy, in Quebec [1].

The FDA-approved "label" for Lexiva (the official prescribing information for physicians) urges caution in using the drug together with proton-pump inhibitors, because experience with other drugs suggested that the blood level of the antiviral might be decreased, making it less effective (no test with Lexiva and Nexium had been done). But the company's random-ized crossover trial in 48 HIV-negative volunteers who took Lexiva for two weeks with Nexium, and later for two weeks without, found that this did not happen, at least with the 20 mg dose of Nexium tested (the drug is supplied in 20 and 40 mg doses)--whether or not the Lexiva was "boosted" with a small dose of Norvir.

There was speculation that the timing of the doses might be important--that because they were taken simultaneously in this study, the Lexiva might be absorbed before the Nexium had time to reduce stomach acidity. A previous study had shown a 30% reduction in blood concentration of amprenavir when Zantac (another drug for reducing stomach acidity) was given one hour before Lexiva [2].

Currently (June 2005) the official prescribing information is still the December 2004 version. Glaxo is seeking FDA permission to change it, in view of the new data. Any version with a later date should reflect the FDA's evaluation of the new information.

The current prescribing information is at http://www.lexiva.com/hcp/prescribingInformation.html. Look for the date at the end of the file.

Note: At the same conference Glaxo presented a similar study showing that tenofovir (Viread) did not affect amprenavir levels when used with Lexiva. This is consistent with prior information.

Comment

We urgently need more drug-interaction trials to guide physicians and reduce guesswork when multiple medications are prescribed. Companies that try to save money by skipping these trials put at risk a far larger investment, if their major drugs cannot be used optimally. These trials could aim for simple rules (such as choice of acid-reducing drugs, and delays when necessary) to avoid problems with antiretrovirals or other critical drugs that require stomach acidity in order to be fully absorbed. And small drug-level tests should check that results developed with HIV-negative volunteers do work for HIV-positive patients.

References

(1.) Shelton MJ, Ford SL, Wire MB, and others. Coadministration of esomeprazole (ESO) with fosamprenavir (fAPV) has no impact on steady-state plasma amprenavir (APV) pharmacokmetics (APV10031). 6th International Workshop on Clinical Pharmacology of HIV Therapy, April 28-30, 2005, Quebec City, Quebec [Abstract 24].

(2.) See "Gastric pH Modifiers" section on the Medscape (you need to register first, but that is free), http://www.medscape.com/viewarticle/505545.

COPYRIGHT 2005 John S. James
COPYRIGHT 2005 Gale Group

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