Direct link found between early sign of cardiovascular disease risk and AZT, possibly other NRTIs

AZT (zidovudine, Retrovir) alone and in combination with the protease inhibitor (PI) indinavir (Crixivan) directly causes damage to the lining of blood vessels, according to a study in rats published in the August 1^st issue of the Journal of Acquired Immune Deficiency Syndrome. This early sign of hardening of the arteries (atherosclerosis) suggests that certain nucleoside reverse transcriptase inhibitors (NRTIs) may directly contribute to the increased cardiovascular risk associated with PI therapy.

Earlier this year, at the Thirteenth Conference on Retroviruses and Opportunistic Infections (CROI) in Denver, an analysis of the Data Collection on Adverse Events of Anti-HIV Drugs (D:A:D) study found that the 16% per-year increased risk of heart attack seen in patients taking potent antiretroviral therapy is caused by PIs and not by non-nucleoside reverse transcriptase inhibitors (NNRTIs).

Although the D:A:D investigators did not determine an effect of the (NRTI) backbone taken by the patients in the study, they did find that that even after adjusting their analysis for blood fat (lipid) levels, there remained a residual risk that could not be accounted for by the PI class' effect on lipid levels alone.

Following initial test tube studies, investigators from Louisiana State University hypothesised that some NRTIs may adversely affect the lining of the blood vessels (vascular endothelium), leading to their reduced flexibility in response to blood flow, even in the absence of lipid abnormalities. This reduced flexibility, called 'endothelial dysfunction', is an early indicator of the hardening and narrowing of the arteries (atherosclerosis), one of the earliest detectable signs of the development of cardiovascular disease.

In order to test their hypothesis, the investigators treated rats with a month of AZT and/or indinavir, achieving levels similar to those taken by humans. They then measured blood fats (plasma lipids) and looked for various signs of endothelial dysfunction. These included measuring the ability of the main artery that carries blood from the heart to the rest of the body (the aorta) to relax, as well as measuring levels of a protein known as endothelin-1 (ET-1).

To test whether these antiretrovirals had a direct effect on the endothelium, they used the drug acetylcholine. This drug relies on the presence and proper functioning of the endothelium to relax blood vessels. They found that the ability of the aorta to relax was dramatically impaired by AZT and AZT plus indinavir.

To confirm that the changes in aortic relaxation were indeed due to the antiretrovirals, the investigators also used the drug sodium nitroprusside. This drug stimulates the blood vessels to relax independently of the endothelial cells. Since this drug caused normal aortic relaxation, the investigators write that this suggests "that a specific impairment of vascular endothelial function was induced by treatment with AZT or AZT plus indinavir."

The investigators also measured levels of ET-1, an amino acid peptide produced by the endothelium to make blood vessels contract. An increase in ET-1 levels indicates damage or injury to endothelial cells, and increases in ET-1 release have been shown to be correlated with atherosclerosis. They found significantly increased levels of ET-1 in the blood of the rats treated with the combination of AZT and indinavir in combination (p

Since endothelial dysfunction and ET-1 levels can also be associated with increased blood fat levels, the investigators also measured total cholesterol and triglyceride levels. They found that only indinavir alone significantly increased total cholesterol levels and that triglycerides were not significantly increased in the presence of either drug.

The investigators explain that since ET-1 levels are usually very low, and are difficult to detect, they were not surprised that differences in ET-1 levels could not be detected in the presence of AZT or indinavir alone. They suspect that the increased levels seen when the drugs are used in combination may confirm that AZT's direct effects on endothelial function add to indinavir's indirect effects through increasing blood cholesterol levels.

There are some limitations to this study. First of all, these experiments were carried out on rats, not in humans, and may not be reproducible or clinically relevant in humans.

In addition, the investigators tested two antiretrovirals that are being used less often in rich countries, although since it is now cheap and off-patent, AZT may well continue to be used worldwide for many years to come, particularly in poorer countries. The investigators used AZT and indinavir because they had previously undertaken test tube experiments with these two drugs (at a time when they were being used routinely in developed countries with access to antiretroviral therapy) and had found they induced endothelial cell toxicity in vitro.

Nevertheless, they write that their "data suggest that clinical treatment with AZT may induce direct vascular endothelial damage" and that the likely mechanism is mitochondrial toxicity. They argue, therefore, that endothelial dysfunction may not be limited to AZT but could also occur with any NRTI that causes mitochondrial toxicity, including ddI (didanosine, Videx / Videx EC) and d4T (stavudine, Zerit). Although these drugs are also being used less often in the developed world because of their association with mitochondrial toxicity, d4T in particular is a common backbone drug in many anti-HIV combinations prescribed in the poorer countries.

In summary, the investigators conclude that their data suggest that "in addition to the lipid disorders induced by protease inhibitors...AZT and perhaps other NRTIs may contribute to the development of cardiovascular complications observed in HIV patients." They add they are undertaking further research in order to discover exactly how this happens.