Two studies have shown contrasting findings on the relationship between the nucleoside reverse transcriptase inhibitor (NRTI) drug abacavir (Ziagen; also in Kivexa (Epzicom) and Trizivir. Ever since a relationship was found between current abacavir use and cardiovascular heart disease in 2008, with abacavir raising the risk of heart attack by 90%, a search has been on to find the reason for this apparent association.
The two contrasting studies were presented at the Sixteenth Conference on Retroviruses and Opportunistic Infections (CROI) in Montreal yesterday. One (Palella) showed that abacavir did not increase inflammatory markers which can damage the lining of blood vessels – one theory for how it might cause heart disease – while the other (Satchell) showed that it did increase tendency of blood to coagulate by making the platelets, the blood cells that form clots, hypersensitive to chemicals that cause coagulation. A review after these presentations (Reiss) explored the reason why studies have given contradictory results. Although cohort studies presented at this conference have confirmed the association (see later in this report), a few studies, including those undertaken by abacavir manufacturers GlaxoSmithKline, have shown no increase in heart problems amongst people on abacavir.
No association between abacavir and inflammation markers
Frank Palella of North Western University in Chicago examined the relationship between abacavir use and three body chemicals that indicate inflammation and are associated with heart problems: D-dimer (a thrombosis marker found at higher-than-normal levels in people with HIV not on antiretroviral therapy (ART)), high sensitivity C-reactive protein (hsCRP, a common marker for inflammation) and the signalling chemical interleukin-6 (IL-6).
He investigated two long-standing cohorts of patients with HIV: the Multi Centre AIDS Cohort (MACS), a group of about 7000 gay men at high risk of HIV which, starting in 1984, is the longest-standing HIV cohort study in the world; and the Women’s Interagency HIV Study (WIHS), a group of 3700 high-risk women, initiated in 1994. He identified study participants who took abacavir either as part of their first-line antiretroviral combination or who later switched to the drug. These participants were matched, in terms of demographics, CD4 count, viral load, treatment history, traditional cardiovascular risk factors like smoking and hepatitis C status, with fellow cohort members who had not taken abacavir.
The study looked at samples from two blood tests, the nearest blood test immediately prior to starting ART and the first after abacavir initiation. The latter was compared with results from matched patients who started ART that did not contain abacavir around the same time. There were 197 female matched pairs and 129 male. As well as comparing abacavir and non-abacavir patients, the study also compared levels of the three proteins in men and women.‘Official’ reference ranges (normal values) for the three proteins are not established, but consensus levels from studies of HIV-negative people indicate a range of 0.1 to 5.0 micrograms per millilitre (mcg/ml) is typical for hsCRP, 0.01 to 0.8 mcg/ml for D-dimer and, for IL-6, 0.1 to 5.0 picograms per millilitre (pg/ml – a picogram is millionth of a microgram).
In a comparison of the blood tests taken before ART and after abacavir initiation (an average of 4.2 years apart), the investigators found significant reductions in both D-dimer and IL-6 levels and significant increases in hsCRP, but there were no differences between those taking and not taking abacavir. Similarly, they found no significant difference in any of the three biomarkers between those on ART who were taking and not taking abacavir. D-dimer levels were 0.26 vs 0.28 mg/ml in those taking and not taking abacavir respectively, hsCRP 1.69 and 1.56 mg/mL and IL-6 2.20 and 2.17 pg/ml. None of these differences were statistically significant. Women had significantly lower hsCRP levels (33%) and significantly lower IL-6 levels (38%) than men. Note that these levels are all in the normal range.
This study only looked at short-term exposure to abacavir – a maximum of six months. In the initial D:A:D data, it appeared that heart attack risk increased immediately after patients started taking abacavir and remained elevated only as long as they were on the drug, but the suggestion in the latest D:A:D figures that abacavir-related risk may be cumulative could mean that marker levels could vary more with increased length of time on the drug.
Abacavir and platelet reactivity
In the second study, Claudette Satchell from Mater Misericordiae Hospital in Dublin compared the function of platelets, the small cell fragments that clump together to form blood clots, in 30 patients taking abacavir and 28 on other antiretrovirals. If platelets are hyper-reactive, they can form clots too easily, increasing the risk of thrombosis.
Pairs of patients with HIV were matched for age, gender, ethnicity and viral load (two thirds were male, the average age 36 and 70 to 80% were white). Only four patients in the abacavir group had a history of cardiovascular disease, compared with eight not on abacavir.
In a poster by the same team at the conference, they reported that platelet function was disordered in HIV-positive people, whether on ART or not, compared with HIV-negative individuals. Abacavir appeared to make this worse.
The researchers exposed blood samples to four chemicals that stimulate clot formation: adenosine diphosphate (ADP), epinephrine (adrenaline), collagen and thrombin receptor-activating peptide (TRAP). They measured platelet reactivity by measuring the number of aggregations (clumps) of platelets three minutes after exposure.
The number of clumps was, for ADP, 30 in patients on abacavir and 18 in non-abacavir patients; for collagen, 13 versus one; for epinephrine, 31 versus 21, and for TRAP 12 versus five. All these differences reached statistical significance, with the exception of TRAP. The associations remained when numerous demographic, HIV and CVD-related factors were controlled for.
“This platelet hyper-reactivity provides a potential underlying mechanism for the increased rates of myocardial infarction (MI: heart attack) in patients on abacavir,” commented Satchell.
This is a small study using a single measurement and it is yet to be determined if taking patients off abacavir could reverse the effect; a prospective study is planned to find this out.
Finally, Peter Reiss from the University of Amsterdam provided an overview of the recent studies showing an association (or not) between abacavir and heart-attack risk. Four studies have found a risk and two have not. He noted that in studies that did not find a risk, patients were on average seven to ten years younger than in ones that did, so the populations may not have been large enough to find a signal in a group which, due to youth, has less overall risk of MI.
Perhaps more significantly, the studies that found a risk were those in which patients switched to abacavir when already on ART, with low viral loads while, in the ones that did not find a risk, patients were drug-naive at baseline. Thus, the effect of abacavir may have been masked by the CVD risk associated with untreated HIV.
Reiss said that thrombosis was a three-stage process. The first stage, which is the most studied, is when inflammation damages arterial walls so they build up fatty plaque linings. This occurs over a period of time and is the pattern followed in the development of normal atherosclerosis (hardening of the arteries) and possibly in HIV infection and with protease inhibitor use. The second is when enzymes cause the fibrous sheath containing the plaque to rupture and the third is when the plaque contents form a clot or thrombosis, which can travel around the body and lodge in an artery or vein. Abacavir may be implicated in these subsequent, more acute processes, which explains why its risk has so far not appeared to be cumulative. Other mysteries remain, such as why abacavir appears to be associated with heart attacks but not strokes.
Palella F et al. Inflammatory markers among abacavir and non-abacavir recipients in the Womens Interagency HIV Study and the Multicenter AIDS Cohort Study. Sixteenth Conference on Retroviruses and Opportunistic Infections, Montreal, abstract 150LB, 2009.
Satchell C et al. Platelet hyper-reactivity in HIV-1-infected patients on abacavir-containing ART Sixteenth Conference on Retroviruses and Opportunistic Infections, Montreal, abstract 151LB, 2009.
Reiss P Abacavir and cardiovascular risk Sixteenth Conference on Retroviruses and Opportunistic Infections, Montreal, abstract 152, 2009.
Sabin CA et al. for the D:A:D Study Group. Use of nucleoside reverse transcriptase inhibitors and risk of myocardial infarction in HIV-infected patients enrolled in the D:A:D study: a multi-cohort collaboration. Lancet. 371:1417-26. 2008.
Satchell C et al. A case-control assessment of platelet function in HIV-1+ and HIV-1- individuals. Sixteenth Conference on Retroviruses and Opportunistic Infections, Montreal, abstract 737, 2009.