HIV positive patients have a two fold increased risk of developing cardiovascular disease (such as heart attacks and strokes). Cardiovascular disease appears to be due in part to both HIV and the side effects from anti-HIV medications.

Abacavir (an important component of current HIV treatment regimens) is one medication shown to be associated with an increase the risk of heart attacks in some studies. The mechanism by which abacavir does this is unknown.

We hypothesise that abacavir is leading to heart disease by interacting with platelets, which then form blood clots within the arteries supplying the heart, the subsequent blockage of the artery causing a heart attack.

This study aims to determine if abacavir increases the activity (or "stickiness") of platelets, and thus provide evidence as to how it may be promoting heart attacks.

It will consist of 23 HIV positive men who currently have well controlled HIV. Participants will take abacavir for 15 days in addition to their usual anti-HIV medications. A blood sample to assess platelet activity will be taken at baseline, following the 15 days of therapy (i.e. at the time of maximal abacavir effect) and again after a 28 day washout period (to determine if any effects are reversible).

Inclusion Criteria:

• > 18 years of age
• Male
• HIV positive
• Stable non-abacavir containing anti-retroviral regimen
• Undetectable HIV Viral load

Exclusion Criteria:

• HLA-B*57*01 allele positivity
• Previous allergy to abacavir
• Known cardiovascular disease
• High Baseline cardiovascular risk (Framingham risk score > 20%)
• Current or recent antiplatelet therapy
• Pre-existing platelet or bleeding disorder (i.e. Thrombophilia, Thrombocytopenia, Von willebrands disease, Haemophilia)
• Significant Chronic liver disease
• Current Methadone use