The US AIDS Drug Assistance Program has reacted angrily to the news that Merck has refused to lower the price of blockbuster HIV/AIDS drug Isentress, accusing the company of engaging in "unethical behaviour".

Isentress, the generic of which is raltegravir, currently costs over $8,000 per patient per year, making it the most expensive first-line drug.

The news comes hot on the heels of Merck’s announcement that it has agreed to pay a $950m fine to settle criminal and civil charges that it misrepresented the safety of its anti-inflammatory drug Vioxx.

The US Justice Department alleged that Merck promoted the drug for treating rheumatoid arthritis before it had been approved for that indication by the US Food and Drug Administration.