Merck & Co reportedly decided on Monday to stop testing its HIV drug Isentress after preliminary trial results showed the treatment was not as effective as the standard treatment or control treatment.

The study was intended to compare a once-daily dose of Isentress with a twice-daily dose of the same drug for its efficacy. Merck thought the once-daily dose could be used to replace the standard twice-a-day treatment with the anti-HIV drug and was prepared to submit its application for regulatory approval next year.

Isentress was approved by the FDA in 2007 and indicated to reduce the HIV load in a twice-daily dose. The drug is the first of a class of HIV drugs known as HIV integrase inhibitors.

Isentress brought the company an annual sales of $777 million for the first nine months in 2009, according to Reuters.

The study showed after a 48-week treatment, 83.2 percent of those treated with the once-daily dose lowered their HIV loads to an undetectable viral level, compared to 88.9 percent of those treated with a twice-daily dose of Isentress.  The difference is statistically significant.

The difference was found  mostly among those with a high viral load, meaning that the once-daily dose may not be enough to suppress the viral growth.

Antiviral drugs are commonly used to reduce the viral load although a low viral load does not always mean the patient is free of risk of developing AIDS.

According to the Centers for Disease Control and  Prevention, more than one million Americans live with HIV and 21 percent percent are unaware  that they are HIV carriers.

An estimated 18,000 people die from AIDS each year in the United States.

There are about 48 drugs the FDA approved to treat AIDS/HIV including AccessPak for HIV PEP Basic, Agenerase, Aptivus, Combivir, Crixivan, Epivir, Epzicom, Gammagard, intelence, Isentress, Kaletra, Norvir, Prezista, Privigen, Retrovir, Reyataz, Sustiva, Truvada, Viramune, Viread, and Ziagen.