Merck and HVTN expand Phase IIb trial

Enrollment in Merck's ongoing Phase IIb AIDS vaccine trial with the MRKAd5 vaccine candidate has been expanded to include double the number of volunteers researchers originally planned at sites in North America, South America and the Caribbean. The test of concept trial (see September 2005 Primer on Understanding Test-of-Concept Trials) started in January of this year and is a collaboration between Merck, the HIV Vaccine Trials Network (HVTN), and the National Institute of Allergy and Infectious Diseases. Final results are not due until 2008 on the vaccine candidate that uses a viral vector to carry three copies of HIV genes to the immune system.

MRKAd5 has previously generated strong cellular immune responses in humans. But because it uses Ad5, a naturally circulating strain or serotype of the virus that can cause the common cold, researchers thought it might be ineffective in those who have already developed adenovirus-directed immunity (see February 2005 Primer onUnderstanding Pre-existing Immunity). However, the results of now completed studies show that MRKAd5 is able to generate immune responses even in volunteers with high levels of pre-existing immunity to Ad5, leading the company to expand the trial to include 1,500 additional volunteers that have high levels of Ad5 antibodies at the start of the trial.

"We have found that the vaccine candidate consistently produces a detectable immune response in 60-70% of people," says Robin Isaacs, executive director of vaccine research at Merck. "If we continue to find that people with high Ad5 antibody levels have a good response to the vaccine then it may make the candidate vaccine useful for a larger number of people."