Merck stops development of CCR5 inhibitor vicriviroc

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Merck has announced that it is to halt all development of the CCR5 inhibitor vicriviroc, following disappointing data from a phase II study of the drug in people with HIV with no previous history of treatment.

Vicriviroc was acquired by Merck as part of its merger with Schering-Plough, which had been developing the drug for over ten years.

Vicriviroc was designed to block the CCR5 receptor on human CD4 cells, which HIV uses to gain entry into CD4 cells.



A protein on the surface of certain immune system cells, including CD4 cells. CCR5 can act as a co-receptor (a second receptor binding site) for HIV when the virus enters a host cell. A CCR5 inhibitor is an antiretroviral medication that blocks the CCR5 co-receptor and prevents HIV from entering the cell.


A person who has never taken treatment for a condition.


A person who has previously taken treatment for a condition. Treatment-experienced people may have taken several different regimens before and may have a strain of HIV that is resistant to multiple drug classes.


In HIV, an individual who is ‘treatment naïve’ has never taken anti-HIV treatment before.

phase II

The second stage in the clinical evaluation of a new drug or intervention, in which preliminary data on effectiveness and additional information about safety is collected among a few hundred people with the disease or condition.

Several other companies have attempted to develop drugs of this type, but only one, Pfizer, has managed to successfully bring a CCR5 inhibitor to market. Maraviroc, marketed as Selzentry in the United States and Celsentri in the rest of the world, has so far failed to find a large market. (The drug is now marketed by Viiv Healthcare, Pfizer and Glaxo SmithKline’s co-venture in HIV drug development).

Vicriviroc had had a difficult development pathway, with a concern over a potentially higher rate of malignancies in one phase II study in treatment-experienced patients, and a failure to show added benefit in phase III studies in which the majority of patients were already receiving highly potent background regimens of recently approved new drugs.

Following these results Merck announced that it was abandoning development of vicriviroc as an option for treatment-experienced patients in January 2010.

An earlier study in treatment-naïve patients was abandoned after a number of cases of early viral load rebound led to concerns about the drug’s potency.

The recently halted study in treatment-naïve patients was testing vicriviroc in an innovative nucleoside analogue-sparing regimen, in combination with atazanavir/ritonavir. Merck provided no details of the study outcomes in a letter to investigators announcing the cancellation of the drug development programme, but said the study results would be presented at a future scientific meeting.

Results from a similar study, in which maraviroc was combined with atazanavir/ritonavir, were presented in a late-breaker session at the Eighteenth International AIDS Conference in Vienna. Find out about the outcome of the study here.