In yet another setback for the star-crossed CCR5 drug class, Merck has announced it will not seek approval for the drug vicriviroc in treatment experienced patients. Merck acquired Vicriviroc, once known as Schering D, when they merged with Schering-Plough in late 2009. While the company's press release promises continued development of the drug for use in people taking HIV drugs for the first time, it remains to be seen whether this drug, long stalled in development, can gain the sort of traction needed to be approved. 

Vicriviroc is an HIV drug that works by attaching to the CCR5 receptor that HIV can use to gain entry in to CD4 and other immune system cells. Not long ago, this drug class was considered to be the 'next big thing' in anti-HIV drugs, holding the potential for strong activity and few side effects. This promise has largely failed to materialize due to a combination of unexpected toxicities, poor performance and company missteps.

A few short years ago, there were three promising CCR5 drugs bunched closely together in the development pipeline. The first to fall was apliviroc, being developed by GlaxoSmithKline. GSK halted development of apliviroc when rare, but catastrophic liver toxicity occurred. Maraviroc- sold in the US as Selzentry- did gain approval a couple years back, but it has been beset by poor sales and the need for an expensive and slow blood test required for its use.

This announcement was based on vicriviroc's failure in two, phase III trials. While disappointing, this is hardly surprising. It had shown marginal benefit at best, and its development was plagued by delays and the lack of a clear development plan by Schering. When Merck and Schering merged, there was a glimmer of hope that Merck's better track record of drug development would finally move vicriviroc forward. This faint glimmer may prove to be a mirage.

The future of HIV drug development is uncertain and troubling. Pharmaceutical companies are looking elsewhere, to markets that are larger (like heart disease), more profitable (like aging) or less crowded (like HCV). While the need for new HIV drugs is undeniable, the simple truth is that more and more companies do not see HIV as an attractive, or even viable option. The failure of another HIV drug is likely to add to this worrying trend.

The HIV drug development pipeline isn't exactly empty. New drugs are being studied. Good drugs, handled correctly by the company developing them can make their mark on the epidemic. HIV is a very data driven market, one where good drugs win out, at least eventually. Whether or not activists and interested scientists can convince the corporate bean counters to invest the necessary capital and other resources to developing these vital new drugs remains to be seen.

In and of itself the latest setback for vicriviroc isn't huge. While Reuters' coverage claimed that 'some industry analysts' considered vicriviroc to be, 'potentially the best' drug of its type, I certainly don't. This drug has languished in development, making halting progress at best. It is more important for the psychology of the HIV drug development field, than for the drug's merits.