US drug developer Tanox will have to carry out another clinical trial before it can file for approval for its candidate HIV/AIDS drug TNX-355, the company said yesterday.

But on the plus side, Tanox said the trial would be relatively small – aimed at providing more information about the best dosing for the drug – and once completed could serve as a pivotal trial in the application. The US Food and Drug Administration typically asks for two pivotal studies supporting the efficacy of a new drug before granting approval.

TNX-355 is a humanised monoclonal antibody that is designed to interfere with HIV by blocking the entry of virus into host cells. Unlike other drugs that use this approach, the antibody blocks CD4 receptors, the primary site of HIV binding.

This step occurs prior to binding with the CCR5 and CXCR4 co-receptors that have been targetted by other drugs, so should be more efficient, and should be less prone to mutation than the other approaches, claims Tanox.

"Despite the potential delay in our development program, we are encouraged that the FDA has recognized the therapeutic potential of TNX-355 in HIV treatment-experienced patients," said Danong Chen, Tanox’ president and chief executive.

"This patient population has a limited number of treatment alternatives, and we continue to believe that TNX-355 could be a valuable option.”

At present the only entry inhibitor approved for marketing is Roche’s Fuzeon (enfuvirtide), cleared for use in patients with advanced HIV in the USA in March 2003. Roche reported first half sales 2006 of 143 million Swiss francs ($116m) for the product, a rise of 19% year-on-year.

In May, Tanox published preliminary data showing that TNX-355 can be combined with Fuzeon treatment to provide a very potent blockade of HIV entry into cells.