Virus's Nemesis

Researchers in the pharmaceutical industry have identified the binding site of a nonpeptide small-molecule drug that inhibits the process by which respiratory syncytial virus (RSV) and related viruses merge with target-cell membranes--the step that initiates viral infection. The drug, BMS-433771, is an orally active inhibitor of cell fusion by RSV, a virus that causes respiratory infections for which current treatments are inadequate.

The work was carried out at Bristol-Myers Squibb Pharmaceutical Research Institute, Wallingford, Conn., by a group led by Director of Discovery Chemistry Nicholas A. Meanwell and Director of Virology Mark Krystal [Proc. Natl. Acad. Sci. USA, 101, 15046 (2004)]. The team also included chemists Kuo-Long Yu and Douglas D. Dischino, virologist Christopher Cianci, and computational chemist David R. Langley.

In the study, the researchers used a technique called photoaffinity labeling to determine the site on RSV's fusion protein at which BMS-433771 binds. The work demonstrates the feasibility of using a small molecule to inhibit formation of a hexameric helix bundle, the folding mechanism by which fusion proteins of RSV and similar viruses gain entry to cells.

"PEPTIDES HAVE long been known as inhibitors of fusion proteins," Meanwell says, "since the first experiments conducted about 10 years ago that ultimately led to the identification and marketing of Fuzeon" (enfuvirtide or T-20) for the treatment of human immunodeficiency virus (HIV) infection. He points out that although nonpeptide small-molecule inhibitors of HIV's gp41 glycoprotein and other viral fusion proteins have been ardently sought, no well-characterized inhibitors of that type have been found.

The PNAS study is the first to confirm that small molecules are capable of interfering with the fusion process, Meanwell says. "The experiments also demonstrate the value of collaboration between chemists and biochemists in elucidating mechanistic pathways."

"This is an outstanding paper," comments chemistry professor and provost Andrew D. Hamilton of Yale University, who led a collaborative group that tried to find a small-molecule inhibitor of gp41. "The authors have not only achieved the rare distinction of finding a small molecule capable of disrupting a protein-protein interaction, but have identified by photoaffinity labeling precisely where it is binding. The target of a transient hexameric helical bundle involved in viral entry into cells is an important one that has been the focus of many groups. This is the first nonpeptide small molecule that has been shown to exert its effect on helical bundle formation through the targeted hot spot region on the protein."

Professor of clinical medicine George Fu Gao of the University of Oxford and the Chinese Academy of Sciences' Institute of Microbiology, who specializes in protein-protein interactions and molecular mechanisms of viral entry, says: "I believe the findings will have a significant effect in this research field. This is something scientists have been looking for. This is the first paper to show a small molecule can occupy the speculated pocket for all the known class I membrane viruses"--the group to which RSV belongs, which includes HIV and the virus that causes SARS (severe acute respiratory syndrome).

According to Gao, the photoaffinity labeling method used in the study could potentially be used to identify inhibitors of other class I viruses. However, he notes that although the study gives indirect evidence of the possible mechanism of the RSV/small-molecule interaction, it is not conclusive, "as we would need a clear crystal structure to show the real binding mode," and such a structure has not yet been obtained.

At this point, Bristol-Myers Squibb does not intend to test and market BMS-433771, but it has made the compound available for licensing.