April 1, 2001
La Jolla, CA – A Salk Institute-led team of scientists has identified a new site on the HIV enzyme integrase for potential drug therapy. Integrase is the only HIV enzyme not targeted by current drugs; reverse transcriptase and protease are blocked by drugs such as AZT and the protease inhibitors.
“Finding integrase inhibitors has been difficult for several reasons,” said Frederic Bushman, a Salk associate professor and author of the study. “We’ve incorporated a rational design approach based on the structure of the integrase molecule.”
The Salk team identified a pocket on the enzyme?s active surface that bore a negative charge and tested a number of small positively charged molecules for ability to bind to the pocket. Those that bound were further modified chemically to increase their efficiency as inhibitors.
“We were able to shut down integrase activity,” said Bushman. He added that the tests were done in vitro, outside of living cells. ?These particular inhibitors don?t work in cells, probably due to interactions with other molecules inside cells. But they do provide a point of departure for structure-based design.?
?And what we’re most excited about is that we’ve learned that this site can be targeted to shut down the enzyme.?
Two other sites on integrase have been found to bind small molecule inhibitors. The finding that inhibitors can bind to three different sites suggests that it may be possible to do combinatorial drug therapy directed at integrase.
The integrase enzyme only acts at one point in the viral life cycle ? the point at which the viral genome is inserted into a cellular chromosome. The kinetics of its activity make integrase a more slippery drug target than its fellow enzymes.
“It’s not like reverse transcriptase (RT) or protease, which must function many times during an infection – thousands of times in the case of RT,” said Bushman. “You decrease the efficiency or protease of RT by 90 per cent with a drug, and you’ve given the cell a fighting chance.
?In contrast, you knock integrase down by 90 per cent, and you may just delay integration a few minutes. But after that, the infection can proceed unabated. Therefore you really have to block it completely, which is more of a challenge.
The study, “Identification of a small-molecule binding site at the dimer interface of the HIV integrase catalytic domain,” appears in the April issue of Acta Crystallographica. Salk co-authors include Jason Greenwald, Denise Rhodees, Young Hwang, Witek Kwiatkowski and Associate Professor Senyon Choe. The study was done in collaboration with Valentina Molteni and Jay Siegel at the University of California, San Diego. The work was supported by the National Institutes of Health. Greenwald is supported by a Markey fellowship and a Howard Hughes Medical Institute predoctoral fellowship.
The Salk Institute for Biological Studies, located in La Jolla, Calif., is an independent nonprofit institution dedicated to fundamental discoveries in the life sciences, the improvement of human health and conditions, and the training of future generations of researchers. The Institute was founded in 1960 by Jonas Salk, M.D., with a gift of land from the City of San Diego and the financial support of the March of Dimes Birth Defects Foundation.