In the January issue of the Journal of Virology, Jing Zhou, of Vanderbilt University in Nashville, reported on a betulinic acid derivative called DSP that had been observed to inhibit HIV replication in vitro. Although previous studies had concluded that DSP was acting at a late stage of virus production to inhibit the release of new virions, Zhou and colleagues have now resolved the activity of the substance to a fine degree, finding that DSP acts to slow processing at a final stage of viral maturation. Apparently, DSP binds to a segment of the gag precursor protein that eventually is cleaved to form the capsid protein (CA) and a small assembly protein called p2. DSP attaches on or near this crucial CA-p2 junction and effectively stops protease (PR) from making the cut. Yet the drug does not otherwise affect PR, and even HIV with resistance to protease inhibitors are blocked by DSP.
The team identified the precise point where DSP acts by growing HIV in the presence of increasing concentrations of the drug until a DSP-resistant mutation was generated and broke free from drug pressure. Finding the site of this mutation allowed them to pinpoint exactly where in the protein chain DSP was acting. Importantly, they learned that, although DSP-resistant mutants were difficult to produce, when they did occur, it appeared that they were less replication-competent than wild-type virus. Furthermore, viruses with the resistance mutation remained sensitive to protease inhibitors.
When DSP is present, new virions may still be produced, but they do not efficiently form stable central cores and are much less infectious than control virus which did not receive DSP treatment. Surprisingly, the virions with incomplete processing of Ca-p2 were still able to fuse to and enter uninfected cells. The impact of the defect, Zhou demonstrated, did not appear until after entry, during reverse transcription. So, although DSP acts at the point when protease is processing a newly formed virus, its inhibitory effect is not realized until the new virus enters a new cell and tries to replicate itself.
They also found that DSP does not inhibit CA-p2 cleavage completely but merely delays maturation; given enough time, mature virions can still be produced despite the presence of the drug, although these never seem to become as infectious as untreated virions. Because the betulinic acid derivative acts on such a specific and highly conserved cleavage point, and because resistance, when it occurs, results in impaired viral fitness, the authors conclude that DSP may be an attractive lead compound for the development of a new class of anti-HIV drugs.
Gilead Files IND for New Protease Inhibitor, GS 9005
Gilead Sciences, makers of Viread (tenofovir) and Emtriva (FTC) have filed an application with the FDA to begin clinical trials with a new protease inhibitor (PI) called GS 9005. The agent was developed by Gilead using a proprietary prodrug technology. Prodrugs are typically metabolized into their active form after entering the body. Based on preclinical studies in animals and in the laboratory, Gilead hopes the drug can be dosed once daily. They also say the drug may have "a resistance profile that is distinct from commercially-available drugs in the PI class."
The next steps will go slowly as Gilead investigates the first use of the drug in humans. The plan is to conduct bioavailability, pharmacokinetic and safety studies of GS 9005 first in non-HIV volunteers, then in HIV-positive persons.
Gilead says their prodrug technology allows a drug to specifically target the white blood cells that are the primary targets of HIV. The technology has already been clinically tested in studies of a prodrug of Tenofovir, called GS 7340. If validated in larger studies, this approach could result in a generation of much more tolerable HIV drugs. Stay tuned.
Back to the GMHC Treatment Issues December 2003 contents page.