An anti-HIV, antisense compound with a stiffened backbone is about tograduate from Phase I to Phase II human trials, in the U.S. and France.Trademarked GEM91 by Hybridon, Inc. of Worcester, Mass., (GEMstands for gene expression modulator), the antiviral DNA derivativewill start Phase II clinical studies next month in France, its inventor,chief scientist Sudhir Agrawal, told BioWorld Today.GEM91, Agrawal explained, is an antisense oligonucleotide 25 baseslong, with a sulfur atom inserted in its phosphate/sugar backbone,replacing one oxygen atom. This chemical switch confers on theresulting phosphorothioate sequence "enormously increased stability,"said immunologist Arthur Krieg, University of Iowa, who co-edits theAntisense Research and Development Journal."Antisense oligos based on natural DNA," he told BioWorld Today,have a half-life of seven minutes in vertebrates, compared to two daysfor the phosphorothioate compound." The sulfur atom, he pointed out,defends the sequence against degradation by enzymes in the nucleus oftarget cells, with a trade-off of slightly stiffening the hybridnucleotide's backbone.Agrawal described another HIV-specific stability of GEM91 at theInternational Conference On AIDS in Yokohama, Japan, last week. Inremarks at the end of his presentation updating preclinical GEM91data, he reported in vitro comparison between Hybridon's antisensecompound and conventional AIDS drugs. All of the pharmaceuticalsdeveloped so far to treat HIV-1, he observed, have shown drugresistance. Their effects don't last very long "because escape mutationsare very fast."In a parallel in vitro experiment between AZT and GEM91," Agrawalreported, "we have seen in about 180 days _ the experiment is stillongoing _ significant resistance to AZT, whereas with GEM91 wehave not found resistance, or any change in anti-HIV activity."His antisense compound takes aim at the HIV-1 gag gene, which isthought to be involved in viral packaging, and is known to be highlyconserved. Ergo, gag offers a non-moving target in HIV's frequentevasive mutations.Agrawal speculates that nevertheless, "it remains possible that HIV-1will develop escape mutants after GEM91 treatment in vivo." But hesurmised that such resistance may take longer to arise in antisenseinhibition than in AZT treatment, "because a single point mutation,which can confer AZT resistance, may not be sufficient for viralescape" from GEM91.A paper by Agrawal et al. in the Aug. 16 Proceedings of the NationalAcademy of Sciences (PNAS) details Hybridon's preclinical researchwith GEM91. Its title: "Antisense oligodeoxynucleotidephosphorothioate complementary to Gag mRNA blocks replication ofhuman immunodeficiency virus type 1 in human peripheral bloodcells."The team pitted GEM91 in vitro against three key cells infected byHIV-1: immortalized T cell lines, cultured macrophages and freshprimary peripheral-blood lymphocytes from infected patients. In thetwo laboratory isolates, the antisense compound blocked virusreplication as effectively as did AZT.In two patient isolates, despite _ by definition _ the team's having noknowledge of the target sequence, GEM91 "suppressed both viruses tothe same level as the lab isolates." Moreover, it prevented thecytopathic effect of HIV-1 in primary CD4+ cells."Those Phase I trials, all of HIV-infected but asymptomatic volunteers,ended two months ago in France, and about four weeks ago in the U.S.,Agrawal said. "The compound," he added, "given both intravenouslyand subcutaneously, was well tolerated. Pharmacokinetics are nowbeing performed."HIV-1 tops the list of Hybridon's "most wanted" pathogens, butparasites are also posted."We were the first to start antisense antiviral research," Agrawal stated.Then last year we published on malaria, and are now looking atschistosomiasis."Another tropical scourge _ leishmaniasis _ is in the sights of aFrench group. Their paper, almost back-to-back with Agrawal's inyesterday's PNAS, is titled "Antisense phosphorothioateoligonucleotides: Selective killing of the intracellular parasiteLeishmania amazonensis." Its lead author is Jean-Jacques Toulm ofINSERM, the French National Institute of Health and MedicalResearch in Bordeaux.They report a 16-base antisense sequence "able to kill amastigotes _the intracellular stage of the parasites . . . About 30 percent infectedmacrophages were cured."Iowa's Arthur Krieg comments, "Both [INSERM and Hybridon] maybe working through an antisense mechanism, or it may be a differentmechanism. But either way they're having effects that may betherapeutically useful."He added, "The basic idea of leishmania is that, like some of theseother protozoan parasites, there's a particular sequence, which ispresent in the parasite's RNA that is not present in any vertebrate RNA.By targeting that sequence, you have the nice specificity _ if youbelieve in an antisense mechanism of action _ that you're going toknock out all of those parasitic RNAs, and not touch any of your goodRNAs." n
-- David N. Leff Science Editor
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