WORCESTER, Mass. -- Several quarantined monkeys arecompleting safety tests of an antisense therapeutic targetingthe AIDS virus. Its results will answer the last of 35 questionsput to Hybridon Inc. of Worcester, Mass., by FDA's anti-viralcenter before it approves Phase I clinical trials.

E. Andrews Grinstead III, Hybridon's chairman, chief executiveofficer and president, told BioWorld that this final simiansafety trial "could be completed within a week," and FDAapproval may follow "any day." He said theoligodeoxynucleotide phosphorothioate, trademarked GEM(gene expression modulation) 91, is the first antisensecompound aimed at inhibiting HIV in humans.

The company filed an investigational new drug (IND)application with FDA last November after initiating dosageprofiling studies in a battery of monkeys. The current primatesare the last in this series, answering the FDA's final dosing-curve question.

The impending Phase I safety and dosage trial will initiallyenroll some 20 asymptomatic or mildly symptomatic AIDSpatients, with CD4 cell counts between 200 and 500, in the U.S.,and an equal number in France. The U.S. study will besponsored by the AIDS Clinical Trial Group of the NationalInstitute of Allergy and Infectious Diseases (NIAID). In France,the Agence National de Recherches sur le SIDA (NationalAgency for AIDS Research) will manage the study and willcontribute its findings to the FDA.

Test results will measure a range of in vivo reactions, fromvital signs to CD4 cell counts, blood pictures and liver enzymes.

Lacking an animal model that mimics the clinical course of HIVinfection, Hybridon's chief scientific officer and vice-presidentof discovery, Sudhir Agrawal, and Robert Gallo's tumor cellbiology laboratory at the National Cancer Institute have testedGEM 91 in petri dishes seeded with human primary peripheralblood lymphocytes, which they first infect with HIV-1.

Their pivotal results, published in last Saturday's Proceedingsof the National Academy of Sciences (PNAS), is titled "Long-term treatment of human immunodeficiency virus-infectedcells with antisense oligonucleotide phosphorothioates."

For more than 12 weeks, GEM prevented the virus fromreplicating in the cells. Agrawal, principal author of the PNASpaper and architect of the GEM antisense phosphate-backbone-modified analogs, told BioWorld that such an unprecedentedlylengthy respite from infection, if achieved in patients, "couldgive their suppressed immune systems a chance to recover andovercome the infection."

The Worcestershire Foundation for Experimental Biology innearby Shrewsbury, Mass., set up Hybridon in 1990 tocommercialize its pioneering development of antisensetechnology. Thoru Pederson, the foundation's president andscientific director, broke the PNAS story on Thursday in anaddress to a science symposium at the annual conference of theMassachusetts Biotechnology Council in Boston.

Pederson began by explaining that Hybridon's antisensecompound acts against HIV, one jump ahead of the existinganti-AIDS drugs AZT, ddI and ddC. These all target the viralenzyme, reverse transcriptase, whereas GEM 91, a 25-nucleotide stretch of "antisense" nucleic acid, hybridizesspecifically to a viral structural gene, gag, and prevents it fromexpressing p24, a protein vital to HIV replication. Once boundto the oligo, the gag segment is degraded by a natural cellenzyme, RNAase H.

GEM would get around HIV's propensity to mutate with highfrequency because new variants would all be vulnerable to thegag-targeting antisense molecule.

Pederson revealed, in work as yet unpublished, that Hybridonhas tried its GEM compounds on blood isolated from 10 AIDSpatients, most of them AZT-resistant. All showed "very stronginhibition of the virus," as measured by p24 expression .

In the model CD4+ T cell line, MOLT 3, Agrawal and his teambegan adding their synthetic antisense compound to the petridishes twice a week, 10 or 11 days after infecting them withthe live virus. When 8 milligrams of GEM 91 per kilogramstopped the infection from spreading for 43 days, they cut thedosage tenfold, to 0.8 mg/kg. After day 56, they dropped itanother order of magnitude, to 0.08 mg/kg, which continued.suppressing the virus beyond 84 days.

To Agrawal, these dosage results "suggest that chemotherapyfor HIV-1 infection with GEM may be achieved by an initialhigh-dose treatment followed by a lower maintenance dose."

"Cost as well must come down tenfold," Pederson told hisBoston conference audience, if the antisense approach toinfection is to succeed commercially.

"Today at Hybridon," he said, "oligos of clinical-grade purity,cost $2,000 for each gram." To attain market penetration,recover R&D costs and create a viable antisense pharmaceuticalindustry, this million-dollar-per-kilogram price tag mustdecrease to $100,000 per kilo, he said.

"The quantity of material needed to treat patients will be acritical issue," said Carl Dieffenbach, chief of developmentaltherapeutics in NIAID's Division of AIDS, who is in charge ofGEM's upcoming Phase I trial.

Dieffenbach added, "Hybridon has gone a long way towardimproving the large-scale synthesis; that's one of the things Dr.Agrawal is very good at. So we can get close to the quantitiesrequired."

Pederson said Hybridon has seen no signs in vitro that GEM 91might randomly enter cells in vivo other than its intendedHIV-infected targets.

To the best of Agrawal's knowledge, of all the 15 start-upcompanies dedicated to antisense R&D, Hybridon alone is goingafter the AIDS virus. It is also targeting Alzheimer's, cancer,cystic fibrosis hepatitis, herpes, influenza, malaria andpapillomavirus, among other diseases.

"I think the AIDS pharmaceutical area is obsessed by a curedrug," Pederson told BioWorld. "A much more likely strategywould be to attenuate the viral system during the mezzaninelevel of the disease, when CD4 counts are plummeting. Thiswould afford a window of time (permitting immune systemrecuperation) during which the virus is attenuated eight totenfold. That's the level of optimism I would have."

Robert Friedman, vice president of Kidder Peabody, whofollows the antisense companies, is also optimistic about GEM,but cautiously. "This entire area of antisense technology, whilevery elegant and very promising, is unproven. So the conceptof being able, in a safe and efficient manner, to go in andinterfere with the genome of the virus, in real life rather than atest tube, is in itself quite an assumption."

To which he adds wryly, "Given that the target here is HIV,which has so far outsmarted the smartest of our scientists, thatmay add another level of precaution."

-- David N. Leff Science Editor

(c) 1997 American Health Consultants. All rights reserved.