WASHINGTON -- Researcher Flossie Wong-Staal on Wednesdaytouted ribozymal gene therapy for treatment of HIV at TheFirst National Conference on Human Retroviruses and RelatedInfections here.

Ribozymes are RNA enzymes that can kill HIV by cleaving thevirus' RNA. The ribozymes' catalytic activity allows a singlemolecule to kill many viruses, a distinct advantage overantisense, said the University of California, San Diego,researcher.

Furthermore, the ribozymes can kill viruses both before andafter they integrate into host DNA, targeting both mRNA andgenomic RNA of new virus progeny. In tissue culture, bytransient transfection, ribozymes inhibited 80-90 percent ofviral expression pre-integration, compared with 10-15 percentfor antisense (see BioWorld, July 12).

Subsequent work in a stable cell line expressing this ribozymeinhibited the HIV expression by three to four logs, including50- to 100-fold reduction of virus at the pre-integration stage.Post-integration, ribozymes inhibited pro-viral synthesis 50-100 fold. Moreover, the two inhibitions appear to bemultiplicative, said Wong-Staal.

In the most promising tissue culture experiment, theresearchers could detect no virus and very little p24 antigenafter 35 days, she said.

One danger is that viruses might escape the ribozyme bymutating. A single nucleotide substitution could degradeefficiency, Wong-Staal said, although her lab found no evidenceof this.

Nonetheless, certain measures can diminish the chance ofescape, she said. First, she suggested, choose a highly conservedtarget within the viral RNA. Second, intervene as soon aspossible following infection. Mutation is less likely to occurwhile virus populations are small.

Wong-Staal suggested that the sheer numbers of virus particlespresent during conventional treatment could account for escapemutants, since slashing a population of 10 to the ninth to 10 tothe tenth by four to five logs leaves ample opportunity formutation.

The Recombinant DNA Advisory Committee (RAC) has approveda Phase I trial of ribozymal therapy in which peripheral bloodlymphocytes and CD4 cells transduced with the ribozyme gene(and controls without it) will be infused into patients.

Whether this kind of T cell therapy can confer benefits is stillcontroversial, said Wong-Staal. "A pessimist would say that youcan never do enough because peripheral blood lymphocytes areonly a fraction of cells in the body. But the optimist would say... that if you reduce the virus load enough, you may allow theimmune system to take over."

Of course, the preferred strategy would be to transduce stemcells, she said, and "a pleasant surprise is the transductionefficiency of CD34 (stem) cells is much better than PBL's(peripheral blood lymphocyte). About 80 percent of thecolonies have detectable expression of the ribozyme gene. ...This is persistent even up to 54 days." Mang Yu, group leaderfor gene therapy in Wong-Staal's laboratory, is performingthese experiments.

Ex vivo therapy is promising, particularly for industrializednations, but for the Third World, ultimately one wants in vivotherapy, said Wong-Staal.

-- David Holzman Washington Editor

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