One in four or five hemophiliacs develop immune inhibition tothe human factor VIII infusions that keep them alive. On thehorizon is a part-porcine protein genetically engineered toblunt their antibody reaction.
Last Saturday in St. Louis, a poster at the American Society ofHematology (ASH) meeting reported "Expression of aRecombinant Hybrid Human/Porcine Factor VIII Molecule WithElimination of Reactivity Toward an Inhibitory Anti-Human A2Domain Antibody."
This data, presented by Emory University hematologist JohnLollar, may someday spell good news for patients who raiseantibodies against their recombinant or plasma-derived humanfactor VIII therapy. But the research is only half way to itsgoal. "Our overall plan is to replace the two antigenic spots onthe fVIII molecule to which inhibitors are directed,"Lollar toldBioWorld.
"We've substituted one of these spots in the A2 domain with aporcine sequence, but in order to move further we have toreplace the C2 epitope as well. "It's clearly a viable approach totreating hemophilia patients with inhibiting antibodies againstA2 and C2," he added. "Both are critical domains in thecoagulant fVIII molecule."
By standard clotting test and enzyme assay alike, their hybriddid not react in vitro with two inhibitors that were able toinhibit human fVIII by binding the A2 domain. "With thosetwo antibodies against A2," Lollar said, "we've eliminatedgreater than 99 percent of the reactivity."
Factor VIII's A2 domain is an amino-acid stretch about 40kilodaltons long that presents a generous target for antibodiesto zero in on. Yet "only one epitope or narrowly clustered set ofepitopes reacts with A2 antibodies, which indicates that theybehave as a general class of inhibitors," Lollar pointed out.
It took Lollar and his co-workers at Emory half a year PP longerthan expected PP to clone the hybrid human/pig gene constructexpressing the A2 domain sequence that they replaced in thefVIII molecule. "It's a discontinuous leap when you try to clonemolecules," he observed. "We're trying now to clone the C2domain, which is a very straightforward exercise, but couldalso take a long time."
"It could even happen as early as next week," he added. "Wewould like to have the complete A2/C2 hybrid stablyexpressed in mammalian culture by next summer."
If and when that happens, Lollar will endeavor to make enoughof the material to try in vivo. "Our initial plan," he said, "is tobypass an animal model, but probably go straight to the humanarena."
Emery filed for a U.S. patent on the hybrid in April 1992.
Hematologist Leon Hoyer, who directs the Red Cross biomedicalresearch institute's Holland Laboratory in Rockville, Md., alsohad a poster at the ASH meeting, representing, in a sense, theflip side of Lollar's. Hoyer reported on hemophiliacs' reactivityto pig rather than human fVIII. He told BioWorld that patientswho develop antibodies against the porcine and/or humanepitopes do this quite soon after being administered either PP"usually following the first 20 to 30 treatments." This makes itvery difficult to treat a patient's bleeding episodes.
Hoyer deemed Lollar's putative pig substitute, hybrid-basedtherapy "a very interesting idea," but doubts that it's apanacea. He noted that "porcine A2 and C2 are also foreign tothe patient. They will also develop antibodies to these, but lessquickly and completely."
Hemophiliacs who totally lack fVIII number an estimated15,000 in the U.S., Hoyer said, but only half of them havesevere forms of the disease. Of these, he estimates that 20percent suffer from antibody inhibition to the molecule.
-- David N. Leff Science Editor
(c) 1997 American Health Consultants. All rights reserved.