By David N. Leff
New therapies for hemophilia racked up a three-base hit in the first two months of the new millennium:
¿ The first three patients to receive recombinant clotting factor IX all achieved clinical improvement, as a data bonus in a Phase I trial testing only safety and dosage. Nature Genetics for March 2000 reported this news in an article titled: "Evidence for gene transfer and expression of factor IX in haemophilia B patients treated with an AAV vector." Its senior author is research hematologist Katherine High at the University of Pennsylvania-affiliated Children's Hospital in Philadelphia.
¿ At a Keystone, Colo., symposium Jan. 8, on "Gene therapy: The next millennium," Charles Ill, director of molecular and cellular biology at The Immune Response Corp. in Carlsbad, Calif., reported, "Optimizing the cDNA sequence of a human B-domain-deleted Factor VIII gene," designed to improve gene therapy of patients with hemophilia A (factor VIII deficiency).
¿ And on the principle, "If you can't lick 'em, bypass 'em," cell biologists at the Uniformed Services University of Medicine in Bethesda, Md., described - in the March issue of Nature Biotechnology - an indwelling pump, or chamber, that gets around clotting factor shortages. Its title: "In vivo bypass of hemophilia A coagulation defect by factor XIIa implant."
Calling these three advances - one clinical, two preclinical - a triple-bagger, compares hemo philia's pitfalls to some of the punishing plays in baseball. Thus, Katherine High of Philadelphia told BioWorld Today:
"Patients whose blood-clotting ability is in the range of 1 percent to 5 percent have what's called 'moderately severe' hemophilia, instead of 'severe.' In general, they have much better preservation of joint function, but of course, they can't play football - or slide into home plate playing baseball. They experience a much lower incidence of spontaneous bleeding, and can actually do fairly well at managing their bleeding disorder.
"Some of those moderately severe hemophiliacs do have more trouble," High added. "For example, they don't even know how to do home treatment, by injecting themselves with exogenous clotting-factor proteins. They bleed so infrequently they just go to the emergency room and get infused if they need it."
It's All in The Percentages
Patients classified as "severe," she explained, "are those whose clotting percentage ranges from 2 percent to under 1 percent. "Their situation," she said, "can be pretty dramatic. That is, people with less than 1 percent for the most part have frequent spontaneous bleeding episodes."
In her Nature Genetics paper, High reported on the first three patients who received gene therapy with human factor IX replacement genes delivered by adeno-associated virus (AAV) vectors: They ranged in age from 23 to 67, and all three had baseline factor IX activity below 1 percent. One of them was infected with the AIDS virus, as a consequence of contaminated blood transfusions. All of them had hepatitis, owing to similar contamination. (See BioWorld Today, June 15, 1999, p. 1.)
"This Phase I trial," High pointed out, "was structured to treat low-dose, mid-dose and high-dose cohorts. Those first three were the low-dose group. We're currently doing the mid-dose subjects, and have enrolled two out of the three patients. The high-dose group will also recruit three patients. Of course," she observed, "most gene therapy trials are Phase I/II dose escalation, with the hope that toward the close of treatment there would be some signs of efficacy."
To the co-authors' surprise, they did see modest but measurable improvement in two of those first three low-dose patients. Both reported that their need to self-administer factor IX was reduced. And High perceived another favorable outcome of this early stage gene therapy:
"This is the first report of putting AAV into skeletal muscle. What that told us is that the vector behaves in a similar way in human muscle as it does in mouse and dog muscle, suggesting that the preclinical data we had developed in these animal models does apply in humans."
From Gene Therapy To Indwelling Implants
Cell biologist Harvey Pollard, at the Uniformed Services University of Medicine, is senior author of the "bypass" paper in Nature Biotechnology. What he reports bypassing is clotting factor VIII, of which the gene is dysfunctional in the majority of hemophiliacs.
"Novo Nordisk in Denmark," Pollard told BioWorld Today, "had come up with the fact that activated VIIa can bypass this factor VIII defect. You can buy recombinant factor VIIa," he pointed out, "but it lasts only two hours in the circulation. So, somebody who's having a bleeding crisis can get literally hundreds of injections. That's not a very good lifestyle. Therefore, we looked for ways to create factor VIIa from factor VII - endogenously.
"Clotting factor XIIa will do it quite specifically," Pollard pointed out. "As there's plenty of factor VII floating around in the bloodstream, we thought maybe we could come up with some little chamber that would actually hold factor XIIa in place, and with membrane holes large enough to let factor VII in, but small enough so cells couldn't get in to generate antibodies, or whatever."
So Pollard and his co-authors ordered up a quantity of thimble-sized, cylindrical, milled teflon receptacles with membranes at either end, and loaded them with factor XIIa poised to activate factor VII. After in vitro testing, and trials in guinea pigs, they inserted three such chambers through small surgical incisions into the abdominal cavities of two 12-kilogram rhesus monkeys.
Pollard recounted: "Would the chambers make VIIa? They did. Would the factor VIIa provide bypass activity - in the face of a possible antibody - to block the endogenous, mutated factor VIII? It did. So the net result was that we were able to generate a very nice bypass activity up to around 20 percent clotting capacity, which lasted over the course of about one month. Then the activity began to trail off. But," he observed, "it's better than every two hours.
"Our next step," Pollard concluded, "is pursuing the question of applying this directly to humans. That's what's going on right now."