By David N. Leff
In 1979, six million Americans suffered from asthma. By 1994, that figure had doubled. Today, says molecular pharmacologist Jonathan Nyce, "We have about 15 million adults with asthma right now, and five or six million children."
No one knows why this sudden jump, but Nyce, who is chairman and chief scientific officer of EpiGeneSis Pharmaceuticals Inc., in Greenville, N.C., has some educated ideas.
"Probably one thing that has caused the increase in asthma," Nyce told BioWorld Today, "is that in the 1970s we had the fuel crunch. So everyone decided to make airtight buildings that would save energy on heating and cooling. Well, those buildings kept us in close contact with some very common allergens that are hard to escape. Things like house-dust mites."
The North American house-dust mite, Dermatophagoides farinae, is a tick-like creature only a tenth of a millimeter long, so it is visible only under a microscope. D. farinae has a lot to answer for.
The mite's favorite abiding place is deep in the texture of carpets and rugs. It feeds on the likes of fungus and dander. Like cockroaches, dust mites shed their own highly allergenic skin or shells.
Which leads to Nyce's second hypothesis: "Virtually everywhere," he pointed out, "we've switched from area rugs to wall-to-wall carpeting. They are much more difficult to clean -- getting rid of house-dust mites and other allergens -- than the old method of 30 years ago, when we had removable carpets that we took outside and beat until they were clean."
In Eastern North Carolina, Nyce's own home turf, "our incidence and mortality from asthma are probably fourfold increased over the national average. There might be some other complicating factors here that we're just trying to figure out now."
Nearly 7,000 people a year die of asthma in the U.S., some from the asphyxiation of the disease itself, others from misuse of the dangerous double-edged drugs, such as the beta adrenergic stimulators, taken to control bronchoconstriction and airway inflammation.
Asthma Needs Safe Double-Edged Drug
"Asthma has two components that need to be treated simultaneously," Nyce said. "The choking constriction of bronchial smooth muscles, and the wheezing gasps of inflamed airway walls."
Adenosine, a ubiquitous molecule in almost every cell of the body, has of late been successfully convicted of culpability in asthma. Nyce described a hypothetical demonstration to prove the prosecutor's case:
"If you sat an asthmatic person down next to a normal person and sprayed adenosine down their throats, the normal subject would have no effects from that challenge," he said. "But you might kill the asthmatic patient, who is hypersensitive to adenosine."
Now a milder version of that drastic test is in current clinical use to distinguish true asthma from other respiratory ailments. "A small amount of adenosine is puffed into the patient's throat, and if his bronchi constrict, yes, he has asthma," Nyce observed.
Adenosine consists of the DNA base adenine coupled to a sugar molecule. "For instance," he explained, "being a component of DNA and RNA, it's involved as cAMP (cyclic adenosine monophosphate) in the second-messenger systems of virtually every cell."
Adenosine comes in two relevant versions. One binds an adenosine-1 (A1) receptor; the other, a receptor for adenosine-2 (A2), which is involved in relaxing bronchodilation.
On its good behavior, the A1 receptor helps maintain the heart's rhythm, mediates many brain processes and does other good works. But overproduced in the wrong places, notably the respiratory tract, the A1 receptor is guilty of asthma beyond a reasonable doubt.
To lay such doubt to rest, Nyce and his associates conducted in vivo experiments to demonstrate, and quantify, the A1 receptor's constrictive and inflammatory activity, and how to abolish it by antisense treatment.
Today's Nature, dated Feb, 20, 1997, carries their report, titled "DNA antisense therapy for asthma in an animal model."
To begin with, working from the GenBank DNA sequence of the A1 receptor, they constructed a computer-assisted model of its single-stranded messenger RNA molecule. From this, they designed a 6,000-molecular-weight, 21-base-pair stretch of antisense DNA that would hybridize to the mRNA and knock out the pathogenic A1 receptor sequence, but not the countervailing A2 stretch.
Sensitized Rabbits Respond to Antisense DNA
"Once that hybridization occurs," Nyce -- who is the Nature article's first author -- explained, "it activates an enzyme that destroys the RNA half of the construct. That frees our antisense DNA drug to dissociate off, find another RNA and set this cascade in motion again."
To test the drug in vivo, the team raised rabbits made permanently allergic by sensitization from birth to house-dust mite allergen. After insufflating the anti-A1 receptor antisense drug by aerosol directly into the animals' lungs, they challenged the rabbit models with aerosolized adenosine, mite dust or other bronchospasm-provoking allergens.
"Then," Nyce recounted, "we measured how much bronchoconstriction had occurred, and how well they were able to breathe. This is the first antisense therapy," he pointed out, "ever delivered effectively to the lung.
"To show specificity," he went on, "after the experiment was done, we sacrificed the animals, removed the bronchial smooth-muscle tissue and actually counted the number of receptors in the tissue -- the A1 receptor versus the A2 receptor. And those studies showed that the A1 receptor is about threefold overexpressed in the asthmatic lung.
"With our antisense drug, we were able to knock it down 75 percent, to the normal level. That had the double effect of inhibiting bronchial constriction and producing apparent anti-inflammatory activity. With this drug," he pointed out, "we have the potential of having one molecule treat both aspects of asthma."
Both are covered by two pending patent applications, assigned to EpiGeneSis by East Carolina University School of Medicine, in Greenville, where Nyce and his co-author are faculty members. The university spun off the company in February 1995.
"We're moving pretty quickly now toward clinical trials," Nyce said. "It looks as if we'll finish our preclinical toxicology and be ready to start Phase I/II studies in January of 1998, then proceed apace from that point." *