Antisense technology has scored its first success in blockingundesired cell growth in live mammals.
In today's edition of the journal Nature, molecular hematologistRobert D. Rosenberg of the Massachusetts Institute ofTechnology reports that antisense gene sequences inhibitedobstructive buildup of arterial smooth muscle cells in rats andrabbits. "To the best of our knowledge," he writes, "thesestudies constitute the first reported use of antisenseoligonucleotides to inhibit synthesis of a normal gene productunder in vivo conditions, with a subsequent effect on a cellularprocess."
To test their antisense therapeutic, Rosenberg and hiscolleagues scraped raw the smooth inner lining, or intima, ofrat carotid arteries. This simulated the side effects often causedin human blood vessels by coronary bypass operations andarterial angioplasty (dilating a constricted artery with a ballooncatheter).
Life-saving surgery can later be life-threatening if arteriesoverreact to the assault on their delicate lining by growingback too much repair tissue, called atherogenesis. Thisrestenosis process narrows the arteries' internal diameter andlimits blood and oxygen flow to the heart. Restenosis afflictedan estimated 10 percent of the 262,000 Americans whounderwent coronary bypass operations in 1990, and 30 percentof the 260,000 who had their occluded arteries reopened byballoon catheter.
Rats responded strikingly to the local application of anantisense oligonucleotide preparation to their woundedcarotids. Of 22 animals, the eight that received antisensetherapy "experienced minimal intimal accumulation." In fivecontrols treated with "sense," or normally coiled,oligonucleotides, "extensive smooth-muscle-cell accumulationwas observed." The nine rats that received no oligonucleotidesor treatment at all experienced the same over-growth.
Rabbits in preliminary experiments responded similarly up totwo months after treatment.
Rosenberg told BioWorld that "antisense oligonucleotides couldpotentially serve as a new class of therapeutic agents forcardiovascular disorders."
The antisense molecule his MIT team has created is a single-stranded stretch of messenger RNA coiled "the wrong way."This sequence, 18 base pairs long, is a fragment of the c-mybproto-oncogene, which regulates cell growth and differentiationin a variety of tissues, notably blood-vessel walls. Whenantisense meets sense mRNA, it cancels the gene's message,preventing proliferation of vascular smooth-muscle cells.
Rosenberg's team administered balloon angioplasty to theirrats' carotid arteries to guarantee that restenosis would occur.Then they doused the injured vessels with an ice-cold solutionof the c-myb antisense sequence. Upon contact the liquidinstantaneously solidified into a gel, which clung to thedenuded arteries. Rosenberg attributes the experiment'ssuccess to the local delivery of high concentrations of anantisense oligonucleotide, modified to slow its degradation inthe rat host.
Juan Badimon, director of cardiovascular biology research atMassachusetts General Hospital, who is familiar withRosenberg's research, cautioned: "Several treatments have beeneffective in the rat or rabbit, and when tested in a large humanclinical trial, they did not work as expected. So it has to berepeated in some animals that more closely resemble thehuman disease, such as the pig and the monkey."
However, Paul DiCorleto, who chairs vascular cell biology at theCleveland Clinic, told BioWorld: "I think in this case you havean agent that appears to be acting in a cell-cycle step in theprocess. It would be pretty remarkable if it were not commonto humans as well."
He added: "What you'd really like to do in the human situationis apply an agent from the inside, not as in rats, around theoutside. In human patients, you're not going to have readyaccess to the exterior of the blood vessel without real surgery."
Rosenberg is planning early studies in pigs and primates,followed "within two years" by trials in humans.
-- David N. Leff Science Editor
(c) 1997 American Health Consultants. All rights reserved.