Coronary bypass operations prolong the lives of thousands each year.In order to perform this life-saving procedure, surgeons need a readysupply of replacement arteries. But the remaining arteries in the bodyare otherwise occupied in the essential process of supplying blood tovarious organs. So surgeons turn to less essential veins to find theirneeded supply of graft material.
However, the use of veins in blood-vessel grafting presents its ownproblems. The engrafted veins are readily remodeled by the bloodcirculation into vessels with artery-like strength. But this remodelingentails the formation within the blood vessel of a new layer of smoothmuscle cells (SMC) _ the neointima _ that is highly susceptible toaccelerated atherosclerosis. This is a form of arteriosclerosis in whichfatty deposits accumulate within the innermost layer of a bloodvessel, and clog it shut. In fact, coronary surgeons have observed thatup to 50 percent of vein bypass grafts fail within 10 years as a resultof restenosis _ occlusion of the remodeled artery.
Curiously, some coronary bypass patients do not develop thissusceptible neointima. They apparently form remodeled arteries, withthe SMCs that are already present increasing in size, rather thanmultiplying their number. In these patients, some grafts have nowstayed on their job for periods exceeding 15 years. The trick isclearly to develop methods ensuring that this effect occurs in anincreasing number of bypass patients.
A significant step along this path is reported in the currentProceedings of the National Academy of Sciences (PNAS), datedMay 9. A paper by Michael Mann and colleagues of Stanford andOsaka Universities, titled, "Genetic engineering of vein graftsresistant to atherosclerosis," describes the use of gene therapytechniques for preventing the eventual failure of vein grafts.
Antisense Blockade Arms Against Atherosclerosis
Mann, a cardiologist and molecular biologist, used a liposomecomplex of antisense oligodeoxynucleotides (oligos) to transfect veinsegments in rabbits before removing them for use as grafts. He andhis colleagues chose antisense sequences for two important genesexpressed during cell division, cell division cycle 2 kinase (cdc2kinase) and proliferating cell nuclear antigen (PCNA). Because theseantisense sequences had prevented vascular SMC growth afterinduced arterial balloon injury, they hoped that the antisensetreatment would do the same in vein grafts.
Between two and 10 weeks after engrafting the veins, Mann and hisco-authors observed outwardly similar vein grafts. But significantdifferences appeared upon closer examination. While the overallthicknesses of the antisense-treated and untreated veins were similar,the SMCs of the treated vein grafts were organized in their normalcircumferential architecture. In contrast, the untreated grafts showeda marked cellular disarray of their SMCs. In parallel, the levels ofcdc2 kinase and PCNA were dramatically reduced by transfection ofantisense oligos; the expression of these cell cycle proteins decreasedby over 90 percent in treated grafts.
In addition, six weeks after surgery, rabbits fed a high cholesteroldiet, and fitted with antisense-treated vein grafts, showed no plaqueformation. In contrast, plaque-like lesions occurred in all controlanimals. "Most remarkably," the PNAS article reported, "thesegenetically modified grafts demonstrated a sustained resistance todiet-induced atherosclerosis."
Onward To Human Vein Graft Therapy
Senior author Victor Dzau, a cardiologist and gene therapist atStanford, told BioWorld Today, "We have now taken humansaphenous veins and looked to see if we can transduce the cells invitro. We have done over 30, and found that labeled antisense oligosget into the cells." Dzau added, "Using a test system in which wehave tried to inhibit interleukin expression, we get a 50 percentreduction in expression levels."
Dzau's group is currently doing toxicity studies, and hopes "to moveto human therapy trials in nine months or a year. Consultants areworking with us to get together the appropriate filings."
He continued: "Stanford University owns the rights to this invention,and they have licensed those rights to our industry collaborators,[Palo Alto-based] CV Therapeutics and Genta Inc. [of San Diego]."Dzau explained that the most important proprietary work under wayat the moment was defining the complex conditions necessary tomaximize the efficiency of transduction. "We want to get the highestefficiency possible in our test systems before proceeding to theclinic," he said. "Interestingly, unlike other vein transductionsystems, we have found that high pressurization of the vein grafts isnot necessary for transduction with our viral vector-liposomecomplexes."
Steven Epstein, a cardiologist and gene therapist, is working onsimilar projects at the National Institutes of Health. He finds Dzau'swork "very exciting." Epstein told BioWorld Today that, "This groupwas able to document the validity of their hypothesis that usingantisense oligos could essentially stop cell division in these veingrafts."
Epstein added, "You wouldn't have necessarily expected thatinhibition of neointima formation would facilitate the adaptation ofthe graft and the changeover to an artery-like structure." He offeredthe opinion that, "This work needs to be confirmed, but if it is, it is avery important result." n
-- Chester Bisbee Special To BioWorld Today
(c) 1997 American Health Consultants. All rights reserved.