Reason for optimism over vascular gene therapy

Reason for

optimism

for such gene-therapy Early diseases AIDS, cystic fibrosis, have shown trials

-

as

and muscular dystrophy that gene transfer in man is possible and apparently safe. But relatively low levels of the genes have been transduced and only small amounts of their protein products expressed, and then usually for only a short time. Hence some critics have called for a halt to human gene trials and a return to basic research.

over

vascular gene

ing chain termination and, thus, cell death. In these experiments, smoothmuscle-cell proliferation decreased by about 50%, significantly reducing

restenosis in these animal models. Another strategy to reduce restenosis has already been used in human trials. Jeffrey Isner and his colleagues at St Elizabeth’s Medical Centre in Boston are treating patients undergoing balloon angioplasty for peripheral vascular disease with a gene encoding for vascular endothelial growth factor (VEGF). , Instead of using a viral the researchers are applying a solution of naked DNA to the hydrogel coating of a : conventional angioplasty 1 Vascular Figure gene therapy technique balloon. Isner says that Double-balloon catheter isolates area targeted for therapy; gene inflating the balloon for vector introduced through instillation port A-li min Rffms to intrn-

vector, simply

However, scientists developing gene therapies for vascular disease are optimistic. They point out that, unlike patients with inherited disorders, for instance, some of whom will require long-term, if not lifelong, therapy, many patients with vascular disorders might obtain substantial therapeutic benefit from gene expression that lasts only a few weeks. "Restenosis is one disease that I think is ideally suited for gene therapy", says Elizabeth Nabel, director of the Cardiovascular Research Centre at the University of Michigan in Ann Arbor. "We know that restenosis after balloon angioplasty occurs clinically within the first 3 to 4 months and that most patients who are going to develop restenosis develop it within 6 months. Therefore, if we can determine what are the right genes to introduce, one would need only transient expression". Nabel and her colleagues have been using gene-transfer techniques to understand the part growth factors play in stimulating the vascular smooth-muscle-cell proliferation seen in restenosis, with the goal of developing gene therapies to control the process. In laboratory animals, for example, they have used an adenovirus vector to introduce the herpes simplex virus thymidine kinase (HSV-tk) gene into smoothmuscle cells at the site of balloon injury, and then treated the animals with ganciclovir (figure 1). The enzyme

phosphorylates ganciclovir, incorporated into dividing cell, caus-

that it can be cellular DNA in a so

752

duce enough VEGF DNA to the vascular wall to stimulate significant endothelial proliferation. Isner hopes that re-endothelialisation rapid induced by VEGF at the angioplasty site will limit smooth-muscle-cell proliferation and other changes that cause restenosis. In another trial in man now underway, Isner is introducing naked VEGF DNA upstream to the blockage to stimulate the growth of collateral vessels to bypass stenotic and occluded femoral arteries. Isner says preliminary results have been promising and that surprisingly little VEGF needs to be

expressed to generate clinically significant angiogenesis. Isner speculates that by some yet un-

therapy

minutes

on

day

two, and that’s all it

takes", Epstein said.

Epstein and his colleagues hope to adapt this approach to gene therapy, partly to reduce the cost of the treatment. At the current market price, would run to about 000 a US$80 dog, Epstein says, "but if you put in cDNA, it will be a trivial

bFGF

treatment

cost".

Epstein believes

an

adenoviral

vector, which would introduce the gene as an episome that would be expressed for only about 21 days before being degraded, would do the job. "That’s perfect for either restenosis or angiogenesis", he says. "We

have an active product there for a short period of time." But research is also underway to develop longer-term applications with transfer. William vascular gene Osborne and his collaborators, David Dale and Alexander Clowes,. at the University of Washington in Seattle, have developed a technique to

only

want to

impregnate

a

porous

synthetic

vascu-

lar graft with gene-altered cells. These cells can be harvested from a small segment of any peripheral vein, Osborne says. "They make this beautiful laminar matrix on top of the graft and in the interstices inside" (figure 2) In one project, they have used a retroviral vector to insert the gene for erythropoietin into the genome of vascular smooth-muscle cells. The goal is to develop vascular shunts that secrete erythropoietin for renal failure

patients haemodialysis.

on

"We ’ know how much

; erythropoietin can make : centimetre

we

per

of

! graft so we can actually tailor the

known mechanism Figure 2 remorai artery gran Trom aog ischaemia upregu- A=endothelial cells. B=transduced smoothlength of the lates the expression muscle cells (blue stain indicates expression graft to give the of VEGF receptors of vector-encoded p-gaiactosidase) patient the required C= synthetic graft material on endothelial cells. haematocrit", says At the US National Heart, Lung Osborne. and Blood Institute, Stephen Epstein, A similar graft has been tested Toren Finkel, and Ellis Unger have in primates, and experiments in a been trying to induce angiogenesis in sheep model of renal failure are patients with coronary-artery disease. underway. Similar vascular grafts, Osborne said, could be used for other Using a dog model of myocardial diseases that involve a defect in ischaemia, they have found that infusions of the angiogenic protein, basic a secreted protein, such as the fibroblast growth factor (bFGF), can haemophilias and cytokine-responsive increase collateral blood flow to haematopoietic disorders. "Basically, ischaemic myocardium distal to a we are looking at it as a drug-delivery stenosis by 50%. "We have found that system". we only have to administer it in two Michael McCarthy sessions, 5 minutes on day one and 5