Vascular graft intimal fibrous hyperplasia: prospects for pharmacological inhibition

The effect of locally administered anti-growth factor antibodies on neointimal hyperplasia formation in expanded polytetrafluoroethylene grafts

The selective blockage of platelet-derived growth factor BB (PDGF-BB), basic fibroblast growth factor (bFGF), and transforming growth factor beta1 (TGF-beta1) by specific antibodies coated into expanded polytetrafluoroethylene (ePTFE) grafts may diminish neointimal hyperplasia. Sixty pigs were divided into two groups (n = 30 each) and then further divided into five subgroups. Group 1 had a bilateral iliac artery ePTFE interposition graft precoated with Matrigel. Three subgroups (A, B, and C) received a specific monoclonal antibody against PDGF-BB, bFGF, or TGF-beta1. One (D) received all antibodies, and one served as control (nonimmune immunoglobulin G [IgG] isotypes) (E). Group 2 had a bilateral iliac artery endothelial cell (EC)-seeded ePTFE interposition graft precoated with Matrigel. Three subgroups (A, B, and C) received a specific antibody against PDGF-BB, bFGF, or TGF-beta1. One (D) received all antibodies, and one served as control (nonimmune IgG isotypes) (E). Light microscopy and immunohistochemical stain showed that neointimal hyperplasia formation was significantly reduced in subgroups D compared to the others (p < 0.05). In subgroups D, the different precoating influenced neointimal hyperplasia formation. It was more pronounced in the prosthesis precoated with EC and Matrigel (p < 0.05). In organ culture, the amount of PDGF-BB, bFGF, and TGF-beta1 release was reduced in subgroup D animals compared to the others (p < 0.05). In subgroups D, the release of PDGF-BB, bFGF, and TGF-beta1 depended on ePTFE seeding. A higher amount of these growth factors was released in the prostheses precoated with EC and Matrigel (p < 0.05), and the bromodeoxyuridine labeling index confirmed higher incorporation in this subgroup (p < 0.001). The combined use of locally administered anti-PDGF-BB, bFGF, and TGF-beta1 monoclonal antibodies reduces neointimal hyperplasia formation.

Heparin inhibits phosphorylation and autonomous activity of Ca(2+)/calmodulin-dependent protein kinase II in vascular smooth muscle cells

Vascular smooth muscle cell (VSMC) hyperproliferation is a characteristic feature of both atherosclerosis and restenosis seen after vascular surgery. A number of studies have shown that heparin inhibits VSMC proliferation in vivo and in culture. To test our hypothesis that heparin mediates its antiproliferative effect by altering Ca(2+) regulated pathways involved in mitogenic signaling in VSMC, we analyzed the effect of heparin on multifunctional Ca(2+)/calmodulin dependent protein kinase II (CaM kinase II) which is abundantly expressed in VSMC. Using activity assays, radioactive labeling, and immunoprecipitation it was found that heparin inhibits the overall phosphorylation of the delta-subunit of CaM kinase II which is consistent with inhibition of autophosphorylation-dependent, Ca(2+)/calmodulin-independent CaM kinase II activity. This effect was less evident in heparin-resistant cells, consistent with a role for CaM kinase II in mediating the antiproliferative effect of heparin. Finally, the effects of pharmacological inhibitors of phosphatases like okadaic acid, calyculin, and tautomycin suggest that heparin inhibits CaM kinase II phosphorylation by activating protein phosphatases 1 and 2A. These findings support the hypothesis that alterations in calcium-mediated mitogenic signaling pathways may be involved in the antiproliferative mechanism of action of heparin.

Does vascular stapling improve compliance of vascular anastomoses?

Elastic properties of vessel walls are altered by vascular anastomoses. Such alterations may lead to neointimal hyperplasia, which is a common cause of reocclusion following vascular surgery. The severity of paraanastomotic hypercompliant zones and anastomotic compliance drop depend on suturing material and on elastic properties of the anastomotic vessel segments. This study compares paraanastomotic hypercompliance and anastomotic compliance drop when using a new vascular closure system (VCS) and a conventional, continuous suture line in the preparation of end-to-end anastomoses. Compliance of artery-artery, vein-artery, and polytetrafluoroethylene-artery anastomoses was measured in an artificial circulation system at mean pressures of 60, 90, and 120 mm Hg, comparing conventional suturing and the VCS. When using the VCS for vein-artery anastomoses, significantly less postanastomotic hypercompliance was achieved at mean pressures of 60 mm Hg (14.2 +/-3.8% above remote postanastomotic area), compared to suture (55.1 +/-14.8%, p<0.05). At 90 mm Hg, respective values were 11.0 +/-2.3% for VCS and 54.7 +/-10.1% for suture, p<0.01. At 120 mm Hg, in polytetrafluoroethylene-artery anastomoses, the anastomotic compliance drop was significantly less when using the continuous suture line (93.9 +/-1.1% below remote postanastomotic compliance), compared to VCS (97.2 +/-0.2%, p<0.05). Compared to conventional suturing, use of the VCS reduced postanastomotic hypercompliance in vein-artery anastomoses.

Vascular cell responses to polysaccharide materials: in vitro and in vivo evaluations

Chitosan has shown promise as a structural material for a number of tissue engineering applications. Similarly the glycosaminoglycans (GAGs) and their analogs have been known to exert a variety of biological activities. In this study we evaluated the potential of GAG-chitosan and dextran sulfate (DS)-chitosan complex materials for controlling the proliferation of vascular endothelial (EC) and smooth muscle cells (SMC). GAG-chitosan complex membranes were generated in vitro and seeded with human ECs or SMCs for culture up to 9d. In addition, porous chitosan and GAG-chitosan complex scaffolds were implanted subcutaneously in rats to evaluate the in vivo response to these materials. The results indicated that while chitosan alone supported cell attachment and growth, GAG-chitosan materials inhibited spreading and proliferation of ECs and SMCs in vitro. In contrast, DS-chitosan surfaces supported proliferation of both cell types. In vivo, heparin-chitosan and DS-chitosan scaffolds stimulated cell proliferation and the formation of a thick layer of dense granulation tissue. In the case of heparin scaffolds the granulation layer was highly vascularized. These results indicate that the GAG-chitosan materials can be used to modulate the proliferation of vascular cells both in vitro and in vivo.

Release of PDGF-BB and bFGF by human endothelial cells seeded on expanded polytetrafluoroethylene vascular grafts

BACKGROUND

The majority of endothelial cell (EC) seeded graft failures are due to anastomotic neointimal fibrous hyperplasia. We investigated the PDGF-BB and bFGF release in vitro by umbilical vein EC seeded on precoated expanded polytetrafluoroethylene (ePTFE) prostheses.

MATERIALS

EC harvested from human umbilical veins were seeded into ePTFE (30 microns internodal distance, 1 cm2 in diameter) disks. ePTFE disks uncoated or precoated with collagen type I, fibronectin, and Matrigel were used, and EC seeded into plastic wells coated as ePTFE disks or uncoated plastic wells served as controls. Scanning electron microscopy study assessed EC coverage. The presence of bFGF and PDGF-BB in serum-free conditioned media from EC seeded into ePTFE grafts and EC seeded into wells was determined by the inhibition antibody-binding assay 72 h after seeding.

RESULTS

EC coverage was similar in uncoated and coated ePTFE grafts. The release of PDGF-BB and bFGF by EC seeded into ePTFE grafts was significantly higher than that observed in EC seeded into plastic wells. The release of PDGF-BB and bFGF was independent from the various substrates used in the experiments in EC seeded into either ePTFE grafts or plastic wells.

CONCLUSIONS

Our findings pointed out that in seeded ePTFE grafts, anastomotic smooth muscle cell proliferation and intimal thickening could take place underneath an intact endothelium because seeded EC may release several growth factors.

Seeding with omental cells prevents late neointimal hyperplasia in small-diameter Dacron grafts

BACKGROUND

The influence of complete endothelialization of a prosthetic graft on development of late neointimal hyperplasia is unknown. This study was designed to investigate the effect of complete coverage with endothelial-like cells on late neointimal hyperplasia in small-diameter Dacron grafts seeded with omental cells in a canine model.

METHODS AND RESULTS

Four-mm-ID Dacron grafts were seeded with cells from omentum and implanted in the carotid arteries in 24 mongrel dogs. Each dog received one seeded and one nonseeded graft. The graft patencies were assessed by angiography at 1, 5, 12, 26, and 52 weeks after surgery. The prostheses were explanted at 5, 12, 26, and 52 weeks after surgery and underwent microscopic studies. The actuarial patency rates at 1, 5, 12, 26, and 52 weeks were 100%, 95%, 95%, 95% and 95% for seeded grafts and 100%, 86%, 49%, 40%, and 13% for nonseeded grafts, respectively. The seeded grafts exhibited a uniform endothelial-like luminal monolayer without the development of late neointimal proliferation or anastomotic neointimal hyperplasia. Neointimal tissue thickness increased up to 6 months; no additional progression of the subendothelial tissue thickness was observed, in fact there was an insignificant decrease.

CONCLUSIONS

Seeding with omental cells prevents development of late neointimal hyperplasia of small diameter prosthetic vascular grafts in a canine model.

Photodynamic therapy inhibition of experimental intimal hyperplasia: acute and chronic effects

PURPOSE

Intimal hyperplasia (IH) is a focal arterial problem that still eludes successful therapy. We have previously demonstrated the feasibility of use of photodynamic therapy (PDT) for the acute treatment of experimental IH with light to activate an otherwise biologically inert photosensitizer. The purpose of this study was to determine the acute and long-term effects of PDT inhibition of IH on the artery wall.

METHODS

Segmental IH was induced by balloon injury localized to the cervical common carotid artery of 33 rats. The photosensitizer chloroaluminum sulfonated phthalocyanine (5 mg/kg) for the experimental group or saline solution for the control group was administered intravenously. Twenty-four hours later, all instrumented portions of arteries were irradiated at 675 nm to induce cytotoxic injury in the PDT-treated arteries as compared with laser only-treated arteries for controls. Animals were killed at 1, 2, 4, and 16 weeks.

RESULTS

There were no untoward side effects in either group. All PDT-treated arteries were devoid of smooth muscle or inflammatory cells in the treated media. There was no evidence of arterial degeneration of PDT-treated arteries. Only three arteries in the PDT group developed IH, whereas it was universal in all controls. In control arteries, immunocytochemistry with bromodeoxyuridine revealed maximal intimal and medial cell proliferation at 1 week, and morphometric analysis demonstrated a maximal IH at 2 weeks. Immunocytochemistry staining for smooth muscle cell actin was positive for the IH in control and when present in PDT-treated arteries, whereas the adventitia of PDT-treated arteries were positive after 2 weeks. Electron microscopy demonstrated early myofibroblast migration to the adventitia, and at 16 weeks occasional myofibroblasts were noted in the media of PDT-treated arteries. There was complete reendothelial cell covering of the intima by 4 weeks.

CONCLUSIONS

These in vivo data demonstrate that PDT is an effective local method for the treatment of experimental IH. There is no evidence of significant recurrence of IH or arterial degeneration. Further studies with PDT may provide novel approaches to the understanding and treatment of arterial IH.

Photodynamic therapy of arteries. A novel approach for treatment of experimental intimal hyperplasia

BACKGROUND

Photodynamic therapy (PDT) uses light activation of otherwise nontoxic dyes for the production of reactive oxygen species that cause cell injury and death.

METHODS AND RESULTS

The inhibition of intimal hyperplasia (IH) by PDT was studied in the balloon injury model of the rat carotid artery. Chloroaluminum-sulfonated phthalocyanine (CASPc) was the drug chosen for PDT because it does not produce skin photosensitivity and has a high absorption peak of light at 675 nm, a wavelength with good tissue penetration. A pilot study indicated that CASPc administration with laser radiant exposure of 100 J/cm2 resulted in a homogeneous, circumferential effect on the whole artery. Male Sprague-Dawley rats received the balloon catheter injury to the left common carotid artery (day 0) and were equally divided into two groups. Nine rats received either CASPc (5 mg/kg i.v., n = 6) or saline (n = 3) at day 2, before IH was present, and nine rats received CASPc or saline in the same manner on day 7, when IH was already present. Twenty minutes after drug injection, the distal left common carotid artery was irradiated under saline with 675-nm laser light at 100 mW/cm2 for 10(3) seconds (100 J/cm2). At this low laser irradiance, there are no thermal effects, but photoactivation of CASPc occurs. The rats were killed at day 14 after balloon injury when IH reaches a maximum. The arteries were harvested after perfusion-fixation for light microscopy, histological and computerized morphometric evaluation, and transmission electron microscopy (TEM) analysis. The cross-sectional areas of the neointima were measured in the PDT-treated arteries and in the laser-only control arteries. There was a significant mean +/- SD decrease of IH in the PDT-irradiated segments of the arteries (0.06 +/- 0.05 mm2) versus the laser-only control ones (0.17 +/- 0.07 mm2) (t test, p less than 0.001), with no statistical difference between the day 2 and day 7 treated rats. Lack of IH was correlated in 90% of cases with histological absence of medial smooth muscle cells or inflammatory cells, but no other structural injury was identified. TEM analysis showed early evidence of PDT-mediated cytotoxic effects at 4 hours and the absence of collagen or elastic tissue structural alterations.

CONCLUSIONS

These data demonstrated that PDT can effectively inhibit the IH response when it is used before or during induction of cellular proliferation in this acute model. Although the long-term implications of PDT in arteries need to be defined, this technique may offer a new method for understanding and treating IH.