Probucol reduces the cellularity of aortic intimal thickening at anastomotic regions adjacent to prosthetic grafts in cholesterol-fed rabbits

Antioxidant therapy reverses impaired graft healing in hypercholesterolemic rabbits

OBJECTIVE

Limited endothelial cell (EC) coverage and anastomotic intimal hyperplasia contribute to thrombosis and failure of prosthetic grafts. Lipid accumulation and lipid oxidation are associated with decreased EC migration and intimal hyperplasia. The goal of this study was to assess the ability of antioxidants to improve graft healing in hypercholesterolemic animals.

METHODS

Rabbits were placed in one of four groups: chow plus N-acetylcysteine (NAC), chow plus probucol, chow with 1% cholesterol plus NAC, or chow with 1% cholesterol plus probucol. After 2 weeks, expanded polytetrafluoroethylene grafts (12 cm long x 4-mm internal diameter) were implanted in the abdominal aorta. Grafts were removed after 6 weeks and analyzed for cholesterol content, EC coverage, anastomotic intimal thickness, and the cellular composition of the neointima. Plasma samples were obtained to assess systemic oxidative stress. The data were compared with previously reported data from animals fed diets of chow and chow with 1% cholesterol.

RESULTS

Prosthetic grafts from rabbits fed chow with 1% cholesterol had significantly greater anastomotic intimal thickening and lower EC coverage than grafts from rabbits fed a regular chow diet. In hypercholesterolemic rabbits, antioxidant therapy decreased global oxidative stress as evidenced by a 40% decrease in plasma thiobarbituric acid reactive substances. In rabbits fed the chow with 1% cholesterol diet, NAC decreased intimal hyperplasia at the proximal anastomosis by 29% and significantly increased graft EC coverage from 46% to 71% (P = .03). Following a similar pattern, probucol decreased intimal hyperplasia by 43% and increased graft EC coverage to 53% in hypercholesterolemic rabbits.

CONCLUSIONS

Global oxidative stress and anastomotic intimal hyperplasia are increased, and endothelialization of prosthetic grafts is significantly reduced in rabbits fed a high-cholesterol diet. Antioxidant treatment improves EC coverage and decreases intimal hyperplasia. Reducing oxidative stress may promote healing of prosthetic grafts.

Impaired graft healing due to hypercholesterolemia is prevented by dietary supplementation with alpha-tocopherol

OBJECTIVE

The patency of prosthetic grafts is partly limited by incomplete endothelial cell coverage and development of anastomotic intimal hyperplasia. The goal of this study was to determine the effect of elevated cholesterol on prosthetic graft healing and the ability of alpha-tocopherol to improve healing.

METHODS

Rabbits were placed on one of four diets: chow, chow plus 1% cholesterol, chow plus alpha-tocopherol, or chow plus 1% cholesterol and alpha-tocopherol. After 2 weeks, expanded polytetrafluoroethylene grafts (12-cm long, 4-mm internal diameter) were implanted in the abdominal aorta. Grafts were removed after 6 weeks and analyzed for cholesterol and alpha-tocopherol content, endothelial coverage, anastomotic intimal thickness, and cellular composition of the neointima.

RESULTS

At the time of graft implantation, plasma cholesterol was 34 +/- 4 mg/dL in the chow group and 689 +/- 30 mg/dL in the 1% cholesterol group (P < .05). Grafts removed from hypercholesterolemic rabbits had marked intimal thickening, with an intima/graft thickness ratio of 0.76 +/- 0.29 compared with 0.14 +/- 0.06 in chow animals (P < .05). Macrophage infiltrate was increased to 45 +/- 11 macrophages/0.625 mm(2) in grafts from hypercholesterolemic rabbits compared with 0 +/- 0.4 in controls (P < .05). Endothelialization of grafts was lower in hypercholesterolemic rabbits than in the chow group, with endothelial cells covering 46% +/- 7% and 62% +/- 7% of the graft surface, respectively (P = .05). When alpha-tocopherol was added to the 1% cholesterol diet, the macrophage count decreased to 12 +/- 8, the intimal/graft thickness ratio decreased to 0.17 +/- 0.09, and endothelial coverage increased to 70% +/- 7% (P < .05 compared with the high-cholesterol group).

CONCLUSION

Anastomotic intimal hyperplasia is dramatically increased and endothelialization is reduced in rabbits on a high-cholesterol diet, but alpha-tocopherol supplementation blocks the augmented neointimal thickening and improves endothelial cell coverage.

Role of probucol in inhibiting intimal hyperplasia after coronary stent implantation: a randomized study

BACKGROUND

Oxygen-free radicals can stimulate smooth muscle cell proliferation and may therefore be involved in the genesis of in-stent restenosis. Thus, treatment with probucol, a potent antioxidant agent that has been shown to reduce restenosis after balloon angioplasty, may be an effective strategy to prevent intimal hyperplasia after stenting.

METHODS

In a prospective double-blind study, 59 patients submitted to coronary stent implantation were randomly assigned to treatment with either probucol (1 g/d) or placebo, starting two weeks before the procedure and continued for 6 months. The primary end point was the intimal hyperplasia volume at 6 months measured by intravascular ultrasound (IVUS) imaging.

RESULTS

Of the 59 randomized patients, 54 underwent successful stent implantation, completed the follow-up period, and underwent repeat angiography, 6.1 +/- 1.1 months after the procedure. Volumetric IVUS analysis revealed similar intimal hyperplasia volumes (403 +/- 26.7 mm3 for probucol vs 44.8 +/- 28.3 mm3 for placebo) and percent volume obstruction of the lumen (30.4% +/- 14.5% for probucol versus 30.7% +/- 17.2% for placebo) in both groups. In addition, quantitative coronary angiography showed no differences in late loss (1.0 +/- 0.8 mm vs 1.1 +/- 0.8 mm), loss index (0.5 +/- 0.4 for both groups), or angiographic restenosis rates (19.4% vs 18.5%) between the probucol and placebo groups, despite the observation of significant changes in the lipid profile and in the plasma antioxidant defenses in patients receiving probucol.

CONCLUSIONS

Treatment with the antioxidant probucol failed to reduce neointimal formation after coronary stent implantation as assessed by IVUS volumetric analysis.

Effect of BO-653 and probucol on c-MYC and PDGF-A messenger RNA of the iliac artery after balloon denudation in cholesterol-fed rabbits

Antioxidants have been proposed as a promising treatment for restenosis after percutaneous transluminal coronary angioplasty (PTCA), but their mechanism of action remains unclear. Here, we investigated the effect of antioxidants on gene expression in the artery after balloon denudation. We developed a sensitive ribonuclease (RNase) protection assay for the messenger RNA (mRNA) levels of immediate early (IE) genes (c-jun, c-fos and c-myc), as well as platelet-derived growth factor-A (PDGF-A), platelet-derived growth factor-beta receptor, transforming growth factor-beta 1, and vascular endothelial growth factor. New Zealand White rabbits were fed a 0.17% cholesterol diet containing vehicle, BO-653 or probucol, and balloon denudation for iliac arteries was performed. The iliac arteries were then removed at 4 h after the denudation, for IE genes, and 10 days after for growth factors and receptors. Both BO-653 and probucol significantly reduced neointimal thickening, compared with the control. In terms of gene expression, BO-653, but not probucol, significantly inhibited c-myc induction. On the other hand, probucol, but not BO-653, significantly inhibited PDGF-A expression. Neither treatment had any effect on the expression of other genes. These results suggest that antioxidants affect the gene expression of the neointimal response and that both BO-653 and probucol inhibit gene expression in specific manners.

Characterization of simple and reproducible vascular stenosis model in hypercholesterolemic hamsters

The importance of low-density lipoprotein (LDL) in the etiology of atherosclerosis is well recognized. We have established a reproducible stenosis model in hypercholesterolemic hamsters, and the process of arterial stenosis by thrombus or neointima was studied and compared with that in normal hamsters. The level of plasma LDL was 4.6 times higher in hamsters fed a high-cholesterol diet than in hamsters fed normal food. Endothelial injury in right common carotid arteries was induced using a modified catheter. Arterial blood flow was monitored continuously using a Doppler flow probe. Arterial patency after the initiation of injury in high-cholesterol hamsters was significantly changed as compared with that of normal hamsters. Neointima was observed 2 wk after the vascular injury. The neointimal area of high-cholesterol hamsters was significantly larger than that of normal hamsters. To characterize the stenosis in hypercholesterolemic hamsters, we measured platelet aggregation, thrombin time, activated partial thromboplastin time, and proliferating smooth muscle cells (SMC) in vitro and in vivo. The half-maximal inhibitory concentration value for platelet aggregation induced by thrombin or collagen, the DNA synthesis stimulated by platelet-derived growth factor (PDGF)-BB, and 5-bromo-2-deoxy-uridine labeling indices (proliferating index of SMC in vivo) in high-cholesterol hamsters were each significantly higher than the comparable value from normal hamsters. However, specific binding of PDGF-BB in SMC was not different between the two types of hamsters. Furthermore, we investigated the inhibitory effects of probucol or losartan on neointima formation using this model. Probucol, but not losartan, significantly reduced the neointimal area in hypercholesterolemic hamsters. These findings indicated that high levels of plasma LDL strongly contributed to the development of thrombus and neointima formation via both up-regulation of platelet aggregation and the enhancement of SMC proliferation. This stenosis model may be useful for the investigation of hypercholesterolemia-associated cardiovascular diseases.

Restenosis: is there a pharmacologic fix in the pipeline?

One of the most frustrating aspects of restenosis is that it is the result of advances in medical care (there was no restenosis before the days of balloon angioplasty), yet it seems to be resistant to all that science has to offer. Still we believe there is reason to be optimistic. We are at last beginning to see some promise from clinical trials, and data being generated confirm some of the hypotheses previously generated from animal experiments. Thus the effects seen with the GP IIb/IIIa antibody 7E3 suggest that thrombosis may be as important in its long-term sequelae as it is for acute reocclusion. The jury is still out on whether antiproliferative approaches will be a therapeutic option, but local delivery paradigms using novel formulations delivered by catheter or impregnated in stents may allow the concept to be tested without the risk of systemic toxicity. Plans are also underway for gene therapy trials, although we may have to wait for better vector technology before taking these into the coronary bed. Perhaps we should move away from the "single pill" approach and accept that, like many infections, malignancies, or even heart failure, a multifaceted approach with combination therapy will provide the first glimmer of that brighter tomorrow.