Model system to study interaction of platelets with damaged arterial wall. II. Inhibition of smooth muscle cell proliferation by dipyridamole and AH-P719

Animal models of restenosis

Percutaneous transluminal coronary angioplasty is a widely used technique for recanalizing arteries that are occluded by atherosclerotic plaque, but its usefulness is limited by the occurrence ofrestenosis in a high proportion of patients. The development of new therapies for this currently intractable problem will be facilitated by the use of animal models of restenosis that are predictive of drug efficacy in humans. Two approaches for improving predictivity can be identified. In the first of these, the goal is to maximize the anatomical and procedural resemblance of the model to humans. The second approach seeks to maximize the pathophysiological and molecular biological resemblance of the model to humans. Tangible progress is being made toward the first goal, but lack of understanding of the basic biology of human restenosis is hampering progress toward the second.

Effect of dipyridamole plus aspirin on hemodialysis graft patency

BACKGROUND

Arteriovenous graft stenosis leading to thrombosis is a major cause of complications in patients undergoing hemodialysis. Procedural interventions may restore patency but are costly. Although there is no proven pharmacologic therapy, dipyridamole may be promising because of its known vascular antiproliferative activity.

METHODS

We conducted a randomized, double-blind, placebo-controlled trial of extended-release dipyridamole, at a dose of 200 mg, and aspirin, at a dose of 25 mg, given twice daily after the placement of a new arteriovenous graft until the primary outcome, loss of primary unassisted patency (i.e., patency without thrombosis or requirement for intervention), was reached. Secondary outcomes were cumulative graft failure and death. Primary and secondary outcomes were analyzed with the use of a Cox proportional-hazards regression with adjustment for prespecified covariates.

RESULTS

At 13 centers in the United States, 649 patients were randomly assigned to receive dipyridamole plus aspirin (321 patients) or placebo (328 patients) over a period of 4.5 years, with 6 additional months of follow-up. The incidence of primary unassisted patency at 1 year was 23% (95% confidence interval [CI], 18 to 28) in the placebo group and 28% (95% CI, 23 to 34) in the dipyridamole-aspirin group, an absolute difference of 5 percentage points. Treatment with dipyridamole plus aspirin significantly prolonged the duration of primary unassisted patency (hazard ratio, 0.82; 95% CI, 0.68 to 0.98; P=0.03) and inhibited stenosis. The incidences of cumulative graft failure, death, the composite of graft failure or death, and serious adverse events (including bleeding) did not differ significantly between study groups.

CONCLUSIONS

Treatment with dipyridamole plus aspirin had a significant but modest effect in reducing the risk of stenosis and improving the duration of primary unassisted patency of newly created grafts. (ClinicalTrials.gov number, NCT00067119.)

Protective Effect of Policosanol on Endothelium and Intimal Thickness Induced by Forceps in Rabbits

Policosanol is a cholesterol-lowering drug isolated from sugar cane wax with concomitant antiplatelet effects that prevents lipofundin-induced atherosclerotic lesions in rabbits and rats, including foam cell formation, and also reduces foam cell formation in carrageenan-induced granulomas in rats, while it inhibits proliferation of smooth muscle cells induced in rabbit cuffed artery. This study was undertaken to determine whether policosanol prevents endothelium damage and increase in arterial wall thickness in rabbits with arterial walls damaged with a forceps. Artery forceps were placed over the central artery of the right ear of all rabbits, and each artery was injured eight times. Animals were randomly distributed into four groups: a positive control group treated with Tween 20/H2O vehicle, two groups treated with policosanol (5 and 25 mg/kg, respectively), and a group treated with aspirin (8 mg/kg). Treatments were given for 30 days. Damaged arteries were examined by light and electron (transmission and scanning) microscopy. To evaluate intimal thickening, areas of intima were measured, and a significant reduction in policosanol-treated animals was observed. The endothelial surface, studied with scanning electron microscopy, revealed several types of damage. In control group, the endothelial surface was severely damaged. De-endothelialized areas were reduced in policosanol-treated animals. Platelet adhesion to subendothelium was seen in all animals of the control group, whereas policosanol-treated groups exhibited significantly reduced platelet adhesion. Policosanol also reduced, dose-dependently, the platelet sequestration induced in the damaged vessel wall, partially preventing the reduction in platelet count. It is concluded that policosanol prevents endothelium injury and reduces significantly intimal thickness of rabbit arteries damaged with forceps.

The nitric oxide/cGMP signaling pathway in pulmonary hypertension

This article briefly reviews the background of endothelium-dependent vasorelaxation, describes the nitric oxide/cGMP/protein kinase pathway and its role in modulating pulmonary vascular tone and remodeling, and describes three approaches that target the nitric oxide/cGMP pathway in the treatment of patients with pulmonary arterial hypertension.

Purinergic signaling and vascular cell proliferation and death

Evidence for the role of purinergic signaling (via P1 and P2Y receptors) in the proliferation of vascular smooth muscle and endothelial cells is reviewed. The involvement of the mitogen-activated protein kinase second-messenger cascade in this action is clearly implicated, although details of the precise intracellular pathways involved still remain to be determined. Synergistic actions of purines and pyrimidines with growth factors occur in promoting cell proliferation. Interaction between purinergic signaling for vascular cell proliferation and cell death mediated by P2X7 receptors is discussed. There is evidence of the release of ATP from endothelial cells, platelets, and sympathetic nerves as well as from damaged cells in atherosclerosis, hypertension, restenosis, and ischemia; furthermore, there is evidence that vascular smooth muscle and endothelial cells proliferate in these pathological conditions. Thus, the involvement of ATP and its breakdown product, adenosine, is implicated; it is hoped that with the development of selective P1 (A2) and P2Y receptor agonists and antagonists, new therapeutic strategies will be explored.

Suppression of arterial intimal hyperplasia by cilostamide, a cyclic nucleotide phosphodiesterase 3 inhibitor, in a rat balloon double-injury model

The effects of cilostamide, a cyclic nucleotide phosphodiesterase 3 (PDE3) selective inhibitor, on vascular intimal hyperplasia were evaluated using a single-balloon injury model and a double-injury model in which the rat common carotid artery was subjected to a second injury at a site injured 14 days previously. In the double-injury model, the second balloon injury caused more severe intimal hyperplasia (intima/media (IM) ratio, 1.88+/-0.10) than in the single-injury model (1.09+/-0.08). Histopathological study revealed that vascular smooth muscle cells (VSMC) were the predominant cell-type in the affected neointimal area. Oral administration of cilostamide for 2 weeks after the second injury suppressed intimal hyperplasia in the double-injury model (30 mg kg(-1) bid, 83% inhibition in terms of the IM ratio, P<0.05; 100 mg kg(-1) bid, 69% inhibition, P<0.05). Similar effects were also observed in the single-injury model with oral administration of cilostamide for 2 weeks (100 mg kg(-1) bid, 36% inhibition, P<0.01). Cilostamide inhibited DNA synthesis of cultured VSMC stimulated by foetal calf serum or different kinds of growth factors, but did not affect their migration stimulated by platelet-derived growth factor (PDGF)-BB. Cilostamide significantly increased the cyclic AMP concentration of VSMC dose-dependently. These results indicate that cilostamide suppresses intimal hyperplasia both in the single- and double-injury models of rat, presumably by inhibiting proliferation rather than migration of VSMC. It is suggested that PDE3 inhibitors might find application in preventing intimal hyperplasia following angioplasty such as percutaneous transluminal coronary angioplasty (PTCA) or stent.

External beam irradiation inhibits neointimal hyperplasia after injury-induced arterial smooth muscle cell proliferation

PURPOSE

Restenosis after catheter-based revascularization has been demonstrated to be primarily caused by smooth muscle cell proliferation. This study examines the effects of external beam irradiation on neointimal proliferation after external injury to the central artery of the rabbit ear.

METHODS AND MATERIALS

Thirty male New Zealand White rabbits were used in this study. Crush lesions were performed on each ear under general anesthesia and bilateral auricular nerve blockade. A single dose of 1200 cGy (n = 10), 1600 cGy (n = 10), or 2000 cGy (n = 10) gamma radiation was delivered to the left or right central artery of the ear 24 hours after injury; the contralateral central artery served as control. All rabbits were sacrificed after 21 days and the central arteries of both ears were fixed for morphometric measurements.

RESULTS

Mean (+/-SD) neointimal area was 0.062 +/- 0.005 mm2 (1200 cGy), 0.022 +/- 0.005 mm2 (1600 cGy), and 0.028 +/- 0.006 mm2 in irradiated arteries compared with 0.081 +/- 0.009 mm2 in the control group. Mean (+/-SD) luminal area was 0.049 +/- 0.004 mm2 (1200 cGy), 0.059 +/- 0.002 mm2 (1600 cGy), and 0.072 +/- 0.006 mm2 (2000 cGy) in irradiated arteries compared with 0.043 +/- 0.008 mm2 in the control group. The differences in neointimal and luminal area between control and irradiated arteries were significant (p < 0.05) for the 1600 and 2000 cGy group only.

CONCLUSION

We conclude that in this model, external beam X-ray irradiation was successful in reducing neointimal proliferation after injury of the central artery of the rabbit ear. Marked reductions in neointimal proliferation were demonstrated in vessels subject to 1600 and 2000 cGy radiation; a less prominent effect was noted for 1200 cGy. Whether this approach can be used successfully to inhibit restenosis in the clinical setting requires further investigation.

Mechanisms of neointima formation--lessons from experimental models

The lamina intima of an artery is the region between the endothelial cell surface and the internal elastic lamina, which forms the luminal margin of the media. In humans the intima of atherosclerosis-prone arteries becomes thicker due to accumulation of smooth muscle cells, which originate from the media. The introduction of percutaneous transluminal coronary angioplasty (PTCA) boosted scientific interest in intimal thickening, because restenosis remains an unresolved problem of this intervention. In order to unravel the mechanisms of intimal thickening there is a need for appropriate animal models. A brief overview of these models is given together with factors that control proliferation and/or migration. Despite intensive research on neointima formation, an effective therapy for restenosis has not emerged to date. This may be due to the fact that other processes, such as acute elastic recoil and chronic constrictive remodeling may contribute to lumen narrowing as well. Other limitations of neointima models are related to species and anatomical differences. Most studies are performed in arteries that are either lesion-free, or contain relatively mild plaques, in contrast to the complicated, stenotic lesions that are the substrate for human PTCA. Other differences are the severity of the injury and incorporation of a mural fibrin-rich thrombus. Nevertheless, studies based on superficial injury, like the frequently used balloon denudation model, are useful. There are similarities with angioplasty, such as endothelial cell damage and proliferation of medial and intimal smooth muscle cells. The use of techniques such as differential display, gene transfer and application of antisense oligonucleotides may provide new therapeutic approaches to reduce neointima formation.

Dipyridamole inhibits PDGF- and bFGF-induced vascular smooth muscle cell proliferation

Dipyridamole is the only pharmacologic agent demonstrated to reduce polytetrafluoroethylene (PTFE) graft occlusion in hemodialysis patients. However, the mechanism of action of dipyridamole in preventing graft occlusion is unknown. The purpose of this study was to examine the direct effects of dipyridamole on both platelet-derived growth factor (PDGF) and basic fibroblast growth factor (bFGF)-induced vascular smooth muscle cell (VSMC) proliferation. Human aortic smooth muscle cells were grown to confluence in 96 well plates. A total of 5 x 10(-6) molar dipyridamole, PDGF 10 ng/ml, or bFGF 10 ng/ml were added to appropriate wells at the start of each experiment. Cell proliferation at 48 hours was determined using tritiated thymidine uptake. Intracellular cyclic AMP (cAMP) was measured using a competitive enzyme immunoassay. Treatment of VSMC with 5 microM dipyridamole dramatically reduced basal proliferation rates compared to controls [5229 +/- 1131 counts per minute (CPM) versus 387 +/- 68 CPM, P < 0.001]. Treatment with dipyridamole also reduced PDGF-stimulated VSMC proliferation (7311 +/- 1655 CPM vs. 593 +/- 110 CPM, P < 0.001) as well as the response to bFGF (5632 +/- 1270 CPM vs. 310 +/- 31 CPM, P < 0.001). Treatment of VSMC with either 5 or 20 microM dipyridamole did not change intracellular cAMP levels. Furthermore, the addition of dibutyryl cAMP to VSMC demonstrated only a modest inhibitory effect on proliferation. We conclude that dipyridamole inhibits both PDGF- and bFGF-stimulated VSMC proliferation. The effects of dipyridamole on VSMC proliferation do not appear to be entirely mediated by changes in intracellular cAMP concentrations. The direct effect of dipyridamole on VSMC proliferation may account for its efficacy in reducing PTFE graft thrombosis in hemodialysis patients.

Pharmacology of smooth muscle cell replication

The suggestion that smooth muscle cell proliferation contributes to hypertension, atherosclerosis, and restenosis after angioplasty has led to a growing interest in the use of drugs to inhibit this process. This review summarizes pharmacological studies of smooth muscle cell proliferation in vitro and in vivo and identifies specific mediators of proliferation that are implicated by drugs binding with high affinity to enzymes or receptors.

A biological basis for re-stenosis after percutaneous transluminal angioplasty: possible underlying mechanisms

Intravascular catheterization is a commonly used diagnostic tool and percutaneous transluminal angioplasty is used to dilate stenosed blood vessels. Although these techniques are very successful diagnostically and therapeutically they may precipitate a number of acute and chronic complications. Chronic intimal changes following balloon angioplasty can result in re-stenosis. Intimal hyperplasia can be a long-term complication of both diagnostic and interventional vascular catheterization. This article details these long-term structural changes, specifically relating experimental in vivo and in vitro changes to those seen clinically.

Rabbit ear model of injury-induced arterial smooth muscle cell proliferation. Kinetics, reproducibility, and implications

Recently, considerable interest has focused on the vascular smooth muscle cell (SMC) response to injury, particularly as it relates to restenosis after angioplasty. In an effort to find an optimal experimental model of arterial SMC proliferation after injury, we examined the effects of external injury to the central artery of the rabbit ear and assessed the reproducibility, morphological changes, and time course of cellular proliferation after such an injury. With rabbits under general anesthesia, direct pressure was applied at two sites along the central artery of the ears of 19 New Zealand White rabbits. Rabbits were maintained on a diet of 2.4% fat and 0.001% cholesterol throughout the experiment. In seven rabbits examined after 21 days, marked SMC proliferation with neointimal formation was observed at all 28 sites (100%). Mean neointimal area, expressed as a percentage of the area of the tunica media, was 82 +/- 40% (range, 21-203%). Compared with the uninvolved artery displaced 2 mm from the injury site, mechanical crush caused a 38% increase in total vessel area (p less than 0.001), a 40% decrease in luminal area (p less than 0.002), and no change in the area of the media. Serial histological studies were performed 1-42 days after injury, using light and electron microscopy and bromodeoxyuridine immunohistochemistry. Beginning at day 3, activated medial SMCs were noted to migrate through defects in the internal elastic membrane, with a gradual increase in neointimal area between days 5 and 12. Peak DNA synthesis was identified in the media 5 days after injury, with proliferative activity shifting almost exclusively to the neointima thereafter.(ABSTRACT TRUNCATED AT 250 WORDS)

Dipyridamole--evaluation of an established antithrombotic drug in view of modern concepts of blood cell-vessel wall interactions

The effect of dipyridamole on the local antithrombotic activities of endothelium has been evaluated. Human whole blood was allowed to flow over an endothelial cell-derived extracellular matrix partially covered by human endothelial cells. Half-maximal suppression of platelet aggregate formation occurred with approximately 5 microM dipyridamole. Similarly, a pronounced inhibition of thrombus formation was observed by in vivo microscopy and computer-assisted morphometric analysis, after oral treatment of non-anesthetized hamsters with dipyridamole, 5 mg/kg. This strong suppression of thrombus formation was maintained in animals on a long-term cholesterol-supplemented diet. The antithrombotic potential of dipyridamole has been clearly demonstrated, both in vitro and in vivo using these more complex approaches employing quantitative microscopy.