Effect of antiplatelet therapy on restenosis after experimental angioplasty

Dipyridamole and vascular healing following stent implantation

Introduction

Patients undergoing coronary stent implantation incur a 2% annual rate of adverse events, largely driven by in-stent restenosis (ISR) due to neointimal (NI) tissue proliferation, a process in which smooth muscle cell (SMC) biology may play a central role. Dipyridamole (DP) is an approved therapeutic agent with data supporting improved vascular patency rates. Pre-clinical data supports that DP may enact its vasculoprotective effects via adenosine receptor-A2B (ADOR-A2B). We sought to evaluate the efficacy of DP to mitigate ISR in a pre-clinical rabbit stent model.

Methods & Results

24 New Zealand White Rabbits were divided into two cohorts-non-atherosclerosis and atherosclerosis (n = 12/cohort, 6 male and 6 female). Following stent implantation, rabbits were randomized 1:1 to control or oral dipyridamole therapy for 6 weeks followed by optical coherence tomography (OCT) and histology assessment of NI burden and stent strut healing. Compared to control, DP demonstrated a 16.6% relative reduction in NI volume (14.7 ± 0.8% vs. 12.5 ± 0.4%, p = 0.03) and a 36.2% relative increase in optimally healed stent struts (37.8 ± 2.8% vs. 54.6 ± 2.5%, p < 0.0001). Atherosclerosis demonstrated attenuated effect with no difference in NI burden (15.2 ± 1.0% vs. 16.9 ± 0.8%, p = 0.22) and only a 14.2% relative increase in strut healing (68.3 ± 4.1% vs. 78.7 ± 2.5%, p = 0.02). DP treated rabbits had a 44.6% (p = 0.045) relative reduction in NI SMC content. In vitro assessment of DP and coronary artery SMCs yielded dose-dependent reduction in SMC migration and proliferation. Selective small molecule antagonism of ADOR-A2B abrogated the effects of DP on SMC proliferation. DP modulated SMC phenotypic switching with ADOR-A2B siRNA knockdown supporting its role in the observed effects.

Conclusion

Dipyridamole reduces NI proliferation and improves stent healing in a preclinical model of stent implantation with conventional antiplatelets. Atherosclerosis attenuates the observed effect. Clinical trials of DP as an adjunctive agent may be warranted to evaluate for clinical efficacy in stent outcomes.

Induction of ferroptosis promotes vascular smooth muscle cell phenotypic switching and aggravates neointimal hyperplasia in mice

BACKGROUND

Stent implantation-induced neointima formation is a dominant culprit in coronary artery disease treatment failure after percutaneous coronary intervention. Ferroptosis, an iron-dependent regulated cell death, has been associated with various cardiovascular diseases. However, the effect of ferroptosis on neointima formation remains unclear.

METHODS

The mouse common right carotid arteries were ligated for 16 or 30 days, and ligated tissues were collected for further analyses. Primary rat vascular smooth muscle cells (VSMCs) were isolated from the media of aortas of Sprague-Dawley (SD) rats and used for in vitro cell culture experiments.

RESULTS

Ferroptosis was positively associated with neointima formation. In vivo, RAS-selective lethal 3 (RSL3), a ferroptosis activator, aggravated carotid artery ligation-induced neointima formation and promoted VSMC phenotypic conversion. In contrast, a ferroptosis inhibitor, ferrostatin-1 (Fer-1), showed the opposite effects in mice. In vitro, RSL3 promoted rat VSMC phenotypic switching from a contractile to a synthetic phenotype, evidenced by increased contractile markers (smooth muscle myosin heavy chain and calponin 1), and decreased synthetic marker osteopontin. The induction of ferroptosis by RSL3 was confirmed by the increased expression level of ferroptosis-associated gene prostaglandin-endoperoxide synthase 2 (Ptgs2). The effect of RSL3 on rat VSMC phenotypic switching was abolished by Fer-1. Moreover, N-acetyl-L-cysteine (NAC), the reactive oxygen species inhibitor, counteracted the effect of RSL3 on the phenotypic conversion of rat VSMCs.

CONCLUSIONS

Ferroptosis induces VSMC phenotypic switching and accelerates ligation-induced neointimal hyperplasia in mice. Our findings suggest inhibition of ferroptosis as an attractive strategy for limiting vascular restenosis.

Antiplatelet therapy for tibial balloon angioplasty: A clinical perspective

Percutaneous transluminal tibial balloon angioplasty has an important role in the therapeutic approach of critical limb ischaemia. Despite a growing number of patients with critical limb ischaemia, there are no trials to guide the pharmacologic approach post intervention. Guidelines pertaining to the antiplatelet therapy post percutaneous transluminal tibial balloon angioplasty have not been developed. In addition, critical limb ischaemia patients have multiple comorbidities and a higher risk of bleeding. To examine the shortest duration of antiplatelet therapy post percutaneous transluminal tibial balloon angioplasty, we reviewed the preclinical data used to develop the standards for the current angioplasty technique.

Adenosine as a Marker and Mediator of Cardiovascular Homeostasis: A Translational Perspective

Adenosine, a purine nucleoside, is produced broadly and implicated in the homeostasis of many cells and tissues. It signals predominantly via 4 purinergic adenosine receptors (ADORs) - ADORA1, ADORA2A, ADORA2B and ADOosine signaling, both through design as specific ADOR agonists and antagonists and as offtarget effects of existing anti-platelet medications. Despite this, adenosine has yet to be firmly established as either a therapeutic or a prognostic tool in clinical medicine to date. Herein, we provide a bench-to-bedside review of adenosine biology, highlighting the key considerations for further translational development of this proRA3 in addition to non-ADOR mediated effects. Through these signaling mechanisms, adenosine exerts effects on numerous cell types crucial to maintaining vascular homeostasis, especially following vascular injury. Both in vitro and in vivo models have provided considerable insights into adenosine signaling and identified targets for therapeutic intervention. Numerous pharmacologic agents have been developed that modulate adenmising molecule.

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.)

Inflammation and restenosis in the stent era

The pathophysiology of restenosis involves early elements of direct injury to smooth muscle cells, deendothelialization, and thrombus deposition. Over time, this leads to smooth muscle cell proliferation/migration and extracellular matrix deposition. There is an increasing body of evidence to suggest that inflammation plays a pivotal role linking early vascular injury to the eventual consequence of neointimal growth and lumen compromise. The widespread use of coronary stents has fundamentally altered the vascular response to injury by causing a more intense and prolonged inflammatory state. Many of the cellular and molecular elements responsible for leukocyte recruitment have been elucidated, providing potential therapeutic targets for restenosis. This review seeks to provide an integrated view of the pathophysiology of restenosis that explains the central role of inflammation.

The mechanisms of coronary restenosis: insights from experimental models

Since its introduction into clinical practice, more than 20 years ago, percutaneous transluminal coronary angioplasty (PTCA) has proven to be an effective, minimally invasive alternative to coronary artery bypass grafting (CABG). During this time there have been great improvements in the design of balloon catheters, operative procedures and adjuvant drug therapy, and this has resulted in low rates of primary failure and short-term complications. However, the potential benefits of angioplasty are diminished by the high rate of recurrent disease. Up to 40% of patients undergoing angioplasty develop clinically significant restenosis within a year of the procedure. Although the deployment of endovascular stents at the time of angioplasty improves the short-term outcome, 'in-stent' stenosis remains an enduring problem. In order to gain an insight into the mechanisms of restenosis, several experimental models of angioplasty have been developed. These have been used together with the tools provided by recent advances in molecular biology and catheter design to investigate restenosis in detail. It is now possible to deliver highly specific molecular antagonists, such as antisense gene sequences, to the site of injury. The knowledge provided by these studies may ultimately lead to novel forms of intervention. The present review is a synopsis of our current understanding of the pathological mechanisms of restenosis.

Coronary-artery stenting compared with balloon angioplasty for restenosis after initial balloon angioplasty. Restenosis Stent Study Group

BACKGROUND

Intracoronary stenting reduces the rate of restenosis after angioplasty in patients with new coronary lesions. We conducted a prospective, randomized, multicenter study to determine whether intracoronary stenting, as compared with standard balloon angioplasty, reduces the recurrence of luminal narrowing in restenotic lesions.

METHODS

A total of 383 patients who had undergone at least one balloon angioplasty and who had clinical and angiographic evidence of restenosis after the procedure were randomly assigned to undergo standard balloon angioplasty (192 patients) or intracoronary stenting with a Palmaz-Schatz stent (191 patients). The primary end point was angiographic evidence of restenosis (defined as stenosis of more than 50 percent of the luminal diameter) at six months. The secondary end points were death, Q-wave myocardial infarction, bypass surgery, and revascularization of the target vessel.

RESULTS

The rate of restenosis was significantly higher in the angioplasty group than in the stent group (32 percent as compared with 18 percent, P= 0.03). Revascularization of the target vessel at six months was required in 27 percent of the angioplasty group but in only 10 percent of the stent group (P=0.001). This difference resulted from a smaller mean (+/-SD) minimal luminal diameter in the angioplasty group (1.85+/-0.56 mm) than in the stent group (2.04+/-0.66 mm), with a mean difference of 0.19 mm (P=0.01) at follow-up. Subacute thrombosis occurred in 0.6 percent of the angioplasty group and in 3.9 percent of the stent group. The rate of event-free survival at 250 days was 72 percent in the angioplasty group and 84 percent in the stent group (P=0.04).

CONCLUSIONS

Elective coronary stenting was effective in the treatment of restenosis after balloon angioplasty. Stenting resulted in a lower rate of recurrent stenosis despite a higher incidence of subacute thrombosis.

Arterial remodeling at the reference site after angioplasty in the atherosclerotic rabbit model

Recent studies suggest that arterial remodeling plays an important role in restenosis and that remodeling at the reference site may also occur. To assess the chronic effect of the reference site remodeling on angioplasty results, we evaluated reference site remodeling in an experimental atherosclerotic restenosis model. Histological sections of iliac stenoses and their associated proximal reference segments from 50 atherosclerotic rabbits killed 4 weeks after angioplasty were analyzed. Lumen area (LA), external elastic lamina area (EEL), and intimal plus medial areas (I+M) were measured at the lesion (L) and reference (R) sites. Angiography was performed preangioplasty, immediately postangioplasty, and 4 weeks postangioplasty. Restenosis was defined as an angiographic loss/gain ratio of greater than 50% at follow-up angiography. Twenty-three lesions were restenotic (R+) and 32 were not (R-). There was no difference in reference site diameters (RD) between these two groups at the time of angioplasty. However, RDs were significantly smaller in the R+ group than in the R- group (1.24+/-0.18 versus 1.52+/-0.28 mm, n=55, P<.01) at 4-week follow-up. Morphometric analysis also showed a smaller LA(R) in the R+ group (0.85+/-0.27 versus 1.06+/-0.37 mm2, n=55, P<.02), whereas there was no difference in I+M(R) between the two groups. EEL(R) significantly correlated with EEL(L), LA(R), and I+M(R) in both groups combined (r=.53, n=55, P<.0001; r=.62, n = 55, P < .0001; and r = .86, n = 55, P < .0001, respectively). Remodeling can favorably and unfavorably affect both the lesion and the reference sites and appears to occur in parallel and proportionately at both sites. These data suggest that angiographic measurement of late percent stenosis using reference site diameters may lead to an underestimation of the percent luminal narrowing in restenotic lesions because unfavorable remodeling occurs in both the lesion and reference sites in restenotic vessels.

Effects of pemirolast and tranilast on intimal thickening after arterial injury in the rat

We previously reported that tranilast, an antiallergic agent, reduced intimal thickening after endothelial injury in rats. In this study, to verify whether or not antiallergic agents inhibit intimal thickening, we investigated the effect of pemirolast on intimal thickening after endothelial injury and compared its effect with that of tranilast. Administration of two antiallergic agents, pemirolast (0.1, 1, and 10 mg/kg, p.o.) and tranilast (300 mg/kg, p.o., daily), was begun 2 days before endothelial injury and continued until the animals were killed. Endothelial injury in the rat femoral artery was induced by a photochemical reaction between localized irradiation by green light and intravenously administered rose bengal. To evaluate intimal hyperplasia, we measured the cross-sectional area of the intima 21 days after endothelial damage. Pemirolast at doses of 0.1, 1, and 10 mg/kg reduced the intimal area to 2.10 +/- 0.33, 1.36 +/- 0.19, and 1.35 +/- 0.18 (x0.01 mm2), respectively, and tranilast showed a tendency to reduce the intimal area, which was 1.86 +/- 0.35 x 0.01 mm2, compared with findings for controls (2.83 +/- 0.49 x 0.01 mm2). In rat A10 vascular smooth-muscle cells, we investigated the effects of antiallergic agents on migration by using a modified Boyden chamber assay and on proliferation by using the bromodeoxyuridine-incorporation assay. Two antiallergic agents inhibited in a concentration-dependent manner both migration and proliferation of smooth muscle cells stimulated by platelet-derived growth factor. These results suggest that antiallergic agents directly inhibit migration of smooth-muscle cells to the intima from the media and proliferation in the intima, and that pemirolast has more potent antihyperplastic action than does tranilast. Antiallergic agents may be effective in preventing restenosis after coronary angioplasty.

Importance of platelets in neutrophil adhesion and vasoconstriction after deep carotid arterial injury by angioplasty in pigs

In previous studies we have shown that platelets can support neutrophil adhesion to the injured vessel wall in vitro and that neutrophils contribute to vascular tone regulation after arterial injury in vivo. In this study, we investigated the implication of platelets in neutrophil adhesion and the vasomotor response to arterial injury in vivo. 111In-labeled neutrophil adhesion and angiographic vasoconstriction were quantified after deep carotid arterial injury by balloon angioplasty in normal (n = 8), thrombocytopenic (n = 7), and aspirin-treated (2 mg/kg IV, n = 7) pigs. Thrombocytopenia was produced by a polyclonal antiplatelet serum that depleted circulating platelet count by 84% without influencing neutrophil count. In the control animals, neutrophil adhesion (x 10(4)/cm2) at the site of deep arterial injury averaged 26.8 +/- 4.0 and decreased significantly to 11.5 +/- 2.3 and 11.2 +/- 2.4 in the thrombocytopenic and aspirin groups, respectively. The degree of vasconstriction was also reduced significantly, from 55.5 +/- 3.8% in the control group to 31.4 +/- 6.2% after platelet depletion and to 23.6 +/- 4.5% in the aspirin-treated group. Neutrophil adhesion to intact noninjured adjacent arterial segments was low in all groups and was not affected by the antiplatelet serum or by aspirin. In in vitro superfusion flow chambers, neutrophil adhesion to damaged arterial segments increased in the presence of platelets in a concentration-dependent manner and was not influenced by the antiplatelet serum. This study demonstrates that platelets can modulate neutrophil adhesion to the deeply injured arterial wall and that both elements may influence the degree of postangioplasty vasoconstriction in vivo.

Tranilast inhibits contraction and Ca2+ movement of porcine coronary arteries

In a recent clinical study, tranilast, an anti-allergic agent, was shown to reduce the rate of coronary restenosis after percutaneous transluminal coronary angioplasty, although the mechanism of this effect is unclear. The present study was undertaken to investigate the effects of tranilast on contraction and Ca2+ movement of the coronary arteries. We characterized the effects of tranilast on isometric force and aequorin-estimated intracellular Ca2+ concentrations ([Ca2+]i) of porcine coronary artery strips. Tranilast concentration-dependently (10-500 microM) inhibited histamine (3 x 10(-5) M)-induced contraction of the coronary arteries. A similar tendency was observed in the response to high K+ (30 mM) stimulation. Histamine caused phasic and tonic increases in [Ca2+]i, and high K+ caused a tonic increase in [Ca2+]i of smooth muscle, both of which were significantly suppressed in the presence of tranilast. These results suggest that tranilast inhibits the contraction of coronary arteries by inhibiting both Ca2+ influx from extracellular environment and Ca2+ release from intracellular Ca2+ stores, which might be related to its preventive effect on restenosis after coronary angioplasty.

Collagen content is significantly lower in restenotic versus nonrestenotic vessels after balloon angioplasty in the atherosclerotic rabbit model

BACKGROUND

It is recognized that restenosis is primarily due to alterations in geometric remodeling of the extracellular matrix rather than intimal hyperplasia. Prior studies have shown that angioplasty stimulates an increase in both synthesis and degradation of collagen in the atherosclerotic vessel. However, differences in collagen content and metabolism between restenotic and nonrestenotic vessels have not been examined.

METHODS AND RESULTS

Four weeks after angioplasty in an atherosclerotic rabbit model, collagen content in restenotic and nonrestenotic vessels was measured both biochemically by hydroxyproline quantitation and histologically by a digital subtraction method with the use of circularly polarized images of picrosirius red-stained sections. Collagenase and gelatinase activity also were measured in the same restenotic and nonrestenotic vessels by use of a radiosubstrate assay. Collagen content was found to be significantly lower in restenotic vessels than in nonrestenotic vessels both biochemically (127.0 +/- 32.6 versus 212.6 +/- 84.3 micrograms/mg tissue; n = 11 vessels; P < .05) and histologically (67.3 +/- 7.9% versus 76.3 +/- 11.8% area fraction; n = 20 sections from 6 vessels; P = .05). There was a significant inverse correlation between biochemically determined collagen content and gelatinase activity (P = .02) and a significant correlation between histologically determined lumen are and percent collagen content (P = .0071).

CONCLUSIONS

Collagen content is significantly decreased in restenotic versus nonrestenotic vessels after angioplasty in the atherosclerotic rabbit model. The increased collagen content in nonrestenotic vessels was associated with preserved lumen area and may play a role in geometric remodeling after angioplasty.

Role of platelets in restenosis after percutaneous coronary revascularization

The role of platelets in the process of restenosis after percutaneous coronary intervention is not fully understood. After vascular injury there is extensive platelet activation, adhesion, aggregation and secretion. Through the liberation of growth factors, such as platelet-derived growth factor, and surface expression of cell adhesion molecules, such as the glycoprotein IIb/IIIa integrin, platelets appear to be a pivotal mediator of the vascular injury response. Experimental models have demonstrated that profound, prolonged thrombocytopenia, or blockade of the IIb/IIIa receptor, may reduce neointimal hyperplasia after arterial balloon injury. However, multiple clinical trials testing conventional or new platelet agents have not yielded any salutary effects. The recent finding that abciximab, a monoclonal antibody fragment directed against IIb/IIIa, reduced clinical restenosis after coronary angioplasty by 26% in patients raises questions about the mechanism of benefit. The alpha v beta 3 vitronectin receptor is responsible for binding endothelial cells to platelets, and it also has a key role in modulating smooth muscle cell migration. It is possible that the antibody fragment exerts its effect on restenosis by means of alpha v beta 3, because abciximab fully cross-reacts to this integrin owing to the shared beta 3 subunit. To date, the other platelet glycoprotein IIb/IIIa inhibitors, including Integrelin, Tirofiban, Lamifiban and Xemilofiban, are specific in binding to this particular integrin. Considerable further study is necessary to unravel the effects of platelets on the restenosis process.

Relationship of vascular thrombosis and inflammatory leukocyte infiltration to neointimal growth following porcine coronary artery stent placement

BACKGROUND

Superficial and intramural thrombosis are reproducible histopathological features of the porcine coronary oversized stent injury model. Fibrin and its degradation products are chemotactic for mononuclear leukocytes, and promote the proliferation and migration of vascular smooth muscle cells (VSMC) in vitro. The goal of this study was to quantitate the serial histomorphologic evolution of thrombosis, leukocyte infiltration, VSMC proliferation and collagen accumulation following porcine coronary artery stent placement in a porcine model.

METHODS

Twenty-four normocholesterolemic swine underwent oversized balloon (3.5-4.0 mm) coronary angioplasty and tantalum metal stent placement. Twenty-six different arterial sites were injured, followed by serial sacrifice at day 4 (n = 6), 8 (n = 6), 14 (n = 6), and 28 (n = 6). Quantitative analysis of the neointima was performed using a high resolution video-microscopy interface and a validated histomorphometric software program. Alpha actin-positive VSMC density (per 10(4) micro(2) neointimal area) and collagen-specific picro-sirius red fluorescence (percent of neointima) were quantitated at sites adjacent to and distant from coronary artery stent placement.

RESULTS

The percent of total neointimal area occupied by resolving thrombus material was greater at days 4-8 compared to 14-28 days (59-63% vs. 1-2%; P = 0.001). Mononuclear leukocytes were also significantly increased at days 4 and 8 (92 +/- 1 and 70 +/- 8%) compared to days 14-28 (both 3 +/- 3%; P = 0.001), as a percentage of the total neointimal cellularity. Total neointimal cell density did not change (20 +/- 10, 12 +/- 6, 23 +/- 7 and 20 +/- 3 cells/10(4) micro(2); P-value, NS), despite progressive cross-sectional vascular area stenosis reduction from 7 +/- 3% at day 4 to 72 +/- 14% at day 28 (P = 0.001). Percent neointimal fibrinoid thrombus content and mononuclear leukocyte cellularity were correlated in this model (R = 0.81; P < 0.001). Peri-stent neointimal collagen staining exceeded that at vascular sites distant from porcine coronary stent placement by 14 days (29 +/- 6 vs. 15 +/- 3%), and remained greater at 28 days (35 +/- 11 vs. 16 +/- 12%) (both P < 0.05).

CONCLUSIONS

Quantitative serial histomorphometry of porcine coronary vascular stent delivery sites demonstrates early (4-8 day) neointimal mononuclear leukocyte infiltration which is histomorphologically and temporally related to intramural fibrinoid thrombosis. Significant vascular stenosis and collagen deposition occurs by 14-28 days at these vascular injury sites. These data suggest a local interaction between thrombotic and inflammatory elements in porcine coronary neointima following oversized stent injury.

Restenosis--an open file

The main procedural drawback to percutaneous coronary angioplasty is restenosis of the treated site within 6 months. Despite advances in equipment, technique, and adjunctive therapies, restenosis has occurred in approximately one-third to one-half of all patients. The biology of restenosis can be divided into plaque persistence and recoil, thrombus formation and transformation, and cellular proliferation and vascular remodeling. Animal models of restenosis have helped to elucidate these mechanisms of restenosis and provide a means to test pharmacologic and mechanical strategies to reduce stenosis recurrence. While numerous agents have been tested in animal models, until recently none has translated into benefit in large-scale clinical trials. Two therapeutic "hopefuls" which have recently emerged in clinical practice are the potent platelet inhibitors, glycoprotein IIb/IIIa receptor antagonists, and intracoronary metallic stents. The IIb/IIIa receptor antagonists target thrombus formation at the angioplasty site, thereby minimizing abrupt vessel closure acutely and neointimal growth chronically, while intracoronary stents safely produce a large coronary arterial lumen acutely and prevent vessel recoil. Separately, these therapeutic strategies have been shown to reduce clinical restenosis 20-30% at 6-month follow-up. With these encouraging results, the future will certainly provide more pharmacologic and mechanical therapies targeting restenosis. With increased understanding of the restenotic process and continued refinement of effective treatments, it may be possible one day to prevent stenosis recurrence.

VCL, an antagonist of the platelet GP1b receptor, markedly inhibits platelet adhesion and intimal thickening after balloon injury in the rat

BACKGROUND

Arterial injury is immediately followed by platelet adhesion at the site of injury, a process that requires the interaction of subendothelial von Willebrand factor with the platelet GP1b receptor. VCL, a recombinant von Willebrand factor GP1b binding domain, inhibits platelet binding to von Willebrand factor. The aim of this study was to determine whether VCL inhibits platelet adhesion at the site of arterial injury and affects neointimal thickening after injury in rats.

METHODS AND RESULTS

Sprague-Dawley rats were randomized to receive VCL, 4 mg/kg bolus followed by a continuous infusion of 2 mg.kg-1.h-1 for 72 hours, or an identical volume of saline. Balloon injury of the femoral artery was performed 15 minutes after the initial bolus injection of VCL. Scanning electron microscopy performed 1 and 3 days after injury indicated that VCL-treated rats had > 80% reduction in the number of platelets adherent to the vessel wall at the site of injury compared with controls (P < .003). Histological examination at day 14 showed that, compared with controls, VCL-treated rats had a 60% reduction in the intima-media ratio (0.21 +/- 0.03 versus 0.53 +/- 0.06, P = .001) and a reduced luminal area stenosis (12 +/- 3% versus 38 +/- 10%, P = .04). At 28 days after injury, there was no rebound of neointimal thickening in VCL-treated rats (intima-media ratio, 0.19 +/- 0.04; luminal stenosis, 17 +/- 5%). The difference between VCL-treated rats and control rats persisted but was attenuated (intima-media ratio, 0.19 +/- 0.04 versus 0.28 +/- 0.1, P = .162; luminal stenosis, 17 +/- 5% versus 31 +/- 5%, P = .058) as neointimal thickening regressed in untreated rats. With the use of proliferating cell nuclear antigen immunohistochemistry on day 3, VCL had no effect on smooth muscle cell (SMC) proliferation.

CONCLUSIONS

Antagonism of the platelet GP1b receptor by VCL profoundly decreased platelet deposition at the site of balloon injury in the rat femoral artery. This effect was associated with a persistent reduction in neointimal thickening. The lack of effect of VCL on SMC proliferation suggests that the decrease in neointimal thickening may have been mediated through inhibition of SMC migration and/or modulation of the extracellular matrix.

Comparison of effects of high-dose and low-dose aspirin on restenosis after femoropopliteal percutaneous transluminal angioplasty

BACKGROUND

Long-term treatment with aspirin is recommended in patients with large-vessel peripheral arterial disease since these patients have a high risk of death from cardiovascular causes. Recent studies have demonstrated the prophylactic effect of low-dose aspirin in reducing the risk of cardiovascular events. Since aspirin is also recommended for prevention of late recurrence after peripheral angioplasty, the present study was undertaken to compare the effects of high-dose (1000 mg/d) and low-dose (100 mg/d) aspirin on long-term patency after femoropopliteal angioplasty.

METHODS AND RESULTS

Two hundred sixteen patients treated successfully by percutaneous transluminal angioplasty for femoropopliteal lesions were randomly allocated to therapy with either 1000 or 100 mg aspirin daily. The follow-up was 24 months. The long-term results were analyzed using the Kaplan-Meier method, and differences between curves of cumulative patency were determined with the Wilcoxon and log-rank statistics. Complete follow-up information (patency after 24 months, restenosis, and death) was obtained in 207 patients. During the 2-year follow-up period, 72 patients--36 in the high-dose and 36 in the low-dose aspirin group, respectively--developed angiographically verified reobstruction within the recanalized segment. By intention-to-treat analysis, the cumulative patency rates at 24 months were 62.5% in the high-dose and 62.6% in the low-dose aspirin group (Wilcoxon, P = .97; log-rank, P = .97). The cumulative survival at 24 months of follow-up was 86.6% in the high-dose and 87.7% in the low-dose aspirin group. The number of patients discontinuing therapy was 30 in the high-dose and 11 in the low-dose aspirin group (P < .01). Fewer patients receiving 100 mg of aspirin discontinued therapy because of gastrointestinal symptoms (4 versus 20).

CONCLUSIONS

The data indicate that 100 mg aspirin is no less effective in the prevention of restenosis after femoropopliteal PTA than a 1000-mg dose and has fewer side effects.

Endothelial cell seeding fails to attenuate intimal thickening in balloon-injured rabbit arteries

BACKGROUND

Restenosis of arteries or bypass grafts after vascular reconstruction is a common clinical entity that significantly limits long-term patency. This process, termed intimal hyperplasia (IH), is characterized by smooth muscle cell proliferation in the intima and subsequent accumulation of intercellular matrix. This study was designed to test the hypothesis that endothelial cell (EC) seeding of acutely injured arteries accelerates reendothelialization of the flow surface and limits the development of IH.

METHODS

ECs were harvested from jugular veins of New Zealand white rabbits (n = 13) and were amplified in tissue culture. Each animal subsequently underwent bilateral balloon catheter injury of the iliofemoral arteries; one side was immediately seeded with cultured autologous ECs at supraconfluent density, whereas the contralateral vessel served as a nonseeded control. Animals were killed 33 +/- 5 days after balloon injury. Intimal thickening was quantitated on histologic sections of vessels (three sections per vessel, total of 60 sections) and percent endothelialization was assessed by SEM; measurements were obtained by use of computer-aided morphometry performed by a blinded observer. Data were analyzed by use of a paired t test for comparison between seeded and control vessels.

RESULTS

Seeded vessels exhibited a greater degree of reendothelialization (93.9% +/- 7.6% of the surface) than their unseeded counterparts (65.1% +/- 22.5%, p < 0.01). Intimal cross-sectional area and the ratio of intimal area to medial area were not significantly different between seeded and control vessels (intima: 0.32 +/- 0.19 vs 0.37 +/- 0.11 mm2, p = 0.28; intimal area to medial area ratio: 0.84 +/- 0.35 vs 1.02 +/- 0.2, p = 0.24).

CONCLUSIONS

We conclude that seeding with autologous venous ECs accelerated restoration of the endothelial monolayer but failed to attenuate IH in balloon-injured rabbit arteries. Further studies are necessary to determine the functional properties of seeded endothelium and to examine the effect of EC seeding on intimal thickening in other clinically relevant models.

Increase of neutrophil adhesion and vasoconstriction with platelet deposition after deep arterial injury by angioplasty

Physiopathologic events after arterial injury are largely influenced by blood element reactions with the injured surface. To determine acute arterial reactivity to injury, simultaneous chromium 51-labeled platelet deposition and indium 111-labeled neutrophil adhesion were quantified at the site of different degrees of carotid arterial injury by balloon dilatation in 21 normal pigs. The degree of vasoconstriction distally to the dilated areas was also quantified angiographically. Arteries were classified histologically as (1) uninjured with intact endothelium; (2) mildly injured with endothelial desquamation; or (3) deeply injured with lesions extending beyond internal elastic lamina, exposing the media. We found that, compared to mild injury, deep injury was associated with greater platelet deposition (38.2 +/- 5.7 x 10(6)/cm2 vs 7.8 +/- 0.9 x 10(6)/cm2; p < 0.05), neutrophil adhesion (30.6 +/- 4.1 x 10(4)/cm2 vs 10.2 +/- 2.9 x 10(4)/cm2; p < 0.05), and vasoconstrictive response (45.5% +/- 3.2% vs 26.7% +/- 2.8%; p < 0.05). Although distally to both types of injuries, noninjured arterial segments with intact endothelium were thromboresistant to platelet deposition, neutrophil adhesion to intact endothelium was much higher after deep injury (2.2 +/- 0.4 x 10(4)/cm2) compared to mild injury (0.36 +/- 0.1 x 10(4)/cm2; p < 0.05). Like platelet deposition, neutrophil adhesion is influenced by the severity of arterial injury; both may therefore be implicated in thrombogenesis and vascular responsiveness after arterial injury in vivo.

Effect of inhibitors of factor Xa or platelet adhesion, heparin, and aspirin on platelet deposition in an atherosclerotic rabbit model of angioplasty injury

Acute thrombotic reocclusion and restenosis after successful coronary angioplasty are limitations of the procedure. Although the restenotic process is not completely understood, acute platelet deposition and thrombosis are considered important initiating mechanisms. The effort to identify pharmacologic agents capable of modifying acute platelet action following mechanical injury requires an animal model mimicking the clinical pathophysiology as closely as possible. We developed a model of angioplasty-induced injury in atherosclerotic rabbit femoral arteries. Acute 111indium-labelled platelet deposition and thrombosis were assessed four hours after balloon-injury in arteries subjected to prior endothelial damage (air desiccation) and cholesterol supplementation (one month). The effects of recombinant tick anticoagulant peptide (rTAP), a blood coagulation factor Xa (fXa) inhibitor and of recombinant leech antiplatelet protein (rLAPP), a platelet adhesion inhibitor, were compared to heparin (HEP) and aspirin (ASA). Recombinant TAP and HEP, but not rLAPP or ASA, successfully prevented thrombus formation and reduced platelet deposition in balloon-injured vessel segments to levels not significantly different from those observed in the contralateral atherosclerotic non-balloon-injured vessels. Therefore, this model, incorporating balloon catheter dilation of arteries exhibiting neointimal growth and atherosclerotic plaque formation, may be useful for evaluation of possible adjunctive therapies during angioplasty.

Impact of biotechnology in the diagnostic and therapeutic management of cardiovascular disorders

Although biotechnology has been useful in the development of new diagnostic methods and drugs for the management of cardiovascular disorders, there are several issues which raise certain questions on the global use of biotechnology based drugs and diagnostic methods (Piascik, 1991; Fareed, 1993a; Fareed, 1994a). The cost is rather prohibitive in the development of this type of technology. Most diagnostic methods and drugs developed utilizing biotechnology based methods are relatively expensive. The second important consideration is the equivalence of the newer biotechnology derived drugs to the natural products. Many of the biotechnology derived drugs are obtained in prokaryotic systems (E. coli). Post-transcriptional modifications such as glycosylation are often important in determining the function of various proteins. On the other hand, biotechnology based diagnostic methods exhibit somewhat different specificity in comparison to conventional methods. Thus, it is rather important to assess the developments in this area in a careful manner. Furthermore, validation of the clinical, diagnostic and therapeutic efficacy of biotechnology derived diagnostic devices and drugs is a prerequisite for their use in cardiovascular medicine.

Experimental therapeutics and clinical studies in (re)stenosis

Restenosis is defined functionally as loss of luminal vessel patency following various methods of interventional cardiology, but ultrastructurally, it represents a wound healing response that involves smooth muscle migration, proliferation and matrix deposition at the site of injury. Currently, despite intensive experimental and clinical studies, there are no therapeutic agents that are able to suppress pharmacologically the clinical restenosis. Ultrastructural pathology and animal modeling have played a pivotal role in defining new experimental therapies and rationales for clinical trials. However, concerns regarding the lack of suitable animal models persist and of the many compounds reported efficacious preclinically, none have demonstrated clinical benefit in preventing restenosis. Animal modeling studies now include the use of true restenosis studies, which may be more clinically relevant and pharmacologically predictive of clinical performance.

Mitogenic effect of serotonin on vascular endothelial cells

BACKGROUND

Recent studies indicate that serotonin (5-HT) has a growth-promoting effect on several different cell types, including smooth muscle cells. After percutaneous transluminal coronary angioplasty, there is damage and denudation of vascular endothelial cells, which promotes platelet aggregation at the site of injury. Aggregating platelets release 5-HT; thus, a high concentration of the amine may be present at sites of endothelial damage, which may act as a mitogen to endothelial cells.

METHODS AND RESULTS

The mitogenic effect of 5-HT was examined on canine and bovine aortic endothelial cells by (1) assessing the increase in [3H]thymidine incorporation into DNA and (2) assessing the increase in the absolute number of cells after stimulation with 5-HT. 5-HT at an added concentration of 200 to 1000 mumol/L in the media induced a significant increase in [3H]thymidine incorporation into endothelial cells and an increase in cell number. This effect was not observed with fibroblasts. As the concentrations of added 5-HT were decreased, the endothelial cells had to be stimulated with 5-HT for longer periods to induce the same degree of cellular proliferation. The precursors and metabolic breakdown products of 5-HT were inactive. The 5-HT-induced endothelial proliferation was reversed by 5-HT2 receptor antagonists and pertussis toxin. These data suggest that the mitogenic effect of 5-HT on endothelial cells is mediated by the 5-HT2 receptor, which is coupled to a G protein.

CONCLUSIONS

5-HT is a mitogen for endothelial cells at concentrations likely to be present at sites of vascular injury. This effect is probably mediated via the 5-HT2 receptor. The growth-promoting effects of 5-HT on endothelial cells may facilitate the healing of intima after vascular damage.

Preparation, characterization, and evaluation of a monoclonal antibody against the rabbit platelet glycoprotein IIb/IIIa in an experimental angioplasty model

The deposition of platelets at the site of balloon angioplasty is thought to play a major role in the pathogenesis of restenosis. The antibody AZ-1, which binds to the rabbit platelet glycoprotein IIb/IIIa receptor and inhibits platelet function both in vitro and in vivo, was produced and tested in an experimental model of angioplasty. Atherosclerosis was induced by desiccation injury of the femoral artery, followed by a 28-day diet with 2% cholesterol and 6% peanut oil. Rabbits were randomized to receive an infusion of saline, a single infusion of 0.5 mg/kg of AZ-1, or an infusion of 0.6 mg/kg AZ-1 before angioplasty. The latter group received a second infusion of 0.6 mg/kg 72 hours later. Functional platelet inhibition was demonstrated by prolongation of the bleeding time in all treated animals. Angiography was performed at baseline, immediately after a standardized angioplasty, and again 28 days after angioplasty on a total of 42 vessels. There were no significant differences between the antibody-treated group and the control group in the mean angiographic minimum luminal diameter at any of the time points. There was also no difference in the initial improvement after angioplasty (acute gain), in the decrease in luminal diameter from immediately after angioplasty to 28 days after angioplasty (late loss), or in the overall improvement from before angioplasty to 28 days after angioplasty. Quantitative histological analysis confirmed the lack of a beneficial effect of AZ-1. There were no significant differences in the area of the intima, the media, or the combined intima and media between the antibody-treated groups and the control group. Thus, potent platelet inhibition for up to 6 days after balloon angioplasty using a monoclonal antibody that inhibits platelet aggregation did not reduce the response to vascular injury after balloon angioplasty in this rabbit model of experimental atherosclerosis.

Decreased platelet adherence of polymer-coated tantalum stents

PURPOSE

To compare the acute thrombogenicity of polyurethane-coated stents with that of bare tantalum stents.

MATERIALS AND METHODS

Thirty stents (15 coated with polyurethane) were balloon expanded in 8-mm x 80-cm sections of polytetrafluoroethylene grafts (six stents per graft). Under sterile conditions, 8- and 10-F vascular sheaths were placed percutaneously in the femoral artery and vein of a 100-lb (45-kg) swine. Platelets were labeled with indium-111 and reinjected for quantitative assay. The stented grafts were connected to the arterial and venous sheaths to create an ex vivo fistula. Each fistula was opened for 5, 10, 30, 60, or 120 minutes, exposing heparinized blood to fully expanded stents. The fistulas were closed, flushed with saline until clear, and fixed with formalin. Stents were explanted, placed in a radionuclide well counter, and scanned with electron microscopy.

RESULTS

Quantitatively, there were considerably fewer platelets on coated versus uncoated stents. At 60 minutes, coated stents averaged 12.93 platelets per 1,000 microns 2 compared with 75.88 platelets per 1,000 microns 2 for bare metallic stents. At 120 minutes, there were 23.22 platelets/1,000 microns 2 versus 102.31 platelets/1,000 microns 2, respectively. Electron microscopy of coated stents demonstrated few scattered platelets at 5, 10, and 30 minutes. There was a uniform layer of platelets at 60 and 120 minutes. Uncoated stents demonstrated random areas of platelet clumping at 5, 10, and 30 minutes. At 60 and 120 minutes, uncoated stents showed extensive layering of platelets and fibrin.

CONCLUSION

Polyurethane coating decreases platelet adhesion, relative to bare tantalum, at all time intervals tested.

Differences in compensatory vessel enlargement, not intimal formation, account for restenosis after angioplasty in the hypercholesterolemic rabbit model

BACKGROUND

In de novo human atherosclerosis, compensatory vessel enlargement limits the effect of intimal plaque formation on lumen narrowing. We hypothesized that arterial remodeling may also play an important role in determining the chronic lumen size after angioplasty and tested this hypothesis using the hypercholesterolemic rabbit iliac artery angioplasty model.

METHODS AND RESULTS

Morphometric analysis of histological cross-sectional areas of vessels from animals killed immediately after angioplasty (acute group, n = 11) were compared with the same areas from animals killed 4 weeks after the procedure (chronic group, n = 37), when restenosis occurs in this model. The area circumscribed by the internal elastic lamina (IEL) increased by 20% from acute to 4 week follow-up after angioplasty (acute group, 2.36 +/- 0.45 mm2, chronic group, 2.84 +/- 0.89 mm2). Over the same time period, intimal area increased by 0.82 mm2. Despite this increase in intimal area, lumen area decreased by only 0.34 mm2 because of the compensatory enlargement of the IEL area. In the chronic group, polynomial regression analysis revealed a quadratic relation between intimal area and lumen area (R2 = .35, P < .001). A lumen area of 0.45 mm2 (the nadir of the quadratic relation) was used to divide the chronic group into two subgroups: restenotic (n = 21; lumen area, < 0.45 mm2) and nonrestenotic (n = 16; lumen area, > 0.45 mm2). By definition, there was a significant difference in lumen area between the two subgroups (0.15 +/- 0.15 mm2 for restenotic; 0.73 +/- 0.18 mm2 for nonrestenotic). Surprisingly, the intimal areas in the two subgroups were virtually identical (2.41 +/- 0.92 mm2 for restenotic, 2.49 +/- 0.69 mm2 for nonrestenotic, P = NS). The difference in the lumen area between restenotic and nonrestenotic vessels was a result of the significantly greater IEL area in the nonrestenotic subgroup (3.22 +/- 0.83 mm2 for nonrestenotic, 2.56 +/- 0.84 mm2 for restenotic, P < .05). In both restenotic and nonrestenotic vessels, the IEL area increased with increases in intimal area. In the restenotic arteries, the slope of this correlation was < 1, showing inadequate compensatory enlargement for the intimal plaque. In the nonrestenotic vessels, the slope was > 1, limiting the effect of intimal plaque on luminal narrowing.

CONCLUSIONS

These data indicate that the iliac artery in an atherosclerotic rabbit model compensates for intimal formation after angioplasty by vessel enlargement. Furthermore, the degree of vessel enlargement is more important than intimal area in determining the chronic lumen size.

Prominent inhibitory effects of tranilast on migration and proliferation of and collagen synthesis by vascular smooth muscle cells

To obtain some ideas about prevention of restenosis after percutaneous transluminal coronary angioplasty (PTCA), we examined the effects of transilast (anti-allergic agent) on migration and proliferation of, and collagen synthesis by, cultured vascular smooth muscle cells (VSMC) from the thoracic aorta of WKY rats. Tranilast was added to culture medium containing 10% fetal calf serum (FCS). The cultures were pulse-labeled with 3H-thymidine (TdR) or 3H-proline (Pro). TdR and Pro uptake into VSMC were measured. The effect of tranilast on migration of VSMC was examined by using culture dishes of an original design. We also examined the inhibitory effects of various drugs, such as a Ca antagonist, an angiotensin converting enzyme (ACE) inhibitor, a phosphodiesterase inhibitor, elastase, colchicine, and mitomycin C, on proliferation and migration of VSMC. Our data showed that the inhibitory effects of tranilast on migration and proliferation of, and collagen synthesis by, VSMC were prominent. Maximal percentage inhibition of proliferation, migration and collagen synthesis was 60.8 +/- 2.3%, 52.7 +/- 14.7% and 62.1 +/- 8.1%, respectively. On the other hand, the inhibitory effects of other drugs, with the exception of colchicine and mitomycin C, on proliferation and/or migration of VSMC were not very strong. Although the inhibitory effects of colchicine and mitomycin C were strong in vitro, their clinical usefulness may be limited by systemic side-effects. These results indicate the potential usefulness of tranilast for prevention of restenosis of coronary arteries after PTCA.

Dipyridamole directly inhibits vascular smooth muscle cell proliferation in vitro and in vivo: implications in the treatment of restenosis after angioplasty

OBJECTIVES

The effect of dipyridamole on smooth muscle cell proliferation and prevention of intimal thickening after arterial injury was investigated.

BACKGROUND

In addition to antiplatelet activity, dipyridamole also inhibits cell proliferation. We examined whether the antiproliferative action of dipyridamole on smooth muscle cells, as demonstrated here, has a direct effect on intimal thickening after vascular injury.

METHODS

Cell proliferation was determined by measuring deoxyribonucleic acid (DNA) synthesis and by cell counting. The in vivo effect of locally delivered dipyridamole was determined in a rabbit model with carotid or femoral artery injury.

RESULTS

Dipyridamole produced a dose-dependent inhibition of smooth muscle cell proliferation, producing 50% inhibition at 7 micrograms/ml. Structural analogues SH-869 and mopamidol were 10 to 100 times less effective than dipyridamole, suggesting that cell growth inhibition may be unrelated to the antiplatelet activity of dipyridamole. Inhibition of cell proliferation by dipyridamole was attenuated by increasing the serum concentration in the culture medium. Bypassing serum by local delivery of dipyridamole at the periadventitial site produced 63% inhibition (p < 0.05) of cell replication in balloon-injured arteries. Locally delivered dipyridamole also inhibited intimal thickening (20%, p < 0.05) after balloon injury.

CONCLUSIONS

Dipyridamole inhibited smooth muscle cell proliferation in vitro. This activity was attenuated by serum proteins. Locally delivered dipyridamole inhibited cell replication in arteries and intimal thickening after balloon injury. These results suggest that although systemic treatment with dipyridamole may not be efficacious because of inadequate serum levels, its antiproliferative action on smooth muscle cells may reduce restenosis when the drug is delivered locally after coronary angioplasty.

Platelet inhibition with ASA/dipyridamole after percutaneous balloon angioplasty in patients with symptomatic lower limb arterial disease. A prospective double-blind trial. Study group on pharmacological treatment after PTA

A randomised double-blind multicentre trial has been performed comparing the influence of 3 months prophylaxis with acetylsalicylic acid/dipyridamole (50 mg/400 mg daily) or placebo on the outcome following percutaneous transluminal angioplasty for lower limb ischaemia. Two hundred and twenty-three patients were included and followed. There were no differences between the groups regarding results 1, 3, 6 and 12 months following the dilatation treatment. It can be concluded that acetylsalicylic acid at the dose of 50 mg daily combined with dipyridamole does not seem to have a prophylactic effect after transluminal angioplasty during a follow-up period of 1 year.

Experimental studies on prevention of atherosclerotic arterial stenosis and restenosis after angioplasty with Andrographis Paniculata Nees and fish oil

In order to search for effective drugs to reduce restenosis incidence after coronary angioplasty, we studied the effects of a Chinese herb, extract of Andrographis Paniculata Nees (APN), and Fish Oil (FO) on atherosclerotic stenosis and restenosis after experimental angioplasty. Preliminary results showed that APN can significantly alleviate atherosclerotic iliac artery stenosis induced by both deendothelialization and high cholesterol diet (control group, stenosis incidence 100%, stenotic severe degree 60.53 +/- 31.03%, of which 30% arteries (6) are total occlusion; FO group: stenotic incidence and severe degree are 77% and 53.00 +/- 21.17%, respectively, and in APN group they are 70% and 25.39 +/- 10.52%, respectively, P < 0.01), and follow-up angiography 4 weeks after angioplasty showed that dilated iliac arteries in control group all had severe restenosis, but in APN group no or only mild restenosis occurs, and in FO group restenosis is as severe as stenotic degree prior to angioplasty. These preliminary results suggest that APN and FO can significantly alleviate stenosis induced by deendothelialization and high cholesterol diet and restenosis after angioplasty, while the former has a more marked effect. The above findings lead the authors to conclude that APN may play an important role in preventing restenosis after coronary angioplasty, but FO may be useful in reducing the extent of of restenosis after coronary angioplasty.

Southwestern internal medicine conference: restenosis: the Achilles heel of coronary angioplasty

Percutaneous transluminal coronary angioplasty has become the treatment of choice for many patients with symptomatic coronary artery disease. Increased experience with the procedure and improvements in equipment have resulted in high initial success rates; however, a significant number of patients develop restenosis. Insights into the pathophysiologic mechanisms of restenosis have led to the use of various pharmacologic agents and devices to prevent its occurrence. Although many have been successful in decreasing the incidence of restenosis in animal studies, none has yet proven successful in decreasing the incidence of restenosis in humans. Newer approaches and novel therapies are needed to prevent restenosis after percutaneous transluminal coronary angioplasty.

Infrainguinal vein graft stenosis

Although knowledge of the biological processes involved in the development of intimal hyperplasia has increased markedly in recent years, the precise aetiology of infrainguinal vein graft stenosis remains undetermined. Current therapy is therefore directed at treatment of the established lesion rather than its prevention. There seems little doubt, however, that recent advances in understanding of the vascular biology of normal and pathological saphenous vein will eventually lead to specific targeted therapy that will allow the prevention of vein graft stenosis.

Pathophysiological mechanisms for restenosis following coronary angioplasty: possible preventive alternatives

Restenosis after successful percutaneous transluminal coronary angioplasty (PTCA) remains an unsolved medical problem. The search for the underlying pathophysiological mechanisms have identified intimal proliferation of smooth muscle cells (SMC) to be the prevailing cause of late restenosis, with endothelial cells (EC) and platelets being important participators in the process. According to the most accepted present theory, SMC would be stimulated to migrate and proliferate shortly after the angioplasty by the release of growth factors from injured EC and accumulated platelets. However, clinical trials of agents interfering with these mechanisms have not significantly diminished the rate of restenosis, which suggest both that our knowledge of the process is incomplete, and that new ways of administering the agents may be required.

Application of photodynamic therapy to the treatment of atherosclerotic plaques

Photodynamic therapy is a therapeutic modality long studies for its application to the treatment of malignant neoplasms. Recently, studies have suggested its potential use in the treatment of atherosclerosis. In this study, two atherosclerotic plaques were induced in the abdominal aortas of 35 rabbits. The animals then received Photofrin II (Quadralogic Technologies Inc., Pearl River, NY), a photosensitizer, at doses of 5 mg/kg and 2.5 mg/kg. After 48 hours, the plaques were irradiated by a fiberoptic connected to an argon ion laser. Fluency rates from 32 mW to 256 mW and energy doses from 1.6 to 60 joules were applied. Only one of the paired plaques was irradiated, the other remaining as a control. Four weeks after treatment, the vessels were assessed. Of 26 plaques treated with photodynamic therapy, 22 were no longer grossly visible, while the nine animals that received light irradiation but no Photofrin II all had visible plaque (P < 0.001). Studies of the vessel sections confirmed a reduction in intimal thickness from 0.74 +/- 0.15 mm in matched controls as compared with 0.51 +/- 0.13 mm in animals with treated plaques. There was a concomitant enlargement of the luminal diameter from 1.13 +/- 0.51 to 1.41 +/- 0.72. On the microscopic level, plaque reduction was most complete in the groups treated with 40 and 60 joules. Different fluency rates and drug dosages did not lead to differing outcomes. Our findings indicate that photodynamic therapy with dihematoporphyrin ether met our goal of reducing plaque size and may represent a means of treating atherosclerotic plaques.

Hirudins: antithrombin anticoagulants

OBJECTIVE

To review the chemistry, pharmacology, available clinical data, and adverse effects of the hirudin anticoagulants.

DATA SOURCES

A MEDLINE search and a review of recent scientific abstracts was conducted to identify pertinent literature.

STUDY SELECTION

Focus was placed on studies conducted in humans. Because hirudin is still an investigational agent, however, relevant animal data, particularly pharmacokinetic studies and studies of preclinical efficacy, were also selected.

DATA EXTRACTION

Data from both human and animal studies were evaluated; emphasis was placed on human trials.

DATA SYNTHESIS

Hirudin has demonstrated potent anticoagulant effects. Although hirudin could have a significant impact on the therapeutic management of patients requiring anticoagulant therapy, only a limited number of human studies have been published to date. Trials comparing hirudin and heparin in specific patient populations are still ongoing.

CONCLUSIONS

Although still in clinical trials, hirudin is a unique agent that may represent a breakthrough in anticoagulant therapy. The specific role that this agent will play in the management of patients has yet to be determined.

The restenosis paradigm revisited: an alternative proposal for cellular mechanisms

Coronary restenosis is a reparative response to arterial injury during angioplasty, and remains a major clinical problem. The reasons for treatment failures likely stem from our incomplete understanding of the cellular mechanisms in restenotic neointimal formation. Restenosis is thought to result from migration and replication of medial smooth muscle cells to form an obstructive neointima, a concept neither observed nor demonstrated in humans. An alternative hypothesis for restenosis is based on observations in the porcine coronary injury model. In this model, there are three cellular stages in neointimal formation: thrombotic (stage I), cellular recruitment (stage II) and proliferative (stage III). The thrombotic stage occurs early and consists of platelets, fibrin and red blood cells accumulating at the vessel injury site. In the recruitment stage, the mural thrombus itself develops an endothelium, followed by a mononuclear leukocytic infiltrate beginning on the lumen side of the vessel. In the proliferative stage, a "cap" of actin-positive cells forms on the lumen surface and progressively thickens. These cells do not arise from media at the injury site. Extracellular matrix secretion and additional recruitment likely add to neointimal volume during this phase. Thrombus assumes a major role in restonosis by providing an absorbable matrix into which smooth muscle cells proliferate. Further studies are needed to validate or modify this hypothesis.

Coronary intimal proliferation after balloon injury and stenting in swine: an animal model of restenosis

OBJECTIVES

This study was designed to compare the proliferative response in coronary arteries after tantalum stent placement or balloon injury in a normolipemic swine model of restenosis.

BACKGROUND

Restenosis remains a significant complication of percutaneous transluminal coronary angioplasty. Efforts to study restenosis have been hampered by the lack of a suitable animal model.

METHODS

In an attempt to create lesions resembling those of human restenosis, normolipemic swine underwent injury of either the left anterior descending or the left circumflex coronary artery with either balloon inflation or deployment of a tantalum stent. At 4 weeks, they were killed and the injured vessels processed for histopathologic analysis. Intimal area, lumen area and maximal intimal thickness were measured. The degree of stenosis was expressed as residual lumen area (lumen area/intimal area ratio).

RESULTS

Vessels injured by either method demonstrated significant intimal smooth muscle proliferation leading to reduction in lumen area. In the 18 stented vessels residual lumen area measured 0.64 +/- 0.18 and maximal intimal thickness measured 0.6 +/- 0.3 mm; in the 15 balloon-injured vessels these values were 0.75 +/- 0.18 and 0.4 +/- 0.3 mm, respectively (p less than 0.05). In addition, most stented vessels had reactive inflammatory infiltrates surrounding the stent wires composed of lymphocytes, histiocytes and many eosinophils.

CONCLUSIONS

These data indicate that coronary artery injury in swine with either balloon inflation or stenting leads to intimal smooth muscle cell proliferation similar to that seen in human restenosis. The degree of intimal proliferation appears to be greater after stenting than after balloon injury. Intracoronary stenting in swine is associated with a marked inflammatory reaction around the stent wires. These models may be helpful in planning systemic and local antirestenosis strategies.

Biology of restenosis and therapeutic approach

Numerous attempts have been made to prevent restenosis after successful transluminal dilation of an atherosclerotic vessel using a variety of pharmacologic and mechanical approaches. This article reviews the pathobiology of the restenosis process, offers a hypothesis as to its cause, reviews attempts to modify the process, and outlines therapeutic approaches to future treatment.

The effect of antiplatelet therapy on platelet accumulation after experimental angioplasty in the rabbit iliac model

Antiplatelet therapy is routinely used in balloon angioplasty. Yet recent clinical trials have not shown these agents to reduce the incidence of restenosis. As this might be due to inadequate antiplatelet effect, this study was designed to evaluate the effectiveness of antiplatelet therapy in reducing platelet accumulation immediately after angioplasty. Sixty-six atherosclerotic rabbits underwent angioplasty of a focal (50-99%) iliac stenosis after pretreatment with an antiplatelet drug. The animals were randomly given aspirin (10 mg/kg orally) (n = 10), aspirin (10 mg/kg intravenously) (n = 7), aspirin (10 mg/kg per os) plus dipyridamole (25 mg orally) (n = 8), a thromboxane synthetase inhibitor (GS13080, 1 mg/kg/h intravenously) (n = 9) or heparin (500 U/kg intravenously) (n = 9). Platelet accumulation, determined by 51Cr-labeled platelet counts 30 min after angioplasty disclosed a significant reduction from control (44.1 +/- 51.2 platelets/cm x 10(6)) with intravenous aspirin (3.3 +/- 1.9; p less than 0.007) and the thromboxane synthetase inhibitor CGS13080 (15.1 +/- 16.6, p less than 0.04). Aspirin plus dipyridamole reduced platelet accumulation as well (10.4 +/- 9.8, p = 0.076), while oral aspirin (109.5 +/- 240) and heparin (34.7 +/- 35.5) were ineffective. The results of this study demonstrate that pretreatment with antiplatelet agents can reduce platelet accumulation significantly after experimental angioplasty. However, the wide variability in effectiveness particularly with oral aspirin may have important clinical implications. Further investigation into more effective antiplatelet therapy may help to reduce the incidence of abrupt vessel closure and restenosis.

Repeat coronary angioplasty as treatment for restenosis

Repeat coronary angioplasty has become the standard approach to a first restenosis. However, the long-term outcome of such a strategy is not well defined. In the present study, 465 patients (mean age 58 years [range 27 to 79], 53% with multivessel disease) underwent a second angioplasty procedure at the same site. The procedure was successful in 96.8% with a 1.5% rate of in-hospital bypass surgery, a 0.9% incidence rate of myocardial infarction and no procedural deaths. Four hundred sixty-three patients (99.6%) were followed up for a mean of 40.5 months. Forty-nine patients (10.6%) underwent a third angioplasty procedure at the same site, 55 (11.8%) had coronary bypass surgery and 33 (7.1%) underwent angioplasty at a different site. During follow-up, 12 patients (2.6%) sustained a myocardial infarction and 21 (4.5%) died including 13 (2.8%) with cardiac death. Of the 442 surviving patients, 88% experienced sustained functional improvement and 78% were free of angina. The actuarial 5-year cardiac survival rate was 96% and the rate of freedom from cardiac death and myocardial infarction was 92%. For the subgroup of 49 patients who had a third angioplasty procedure at the same site, the success rate was 93.9% with a 2% incidence rate of myocardial infarction. There were no in-hospital deaths or coronary artery bypass operations. The mean follow-up interval for this subgroup was 30.5 months with a 22.4% cross-over rate to coronary bypass surgery, a 4.1% incidence rate of myocardial infarction and a 2% cardiac mortality rate. At last follow-up, 89% of patients had sustained functional improvement and 76% were free of angina. The combined angiographic and clinical restenosis rate was 48%. Repeat angioplasty as treatment for restenosis is an effective approach associated with a high success rate, low incidence of procedural complications, and sustained functional improvement in combination with an acceptable rate of bypass surgery. However, there is a trend toward diminished angioplasty efficacy after a second restenosis. Thus, decisions for further revascularization should be made after careful review of available options.