Development of a platform to evaluate and limit in-stent restenosis

Local Stent-Based Release of Transforming Growth Factor-β1 Limits Arterial In-Stent Restenosis

The long-term success of intra-arterial stenting remains limited by in-stent restenosis (ISR). Transforming growth factor-β1 (TGF-β1) can inhibit smooth muscle cell (SMC) proliferation and migration and convert SMCs into extracellular matrix (ECM)-synthesizing cells. Here, we evaluate the effects of stent-based delivery of TGF-β1 on ISR in a rabbit model. Channeled stents loaded with TGF-β1 or control microspheres were deployed in rabbit aortas. Stented aortas were harvested at 7 and 28 d and evaluated for Ki-67-positive cells, collagenous ECM production, and intima-to-media (I/M) ratio. At 7 d, the TGF-β1 group exhibited fewer Ki-67-positive cells were found for the TGF-β1 group (17.87 ± 2.18 cells per mm(2)) relative to control (25.07 ± 2.65 cells per mm(2), p = 0.04), but increased collagen content (31.4 ± 2.5 percentage area) compared with control (29.3 ± 1.2 percentage area, p = 0.019). The I/M ratio in the TGF-β1 group was reduced by 50% and 9.1% versus control at 7 d (0.13 ± 0.02 vs. 0.26 ± 0.02, p = 0.0001) and 28 d (1.80 ± 0.05 vs. 1.98 ± 0.08, p = 0.0038), respectively. Stent-based controlled release of TGF-β1 limits ISR and is associated with inhibition of SMC proliferation but an increase in ECM production.

Microfabrication and nanotechnology in stent design

Intravascular stents were first introduced in the 1980s as an adjunct to primary angioplasty for management of early complications, including arterial dissection, or treatment of an inadequate technical outcome due to early elastic recoil of the atherosclerotic lesion. Despite the beneficial effects of stenting, persistent high rates of restenosis motivated the design of drug-eluting stents for delivery of agents to limit the proliferative and other inflammatory responses within the vascular wall that contribute to the development of a restenotic lesion. These strategies have yielded a significant reduction in the incidence of restenosis, but challenges remain, including incomplete repair of the endothelium at the site of vascular wall injury that may be associated with a late risk of thrombosis. A failure of vessel wall healing has been attributed primarily to the use of polymeric stent coatings, but the effects of the eluted drug and other material properties or design features of the stent cannot be excluded. Improvements in stent microfabrication, as well as the introduction of alternative materials may help to address those limitations that inhibit stent performance. This review describes the application of novel microfabrication processes and the evolution of new nanotechnologies that hold significant promise in eliminating existing shortcomings of current stent platforms.

Directed Migration of Smooth Muscle Cells to Engineer Plaque-Resistant Vein Grafts

PURPOSE

To test the hypothesis that controlled perivascular release of tissue plasminogen activator (tPA) can generate cleaved extracellular matrix (ECM) chemotactic gradients to guide the migration of vascular smooth muscle cells (SMCs) away from the lumen, thereby limiting neointima formation.

METHODS

This hypothesis was tested in rabbit models in which the perivascular surface of vein bypass grafts was treated with microspheres releasing tPA (MS-tPA), microspheres containing no drug (MS-blank), or phosphate buffered saline (PBS). Vein graft segments harvested after 7 days were then evaluated for elastin content, proliferating SMCs, intima-to-media (I/M) ratio, and inflammation; late impact on neointima formation was also examined.

RESULTS

The 7-day results demonstrated cleaved elastin gradients and proliferating SMCs that assumed a more peripheral distribution in the MS-tPA group than MS-blank and PBS controls (p<0.05). At 28 days, vein grafts treated with MS-tPA showed a mean I/M ratio (0.35+/-0.04) that was 63.5% lower than PBS controls (0.96+/-0.07, p<0.005) and 43.5% lower than MS-blank specimens (0.62+/-0.08, p<0.05).

CONCLUSIONS

Perivascular release of tPA modifies ECM gradients, directionally guides SMC migration away from the lumen, and limits neointima formation.

Efficient inhibition of in-stent restenosis by controlled stent-based inhibition of elastase: a pilot study

PURPOSE

It is proposed that local elastase inhibition could suppress the extracellular matrix (ECM) degradation and subsequent smooth muscle cell migration and limit subsequent in-stent restenosis. This study evaluated the effect of stent-based controlled elastase inhibition on restenosis after stent implantation in a rabbit model.

MATERIALS AND METHODS

Biodegradable microspheres containing the potent elastase inhibitor alpha-1-antitrypsin (AAT) were prepared. Daily release of AAT from the microspheres was confirmed in vitro. The microspheres were loaded into stents with an abluminal polymer reservoir. Implantation of the stent with AAT microspheres and blank microspheres (control) was performed in the abdominal aortae of six rabbits in each group. After stent deployment, all stents were overdilated to 125% diameter. Stent-implanted arteries were harvested after 7 days (n = 3 each) or 28 days (n = 3 each). To assess the effect of local delivery of AAT, elastase activity and elastin content of the stent-implanted aortae were analyzed. As an endpoint, intima-to-media (I/M) ratio was determined in the 7-day and 28-day specimens.

RESULTS

Significant inhibition of elastase was confirmed in treated vessels versus controls at 7 days after stent implantation (P < .05). This reduction in elastase activity was sufficient to afford early and late reduction of in-stent neointima. Plaque progression in the 28-day specimens decreased to 67% with elastase inhibition relative to controls (P < .05).

CONCLUSION

Stent-based controlled release of elastase inhibitor may significantly reduce ECM degradation and might limit in-stent restenosis.

Liver tissue engineering at extrahepatic sites in mice as a potential new therapy for genetic liver diseases

Liver tissue engineering using hepatocyte transplantation has been proposed as an alternative to whole-organ transplantation or liver-directed gene therapy to correct various types of hepatic insufficiency. Hepatocytes are not sustained when transplanted under the kidney capsule of syngeneic mice. However, when we transplanted hepatocytes with the extracellular matrix components extracted from Engelbreth-Holm-Swarm cells, hepatocytes survived for at least 140 days and formed small liver tissues. Liver engineering in hemophilia A mice reconstituted 5% to 10% of normal clotting activity, enough to reduce the bleeding time and have a therapeutic benefit. Conversely, the subcutaneous space did not support the persistent survival of hepatocytes with Engelbreth-Holm-Swarm gel matrix. We hypothesized that establishing a local vascular network at the transplantation site would reduce graft loss. To test this idea, we provided a potent angiogenic agent before hepatocyte transplantation into the subcutaneous space. With this procedure, persistent survival was achieved for the length of the experiment (120 days). To establish that these engineered liver tissues also retained their native regeneration potential in vivo, we induced two different modes of proliferative stimulus to the naive liver and confirmed that hepatocytes within the extrahepatic tissues regenerated with activity similar to that of naive liver. In conclusion, our studies indicate that liver tissues can be engineered and maintained at extrahepatic sites, retain their capacity for regeneration in vivo, and used to successfully treat genetic disorders.

Stent-based Controlled Release of Intravascular Angiostatin to Limit Plaque Progression and In-Stent Restenosis

PURPOSE

To evaluate the importance of angiogenesis in plaque progression after stent placement, this study examines stent-based controlled delivery of the antiangiogenic agent, angiostatin, in a rabbit model.

MATERIALS AND METHODS

Controlled release biodegradable microspheres delivering angiostatin or polymer-only microspheres (polylactic-co-glycolic-acid-polyethylene glycol; PLGA/PEG) were loaded in channeled stents, anchored, and deployed in the aorta of adult New Zealand white rabbits (n = 6 animals per group, three each per time point). The stented aortas were harvested at 7 days and 28 days and evaluated for neovascularization, local inflammation, vascular smooth muscle cell proliferation, and in-stent plaque progression.

RESULTS

At 7 days, neovascularization was significantly decreased in the angiostatin groups (1.6 +/- 1.6 neovessels per mm(2) plaque) versus the control group (15.4 +/- 2.6 neovessels per mm(2) plaque; P =.00081), as were local inflammation where angiostatin-treated groups demonstrated significantly lower macrophage recruitment per cross section (34.9 +/- 4.9 cells per cross section) relative to the control group (55.2 +/- 3.84 cells per cross section; P =.0037). And a significant decrease in the overall vascular smooth muscle cell proliferation (143.8 +/- 26.3 Ki-67 positive cells per mm(2)) relative to the control group (263.2 +/- 16.6 Ki-67 positive cells per mm(2); P =.00074). At both 7 and 28 days, in-stent plaque progression in the angiostatin groups was successfully limited relative to the control group by 54% (0.255 +/- 0.019% of cross section; P =.00016) and 19% (1.981 +/- 0.080; P =.0033) respectively and resulted in reduction of in-stent restenosis relative to the control group.

CONCLUSION

Angiostatin-eluting stents may limit neovascularity after arterial implantation, offer insight into in-stent restenosis, and allow future refinement of bioactive stent designs and clinical strategies, particularly in light of evidence that intimal smooth muscle cells may in part be marrow-derived.

In-stent restenosis limitation with stent-based controlled-release nitric oxide: initial results in rabbits

PURPOSE

To evaluate effect of controlled stent-based release of an NO donor to limit in-stent restenosis in rabbits.

MATERIALS AND METHODS

Bioerodable microspheres containing NO donor or biodegradable polymer (polylactide-co-glycolide-polyethylene glycol) were prepared and loaded in channeled stents. Daily concentrations of NO release from NO-containing microspheres were assayed in vitro. NO- and polymer-containing (control) microsphere-loaded stents were deployed in aortas of New Zealand white rabbits (n = 8). Aortas with stents were harvested at 7 (n = 5) and 28 days (n = 3) and evaluated for cyclic guanosine monophosphate (cGMP) levels (7 days), number of proliferating cell nuclear antigen-positive cells (7 days), and intima-to-media ratio (7 and 28 days), with statistical significance evaluated by using one-way analysis of variance.

RESULTS

NO-containing microspheres released NO with an initial bolus in the 1st week, followed by sustained release for the remaining 3 weeks. Significant increase in cGMP levels and decrease in proliferating cell nuclear antigen-positive cells were found at 7 days for the NO-treated group relative to controls (P <.05). Intima-to-media ratio in the NO-treated group was reduced by 46% and 32% relative to controls at 7 and 28 days, respectively (mean, 0.14 +/- 0.01 [standard error] vs 0.26 +/- 0.02 at 7 days, P <.01; 1.34 +/- 0.05 vs 1.98 +/- 0.08 at 28 days, P <.01).

CONCLUSION

Stent-based controlled release of NO donor significantly reduces in-stent restenosis and is associated with increase in vascular cGMP and suppression of proliferation.