Local effects of nitric oxide supplementation and suppression in the development of intimal hyperplasia in experimental vein grafts

Nitric oxide releasing nanomatrix gel treatment inhibits venous intimal hyperplasia and improves vascular remodeling in a rodent arteriovenous fistula

Vascular access is the lifeline for hemodialysis patients and the single most important component of the hemodialysis procedure. Arteriovenous fistula (AVF) is the preferred vascular access for hemodialysis patients, but nearly 60% of AVFs created fail to successfully mature due to early intimal hyperplasia development and poor outward remodeling. There are currently no therapies available to prevent AVF maturation failure. First, we showed the important regulatory role of nitric oxide (NO) on AVF development by demonstrating that intimal hyperplasia development was reduced in an overexpressed endothelial nitric oxide synthase (NOS3) mouse AVF model. This supported the rationale for the potential application of NO to the AVF. Thus, we developed a self-assembled NO releasing nanomatrix gel and applied it perivascularly at the arteriovenous anastomosis immediately following rat AVF creation to investigate its therapeutic effect on AVF development. We demonstrated that the NO releasing nanomatrix gel inhibited intimal hyperplasia formation (more than 70% reduction), as well as improved vascular outward remodeling (increased vein diameter) and hemodynamic adaptation (lower wall shear stress approaching the preoperative level and less vorticity). Therefore, direct application of the NO releasing nanomatrix gel to the AVF anastomosis immediately following AVF creation may enhance AVF development, thereby providing long-term and durable vascular access for hemodialysis.

Experimental implantation of an arterial substitute made of silicone reinforced with polyester fabric in rabbits

OBJECTIVES

The aim of this study was to analyze silicone tubes with an internal diameter of 4 mm as a possible material for vascular prostheses.

METHODS

Grafts were implanted into the infrarenal aortas of 33 rabbits. Fluoroscopic examinations were performed within 150 days after surgical implantation. Sample grafts were analyzed via electron microscopy to evaluate the eventual endothelialization of the prostheses.

RESULTS

The patency rates of the prostheses were 87% (±6.7%) after 30 days, 73% (±9.3%) after 60 days and 48% (±12%) after 120 days. The material presented characteristics that support surgical implantation: good tolerance promoted by polyester tear reinforcement, ease of postoperative removal and a lack of pseudoaneurysms. However, intimal hyperplasia was a limiting factor for the patency rate.

CONCLUSIONS

We concluded that polydimethylsiloxane has limited potential as an alternative material for small vascular prostheses.

Targeted Nitric Oxide Delivery by Supramolecular Nanofibers for the Prevention of Restenosis After Arterial Injury

AIMS

Cardiovascular interventions continue to fail as a result of arterial restenosis secondary to neointimal hyperplasia. We sought to develop and evaluate a systemically delivered nanostructure targeted to the site of arterial injury to prevent neointimal hyperplasia. Nanostructures were based on self-assembling biodegradable molecules known as peptide amphiphiles. The targeting motif was a collagen-binding peptide, and the therapeutic moiety was added by S-nitrosylation of cysteine residues.

RESULTS

Structure of the nanofibers was characterized by transmission electron microscopy and small-angle X-ray scattering. S-nitrosylation was confirmed by mass spectrometry, and nitric oxide (NO) release was assessed electrochemically and by chemiluminescent detection. The balloon carotid artery injury model was performed on 10-week-old male Sprague-Dawley rats. Immediately after injury, nanofibers were administered systemically via tail vein injection. S-nitrosylated (S-nitrosyl [SNO])-targeted nanofibers significantly reduced neointimal hyperplasia 2 weeks and 7 months following balloon angioplasty, with no change in inflammation.

INNOVATION

This is the first time that an S-nitrosothiol (RSNO)-based therapeutic was shown to have targeted local effects after systemic administration. This approach, combining supramolecular nanostructures with a therapeutic NO-based payload and a targeting moiety, overcomes the limitations of delivering NO to a site of interest, avoiding undesirable systemic side effects.

CONCLUSION

We successfully synthesized and characterized an RSNO-based therapy that when administered systemically, targets directly to the site of vascular injury. By integrating therapeutic and targeting chemistries, these targeted SNO nanofibers provided durable inhibition of neointimal hyperplasia in vivo and show great potential as a platform to treat cardiovascular diseases.

Nitric oxide delivery via a permeable balloon catheter inhibits neointimal growth after arterial injury

BACKGROUND

Neointimal hyperplasia limits the longevity of vascular interventions. Nitric oxide (NO) is well known to inhibit neointimal hyperplasia. However, delivery of NO to the vasculature is challenging. Our study aims to evaluate the efficacy of delivering NO to the site of injury using a permeable balloon catheter. Our hypothesis is that ultra-short duration NO delivery using a permeable balloon catheter will inhibit neointimal hyperplasia.

MATERIALS AND METHODS

Ten-week-old male Sprague-Dawley rats underwent carotid artery balloon injury. Groups included: (1) control, (2) injury, (3) injury + periadventitial NO, and (4) injury + endoluminal NO via permeable balloon catheter. The catheter was inflated to 5 atm pressure for 5 min. Arteries were harvested 2 wk following injury. Morphometric assessment for neointimal hyperplasia and immunohistochemical staining for inflammatory markers were performed.

RESULTS

Injury increased neointimal hyperplasia compared with control (intima/media area [I/M] ratio 1.07 versus 0.11, respectively, P < 0.001). Periadventitial delivery of NO reduced the I/M area ratio compared with injury alone (55% decrease, P < 0.001). Endoluminal delivery of NO also reduced the I/M area ratio compared with injury alone (65% decrease; P < 0.001). Both endoluminal and periadventitial NO affected the I/M ratio by reducing the intimal area (64% and 46%, respectively, P < 0.001) whereas neither affected the medial area. Periadventitial NO delivery increased lumen area (P < 0.05), whereas endoluminal NO delivery increased circumference (P < 0.05). Periadventitial NO delivery inhibited macrophage intimal infiltration compared with injury alone (P < 0.05).

CONCLUSIONS

These data demonstrate that short-duration endoluminal NO delivery via permeable balloon catheters inhibits neointimal hyperplasia following arterial interventions. Endoluminal delivery of NO could become a focus for future clinical interventions.

Nitric oxide and nitrite-based therapeutic opportunities in intimal hyperplasia

Vascular intimal hyperplasia (IH) limits the long term efficacy of current surgical and percutaneous therapies for atherosclerotic disease. There are extensive changes in gene expression and cell signaling in response to vascular therapies, including changes in nitric oxide (NO) signaling. NO is well recognized for its vasoregulatory properties and has been investigated as a therapeutic treatment for its vasoprotective abilities. The circulating molecules nitrite (NO(2)(-)) and nitrate (NO(3)(-)), once thought to be stable products of NO metabolism, are now recognized as important circulating reservoirs of NO and represent a complementary source of NO in contrast to the classic L-arginine-NO-synthase pathway. Here we review the background of IH, its relationship with the NO and nitrite/nitrate pathways, and current and future therapeutic opportunities for these molecules.

Insights into the effect of nitric oxide and its metabolites nitrite and nitrate at inhibiting neointimal hyperplasia

OBJECTIVE

Periadventitial delivery of the nitric oxide (NO) donor PROLI/NO following arterial injury effectively inhibits neointimal hyperplasia. Given the short half-life of NO release from PROLI/NO, our goal was to determine if inhibition of neointimal hyperplasia by PROLI/NO was due to NO, or its metabolites nitrite and nitrate.

METHODS AND RESULTS

In vitro, the NO donor DETA/NO inhibited proliferation of rat aortic vascular smooth muscle cells (RASMC), but neither nitrite nor nitrate did. In vivo, following rat carotid artery balloon injury or injury plus the molar equivalents of PROLI/NO, nitrite, or nitrate (n=8-11/group), PROLI/NO was found to provide superior inhibition of neointimal hyperplasia (82% inhibition of intimal area, and 44% inhibition of medial area, p<0.001). Only modest inhibition was noted with nitrite or nitrate (45% and 41% inhibition of intimal area, and 31% and 29% inhibition of medial area, respectively, p<0.001). No effects on blood pressure were noted with any treatment groups. In vivo, only PROLI/NO inhibited cellular proliferation and increased arterial lumen area compared to injury alone (p<0.001). However, all three treatments inhibited inflammation (p<0.001).

CONCLUSIONS

PROLI/NO was more effective at inhibiting neointimal hyperplasia following arterial injury than nitrite or nitrate. However, modest inhibition of neointimal hyperplasia was observed with nitrite and nitrate, likely secondary to anti-inflammatory actions. In conclusion, we have demonstrated that the efficacy of NO donors is primarily due to NO production and not its metabolites, nitrite and nitrate.

Poly(diol-co-citrate)s as novel elastomeric perivascular wraps for the reduction of neointimal hyperplasia

The synthesis of poly(diol-co-citrate) elastomers that are biocompatible with vascular cells and can modulate the kinetics of the NO release based on the diol of selection is reported. NO-mediated cytostatic or cytotoxic effects can be controlled depending on the NO dose and the exposure time. When implanted in vivo in a rat carotid artery injury model, these materials demonstrate a significant reduction of neointimal hyperplasia. This is the first report of a NO-releasing polymer fabricated in the form of an elastomeric perivascular wrap for the treatment of neointimal hyperplasia. These elastomers also show promise for other cardiovascular pathologies where NO-based therapies could be beneficial.

Zoledronate inhibits intimal hyperplasia in balloon-injured rat carotid artery

BACKGROUND AND OBJECTIVE

Zoledronate has been reported to inhibit the proliferation, adhesion and migration of vascular smooth muscle cells. In the present study, we assessed whether systemic and local delivery of zoledronate would be sufficient to prevent intimal hyperplasia.

METHODS

Twenty-four male Sprague-Dawley rats were assigned into four groups: non-treated group, systemic zoledronate-treated group, local collagen-treated group and local zoledronate-treated group. All four groups underwent balloon injury to the right common carotid artery. The left uninjured carotid arteries of the non-treated group were considered as normal artery samples. Twenty-one days after arterial injury and treatment, the right and left common carotid arteries were fixed, sectioned, stained and measured by computer-aided image analysis.

RESULTS

At 3 weeks, there was a 59% reduction of the intima/media area ratio in the systemic zoledronate-treated group compared with the non-treated group (P < 0.01). There was an 87% reduction of the intima/media area ratio in the local zoledronate-treated group compared with the local collagen-treated group (P < 0.01).

CONCLUSIONS

Both systemic and local delivery of zoledronate correspond to a significant reduction in intimal hyperplasia seen at 3 weeks.

Heightened efficacy of nitric oxide-based therapies in type II diabetes mellitus and metabolic syndrome

Type II diabetes mellitus (DM) and metabolic syndrome are associated with accelerated restenosis following vascular interventions due to neointimal hyperplasia. The efficacy of nitric oxide (NO)-based therapies is unknown in these environments. Therefore, the aim of this study is to examine the efficacy of NO in preventing neointimal hyperplasia in animal models of type II DM and metabolic syndrome and examine possible mechanisms for differences in outcomes. Aortic vascular smooth muscle cells (VSMC) were harvested from rodent models of type II DM (Zucker diabetic fatty), metabolic syndrome (obese Zucker), and their genetic control (lean Zucker). Interestingly, NO inhibited proliferation and induced G0/G1 cell cycle arrest to the greatest extent in VSMC from rodent models of metabolic syndrome and type II DM compared with controls. This heightened efficacy was associated with increased expression of cyclin-dependent kinase inhibitor p21, but not p27. Using the rat carotid artery injury model to assess the efficacy of NO in vivo, we found that the NO donor PROLI/NO inhibited neointimal hyperplasia to the greatest extent in type II DM rodents, followed by metabolic syndrome, then controls. Increased neointimal hyperplasia correlated with increased reactive oxygen species (ROS) production, as demonstrated by dihydroethidium staining, and NO inhibited this increase most in metabolic syndrome and DM. In conclusion, NO was surprisingly a more effective inhibitor of neointimal hyperplasia following arterial injury in type II DM and metabolic syndrome vs. control. This heightened efficacy may be secondary to greater inhibition of VSMC proliferation through cell cycle arrest and regulation of ROS expression, in addition to other possible unidentified mechanisms that deserve further exploration.

Nitric oxide: what a vascular surgeon needs to know

Atherosclerosis in the form of peripheral arterial disease results in significant morbidity and mortality. Surgical treatment options for peripheral arterial disease include angioplasty with and without stenting, endarterectomy, and bypass grafting. Unfortunately, all of these procedures injure the vascular endothelium, which impairs its ability to produce nitric oxide (NO) and ultimately leads to neointimal hyperplasia and restenosis. To improve on current patency rates after vascular procedures, investigators are engaged in research to improve the bioavailability of NO at the site of vascular injury in an attempt to reduce the risk of thrombosis and restenosis after successful revascularization. This article reviews some of the previous research that has aimed to improve NO bioavailability after vascular procedures whether through systemic or local delivery, as well as to describe some of the NO-releasing products that are currently undergoing study for use in clinical practice.

Nitric oxide and nanotechnology: a novel approach to inhibit neointimal hyperplasia

OBJECTIVE

Nitric oxide (NO) has been shown to inhibit neointimal hyperplasia after arterial interventions in several animal models. To date, however, NO-based therapies have not been used in the clinical arena. Our objective was to combine nanofiber delivery vehicles with NO chemistry to create a novel, more potent NO-releasing therapy that can be used clinically. Thus, the aim of this study was to evaluate the perivascular application of spontaneously self-assembling NO-releasing nanofiber gels. Our hypothesis was that this application would prevent neointimal hyperplasia.

METHODS

Gels consisted of a peptide amphiphile, heparin, and a diazeniumdiolate NO donor (1-[N-(3-Aminopropyl)-N-(3-ammoniopropyl)]diazen-1-ium-1,2-diolate [DPTA/NO] or disodium 1-[(2-Carboxylato)pyrrolidin-1-yl]diazen-1-ium-1,2-diolate [PROLI/NO]). Nitric oxide release from the gels was evaluated by the Griess reaction, and scanning electron microscopy confirmed nanofiber formation. Vascular smooth muscle cell (VSMC) proliferation and cell death were assessed in vitro by (3)H-thymidine incorporation and Personal Cell Analysis (PCA) system (Guava Technologies, Hayward, Calif). For the in vivo work, gels were modified by reducing the free-water content. Neointimal hyperplasia after periadventitial gel application was evaluated using the rat carotid artery injury model at 14 days (n = 6 per group). Inflammation and proliferation were examined in vivo with immunofluorescent staining against CD45, ED1, and Ki67 at 3 days (n = 2 per group), and graded by blinded observers. Endothelialization was assessed by Evans blue injection at 7 days (n = 3 per group).

RESULTS

Both DPTA/NO and PROLI/NO, combined with the peptide amphiphile and heparin, formed nanofiber gels and released NO for 4 days. In vitro, DPTA/NO inhibited VSMC proliferation and induced cell death to a greater extent than PROLI/NO. However, the DPTA/NO nanofiber gel only reduced neointimal hyperplasia by 45% (intima/media [I/M] area ratio, 0.45 +/- 0.07), whereas the PROLI/NO nanofiber gel reduced neointimal hyperplasia by 77% (I/M area ratio, 0.19 +/- 0.03, P < .05) vs control (injury alone I/M area ratio, 0.83 +/- 0.07; P < .05). Both DPTA/NO and PROLI/NO nanofiber gels significantly inhibited proliferation in vivo (1.06 +/- 0.30 and 0.19 +/- 0.11 vs injury alone, 2.02 +/- 0.20, P < .05), yet had minimal effect on apoptosis. Only the PROLI/NO nanofiber gel inhibited inflammation (monocytes and leukocytes). Both NO-releasing nanofiber gels stimulated re-endothelialization.

CONCLUSIONS

Perivascular application of NO-releasing self-assembling nanofiber gels is an effective and simple therapy to prevent neointimal hyperplasia after arterial injury. Our study demonstrates that the PROLI/NO nanofiber gel most effectively prevented neointimal hyperplasia and resulted in less inflammation than the DPTA/NO nanofiber gel. This therapy has great clinical potential to prevent neointimal hyperplasia after open vascular interventions in patients.

Beneficial effect of a short-acting NO donor for the prevention of neointimal hyperplasia

Nitric oxide (NO)-based therapies effectively inhibit neointimal hyperplasia in animal models of arterial injury and bypass grafting, but are not available clinically. We created a simple, effective, locally applied NO-eluting therapy to prevent restenosis after vascular procedures. We investigated the efficacy of perivascular delivery of two distinctly different diazeniumdiolate NO donors, 1-[2-(carboxylato)pyrrolidin-1-yl]diazen-1-ium-1,2-diolate (PROLI/NO) (short half-life) and diazeniumdiolated poly(acrylonitrile) (PAN/NO) (long half-life), in powder or gel form (30% poloxamer 407), at inhibiting neointimal hyperplasia using the rat carotid artery injury model. Two weeks postinjury, all of the NO-eluting therapies successfully reduced neointimal hyperplasia. However, most dramatically, PROLI/NO powder reduced intimal area by 91.2% (p<0.05) versus injury alone. PROLI/NO powder was noted to reduce the medial area (40.2% vs injury alone, p<0.05), whereas other groups showed no such effect. Three days postinjury, each NO treatment group significantly reduced cellular proliferation. However, inflammatory markers revealed a distinct pattern: PAN/NO groups displayed increased leukocyte infiltration (p<0.05), whereas PROLI/NO groups displayed less macrophage infiltration (p<0.05). In conclusion, perivascular delivery of diazeniumdiolate NO donors in powder or gel form effectively inhibits neointimal hyperplasia. Application of short-acting PROLI/NO powder most effectively inhibited neointimal hyperplasia and inflammation and may represent a simple, clinically applicable NO-eluting therapy to prevent neointimal hyperplasia and restenosis after open vascular interventions.

The role of nitric oxide in the pathophysiology of intimal hyperplasia

Since its discovery, nitric oxide (NO) has emerged as a biologically important molecule and was even named Molecule of the Year by Science magazine in 1992. Specific to our interests, NO has been implicated in the regulation of vascular pathology. This review begins with a summary of the molecular biology of NO, from its discovery to the mechanisms of endogenous production. Next, we turn our attention to describing the arterial injury response of neointimal hyperplasia, and we review the role of NO in the pathophysiology of neointimal hyperplasia. Finally, we review the literature regarding NO-based therapies. This includes the development of inhalational-based NO therapies, systemically administered L-arginine and NO donors, NO synthase gene therapy, locally applied NO donors, and NO-releasing prosthetic materials. By reviewing the current literature, we emphasize the tremendous clinical potential that NO-based therapies can have on the development of neointimal hyperplasia.

Influence of ischemic injury on vein graft remodeling: role of cyclic adenosine monophosphate second messenger pathway in enhanced vein graft preservation

OBJECTIVE

Endothelial injury during the harvest of saphenous vein grafts might play an important role in the development of vein graft disease after coronary artery bypass grafting. Using a murine autologous arterialized vein patch model, we tested whether the initial ischemic insult of vein grafts was linked to the later development of graft neointimal hyperplasia and whether the restoration of the cyclic adenosine monophosphate second messenger pathway would attenuate the development of neointimal hyperplasia.

METHODS

A segment of the external jugular vein of a mouse was grafted onto its abdominal aorta. Three weeks after the operation, the degree of neointimal hyperplasia of the implanted graft was compared among (1) grafts without preservation, (2) grafts with 2 hours of preservation (25 degrees C) in heparinized saline, and (3) grafts with 2 hours of preservation in heparinized saline in the presence of a cyclic adenosine monophosphate analog. In addition, cyclic adenosine monophosphate contents of vein grafts and leukocyte adherence to the graft endothelium were assessed.

RESULTS

Cyclic adenosine monophosphate contents were significantly decreased after 2 hours of preservation (212 +/- 8 vs 156 +/- 5 pmol/L, P < .01). The grafts preserved for 2 hours showed greater neointimal hyperplasia compared with the grafts without preservation (neointimal expansion, 68.7% +/- 9.6% vs 46.1% +/- 4.8%; P < .01). The addition of a cyclic adenosine monophosphate analog to the preservation solution significantly suppressed neointimal hyperplasia of grafts preserved for 2 hours (44.3% +/- 5.0%). Inhibiting the cyclic adenosine monophosphate-dependent protein kinase by adding Rp-cAMPS abrogated the beneficial effects. Furthermore, grafts preserved for 2 hours had significantly more leukocytes adhering to the graft endothelium 24 hours after the operation compared with nonpreserved grafts, which was significantly reduced by the cyclic adenosine monophosphate treatment.

CONCLUSIONS

Ischemic insult during vein graft harvest and preservation is a key factor in the development of vein graft neointimal hyperplasia at least in part caused by the depletion of cyclic adenosine monophosphate. We conclude that stimulation of the cyclic adenosine monophosphate second messenger pathway might be a potential strategy for the prevention of vein graft disease.

Transduction of peptide analogs of the small heat shock-related protein HSP20 inhibits intimal hyperplasia

BACKGROUND

Human saphenous vein (HSV) is the autologous conduit of choice for peripheral vascular reconstructions. However, vasospasm can lead to early graft failure. The leading cause of delayed graft failure is intimal hyperplasia.

OBJECTIVE

To develop a proteomic approach to prevent vein-graft spasm and intimal hyperplasia.

METHODS

Biomimetic peptide analogs of the small heat shock-related protein HSP20, containing a protein transduction domain (PTD), a phosphorylated serine, and a sequence of HSP20 surrounding the phosphorylation site (PTD-pHSP20), or a scrambled sequence of the same amino acids surrounding the phosphorylation site (PTD-scHSP20) were synthesized. The peptides were used in muscle bath and organ culture experiments with human saphenous vein (HSV) segments. Cultured smooth muscle cell lines were used to determine the effect of the peptides on proliferation and migration.

RESULTS

In HSV rings precontracted with norepinephrine, PTD-pHSP20 but not PTD-scHSP20 led to relaxation. There was no significant difference in smooth muscle cell proliferation in cells treated with PTD-pHSP20 compared with PTD-scHSP20. Treatment with PTD-pHSP20 significantly inhibited cellular migration compared with PTD-scHSP20. Control, untreated, and PTD-scHSP20-treated saphenous veins had significant increases in intimal thickness after culture. This intimal thickening was completely inhibited by treatment with PTD-pHSP20.

CONCLUSIONS

Protein transduction of biologically active motifs of HSP20 can affect pathologic and physiologic responses of HSV and represents a novel proteomic-based therapeutic approach.

CLINICAL RELEVANCE

We have been a part of the genomics era and are now viewing the emergence of "proteomics." The genome is linear and relatively easy to examine; however the proteome is much more complex and dynamic. In essence, the purpose of gene therapy is to manipulate the genome to produce a particular protein. This manuscript describes a new proteomic approach in which the biologically active part of a protein is directly introduced into vascular cells. Peptides were synthesized which contained a total of 24 amino acids, 11 of which represent a protein transduction domain or "carrier" while the other 13 are the biologically active "cargo." These synthetic peptides prevent spasm (contraction) and intimal hyperplasia in segments of human saphenous vein treated ex vivo. Preclinical development is currently underway to develop these molecules as a proteomic-based vein harvest solution to enhance vein-graft patency.

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.

Thermosensitive sol-gel reversible hydrogels

Aqueous polymer solutions that are transformed into gels by changes in environmental conditions, such as temperature and pH, thus resulting in in situ hydrogel formation, have recently attracted the attention of many investigators for scientific interest and for practical biomedical or pharmaceutical applications. When the hydrogel is formed under physiological conditions and maintains its integrity for a desired period of time, the process may provide various advantages over conventional hydrogels. Because of the simplicity of pharmaceutical formulation by solution mixing, biocompatibility with biological systems, and convenient administration, the pharmaceutical and biomedical uses of the water-based sol-gel transition include solubilization of low-molecular-weight hydrophobic drugs, controlled release, labile biomacromolecule delivery, such as proteins and genes, cell immobilization, and tissue engineering. When the formed gel is proven to be biocompatible and biodegradable, producing non-toxic degradation products, it will provide further benefits for in vivo applications where degradation is desired. It is timely to summarize the polymeric systems that undergo sol-gel transitions, particularly due to temperature, with emphasis on the underlying transition mechanisms and potential delivery aspects. This review stresses the polymeric systems of natural or modified natural polymers, N-isopropylacrylamide copolymers, poly(ethylene oxide)/poly(propylene oxide) block copolymers, and poly(ethylene glycol)/poly(D,L-lactide-co-glycolide) block copolymers.

L-arginine polymers inhibit the development of vein graft neointimal hyperplasia

OBJECTIVE

We sought to determine whether L -arginine polymer treatment of vein grafts enhances vascular production of nitric oxide and inhibits the development of neointimal hyperplasia.

METHODS

External jugular veins of New Zealand White rabbits (n = 42) were harvested; treated intraluminally for 15 minutes with phosphate-buffered saline solution or L -arginine polymer 5, 7, or 9 at either 10 or 100 micromol/L; and then grafted into the contralateral carotid artery. Rabbits were killed after 28 days, and 5-microm sections of vessels were stained with hematoxylin and scored for intima/media ratio by using computerized morphometric analysis. Separate veins were treated in a similar fashion with biotinylated polymers and phosphate-buffered saline solution to assess for translocation efficiencies. Finally, vein segments pretreated with either phosphate-buffered saline solution or L -arginine polymers were cultured in Dulbecco's modified Eagle's medium containing lipopolysaccharide (100 microg/mL) and interferon gamma (200 U/mL) for 48 hours before measuring nitric oxide levels by means of the Griess reaction.

RESULTS

Biotinylated L -arginine polymers demonstrated a dose- and length-dependent uptake into intimal and medial cells of treated vessels. Nitric oxide levels were significantly higher in vein segments treated with 100 micromol/L of L -arginine polymer 9 compared with control segments. Finally, the intima/media ratio also reflected both length- and concentration-dependent inhibition of neointimal hyperplasia.intima/media ratio PBS R5 R7 R9 10 micromol/L 0.909 +/- 0.072 0.920 +/- 0.073 0.861 +/- 0.138 0.710 +/- 0.122 100 micromol/L 0.924 +/- 0.061 0.581 +/- 0.089* 0.529 +/- 0.093* PBS, Phosphate-buffered saline solution; R, L -arginine polymer. *P <.001 versus phosphate-buffered saline solution and L -arginine polymer 5 controls (Bonferroni-corrected value).

CONCLUSIONS

Arginine polymers of sufficient length and concentration were effective in increasing nitric oxide levels and reducing neointimal hyperplasia in this vein graft model.

Adventitial versus intimal liposome-mediated ex vivo transfection of canine saphenous vein grafts with endothelial nitric oxide synthase gene

PURPOSE

Experiments were designed (1) to evaluate liposome-mediated endothelial constitutive nitric oxide synthase (ecNOS) transfection in vein grafts and (2) to compare intimal and adventitial routes of transfection.

METHODS

Male mongrel dogs (N = 36) underwent bilateral femoral artery bypass grafting with the lateral saphenous vein. In each animal one vein was transfected with plasmid (pVR1012) containing the ecNOS gene, and another vein was transfected with plasmid alone (control). Gene transfer was performed from either the intimal surface (Group I, n = 18) or the adventitial surface (Group II, n = 18). In each group there were three transfection subgroups (n = 6 each): (a ) 10 microg/mL naked plasmid DNA, (b ) 10 microg/mL plasmid DNA + liposome (LipofectAMINE PLUS), and (c ) 100 microg/mL plasmid DNA + LipofectAMINE PLUS. Grafts were harvested on the third postoperative day, and the transfection was assessed with molecular techniques and enzyme assay for activity of NOS by conversion of tritiated l-arginine to tritiated l-citrulline. Proliferating cells were quantified with a digital analysis of histologic sections after nuclear antigen Ki-67 (MIB1) immunohistochemistry.

RESULTS

Transgene was identified with polymerase chain reaction in all ecNOS-transfected grafts, regardless of transfection modality. However, significant transcription of the ecNOS transgene was observed only in Group IIc (mean ecNOS messenger RNA, 8.7+/-1.7 vs. 3.1+/-0.7 x 10(-2) attomole/microL, in transfected compared with control grafts, respectively, P =.01). NOS activity increased approximately twofold in this group (11.58+/-2.1 and 6.3+/-1.0 pmol tritiated l-citrulline per milligram protein per hour in transfected and control grafts, respectively, P = .05). Numbers of proliferating cells did not differ among ecNOS-transfected and control grafts in any transfection group.

CONCLUSION

These results suggest that ecNOS transfection of vein grafts is feasible through intimal and adventitial routes with naked DNA or a liposomal vector. However, efficient transcription of the transgene is evident at postoperative day 3 only after adventitial transfection of 100 microg/mL of the gene.

Mechanisms of vascular atrophy and fibrous cap disruption

The process of plaque destabilization and rupture remains an area of intense investigation. While reductions in lumen cross-sectional area induced by early, non-occlusive lesions are compensated by remodeling and expansion of the artery, further plaque enlargement leads to an uncompensated reduction in lumen area and an increase in surface shearing forces. We hypothesize that these local increases in wall shear stress lead to a reduction in smooth muscle cell proliferation and increase in cell death. Using a primate prosthetic graft model, we have observed that alterations in nitric oxide and platelet-derived growth factor metabolism are important regulators of intimal growth and regression. We suggest that these factors may also be influential in the process of fibrous cap atrophy and plaque rupture.

Combination therapy of cholesterol reduction and L-arginine supplementation controls accelerated vein graft atheroma

Hyperlipidemia contributes to the development of intimal hyperplasia and accelerated atheroma in vein bypass grafts. Dietary cholesterol reduction and oral supplementation with L-arginine have been shown to reduce accelerated atheroma in experimental vein grafts. This study extends these observations by examining the effect of the combination therapy of cholesterol reduction and L-arginine supplementation on the development of intimal hyperplasia in vein grafts in hypercholesterolemic animals. Thirty New Zealand White rabbits had a carotid vein bypass graft performed and were sacrificed at 28 days postoperatively either for morphology (light and electron microscopy) and videomorphometry, or for in vitro contractile studies. Twenty animals received a 1% cholesterol diet for 4 weeks prior to surgery. This diet was continued until harvest in ten animals. Ten cholesterol-fed animals received L-arginine supplementation (2 g/kg/day, p.o.) for 7 days preoperatively and thereafter until harvest and in addition were returned to a normal diet on the day of surgery. The last ten animals were controls (normal diet). Combined cholesterol reduction and L-arginine supplementation prevented accelerated atheroma in vein grafts, halted the change in enhanced smooth muscle cell contractility, and improved endothelial cell function. Early postoperative therapy targeting atheroma development in the high-risk patient could offer significant morphological and functional benefits.

Local delivery of a tissue factor antibody reduces early leukocyte infiltration but fails to limit intimal hyperplasia in experimental vein grafts

BACKGROUND

Tissue Factor-mediated thrombin generation involves the generation of VIIa and Xa and has been implicated in the pathogenesis of intimal hyperplasia. In experimental vein grafts, Tissue Factor protein is increased over the first 3 days and colocalized with CD18-positive leukocytes; this increase in Tissue Factor precedes the development of intimal hyperplasia. This study further evaluates the potential role of Tissue Factor in vein graft intimal hyperplasia by directly inhibiting Tissue Factor protein.

METHODS

New Zealand white rabbits underwent interpositional bypass grafting of the common carotid artery using the external jugular vein. Perioperatively, murine anti-rabbit Tissue Factor antibody (109 microg/ml gel, 12,500x IC50 of Tissue Factor activity) was applied to the adventitial surface of the graft, using a pluronic gel (30% soln.). Tissue Factor antibody treated vein grafts were compared to control and empty gel-treated vein grafts. Vein grafts were examined at 3 days to assess CD18-positive leukocyte infiltration and the presence of residual antibody by Western blotting. At 28 days, intimal and medial dimensions were quantified using videomorphometry.

RESULTS

At day 3, there was marked reduction in CD18-positive leukocytes in the Tissue Factor antibody versus control vein grafts (6.3 +/- 4.7 vs 20.8 +/- 7.4 per 200x field, P < 0.05). At 28 days, intimal hyperplasia was similar for the control (70 +/- 4 microm, mean +/- SEM), gel (73 +/- 4 microm), and Tissue Factor antibody (75 +/- 4 microm) vein grafts. However, medial thickness (76 +/- 4 microm;, P < 0.05) was significantly increased compared to the gel treated vein graft (61 +/- 5 microm).

CONCLUSION

Local delivery of pharmacologic doses of an anti-rabbit Tissue Factor antibody decreased CD18-positive leukocyte infiltration but failed to limit intimal hyperplasia in experimental vein grafts. The results suggest that inhibition of Tissue Factor protein modulates polymorphonuclear leukocyte-endothelial interactions but not in the subsequent development of intimal hyperplasia. It implies that the relationship between the extrinsic coagulation cascade and intimal hyperplasia in vein grafts is complex.