Deon Bezuidenhout

Cell-demanded release of VEGF from synthetic, biointeractive cell ingrowth matrices for vascularized tissue growth

Local, controlled induction of angiogenesis remains a challenge that limits tissue engineering approaches to replace or restore diseased tissues. We present a new class of bioactive synthetic hydrogel matrices based on poly(ethylene glycol) (PEG) and synthetic peptides that exploits the activity of vascular endothelial growth factor (VEGF) alongside the base matrix functionality for cellular ingrowth, that is, induction of cell adhesion by pendant RGD‐containing peptides and provision of cell‐mediated remodeling by cross‐linking matrix metalloproteinase substrate peptides. By using a Michael‐type addition reaction, we incorporated variants of VEGF121 and VEGF165 covalently within the matrix, available for cells as they invade and locally remodel the material. The functionality of the matrix‐conjugated VEGF was preserved and was critical for in vitro endothelial cell survival and migration within the matrix environment. Consistent with a scheme of locally restricted availability of VEGF, grafting of these VEGF‐modified hydrogel matrices atop the chick chorioallontoic membrane evoked strong new blood vessel formation precisely at the area of graft‐membrane contact. When implanted subcutaneously in rats, these VEGF‐containing matrices were completely remodeled into native, vascularized tissue. This type of synthetic, biointeractive matrix with integrated angiogenic growth factor activity, presented and released only upon local cellular demand, could become highly useful in a number of clinical healing applications of local therapeutic angiogenesis.

Long-term experience in autologous in vitro endothelialization of infrainguinal ePTFE grafts

OBJECTIVE Based on a previous randomized study showing significantly superior patency rates for in vitro endothelialized expanded polytetrafluoroethylene (ePTFE) grafts we investigated whether it was feasible for a nontertiary institution to offer autologous in vitro endothelialization to all elective infrainguinal bypass patients who had no suitable saphenous vein available. METHODS Over a period of 15 years, 310 out of 318 consecutive nonacute patients (age 64.7 +/- 8.6) received 341 endothelialized ePTFE grafts (308 femoropopliteal: 153 above knee [AK] and 155 below knee [BK] and 33 femorodistal). Autologous endothelial cells were harvested from short segments (3.9 +/- 1.1 cm) of subcutaneous veins (80% cephalic, 11% basilic, 2% external jugular, and 7% saphenous) and grown to mass cultures within 18.9 +/- 4.5 days before being confluently lined onto fibrin glue-coated ePTFE grafts. The graft diameter was 6 mm (64%) or 7 mm (36%). The overall procedure-related delay for graft implantation was 27.6 + 7.8 days. Growth failure prevented 2.5% of patients from receiving an endothelialized graft. The mean observation period was 9.6 years. Primary patencies were obtained from Kaplan-Meier survivorship functions. Explants for morphological analysis were obtained from eight patients. RESULTS The overall primary patency rate of femoropopliteal grafts was 69% at 5 years (68% [AK] vs 71% [BK]) and 61% at 10 years (59% [AK] vs 64% [BK]). Primary patency of 7 mm vs 6 mm grafts was 78%/62% at 5 years and 71%/55% at 10 years. The difference between the two groups was statistically significant (log rank test P = .023; Breslow test P = .017). Stage I vs II/III patients showed 5-year patencies of 67% vs 73% (N.S.) and 10-year patencies of 61%% vs 53% (N.S.). The primary patency of femorodistal grafts was 52% at 5 years and 36% at 10 years. The limb salvage rate was 94% (fempop) vs 86% (femdistal) at 5 years and 89% vs 71% at 10 years. All retrieved samples showed the presence of an endothelium after 38.9 +/- 17.8 months. CONCLUSION Autologous in vitro endothelialization was shown to be a feasible routine procedure at a nontertiary hospital. Explants confirmed the presence of an endothelium years after implantation while the primary patency in the particularly challenging subgroup of patients without a suitable saphenous vein resembles that of vein grafts.

A Synthetic Non-degradable Polyethylene Glycol Hydrogel Retards Adverse Post-infarct Left Ventricular Remodeling

BACKGROUND Left ventricular remodeling after myocardial infarction is a key component of heart failure and it has long been postulated that it may result from increased wall stress. It has recently been suggested that an injectable, non-degradable polymer may limit pathological remodeling in a manner analogous to that of cardiac support devices. We have tested a non-degradable polyethylene glycol (PEG) gel in a rat infarction model. METHODS AND RESULTS After permanent ligation of the left anterior descending artery in male Wistar rats, PEG gel reagents were injected into the infarcted region and polymerized in situ. At 4 weeks, fractional shortening and infarct volume were unchanged relative to a saline injected control, but the infarct-induced left ventricular end-diastolic diameter (LVEDD) increase was substantially reduced (43%, P < .05) and wall thinning was completely prevented. At 13 weeks, the LVEDD were similar for both saline- and PEG-injected hearts. The non-degradable PEG gels did elicit a macrophage-based inflammatory reaction. CONCLUSIONS The injection of non-degradable synthetic gel was effective in ameliorating pathological remodeling in the immediate postinfarction healing phase, but was unable to prevent the dilation that occurred at later stages in the healed heart.

The dosage dependence of VEGF stimulation on scaffold neovascularisation

Growth factors are often used in tissue regeneration to stimulate vascularisation of polymeric scaffolds, with vascular endothelial growth factor (VEGF) having been extensively studied for short-term vessel ingrowth. We have therefore evaluated the effect of different concentrations of VEGF on the vascularisation of a porous scaffold in the short-, intermediate- and long-term, by delivering 15, 150 and 1500ng VEGF/day to polyurethane scaffolds by osmotic pumps for up to 6 weeks. An increased vascularisation months after termination of VEGF delivery was only achieved with 150ng/day (46%, p<0.05). This dosage consistently showed elevated levels of vascularisation (144, 125, 160 and 60% above PBS controls at 10, 20, 30 and 42 days, respectively, p<0.05), whilst the vessels induced by the highest dosage, though initially maximally elevated (265 and 270% at 10 and 20 days, p<0.05) tended to regress after 20 days of VEGF delivery. Pericyte coverage was decreased at 20 days for the highest dosage (30%, p<0.05). Lectin perfusion demonstrated that vessels within the scaffold were connected to the host vasculature at all time points and perfusion was substantially raised by VEGF delivery at day 20. These results suggest concentration of VEGF plays a critical role in the nature and persistence of vasculature formed in a tissue regenerative scaffold.

Effect of well defined dodecahedral porosity on inflammation and angiogenesis.

Porosity is an important factor in the healing of prosthetic devices. To better understand this phenomenon, porous polyurethane scaffolds were produced by a variation of the phase inversion/porogen extraction technique in which a prepacked column of spherical porogen particles was infiltrated with a polymer solution before polymer precipitation and porogen extraction. Scaffolds contained pores of well defined shape (approaching open faced pentagonal dodecahedra), narrow size distributions (66.1 ± 1.3 &mgr;m, 84.2 ± 1.7 &mgr;m, and 156.9 ± 1.2 &mgr;m) and high interconnectivity (interconnecting windows of 30.1 ± 0.8 &mgr;m, 41.9 ± 1.5 &mgr;m, and 76.4 ± 2.0 &mgr;m, respectively). A high degree of accessible macroporosity (>80%) could be achieved while limiting the mostly inaccessible microporosity to below 2%.The neovascularization and inflammatory responses to the scaffolds were evaluated in the subcutaneous rat model for 4 weeks. The inflammatory response index and foreign body giant cell index could be reduced by 56% (p < 0.05) and 21% (p < 0.02), respectively, when the pore size was increased from 66 &mgr;m to 157 &mgr;m, whereas the vascularization index and arteriolar index remained unchanged.Thus, a significant decrease in inflammatory response could be achieved without adversely affecting the degree of neovascularization by increasing the size of the pores.

Modification, crosslinking and reactive electrospinning of a thermoplastic medical polyurethane for vascular graft applications.

Thermoplastic polyurethanes are used in a variety of medical devices and experimental tissue engineering scaffolds. Despite advances in polymer composition to improve their stability, the correct balance between chemical and mechanical properties is not always achieved. A model compound (MC) simulating the structure of a widely used medical polyurethane (Pellethane) was synthesized and reacted with aliphatic and olefinic acyl chlorides to study the reaction site and conditions. After adopting the conditions to the olefinic modification of Pellethane, processing into flat sheets, and crosslinking by thermal initiation or ultraviolet radiation, mechanical properties were determined. The modified polyurethane was additionally electrospun under ultraviolet light to produce a crosslinked tubular vascular graft prototype. Model compound studies showed reaction at the carbamide nitrogen, and the modification of Pellethane with pentenoyl chloride could be accurately controlled to up to 20% (correlation: rho=0.99). Successful crosslinking was confirmed by insolubility of the materials. Initiator concentrations were optimized and the crosslink densities shown to increase with increasing modification. Crosslinking of Pellethane containing an increasing number of pentenoyl groups resulted in decreases (up to 42%, p<0.01) in the hysteresis and 44% in creep (p<0.05), and in a significant improvement in degradation resistance in vitro. Modified Pellethane was successfully electrospun into tubular grafts and crosslinked using UV irradiation during and after spinning to render them insoluble. Prototype grafts had sufficient burst pressure (>550 mm Hg), and compliances of 12.1+/-0.8 and 6.2+/-0.3%/100 mm Hg for uncrosslinked and crosslinked samples, respectively. It is concluded that the viscoelastic properties of a standard thermoplastic polyurethane can be improved by modification and subsequent crosslinking, and that the modified material may be electrospun and initiated to yield crosslinked scaffolds. Such materials hold promise for the production of vascular and other porous scaffolds, where decreased hysteresis and creep may be required to prevent aneurismal dilation.

Constrictive external nitinol meshes inhibit vein graft intimal hyperplasia in nonhuman primates

OBJECTIVE External mesh support of vein grafts has been shown to mitigate the formation of intimal hyperplasia. The aim of the present study was to address the issue of optimal mesh size in a nonhuman primate model that mimics the dimensional mismatch typically encountered between clinical vein grafts and their target arteries. METHODS The effect of mesh size on intimal hyperplasia and endothelial preservation was assessed in bilateral femoral interposition grafts in Chacma baboons (n(Sigma) = 32/n = 8 per mesh size). No mesh support (group I) was compared with external nitinol meshes at three different sizes: loose fitting (group II), 25% diameter constricting (group III), and 50% diameter constricting (group IV). Mesh sizes were seen not only in isolation but also against the background of anastomotic size mismatch at implantation, expressed as quotient of cross-sectional area of host artery to vein graft (Q(C)). RESULTS Significant amounts of intimal hyperplasia were found in group I (Q(C) median 0.20; intimal hyperplasia 6 weeks = 1.63 +/- 0.34 mm(2); intimal hyperplasia 12 weeks = 1.73 +/- 0.5 mm(2)) and group II (Q(C) median 0.25; intimal hyperplasia 6 weeks = 1.96 +/- 1.64 mm(2); intimal hyperplasia 12 weeks = 2.88 +/- 1.69 mm(2)). In contrast, group III (Q(C) median 0.45; intimal hyperplasia 6 weeks = 0.08 +/- 0.13 mm(2); intimal hyperplasia 12 weeks = 0.18 +/- 0.32 mm(2)) and IV (Q(C) median 1.16; intimal hyperplasia 6 weeks = 0.02 +/- 0.03 mm(2); intimal hyperplasia 12 weeks = 0.11 +/- 0.10 mm(2)) showed dramatically suppressed intimal hyperplasia (P < .01) at both time points. Endothelial integrity was only preserved in group IV (P < .05). There were no significant differences in vascularization and inflammation in either interlayer or intergroup comparisons. CONCLUSION By using an animal model that addressed the clinical phenomenon of diameter discrepancy between vein graft and bypassed artery, we could demonstrate that suppression of intimal hyperplasia required constrictive mesh sizes.

Bioprosthetic heart valves: the need for a quantum leap

More than 250000 bioprosthetic heart valves are being implanted annually. Although the majority of recipients are elderly developed‐world patients, the most urgent need for tissue valves is in younger patients, where rapid degeneration of contemporary prostheses remains a serious obstacle. After decades of empirical and mostly futile attempts to extend the longevity of tissue valve prostheses, new insights and solutions are on the horizon. Aetiologically, a shift of focus from mineralization to immune responses and inflammation emerges. On the development side, new engineering approaches to both selective extraction of tissue components and cross‐links are increasingly defining the new direction. In order to dramatically improve the performance of bioprosthetic heart valves, these new developments need to lead to a broad consensus for a paradigm shift in a hitherto rather stagnant field of medical research.

The beneficial effects of deferred delivery on the efficiency of hydrogel therapy post myocardial infarction

Biomaterials are increasingly being investigated as a means of reducing stress within the ventricular wall of infarcted hearts and thus attenuating pathological remodelling and loss of function. In this context, we have examined the influence of timing of delivery on the efficacy of a polyethylene glycol hydrogel polymerised with an enzymatically degradable peptide sequence. Delivery of the hydrogel immediately after infarct induction resulted in no observable improvements, but a delay of one week in delivery resulted in significant increases in scar thickness and fractional shortening, as well as reduction in end-systolic diameter against saline controls and immediately injected hydrogel at both 2 and 4 weeks post-infarction (p < 0.05). Hydrogels injected at one week were degraded significantly slower than those injected immediately and this may have played a role in the differing outcomes. The hydrogel assumed markedly different morphologies at the two time points having either a fibrillar or bulky appearance after injection immediately or one week post-infarction respectively. We argue that the different morphologies result from infarction induced changes in the cardiac structure and influence the degradability of the injectates. The results indicate that timing of delivery is important and that very early time points may not be beneficial.

Sustaining Neovascularization of a Scaffold Through Staged Release of Vascular Endothelial Growth Factor-A and Platelet-Derived Growth Factor-BB

Tissue regeneration into a three-dimensional scaffold requires the stimulation of blood vessel ingrowth. We have employed a freely interconnecting porous scaffold developed by us to determine the utility of a covalently bound heparin surface coating for the delivery of vascular endothelial growth factor (VEGF) and platelet-derived growth factor BB (PDGF-BB) in vivo. The heparin surface was shown to release VEGF far more rapidly than PDGF-BB in vitro (VEGF: 75 ng/h for 24 h; PDGF-BB: 86 pg/h for >7 days). In rat subcutaneous implants, at 10 days the heparin surface alone increased vessel ingrowth substantially (p<0.05 vs. unmodified scaffold), release of VEGF resulted in a further increase (p<0.05 vs. heparinized scaffold), whereas PDGF-BB had no additional effect. The increase induced by the combination of growth factors was similar to VEGF alone. After 2 months, PDGF-BB, but not VEGF delivery, resulted in a substantial increase in vascularization above that induced by heparin (p<0.05). At the longer time point the combination of growth factors was similar to PDGF-BB. However, only the combination of growth factors significantly elevated the number of ingrowing arterioles (p<0.05 vs. heparinized scaffold). Thus, the covalent modification of a porous scaffold with heparin allows for the differential release of VEGF and PDGF-BB that results in both a rapid and sustained increase in scaffold vascularization.