Onset and progression of calcification in porcine aortic bioprosthetic valves Implanted as orthotopic mitral valve replacements in juvenile sheep

Swine models in translational research and medicine

Swine are increasingly studied as animal models of human disease. The anatomy, size, longevity, physiology, immune system, and metabolism of swine are more like humans than traditional rodent models. In addition, the size of swine is preferred for surgical placement and testing of medical devices destined for humans. These features make swine useful for biomedical, pharmacological, and toxicological research. With recent advances in gene-editing technologies, genetic modifications can readily and efficiently be made in swine to study genetic disorders. In addition, gene-edited swine tissues are necessary for studies testing and validating xenotransplantation into humans to meet the critical shortfall of viable organs versus need. Underlying all of these biomedical applications, the knowledge of husbandry, background diseases and lesions, and biosecurity needs are important for productive, efficient, and reproducible research when using swine as a human disease model for basic research, preclinical testing, and translational studies.

Multimodal preoperative imaging for transcatheter mitral valve replacement in the domestic sheep model

Preclinical in vivo evaluation is an essential step in the progression of new cardiac devices into patient use, with studies predominantly performed in the domestic sheep model. A growing area of interest in cardiac device development is transcatheter mitral valve replacement (TMVR). Clinically, multimodal imaging, or computed tomography (CT) and echocardiography (echo) are used extensively to preoperatively determine mitral valve morphology prior to an intervention, but there is no description on how these modalities can be implemented to support preclinical studies. The purpose of this study is to apply clinically relevant CT and echo acquisition and assessment techniques to a large group of naive research sheep in order to analyze and report modality-related effects on mitral valve dimensional reference intervals in the sheep model. To this end, fifty-five adult domestic sheep underwent preoperative CT and echo exams and resultant images were analyzed using a landmark-based multiplanar measurement protocol and compiled into a master dataset for statistical analysis. We found moderate agreement between CT and echo-derived measurements of the mitral valve in sheep and propose the first clinically-relevant dimensional indices for the sheep's naive mitral valve which can be used to guide future studies evaluating novel TMVR devices. This study is the first of its kind in proposing a reproducible method for detailed examination of the mitral valve in the sheep model using clinically-relevant multimodal imaging. As in patients, CT and echo can reveal accurate native mitral valve dimensions in the sheep prior to preclinical TMVR studies.

Characterization of main pulmonary artery and valve annulus region of piglets using echocardiography, uniaxial tensile testing, and a novel non-destructive technique

Characterization of cardiovascular tissue geometry and mechanical properties of large animal models is essential when developing cardiovascular devices such as heart valve replacements. These datasets are especially critical when designing devices for pediatric patient populations, as there is often limited data for guidance. Here, we present a previously unavailable dataset capturing anatomical measurements and mechanical properties of juvenile Yorkshire (YO) and Yucatan (YU) porcine main pulmonary artery (PA) and pulmonary valve (PV) tissue regions that will inform pediatric heart valve design requirements for preclinical animal studies. In addition, we developed a novel radial balloon catheter-based method to measure tissue stiffness and validated it against a traditional uniaxial tensile testing method. YU piglets, which were significantly lower weight than YO counterparts despite similar age, had smaller PA and PV diameters (7.6–9.9 mm vs. 10.1–12.8 mm). Young’s modulus (stiffness) was measured for the PA and the PV region using both the radial and uniaxial testing methods. There was no significant difference between the two breeds for Young’s modulus measured in the elastic (YU PA 84.7 ± 37.3 kPa, YO PA 79.3 ± 15.7 kPa) and fibrous regimes (YU PA 308.6 ± 59.4 kPa, YO PA 355.7 ± 68.9 kPa) of the stress-strain curves. The two testing techniques also produced similar stiffness measurements for the PA and PV region, although PV data showed greater variation between techniques. Overall, YU and YO piglets had similar PA and PV diameters and tissue stiffness to previously reported infant pediatric patients. These results provide a previously unavailable age-specific juvenile porcine tissue geometry and stiffness dataset critical to the development of pediatric cardiovascular prostheses. Additionally, the data demonstrates the efficacy of a novel balloon catheter-based technique that could be adapted to non-destructively measure tissue stiffness in situ.

Mechanisms and Drug Therapies of Bioprosthetic Heart Valve Calcification

Valve replacement is the main therapy for valvular heart disease, in which a diseased valve is replaced by mechanical heart valve (MHV) or bioprosthetic heart valve (BHV). Since the 2000s, BHV surpassed MHV as the leading option of prosthetic valve substitute because of its excellent hemocompatible and hemodynamic properties. However, BHV is apt to structural valve degeneration (SVD), resulting in limited durability. Calcification is the most frequent presentation and the core pathophysiological process of SVD. Understanding the basic mechanisms of BHV calcification is an essential prerequisite to address the limited-durability issues. In this narrative review, we provide a comprehensive summary about the mechanisms of BHV calcification on 1) composition and site of calcifications; 2) material-associated mechanisms; 3) host-associated mechanisms, including immune response and foreign body reaction, oxidative stress, metabolic disorder, and thrombosis. Strategies that target these mechanisms may be explored for novel drug therapy to prevent or delay BHV calcification.

Poly-2-methyl-2-oxazoline–modified bioprosthetic heart valve leaflets have enhanced biocompatibility and resist structural degeneration

Bioprosthetic heart valves (BHV) fabricated from glutaraldehyde-fixed heterograft tissue, such as bovine pericardium (BP), are widely used for treating heart valve disease, a group of disorders that affects millions. Structural valve degeneration (SVD) of BHV due to both calcification and the accumulation of advanced glycation end products (AGE) with associated serum proteins limits durability. We hypothesized that BP modified with poly-2-methyl-2-oxazoline (POZ) to inhibit protein entry would demonstrate reduced accumulation of AGE and serum proteins, mitigating SVD. In vitro studies of POZ-modified BP demonstrated reduced accumulation of serum albumin and AGE. BP-POZ in vitro maintained collagen microarchitecture per two-photon microscopy despite AGE incubation, and in cell culture studies was associated with no change in tumor necrosis factor-α after exposure to AGE and activated macrophages. Comparing POZ and polyethylene glycol (PEG)–modified BP in vitro, BP-POZ was minimally affected by oxidative conditions, whereas BP-PEG was susceptible to oxidative deterioration. In juvenile rat subdermal implants, BP-POZ demonstrated reduced AGE formation and serum albumin infiltration, while calcification was not inhibited. However, BP-POZ rat subdermal implants with ethanol pretreatment demonstrated inhibition of both AGE accumulation and calcification. Ex vivo laminar flow studies with human blood demonstrated BP-POZ enhanced thromboresistance with reduced white blood cell accumulation. We conclude that SVD associated with AGE and serum protein accumulation can be mitigated through POZ functionalization that both enhances biocompatibility and facilitates ethanol pretreatment inhibition of BP calcification.

Pulmonary valve tissue engineering strategies in large animal models

In the last 25 years, numerous tissue engineered heart valve (TEHV) strategies have been studied in large animal models. To evaluate, qualify and summarize all available publications, we conducted a systematic review and meta-analysis. We identified 80 reports that studied TEHVs of synthetic or natural scaffolds in pulmonary position (n = 693 animals). We identified substantial heterogeneity in study designs, methods and outcomes. Most importantly, the quality assessment showed poor reporting in randomization and blinding strategies. Meta-analysis showed no differences in mortality and rate of valve regurgitation between different scaffolds or strategies. However, it revealed a higher transvalvular pressure gradient in synthetic scaffolds (11.6 mmHg; 95% CI, [7.31-15.89]) compared to natural scaffolds (4,67 mmHg; 95% CI, [3,94-5.39]; p = 0.003). These results should be interpreted with caution due to lack of a standardized control group, substantial study heterogeneity, and relatively low number of comparable studies in subgroup analyses. Based on this review, the most adequate scaffold model is still undefined. This review endorses that, to move the TEHV field forward and enable reliable comparisons, it is essential to define standardized methods and ways of reporting. This would greatly enhance the value of individual large animal studies.

Expression of Calcification and Extracellular Matrix Genes in the Cardiovascular System of the Healthy Domestic Sheep (Ovis aries)

The maintenance of a healthy cardiovascular system requires expression of genes that contribute to essential biological activities and repression of those that are associated with functions likely to be detrimental to cardiovascular homeostasis. Vascular calcification is a major disruption to cardiovascular homeostasis, where tissues of the cardiovascular system undergo ectopic calcification and consequent dysfunction, but little is known about the expression of calcification genes in the healthy cardiovascular system. Large animal models are of increasing importance in cardiovascular disease research as they demonstrate more similar cardiovascular features (in terms of anatomy, physiology and size) to humans than do rodent species. We used RNA sequencing results from the sheep, which has been utilized extensively to examine calcification of prosthetic cardiac valves, to explore the transcriptome of the heart and cardiac valves in this large animal, in particular looking at expression of calcification and extracellular matrix genes. We then examined genes implicated in the process of vascular calcification in a wide array of cardiovascular tissues and across multiple developmental stages, using RT-qPCR. Our results demonstrate that there is a balance between genes that promote and those that suppress mineralization during development and across cardiovascular tissues. We show extensive expression of genes encoding proteins involved in formation and maintenance of the extracellular matrix in cardiovascular tissues, and high expression of hematopoietic genes in the cardiac valves. Our analysis will support future research into the functions of implicated genes in the development of valve calcification, and increase the utility of the sheep as a large animal model for understanding ectopic calcification in cardiovascular disease. This study provides a foundation to explore the transcriptome of the developing cardiovascular system and is a valuable resource for the fields of mammalian genomics and cardiovascular research.

Preclinical Assessment of Cardiac Valve Substitutes: Current Status and Considerations for Engineered Tissue Heart Valves

Tissue engineered heart valve (TEHV) technology may overcome deficiencies of existing available heart valve substitutes. The pathway by which TEHVs will undergo development and regulatory approval has several challenges. In this communication, we review: (1) the regulatory framework for regulation of medical devices in general and substitute heart valves in particular; (2) the special challenges of preclinical testing using animal models for TEHV, emphasizing the International Standards Organization (ISO) guidelines in document 5840; and (3) considerations that suggest a translational roadmap to move TEHV forward from pre-clinical to clinical studies and clinical implementation.

In Search of the Ideal Valve: Optimizing Genetic Modifications to Prevent Bioprosthetic Degeneration

BACKGROUND

Bioprosthetic heart valves undergo structural degeneration and calcification. Similarities exist in the histopathologic features of explanted bioprosthetic valves and rejected pig tissues and organs after xenotransplantation into nonhuman primates. The development of more durable bioprosthetic valves, namely from genetically modified pigs, could negate the need for the insertion of mechanical prostheses in children and young adults with the requirement for life-long anticoagulation and might avoid the need for reoperation in elderly patients.

METHODS

We reviewed the literature (MedlinePlus, PubMed, Google Scholar) through September 1, 2018, under four key terms: (1) bioprosthetic heart valves, (2) xenograft antigens, (3) immunologic responses to bioprosthetic valves, and (4) genetic modification of xenografts.

RESULTS

Advances in tissue and organ xenotransplantation have elucidated important immunologic barriers that provide innovative approaches to prevent structural degeneration of bioprosthetic heart valves. The current evidence suggests that bioprosthetic valves derived from genetically modified pigs lacking xenogeneic antigens (namely Gal, Neu5Gc, and Sda), termed triple-knockout pigs, would function considerably longer than current wild-type (genetically unmodified) porcine valves in human recipients.

CONCLUSIONS

Preclinical and clinical studies to determine the safety and efficacy of triple-knockout porcine bioprosthetic valves will likely establish that they are more resistant to human immune responses and thus less susceptible to structural degeneration.

In Vitro Calcification Model (2): Apatite Formation on Segmented Polyurethane Thin Films by Using an Alternate Soaking Process: The Effect of Adsorbed Serum Proteins on Calcification

In the present study, the effect of serum proteins on the calcification (apatite deposition) of commercially available polyurethane (PU) films, Pellethane® 2363-80AE, Pellathane® 2363-55DE, and K-III, was evaluated by using an alternating soaking process. Bovine serum albumin, γ-globulin and fibrinogen were used as typical serum proteins. After the proteins were adhered to PU films, the films were used in the apatite formation process. The thin-film X-ray diffraction spectra of a fibrinogen adhered K-III sample, after 200 calcification sequences, showed HAp formation. The calcium content on the surface of all films to which protein adhered was greater than that of films incubated in protein free solutions. The films to which fibrinogen adhered showed remarkable apatite formation, though the amount of adhered fibrinogen was much smaller than the amounts of the other proteins. K-III films to which fibrinogen had adhered showed small globular apatite aggregations, whereas the Pellethane® 2363 series films showed confetti-shaped apatite. Different morphologies exhibited by the deposits depended on the types of serum protein and on the substrate chemistry. The reason for the different apatite morphology on different PU films is not clear, however, the kind of serum protein seems to play an important role in the calcification of a PU.

Current Progress in Anticalcif ication for Bioprosthetic and Polymeric Heart Valves

The use of bioprosthetic valves fabricated from fixed heterograft tissue (porcine aortic valves or bovine pericardium) in heart valve replacement surgery is limited because of calcification-related failures. The mechanism of calcification of bioprosthetic valves is quite complex and has a variety of determinants, including host factors, tissue fixation conditions, and mechanical effects. Currently, there is no effective therapy to prevent calcification in clinical settings. This article reviews a variety of anticalcification strategies that are under investigation either in advanced animal models or in clinical trials. Bisphosphonates, such as ethan hydroxybisphosphonate (EHBP), inhibit calcium phosphate crystal formation. However, because of their systemic toxicity, they are used as either tissue treatments or polymeric site-specific delivery systems. Detergent treatment, such as sodium dodecyl sulfate (SDS), extracts almost all phospholipids from bioprosthetic heart valve cuspal tissue. Procedures, such as amino oleic acid pretreatment, inhibit calcium uptake. Polyurethane trileaflet valves, investigated as alternatives to bioprosthetic or mechanical valve prostheses, undergo intrinsic and thrombus-related calcification and degradation. Calcification- and thrombus-resistant polyurethanes synthesized in our laboratory by covalent linking of EHBP or heparin (either in bulk or on surface) by unique polyepoxidation chemistry are attractive candidates for further research. Tissue-engineered heart valves may have an important place in the future.

Successful matrix guided tissue regeneration of decellularized pulmonary heart valve allografts in elderly sheep

In vivo repopulation of decellularized allografts with recipient cells leads to a positive remodeling of the graft matrix in juvenile sheep. In light of the increasing number of heart valve replacements among older patients (>65 years), this study focused on the potential for matrix-guided tissue regeneration in elderly sheep. Pulmonary valve replacement was performed in seven-year old sheep using decellularized (DV), decellularized and CCN1-coated (RV), or decellularized and in vitro reendothelialized pulmonary allografts (REV) (n=6, each group). CCN1 coating was applied to support re-endothelialization. In vitro re-endothelialization was conducted with endothelial-like cells derived from peripheral blood. Echocardiograms of all grafts showed adequate graft function after implantation and at explantation 3 or 6 months later. All explants were macroscopically free of thrombi at explantation, and revealed repopulation of the allografts on the adventitial side of valvular walls and proximal in the cusps. Engrafted cells expressed vimentin, sm α-actin, and myosin heavy chain 2, while luminal cell lining was positive for vWF and eNOS. Cellular repopulation of valvular matrix demonstrates the capacity for matrix-guided regeneration even in elderly sheep but is not improved by in vitro endothelialization, confirming the suitability of decellularized matrix for heart valve replacement in older individuals.

Concomitant dual intravascular and subcutaneous microsurgical implantation of xenograft tissue in a rodent model for evaluation of structural degeneration

INTRODUCTION

An economical animal model to study xenograft tissue degeneration and calcification and the durability of biological vascular patch material and bioprosthetic valve leaflets is desirable.

OBJECTIVE

A cost-effective model to analyze xenograft degeneration, calcification, immunologic reaction, and anticalcification treatment was developed. Furthermore, a technique for implant into the vascular lumen of the abdominal aorta in rats is presented.

METHODS

Twelve Lewis rats were used as recipients. The microsurgical procedure was performed using a high-definition optical system. Anesthesia was induced and maintained with isoflurane inhalation. The suprarenal and infrarenal portion of the abdominal aorta was isolated, the abdominal aorta was cross-clamped, and a 4-mm square portion of the abdominal aorta was removed. Subsequently, a complementary-sized piece of porcine or bovine glutaraldehyde-fixed bioprosthetic valve leaflet tissue was sutured as a patch in the abdominal aorta.

RESULTS

The mean operating time was 45 ± 10 minutes and the mean ischemic time was 25 ± 5 minutes. Early and 3-month survivals were 100%. One rat had intraoperative bleeding. No paralysis or thrombosis was observed.

CONCLUSION

Feasibility and reproducibility of removing a portion of the abdominal aorta and replacing it with a patch of xenograft tissue was demonstrated in a rodent model with 100% survival at 3 months. Concomitant dual intravascular and subcutaneous microsurgical implantation of xenograft tissue in a small-animal (rat) model is a cost-effective approach for investigation of xenograft tissue degeneration.

Aortic valve disease and treatment: the need for naturally engineered solutions

The aortic valve regulates unidirectional flow of oxygenated blood to the myocardium and arterial system. The natural anatomical geometry and microstructural complexity ensures biomechanically and hemodynamically efficient function. The compliant cusps are populated with unique cell phenotypes that continually remodel tissue for long-term durability within an extremely demanding mechanical environment. Alteration from normal valve homeostasis arises from genetic and microenvironmental (mechanical) sources, which lead to congenital and/or premature structural degeneration. Aortic valve stenosis pathobiology shares some features of atherosclerosis, but its final calcification endpoint is distinct. Despite its broad and significant clinical significance, very little is known about the mechanisms of normal valve mechanobiology and mechanisms of disease. This is reflected in the paucity of predictive diagnostic tools, early stage interventional strategies, and stagnation in regenerative medicine innovation. Tissue engineering has unique potential for aortic valve disease therapy, but overcoming current design pitfalls will require even more multidisciplinary effort. This review summarizes the latest advancements in aortic valve research and highlights important future directions.

Stentless valves treated by the L-hydro process in the aortic position in sheep

Calcification of glutaraldehyde-treated bioprosthetic heart valves is a major cause of long-term failure. We studied porcine aortic valves treated by the L-Hydro process and implanted into 14 juvenile sheep (group 1). Another 10 sheep were implanted with glutaraldehyde-treated porcine bioprostheses (group 2). The animals were sacrificed after 150 days and the explanted valves were analyzed for calcification. Hemodynamic measurements by echocardiography and angiography were carried out prior to sacrifice. Macroscopic analysis showed calcification and loss of mobility of the leaflets in all group 2 implants and in one group 1 implant. Light microscopy showed foci of calcification in all group 2 implants and in 3 valves from group 1. A significant reduction in the level of calcification was found in porcine bioprostheses treated by the L-Hydro process and implanted into the juvenile sheep model.

Glutaraldehyde-Fixed Bioprosthetic Heart Valve Conduits Calcify and Fail From Xenograft Rejection

BACKGROUND

Glutaraldehyde fixation (G-F) decreases but likely does not eliminate the antigenicity of bioprosthetic heart valves. Rejection (with secondary dystrophic calcification) may be why G-F xenograft valves fail, especially in young patients, who are more immunocompetent than the elderly. Therefore, we sought to determine whether rejection of G-F xenograft occurs and to correlate this with graft calcification.

METHODS AND RESULTS

Ascending aortas/valves (from rats [syngeneic] or guinea pigs [xenogeneic]) were transplanted (fresh or after 48 hour of G-F) into the infrarenal aortas of young rat recipients for 20 days. A xenogeneic group was also treated with steroids until graft harvest. The valves and media/adventitia were scored blindly for inflammation (0 to 4). Percent graft infiltration by T cells/macrophages was determined (immunohistochemistry), and rat IgG ELISAs were performed. There was >3 times more valve inflammation, >10 times more valve T-cell/macrophage infiltrate, and >3 times antibody rise in the G-F xenogeneic groups compared with the fresh syngeneic or the G-F syngeneic groups (P<0.05). There was >2 times more adventitial inflammation and T-cell/macrophage infiltrate in the xenogeneic groups (P<0.05). Steroid treatment decreased inflammation and antibody rise in the xenogeneic groups (P<0.05). Correlation analysis revealed media/adventitia inflammation (P=0.02) and percent macrophage (P=0.01) infiltration to be predictors of calcification.

CONCLUSIONS

G-F xenografts have cellular/humoral rejection and calcify secondarily.

Factors influencing calcification of cardiac bioprostheses in adolescent sheep

OBJECTIVE

We determined the possible effects of age, antimineralization treatments, circulatory implant conditions, prosthesis design, and valve-related structural aspects on valve calcification in adolescent sheep.

METHODS

Calcium content was measured by means of atomic absorption spectrometry in bioprostheses implanted in 120 sheep (age <1 year) for a period of 3 or 6 months.

RESULTS

Bioprostheses calcified significantly in adolescent sheep, but the extent of calcification was multifactorial. Multivariate analysis of the calcium content reveals that age, mitral or pulmonary implant position, prosthesis design (stented or stentless), structure (porcine or pericardial, wall portion or cusp), and antimineralization treatment are independent factors influencing calcification; implant duration beyond 3 months was not. In juvenile sheep (age 5 months) the wall portion, as well as the cusps of the prosthesis, calcified significantly more than in adolescent sheep (age 11 months). Irrespective of age, the cusps of valves implanted in the mitral position calcified more than those in the pulmonary position. The wall portion of stentless valves calcified more than that of stented valves, and pericardial valves calcified less than porcine valves. The surfactant (Tween 80, No-React, and alpha-amino-oleic acid) and alcohol (ethanol and octanediol) treatment significantly reduced cusp calcification; sodium dodecylsulfate did not. None of the anticalcification treatments was able to prevent wall calcification in stentless porcine valves.

CONCLUSION

These findings suggest that tissue valve calcification is determined by many independent factors, which can be identified by using adolescent sheep as a preclinical in vivo model.

Biochemical and mechanical behavior of ostrich pericardium as a new biomaterial

We have performed a comparative analysis of glutaraldehyde-preserved ostrich pericardium, as a novel biomaterial, with bovine pericardium. The biochemical characteristics (histology, water content, amino acid composition, and collagen and elastin contents), mechanical properties, and in vivo calcification in a subcutaneous rat model were examined. Ostrich pericardium is slightly thinner and shows a higher water content (70+/-2% vs. 62+/-2%) than bovine pericardium. Additionally, ostrich pericardium presents 1.6-fold lower elastin content and a lower percentage of collagen in reference to the total protein content (68+/-2% vs. 76+/-2%). However, ostrich pericardium shows better mechanical properties, with higher tensile stress at rupture (32.4+/-7.5 vs. 11.5+/-4.6) than calf pericardium. In vivo calcification studies in a rat subcutaneous model show that ostrich pericardium is significantly less calcified than bovine pericardium (23.95+/-13.30 vs. 100.10+/-37.36 mg/g tissue) after 60 days of implantation. In conclusion, glutaraldehyde-stabilized ostrich pericardium tissue shows better mechanical properties than calf tissue. However, calcium accumulation in implanted ostrich tissue is still too high to consider it a much better alternative to bovine pericardium, and anticalcification treatments should be considered.

Comparative study of the L-hydro process and glutaraldehyde preservation

Commercial bioprosthetic heart valves are commonly preserved in glutaraldehyde and are cytotoxic to host cells, preventing spontaneous endothelialization. The aim of this study was to demonstrate the potential for in vivo endothelialization of bioprostheses treated by the L-Hydro process which consists of mild extraction of antigenic substances and incorporation of antiinflammatory and antithrombotic agents. Seven stented porcine heart valves treated by the L-Hydro process and 3 glutaraldehyde-fixed porcine heart valves were implanted in the mitral position in juvenile sheep. The valves were evaluated by echocardiography, angiography, histology, and histochemistry. No hemodynamic differences were observed, but scanning and transmission electron microscopy showed nearly complete coverage by endothelial cells of all leaflets in the L-Hydro-treated valves after 5 months of implantation. The endothelial cells were in direct contact with the underlying collagen and expressed von Willebrand-related antigens. The surfaces of the glutaraldehyde-treated valves were covered by fibrin, macrophages, calcium, and thrombotic material; only sparse endothelial cells were observed and contact with the underlying tissue was incomplete. These data indicate that L-Hydro-treated porcine valves are capable of inducing spontaneous endothelialization.

Kangaroo versus freestyle stentless bioprostheses in a juvenile sheep model: hemodynamic performance and calcification behavior

BACKGROUND

Glutaraldehyde-preserved bioprosthetic heart valve substitutes have limited performance and longevity due to tissue degeneration and calcification. The Freestyle valve (Medtronic Heart Valves, Inc, Minneapolis, MN) combines zero fixation pressure and proportional, variant-amino oleic acid (AOA) as antidegeneration and antimineralization measures. The aim of this study was to compare the calcification behavior of glutaraldehyde-preserved kangaroo aortic valves with Freestyle stentless bioprostheses in a juvenile sheep model.

METHODS

Pulmonary artery and valve replacements were performed in juvenile sheep with Freestyle stentless aortic valves (n = 4) or glutaraldehyde-preserved kangaroo stentless aortic valves with no antimineralization measures (n = 6), and explanted at 200 postoperative days.

RESULTS

Freestyle stentless valves and stentless kangaroo aortic valves showed normal valve function immediately postoperatively and up to 120 days. Valve leaflets of all valves were macroscopically free of visible calcification with normal histology. Valve leaflet calcification (microg Ca/mg tissue) was less in kangaroo than Freestyle (1.27 +/- 0.43 versus 2.38 +/- 1.37, p = 0.856). Aortic wall tissue calcification was severe in kangaroo and Freestyle (127.93 +/- 12.22 versus 122.19 +/- 11.99, p = 0.596).

CONCLUSIONS

We conclude that glutaraldehyde-preserved kangaroo aortic valve leaflets are equal to AOA-treated Freestyle stentless valve leaflets with regard to calcification in juvenile sheep. Both bioprostheses are prone to aortic wall calcification. The low calcification features of the kangaroo aortic valve leaflets without antimineralization treatment may benefit the longevity of the valve.

The potential calcification of octacalcium phosphate on intraocular lens surfaces

Recently, calcification was observed on implanted intraocular lens (IOL) surfaces when viscoelastic substances were applied during surgery. To elucidate the mechanisms of mineral formation, the crystallization of calcium phosphates on IOL surfaces was studied in vitro with nanomolar sensitivity using a constant composition method. Three different commercial viscoelastic materials (Viscoat, OcuCoat, and Amvisc Plus) were investigated and it was found that some IOLs treated with Viscoat or Amvisc Plus induced the nucleation and growth of octacalcium phosphate crystallites under biological conditions. After treatments, the IOL surfaces became more hydrophilic probably because of the high viscoelastic phosphate and carboxylate contents. In contrast to Viscoat, the use of OcuCoat during surgery resulted in virtually no octacalcium phosphate nucleations. Calcification studies of IOL surfaces treated with fatty acids, which are present in human aqueous humor, suggest that hydrophobic cyclic silicones adsorbed on the IOL surfaces interact strongly with hydrophobic hydrocarbon chains of the fatty acids, creating a layer of amphiphiles oriented with functional carboxylate groups exposed to the aqueous solution and serving as active calcification sites.

Triglycidylamine crosslinking of porcine aortic valve cusps or bovine pericardium results in improved biocompatibility, biomechanics, and calcification resistance: chemical and biological mechanisms

We investigated a novel polyepoxide crosslinker that was hypothesized to confer both material stabilization and calcification resistance when used to prepare bioprosthetic heart valves. Triglycidylamine (TGA) was synthesized via reacting epichlorhydrin and NH(3). TGA was used to crosslink porcine aortic cusps, bovine pericardium, and type I collagen. Control materials were crosslinked with glutaraldehyde (Glut). TGA-pretreated materials had shrink temperatures comparable to Glut fixation. However, TGA crosslinking conferred significantly greater collagenase resistance than Glut pretreatment, and significantly improved biomechanical compliance. Sheep aortic valve interstitial cells grown on TGA-pretreated collagen did not calcify, whereas sheep aortic valve interstitial cells grown on control substrates calcified extensively. Rat subdermal implants (porcine aortic cusps/bovine pericardium) pretreated with TGA demonstrated significantly less calcification than Glut pretreated implants. Investigations of extracellular matrix proteins associated with calcification, matrix metalloproteinases (MMPs) 2 and 9, tenascin-C, and osteopontin, revealed that MMP-9 and tenascin-C demonstrated reduced expression both in vitro and in vivo with TGA crosslinking compared to controls, whereas osteopontin and MMP-2 expression were not affected. TGA pretreatment of heterograft biomaterials results in improved stability compared to Glut, confers biomechanical properties superior to Glut crosslinking, and demonstrates significant calcification resistance.

Calcification of Tissue Heart Valve Substitutes: Progress Toward Understanding and Prevention

Calcification plays a major role in the failure of bioprosthetic and other tissue heart valve substitutes. Tissue valve calcification is initiated primarily within residual cells that have been devitalized, usually by glutaraldehyde pretreatment. The mechanism involves reaction of calcium-containing extracellular fluid with membrane-associated phosphorus to yield calcium phosphate mineral deposits. Calcification is accelerated by young recipient age, valve factors such as glutaraldehyde fixation, and increased mechanical stress. Recent studies have suggested that pathologic calcification is regulated by inductive and inhibitory factors, similar to the physiologic mineralization of bone. The most promising preventive strategies have included binding of calcification inhibitors to glutaraldehyde fixed tissue, removal or modification of calcifiable components, modification of glutaraldehyde fixation, and use of tissue cross linking agents other than glutaraldehyde. This review summarizes current concepts in the pathophysiology of tissue valve calcification, including emerging concepts of endogenous regulation, progress toward prevention of calcification, and issues related to calcification of the aortic wall of stentless bioprosthetic valves.

Prevention of Calcification of Bioprosthetic Heart Valve Cusp and Aortic Wall With Ethanol and Aluminum Chloride

BACKGROUND

Calcification is frequently associated with device failure of bioprostheses fabricated from either glutaraldehyde pretreated porcine aortic valves or bovine pericardium. It was hypothesized that differential pretreatment with ethanol-aluminum chloride will prove safe and efficacious for inhibiting the calcification of both the porcine aortic valve bioprosthetic cusp and the aortic wall.

METHODS

Glutaraldehyde-fixed porcine aortic valves were subjected to differential aluminum chloride (AlCl3) and ethanol pretreatment; aortic wall segments were treated exclusively with AlCl3 (0.1 moles/L) for 45 minutes, 6 hours, or 8 hours (groups 3A, B, and C, respectively), followed by valve cusp incubations in ethanol (80%, pH 7.4). Nontreated control bioprosthetic valves were either stent-mounted porcine aortic valve bioprostheses (Carpentier-Edwards, group 1) (Edwards, Santa Anna, CA) or St. Jude Toronto SPV valves (St. Jude Medical, St. Paul, MN) (group 2). Mitral valve replacements were carried out in juvenile sheep for 150 days.

RESULTS

Calcium in cusps from group 3A was 2.84 +/- 0.62 mg calcium/g tissue versus control, 22.79 +/- 8.46 mg calcium/g tissue, p = 0.04. Valves pretreated with AlCl3 for 45 minutes, 6 hours, and 8 hours had significantly lower levels of calcium in the aortic wall compared to controls (40.38 +/- 5.66, 26.77 +/- 4.02, and 28.94 +/- 8.25 mg calcium/g tissue for groups 3A, 3B, and 3C, respectively, vs 95.47 +/- 17.14 mg calcium/g tissue for group 1, p < 0.001, and 133.42 +/- 3.96 mg calcium/g tissue for group 2, p < 0.001).

CONCLUSIONS

Differentially applied ethanol and aluminum chloride pretreatment significantly inhibited calcification of both the glutaraldehyde-fixed porcine aortic valve bioprosthetic cusp and the aortic wall.

Calcification of intraocular implant lens surfaces

Calcification of octacalcium phosphate [Ca8H2(PO4)6 x 5H2O, OCP] on differently packaged "Ultem" and "Surefold" intraocular implant lens surfaces has been studied in vitro in solutions supersaturated with respect to OCP at pH = 7.10 and 37 degrees C. No mineral deposition was observed on the lenses packaged in Ultem vials even after treatment with behenic acid, one of the fatty acids identified on explanted lenses. Following treatment with behenic acid, nucleation of OCP occurred on the lenses from Surefold vials, which incorporate silicone gaskets; induction periods preceding calcification were about 6 h. No mineralization was found on the lenses in vials with other gasket materials, including polytetrafluoroethylene, fluorocarbon elastomer, and polypropylene. The results of this study indicate that both silicone and fatty acids such as behenic acid play important roles in inducing the in vivo calcification of OCP on IOL lenses; all of the lens treatment steps were necessary for nucleation induction.

Role of elastin in pathologic calcification of xenograft heart valves

Bioprosthetic heart valves fabricated from glutaraldehyde crosslinked porcine aortic valves often fail because of calcific degeneration. Calcification occurs in both cusp and aortic wall portions of bioprosthetic heart valves. The purpose of this study was to discern the role of different aortic wall components in the calcification process. Thus, we selectively extracted cells and other extracellular matrix proteins from porcine aorta using trypsin/DNase/RNase, cyanogen bromide (CNBr), and sodium hydroxide (NaOH) treatments and subdermally implanted these pretreated aortas in young rats. Total DNA and phospholipid data showed complete removal of cells by CNBr and NaOH treatments, whereas trypsin/DNase/RNase treatment was effective in removing DNA but not phospholipids. As shown by amino acid data and Masson's trichrome staining, collagen was removed in CNBr and NaOH treatments. Control fresh porcine aorta calcified significantly after 21 days of implantation (Ca 26.4 +/- 2.4 microg/mg). Removal of cells and collagen from the aorta by CNBr treatment did not lead to a statistically significant reduction in aortic calcification (Ca 20.8 +/- 3.0 microg/mg). Moreover, partial degradation of elastin fibers caused by NaOH (during extraction) and trypsin treatment (after implantation) of the aorta significantly increased elastin-oriented calcification (Ca 94.4 +/- 9.3 and 58.4 +/- 4.6 microg/mg, respectively). Our results indicate that the elastin component of the aorta may undergo independent calcification irrespective of devitalized cell-mediated calcification observed in glutaraldehyde crosslinked aortas. Our results also demonstrate the importance of studying elastin-oriented calcification in decellularized elastin-rich aortic matrices currently used in tissue-engineering applications.

In vivo repopulation of xenogeneic and allogeneic acellular valve matrix conduits in the pulmonary circulation

BACKGROUND

Approaches to in vivo repopulation of acellularized valve matrix constructs have been described recently. However, early calcification of acellularized matrices repopulated in vivo remains a major obstacle. We hypothesised that the matrix composition has a significant influence on the onset of early calcification. Therefore, we evaluated the calcification of acellularized allogenic ovine (AVMC) and xenogenic porcine (XVMC) valve matrix conduits in the pulmonary circulation in a sheep model.

METHODS

Porcine (n = 3) and sheep (n = 3) pulmonary valve conduits were acellularized by trypsin/EDTA digestion and then implanted into healthy sheep in pulmonary valve position using extracorporeal bypass support. Transthoracic echocardiography (TTE) was performed at 12 and 24 weeks after the implantation. The animals were sacrificed at week 24 or earlier when severe calcification of the valve conduit became evident by TTE. The valves were examined histologically and biochemically.

RESULTS

All AVMC revealed severe calcification after 12 weeks with focal endothelial cell clustering and no interstitial valve tissue reconstitution. In contrast, after 24 weeks XVMC indicated mild calcification on histologic examination (von Kossa staining) with histologic reconstitution of valve tissue and confluent endothelial surface coverage. Furthermore, immunohistologic analysis revealed reconstitution of surface endothelial cell monolayer (von Willebrand factor), and interstitial myofibroblasts (Vimentin/Desmin).

CONCLUSIONS

Porcine acellularized XVMC are resistant to early calcification during in vivo reseeding. Furthermore, XVMC are repopulated in vivo with valve-specific cell types within 24 weeks resembling native valve tissue.

Calcification resistance with aluminum-ethanol treated porcine aortic valve bioprostheses in juvenile sheep

BACKGROUND

Calcification of glutaraldehyde fixed bioprosthetic heart valve replacements frequently leads to the clinical failure of these devices. Previous research by our group has demonstrated that ethanol pretreatment prevents bioprosthetic cusp calcification, but not aortic wall calcification. We have also shown that aluminum chloride pretreatment prevents bioprosthetic aortic wall calcification. This study evaluated the combined use of aluminum and ethanol to prevent both bioprosthetic porcine aortic valve cusp and aortic wall calcification in rat subcutaneous implants, and the juvenile sheep mitral valve replacement model.

METHODS

Glutaraldehyde fixed cusps and aortic wall samples were pretreated sequentially first with aluminum chloride (AlCl3) followed by ethanol pretreatment. These samples were then implanted subdermally in rats with explants at 21 and 63 days. Stent mounted bioprostheses were prepared either sequentially as previously described or differentially with AlCl3 exposure restricted to the aortic wall followed by ethanol pretreatment. Mitral valve replacements were carried out in juvenile sheep with elective retrievals at 90 days.

RESULTS

Rat subdermal explants demonstrated that sequential exposure to AlCl3 and ethanol completely inhibited bioprosthetic cusp and aortic wall calcification compared with controls. However the sheep results were markedly different. The differential sheep explant group exhibited very low levels of cusp and wall calcium. The glutaraldehyde group exhibited little cusp calcification, but prominent aortic wall calcification. All sheep in the two groups previously described lived to term without evidence of valvular dysfunction. In contrast, animals in the sequential group exhibited increased levels of cusp calcification. None of the animals in this group survived to term. Pathologic analysis of the valves in the sequential group determined that valve failure was caused by calcification and stenosis of the aortic cusps.

CONCLUSIONS

The results clearly demonstrate that a combination of aluminum and ethanol reduced aortic wall calcification and prevented cuspal calcification. Furthermore, this study demonstrates that exclusion of aluminum from the cusp eliminated the cuspal calcification seen when aluminum and ethanol treatments were administered in a sequential manner.

Aluminum chloride pretreatment of elastin inhibits elastolysis by matrix metalloproteinases and leads to inhibition of elastin-oriented calcification

Calcification of elastin occurs in many pathological cardiovascular diseases including atherosclerosis. We have previously shown that purified elastin when subdermally implanted in rats undergoes severe calcification and aluminum chloride (AlCl(3)) pretreatment of elastin inhibits calcification. In the present study we investigated whether matrix metalloproteinase (MMP) binding to elastin and elastin degradation is prevented by AlCl(3) pretreatment. Subdermal implantation of AlCl(3)-pretreated elastin showed significantly lower MMP-9 and MMP-2 activity surrounding the implant as compared to the control implants. AlCl(3) pretreatment also significantly inhibited elastin implant calcification at the seven-day implant period (AlCl(3)-pretreated 4.07 +/- 1.27, control 23.82 +/- 2.24 microg/mg; p<0.0001). Moreover, elastin gel zymography studies showed that gel pretreatment with AlCl(3) inhibited elastolysis by MMP-9. We also demonstrate significant suppression of MMP-2 activity in aortic wall segments of AlCl(3)-pretreated porcine bioprosthetic heart valve implants as compared to control valve implants in sheep mitral valve replacement studies. AlCl(3) pretreatment also significantly inhibited calcification of elastin in this model. Thus, we conclude that aluminum ion binding to elastin prevents MMP-mediated elastolysis and thus prevents elastin calcification.

Tissue reactions to epoxy-crosslinked porcine heart valves post-treated with detergents or a dicarboxylic acid

Calcification limits the long-term durability of xenograft glutaraldehyde (GA)-crosslinked heart valves. Previously, a study in rats showed that epoxy-crosslinked heart valves reduced lymphocyte reactions to the same extent as the GA-crosslinked control and induced a similar foreign-body response and calcification reaction. The present study was aimed at reducing the occurrence of calcification of epoxy-crosslinked tissue. Two modifications were carried out and their influence on cellular reactions and the extent of calcification after 8 weeks' implantation in weanling rats was evaluated. First, epoxy-crosslinked valves were post-treated with two detergents to remove cellular elements, phospholipids and small soluble proteins, known to act as nucleation sites for calcification. The second approach was to study the effect of the impaired balance between negatively and positively charged amino acids by an additional crosslinking step with a dicarboxylic acid. The detergent treatment resulted in a washed-out appearance of especially the cusp tissue. With the dicarboxylic acid, both the cusps and the walls had a limited washed-out appearance. The wall also demonstrated some detachment of the subendothelium. After implantation, both detergent and dicarboxylic acid post-treatment histologically resulted in reduced calcification at the edges of cusps and walls. However, total amounts of calcification, measured by atomic emission spectroscopy, were not significantly reduced. Data concerning the presence of lymphocytes varied slightly, but were in the same range as the GA-crosslinked control, i.e., clearly reduced compared with a noncrosslinked control. It is concluded that both the double detergent and the dicarboxylic acid post-treatment of epoxy-crosslinked heart valve tissue do not represent a sound alternative in the fabrication of heart valve bioprostheses.

In vivo behavior of epoxy-crosslinked porcine heart valve cusps and walls

Calcification limits the long-term durability of xenograft glutaraldehyde-crosslinked heart valves. In this study, epoxy-crosslinked porcine aortic valve tissue was evaluated after subcutaneous implantation in weanling rats. Non-crosslinked valves and valves crosslinked with glutaraldehyde or carbodiimide functioned as control. Epoxy-crosslinked valves had somewhat lower shrinkage temperatures than the crosslinked controls, and within the series also some macroscopic and microscopic differences were obvious. After 8 weeks implantation, cusps from non-crosslinked valves were not retrieved. The matching walls were more degraded than the epoxy- and control-crosslinked walls. This was observed from the higher cellular ingrowth with fibroblasts, macrophages, and giant cells. Furthermore, non-crosslinked walls showed highest numbers of lymphocytes, which were most obvious in the capsules. Epoxy- and control-crosslinked cusps and walls induced lower reactions. Calcification, measured by von Kossa-staining and by Ca-analysis, was always observed. Crosslinked cusps calcified more than walls. Of all wall samples, the non-crosslinked walls showed the highest calcification. It is concluded that epoxy-crosslinked valve tissue induced a foreign body and calcification reaction similar to the two crosslinked controls. Therefore, epoxy-crosslinking does not represent a solution for the calcification problem of heart valve bioprostheses.

Inhibition of matrix metalloproteinase activity attenuates tenascin-C production and calcification of implanted purified elastin in rats

Elastin, a major extracellular matrix protein present in arterial walls provides elastic recoil and resilience to arteries. Elastin is prone to calcification in a number of cardiovascular diseases including atherosclerosis and bioprosthetic heart valve mineralization. We have recently shown that purified elastin when implanted subdermally in rats undergoes severe calcification. In the present study, we used this elastin implant model to investigate the molecular mechanisms underlying elastin calcification. Intense matrix metalloproteinase (MMP-2) and tenascin-C (TN-C) expression were seen in the proximity of the initial cal-cific deposits at 7 days. Gelatin zymography studies showed both MMP-2 (latent and active form) and MMP-9 expression within the implants. To investigate the role of MMPs in calcification, rats were administered a MMP inhibitor, (2S:-allyl-N:-hydroxy-3R:-isobutyl-N:-(1S:-methylcarbamoyl-2-ph enylet hyl)-succinamide (BB-1101) by daily injection, either systemically or at the implant site. The site-specific BB-1101 administration almost completely suppressed TN-C expression, as shown by immunohistochemical staining, within the implants. The systemic BB-1101 injections also significantly reduced TN-C expression within the elastin implants. Moreover, calcification of elastin implants was significantly reduced in the site-specific administration group (5.43 +/- 1.03 microg/mg Ca for BB-1101 group versus 21.71 +/- 1.19 for control group, P: < 0.001). Alizarin Red staining clearly showed that the elastin fibers were heavily calcified in the control group, whereas in BB-1101 group the calcification was scarce with few fibers showing initial calcification deposits. The systemic administration of BB-1101 also significantly reduced elastin calcification (28.07 +/- 5.81 control versus 16.92 +/- 2.56 in the BB-1101 group, P: < 0.05), although less than the site-specific administration. Thus, the present studies indicate that MMPs and TN-C play a role in elastin-oriented calcification.

The radiographic quantitation of aortic valve calcification: implications for assessing bioprosthetic valve calcification in vitro

Calcification of natural aortic and bioprosthetic heart valves is a poorly understood phenomenon that results in valvular obstruction and tissue failure. We describe a non-destructive quantitative computed microtomographic (QCT) technique for determining both calcium content and local calcium distribution within explanted valves. As a reference standard, a dual-energy x-ray absorptiometry (DEXA) system with an accuracy demonstrated to be within 1% of the true calcium mass of test material was used to obtain the total calcium content of 24 human aortic valve cusps recovered at autopsy from patients aged 51-80 years. These cusps were then scanned using our unique volume QCT scanner, with multiple x-ray projections acquired by rotating the explanted tissue through a single axis of rotation. A three-dimensional cross-sectional map was reconstructed for each cusp. Voxel size was 0.003 mm3 and a calibration phantom was used to calculate calcium content. The minimum detection limit for calcium mass was 1 mg within the whole cusp. The DEXA and QCT scans were compared with respect to total calcium content, which ranged from 0 to 15 mg. An excellent correlation between the two independent techniques was demonstrated with an r2 value of 0.94 (p < 0.001). Non-destructive microtomographic CT scanning provided excellent volumetric density measurements, with quantitative 3D images permitting an assessment of any individual area of the cusp for calcium content and spatial distribution. This new approach to valve tissue analysis allows for subsequent histologic assessment.

In Vitro Calcification Model—Part 1: Apatite Formation on Segmented Polyurethane Containing Silicone Using an Alternate Soaking Process

The effect of silicone on calcification on polyetherpolyurethane (PU) surfaces was studied using an alternate soaking process in vitro. The process is based on a wet process of hydroxyapatite formation that involves alternately soaking in CaCl2/Tris-HCl (pH 7.4) and Na2HPO4 solutions (pH 7.4) at 37°C. We used K-III and Pellethane® 2363 as samples. K-III is a complex of PU containing dimethyldiacetoxysilane and methyltriacetoxysilane. Pellethane® 2363 series contain less Si. Si content was assessed by X-ray photoelectron spectroscopy (XPS). The gravimetric measurements and scanning electron microscopic observations with energy dispersed X-ray analyzer (EDX) were performed after specific reaction cycles. Calcified deposits formed on the surface of K-III were 20 μg/cm2 after 10 reaction cycles. EDX results showed remarkable Ca (Kα, Kb), P (Kα), and Si (Kα) peaks by the deposits formed on the surface of K-III. On the other hand, no peaks for Ca of P were observed on the surface of Pellethane® after ten reaction cycles. The reason for calcified deposit formation on PU is not clear. However, it was suggested that Si on the surface of PU is one of the key factors for calcified deposit formation. These results suggest that an alternate soaking process could be a useful evaluation method for an accelerated evaluation of fatigue induced by calcification on polymer substrates.