Weiliam Chen

Mechanism of efficacy of 2-amino oleic acid for inhibition of calcification of glutaraldehyde-pretreated porcine bioprosthetic heart valves.

Calcification is a frequent cause of the clinical failures of glutaraldehyde-pretreated bioprosthetic heart valves (BPHV) fabricated from glutaraldehyde-cross-linked porcine aortic valves. 2-Amino oleic acid (AOA) has been shown in previous in vivo studies to be a promising anticalcification agent. Our objective was to investigate the mechanism of calcification inhibition mediated by AOA pretreatment of porcine aortic valve bioprostheses. Methods and ResultsBPHV tissues were treated with an AOA solution for 72 hours before experimentation. The diffusion of AOA across both cusp and aortic wall was evaluated. The lag time for AOA to diffuse across the aortic wall was prolonged compared with that of the cusp. An extraction study was performed to determine the stability of AOA binding; the results indicated that the binding was relatively stable regardless of solvent extraction conditions. The interaction between ionic calcium and AOA on treated tissue also was investigated by evaluating the patterns of calcium diffusion across both treated and untreated tissues. The results showed that AOA significantly reduced the diffusion of calcium. AOA inhibition of aortic valve calcification (calcium level, 5.5±3.0 mg/g of tissue compared with control; calcium level, 91.2±19.5 mg/g of tissue) but not aortic wall (calcium level, 158.7±10.3 mg/g of tissue compared with control; calcium level, 157.5±7.9 mg/g of tissue) was demonstrated on representative specimens from valves implanted in left ventricular apicoaortic shunts explanted after 150 days. ConclusionsAOA covalently binds to glutaraldehyde-pretreated bioprosthetic heart valve tissue, presumably as the result of an aldehyde-amino reaction. Covalently bound AOA diminishes Ca2+ diffusion compared with non-AOA-pretreated bioprosthetic tissues. This may explain in part the anticalcification mechanism of AOA. Furthermore, AOA inhibits calcification of porcine BPHV cusps in the circulation.

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.

Refinement of the Alpha Aminooleic Acid Bioprosthetic Valve Anticalcification Technique

BACKGROUNDnAminooleic acid treatment has been demonstrated to prevent porcine valve calcification and to protect valvular hemodynamic function. Initial enthusiasm was tempered by histologic studies of these AOA valves, which showed cuspal hematomas, structural loosening, and surface roughening. This prompted a systematic review of the AOA treatment process. Unsolubilized particles of alpha aminooleic acid present in the treatment solution were identified as the cause of mechanical abrasion of valve cusps during processing. These particles were eliminated with a revamped protocol, which included filtration of the AOA solution before valve preparation.nnnMETHODSnPorcine aortic valve cusps treated with this modified AOA protocol (AOA II) were studied in a rat subdermal implant model of mineralization. A juvenile sheep trial was then used to confirm the antimineralization effects of AOA II on glutaraldehyde-fixed porcine aortic roots in a circulatory model of accelerated calcification.nnnRESULTSnRetrieved AOA II-treated cusps from the subdermal model were markedly less calcified than control cusps (AOA II, 1 +/- 0, 17 +/- 4, 23 +/- 6, and 17 +/- 10 versus control, 189 +/- 14, 251 +/- 16, 250 +/- 14, and 265 +/- 10 mg calcium/mg sample at 4, 8, 12, and 16 weeks, respectively; p < 0.0001). Morphologic examination of the AOA II cusps of the valves retrieved from the sheep demonstrated freedom from the structural loosening, surface roughening, and hematoma formation that had limited the utility of the original AOA preparation technique. Cusps from AOA II-treated porcine roots had significantly less calcium than control cusps (AOA II, 5.5 +/- 3.0 mg/g; control, 91.2 +/- 19.5 mg/g; p = 0.0004). The aortic walls had similar levels of calcification (AOA II, 156 +/- 73 mg/g; control, 159 +/- 10 mg/g; p = not significant).nnnCONCLUSIONSnThese data suggest that the modified AOA technique warrants further evaluation as an antimineralization treatment for glutaraldehyde-fixed porcine bioprostheses.

Polymeric drug delivery systems for treatment of cardiovascular calcification, arrhythmias and restenosis

Abstract Cardiovascular controlled release, utilizing drug-polymer composites implanted in direct contact with the heart, has recently come into clinical use with a dexamethasone eluting cardiac pacemaker lead tip. Furthermore, cardiovascular controlled release systems are under active investigation in a number of other areas of possible application. The general working hypothesis of this approach is that regionally administered drug delivered directly to the heart or a blood vessel will more efficiently and effectively treat localized disease processes of interest, while avoiding systemic side effects. Successful experimental examples illustrating the validity of this hypothesis have involved investigations into cardiovascular calcification, therapy of cardiac arrhythmias, and treatment of arterial restenosis following angioplasty. Efficacious results in each of these areas, with some limitations, have been noted, and are discussed in detail in this paper. An ideal cardiovascular controlled release system will consist of a feedback responsive implant in which drug release kinetics could be varied according to disease activity, or other considerations such as side effects of the therapeutic agent. Furthermore, cardiovascular drug delivery should be ideally extremely long acting and this may be possible through the use of therapeutic agents in a refillable reservoir configuration, or local gene therapy with long standing expression of the gene of interest.

Synergistic inhibition of calcification of porcine aortic root with preincubation in FeCl3 and α-amino oleic acid in a rat subdermal model

Postimplant calcific degeneration is a frequent cause of clinical failure of glutaraldehyde crosslinked porcine aortic valve bioprostheses. We demonstrated previously in rat subdermal and circulatory implants that alpha-amino oleic acid used as a bioprosthesis pretreatment was highly effective in mitigating aortic valve cusp but not aortic wall calcification. In this study we investigated the feasibility of synergistically applying two proven anticalcification agents (alpha-amino oleic acid and FeCl3) as pretreatments for mitigating both bioprosthetic cusp and aortic wall calcification. alpha-Amino oleic acid is hypothesized to prevent calcification by disrupting calcium phosphate formation kinetics, whereas suppression of alkaline phosphatase activity and ferric-phosphate complexation at a cellular membrane initiation sites may be important factors in ferric ions inhibition of calcification. In vivo implant studies (21-day rat subdermal model) indicated that individually FeCl3 (0.01 or 0.1 M for 24 h) or alpha-amino oleic acid (saturated solution) treatments were equally effective in mitigating cuspal calcification (tissue calcium levels: 30.2 +/- 10.2, 29.8 +/- 2.7, and 31.6 +/- 7.8 micrograms/mg tissue, respectively). However, sequential application of first alpha-amino oleic acid and then FeCl3 synergistically reduced aortic wall calcification more effectively than either of the agents alone. The benefit of a synergistic application of two anticalcification treatments, alpha-amino oleic acid and FeCl3, was demonstrated. However, the synergistic effect was observed on aortic wall only at a higher FeCl3 concentration. (i.e., 0.1 M).

The Development of a Cellulosic Material Based Method for Heparin Therapy Monitoring

We have developed a non-clotting based heparin assay. It is established by the attachment of protamine (a heparin antidote) to a porous filter paper strip; and the subsequent migration of heparin sample through the filter paper in an ascending or descending manner. The area of paper to which heparin adsorbed will be proportional to the heparin level in the sample. This region is visualized by spraying methylene blue NNX (a dye that interacts with heparin and the interaction causes a metachromatic shift of the dye’s absorption maximum from blue to purple) solution onto the paper strip upon the exhaustion of the sample reservoir. This approach has proven to be capable of detecting and differentiating plasma heparin level within clinical range in a relatively time efficient and accurate manner.