Prevention of calcification of bioprosthetic heart valve leaflets by Ca2+ diphosphonate pretreatment

EFFECT OF ALENDRONATE-CONTAINING COATINGS ON OSTEOINTEGRATION INTO POROUS TANTALUM IN A CORTICAL BONE MODEL

Hip replacement is extensively performed in hips with serious damages. The clinical outcomes of hip implants remain to be improved. Local delivery of bisphosphonates may improve implant fixation by positively affecting local bone modeling. In this study, two alendronate-containing coatings were prepared on porous tantalum by electrolytic deposition. Calcium phosphate coating was deposited and adsorbed with alendronate; the resulting coating had a low drug dose and slow release rate. Solid calcium alendronate coating was also deposited on tantalum; the resulting coating had high drug dose and faster release rate. The effects of the two coatings on new bone formation and implant fixation were studied in the rabbit tibial cortex. Four weeks after implantation, the implants with adsorbed alendronate showed the highest total new bone formation and mechanical fixation, whereas the implants with solid drug coating showed slightly lower fixation and total new bone formation than control bare implants. The improvement by the alendronate-adsorbed calcium phosphate coating provides potentials of enhancing early fixation of porous implants. The solid drug coating warranted further studies to exploit its high drug dose for inhibiting future osteolysis.

A review of the biocompatibility of implantable devices: current challenges to overcome foreign body response

In recent years, a variety of devices (drug-eluting stents, artificial organs, biosensors, catheters, scaffolds for tissue engineering, heart valves, etc.) have been developed for implantation into patients. However, when such devices are implanted into the body, the body can react to these in a number of different ways. These reactions can result in an unexpected risk for patients. Therefore, it is important to assess and optimize the biocompatibility of implantable devices. To date, numerous strategies have been investigated to overcome body reactions induced by the implantation of devices. This review focuses on the foreign body response and the approaches that have been taken to overcome this. The biological response following device implantation and the methods for biocompatibility evaluation are summarized. Then the risks of implantable devices and the challenges to overcome these problems are introduced. Specifically, the challenges used to overcome the functional loss of glucose sensors, restenosis after stent implantation, and calcification induced by implantable devices are discussed.

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.

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.

Electrolytic deposition of calcium etidronate drug coating on titanium substrate

Wear debris-induced osteolysis is the major cause of aseptic loosening and failure of hip implants. One of the promising therapeutic interventions to improve the longevity of hip implants is to administrate bisphosphonate drug to inhibit osteoclastic bone resorption. This study aimed at developing new techniques of directly combining bisphosphonate with implants to achieve local delivery and controlled release of the drug. Instead of using soluble sodium salt, we proposed to apply sparingly soluble calcium salt of bisphosphonate as a potential long-term antiosteolysis coating on hip implants. Calcium salt of etidronate, a member of the bisphosphonate family of potent osteoclast inhibitors, was used in this pilot study. By adopting the electrolytic deposition (ELD) technique, which was developed for ceramic coatings including calcium phosphates, we demonstrated that a thin layer of calcium bisphosphonate could be deposited onto titanium surface. The drug coating is amorphous as characterized with X-ray diffraction, and has globular morphology under the scanning electron microscope. Electrospray-ionization mass-spectrometry (ESI-MS) and Fourier-transformed infrared spectroscopy confirmed that the molecular structure of the etidronate (m/z 205, H3L-, the single dissociated form of parent etidronic acid, denoted as H4L) was preserved after the ELD process. In vitro release into a "physiological" buffer solution confirmed that the etidronate concentration was limited by its low solubility. The etidronate concentration was 8 x 10(-5) M at day 1 and kept relatively stable at approximately 6 x 10(-5) M from day 2 to day 8. The deposition mechanisms of the drug coating and its potential efficacy as an antiosteolytic release source were discussed.

Reduction of calcification by various treatments in cardiac valves

The importance of glutaraldehyde pretreated bioprosthetic heart valves fabricated from bovine pericardium or porcine aortic valves is well realized in the management of valvular heart diseases. But, calcification limits the durability and is the most frequent cause of failure of these bioprosthetic heart valves. Various research groups in the world are actively involved in describing, understanding, and preventing calcification of bioprosthetic heart valves. Since there is no satisfactory clinical means for preventing or treating this disorder, attempts are made to improve the anticalcification properties of the replacement valves in the preparation stage itself. Research in this area is very active, and many newer approaches are made to mitigate the problem. An attempt has been made in the present article to review various theories put forward to explain the causative factors involved and mechanistic aspects of biocalcification and to present various strategies attempted for the prevention of calcification with the special feature on the work done in the area in our laboratory.

Identification of specific calcium-binding noncollagenous proteins associated with glutaraldehyde-preserved bovine pericardium in the rat subdermal model

Calcification of glutaraldehyde-preserved bioprosthetic heart valves (BHVs) results in their clinical failure. The mechanism of this pathologic calcification is not well defined. Since serum proteins are known to be taken up in mineralized tissue, we hypothesized that serum proteins derived from several calcium-binding noncollagenous proteins (NCPs) of bone and teeth also may be associated with pathologically mineralized BHVs. Using a rat subdermal model of BHV calcification, glutaraldehyde-preserved bovine pericardium (GPBP) was implanted for 1, 3, 14, and 60 days, and then subjected to an extraction procedure designed to isolate only NCPs tightly bound to the mineral phase. Gel electrophoresis and Coomassie Brilliant Blue staining demonstrated that these proteins became associated with GPBP over time, paralleling reported calcium uptake by the tissue. Stains-All staining demonstrated a marked accumulation of highly acidic, phosphorylated NCPs associated with 60-day GPBP extracts. Some of these proteins were detected in rat serum but were absent from extracts of GPBP incubated in rat serum in vitro. Western blotting with antibodies to three NCPs found in bone and teeth-bone acidic glycoprotein 75 (BAG 75), osteopontin, and SPARC-demonstrated that these NCPs were tightly bound to the mineral phase of calcified GPBP. A fourth NCP, bone sialoprotein II (BSP II) was barely detectable. Thus each identified NCP showed a different pattern of GPBP association relative to mineral deposition, suggesting unique roles for each in pathologic calcification. SPARC increased within 3 days of GPBP implantation but decreased by 2 weeks. BAG 75 and osteopontin uptake was detected in the initial mineral deposits and increased mineralization proceeded. BSP II never increased significantly over the entire-period. Further studies, which should include immunohistochemistry, will be important for delineating the source, location, and function of these three NCPs and for identifying others that also may be involved in this pathological process. Most important, the new insights into the mechanism of pathologic calcification described here present exciting opportunities for novel approaches to BHV calcification prevention.

Heterogeneous nucleation of calcium phosphates on solid surfaces in aqueous solution

The heterogeneous nucleation of calcium phosphates on solid surfaces of poly(methyl methacrylate) (PMMA), poly-(tetrafluoroethylene-co-hexafluoropropylene) (FEP), silicone rubber, mica, and radiofrequency glow discharge (RFGD)-treated PMMA, FEP, and silicone rubber has been studied in solutions supersaturated with respect to hydroxyapatite. The surface properties of the substrates were characterized by contact angle measurements. For the RFGD-treated surfaces, the Lifshitz-Van der Waals surface tension component changes very little, but the Lewis acid-base surface tension parameters vary greatly depending upon the materials. With scanning electron microscopy, nucleation of calcium phosphates was observed only on the surfaces: mica, RFGD-treated PMMA and FEP, with relatively high values of the Lewis base surface tension parameter. The more hydrophobic surfaces having low Lewis acid-base surface tensions, untreated PMMA and FEP, silicone rubber, and even RFGD-treated silicone rubber showed no nucleation.

Inhibition of calculi fragment growth by metal-bisphosphonate complexes demonstrated with a new assay measuring the surface activity of urolithiasis inhibitors

To evaluate compounds for the long-term inhibition of urinary calculi growth, we applied a newly developed in vitro assay to various metal-ligand complexes. The new experimental model--the preadsorbed calculi growth assay--was based upon the initiation of crystal growth in a metastable solution of calcium and oxalate with uniform granules derived from human renal calculi. Potential inhibitors were first absorbed onto the surface of the calculi granules, following which the loss of calcium from the seeded metastable solution was monitored as the indicator of calcium oxalate deposition. This assay allowed participation in the reaction by any matrix components present in the human calculi granules and limited the effect of the inhibitors to the calculi surface. Some complexes of metal ions with bisphosphonates had strong inhibitory effects, as opposed to citrate complexes which had minimal effect in this assay. Tin was the most potent metal ion, and pamidronate was the most potent bisphosphonate; together they slowed the growth of calculi granules to 9% of control. The inhibition by Tin-bisphosphonate complexes persisted despite a week of continual rinsing with a solution of sodium chloride and calcium. If the metal-bisphosphonate complexes are active in vivo as well, they might be considered for prophylaxis of calcium oxalate calculi or the prevention of regrowth of residual fragments following lithotripsy.

Inhibition of calcification of glutaraldehyde pretreated porcine aortic valve cusps with sodium dodecyl sulfate: preincubation and controlled release studies

Calcification of bioprosthetic heart valves fabricated from glutaraldehyde pretreated bovine pericardium or porcine aortic valves (PAV) is a frequent cause of the failure of these devices. Of all strategies considered thus far, only detergent preincubations using compounds such as sodium dodecyl sulfate (SDS) inhibited PAV bioprosthetic mineralization in circulatory sheep bioprosthetic valve replacements. The present study sought to characterize the mechanism of action of SDS preinicubation. Results of transport and material characterization studies showed that SDS had a relatively high affinity for PAV, with a maximum uptake of 167.1 +/- 6.8 micrograms SDS/mg tissue over 24 h at 37 degrees C with a partition coefficient of 19.3. The PAV diffusion of SDS was 1.95 +/- 0.35 10(-6) cm2/sec. The principal effect of SDS on PAV was phospholipid extraction. The residual organic phosphate in the SDS pretreated tissue was 2.22 +/- 0.72 nmol/mg tissue compared to the control untreated group with 18.52 +/- 2.1 nmol/mg tissue. Incubations of PAV specimens in a 1% SDS solution for 24 h significantly inhibited calcification after 21 days in subdermal implants in 3-week-old male rats (PAV Ca2+ = 18.0 +/- 11.8 micrograms/mg) compared to control (177.8 +/- 6.0 micrograms/mg). In contrast, coimplants of 30% SDS silicone rubber polymers, for regional sustained SDS administration, did not impede PAV calcification in 21 day implants Ca2+ = 166.0 +/- 14.0 micrograms/mg compared to the nondrug silicone matrix controls, Ca2+ = 173.0 +/- 6.6 micrograms/mg). Thus, we conclude that the mechanisms of SDS inhibition of PAV calcification is due to material effects which occur during preincubation, and is not facilitated by sustained SDS administration.

Synergistic inhibition of the calcification of glutaraldehyde pretreated bovine pericardium in a rat subdermal model by FeCl3 and ethanehydroxydiphosphonate: preincubation and polymeric controlled release studies

Calcification is a frequent cause of the clinical failure of bioprosthetic heart valves fabricated from glutaraldehyde-pretreated porcine aortic valves or glutaraldehyde-pretreated bovine pericardium (GPBP). We investigated the hypothesis that ferric chloride (FeCl3) and sodium-ethanehydroxydiphosphonate (EHDP) may act synergistically to prevent bioprosthetic tissue calcification. Pre-incubations and controlled release systems were studied individually. FeCl3-EHDP polymeric controlled release matrices were formulated using silicone rubber and evaluated for in vitro release kinetics at pH 7.4 and 37 degrees C. The effects of Fe-EHDP synergism on GPBP calcification were investigated with 21 d subdermal implants in 3 wk-old male rats. Results demonstrated that levels of Fe3+ and EHDP uptake, measured in GPBP tissues pre-incubated first in an FeCl3 solution (10(-5) M) followed by an EHDP solution (0.1 M), were higher than in the reverse order of incubation. In the first series of rat implants, GPBP was pre-incubated in either FeCl3 or Na2EHDP solutions, or sequential pre-incubations of first FeCl3 and then Na2EHDP solutions, or the reverse. The inhibition of calcification was greatest when FeCl3 (first pre-incubation, 10(-5) M) was combined with Na2EHDP (second pre-incubation, 0.1 M) (1.78 +/- 0.2 micrograms of Ca2+/mg of dried tissue) compared with the other pre-incubation groups: EHDP (first pre-incubation) combined with FeCl3 (second pre-incubation) (21.7 +/- 6.4), FeCl3 solution alone at 10(-5) M (27.9 +/- 10.7), Na2EHDP solution alone at 0.1 M (52.3 +/- 11.9) and the control group (72.3 +/- 10.2). In a second series of implants, GPBP specimens were co-implanted with individual controlled release systems containing one of the following formulations (weight percentage in silicone rubber): 1% FeCl3, 20% CaEHDP, 20% protamine sulphate, 1% FeCl3-20% CaEHDP, and 1% FeCl3-20% protamine sulphate. The 1% FeCl3-20% CaEHDP silicone-rubber matrices were the most effective for inhibiting GPBP mineralization (13.7 +/- 3.0 micrograms Ca2+/mg of dried tissue) compared with non-drug silicone co-implant controls (74.7 +/- 5.58 micrograms Ca2+/mg of dried tissue) and other polymeric treatment groups (32.3 +/- 2.3-80.0 +/- 19.7). No adverse effects on bone or overall growth of any treatment protocols were noted. Thus, combinations of FeCl3 and EHDP, using either pre-incubations or polymeric controlled release, were synergistic for inhibiting GPBP calcification.

Effects of metallic ions and diphosphonates on inhibition of pericardial bioprosthetic tissue calcification and associated alkaline phosphatase activity

This study focused on the association of extrinsic alkaline phosphatase (AP) activity with both early and advanced calcification of glutaraldehyde-pretreated bovine pericardial bioprosthetic (GPBP) tissue, and the inhibition of both calcification and AP activity by pre-incubation in diphosphonates (sodium-ethanehydroxydiphosphonate [NaEHDP], aminopropanehydroxydiphosphonate [APD]) and metallic salts (FeCl3, Ga(NO3)3, AICI3). GPBP specimens were implanted subcutaneously in 3 wk old male rats after pre-incubation. Following explantation of the tissue at 72 h and 21 d, calcification was assessed morphologically by light microscopy and chemically by atomic adsorption spectroscopy for calcium content and by molybdate complexation for phosphorus. AP activity was characterized by enzymatic hydrolysis of paranitrophenyl phosphate and by histochemical studies. In both control and pretreated groups, AP levels were greater in 72 h explants than 21 d retrievals, which demonstrated extensive calcification in control explants. All pre-incubations that resulted in inhibition of calcification after 21 d, except for APD, were associated with 72 h AP content which was lower than control specimens. The typical time of initiation of calcification was 72 h, as determined by previous studies with this model system. Covalently bound APD inhibited calcification. Increased AP activity in the APD group may be due to the toxicity of this agent with resultant acute inflammation, or other incompletely understood effects of diphosphonates on calcification and AP. Furthermore, EHDP and Ga3+ incubations were also associated with decreased GPBP AP at 72 h compared to control, but were not effective for inhibiting calcification after 21 d. We concluded that inhibition of peak GPBP AP activity is not necessarily associated with the prevention of GPBP mineralization.