Borohydride reduction of biological tissues

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FIELD OF INVENTION

The invention is in the field of processes for preparing biological tissues of extracellular matrices for implantation as bioprosthetic implants. The invention relates to a process for using sodium or potassium borohydride to modify reactive chemical groups in biological tissues or protein matrices that have been cross-linked or treated by aldehydes or other chemical reagents.

BACKGROUND

Treatment of biological tissues with glutaraldehyde is a preferred method for manufacturing bioprosthetic implants. Glutaraldehyde forms cross-links and polymers with a variety of matrix components of biological tissues, rendering these tissues less immunogenic when implanted in a living host. Glutaraldehyde cross-linking also alters the mechanical properties of biological tissues.

Glutaraldehyde cross-linked porcine heart valves and bovine pericardial tissues have been used as material for bioprosthetic human heart valves. Glutaraldehyde treated blood vessels and pericardial patches have also been used in other applications.

Glutaraldehyde treated biological tissues, especially porcine heart valves and bovine pericardial tissues, have been used as materials for bioprosthetic heart valves for more than 20 years. It has become evident that calcification of the glutaraldehyde treated bioprosthetic heart valves has been a major cause of long term device failure.

The exact mechanism underlying the calcification of glutaraldehyde treated heart valves is not known. It has been suggested that the toxicity of the glutaraldehyde polymers or the breakdown products of the glutaraldehyde polymers cause the death of connective tissue cells arriving at the implant site. The remnants of these dead cells may then act as nidi for the nitiation of calcificaiton. The toxicity of the glutaraldehyde polymers which leach out to tissue surrounding a glutaraldehyde cross-linked bio-implant may, through a similar mechanism, further complicate the integration of the implant into the host tissue. The free aldehyde groups of the glutaraldehyde polymers may contribute to the toxicity of the polymers.

A variety of approaches have been taken to counteract calcification of glutaraldehyde treated implants. For example, aldehyde treated tissues have been reacted with polyols (U.S. Pat. No. 5,476,516, issued to Seitter et al., 19 Dec. 1995), treated with alipathic carboxylic acids (U.S. Pat. No. 4,976,733 issued to Girardot 11 Dec, 1990) treated with partially degraded heparin (U.S. Pat. No. 5,645,587 issued to Chanda et al. 8 Jul. 1997), treated with sodium dodecyl sulfate (U.S. Pat. No. 4,323,358 issued to Lentz et al. 6 Apr. 1982), treated with polysaccharides, disphsphonates or phosphoproteins (U.S. Pat. No. 5,104,405 to Nimni 14 Apr. 1992), impregnated with ferric or stannic salts (U.S. Pat. No. 5,002,566 to Carpentier et al. 26 Mar. 1991), and treated with trivalent aluminum cations (U.S. Pat. No. 5,094,661 to Levy et al. 10 Mar. 1992).

Sodium borohydride in aqueous solution is a known reducing agent. Sodium borohydride in solution has previously been used as a reducing agent to treat glutaraldehyde fixed tissues. U.S. Pat. No. 4, 553,974 issued to Dewanjee on 19 Nov. 1985 discloses a process for treatment of glutaraldehyde fixed tissue with a calcification inhibiting agent, and then reducing the tissue with sodium borohydride. The calcification inhibiting agent contains reactive amino groups that bond to free reactive groups in the fixed tissue. Dewanjee '974 discloses a variety of amino diphosphonate compounds as effective calcification inhibiting agents (as do U.S. Pat. Nos. 5,296,583 and 5,436,291 issued to Levy et al. on 22 Mar. 1994 and 25 Jul. 1995 respectively). Dewanjee '974 teaches that a solution of sodium borohydride may be used to stabilize the bonding of such amino diphosphonates and glutaraldehyde to protein molecules.

Chen et al. ("Effect of 2-amino oleic acid exposure conditions on the inhibition of calcification of glutaraldehyde cross-linked porcine aortic valves" Journal of Biomedical Materials Research, Vol. 28, 1485-1495 (1994)) teach that the use of a sodium borohydride solution alone to treat glutaraldehyde fixed aortic tissues is not effective to reduce calcification in the rat subdermal implant model. In fact, when used on aortic wall tissue, the sodium borohydride treatment resulted in increased calcification compared to controls with no treatment.

SUMMARY OF THE INVENTION

The invention provides a method of preparing biological tissue that includes the following steps:

(1) providing a biological tissue having reactive groups that will react with an aldehyde. The biological tissue may be fibrous material derived from animal origins, such as mammalian pericardium.

(2) Treating the biological tissue with an aldehyde, such as glutaraldehyde or other polymeric forms of aldehydes, or polymers containing aldehydes and/or ketones. Biological tissues containing collagen and other proteins may be cross-linked by such treatment. Following such treatment, the tissue typically includes substituents such as aldehyde groups and Schiff's bases.

(3) Dehydrate the biological tissue. The tissue may be dehydrated by treating it with a polar organic solvent, such as ethanol, increasingly anhydrous solutions of the polar organic solvent may be used for this purpose.

(4) Treating the tissue with sodium or potassium borohydride in a polar organic solvent at a pH of less than about 8 to reduce substituents on the tissue such as free aldehyde groups and Schiff bases.

Additional steps may be useful:

Following treatment of the tissue with the borohydride, the borohydride may be removed from the tissue. Removal of the borohydride may be accomplished by extraction into an appropriate organic solvent (such as ethanol, other alcohols, acetonitrile or pyridine). Removal of the hydride from the tissue may be used to stop the reduction reaction.

To prepare tissue treated in accordance with the invention for use, the tissue may be rehydrated. For example, the tissue may be treated with increasingly dilute concentrations of a polar organic solvent in aqueous solution.

The concentration of reactants, pH, temperature of reaction and duration of reaction are all parameters of the process of the invention that may be varied by those skilled in this art in order to vary the results achieved with the process of the invention. Generally, the reduction should be carried out at temperatures below those which can cause macromolecules in the tissue to loose their tertiary and quaternary structures. The reduction may preferably be carried out for a sufficient period of time, and under appropriate conditions, to reduce substantially all of the aldehydes and Schiff's bases in the tissue. A quantity of borohydride may be used that is approximately stoichiometrically equivalent to the theoretical amount of aldehydes and Schiff'bases in the tissue. The borohydride may be provided in an amount that exceeds the saturation concentration of the borohydride in the polar organic solvent. Providing such an excess of borohydride may help to ensure that there is a continual supply of the sodium borohydride as the reduction reaction progresses.

In a preferred embodiment, dehydration may be performed in a stepwise procedure by immersing the biological tissues in a series of solutions with increasing concentrations of alcohol (or other polar organic solvents) in water, or in a continuously concentrating solvent solution, such as a gradient solution system. As the final dehydration step, the tissues may be immersed in 100% alcohol. The alcohol may be aspirated away from the tissue and a solution of sodium borohydride in 100% alcohol added to the dehydrated biological material. Reduction of the tissues with borohydride may be allowed to proceed at 4.degree. C. for approximately 24 hours (overnight), fresh sodium borohydride in ethanol solution may then be added and the reaction continued at 4.degree. C. for a further period of approximately 24 hours (overnight). The borohydride solution may then be aspirated away from the tissues. The excess borohydride in the tissue may be extracted by washing the tissues in 100% alcohol. The borohydride reduced material may then be rehydrated by immersing the material in a series of alcohol solutions with decreasing concentrations of alcohol in water.

The process of the present invention reduces the aldehyde groups in an aldehyde treated biological tissue to improve the biocompatability of the tissue. Although the process uses a strong reducing agent, sodium or potassium borohydride, the structural integrity of the tissues is preserved, as evidenced by the results disclosed herein. Accordingly, use of the borohydride in accordance with the process of the invention does not appear to adversely affect structural components of the tissues. However, if borohydride is used to reduce tissues in methods which differ from the process of the present invention, adverse effects on the structural components of the tissues, such as collagen, elastin or proteoglycan, may occur.

The process of the invention has the additional advantage of stabilizing the potentially reversible Schiff's base cross-links between the aldehyde and the treated tissues, for example between glutaraldehyde polymers and protein components of the tissue. It has been shown that glutaraldehyde cross-linked biological tissues release toxic components to cells in vitro even after extensive and prolonged washing (up to 6 months). This is evidence that the attachment of glutaraldehyde or its polymers to tissue fibers is reversible. The reversibility of the reaction is counteracted by reducing the Schiff's bases. This reduction, converting the C.uparw.N double bond to a C-N single bond also increases the rotational freedom of the glutaraldehyde polymer which is attached to the tissue fibre. This aspect of the reduction of the Schiff's bases may underlie another unanticipated advantage of the invention: the glutaraldehyde cross-linked biological tissue after reduction has a compliance characteristic closer to native tissue than non-reduced tissue.

Biological tissues reduced in accordance with the invention may be used as implanted materials, such as heart valve prostheses, patches or conduits. In addition, such tissues may be used in other biological processes, including instrumentation and biological test support materials.

DETAILED DESCRIPTION

Glutaraldehyde treated bovine pericardia were prepared by immersing freshly harvested pericardial tissue in glutaraldehyde solution in phosphate buffered saline for several days. The glutaraldehyde tissues were dehydrated by immersing the tissues in 25% ethanol in water (tissue: volume=5 g wet weight: 100 ml) for 15 minutes the 25% ethanol was aspirated and a 50% ethanol solution was added. After 15 minutes the 50% ethanol was aspirated and a 75% ethanol was added. After another 15 minutes the 75% ethanol was aspirated and 100% ethanol was added. In the same manner the 100% ethanol was aspirated and replaced by new 100% ethanol two additional times.

Those skilled in the art of this invention will understand that there are a variety of ways in which a tissue may be dehydrated in accordance with this invention. The dehydration step should preferably remove as much water as possible, since the presence of water will interfere with the aldehyde reduction reaction. The tissue may be dehydrated by treating it with polar organic solvents other than ethanol, such as other alcohols like isopropanol (because of their physical properties, methanol and alcohols with more than four carbons would generally not be practical as the major constituents of the dehydrating agent), acetonitrile, pyridine or mixtures of such solvents. Such solvents may be selected in accordance with the present invention by virtue of their ability to extract water from the treated tissue, while not reacting adversely with the tissue. Increasingly anhydrous solutions of the polar organic solvent may be used for this purpose. In each case, it will be appreciated that the efficacy of the dehydrating agent may be tested according to its ability to dry the tissue while not interfering with the other aspects of the process of the invention.

Dehydration of glutaraldehyde treated tissues prior to reduction with borohydride is an important aspect of the invention. Sodium or potassium borohydride may reduce aldehydes, Schiff's bases and similar functional groups in aqueous solution and neutral pH. However, under these conditions, the borohydrides are very labile and are themselves readily hydrolyzed by water. Another difficulty is that such a reduction reaction generates hydrogen gas as a by-product which can be trapped in the tissue matrices. As the trapped gas accumulates as bubbles in the matrices, the bubbles may disrupt the tissue matrices as well as create physical voids. They may also create physical barriers for fluid diffusion, preventing reduction agents from entering the tissue. In addition, bubbles may make it difficult for reduction by-products to diffuse out of the tissue, thus causing a rise of the pH in the micro environment inside the tissue. When the pH of a borohydride solution rises above 8, cleavage of peptide bonds may occur. The cleavage of peptide bonds would significantly weaken the tissue's fibrous structure. It is therefore important to control the pH environment of the reduction reaction and prevent the formation of trapped hydrogen gas bubbles.

To initiate the reduction reaction, the 100% ethanol is aspirated and sodium or potassium borohydride reduction reagent added. In a preferred embodiment, the reduction reagent may be prepared by adding 0.2 grams of sodium borohydride to 100 ml of 100% ethanol. The reduction reaction may be allowed to continue at 4.degree. C. for approximately 24 hours (overnight). Fresh reduction reagent may then be added and the reaction continued at 4.degree. C. for a further period of approximately 24 hours (overnight). It will be appreciated that the duration and conditions of the reduction reaction are preferably sufficient to ensure that substantially all of the free aldehydes in the tissue are reduced.

To control pH in accordance with the present invention during the reduction reaction, the pH of the reducing reaction solution may be monitored. If the pH rises to 8 or above, the sodium borohydride solution may be removed and replaced with a fresh solution of sodium borohydride (preferably sodium borohydride in ethanol).

At the end of the reduction reaction, the reduction reagent may be aspirated and 100% ethanol used to wash the tissues for 15 min. The washing procedure may be repeated two more times. The washed tissues were re-hydrated by removing the 100% ethanol by aspiration and adding a 75% ethanol solution. Fifteen minutes later, the 75% ethanol was aspirated and a 50% ethanol solution was added. The reduced tissues were then stored in 50% ethanol.

For use, the tissue treated in accordance with the method of the invention may be sterilized. The tissue may also be treated with heparin, for example by dipping in a heparin bath. In one embodiment, the tissue is sterilized and then treated with heparin.

In accordance with known techniques for using glutaraldehyde fixed tissues, tissues treated in accordance with the invention may be fashioned into a variety of bioprosthetic implants, such as heart valves and pericardial patches.

For evaluation of the reduced tissues by in vitro and in vivo tests, as disclosed below, tissue samples were immersed and rinsed in phosphate buffered saline at 4.degree. C. for 15 min. The reduced tissues appear white and very flexible, unexpectedly resembling fresh non-glutaraldehyde treated tissues.

The effectiveness of the reduction reaction in reducing free aldehydes was determined by immersing a piece of the reduced tissue in Fuchsin-sulfite reagent (Schiff's reagent) for 15 min. Non-reduced tissues turn pink and purple quickly. Reduced tissue samples remain white and turned slightly pink after one hour, indicating that the concentration of free aldehydes in the tissue had been substantially reduced.

The integrity of collagen fibers in the reduced tissues was tested by several methods. The first method evaluated the quarter scattered banding of collagen fibers using transmission electron microscopy. Reduced tissues were dehydrated and embedded in epoxy resin directly prior to sectioning and staining. The resulting electron micrographs show that the collagen fibers in the reduced tissues were intact since the banding pattern appeared normal.

The second method was to compare the shrink temperature of the reduced samples with that of the native fresh tissues as well as tissues cross-linked with glutaraldehyde but not reduced. Tissues were sandwiched between two glass slides (using the glass slides as support) and immersed in physiological 0.9% saline. The saline was gradually heated at approximately 1.degree. C. per minute. The length of the tissue strips were measured at frequent intervals. The exact temperature at which the tissue's shrinkage rate was at least 1 mm/min was recorded as the shrink temperature. From this inflection point on a tissue length vs. temperature graph the shrinkage rate increases dramatically. The results show that the shrink temperature of the reduced tissues was 79.4.degree..+-.0.3.degree. C. compared to 83.7.degree..+-.0.2.degree. C. for the non-reduced cross-linked tissues and 62.3.degree..+-.0.5.degree. C. for the fresh native tissues. These results provide evidence that the collagen fibers as well as the bulk of the glutaraldehyde cross-links remain intact following reduction of the tissue.

The mechanical properties of the reduced tissues were examined by two different methods. In the first method, dumb-bell shaped tissues were stamped from pericardial tissues treated by glutaraldehyde and post-glutaraldehyde reduction. The tissues were pulled on an Instron Instrument at a load speed of 10 mm/minute. The results in Table 1 show that there is no significant difference between the stress/strain ratios of the two groups. There is also no difference in the maximum load strength of the two groups. In the second method, tissues were pulled to 2.5 kg and held at that distance for 15 min. while the reduction in load was measured. The result in Table 2 shows that the initial slope of the relaxation curves of the two sample groups were not significantly different. These results indicate that the post-glutaraldehyde reduction process did not alter or weaken the mechanical properties of the tissues.

The biocompatibility of the glutaraldehyde treated bovine pericardium after reduction by borohydride was examined in tissue culture studies. Reduced and non-reduced tissues were cut into round disks which fit snugly in the wells of a multi-well tissue culture plate. After the tissues w