Synthetic Protein as Implantables

The field of this invention is the production and use of bioresorbable polypeptide polymers.

The rate at which an implanted material resorbs or biodegrades within the body can be a major factor in determining its utility as a biomaterial. So called inert materials, such as metals, ceramics and plastics have been shown to be useful for permanent implants. However, in applications in which a device serves as an aid to healing or as a temporary aid in surgical repair, a resorbable material has the advantage of not having to be removed, once healing has occurred. Resorbable sutures and staples, bone pins and screws, wound dressings, and injectable drug delivery systems or depots are examples of such devices. There are very few materials available today which have the physical, chemical and biological properties necessary for the fabrication of medical devices, which must degrade and resorb in the body without detrimental consequences.

Various synthetic organic polymers have found use, such as polylactides, polyglycolides, polyanhydrides and polyorthoesters, which degrade in the body by hydrolysis. Collagen, glycosaminoglycans and hyaluronic acid are examples of natural implantable materials which resorb at least partially by enzymatic degradation. The rates of resorption are limited to the nature of the particular material and modifications can change the rate of resorption, but at the same time may adversely affect the desired properties of the product.

Illustrative of efforts to vary resorption characteristics by compositional changes are synthetic resorbable sutures composed of copolymers of lactide and glycolide. By varying the ratio of lactic acid to glycolic acid, the rate of resorption may be varied. Unfortunately, rapidly resorbing compositions tend to be soft and weak. Slow resorbing compositions are stiff and strong. However, their resorption, which is hydrolytic, produces acid buffered by the tissue medium, where erosion occurs at the polymer surface. In addition, however, hydrolysis may occur internally, where the resulting acid catalyzes and accelerates the degradation of the polymer. Thus, internal pockets of degradation can lead to rapid and catastrophic failure of mechanical properties.

There is, therefore, a need for products which can be used in the production of implantable devices. Such products should have the desired mechanical properties of tensile strength, elasticity, formability, and the like, provide for controlled resorption, and be physiologically acceptable.

Relevant Literature

U.S. Pat. No. 5,243,038 describes the preparation of high molecular weight, protein polymers and copolymers comprising long segments of small repeating units. Bioactive Polymeric Systems, Gebelein, C. G. and Carraher, C. E., eds., Plenum Press, New York, 1985; Contemporary Biomaterials, Boretos, John W. and Eden, Murray, eds., Noyes Publications, New Jersey, 1984; and Concise Guide to Biomedical Polymers: Their Design, Fabrication and Molding, Boretos, John W., Thomas pub., Illinois, 1973, describe compositions, characteristics, and applications of biomaterials.

Protein copolymers are provided having segments varying in the number of repetitive units, based on fibroin and elastin. The protein copolymers and silk homopolymers find use in the production of a wide variety of implantable devices and components thereof.

Implantable devices and components thereof are provided comprised of recombinant novel copolymers having alternating segments of repetitive units based on fibroin (silk) in combination with elastin or recombinant polymers of fibroin. Particularly, the units for the most part are GAGAGS and VPGVG, respectively, although some variations are permitted, such as the particular order of the amino acids in the sequence and conservative substitutions, such as, but not limited to, replacing serine with threonine and glycine with alanine.

In the copolymers, by varying the ratio of the two different units, the length of the segments comprising each of the units, the molecular weight, any intervening sequences, modifications to the individual repeating units, and the like, one can vary the tensile properties of the product only moderately, such as elasticity, stiffness, hardness, ease of processing, and flexibility, while one can substantially vary the rate of resorption. Faster resorption can be achieved by reducing the number of repeating units of silk in the silk segment below about 8 units or increasing the number of elastin units per elastin segment to greater than 8, individually or in combination.

For the copolymers, the ratio of the average number of elastin units to the average number of silk units per segment of the repetitive units will be in the range of about 0.5, usually about 1-5. For the most part, there will be at least two fibroin units per segment and not more than about 12, usually not more than about ten, preferably ranging from about 2-8. For the elastin units, there will usually be at least two, more usually at least about four, generally ranging from about 6-32, more usually from about 6-18, preferably from about 6-16. The percent of amino acids contributed by the silk units will generally range from about 15-65%, more usually from about 15-60%, preferably about 20-55%.

The copolymers which find use in the invention will generally range from about 15-80% of amino acids provided by fibroin units,

The polymers will be at least about 15 kDa and generally not more than about 150 kDa, usually not more than about 125 kDa, preferably ranging from about 35-100 kDa. In order to achieve the copolymers, the number of segments will provide for the desired molecular weight. Therefore, the number of segments can vary widely, depending upon the size of each individual segment. Thus, the number of segments may vary from about 2-40, more usually ranging from about 6-20.

Based on the method of preparation, there may be non-repetitive units at the N- and C- termini. Usually, the terminal sequences will contribute fewer than ten number percent of the amino acids, more usually fewer than five number percent of the amino acids. Generally, the sequence will range from about 0-125 amino acids, more usually from about 0-60 amino acids, where the total number of amino acids will generally not exceed about 100 amino acids, more usually not exceed about 50 amino acids.

For special applications the polymers may be modified by introducing intervening sequences between segments or blocks of segments, where the total number of repeating units per block may vary from about 4 to 40, thus involving two or more segments. The intervening sequences may include from about 1 to 60, usually about 3 to 40 amino acids, and may provide for a wide variety of properties. For example, by including amino acids which have chemically reactive sidechains, one may provide for sites for linking a variety of chemically or physiologically active compounds, for cross-linking, for covalently bonding compound which may change the rate of resorption, tensile properties or the like. Thus amino acids, such as cysteine, aspartic acid, glutamic acid, lysine and arginine may be incorporated in these intervening sequences. Alternatively, the sequence may provide for sequences which have physiological activity, such as cell binding, specific protein binding, enzyme substrates, specific receptor binding, and the like. In this manner, the useful properties of the basic protein may be greatly varied in accordance with the intended use, being tailored for specific applications.

The polymers have good mechanical properties to form a wide variety of products. The protein polymers may be drawn, molded, cast, spun, extruded, or the like, in accordance with known ways for forming structures such as films, formed objects, fibers, or unformed structures, such as amorphous masses, and the like. Also, gels may be formed which may be shaped in a variety of ways, depending upon the particular application. The compositions can be sterilized by conventional ways to provide sterile products.

The subject compositions can be used to provide a wide variety of devices, such as membranes, sutures, staples, bone pins, screws, wound dressings, and as drug depots, where the products may be formed prior to implantation or in situ. The compositions as formed are found to provide the necessary mechanical properties for the particular applications, the resorption times can be controlled so as to ensure mechanical maintenance during the time required for structure integrity, and at the same time ensuring that the device or material need not be manually removed, since the material undergoes resorption.

The subject compositions may be used in combination with other materials, such as collagen, fibrinogen, and other natural proteins; hyaluronic acid, dextran, or other polysaccharides; or polyethylene oxide, polyhydroxyalkanoates, or other polyesters, to produce blended materials to provide a larger range of physical and biological properties, for applications, such as wound dressings or membranes for the prevention of surgical adhesions. For example, the protein polymer SELP3 combined with sodium hyaluronate, in equal proportions by weight, may be used to prepare a film, which compared to pure hyaluronate gels, exhibits greater mechanical toughness and a decreased resorption rate.

The compositions may be prepared in accordance with the manner described in U.S. Pat. No. 5,243,038. This procedure involves synthesizing small segments of single stranded DNA of from about 15-150 nucleotides to provide a plurality of fragments which have cohesive ends, which may be ligated together to form a segment or a plurality of segments. The first dsDNA fragment is cloned to ensure the appropriate sequence, followed by the addition of successive fragments, which are in turn cloned and characterized, to ensure that the integrity of the sequence is retained. The fragments are joined together to form a "monomer" which then becomes the major repeating building block of the polymer gene.

Alternatively, long single strands may be prepared, cloned and characterized, generally being of at least 100 nucleotides and up to about 300 nucleotides, where the two single strands are hybridized, cloned and characterized and may then serve as the monomer or the building block. The monomers may then be multimerized, having complementary termini, particularly cohesive ends, so that the polymers will have two or more monomers present. The multimers may then be cloned in an appropriate vector and characterized to determine the number of monomers and the desired size polymer selected. Expression can be achieved in an expression host using transcriptional regulatory regions functional in the expression host. The expression host can be prokaryotic or eukaryotic, particularly bacterial, e.g. E. coli, B. subtills, etc.; yeast, e.g. Saccharomyces, Neurospora, etc.; insect cells, plant cells, and the like. If desired, a signal sequence may be provided for secretion of the polymer. A wide variety of signal sequences are known and have been used extensively for secreting proteins which are not normally secreted by the expression host.

After completion of expression, where the protein is retained in the host, the cells are disrupted and the product extracted from the lysate. Where the product is secreted, the product may be isolated from the supernatant. In either case, various techniques for purifying the products may be employed, depending upon whether the products are soluble or insoluble in the medium. Where insoluble, impurities may be extracted from the polymer, leaving the polymer intact. Where soluble, the polymer may be purified in accordance with conventional ways, such as extraction, chromatography, or the like.

The following examples are offered by way of illustration and not by limitation.

Preparation of polymers.

E. coli strain EC3 containing the respective plasmid encoding each polymer shown in Table 1 below, was prepared in accordance with the methods described in U.S. Pat. No. 5,243,038. Each strain was then fermented using a fed-batch method.

Biomass for each polymer was harvested from the fermentation broth by centrifugation in a Sorval RC3B using a H6000A rotor at 5,000 rpm for 30 minutes at 10.degree. C. to yield a packed cell paste. 500 grams of cell paste was resuspended in 2 liters of 50 mM Tris buffer (pH=8.0). The cell slurry was homogenized using a Manton Gaulin cell disrupter at 7-8,000 psi with three complete passes of the liquid. The cell homogenate was passed through a chilled heat exchanger to maintain the temperature at 15.degree. C. or less. Pancreatic DNAse was added to the homogenate to a final concentration of 1 .mu.g/ml and stirred at room temperature for 2 hours. The homogenate was centrifuged in a Sorval RC3B centrifuge using a H6000A rotor at 5,000 rpm for 1 hour at 10.degree. C.

For SELPO, 3, 7, and 8, the supernatant was placed into 12-14,000 molecular weight cut-off dialysis bags and dialyzed against 2 changes of 100.times.volume of 20 mM sodium acetate buffer (pH=4.7) for 24 hours. The contents of the bags were transferred to centrifuge bottles and centrifuged in a Sorval RC3B centrifuge using a H6000A rotor at 5,000 rpm for 1 hour at 10.degree. C. The supernatant was removed to a large beaker and the pH adjusted to 8.0 by addition of 30% ammonium hydroxide. Saturated ammonium sulfate was then added to reach a final concentration of 20% for SELPO, 25% for SELP8 and 3, and 33% for SELP7. The solution was stirred at room temperature for 1 hour. The solution was centrifuged in a Sorval RC3B using a H6000A rotor at 5,000 rpm for 30 minutes at 10.degree. C. The pellet was resuspended in 2 liters of deionized water, placed in dialysis bags, and dialyzed against 3 changes of deionized water of 100.times.volume over 48 hours. The contents of the bags were shell frozen and lyophilized to dryness.

For SELP4 and 5, the centrifuged homogenate supernatant was directly precipitated with ammonium sulfate at a concentration of 25%. The solution was then centrifuged in a Sorval RC3B using a H6000A rotor at 5,000 rpm for 1 hour at 10.degree. C. The pellet was resuspended in 5 liters of 4M LiBr and stirred at 4.degree. C. for 16 hours. The solution was centrifuged in a Sorval RC3B centrifuge using a H6000A rotor at 5,000 rpm at 10.degree. C. for 1 hour. The pH of the supernatant was adjusted to pH 3.7 by slow addition of 1M acetic acid at 4.degree. C. The solution was centrifuged in a Sorval RC3B using a H6000A rotor at 5,000 rpm at 10.degree. C. for 1 hour. The supernatant pH was adjusted to 8.0 by addition of ammonium hydroxide and then dialyzed against 3 changes of 100.times.volume deionized water over 48 hours. The solution was removed from dialysis and centrifuged in a Sorval RC3B using a H6000A rotor at 5,000 rpm at 10.degree. C. for 1 hour. Saturated ammonium sulfate was added to the supernatant to reach 25% of saturation and stirred for 1 hour. The solution was centrifuged in a Sorval RC3B using a H6000A rotor at 5,000 rpm at 10.degree. C. for 1 hour. The pellet was dissolved in 4.5M LiBr, placed in dialysis bags, and dialyzed against 3 changes of 100.times.volume of deionized water. The contents of the bags were shell frozen and lyophilized to dryness.

All reagent solutions used in the following procedures were depyrogenated prior to use by filtration through a 10,000 nominal molecular weight cut-off hollow fiber cartridge (AG Technologies). All glassware and utensils used were sterilized and depyrogenated by heating at 180.degree. C. for 4 hours. 4-5 grams of all SELP dried polymers were dissolved in 1.2 liters of 10M urea. 20 mls of 2M Tris pH 8.0 and 780 mls of milli-Q water were added. The solution was sonicated to promote full dissolution of the protein. 500 grams of Whatman DE52 ion exchange resin was prepared by precycling through acid and base treatment as recommended by manufacturer prior to and in between each usage. The resin was finally equilibrated with 6M urea, 20 mM Tris pH 8.0 in a beaker with gentle stirring. The resin was filtered in a buchner funnel until excessive liquid was removed. The cake of resin was placed in a beaker and the protein solution was added. The slurry was stirred gently for 1 hour. The slurry was filtered in a buchner funnel and the liquid was collected in a cleaned vacuum flask. 500 grams of fresh precycled and equilibrated resin was added to a clean beaker and the filtered solution was added. The slurry was stirred gently for 1 hour and filtered again. The filtered solution was once more combined with 500 grams of freshly precycled and equilibrated resin, stirred for 1 hour, and filtered. The final filtered solution was placed in 6,000 molecular weight cut-off dialysis bags which had been soaked in 0.5N NaOH for at least 24 hours. The solution was dialyzed against 3 changes of 100.times.volume of deionized water. The dialyzed solution was removed from the bags, placed in depyrogenated lyophilization flasks and lyophilized to dryness. Employing the above procedure, the following polymers were prepared.

Other polymers which were prepared include [(VPGVG).sub.32 (GAGAGS).sub.8 ] (SEQ. ID NO:09), referred to as SELP6

SELP films.

SELP films that were approximately 0.05 mm thickness were produced by solvent evaporation.

Approximately 1.7 grams of each polymer, except for SELP7 where only 1.05 grams was used, were solubilized in 34 mls of 88% formic acid. The solution was stirred for 7 hours at room temperature to insure complete solubilization. The solution was then poured into a film casting apparatus consisting essentially of a rectangular polyethylene trough with a removable polyethylene bottom. The casting apparatus was placed in a vacuum oven attached to a nitrogen gas source for sparging the atmosphere. The films were dried in the sealed oven drawing a 10-15 micron vacuum with a slow continual influx of nitrogen gas at 60.degree.-75.degree. C. After 15-18 hours of drying, the apparatus was disassembled and the film was peeled off the polyethylene bottom. The films were exposed for 5 minutes to a basic atmosphere (5% open solution of ammonium hydroxide in a sealed desiccator) to neutralize any residual formic acid.

A polyethylene sheet of the same area dimensions as the protein film was roughened by hand using fine grit sand paper and a fine film of cyanoacrylate glue was spread over its surface. The protein film was applied to the wet surface. A teflon sheet was placed on top and bottom of the polyethylene and protein layers and stainless steel plates were placed around those. The entire assembly was pressed in a Carver laboratory press at a force of 0.8 metric tons for 18 hours at room temperature. The polyethylene/protein film laminated sheet was placed on a cutting board and 1.3 cm diameter discs were punched out using a stainless steel punch and rubber mallet. The discs were placed individually in stoppered glass vials.

Specimens were produced from each of the polymers as well as denatured collagen protein (DCP) produced identically as described for the SELP films. Bovine collagen (fibrillar form, lot number 921101 ) was obtained from Colla-Tec, Inc. (Plainsboro, N.J.). It was completely solubilized in 88% formic acid producing a clear but viscous solution. All specimens were sterilized by electron beam irradiation at 2.5.+-.0.2 Mrads. Each disk was implanted subcutaneously in the back of rats such that the protein film was in direct contact with the muscle tissue. The specimens remained in the animals for different periods of time: one, four and seven weeks post implantation. At each time interval six specimens per polymer group were retrieved for protein analysis. Additional specimens from each group were evaluated for tissue reaction by histology.

Non-implanted and retrieved specimens were analyzed to determine the mass of SELP film contained per specimen. Amino acid analysis was performed on each specimen by sealing them individually in an hydrolysis vial with constant boiling hydrochloric acid and heating for 24 hr at 100.degree.-110.degree. C. After hydrolysis, the specimen was extracted and an aliquot of the extract was derivatized with PTC. The derivatized amino acids were separated by reverse phase HPLC and quantified by their absorbance at 254 nm according to the methods of Henrickson and Meredith (Anal Biochem. 137, 65-74, 1984).

The mass of the SELP film present on each specimen was determined. The amino acid contribution of the SELP protein was estimated based on the total content of the amino acids G,A,S,V and P which for the pure polymers is>95%. Other amino acids potentially contributed by extraneous protein deposited onto the specimens during residence in the body were excluded from these analyses. Average SELP film mass for non-implanted specimens was determined from the same batch of specimens used for implantation. Average SELP film mass for retrieved specimens was similarly calculated except that replicates having values greater than two standard deviations from the mean were discarded. Deviations in many cases were due to partial retrieval of specimens that had fragmented in the tissue after implantation and may not reflect true resorption. Resorption Analysis and Results

Resorption analysis was conducted statistically by analyzing four specimen population treatment groups. These were: (1) non-implanted; (2) one week post-implantation; (3) four weeks post-implantation; and (4) seven weeks post-implantation.

The results from Table 2 are the values for the mass of protein film contained on specimens after implantation. Each value is the mean of at least five specimen masses as determined by amino acid composition. Table 3 displays the same values as a percent of the initial weight prior to implantation as determined by the mean mass of six specimens of the non-implanted specimens. The results indicate that upon implantation, SELP0 and DCP are substantially resorbed by one week, falling below 5% of their non-implanted masses. SELP7 is substantially resorbed by four weeks with only 5.8% remaining. SELP8 and SELP3 are resorbing by seven weeks with mean values of 18.1% and 58.2% remaining, respectively. SELP4 and SELP5 films show no evidence of resorption at seven weeks.

From the above results one may conclude the following. Faster resorption correlates with compositions containing domains of silk-like blocks fewer than eight. The polymers containing eight silk-like blocks have substantially reduced rates of resorption. However, the total content of silk-like blocks in the copolymer composition does not correlate with resorption rate. While very similar compositionally, SELP7 and SELP8 resorbed quickly, while SELP4 and SELP5 do not resorb in seven weeks. The lack of resorption of SELP4 and SELP5 films at seven weeks post-implantation corresponds with repeating domains containing greater than eight elastin-like blocks. Although their silk-like block lengths are identical at eight, SELP4 and 5 with elastin-like block lengths of 12 and 16 resorb to a lesser degree than SELP3, which has an elastin-like block length of 8.

The subject polymers, regardless of their composition, form free-standing films with strength enough to allow easy handling. SELP7 and SELP4 films have tensile strengths of 19.+-.1 and 21.+-.8 MPa, respectively. The compositional difference between them that causes SELP7 to resorb in four weeks and SELP4 to remain intact beyond seven weeks makes little apparent difference in their tensile properties. These strengths are adequate for their use in surgical and wound healing applications.

The observed resorption of these polymers occurs via surface erosion. This is consistent with the mechanism of degradation of SELP proteins within the body. At physiological conditions, proteins will degrade only through the action of proteases. Because endogenous proteases are high molecular weight compounds of approximately 20 kDa or greater, their diffusion into the dense SELP films will be limited. The degradation of SELP films is, therefore, progressive from the external surfaces of the material. The subject materials therefore should undergo a slow loss of mechanical integrity while being reduced in mass.

It is evident from the above results, that the subject compositions have particularly desirable properties for uses in implants. By varying compositional ratios, the rate of resorption can be varied greatly, without significant changes in tensile properties. The compositions can be formed in a wide variety of devices or objects, to find extensive use for a variety of purposes and context as implants.

All publications and patent applications cited in this specification are herein incorporated by reference as if each individual publication or patent application were specifically and individually indicated to be incorporated by reference.

Although the foregoing invention has been described in some detail by way of illustration and example for purposes of clarity of understanding, it will be readily apparent to those of ordinary skill in the art in light of the teachings of this invention that certain changes and modifications may be made thereto without departing from the spirit or scope of the appended claims.