Process for Augmenting Soft Tissue With Cross-Linked Polyvinyl Pyrrolidone

This invention relates to a method for augmenting soft tissue in mammals, including humns, with crosslinked polyvinyl pyrrolidone.

Soft tissue augmentation for cosmetic purposes and to repair congenital or trauma-caused defects is a growing technique in the medical profession. Solid or liquid silicone implants have been used for some years, and more recently, a collagen preparation ("Zyderm") that is specifically described in Daniels et al., U.S. Pat. No. 3,949,073, has been used in cosmetic facial repair to remove wrinkles, etc. The silicone implants have never been entirely satisfactory because they tend to harden or calcify, and the collagen preparation elicits an inflammatory immune response in a small but significant number of patients. The collagen treatment is usually not permanent, so it must often be repeated periodically, e.g., every 12 to 24 months. An inflammatory immune response is sometimes elicited with collagen upon retreatment, even if it was absent in earlier treatments. [Ref., Webster et al., Arch. Otolaryngol., 110, 656(1984), Cohen et al., Plast. Reconst. Surg., 73(5), 857(1984), and Aran et al., Plast. Reconst. Surg., 71(5), 731(1983).]

The present invention is based on the discovery that crosslinked polyvinyl pyrrolidone ("PVP") can be used to augment soft tissue in mammals, including humans, to treat defects therein, with a remarkable lack of tissue reaction. The treatment is long-lasting, if not permanent, so the need to re-treat as one must do in most cases with collagen, is much reduced if not eliminated entirely.

The process of the invention comprises administering to a soft tissue site in a mammal, including humans, in need of augmentation, an aqueous gel of crosslinked PVP.

Cross-linked vinyl pyrrolidone polymers are known. See for instance, Folkman et al., U.S. Pat. No. 4,391,797 (col. 1, line 68) and Cordrey et al., U.S. Pat. No. 3,943,045, who disclose the use of crosslinked vinyl pyrrolidone copolymers as contact lenses, prosthetic devices, surgical implants, protective corneal fittings, and protective membranes or bandages.

Seiderman, in U.S. Pat. No. 3,966,847, discloses contact lenses made of crosslinked vinyl pyrrolidone copolymers.

Boghosian, in U.S. Pat. No. 3,150,045, discloses the use of uncrosslinked PVP in aqueous solution as a material to stimulate growth of epithelial cells in the eye.

Amkraut et al., in U.S. Pat. No. 4,439,199, disclose the use of uncrosslinked PVP as a delivery system for immunopotentiator medicaments.

The use of collagen or cartilage based materials to augment soft tissue is disclosed in U.S. Pat. Nos. 3,949,073 (Daniels et al.), 4,424,208 (Wallace et al.), and 4,469,676 (Hecmati).

Polyvinyl pyrrolidone is commercially available as a high melting water soluble polymeric powder in viscosity ranges which correspond to number average molecular weights from 10,000 to 700,000. It is sold under a variety of trade names: Povidone, PVP, Plasdone, and Kollidon are examples. These powders are hygroscopic and generally contain about 5% water. There are purified grades available which are used in the pharmaceutical industry and as blood plasma volume extenders. (Encyclopedia of Polymer Science and Technology, Vol. 14, "N-Vinyl Amide Polymers", H. F. Mark, N. G. Gaylord, N. M. Bikales, Eds., J. Wiley and Sons, New York, (1964), pp. 239-251). Solutions of PVP in an aqueous carrier can be subjected to ionizing radiation, preferably gamma rays, and crosslinked into insoluble swollen gels. The concentration range suitable for producing homogeneous soft gels suitable for use as a soft tissue filler implant is 5-40%, and preferably 10-20%, by weight of the solution. Concentrations of less than 5% PVP tend to produce heterogeneous systems as the amount of water in the system is too great to be included in the gel structure. The type of ionizing radiation used to produce crosslinking is preferably gamma rays in the range of 2-15 Mrad dose, preferably 2.4-3.4 Mrad.

The PVP is dissolved in the aqueous carrier and then the solution is loaded into a syringe equipped with a 26 gauge or smaller needle and subjected to irradiation. After the irradiation treatment the gel is sterile and ready for use, for example, for injection under a wrinkle to plump up the skin.

Optionally, the PVP solution can be irradiated in a glass ampoule, flask, or vial and then transferred to a syringe fitted with a 26 gauge or smaller needle, and sterilized, for example, by subjecting the gel to heat treatment of 70.degree.-75.degree. C. for 6 hours before use. This heat treatment has no noticeable effect on the consistency of the PVP hydrogel. This treatment is presented as an example only and is not meant to limit the scope of the invention.

As the concentration of PVP is increased from 5 to 40% by weight in the aqueous carrier, the gels become softer and less friable. An explanation of this phenomenon, which is not meant to limit the scope of the invention, is that a particular dose of ionizing radiation will produce a given number of crosslinks. As the concentration of polymer chains increases, the number of crosslinks per chain decreases and a looser network forms. Therefore, it is possible to alter the consistency of the hydrogels by manipulating concentrations of polymer or other additives which change the crosslink density or by changing the total dose of irradiation. The consistency of several hydrogels is given in Table I, below.

In order to determine the permanence and consistency of PVP hydrogel in vivo, a relatively large quantity (0.25 cc) was injected into the rat dorsal subcutis. Adult female Woodlyn Wistar albino rats were used. All rats were held in the animal colony for at least one week prior to being employed in the study. They were maintained according to the guidelines set down by the Canadian Association for Laboratory Animal Sciences and had food and water ad libitum. All implantation procedures were carried out with the test animals under general anesthesia (Metofane). The abdominal skin of the rat was shaved, washed with GAMOPHEN* leaves and painted with IOPREP*.

Injections of the sterile PVP hydrogels (dry heat sterilized at 70.degree.-75.degree. C. for 6 hours) were made in the abdominal subcutis from pre-loaded syringes equipped with hypodermic needles. All injections were 0.25 cc in size. Injections of commercial ZYDERM* collagen based soft tissue filler were made in the exact same manner to serve as controls. The observation procedures involved first the evaluation of the gross tissue response and then the estimation of absorption having occurred. In order to accomplish this the test animals were placed under light general anesthesia (Metofane). Transillumination of the abdominal skin was employed to evaluate the tissue response. Progressive measurement of the sizes of the implants were taken and recorded in order to determine absorption patterns. At the end of this study the implants were examined histologically. The results of this study are described below in Tables II and III.

In another study of long term in vivo performance and tissue response, small volume injections (<0.1 cc) of a 13% PVP gel and ZYDERM* were made in the dorsal skin of rats. The PVP was injected to the left of the spinal column and the ZYDERM to the right of the spinal column. An attempt was made to inject the samples intradermally, although subsequent histological analysis showed sample present in the dermis, the hypodermis, and the subcutis (Table IV, below).

The following examples are provided to illustrate embodiments of the invention.

Samples of polyvinyl pyrrolidone (MW=44,000) from two lots were dissolved in distilled water to produce solutions of concentrations ranging from 4.9%, 10%, 13%, and 20%, based on total solution weight. The solutions were stored in glass ampoules and irradiated to 2.5 Mrad in a Co.sup.60 gamma irradiator. Transparent gels were formed and their appearance and consistency as well as viscosity characterization of the starting polymer are described in Table I.

The gels of Example 1 of concentration 4.9% and 13% were loaded into 10 cc hypodermic needles and heat sterilized at 70.degree.-75.degree. C. for six hours. These gels were then implanted into the subcutis of rats and their permanence and tissue response studied as described in Tables II and III. The 13% gels were also injected into the dermis, and subcutis of rates and studied as described in Table IV.

One half gram samples of the 4.9% and 13% gels of Example I were stored in 25 ml of phosphate buffer solution (pH=7.4) at 50.degree. C. for two weeks. The gels appeared unchanged except for minor swelling at the end of this time period.

Samples of polyvinyl pyrrolidone (MW=44,000) from one of the lots described in Example 1 was dissolved in distilled water to produce solutions of concentrations 1% and 2%. The solutions were loaded into glass vials and irradiated to 2.5 Mrad dose in a Co.sup.60 irradiator. The gels formed were opaque and had excluded water which could be decanted from the gel. The water was removed from the 2% gel in a forced air oven and the actual solids content measured to be 2.7%.

A sample of pharmaceutical grade PVP, Plasdone K 29-32, was dissolved in distilled water to produce a 15% w/w solution. This was loaded into a glass vial and irradiated in a Co.sup.60 irradiator to a 2.5 Mrad dose. This produced a transparent gel similar in consistency to the 13% gel produced in Example 1.

A sample of 700,000 M.W. PVP was dissolved in distilled water to produce a 15% w/w solution. This was irradiated as in Example 5. A transparent gel was produced which was stiffer and more friable than the gels of Examples 1 and 5.

A sample of 44,000 M.W. PVP was dissolved in distilled water to produce a 40% w/w solution. This sample was loaded into a glass ampoule and irradiated to 5 Mrad in a Co.sup.60 gamma irradiator. A transparent gel was formed which was firmer than the samples prepared in Example 1.

A sample of 360,000 M.W. PVP was dissolved in distilled water to produce a 15% w/w solution. This sample was loaded into a glass ampoule and irradiated to a 2.5 Mrad in a Co.sup.60 gamma irradiator. A transparent gel was formed which was stiffer and more friable than the gels produced in Examples 1 and 5.

A sample of 700,000 M.W. PVP was dissolved in distilled water to produce a 40% w/w solution. This was loaded into a glass ampoule and irradiated to 2.5 Mrad in a Co.sup.60 gamma irradiator. A transparent gel formed.

A sample of 10,000 M.W. PVP from Polysciences Inc. was dissolved in distilled water to produce a 5% w/w solution. The solution was loaded into a glass ampoule and irradiated to 15 Mrad in a Co.sup.60 gamma irradiator. A soft transparent gel formed.