DOW CORNING SILICONE GEL RAT STUDY

DOW CORNING SILICONE GEL RAT STUDY demonstrates that "under specific experimental conditions, silicone gel can function as an immune adjuvant," according to a March 17 statement by the company. An adjuvant is a substance which enhances the immune response to a foreign material, such as a protein. Completed March 9, the study involved 69 rats, of which 20 received injections of a silicone gel/antigen mixture into muscle tissue. According to Dow Corning, the gel injected into 10 of these rats was derived from the type used in Dow Corning's breast implants; the gel injected into the other 10 rats was derived from a McGhan breast implant material. In addition to the 20 rats injected with gel, 20 were injected with a silicone fluid/antigen mixture, 10 were injected with "Freund's complete adjuvant" as a "positive control," and 10 were injected with a saline solution as a "negative control." A group of nine rats that did not receive any injections served as a "naive control." Two, four, six, and eight weeks after the injections, blood samples were taken from the rats and measured for antibody reaction to the antigen. "The adjuvancy of silicone gel was demonstrated by an increased production of antibodies in laboratory rats when [the] antigen called BSA (bovine serum albumin) was mixed with the gel," Dow Corning states. With both the Dow Corning gel and McGhan gel, the rats' antibody production was greater than that in the positive control group, the company says. No adjuvancy was associated with the silicone fluid, which generally is 80% of the filler in silicone gel-filled breast implants, according to Dow Corning. The remaining 20% is comprised of silicone gel. Dow Corning conducted its study after John Naim, MD, Rochester University Hospital, Rochester, NY, et al. completed a study that identified silicone gel as an immune adjuvant. Results of the Naim study are slated for publication in the journal Immunologic Investigation the week of March 22, according to Dow Corning. The company says its study was patterned after Naim's study. "The heightened antibody production caused by silicone gel acting as an adjuvant" in the rat study "does not mean that silicone causes immune disease," Dow Corning says in its statement. In addition, the study also does "not determine whether silicone gel could function as an adjuvant in women with silicone breast implants," the company maintains. However, a Dow Corning topsider says that the study "raises concern" that silicone gel could be "associated with" or "cause" autoimmune disease. Dow Corning is conducting additional studies designed to determine if the rat data has relevance for women with breast implants. For example, the firm is conducting additional research on rats using gel that "more closely resembles the physical form of gel actually used in implants," according to the March 17 statement. The company notes that "the physical form of the gel used" in the rat study completed March 9 "differs markedly from its physical form in a breast implant." Dow researchers say the gel in the completed study was refined into smaller particles in order to mix it with the antigen. A study slated to begin in the next several months will evaluate "whether silicone gel could cause an increased production of antibodies in the absence of added antigen." The firm points out that "silicone gel implants do not have an antigen mixed in with the material" like the gel in the rat study did. The planned study protocol will call for rats to be injected with a silicone gel only. Dow Corning estimates this study will be completed in roughly five months. The firm sent the rat study results to FDA via facsimile on March 10 and met in person with agency staffers on March 15 to discuss the study in greater detail. Myron Harrison, Dow's chief medical officer, says the company "will continue to brief" the agency "about ongoing research as it is completed." The company also is in the process of delivering the test results to physician groups such as the American Society of Plastic and Reconstructive Surgeons and the American Society for Aesthetic Plastic Surgery. Harrison says the company will continue research "so that physicians and the agency can provide informed advice to patients." The series of animal studies under way at Dow Corning is part of an "integrated research plan" discussed by the company at an FDA panel meeting in February 1992 ("The Gray Sheet" Feb. 24, 1992, p. 7). The plan is comprised of roughly 20 ongoing or planned studies, including a 30-month retrospective study of breast implant recipients and a two-year study of silicone gel carcinogenicity in rats. Results from a silicone carcinogenicity rat study completed in 1988 found that roughly one in four rats injected with the material developed tumors. At the time the data was released, Dow Corning said that the "solid state tumorigenesis" likely is unique to rats and that the results "showed no significant risk to humans" ("The Gray Sheet" Nov. 14, 1988, p. 19).