Re: Felix,K., Lin,S., Bornkamm,G.W. and Janz,S. (1998) Tetravinyl-tetramethylcyclo-tetrasiloxane (tetravinyl D4) is a mutagen in Rat2{lambda}lacI fibroblasts. Carcinogenesis, 19, 315–320

Kenneth Kulig

Department of Surgery, Division of Emergency Medicine and Trauma, University of Colorado Health Sciences Center, Denver, CO 80210-5817, USA

Dear Sir,

I read with interest the research findings of Dr Felix and coworkers (1) regarding the possible mutagenicity of tetravinyl D4. Because of the possible concerns this article might raise among women who have silicone breast implants, several issues deserve comment.

There is no evidence that tetravinyl D4 is found in silicone gel

The authors cite three articles by a Nuclear Magnetic Resonance group in support of the proposition that vinylated low molecular weight compounds such as tetravinyl D4 are found within the gel matrix. However, these articles contain no evidence for the presence of tetravinyl D4 or other such compounds within silicone gel (2,3), and they do not conclude that they are present.

The vinyl groups involved in the crosslinking of the linear polymers as Felix et al. accurately point out, are located randomly along the polymer chains. To create tetravinyl D4, there must be four vinyl groups located sequentially along four siloxanes, all of which have either remained uncrosslinked or have spontaneously depolymerized. The authors have claimed that tetravinyl D4 is the most abundant vinylated derivative of D4, but offer no supportive data for this highly unlikely proposition.

Felix et al. used a concentration of tetravinyl D4 in their experiments of 50 µM, clearly a dose without biologic plausibility, with a resultant very modest increase of mutant frequency of only 1.7. The 50 µM of tetravinyl D4 used in these experiments would translate into a theoretical quantitation of 4.69 mg in a typical 300 g silicone gel breast implant. There is no evidence that there is any amount of tetravinyl D4 in silicone gel, let alone a quantitation of this magnitude. In addition, because no dose–response specific for mutagenicity was determined, one cannot conclude that tetravinyl D4 is likely to be a mutagen at any relevant concentrations.

D4, the most important and relevant of the six materials tested, was clearly not mutagenic

The authors cite one article as evidence for certain siloxanes such as D4 being abundant in gel (4), while another article cited (5) has quantitated the D4 concentrations in two gels of 300 and 700 p.p.m. The importance of this is that D4 has been quantitated in silicone gel, while the other five siloxanes tested have not. Of those five, decamethyltetrasiloxane resulted in a statistically significant lower rate of mutagenesis than controls. The four other silicone compounds (other than tetravinyl D4) were neither mutagenic nor protective in this model.

The title of the article emphasised the only compound out of the six tested, tetravinyl D4 (which has never been demonstrated to exist in silicone gel), which appeared to result in a very modest increase in mutation frequency, instead of emphasising the lack of mutagenicity of the other five, including the most important and biologically relevant, D4. The massive literature concerning both animal and human exposure to all forms of silicones, where no tumorigenic effect has been seen, is consistent with these findings.

The use of {gamma}-cyclodextrin to create cellular interactions with hydrophobic silicone compounds that would not otherwise occur is an artificial model that is unlikely to be biologically relevant

The authors acknowledge that because the siloxanes tested were so hydrophobic, adding them to cell culture media would not be appropriate because they would not interact with the cells in vitro. They therefore, in essence, `forced' whatever interaction they could be complexing the siloxanes with {gamma}-cyclodextrin. The biologic relevance of this is unclear, particularly if trying to determine the possibility of an intracellular phenomenon such as mutagenesis. There was no control group where only {gamma}-cyclodextrin was used as a test material in this model. Instead, it was assumed that, because {gamma}-cyclodextrin alone had no mutagenic or other significant biologic effect in other models, the same would be true in this model.

The mouse plasmacytoma model, which was the impetus for this research, has little relevance to the use of silicone gel as a biomedical device in humans

While plasmacytomas could be induced in this model using a variety of materials which persist in situ when injected i.p. in BALB/c and CD2-Idh 1 mice, they could not be induced by silicone gel in CDF1, DBA/2N, C57BL/6N or C3H/HeJ strains (5). The extramedullary plasmacytomas seen in the murine model were clearly different from those which might be found in humans (5). Mutagenesis in the rat{lambda}lacI fibroblast model employed by Felix et al., even if it occurred, does not mean that it would occur in mouse plasma cells, nor in any human tissue. There are currently no data to support the theory that silicone gel breast implants might be associated with human plasmacytomas, MGUS or multiple myeloma (7,8), whose true cause remains unknown (9,10).

In conclusion, the study by Felix et al. should be considered as a negative study for the possible mutagenicity of materials known or likely to be found in silicone gel. It should not be interpreted to mean that silicone gel itself, or any known component of it, has been shown to be mutagenic.

References

  1. Felix,K., Lins,S., Bornkamm,G.W. and Janz,S. (1998) Tetravinyl-tetramethylcyclo-tetrasiloxane (tetravinyl D4) is a mutagen in Rat2{lambda}lacI fibroblasts. Carcinogenesis, 19, 315–320.[Abstract]
  2. MacDonald,P., Plavac,N., Peters,W., Lugowski,S. and Smith,D. (1995) Failure of 29 Si NMR to detect increased blood silicone levels in silicone gel breast implant recipients. Anal. Chem., 67, 3799–3801.[ISI][Medline]
  3. Garrido,L., Pfleiderer,B., Jenkins,B.G., Hulka,C.A. and Kopans,D.B. (1998) Erratum: Migration and chemical modification of silicone in women with breast prostheses. Mag. Reson. Med., 40, 689.[ISI]
  4. Batich,C., DePalma,D., Marotta,J. and Latorre,G. (1996) Silicone degradation reactions. Curr. Top. Microbiol. Immunol., 194, 83–91.
  5. Potter,M., Morrison,S., Wiener,F., Zhang,X.K. and Miller,F. (1994) Induction of plasmacytomas with silicone gel in genetically susceptible strains of mice. J. Natl Cancer Inst., 86, 1058–1065.[Abstract]
  6. Gabriel,S.E., O'Fallon,W.M., Kurland,L.T., Beard,C.M., Woods,J.E. and Melton,L.J. (1994) Risk of connective tissue diseases and other disorders after breast implantation. N. Engl. J. Med., 330, 1697–1702.[Abstract/Free Full Text]
  7. McLaughhlin,J.K., Nyren,O., Blot,W.J., Yin,L., Josefsson,S., Fraumeni,J.F. and Adami,H.O. (1998) Cancer risk among women with cosmetic breast implants: a population based cohort study in Sweden. J. Natl Cancer Inst., 90, 156–158.[Free Full Text]
  8. Deapen,D. and Brody,G. (1995) Re: Induction of plasmacytomas with silicone gel in genetically susceptible strains of mice. J. Natl Cancer Inst., 87, 315.[ISI][Medline]
  9. Bataille,R. and Harousseau,J.L. (1997) Multiple myeloma. N. Engl. J. Med., 23, 1657–1664.
  10. Riedel,D.A. and Pottern,L.M. (1992) The epidemiology of multiple myeloma. Hematol./Oncol. Clin. N. Am., 6, 225–247.[ISI][Medline]

 

Response

Klaus Felix

Dear Sir,

The starting point of our studies on the potential mutagenicity of siloxanes was the observation by Potter and associates (1) that small fragments of silicone gels injected i.p. into genetically susceptible BALB/c mice can induce plasmacytomas (i.e. malignant lymphomas of terminally differentiated B lymphocytes). The mechanism of silicone gel-induced plasmacytoma development is unclear, but the extensive literature on the carcinogenicity of vinyl compounds suggested to us that vinyl-substituted siloxanes may be the culprits by acting as mutagenic/plasmacytomagenic compounds. Hence, we decided to examine the possibility that low molecular weight, vinyl-substituted silicone compounds (e.g. vinylated siloxanes), once released from the complex silicone gel matrix and penetrating the surrounding tissue, may be mutagenic in rodent target cells. We selected for our study various siloxanes that were substituted with vinyl moieties or not substituted with vinyl moieties (used as controls) as test compounds because they were known to be used in the manufacture of silicone gels. We evaluated the mutagenicity of these compounds in short-term in vitro assays with Rat2{lambda}lacI fibroblasts, speculating that the putative mutagenicity of vinylated siloxanes may be a critical determinant of the plasmacytoma-inducing potency of silicone gels.

Dr Kulig's statement that there is no clear evidence for the presence of tetravinyl D4 (TVD4) in silicone gels is probably correct, as we were not able to detect this compound in our own limited GC/MS analyses of silicone gels. We have, however, found a close relative of TVD4 in commercially available gels, namely heptamethylvinyl-cyclotetrasiloxane. Whatever type of contaminant present, we believe there is no guarantee that the silicone polymerization processes that build the envelope or the matrix of the gel will ever be complete and thus not leave behind residual amounts of reactants, catalysts and moderators. Upon delayed release, it is entirely conceivable that the siloxanes may reach millimolar concentrations in micro-environments that comprise the critical contact zones between gel compounds and cells. In retrospect, we were probably incorrect to suggest that Garrido's laboratory (see refs 15–17 in the Discussion section of our paper) found TVD4 in the silicone gels, but we also think that Dr Kulig is not correctly interpreting our statement in the Discussion section when he writes that we made it appear that TVD4 is the most abundant vinylated derivative of D4. Instead, we stated that TVD4 is probably the most abundant cyclic vinylsiloxane in the synthesis of the gel, which does not necessarily mean that it is the most abundant compound in the manufactured and completely cured gel.

We agree with Dr Kulig's comment that D4 was not mutagenic in Rat2 fibroblasts. However, we are not ready yet to generalize this finding, as other target cells (e.g. B lymphocytes) that may have different sensitivities have not been evaluated. On another note, the bioavailability of low molecular weight, lipophilic, volatile or UV light-sensitive compounds in cell cultures is very limited. It is widely accepted that the biosolubilization processes that occur in vivo to make even extremely hydrophobic compounds available for interaction with cells and tissues are very difficult, if not impossible, to reproduce in a Petri dish in vitro. Dr Kulig fails to appreciate this well-established fact when he objects to the complexation of the lipophilic siloxanes by cyclodextrins in our study. The cyclodextrin complexation method is a sophisticated one that permits the effective delivery of problem compounds in vitro. Indeed, cyclodextrins have gained widespread use in the food, pharmaceutical and chemical industry. A major benefit of cyclodextrins is the lack of adverse effects often seen with organic solvents and the almost total absence of toxicity, as demonstrated in our study by the finding that millimolar amounts of {gamma}-cyclodextrin were nontoxic and nonmutagenic in Rat2{lambda}lacI cells.

Last, but not least, we hasten to point out that our study was strictly addressed to the mouse plasmacytoma model. No further conclusions with respect to malignancies in humans should be taken into consideration. We thank Dr Kulig for his interest and critical review of our publication.

References

  1. Potter,M., Morrison,S., Wiener,F., Zhang,X.K. and Miller,F.W. (1994) Induction of plasmacytomas with silicone gel in genetically susceptible strains of mice. J. Natl Cancer Inst., 86, 1058–1065.[Abstract]




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