Efficacy of thermoplastic elastomer and latex condoms as viral barriers

Efficacy of thermoplastic elastomer and latex condoms as viral barriers

Contraception 47:559-567, EFFICACY 1993 OF THERMOPLASTIC ELASTOMER CONDOMS AS VIRAL BARRIERS AND LATEX James Kettering, Ph.D. Department of Mic...

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James Kettering, Ph.D. Department of Microbiology School of Medicine Loma Linda University 92354 Loma Linda, California

ABSTRACT The barrier efficacy of a thermoplastic elastomer (TPE) and three brands of latex condoms was compared in a passiveleak test and in a dynamic model of simulated intercourse. Fifteen replicates of each of the condoms were challenged with bacteriophage T7 (100 nm) and the polio virus Type 1 (PV-1, 27 In the passive test, no condom leaked either virus. nm). In the dynamic model, no TPE condoms leaked either virus and no latex condoms leaked T7. Two samples of one commercially These data available latex condom leaked T7 but not PV-1. support that intact latex condoms are effective in vivo viral barriers and extend the finding to TPE condoms as well. Given its substantial equivalence to latex as a viral barrier, TPE condoms are an alternative choice for individuals with latex allergies. The TPE material is also more resistant to common that affect latex adversely and environmental conditions should therefore be a superior choice if condoms must be stored for extended periods in suboptimal conditions.

Submitted for publication Ilarch 19, 1993 Accepted for publication April 20, 1993

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Despite the efficacy of intact latex condoms as a barrier to viral infections, certain hindrances may limit their successful use. First, accelerators (e.g., thiurams, mercaptobenzothiazoles, carbamates) and antioxidants (e.g., phenylenediamines) added during the manufacture of latex condoms can cause cell-mediated allergic reactions or irritant reactions in some individuals1 and thereby discourage use. serious antibody-mediated More reactions, including anaphylaxis, can be induced by exposure to the proteins found in natural rubber latex.' Such hypersensitive individuals, a subset of the population, lack an effective growing alternative condom since natural membrane condoms are not protection considered adequate against the human immunodeficiency virus (HIV).3*4 Second, normal oxidative processes cause latex condoms to For example, a breakage rate of 49% was found deteriorate. for latex condoms with a field age of 40 months.5 Clearly, suboptimal storage conditions or extended storage can increase the probability that latex condoms will break during use. Conscientious users can therefore be placed at risk for sexually transmitted diseases or pregnancy. Latex condoms based also deteriorate rapidly if used with mineral-oil lubricants6--a demonstrated source of confusion for some users.' Recently, however, a new thermoplastic elastomer (TPE) condom, Tactylon*', was introduced. This synthetic, nonvinyl TPE is based on triblock copolymers, such as polystyrene-b(ethylene-co-butylene)-b-polystyrene(S-EB-S) and contains none of the allergens or chemical additives associated with latex It is also impervious to the common environmental condoms.1 factors that weaken latex.' A preliminary clinical trial showed that this S-EB-S TPE and latex condoms had similar breakage rates (1.2% and 1.3%, respectively).' Given the substantial equivalence of Tactylon's physical properties to latex and its potential advantages in other respects, this study was undertaken to compare the efficacy of the viral barrier of latex and TPE condoms. MATERIALS


TactylonTM condoms (Tactyl Technology, Vista, Condoms. CA) were compared with three brands of natural rubber latex condoms (Shiek@, Schmid, Sarasota, FL; Trojan-Enz@, Carter All Products, NY; and Lifestyles@, Ansell, Dotham, AL). condoms had reservoir tips and were nonlubricated. Fifteen condoms of each brand were tested against the bacteriophage, T7, and 14 condoms of each brand were tested against the poliovirus. The T7 was grown in liquid culture from stock Viruses. suspensions maintained in the Microbiology Department of Loma Linda University. T7, a virulent bacteriophage of Escherichiacoli

Contraception B, is about 100 nm in diameter, comparable to HIV. Lysates of the virus were grown to lOlo viruses/ml and filtered through a 0.45 pm-pore membrane. The poliovirus type 1 (PV-l), a clinical isolate identified by the neutralization procedure of the World Health PV-1 was Organization,1° was also used to test the condoms. grown in human fibroblasts and used at a concentration of 106The human fibroblasts were cultured in 10' viruses/ml. Dulbecco's minimal essential medium (MEM), containing 5% fetal calf serum and antibiotics, at 360 C in a CO, incubator. Tris-buffered saline fortestingwith Passive Viral Challenge. T7 or Dulbecco's MEM (50 ml) for testing with PV-1 was placed Ten ml of in a disposable, capped sterile specimen container. the test virus was placed in a condom, which was sealed, immersed in the specimen container, and maintained at room temperature or 370 C for 10 days. Daily samples (100 ~1) were removed and assayed for virus. At least 10 replicates of each sample were tested. artificial A simulated, model of Active Viral Challenge. intercourse was used in the active challenge (Fig. 1). A sterile, 50-ml, disposable centrifuge tube (30 x 115 mm, Falcon 2098) was covered with a condom containing 4 ml of virus. This system was placed into an outer condom, containing 2 ml of virus-free medium (Fig. 1). The apparatus was introduced into a commercially produced latex vagina. A standard procedure of 30 strokes (within 30 set) was used. from the inner and outer were then withdrawn Samples reservoirs by sterile syringe and assayed for the presence of virus. At least 14 replicates of each sample were tested against T7 and PV-1. Controls. Positive and negative controls were used with both the active and passive challenges. Positive controls consisted of three condoms of each brand punctured six times with a 27-gauge needle and served to insure the free passage and viability of the virus. Negative controls consisted of condoms containing only sterile medium. T7 was assayed by diluting the virus stock in Assays. sterile saline in a ten-fold dilution scheme. The standard assay procedure used was the agar layer technique described by Adams.ll Plaques were counted, and viruses per ml were calculated by the following formula: viruses/ml

= number of plaques dilution x volume

PV-1 was assayed in a standard manner, using the plaquing technique described by Landry andHsiung.l' Plaques were counted and viruses per ml calculated by the above formula.



Sterile 50 ml disposable centrifuge tube

Inner Condom (4 ml virus)

Outer Condom (2 ml medium virus free)

Figure 1. Condom tester. Viral-inoculated medium was placed inside the inner condom, which covered a disposable centrifuge To simulate intercourse, a condom-covered tube. tube was withdrawn and inserted 30 times into a commercially produced latex vagina. See materials and methods for details.


RESULTS No virus was detected in any of the condoms tested After the active challenge, two latex condoms passively. (13%) of one brand (Trojan-Enz@) leaked T7 (Table I). None of the condoms leaked PV-1. The Lifestyles@ condom, however, had a toxic effect on the human fibroblasts and could not be All positive controls in both active and passive evaluated. No virus was detected in any negative challenges leaked. control. TABLE I CONDOM LEAKAGE RESULTS OF ACTIVE VIRAL CHALLENGE ============================================================ T7 Sample PV-1 No. Leaked N N No. Leaked ============================================================ Shiek@ 15 14 0 Trojan Enz@ 15 z (13.3%) 14 0 Lifestyles@ 15 0 14 _a Tactylon" 15 0 14 0 ============================================================ 'Unable to evaluate because of toxicity to human fibroblast cells DISCUSSION Models of infectivity suggest that the widespread use of condoms would significantly decrease the spread of AIDS."' Consequently, numerous studies have evaluated the efficacy of the protection conferred by the condom barrier. Intact natural rubber latex condoms appear to prevent the passage of virus,",15 cytomegalovirus,16 human simplex herpes virus (HIV),'4*17t1sand the hepatitis B immunodeficiency virus.3,4'19'20 In contrast, natural membrane condoms (made from sheep caecum) permit small viruses to leak and are not considered adequate protection from HIV.3J4*1gJ20The current supports that resist further latex condoms experiment penetration by both small and large viruses but extends the findings to TPE condoms as well. The results of the passive condom test cannot easily be generalized to in vivo condom use because such designs disregard many pertinent variables. The results, however, can be interpreted in terms of porosity alone.' Consequently, the failure of the condoms to leak the T7 bacteriophage implies that the effective maximum pore size in all of the TPE condoms and in all but two of the latex condoms was too small to At 100 nm, T7 is a reasonable permit the passage of HIV. surrogate for HIV, the size of which has been reported to



range from 80 to 120 nm.14 The failure of condoms to leak PV1, a typical enterovirus, 28 nm in diameter, likewise suggests that TPE condoms would be an equally effective in vitro barrier against hepatitis B, which is about 42 nm. That the TPE condoms also did not leak during the active challenge further strengthens the conclusion that they are substantially equivalent to latex condoms as a viral barrier. Nonetheless, the relevance of such simple plunger systems tests to actual coitus has been questioned.'l In this case, the model incorporated some, but not all, of the conditions that could affect condom permeability in vivo. For example, the model was dynamic to approximate the physical stressing of The overall duration of the condoms during intercourse. reciprocation cycles was 30 set, a brief but not unreasonable As in other duration of thrusting before ejaculation." studies, however, the condom was loaded with virus before condition analogous to postejaculation stroking stroking 16,21--a and therefore somewhat unrealistic. Stretch and friction, although unquantified, were also present. Unfortunately, so many facets of human sexual physiology (e.g., thrust distance and rates, duration, vaginal or rectal pressures, viscosity of seminal fluids), anatomy, (e.g., penis length and girth), and practices (e.g., range and incidence) are unknown or known imprecisely 'lizhat even the most sophisticated models of coitus necessarily left important variables undefined or have approximated.23 Nonetheless, when data from condom testing are considered in combination with epidemiological data on seroconversion from exposure to HIV positive sexual partners," condom leakage rates would appear to correspond to an increased risk of infection as, for example, with natural membrane condoms. Overall, however, the risk of viral permeability when intact, high-quality condoms are used correctly seems to be low. In fact, if natural membrane condoms are avoided and synthetic condoms are used correctly, breakage rather than permeability seems to be the major potential risk.23 It is encouraging, then, that the breakage rate of the new TPE condom does not appear to differ significantly from the breakage rate of latex condoms in clinical trials.g The TPE condom, however, has certain advantages that could translate into improved safety and compliance. Unlike latex condoms, which are dipped from a nonuniform emulsion, condoms from a homogenous solution. TPE are dipped Consequently, control of film uniformity is inherently easier with the TPE than with latex. Scanning electron micrographs of latex films show microscopic pitting on the order of 1.5 flm.2o In contrast, electron micrographs of TPE condoms reveal a smooth, glassy surface (unpublished observation). Given sample-to-sample variation among the inherent condoms, uniformity of the TPE enhances the integrity of the finished product. Furthermore, latex is subject to deterioration when exposed to a variety of factors easily encountered in the environment: humidity, heat, ultraviolet light, ozone, and



These conditions can damage rubber mechanical stress.24 severely by cleaving its unsaturated polyisoprene bonds. Consequently, the latex surface is damaged and condom strength decreases. Stretching latex only 3%, for example, exposes the underlying unsaturated bonds to further ozone cracking damage.25 High levels of heat, humidity, and ozone are especially prevalent in developing countries where appropriate transport, storage, and use of condoms cannot be guaranteed. In contrast, the chemical structure of the S-EB-S TPE makes it impervious to weakening from any of these conditions and hence more durable.' The accelerators and anti-oxidants added to latex during vulcanization partially offset the deleterious effects of aging. However, they also serve as irritants or allergens for cell-mediated (type IV) allergic reactions, causing contact dermatitis.' Antibody-mediated (type 1) allergic reactions, including anaphylaxis and shock, can be induced by the proteins that occur naturally in latex.' Such reactions preclude the use of latex condoms by many individuals. The SEB-S TPE condoms, which are free of these allergens, may be the only alternative condom for latex-sensitive individuals.

Acknowledgment Thanks to Curt Hamann, M.D., Director, Clinical Research, Tactyl Technologies, Vista, CA,for supplying the condoms used in this study.


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