We tested four commercially available vaginal gels to assess their HSV-2 growth modulating activity in vitro. Our data showed that the tested gels either had an approximately neutral effect (Gel-1, Gel-2) or strong inhibitory effect (Gel-3, Gel-4) at the highest tested concentration (10–20 w/v%). Since for Gel-3 and Gel-4, the tested 10–20 w/v% concentrations mean in effect a 5–10 fold dilution, and considering that the volume of vaginal fluid and sperm lies in the 1–3 ml range  , one may expect that in vivo these gels can achieve 10–20 w/v% or higher concentrations and have significant antiviral activity. As during the symptomatic infection the sexual activity is likely abandoned, HSV-2 transmission via shedding during the asymptomatic periods and in the very early phase of symptomatic infections is probably more frequent. Previous studies showed that HSV-2 shedding is a frequent event, occurring 11%-26% of days in HSV-2 seropositive individuals . Schiffer et al. showed that for 14685 swab samples, 18% were HSV-2 positive (> 150 DNA copies/ml), and close to 90% of the samples contained more than 104 DNA copies/ml . Since the median HSV-2 load was 104.8 DNA copies/ml  and the threshold of HSV-2 transmission was calculated previously as > 104 infectious unit (IFU) , even a low level HSV-2 load decrease could be significant in preventing the transmission. Gel-3 and Gel-4 were able to cause ~ 2 logs decrease in HSV-2 IFU and were effective at least at 6.4 MOIs (~ 400.000 IFUs), therefore these gels might lower the risk of transmission, despite the fact that they were not designed for prevention. These results should also be considered in the evaluation of epidemiological studies where the application of vaginal lubricants did not decrease HSV-2 transmission, while the presence or the lack of anti-HSV-2 activity of the lubricants were not determined .
As the commercially available gels generally contain several ingredients in unknown concentrations, the exact sources of the cumulative inhibitory activity are not known. As an example, a potentially antimicrobial unique component of Gel-4 gel was the “citrus aroma”. It was described previously, that citrusinine-I, an alkaloid isolated from the citrus plant Rutaceae displayed antiviral activity against HSV-1 and HSV-2 . Among the physicochemical attributes of the vaginal gels, one is their hydrophilic, hydrophobic or amphipathic nature. Amphipathic gel components can behave as surfactants, providing a detergent-like activity for the gels. HSV-2 is an enveloped virus, hence the detergent activity could destabilize the viral membrane and decrease viral infectivity . We measured the surface tension decreasing activity of the gels which correlates with their detergent-like activity. Gel-3 and Gel-4 showed a marked detergent-like activity, while Gel-1 and Gel-2 had negligible effect. Our results also showed that detergent-like activity and the in vitro antiviral activity of the gels were strongly correlated, a finding that may be used in future gel developments.
A limitation of our study, that we did not investigate the in vivo antiviral effects of the gels. Previous data show that in vivo a marked detergent-like activity may actually increase HSV-2 susceptibility. In a mouse model of HSV-2 infection, all the five intravaginally applied detergent containing gels increased HSV-2 susceptibility . This effect was likely due to the in vivo detergent cytotoxicity, which led to injury and shedding of the epithelial cells from the upper layers of the mucosa. It is possible that there is an optimal detergent concentration where the viral membrane is destabilized, but the epithelial layer of the cervix remains intact. Regarding the selective toxicity, two of the four tested gels showed significant anti-HSV-2 activity but had no impact on the viability of the host cells in the 1:5 and 1:10 dilutions (20 w/v% and 10 w/v%). However there is room for further development, e.g Gel-3 showed anti-HSV-2 activity only in its first non-toxic concentration.
In conclusion, our experiments revealed that there are substantial differences among commercially available vaginal gels regarding their anti-HSV-2 activity. From the tested four gels we found two with a significant antiviral activity, suggesting that these gels might be able to decrease the frequency of HSV-2 transmission. Further experiments are needed to evaluate their overall effect on HSV-2 infectivity in vivo.