in 2005 , an estimated 4.1 million people worldwide were newly infected with human immunodeficiency virus ( hiv ) ( unaids 2006 ) . unfortunately , the development of a vaccine has proven elusive ; therefore prevention has relied primarily on behavioral changes including condom use . many males are unwilling to use condoms thus placing their sex partner(s ) at risk . microbicides are compounds that when applied vaginally or rectally would protect the user from sexually transmitted infections ( stis ) . microbicides could offer additional protection when combined with condoms or be used alone as a means of protection for women unable to negotiate condom use . although the emphasis has been on preventing hiv , protection from other stis is desirable in terms of avoiding illness and associated sequelae . importantly , many stis may increase vulnerability to hiv infection ( eg , genital herpes infection facilitates the transmission of hiv ) . microbicides , like vaccines , will be used by healthy individuals to protect against infections . like vaccines , microbicides will need to be both highly efficacious and display an excellent safety profile . these products should have little or no effect on host defenses including the normal vaginal or rectal microflora , the mechanical barrier provided by intact epithelium and a variety of innate and adaptive immune responses ( turpin 2002 ) . studies using in vitro systems and animal models are important tools in the preclinical development of microbicides . dendrimers are a relatively new class of compounds that hold the potential to be safe and effective microbicides ( mccarthy 2005 ) . many biologic targets are large complex macromolecules ( eg , membrane receptors ) on the scale of 1100 nm . , most pharmaceutical development has focused on low molecular weight drugs that lack the capacity to interact with such targets in a multivalent manner . dendrimers are  nanoscale  macromolecules that can be built with various properties including the ability to interact polyvalently with a target . dendrimer production is a highly controlled staged process that allows for the synthesis of precisely defined macromolecular structures . the result is a highly branched three - dimensional molecule that may possess various surface features . spl7013 is a dendrimer that was developed by starpharma ( melbourne , victoria , australia ) and contains a specifically designed polyanionic surface . this highly charged surface allows spl7013 to attach to targets on viruses , blocking viral attachment and/or adsorption to cells thereby preventing infection . in the case of hiv , spl7013 is thought to bind gp120 proteins on the surface of the virus , through which the virus normally attaches to cd4 receptors on human cells . spl7013 is the active product in the candidate microbicide , vivagel ( spl7013 gel ) . recognizing its potential , the united states food and drug administration ( fda ) granted vivagel fast track status in january 2006 for the prevention of hiv indication ( starpharma 2006a ) . the fast track program facilitates and expedites development of products that serve unmet medical needs of serious or life - threatening conditions . recently , in collaboration with starpharma , the united states national institutes of health ( nih ) submitted a second investigational new drug application ( ind ) to fda for the prevention of hsv-2 indication . vivagel is a water - based vaginal product of 3% weight / weight ( w / w ) spl7013 mixed in carbopol gel buffered to a ph physiologically compatible with the normal human vagina . spl7013 is built from a divalent core , the benzhydrylamine amide of l - lysine ( mccarthy 2005 ) . to this core , four successive layers of the branching units , l - lysine are added creating a dendrimer with 32 amine groups on the surface . a sodium 1-(carboxymethoxy ) naphthalene-3,6-disulfonate group is attached to each of the 32 amine surface groups via amide linkers . the chemical product is bha.lys15lys16(nhcoch2o)1-(3,6-naphth(so3na)32 ( bha = benzhydrylamine ) with a molecular weight of 16,581 daltons . high performance liquid chromatography ( hplc ) and liquid chromatography / mass spectrometry are employed in manufacturing to ensure a single molecular entity is synthesized . carbopol gel was chosen because of its mucoadhesive properties and its use in other vaginal products and microbicide formulations . carbopol 971p nf is a cross - linked acrylic acid listed in the usp monograph as carbomer 941 and is categorized by the fda as a generally regarded as safe ( gras ) excipient . it is a weak acid with high buffering capacity and has been shown to be partially effective at inhibiting herpes simplex virus in mice and partially effective against hiv-1 in vitro . the results of in vitro and in vivo studies of spl7013 and the formulated product vivagel are discussed in the text below . gong et al ( 2005 ) evaluated spl7013 for efficacy against herpes simplex virus ( hsv ) and for cell cytotoxicity . efficacy against infection was tested by incubating vero cells ( african green monkey kidney cells ) with spl7013 prior to the addition of the viral inoculum ( pre - infection prophylaxis ) . plaque reduction assays demonstrated the 50% effective concentration ( ec50 ) against infection as 2.0 g / ml for hsv type-1 ( hsv-1 ) and 0.5 g / ml for hsv type-2 ( hsv-2 ) . viral absorption to the vero cells was completely inhibited at spl7013 concentrations > 3 g / ml . antiviral activity in cells already infected with hsv-1 or hsv-2 was demonstrated as well ( post - infection treatment ) . ec50 for treating infections were 6.1 g / ml for hsv-1 and 3.8 g / ml for hsv-2 . time - of - addition studies indicated that spl7013 inhibited both cell entry and hsv replication suggesting that it may have potential as a therapeutic agent for hsv infections . spl7013 was shown to provide comparable protection and antiviral activity against a hsv-2 strain resistant to penciclovir / acyclovir and a hsv-1 strain resistant to foscarnet . spl7013 was not toxic to vero cells up to the highest concentration tested , 10,000 g / ml . effects on cell proliferation were tested on two epithelial cell lines ( hela-229 and hep-2 ) in both stationary and dividing phases . the 50% cytotoxic concentrations ( cc50 ) on both cell lines were greater than the highest concentration tested ( 10,000 g / ml ) . by serial passage in the presence of increasing concentrations of spl7013 , gong et al ( 2005 ) generated an hsv-1 strain that was resistant to post - infection dendrimer treatment but not pre - infection dendrimer prophylaxis . using this approach this suggested it was possible to develop resistance to the effects of spl7013 on viral replication but not on the mechanism(s ) responsible for preventing infection . dezzutti et al ( 2004 ) evaluated the cytotoxicity and anti - hiv activity of serial dilutions of 5% w / w spl7013 in the vivagel formulation . cellular toxicity was appraised in peripheral blood mononuclear cells ( pmbcs ) , macrophages ( m ) , urogenital and colorectal cell lines . spl7013 was found to be relatively nontoxic and its toxicity was similar to that of the base carbopol gel . the ability of mucosal epithelial cells to maintain an intact , polarized monolayer in the presence of the various candidates was evaluated by measuring transepithelial resistance ( ter ) . anti - hiv activity was evaluated in assays for prevention of infection of pmbcs and m as well as for prevention of the transfer of infectious hiv-1 from epithelial cells to activated pmbcs . the safety and efficacy of spl7013 for rectal use was evaluated using colorectal explants obtained from patients undergoing bowel surgical resection for non - inflammatory disease . explants exposed to 5% spl7013 underwent epithelial sloughing leaving the lamina propria intact whereas the control explants treated with the nonoxynol-9 suffered sloughing and necrosis of the lamina propria . as is described below , any epithelial sloughing attributed to spl7013 in these explant studies is in contrast to the safety profile of vivagel following repeated rectal administration in nonhuman primate studies . a decrease in explant viability was detected after application of both spl7013 and the placebo carbopol gel by a mtt ( 1-(4,5-dimethylthiazol-2yl)-3,5-diphenylformazan ) assay . anti - hiv efficacy was evaluated utilizing hiv-1 cxcr4-using ( r4 ) and ccr5-using ( r5 ) strains . transmission of hiv-1 infection to the rectal epithelium was reduced by > 85% with spl7013 and by 50% with the base carbopol formulation alone . the in vitro studies utilized vero cell cultures incubated with different concentrations of unformulated spl7013 to evaluate cell cytotoxicity and effectiveness against hsv-2 infection . the group found an ec50 of 0.6 g / ml while the cc50 was greater than the maximum 1000 g / ml tested . mice were treated vaginally with 100 mg / ml , 10 mg / ml , 1 mg / ml spl7013 or phosphate buffered saline ( pbs ) and challenged 20 seconds later with hsv-2 . a dose related response was noted with 12/12 , 10/12 , 6/12 and 0/12 mice being protected from infection , respectively . the 10 mg / ml concentration was tested with hsv-2 challenges at 5 , 30 , or 60 minutes following spl7013 prophylaxis . protection against infection was seen at each respective time in 14/16 , 12/16 and 4/15 mice . vaginal gel prototypes were prepared with various concentrations of spl7013 in three different carbopol formulations differing in the amount of propylene glycol and glycerin . gels containing 5 , 1 and 0% w / w spl7013 were evaluated in a 5-day repeat dose rabbit vaginal irritation model and all elicited the minimal vaginal irritation . the 5 , 1 and 0% w / w spl7013 concentrations in the different carbopol formulations were tested in mice against a hsv-2 challenge at 5 and 30 minutes following prophylaxis . based on these animal studies the carbopol formulation containing 1% w / w glycerin and propylene glycol was selected for further vaginal testing in the guinea pig model of genital herpes disease and is now referred to as vivagel . the guinea pig model is felt to closer approximate human genital herpes disease than the mouse model . concentrations of 1%5% w / w spl7013 were evaluated with a hsv-2 challenge at 5 minutes following prophylaxis . the results showed high levels of protection were provided by 3% and 5% w / w spl7013 formulations . this same carbopol formulation was selected for evaluation in subsequent primate model studies . in june 2006 , starpharma ( 2006b ) announced that preliminary studies suggest that spl7013 exhibits contraceptive activity in a rabbit model . the study , performed at johns hopkins university ( baltimore , maryland , usa ) , used two formulations of 3% spl7013 . a 75% reduction in embryos was seen with the spl7013 carbopol gel formulation and a 95% reduction with the spl7013 hec ( hydroxyethyl cellulose ) gel formulation . the reduction in the number of embryos in rabbits was similar to that produced by contraceptive products containing nonoxynol-9 . investigators at the university of washington at seattle ( patton et al 2006 ) studied the safety and efficacy of spl7013 vaginal gels in pigtailed macaques . in a comparative study , three concentrations ( 1% , 3% and 5% w / w spl7013 ) and the base carbopol gel alone ( placebo ) were administered intravaginally once daily for four consecutive days . cervicovaginal irritation was seen on colposcopy in four of the six animals administered the 5% w / w spl7013 formulation but in none of the animals receiving the 3% w / w , 1% w / w spl7013 or placebo gel . vaginal and cervical biopsies collected 24 hours after the fourth application of the 3% gel and placebo appeared similar to those obtained at baseline . a transient impact on vaginal microflora was seen 30 minutes after application in all groups but no pattern of a sustained shift in organisms was noted . vaginal ph was significantly lower 30 minutes after application of the spl7013 formulations but had returned to baseline by the fourth day after the final application . the ph change is due to the difference between the ph of the formulations ( ph 5 ) and the higher ph of the normal macaque vagina . vivagel has been formulated at ph 5 to be physiologically compatible with the normal acidic nature of the human vagina . based on the results of the vaginal tests , the 3% w / w spl7013 formulation was selected for study in the rectal model . the macaques received either the 3% w / w spl7013 formulation or the base carbopol gel alone ( placebo ) intrarectally on three consecutive days or received no treatment at all . the safety of vivagel in this model was demonstrated following collection of rectal lavages and their evaluation for evidence of epithelial damage ( ie , cellular debris , epithelial sheets and blood and/or stroma associated with the epithelial sheets ) . this analysis showed there were no significant differences between the animals in the nontreated control group and those animals treated with the 3% w / w spl7013 and placebo gels . rectal ph was significantly lower in the placebo and 3% w / w spl7013 gel groups , which reflects the phs of these formulations relative to the ph of the macaque rectum , and recovered by 24 hours after the last application . the efficacy of the 3% w / w spl7013 gel for the prevention of cervical chlamydia trachomatis infection was assessed . four of the six animals that received the 3% w / w spl7013 gel and five of the six animals that received no product developed infection following intravaginal inoculation . jaing et al ( 2005 ) demonstrated anti - retroviral activity in vitro against both cxcr4-using ( r4 ) and ccr5-using ( r5 ) hiv and shiv ( chimeric simian - human immunodeficiency-1 virus ) strains in human and simian cell lines . based on these findings for unformulated spl7013 , the ability of a single intravaginal application of vivagel to block vaginal shiv transmission was tested . following challenge , all untreated pigtailed macaques ( 8/8 ) and seven of the eight animals given the base carbopol formulation without spl7013 ( placebo gel ) developed shiv infection as indicated by plasma viremia and cd4 cell decline . a dose - dependent response was observed among the spl7013 treated macaques ; 6/6 , 5/6 , and 2/6 animals treated with the 5% , 3% and 1% w / w spl7013 gels , respectively , were protected against infection . the uninfected animals were followed for over a year and remained shiv negative by serology and lymph node biopsy . clinical observation did not reveal any signs of toxicity or vaginal irritation due to the single application of the spl7013 formulations . a phase i clinical trial involving 36 healthy women was completed in 2004 ( mccarthy 2005 ) . vivagel preparations containing 0.5%3.0% w / w spl7013 were evaluated for safety in comparison to the base carbopol formulation without spl7013 ( placebo gel ) . all spl7013 concentrations of vivagel were found to be safe and as well tolerated as placebo . spl7013 was not absorbed into the systemic circulation simplifying the toxicological aspects about this drug . spl7013 is a dendrimer developed specifically to work as a microbicide against hiv and hsv-2 . in vitro and animal model studies have demonstrated the efficacy of the unformulated dendrimer in protecting against both hsv and hiv infections . vivagel , at spl7013 concentrations up to and including 3% w / w , was shown to be safe following vaginal or rectal application in macaques and protected macaques against vaginal shiv infection . further trials are planned to expand on the initial phase i clinical trial and to evaluate vivagel for the prevention of hiv and hsv-2 genital infections in women .
microbicides are compounds that applied vaginally or rectally , protect the user from sexually transmitted infections . 
 although no commercial product is yet available , many candidates are under development . 
 a leading candidate , vivagel ( spl7013 gel ) is the product of nanotechnology . 
 the active ingredient is spl7013 , a dendrimer that was designed specifically with hiv and hsv antiviral activity and human safety in mind . 
 spl7013 has demonstrated efficacy against human immunodeficiency virus and herpes simplex virus in in vitro and animal models . 
 vivagel appears to be well tolerated in both animals and humans . 
 this review summarizes the studies of vivagel and its active ingredient , spl7013 .