Structural Determinants of Activity of Chlorhexidine and Alkyl Bisbiguanides Against the Human Oral Flora

Abstract

We assayed chlorhexidine and a series of its analogues, in which the chlorophenyl terminal substituents were replaced with alkyl chains, for their in vitro antimicrobial activity against the Gram-negative and Gram-positive oral bacteria. Changes in antimicrobial activity were correlated with changes in agent structure for identification of structural criteria which may be important in the optimization of agent activity. Chlorhexidine showed substantial antimicrobial activity against the Gram-negative as well as the Gram-positive oral bacteria. The alkyl agents were comparable with chlorhexidine in their activity against Bacteroides gingivalis and Bacteroides intermedius, black-pigmented Gram-negative obligate anaerobes associated with periodontal disease in adults. Alkyl agents alexidine, heptihexidine (1,6-bis-n-heptylbiguanidohexane), hexoctidine (1,8-bis-n-hexylbiguanidooctane), and hexhexidine (1,6-bis-n -hexylbiguanidohexane), as well as chlorhexidine, were active against Actinobacillus actinomycetemcomitans, a Gram-negative organism associated with localized juvenile periodontitis. Hexidecidine (1,10-bis-n-hexylbiguanidodecane) and heptoctidine (1,8-bis-n-heptylbiguanidooctane) were more active, and hexhexidine was as active as chlorhexidine against Fusobacterium nucleatum, also associated with periodontal disease. Seven of the agents were more active than chlorhexidine against Actinomyces species. All test agents were active against Streptococcus mutans, a Gram- positive coccus associated with dental caries. Hexidecidine had activity equal to that of chlorhexidine when evaluated against the entire battery of organisms. Analysis of structure-activity relationships revealed that alkyl chains could replace chlorophenyl groups with retention or improvement of antimicrobial activity. Agents with lower lipophilicity were generally more active than those with higher lipophilicity. Within an isolipophilic series, activity increased as methylene bridge length increased, with a requirement for a bridge of at least six carbons for activity against most bacteria. Agents with branched terminal groups were more active than agents of the same bridge length but with unbranched terminal groups. The alkyl bisbiguanides, agents which do not contain chlorophenyl moieties, have activity against the oral flora comparable with that of chlorhexidine. Of the alkyl agents evaluated in this study, hexidecidine, hexhexidine, and alexidine offer the most promise for use in control of oral diseases.