Streptococcus mutans x-prolyl dipeptidyl peptidase as a target against biofilm formation unravelled by antihuman dpp iv drugs: a new paradigm for the synthesis of innovative anti-caries agents.

Introduction: Dental caries, a multifactorial global disease caused mainly by Streptococcus mutans may exhibit higher morbidity in diabetic patients with poor glycemic control. We speculated that gliptins, commonly used as anti-dipeptidyl peptidase (DPPIV) drugs in diabetics, may interfere with S. mutans. Materials and Methods: We have determined the ex vivo antimicrobial and anti-biofilm activity on S. mutans (reference strain UA159) of three gliptins, namely saxagliptin, sitagliptin, and vildagliptin. The bacterial dipeptidyl peptidase (Sm-DPP IV) was over-expressed, purified by affinity chromatography, characterized and evaluated for its inhibition by the same drugs. Also isogenic deletion mutants of the gene expressing the Sm-DPP IV, pepX, were obtained and used to perform CLSM studies aimed at evaluating the biofilm structure and viability formed at different stages of development. Concurrently, we performed modeling and docking studies in parallel with synthesizing new molecules, which may be effective against the bacterial enzyme and not against the human homologue. Results: Saxagliptin was quite effective in inhibiting biofilm at 128 μg/mL, a concentration close to the inhibition constant of Sm-DPP IV (Ki = 129μM ± 16), while vildagliptin and sitagliptin inhibited at higher concentrations (~256 μg/mL). In CLSM studies, biofilm of pepX isogenic mutant was comparable to that formed in presence of saxagliptin, establishing a probable role of this enzyme in sucrose independent biofilm formation by S. mutans. The purified enzyme has shown to be strongly inhibited by the known DPP IV inhibitor valine-pyrrolidide. In view of these promising outcomes, a series of compounds, designed by modeling and docking experiments on the computed structure of the Sm-DPP IV, were synthesized and tested for their inhibition of the Sm-DPP IV. Among them some lead compounds have been identified to foster the development of additional, more potent and selective inhibitors of the streptococcal biofilm. Conclusions: In an era of multidrug resistant pathogens, it is urgent to look for new anti-virulence drug targets. Sm-DPP IV might be one of them. Our work is now waiting this paradigm to be extended to other streptococci. Additionally, this study added evidence to the concept that routinely used nonantibiotic medicinal drugs might affect the human microbiota.