Endodontic applications of the metallo-drug, Auphen.

Abstract

Background: Root canal treatment requires adequate removal of the infection-causing microorganisms within the intraradicular system. Conventional root canal treatment involves mechanical debridement, using hand and rotary instruments, in conjunction with antimicrobial solutions to decrease the bacterial load to an acquiescent level. Between appointments, medicaments are regularly injected into the root canal space for continued antimicrobial action and to prevent bacterial repopulation. However, the ideal irrigant or medicament does not exist, as many possess cytotoxic properties that can damage the hosts tissues, leading to an inflammatory response. Metallo-drugs, particularly Auphen, impart minimal side-effects on host cells when used in small dosages. Auphen targets aquaporins; present in human cells, plants and bacteria; and has been previously trialled in anti-cancer and rheumatoid arthritis treatment. Aquaporins are channels that regulate the inward and outward movement of water and glycerol across the cytoplasmic membrane. Auphen binds to the aquaporin within the channel, inhibiting its function, leading to cellular instability and death. The antibacterial applications of Auphen in endodontics have yet to be assessed. Aim: The aim of this study was to analyse the ex vivo antibacterial efficacy of Auphen combined with an injectable hydrogel carrier (Poloxamer 407®) against two common bacteria associated with root canal infection. Methodology: Two species, Streptococcus gordonii and Streptococcus mutans, were sourced from frozen glycerol stocks provided by the Department of Oral Sciences Culture Collection (University of Otago, NZ). An aqueous solution of Auphen at varying concentrations was prepared and the minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) determined using a broth microdilution assay. Auphen was incorporated into Poloxamer 407 (P407) gels at 25% (w/v) and the gel stability evaluated. The ex vivo release profile of Auphen from the P407 formulations and the gel dissolution profile were obtained using inductively coupled plasma-mass spectroscopy (ICP-MS). The antimicrobial efficacy of Auphen in a P407 gel formulation was assessedon planktonic cell cultures and on mono-species biofilms using a resazurin metabolic biofilm assay. MIC and MBC comparisons were made between Auphen (aqueous and gel state) formulations, saturated calcium hydroxide, sodium hypochlorite and sterile water. Results: Stable Auphen-loaded P407 gels were successfully prepared. ICP-MS analysis indicated a linear release of Auphen from the dissolving P407 gel over the 7-day period. Aqueous Auphen ≥ 0.5 mM completely inhibited the growth of both species after antimicrobial exposure. Auphen retained its antimicrobial activity when incorporated with P407 gel, with both species of Streptococcus susceptible at the same concentration (MIC 0.1 mM, MBC 0.5 mM). Both aqueous Auphen and Auphen-loaded gels impaired growth of biofilm at concentrations ≥ 0.05 mM. The viability of mono-species biofilms after 24-hours exposure to Auphen ( ≥ 0.1 mM) in both aqueous and gel-form was comparable to sodium hypochlorite, completely inhibiting growth. Saturated calcium hydroxide slowed growth of both Streptococcus biofilms but did not completely inhibit growth. Conclusions: Aqueous Auphen and Auphen-loaded P407 gel at 0.5 mM resulted in antimicrobial activity against both planktonic cultures and mono-species biofilms of S. gordonii and S. mutans. Auphen ( ≥ 0.5 mM), in both aqueous and gel form, resulted in superior antimicrobial ability compared to saturated calcium hydroxide. Within the limitations of this study, Auphen in a polymer gel demonstrates potential for use as an antimicrobial agent in the treatment of root canal infections.