Abstract
<title>Abstract</title> <p> Background Chronic respiratory infections caused by pathogens including <italic>Pseudomonas aeruginosa</italic> pose a major therapeutic challenge in patients with pulmonary diseases such as cystic fibrosis (CF) and non-CF bronchiectasis, where treatment efficacy is limited by the formation of biofilms, the emergence of multidrug resistant pathogens and a shortage of new and effective antibiotic classes. One approach to address this problem involves the use of “antibiotic enhancers”, which are compounds that potentiate the activity of existing antibiotics without possessing direct antimicrobial effects of their own. The mucoactive thiol-based prodrug erdosteine, currently approved for the treatment of acute and chronic respiratory diseases, has been reported to exhibit anti-adhesive activity against respiratory pathogens, suggesting a clinically relevant contribution to the prevention of bacterial colonization. Furthermore, erdosteine has demonstrated the ability to potentiate the activity of various antimicrobial drugs, but to date has not been investigated for its ability to influence the effect of tobramycin, an antibiotic agent widely used to treat pathogens in the respiratory tract. Furthermore, whilst erdosteine exhibits a wide range of pharmacological actions, including mucolytic, anti-inflammatory, antioxidant activity, the ability to reduce viral load and inhibit bacterial adhesion to epithelial cells, the mechanism of its antibiotic potentiating properties remains poorly understood. In this study, we investigated the ability of MET-1, the primary active metabolite of erdosteine, to enhance the activity of tobramycin against <italic>P. aeruginosa</italic> strains using a panel of both <italic>in vitro</italic> and <italic>in vivo</italic> assays. Methods The activity of both mono- and combination therapies was measured in planktonic <italic>P. aeruginosa</italic> (strains RP73 & PA01) using <italic>in vitro</italic> microdilution assays including chequerboard analysis and time-kill assays, whilst activity against biofilms were analysed using microtiter plate assays with readouts including total biomass, metabolic activity, and bacterial enumeration. Combination therapy of 40mg/kg tobramycin and 100mg/kg erdosteine was also assessed <italic>in vivo</italic> using a murine model of chronic pulmonary infection induced by <italic>P. aeruginosa.</italic> Results 10mg/L MET-1 significantly enhanced the antimicrobial activity of tobramycin in planktonic cultures of RP73 at 24 hrs and PA01 at 4 and 7 hrs, leading to a greater than 2 log reduction in bacterial numbers compared to monotherapy, while MET-1 alone showed no measurable reduction under the same experimental conditions. In biofilm cultures, MET-1 significantly reduced the IC50 of tobramycin in mature RP73 and PA01 biofilms by 15 and 5-fold respectively. Finally, combination therapy of 40mg/kg tobramycin and 100mg/kg erdosteine significantly reduced bacterial load in a murine model of chronic pulmonary infection. Conclusions Our <italic>in vitro</italic> and <italic>in vivo</italic> studies highlight the ability of erdosteine to potentiate the activity of tobramycin suggesting that erdosteine has the potential to reduce the burden of long-term antimicrobial therapies in patients with chronic respiratory infections and thus improve clinical outcomes. </p>