Combination therapy of the quinolone-derived compound HT61 and Tobramycin demonstrates synergism against a range of clinical isolates of Pseudomonas aeruginosa in vitro

Antimicrobial resistant pathogens such as Pseudomonas aeruginosa (P. aeruginosa) represent a significant challenge to patients with respiratory diseases including Cystic Fibrosis (CF) and COPD, where treatment of such infections is exacerbated by a shortage of new antibiotic classes. One novel approach utilises ‘antibiotic enhancers’ that potentiate the antimicrobial activity of existing antibiotics. HT61, a small quinolone-derived compound, potentiates the activity of the aminoglycosides tobramycin and gentamicin against Staphylococcus aureus and P. aeruginosa. In this study, we have investigated synergism between tobramycin and HT61 using a panel of tobramycin-sensitive and resistant clinical isolates of P. aeruginosa from CF patients. Microdilution methods and chequerboard analysis were used to evaluate antimicrobial synergy of drug combinations against 63 isolates. HT61’s ability to potentiate tobramycin’s bactericidal activity was subsequently assessed using bacterial time-kill assays, and biofilm eradication assays. 74% of isolates (47/63) demonstrated evidence of either positive interactions (29/63) determined by a Fractional Inhibitory Concentration (FIC) below 1.0, or synergy (18/63) determined by an FIC value of 0.5 or below. Using a sub-selection of these isolates, clear augmentation of Tobramycin’s antimicrobial activity was observed in both time-kill assays and biofilm eradication assays regardless of FIC classification. The expansion of previous studies highlighting the potentiating capabilities of HT61 enhancement of antibiotics in vivo across a further 63 clinical isolates of P. aeruginosa in a laboratory setting further highlights the potential therapeutic benefits of HT61:Tobramycin combinations in respiratory diseases associated with drug-resistant P. aeruginosa .