Towards optimising use of colistin against Pseudomonas aeruginosa

The use of colistin, a 50-year old polymyxin antibiotic with a reportedly high incidence of nephrotoxicity and neurotoxicity, declined with the development of potentially less toxic anti pseudomonal agents. Colistin retains significant activity against emerging multidrug resistant (MDR) Gram-negative bacteria and is often the only therapeutic option available to treat infections by these pathogens. As a consequence, use of colistin as a ‘last-line’ of defence has increased dramatically over recent years. However, there has been a dearth of pharmacological information to inform rational use of colistin in order to maximise antibacterial activity and minimise toxicity and the development of resistance. As resistance to colistin is beginning to emerge, there is an urgent need to optimise use of colistin to prolong its therapeutic utility. To increase our understanding of colistin pharmacokinetics (PK) and pharmacodynamics (PD) this thesis examined the activity of colistin against Pseudomonas aeruginosa. Colistin is administered parenterally as its sulphomethylated derivative, sodium colistin methanesulphonate (CMS); colistin is subsequently formed from CMS in vivo. The contribution to bacterial killing of both CMS and formed colistin was unknown, despite the important implications for susceptibility and PK/PD studies. The relative antibacterial activity of CMS and colistin against P. aeruginosa was thus investigated. The time-course of the killing effect achieved with CMS (and formed colistin) was very similar to that observed with colistin when added to achieve the same colistin concentration-time course resulting from the conversion of CMS. Killing with CMS did not begin until significant concentrations of colistin were achieved, indicating that CMS possesses little antibacterial activity. As the time-course of antibacterial activity from CMS could be accounted for by the appearance of colistin, it was clearly demonstrated that antipseudomonal activity was due to formation of colistin. CMS may therefore be regarded as an inactive pro-drug of colistin. Confusion surrounds the optimal dosing regimen of CMS. Three clinically relevant dosage regimens of CMS (8, 12 and 24 hourly) were simulated in an in vitro PK/PD model to evaluate the PD of colistin against P. aeruginosa. Overall bacterial killing and regrowth throughout the experimental period (72 h) was generally similar among the three regimens, with regrowth observed with all regimens. Population analysis profiles (PAPs) revealed the presence of colistin resistant subpopulations with each regimen at 72 h. In the two regimens which employed the greater dosage interval (12 and 24 h), the emergence of resistance was substantially greater and occurred earlier than for the 8 hourly regimen. Information on the PK/PD index that best predicts colistin efficacy is important for rational design of optimal dosing strategies. Dose fractionation was employed in an in vitro PK/PD model to identify the PK/PD index (i.e., the area under the unbound concentration-time curve to MIC ratio [ƒAUC/MIC], the unbound maximal concentration to MIC ratio [ƒCmax/MIC], or the cumulative percentage of a 24-h period that unbound concentrations exceed the MIC [ƒT>MIC]) that best predicts colistin efficacy against P. aeruginosa and to determine the values for the predictive index required to achieve various magnitudes of killing effect. Overall killing effect was best correlated with ƒAUC/MIC. The magnitudes of ƒAUC/MIC required for 1- and 2-log10 reductions in the area under the cfu/mL curve relative to growth control were identified. Use of combination antibiotic therapy may be beneficial against rapidly emerging resistance in P. aeruginosa. Bacterial killing and resistance emergence with colistin monotherapy and in combination with imipenem was systematically investigated at two inocula (~10^6 and ~10^8 cfu/mL) using static time-kill methodology. The bacterial strains examined included colistin heteroresistant and colistin- and imipenem-resistant strains; colistin-heteroresistant P. aeruginosa was first identified in this study. Colistin combined with imipenem at clinically achievable concentrations substantially increased bacterial killing against MDR and colistin heteroresistant isolates at both inocula. Combination therapy against colistin-susceptible isolates generally had little effect on the proportion of colistin-resistant subpopulations, with PAPs very similar to that obtained with equivalent colistin monotherapy. Finally, to further investigate the combination of colistin and carbapenems, a systematic investigation examining the bacterial killing and emergence of colistin resistance with colistin alone and in combination with doripenem at both high and low inocula against P. aeruginosa was undertaken using an in vitro PK/PD model. The addition of doripenem to colistin using clinically relevant dosage regimens resulted in substantial improvements in bacterial killing over equivalent monotherapy against the MDR colistin-resistant isolate at both inocula. Although the benefits in overall antibacterial activity with the combination were slightly less pronounced against the colistin-susceptible but -heteroresistant strain, combination regimens nevertheless resulted in substantial improvements in bacterial killing. Combination therapy substantially reduced and delayed the emergence of colistin-resistant subpopulations in the colistin susceptible strain, but had no effect on the colistin resistance of a MDR colistin-resistant isolate. In brief, this thesis was the first to demonstrate CMS is an inactive prodrug of colistin and to systematically investigate colistin combinations. The findings contained therein improve the current understanding of the PK/PD determinants of colistin activity and resistance development. This work provides important information that will assist in designing optimal dosing strategies to maximise the efficacy of, and reduce the development of resistance to, this increasingly important therapeutic agent.
Awards: Winner of the Mollie Holman Doctoral Medal for Excellence, Faculty of Pharmacy and Pharmaceutical Sciences, 2012.