Balancing Antibacterial Efficacy and Reduction in Renal Function to Optimise Initial Gentamicin Dosing in Paediatric Oncology Patients.

This study aimed to determine the optimal starting dose of gentamicin in paediatric oncology patients. A population pharmacokinetic model describing drug exposure, a semi-mechanistic model describing bacterial killing and an Emax model describing renal cortex accumulation were linked in a utility function using NONMEM®. The optimal gentamicin starting dose was estimated in patients aged from 0.1 to 18.2 years, by balancing the probability of efficacy on day 1 against relative renal function reduction on day 7 with continued dosing. Using achievement of a gentamicin area under the concentration time curve to bacterial minimum inhibitor concentration (MIC) ratio of ≥ 100 and maximum concentration to MIC ratio of ≥ 10 as the efficacy endpoints, a starting dose of 7.1, 9.5, 10.8 and 14.6 mg/kg/q24h was optimal at a MIC of 0.5, 1, 2 and 4 mg/L respectively, with ≥ 75% probability of obtainment. Using achievement of a 2-log10 bacterial count reduction at 24-h post-dose as the efficacy endpoint, a starting dose of 12.8 mg/kg/q24h was optimal, with 85.6% probability of obtainment. Under these different dosing scenarios, relative reduction in renal function ranged on average from 6.9 to 14.5% on day 7. The current recommended starting dose of gentamicin of 7.5 mg/kg/q24h may not be sufficient to achieve efficacy on day 1 if bacterial MIC is > 0.5 mg/L. A higher initial dose (up to 14.6 mg/kg/q24h), in less sensitive microorganisms, would likely cause only a relatively small reduction in renal function at day 7. Close monitoring is crucial if high doses are given, especially for longer than 7 days.