Gentamicin concentration gradients in scala tympani perilymph following systemic applications

Abstract

It has been shown in prior studies that round window membrane (RWM) application of gentamicin produced a robust basal-apical concentration gradient in the perilymph of scala tympani (ST) with peak concentrations in the basal turn of ST. These gradients potentially contribute to the clinical efficacy and safety of intratympanic gentamicin applications for the treatment of Ménière's disease. The present study aimed to establish the distribution of gentamicin along ST perilymph after systemic applications. Gentamicin sulfate was applied intravenously in the amounts of 100, 300 and 600 mg/kg body weight (BW) over a period of 3 h or as a 300 mg/kg BW subcutaneous bolus injection. At 3 and 5 h after the start of the application perilymph of ST was aspirated from the cochlea apex of the right and left cochlea, respectively, and 10 sequential 1-µl perilymph samples from the apex of each cochlea were quantitatively analyzed using a fluorescence polarization immunoassay. In contrast to local RWM delivery, systemic application of gentamicin resulted in the highest perilymph levels in the apex of the cochlea with decreasing concentrations towards the basal regions of ST. The absolute gentamicin concentrations increased with the amount of drug applied and time before sampling. While it is likely that the basal-apical gradient measured after local drug applications to the round window niche is the result of the direct uptake of drugs into the perilymph of the ST, distribution by diffusion and a very low perilymph flow towards the cochlear apex, computer simulations suggested that the apical-basal gradient observed with these systemic applications can be explained by higher entry rates of gentamicin in the apex compared to the basal turns of the cochlea. It is also possible that gentamicin enters perilymph indirectly from the blood via the endolymph. In this case the faster kinetics in apical turns could be due to the smaller cross-sectional area of ST relative to endolymph in the apical turns.

Fig. 1

Gentamicin serum concentrations with intravenous (i.v.) and subcutaneous (s.c.) applications: With continuous i.v. application peak values were observed at 150 min after start of application. After the termination of application (180 min) serum values started to decline slowly. With single s.c. injections (300 mg/kg/bw) peak concentrations in serum were observed approximately 60 min after application. For the same dose applied higher peak concentration were measured with i.v. applications compared to the s.c. paradigm. Error bars: standard deviation

Fig. 2

Gentamicin gradients in ST perilymph revealed by sequential sampling from the cochlear apex after continuous intravenous application three hours (left) and five hours (right) after the start of drug application. The first 4 samples (gray area) correspond to perilymph from different regions of ST. Sample 1, originating from apical perilymph, always contained higher gentamicin concentration than sample 4 , originating from the basal turn, demonstrating the existence of an apical-to-basal concentration gradient along the scala. This is completely opposite to the basal-apical gradients that were measured in previous experiments following local (RWM) applications (Plontke et al. 2007). Subsequent samples (numbers 5 to 10) contain CSF (which itself may contain gentamicin) that has passed through ST. The measured gentamicin levels in these later samples can be explained by repartitioning of the drug from adjacent cochlear compartments back into ST. With increasing dose higher concentrations were observed in perilymph at both 3 hours and 5 hours. Error bars indicate standard deviation. The pale shaded area at the lower part of each graph indicates LOQ. (***p< 0.001; **p< 0.01; *p< 0.05: for comparison of doses at one sampling time; (⁺⁺⁺p< 0.001; ⁺⁺p< 0.01; ⁺p< 0.05: for comparison of same doses but between the 3h and 5h sampling times).

Fig. 3

Gentamicin gradients in ST perilymph after subcutaneous bolus injections. Although lower drug concentrations were measured in the perilymph after s.c. applications compared to the i.v. protocol with the same dose (300 mg/kg/bw), gentamicin distribution indicated an apical to basal gradient as in the continuous i.v. infusion protocol. Grey area: samples originating from ST perilymph. Pale gray shaded area at the bottom of the graph indicates the LOQ.

Fig. 4

Calculated sample concentration curves for four combinations of entry kinetics from blood to scala tympani perilymph. “Fast” entry indicates a 200 min entry half time and “Slow” entry indicates a 2000 min entry half time at the location. A: Curve for fast entry throughout ST. Initial samples, originating from the apex, are lower than sample 4, originating from the basal turn, as solute losses to adjacent compartments are more significant at the apex, where ST cross-section is small. B: Curve for fast entry in the basal turn and slow entry in the apical turn with entry rates graded in between. The difference between Sample 1, from the apex, and sample 4, from the base, is even greater. C: Curve for slow entry in the basal turn and fast entry at the apex with rates graded in between. Sample 1, from the apex is now higher than sample 4 from the base. D: Curve for slow entry throughout ST. The curve is comparable to curve A, but lower. If direct gentamicin entry into ST was assumed, sample concentration curves comparable to the measured data (Figs. 2 and 3) could only be achieved with a higher entry rates at the apex relative to the base (i.e. curve C with solid symbols).

Fig. 5

Gray lines: Mean Gentamicin concentration in sequential perilymph samples after continuous intravenous application (300 mg/kg/bw) five hours after start of drug application (gray, from Figure 2). Black lines show calculated sample values. Left: Gentamicin entry from blood into ST with slower entry kinetics at the base than at the apex of the cochlea. Right: Gentamicin reaching ST by an endolymphatic route with uniform entry rates along the cochlea. In this case the faster kinetics in apical ST perilymph is due to the far smaller ST cross-sectional area in apical turns compared to the base, while endolymph remains relatively uniform in cross-section with distance.

Fig. 6

Mean gentamicin concentrations in scala tympani in sequential apical samples taken from ST after 3 hours of continuous intravenous (i.v.) application (300 mg/kg/bw group from Fig. 2 left, full symbols) compared with the mean gentamicin concentrations after continuous irrigation of the RWM for 3 hours (historical data from a previous study of our group: Plontke et al. 2007, ©Laryngoscope). The distance along ST plotted is that of the midpoint of the estimated region of origin for 1-μL-samples (half the sample volume apical and half the volume basal to the location). Note logarithmic scale.