Object. Although recent studies have shown that convection can be used to distribute macromolecules within the central nervous system (CNS) in a homogeneous, targeted fashion over clinically significant volumes and that the volume of infusion and target location (gray as opposed to white matter) influence distribution, little is known about other factors that may influence optimum use of convection-enhanced distribution. To understand the variables that affect convective delivery more fully, we examined the rate of infusion, delivery cannula size, concentration of infusate, and preinfusion sealing time.
Methods. The authors used convection to deliver 4 µl of 14C-albumin to the striatum of 40 rats. The effect of the rate of infusion (0.1, 0.5, 1, and 5 µl/minute), cannula size (32, 28, and 22 gauge), concentration of infusate (100%, 50%, and 25%), and preinfusion sealing time (0 and 70 minutes) on convective delivery was examined using quantitative autoradiography, National Institutes of Health image analysis software, scintillation analysis, and histological analysis.
Higher rates of infusion (1 and 5 µl/minute) caused significantly (p < 0.05) more leakback of infusate (22.7 ± 11.7% and 30.3 ± 7.8% [mean ± standard deviation], respectively) compared with lower rates (0.1 µl/minute [4 ± 3.6%] and 0.5 µl/minute [5.2 µ 3.6%]). Recovery of infusate was significantly (p < 0.05) higher at the infusion rate of 0.1 µl/minute (95.1 ± 2.8%) compared with higher rates (85.2 ± 4%). The use of large cannulae (28 and 22 gauge) produced significantly (p < 0.05) more leakback (35.7 ± 8.1% and 21.1 ± 7.5%, respectively) than the smaller cannula (32 gauge [5.2 ± 3.6%]). Varying the concentration of the infusate and the preinfusion sealing time did not alter the volume of distribution, regional distribution, or infusate recovery.
Conclusions. Rate of infusion and cannula size can significantly affect convective distribution of molecules, whereas preinfusion sealing time and variations in infusate concentration have no effect in this small animal model. Understanding the parameters that influence convective delivery within the CNS can be used to enhance delivery of potentially therapeutic agents in an experimental setting and to indicate the variables that will need to be considered for optimum use of this approach for drug delivery in the clinical setting.