MICRODIALYSIS CALIBRATION USING RETRODIALYSIS AND ZERO-NET FLUX - APPLICATION TO A STUDY OF THE DISTRIBUTION OF ZIDOVUDINE TO RABBIT CEREBROSPINAL-FLUID AND THALAMUS

A retrodialysis (RD) method for the real-time calibration of on-line m icrodialysis (MD) procedures was investigated in vitro and in vivo. Ca libration by retrodialysis was simultaneously validated through the us e of a zero-net flux (ZNF) method, which assumes directional independe nce of diffusion of the solute. In RD, a calibrator with dialysance (P eA; effective permeability-surface area product) similar to that of th e compound of interest is introduced into the perfusate. If the calibr ator is suitable, its loss from the perfusate during RD is identical t o the recovery of the solute of interest determined simultaneously by normal MD. Two antiviral nucleosides (AZT and AZdU) which differ struc turally by only a methylene group were utilized as solute and calibrat or, respectively. Both nucleosides exhibited similar recovery and loss at flow rates of 0.5 to 5 muL/min in vitro, indicating a similar PeA product in this flow domain. Furthermore, both compounds showed simila r loss into the lateral ventricle or thalamus of rabbits (n = 4) durin g RD at a flow rate of 1 muL/min for 6 hr. The relative loss decreased rapidly within the first hour, reaching a relatively stable value aft er 2 hr. The significant reduction in the loss of AZdU and AZT in vivo compared with that in vitro likely results from a lower diffusion coe fficient in tissue. The distribution of AZT between plasma and cerebro spinal fluid (CSF) in the ventricle and extracellular fluid (ECF) in t halamus was determined at steady state using calibration by RD and ZNF simultaneously. The relative loss of AZdU during continuous RD was no t significantly different from the recovery of AZT determined by ZNF i n the same animal. Since RD may allow for continuous monitoring of mic rodialysis recovery in real time, it may offer an advantage over the Z NF method of system calibration. The steady-state C(csf)/C(p) and C(ec f)/C(p) ratios for AZT in this study were 0.26 +/- 0.08 and 0.18 +/- 0 .08. That these ratios are much less than unity suggests that carrier- mediated transport of AZT exists in the brain-to-plasma direction.