Kinetics and sodium independence of [3H]-tiazofurin blood to brain transport in the guinea pig.

Tiazofurin is a nucleoside analog with potent antitumor activity. The objective of this study was to define the kinetic parameters of tiazofurin transport from blood into the guinea pig brain. The second aim was to define kinetic parameters of tiazofurin transport inhibition by adenosine (Ki and Kda), as well as whether tiazofurin blood-to-brain transport was performed in countertransport with Na+. In order to determine these parameters, the in situ method of perfusion of guinea pig brain was used. Addition of increasing concentrations of unlabelled tiazofurin to perfusing medium (0.05-2.7 mmol/l) caused progressive decrease of [3H]-tiazofurin brain clearance with Km values ranging from 119.57 +/- 40.12 to 150.17 +/- 51.68 x 10(-6) mol/l. Maximal transport capacity ranged from 325.03 +/- 113.93 to 417.50 +/- 151.53 pmol/min/g. Introduction of adenosine into the perfusing medium (0.005-0.2 mmol/l) caused similar but more rapid decrease of [3H]-tiazofurin brain clearance (Ki = 6.36 +/- 3.14 x 10(-6) mol/l for nucleus caudate and 11.74 +/- 4.85 x 10(-6) mol/l for cerebral cortex). Therefore, it seems that transport system for adenosine plays the main role in tiazofurin brain uptake, but another transport mechanism is also involved in this process. Depletion of perfusing medium from sodium ions did not cause significant change in volume of distribution values for [3H]-tiazofurin in guinea pig cerebral cortex.