The use of avidin-biotin technology in drug delivery facilitates the conjugation of biotinylated therapeutics to transport vectors that are enabled to undergo receptor-mediated transcytosis through the brain capillary endothelial wall, which makes up the blood-brain barrier (BBB) in vivo. However, the conjugation of avidin, a cationic glycosylated protein, to transport vectors greatly increases the rate of removal of the vector from the bloodstream, owing to rapid uptake of avidin by peripheral tissues such as liver and kidney. However, modified avidins may retain high affinity biotin binding properties, but may not be rapidly removed from plasma by peripheral tissues, and such avidin analogues would provide preferred plasma pharmacokinetic profiles. Therefore, the present studies investigate the pharmacokinetics of plasma removal of [3H]biotin bound to one of six different avidin analogues: streptavidin, Neutra-lite avidin, avidin, neutral avidin, Lite-avidin, and succinylated avidin. Isoelectric focusing studies show that avidin and Lite-avidin were highly cationic proteins, whereas neutral avidin, Neutra-lite avidin, and streptavidin were neutral proteins, and succinylated avidin had an acidic isoelectric point. The avidin analogues fell into two groups with respect to rate of biotin removal from plasma. The low clearance group included streptavidin and Neutra-lite avidin, which had a mean plasma clearance of 0.41 mL/min/kg. The high clearance group consisted of succinylated avidin, neutral avidin, and Lite-avidin and had a mean plasma clearance of 17 mL/min/kg, or 40-fold faster than the low clearance avidins.(ABSTRACT TRUNCATED AT 250 WORDS)