PHARMACOKINETICS OF [H-3] BIOTIN BOUND TO DIFFERENT AVIDIN ANALOGS

The use of avidin-biotin technology in drug delivery facilitates the c onjugation of biotinylated therapeutics to transport vectors that are enabled to undergo receptor-mediated transcytosis through the brain ca pillary 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 p eripheral tissues such as liver and kidney. However, modified avidins may retain high affinity biotin binding properties, but may not be rap idly removed from plasma by peripheral tissues, and such avidin analog ues would provide preferred plasma pharmacokinetic profiles. Therefore , the present studies investigate the pharmacokinetics of plasma remov al of [H-3]biotin bound to one of six different avidin analogues: stre ptavidin, Neutra-lite avidin, avidin, neutral avidin, Lite-avidin, and succinylated avidin. Isoelectric focusing studies show that avidin an d Lite-avidin were highly cationic proteins, whereas neutral avidin, N eutra-lite avidin, and streptavidin were neutral proteins, and succiny lated avidin had an acidic isoelectric point. The avidin analogues fel l 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 clearanc e group consisted of succinylated avidin, neutral avidin, and Lite-avi din and had a mean plasma clearance of 17 mL/min/kg, or 40-fold faster than the low clearance avidins. In conclusion, these studies show tha t the rate of removal of avidin analogues differs by more than a log o rder of magnitude depending on the charge and the degree of glycosylat ion of the avidin analogue. Use of high clearance avidin analogues may be preferred when it is desired to rapidly remove biotinylated therap eutics from the plasma, whereas the use of low clearance avidins may b e desired in targeted drug delivery.