BIOCHEMICAL MODIFICATION OF STREPTAVIDIN AND AVIDIN - IN-VITRO AND IN-VIVO ANALYSIS

The high affinity streptavidin (or avidin)/biotin system is being inve stigated for imaging and radiotherapy procedures, Streptavidin (SA) an d avidin exhibit markedly different pharmacokinetics, with avidin clea ring from the blood much faster than SA. To optimize blood clearance k inetics, SA and avidin were biochemically modified and analyzed in vit ro and in vivo. Methods: Galactose moieties were covalently attached t o promote binding by hepatocyte galactose receptors and hasten SA clea rance. To prolong avidin clearance, avidin was deglycosylated and/or n eutralized by acetylation of its lysine amino acids. In vitro, the mod ified proteins were analyzed by isoelectric focusing, SDS polyacrylami de electrophoresis and a biotin binding saturation assay, The modified and native proteins were radiolabeled with I-131 and injected into ra bbits for pharmacokinetic, biodistribution and imaging analysis. Resul ts: For SA, the resulting increase in blood clearance and liver accumu lation was correlated to the amount of galactose bound to SA. For avid in, each type of modification increased its circulation time, with the slowest clearance resulting from a combination of deglycosylation and neutralization. Conclusion: Biochemical modification of SA and avidin resulted in altered pharmacokinetics compared to the native proteins, Modified SA or avidin, when cross-linked with a lesion-specific targe ting agent, may be applicable for rapid two-step in vivo imaging techn iques.