INFLUENCE OF ENDOGENOUS BIOTIN ON THE BIODISTRIBUTION OF LABELED BIOTIN DERIVATIVES IN MICE

Previously, this laboratory reported that in mice pre-targeted with unlabelled streptavidin, the biodistribution of In-111 administered on one biotin derivative (EB1) was superior to that of another derivative (DB2). In addition, a Scatchard analysis showed that the affinity constant of In-111-EB1 is lower by seven orders of magnitude from that of In-111-DB2. Therefore, this paper considers the role that endogenous biotin may play in these observations. Both In-111-labelled EB1 and DB2 were bound to streptavidin and incubated at 37 degrees C in mouse blood with increasing concentrations of d-biotin. As determined by Sephadex G-50 chromatography, only an 8-fold molar excess of d-biotin relative to labelled streptavidin was required to displace 90% of label in the case of EB1, whereas even a 20-fold molar excess provided no detectable displacement of DB2. That this displacement was also occurring in vivo was established in a mouse model bearing an infected thigh: increasing the serum biotin level (by intraperitoneal administration of d-biotin) had no effect on the biodistribution of In-111 when administered on DB2; however, the target to non-target ratio decreased in the case of EB1. We have also observed that the biodistribution is no longer favourable when EB1 is administered radiolabelled with Tc-99(m). When In-111 was substituted with Tc-99(m) on EB1, chromatography of blood samples showed that similar displacement was occurring; however, in this case, the displaced label bound to serum proteins. We conclude that endogenous biotin is responsible for the more favourable biodistribution obtained with In-111-labelled EB1 versus DB2 in mice, as the former derivative is more easily displaced from streptavidin and thereafter cleared from circulation rapidly. The biodistribution of Tc-99(m)-EB1 is not as favourable because, once displaced, the clearance is restricted by binding to serum proteins.