Synthesis, plasma clearance, and in vitro stability of protein containing a conjugated indium-111 chelate

A new bifunctional chelating agent, N′-(p-diazoniumbenzyl)-N,N N″,N″-diethylenetri-aminetetraacetic acid (DTTA) was synthesized. The compound as coupled to methyl p-hydroxybenzimidate and the resulting azoimidate was attached to lysine residues of monomeric human serum albumin (HSA) via the amidination reaction. Blood clearnace of111In-labelled DTTA conjugated to HSA (AAHSA) in rabbits was biphasic. The first phase has a clearance indistinguishable from that of125I-labeled HSA. During the second phase, the111In-labeled AAHSA was cleared more rapidly so that between 24 hours and 48 hours the percent of the injected dose of111In-labeled AAHSA in the blood was significantly lower than that of125I-labeled HSA. Highly conjugated111In-labeled AAHSA (0.9 DTTA/HSA) showed accelerated clearance at 24 and 48 hours compared to lightly conjugated protein (<0.9 DTTA/HSA). As a result we postulate that the level of conjugation is the critical parameter controlling the blood clearance.