Objective: Monoclonal antibodies (mAbs) have been radiolabeled with a variety of radioisotopes
utilizing various kinds of bi-functional chelating agents (BFCAs) with an aim to develop suitable agents for
radioimmunotherapy. The number of BFCA moieties present per antibody molecule plays a significant role in
determining the pharmacokinetics and immunoreactivity exhibited by the radiolabeled antibodies. The objective
of the present study is to evaluate the effect of the number of BFCA moieties present per antibody molecule on
the pharmacokinetics and immunoreactivity of the 177Lu-labeled Rituximab.
Methods: Three different mAb-BFCA conjugates were prepared using different molar ratios of Rituximab
(mAb) to p-NCS-benzyl-DOTA (BFCA) viz. 1:5, 1:10 and 1:50 employing different reaction conditions.
Studies were carried out to determine the average number of BFCAs attached per mAb molecule. All the three
mAb-BFCA conjugates were labeled with 177Lu(III) and were subsequently evaluated in normal Swiss mice to
ascertain their respective pharmacokinetic behavior. In-vitro studies were also performed in Raji cell lines (human
burkitt's lymphoma) for determining the effect of increasing number of BFCAs attached per mAb molecule
on the immunoreactivity of the resultant 177Lu-labeled mAb-BFCA complexes.
Results: 177Lu-labeled mAb-BFCA complex prepared corresponding to 1:50 mAb to BFCA ratio exhibited the
least non-specific uptake and rapid clearance from majority of the organs, but also exhibited least immunoreactive
fraction (IRF = 19.37%). On the other hand, 177Lu-labeled mAb-BFCA complex prepared corresponding
to 1:5 mAb to BFCA ratio exhibited the highest non-specific uptake and slower clearance pattern, but
highest IRF (71.17%). 177Lu-labeled mAb-BFCA complex prepared corresponding to 1:10 mAb:BFCA ratio
exhibited intermediate pharmacokinetic behaviour with moderate IRF (53.05%).
Conclusions: The present study indicates that antibody to BFCA ratio plays a crucial role in determining the
immunoreactivity and pharmacokinetic behavior of the radiolabeled antibodies and must be prudently chosen
for their successful therapeutic application.