A Comparison of the Cyclic Anhydride and Mixed Anhydride Methods for 111In-DTPA Chelation to Monoclonal Antibodies

Indium selectively increases the cytochrome P-450 dependent O-dealkylation of coumarin derivatives in male mice

Indium pretreatment of rats and mice has been reported to decrease the concentration of cytochrome P-450, thereby reducing the activity of some cytochrome P-450 dependent enzymatic reactions. The present study reveals that pretreatment of C57Bl/6JHan mice of both sexes with one s.c. dose of 120 mg of In2(SO4)3.5 H2O per kg of body weight decreases the concentration of cytochrome P-450 to about 65% of control levels. Neither cytochrome b5 nor NADPH-cytochrome P-450 reductase is affected. Hepatic microsomal ethoxyresorufin O-deethylase activity declines to about 75% of control values. In contrast, with coumarin substrates, a sex dependence in the direction of change is observed: in female mice indium decreases the activity to about 75%, whereas in males it enhances the activity to 140%. Moreover, with 7-(methoxy-14C)coumarin as substrate, indium-pretreated male mice exhale about 180% and females about 65% of 14CO2 compared to the corresponding controls. A close correlation between the in vivo and in vitro effects of indium on the metabolism of the coumarin derivatives is suggested. After isolation and purification of cytochrome P-450, SDS-PAGE indicates in indium-pretreated male mice an intensification of a 48.5 kDa protein band which is decreased in females. Immunological studies using antibodies raised against control female cytochrome P-450 show cross reactivity among all microsomes used in these experiments. High percentages of inhibition occur in microsomes with high molecular activity towards coumarin derivatives. The in vitro kinetics of antibody-inhibited O-deethylation of 7-ethoxycoumarin seems to obey a non- or partial-competitive type of inhibition. Indium pretreatment of mice produces sex-dependent effects on the metabolism of coumarin derivatives.

DTPA-coupled proteins--procedures and precautions

While demonstrating that the cyclic anhydride of diethylenetriaminepentaacetic acid (DTPA) may be used to covalently attach this chelator to proteins, it was necessary to develop several new procedures and techniques. We were able to show that coupled proteins may be labeled with indium-111 (111In) simply by transcomplexation from a weaker complex such as the acetate to avoid subjecting the protein, even momentarily, to the acidity of the chloride (however, we have also observed that too high a concentration of acetate and other complexing agents such as citrate will interfere with protein labeling by competing with DTPA for the radioactivity). In agreement with other investigators using bifunctional chelate methodology we have observed that trace metals may interfere with the labeling. Accordingly, we developed a simple paper chromatographic assay which may be used to establish whether trace metals are present at interfering concentrations and to identify their sources. We have also employed a hydrolyzed control assay to determine whether and to what degree the radioactivity is bound to protein other than at the DTPA groups. We have developed a simple method of measuring the average number of DTPA groups per protein molecule which involves labeling the products of coupling with 111In. Procedures used and precautions observed in the radiolabeling of proteins by these methods are discussed.

A method for direct quantification of the amount of DTPA in 111In monoclonal antibody preparations

The bicyclic anhydride of DTPA-1-14C (BADTPA-1-14C) was synthesized and reacted with an antibody to human melanoma associated antigen (MAA) and with one to human class II major histocompatibility complex antigen (HLA-DR). DTPA-1-C incorporation per mole of anti MAA at molar ratios of 1:1 to 200:1 ranged from 0.5 to 25, while immunoreactivity ranged from 49 to 9%. With antibody to HLA-DR, results were similar. Anti MAA, but not anti HLA-DR, demonstrated polymerization upon conjugation. BADTPA-1-14C provides a convenient and accurate method for measuring the amount of DTPA in monoclonal antibody preparations and its effect on immunoreactivity.