Structure-function relationships in indium-111 radioimmunoconjugates

Abstract

Conjugates formed by reaction of monoclonal antibody B72.3 with benzyl isothiocyanate derivatives of four amino polycarboxylate chelators (NTA, EGTA, EDTA, DTPA) were labeled with indium-111 and administered iv to athymic mice bearing antigen-positive (LS174T) and antigen-negative (A375) human tumor xenografts. Conjugate immunoreactivities, antibody dose, and xenograft size were controlled, so that the effects of varying chelate structure could be evaluated under conditions where immunological and physiological factors were effectively held constant. Tissue distribution and excretion of the radiometal at 24 and 48 h postinjection were shown to correlate directly with chelate thermodynamic stability (NTA less than EGTA less than EDTA less than DPTA). Radioactivity levels in the blood and the LS174T xenograft increased, while kidney levels and excretion levels decreased, with increasing chelate stability. The kidney was the only normal organ that accumulated non-antibody-bound 111In, uptake of radioactivity into all other tissues, and in particular the liver, being unaffected by changes in chelate structure. Mean transferrin saturation in the tumor-bearing athymic mice was found to be 65%. It is proposed that uptake of free 111In by serum transferrin is precluded in this model, leading to the observed renal localization of unbound label. Kidney:blood and kidney:LS174T activity ratios at 48 h postinjection provided the most sensitive indices of conjugate instability in vivo, spanning 50- and 20-fold ranges, respectively, between the least stable and the most stable conjugate. It is concluded that this antigen/antibody system and mouse model are well-suited to structure-function studies of immunoglobulin labels.

H4octapa: An Acyclic Chelator for 111In Radiopharmaceuticals

“…56 Surprisingly, despite the exceptional mouse serum stability of [In(decapa)] 2− , the in vivo stability in mice was found to be suboptimal, with slower clearance from the blood pool and most notably high persistent kidney (3.68 ± 0.54% ID/g) and increasing bone uptake (1.95 ± 0.12% ID/g) over 24 h; however, all other organs contained less than 1% ID/g after 24 h (Figure 4, Table 3). Although these values are inadequate when compared to the in vivo stability and clearance of exceptionally stable chelators like [ 111 In(DOTA)] − and [In(octapa)] − , it is still fair when compared to the very unstable 111 In-citrate complex, which has shown values of 18.7 ± 3.7% ID/g in the kidneys after 24 h. 57 Often 111 InCl 3 and 111 In-citrate are used to emulate the conditions of an unstable chelator that would undergo complete and rapid decomposition/transchelation in vivo. One commonly cited study (Ando et al) has shown 111 InCl 3 to have higher liver, spleen, and kidney uptake than 111 Incitrate; 56 however, another study has shown the kidney uptake of 111 InCl 3 at 24 h in healthy rats to be 2.58 ± 0.83% ID/g, 58 which is significantly less than the value for 111 In-citrate cited above of 18.7 ± 3.7% ID/g, 57 and contradicts the observations from Ando et al 56 Additionally, the highly anionic 111 In-citrate clears much more quickly through the kidneys than 111 InCl 3 , demonstrating that the hypothesis of rapid and complete dissociation upon introduction of these radiometal species into an animal is not accurate.…”

“…Although these values are inadequate when compared to the in vivo stability and clearance of exceptionally stable chelators like [ 111 In(DOTA)] − and [In(octapa)] − , it is still fair when compared to the very unstable 111 In-citrate complex, which has shown values of 18.7 ± 3.7% ID/g in the kidneys after 24 h. 57 Often 111 InCl 3 and 111 In-citrate are used to emulate the conditions of an unstable chelator that would undergo complete and rapid decomposition/transchelation in vivo. One commonly cited study (Ando et al) has shown 111 InCl 3 to have higher liver, spleen, and kidney uptake than 111 Incitrate; 56 however, another study has shown the kidney uptake of 111 InCl 3 at 24 h in healthy rats to be 2.58 ± 0.83% ID/g, 58 which is significantly less than the value for 111 In-citrate cited above of 18.7 ± 3.7% ID/g, 57 and contradicts the observations from Ando et al 56 Additionally, the highly anionic 111 In-citrate clears much more quickly through the kidneys than 111 InCl 3 , demonstrating that the hypothesis of rapid and complete dissociation upon introduction of these radiometal species into an animal is not accurate. 56 Considering these inconsistencies, it is important to utilize internal standards (such as [ 111 In-(DOTA)] − in this study), and additionally there is a need for more clear and reliable baseline data for the biodistribution of 111 In in its most commonly used forms.…”

“…Surprisingly, despite the exceptional mouse serum stability of [In(decapa)] 2– , the in vivo stability in mice was found to be suboptimal, with slower clearance from the blood pool and most notably high persistent kidney (3.68 ± 0.54% ID/g) and increasing bone uptake (1.95 ± 0.12% ID/g) over 24 h; however, all other organs contained less than 1% ID/g after 24 h (Figure , Table ). Although these values are inadequate when compared to the in vivo stability and clearance of exceptionally stable chelators like [ 111 In(DOTA)] − and [In(octapa)] − , it is still fair when compared to the very unstable 111 In-citrate complex, which has shown values of 18.7 ± 3.7% ID/g in the kidneys after 24 h …”

Disposition of radioactivity after injection of liver-targeted proteins labeled with 111In or 125I. Effect of labeling on distribution and excretion of radioactivity in rats

“…In addition, several other modifications of these BCAs have been investigated to improve labeling efficiency and in vivo stability. [5][6][7][8] When internalized by cells, these proteins undergo intracellular catabolism with subsequent formation of radiolabeled metabolites. In particular, after endocytotic internalization, the ligand is delivered to the endosome and then to the lysosome.…”

Residualizing Indium-111-Radiolabel for Plasmid DNA and Its Application to Tissue Distribution Study

“…For studies involving macromolecules such as antibodies, biologically active proteins, polymers, and their drug conjugates, residualizing radiolabels have been developed to trace their tissue distribution. So far, hydrophilic bifunctional chelate-radioactive metal complexes or radioiodination using sugar-containing spacers has been used as residualizing radiolabels for such compounds ( − ). In these approaches, relatively large, hydrophilic molecules are incorporated into the structure of the radiolabeled adducts.…”