Influence of the Linker on the Biodistribution and Catabolism of Actinium-225 Self-Immolative Tumor-Targeted Isotope Generators

Abstract

Current limitations to applications of monoclonal antibody (mAb) targeted isotope generators in radioimmunotherapy include the low mAb labeling yields and the non-specific radiation of normal tissues by non-targeted radioimmunoconjugates (RIC). Radiotoxicity occurs in normal organs that metabolize radiolabeled proteins and peptides, primarily liver and kidneys, or in radiosensitive organs with prolonged exposure to the isotope from the blood, such as the bone marrow. Actinium-225 nanogenerators also have the problem of released alpha emitting daughters. We developed two new bifunctional chelating agents (BCA) in order to address these issues. Thiol-maleimide conjugation chemistry was employed to increase the efficiency of the mAb radiolabelings by up to 8 fold. In addition, one bifunctional chelating agent incorporated a cleavable linker to alter the catabolism of the alpha particle emitting mAb conjugate. This linker was designed to be sensitive to cathepsins to allow release and clearance of the chelated radiometal after internalization of the radioimmunoconjugate into the cell. We compared the properties of the cleavable conjugate (mAb-DOTA-G3FC) to non-cleavable constructs (mAb-DOTA-NCS and mAb-DOTA-SH). The cleavable RIC was able to release 80% of its radioactive payload when incubated with purified cathepsin B. The catabolism of the constructs mAb-DOTA-G3FC and mAb-DOTA-NCS was investigated in vitro and in vivo. RIC integrity was retained at 85% over a period of 136 hours in mouse serum in vivo. Both conjugates were degraded over time inside HL-60 cells after internalization and in mouse liver in vivo. While we found that the rates of degradation of the two RICs in those conditions were similar, the amounts of the radiolabeled product residues were different. The cleavable mAb-DOTA-G3FC conjugate yielded a larger proportion of fragments below 6kDa in size in mouse liver in vivo after 12 hours than the DOTA-NCS conjugate. Biodistribution studies in mice showed that the mAb-DOTA-G3FC construct yielded a higher liver dose and prolonged liver retention of radioactivity compared to the mAb-DOTA-NCS conjugate. The accumulation in the liver seemed to be in part caused by the maleimide functionalization of the antibody, since the non-cleavable mAb-DOTA-SH maleimide-functionalized control conjugate displayed the same biodistribution pattern. These results provide an insight into the catabolism of RICs, by demonstrating that the release of the radioisotope from a RIC is not a sufficient condition to allow the radioactive moiety to clear from the body. The excretion mechanisms of radiolabeled fragments seem to constitute a major limiting step in the chain of events leading to their clearance.

Anti-CD45 pretargeted radioimmunotherapy using bismuth-213: high rates of complete remission and long-term survival in a mouse myeloid leukemia xenograft model

“…30,31 More recently, longer-lived ␣-emitting radionuclides, such as 225 Ac (t 1 ⁄2 ϭ 10 days), 223 Ra (t 1 ⁄2 ϭ 11.4 days), and 227 Th (t 1 ⁄2 ϭ 18.7 days), have been investigated. [40][41][42][43][44] These radionuclides do generate short-lived daughter therapeutic ␣-particles ( 221 Fr, 217 At, and 213 Bi) from 225 Ac inside a targeted leukemia cell, which largely account for the increased potency of 225 Ac-atomic nanogenerators over 213 Bi constructs. 16 Importantly, however, studies in mice and cynomologous monkeys suggest that these ␣-daughters also have significant potential to inflict renal damage because of uptake of bismuth radioisotopes from the decay chain.…”

Self‐Immolative Spacers: Kinetic Aspects, Structure–Property Relationships, and Applications

“…Since their introduction in 1981, [11][12][13][14][15][16][17] self-immolative spacers found applications in prodrugs, [4,[18][19][20][21][22][23][24][25][26][27][28][29][30][31][32] sensors for chemicals or enzymes, [33][34][35][36][37][38][39][40][41] and for drug delivery. [26-28, 30-32, 42-46] Self-immolative spacers are also increasingly used in materials science, [47][48][49][50][51] where their unique feature of kinetically programmable auto-destruction provides unprecedented opportunities for sustainable matter management( i.e.…”

High Efficiency Diffusion Molecular Retention Tumor Targeting

“…The DOTA functional group can chelate a range of trivalent metals SPECT or PET ( 111 In, 68 Ga) [39] or for radiotherapy (e.g. 213 Bi [40], [41], 177 Lu [42], [43], 90 Y [42], [44], [45] or 225 Ac [46], [47]. DMR maybe particularly well suited to the delivery of alpha particle emitters, with their high toxicity and short range of action [12], [48].…”