Abstract
1281
Objectives: One the key factors of developing safe and effective radioimmunoconjugates is maximizing efficacy while minimizing off-target toxicity in normal tissue. While this statement is one of the core tenants of developing new drugs, the application to radioimmunotherapeutics presents new challenges. Herein we describe a means to more effectively eliminate radioactivity from the body after catabolism and/or metabolism of radioimmunoconjugates by making modifications to the linker region of bifunctional DOTA chelates.
Methods: FPI-1175 humanized mAb (anti-IGF-1R) or non-specific human IgG were conjugated with lysine directed bifunctional DOTA chelates constructed with either a PEG or alkyl-chain linker or a commercial anhydride-DOTA-GA. The resulting immunoconjugates were radiolabeled with Lu-177. Biodistribution studies with urine and feces collection were carried out using Colo-205 (colorectal cancer) xenograft bearing athymic nude mice or normal immunocompetent mice. The metabolic excretion profile of a FPI-1175 conjugated with an alkyl-DOTA or PEG-DOTA chelate was compared to FPI-1175 conjugated with commercial DOTA-GA. Results: Biodistribution studies of the radioimmunoconjugates revealed that the linker type altered the route, rate, and extent of radioactivity excretion but not the overall tissue distribution. The DOTA-GA-mAb was excreted slowly with 13% of the injected dose (ID) eliminated over 7 days predominantly via the urine (9.9 %). In contrast, rate of excretion of the alkyl-DOTA-mAb occurred faster with 26% ID eliminated over 7 days predominantly via the feces (22.9%). The PEG-DOTA-mAb demonstrated an excretion profile distinct from the other two radioimmunoconjugates tested. Overall excretion was higher at 40.2% ID eliminated over 7 days and excretion was divided between urinary (20.6%) and fecal routes (19.6%). Similar results were obtained using analogous radioimmunoconjugates based on non-targeted human IgG. Overall excretion of the PEG-DOTA-IgG was 28.5% ID eliminated over 7 days and excretion was divided between urinary (14.2%) and fecal routes (14.3%). In vitro experiments demonstrated that all FPI-1175 radioimmunoconjugates were effectively identical in terms of cellular internalization and retention and are likely to undergo similar catabolic degradation inside tumor cells. Furthermore, excretion data for lysine-derivatives alkyl-DOTA and PEG-DOTA small molecule compounds which mimic catabolic byproducts of the respective radioimmunoconjugates was found to be via the urinary tract. Conclusion: The improved excretion profile of the novel alkyl-DOTA or PEG-DOTA chelates when conjugated to monoclonal antibodies would be expected to reduce the body burden of radioactivity when such conjugates are used as diagnostic or therapeutic radiopharmaceuticals. The in vivo behavior of antibodies conjugated with the novel alkyl-DOTA or PEG-DOTA chelates would not have been predicted by their behavior at the cellular level or by the in vivo behavior of potential small molecule DOTA catabolic compounds alone.