Alpha radioimmunotherapy using ²²⁵Ac-proteus-DOTA for solid tumors - safety at curative doses

This is the initial report of an α-based pre-targeted radioimmunotherapy (PRIT) using ²²⁵Ac and its theranostic pair, ¹¹¹In. We call our novel tumor-targeting DOTA-hapten PRIT system "proteus-DOTA" or "Pr." Herein we report the first results of radiochemistry development, radiopharmacology, and stoichiometry of tumor antigen binding, including the role of specific activity, anti-tumor efficacy, and normal tissue toxicity with the Pr-PRIT approach (as α-DOTA-PRIT). A series of α-DOTA-PRIT therapy studies were performed in three solid human cancer xenograft models of colorectal cancer (GPA33), breast cancer (HER2), and neuroblastoma (GD2), including evaluation of chronic toxicity at ~20 weeks of select survivors. Methods: Preliminary biodistribution experiments in SW1222 tumor-bearing mice revealed that ²²⁵Ac could not be efficiently pretargeted with current DOTA-Bn hapten utilized for ¹⁷⁷Lu or ⁹⁰Y, leading to poor tumor uptake in vivo. Therefore, we synthesized Pr consisting of an empty DOTA-chelate for ²²⁵Ac, tethered via a short polyethylene glycol linker to a lutetium-complexed DOTA for picomolar anti-DOTA chelate single-chain variable fragment (scFv) binding. Pr was radiolabeled with ²²⁵Ac and its imaging surrogate, ¹¹¹In. In vitro studies verified anti-DOTA scFv recognition of [²²⁵Ac]Pr, and in vivo biodistribution and clearance studies were performed to evaluate hapten suitability and in vivo targeting efficiency. Results: Intravenously (i.v.) administered ²²⁵Ac- or ¹¹¹In-radiolabeled Pr in mice showed rapid renal clearance and minimal normal tissue retention. In vivo pretargeting studies show high tumor accumulation of Pr (16.71 ± 5.11 %IA/g or 13.19 ± 3.88 %IA/g at 24 h p.i. for [²²⁵Ac]Pr and [¹¹¹In]Pr, respectively) and relatively low uptake in normal tissues (all average ≤ 1.4 %IA/g at 24 h p.i.). Maximum tolerated dose (MTD) was not reached for either [²²⁵Ac]Pr alone or pretargeted [²²⁵Ac]Pr at administered activities up to 296 kBq/mouse. Single-cycle treatment consisting of α-DOTA-PRIT with either huA33-C825 bispecific anti-tumor/anti-DOTA-hapten antibody (BsAb), anti-HER2-C825 BsAb, or hu3F8-C825 BsAb for targeting GPA33, HER2, or GD2, respectively, was highly effective. In the GPA33 model, no complete responses (CRs) were observed but prolonged overall survival of treated animals was 42 d for α-DOTA-PRIT vs. 25 d for [²²⁵Ac]Pr only (P < 0.0001); for GD2, CRs (7/7, 100%) and histologic cures (4/7, 57%); and for HER2, CRs (7/19, 37%) and histologic cures (10/19, 56%) with no acute or chronic toxicity. Conclusions: [²²⁵Ac]Pr and its imaging biomarker [¹¹¹In]Pr demonstrate optimal radiopharmacologic behavior for theranostic applications of α-DOTA-PRIT. For this initial evaluation of efficacy and toxicity, single-cycle treatment regimens were performed in all three systems. Histologic toxicity was not observed, so MTD was not observed. Prolonged overall survival, CRs, and histologic cures were observed in treated animals. In comparison to RIT with anti-tumor IgG antibodies, [²²⁵Ac]Pr has a much improved safety profile. Ultimately, these data will be used to guide clinical development of toxicity and efficacy studies of [²²⁵Ac]Pr, with the goal of delivering massive lethal doses of radiation to achieve a high probability of cure without toxicity.