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

Abstract

This is the initial report of an α-based pre-targeted radioimmunotherapy (PRIT) using 225 Ac and its theranostic pair, 111 In. 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 225 Ac could not be efficiently pretargeted with current DOTA-Bn hapten utilized for 177 Lu or 90 Y, leading to poor tumor uptake in vivo . Therefore, we synthesized Pr consisting of an empty DOTA-chelate for 225 Ac, 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 225 Ac and its imaging surrogate, 111 In. In vitro studies verified anti-DOTA scFv recognition of [ 225 Ac]Pr, and in vivo biodistribution and clearance studies were performed to evaluate hapten suitability and in vivo targeting efficiency. Results: Intravenously (i.v.) administered 225 Ac- or 111 In-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 [ 225 Ac]Pr and [ 111 In]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 [ 225 Ac]Pr alone or pretargeted [ 225 Ac]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 [ 225 Ac]Pr only ( P < 0.0001); for GD2, CRs (7/7, 100%) and histologic cures (4/7, 57%); and for HER2, CRs (7/19, ...

Engineered Cells as a Test Platform for Radiohaptens in Pretargeted Imaging and Radioimmunotherapy Applications

“…Recently, our group has developed a new theranostic pair of DOTA-haptens, the designated “Proteus” (Pr) haptens [ 111 In]­In-Pr/[ 225 Ac]­Ac-Pr, in addition to the already established theranostic pair based on benzyl-DOTA (DOTA-Bn) [ 86 Y]­Y-DOTA-Bn/[ 177 Lu]­Lu-DOTA-Bn . Various other probes are in development.…”

Pretargeted Theranostics

“…While the hapten DOTA-Bn allows for straight forward incorporation of yttrium and lutetium, it comes with severe limitations to the modularity of the system as the affinity of the hapten towards scFv C825 varies depending on the chelated metal. This effect was observed in a study on the anti-DOTA antibody scFv -hu3F8-C825 (Figure 5), which bound [ 177 Lu]Lu-DOTA-Bn with picomolar affinity, whereas [ 225 Ac]Ac-DOTA-Bn was found to have severely decreased binding [38]. This resulted in a decreased tumor accumulation.…”

Pretargeting: A Path Forward for Radioimmunotherapy

“…Between 2014 and 2018, Cheal et al demonstrated DOTA-PRIT with 177 Lu-DOTA-hapten targeting a wide variety of solid tumors (antigen targets: GD2 (120), glycoprotein A33 (121), HER2 (122)). More recently, that team developed a 225 Ac-DOTA-hapten for a-PRIT (123). When DOTA-PRIT was used to pretarget 177 Lu-DOTA-hapten in nude mice bearing human xenografts, TIs of 28-142 for blood, 7-23 for kidney, and 12-47 for liver were achieved, with estimated tumor-absorbed doses ranging from 1,476 to 8,473 cGy/37 MBq (120,122,124).…”