Pretargeted alpha radioimmunotherapy (DOTA-PRIT) for HER2-expressing human gastric cancer: a theranostic approach using 111In/225Ac-radiohapten.

288 Objectives: We have reported CRs and cures using anti-HER2/anti-DOTA hapten bispecific pretargeted radioimmunotherapy (anti-HER2-DOTA-PRIT) approach for treatment of HER2-expressing breast cancer using beta- or alpha-emitting isotopes. As part of this work, we documented identical pharmacodynamics of the theranostic pairs: In-111- versus Ac-225 - radiohapten chelates (proteus DOTA, PrDOTA; Mol Imaging Biol 2019 21: 167-335). Herein we report on an extension of our studies to a second HER2-expressing solid tumor using a NCI-N87 gastric cancer mouse model. Methods: Initial biodistribution (using In-111) and therapy studies (using Ac-225) were performed in groups of nude mice bearing subcutaneous (s.c.) HER2-positive (HER2(+)) NCI-N87 human gastric carcinoma xenografts (5 x 106 cells/mouse; treatment initiated on day 14 post-inoculation). The anti-HER2-DOTA-PRIT targeting regimen was administered intravenously (i.v.): 0.25 mg (1.19 nmol) of anti-HER2-C825 at t = -28 hours (h), 25 µg of CCA16-DOTAY clearing agent (2.76 nmol) at t = -4 h, and [111In]PrDOTA or [225Ac]PrDOTA at t = 0 h. A biodistribution study was performed at 24 h post-injection of [111In]PrDOTA (n = 5, 24-26 µCi (888-962 kBq), 120-130 pmol). A separate preliminary therapy study with one or two cycles of [225Ac]PrDOTA (0.74-0.79 nmol, 1 µCi (37 kBq)) was performed with appropriate controls (no treatment, anti-HER2-C825 only, non-specific-DOTA-PRIT with anti-GPA33 huA33-C825 in place of anti-HER2-C825). Survival was characterized using Kaplan-Meier curves, and comparison was performed using the log-rank test (significance: P Results: Biodistribution of the therapeutic surrogate pretargeted 111In-radiolabeled proteus-DOTA (as [111In]PrDOTA to mimic [225Ac]PrDOTA) at 24 h post-injection of 111In-activity showed tumor, blood, and kidney uptakes of 9.32 ± 1.25 % injected activity per gram (%IA/g), 1.14 ± 0.33 %IA/g, and 0.90 ± 0.13 %IA/g, respectively. With regard to alpha-DOTA-PRIT, all treatments were well tolerated, with noweight loss >10%. Anti-HER2-DOTA-PRIT was highly efficacious in regard to shrinkage of tumor and extended survival compared with controls. Except for 3/10 non-specific-DOTA-PRIT, all controls died by 86 d of tumor progression. In contrast, in the treated group, as of 86 d post treatment-initiation, 9/10 mice for each group treated with either one or two cycles of anti-HER2-DOTA-PRIT were still alive, with tumor volumes of 444 ± 170 mm3 and 154 ± 93 mm3 for single-cycle and double-cycle treatment, respectively (significance for greater effectiveness of single vs. double cycle, P = 0.0002). No complete responses (CR; tumor volume ≤10 mm3) were observed in controls or single-cycle anti-HER2-DOTA-PRIT, and 3/10 CRs were observed in mice treated with double-cycle anti-HER2-DOTA-PRIT. Furthermore, median survival (MS; in days (d) after treatment initiation, defined as time to tumor progression to a volume of 1000 mm3 or death for any reason) was 30 d, 36 d, and 56 d for no treatment, non-radioactive anti-HER2-C825 only, and non-specific-DOTA-PRIT controls, respectively. Survivors will be evaluated for tumor status and long-term toxicity at 20 weeks. Conclusions: Alpha-DOTA-PRIT targeting the HER2 antigen, and based on Indium-111 surrogate radiohapten, achieves substantial % injected activity per gram in tumors, and is well-tolerated and effective for treatment of HER2(+) human gastric cancer mouse models, leading to CRs and significantly prolonged survival (P ≤ 0.0043 for all controls versus single-cycle anti-HER2-DOTA-PRIT). In the treated group, no deaths were observed from toxicity, and dose escalation studies continue with the goal of achieving 100% CR’s and histologic cures in preclinical models of gastric cancer.