Evaluation of the Anti Glypican 1 Chimeric Antibody Miltuximab for Delivery of Radioisotopes in Cancer Treatment

Objectives: Monoclonal antibody-based immunotherapeutics are a promising class of targeted anti-cancer drugs due to their high target specificity. These characteristics also make them a powerful platform for the delivery of radioactive metals to tumors for imaging and treatment. Miltuximab (Mil) is an antibody specific to the extracellular matrix protein glypican 1 (GPC1) that is specifically upregulated in several cancers, including prostate, esophageal, and pancreatic cancers [1,2]. Our aim was to prepare two series of antibody-chelator constructs using Mil and the metal chelators S-2-(4-isothiocyanatobenzyl)-1,4,7,10-tetraazacyclododecane tetraacetic acid (DOTA) or a bifunctional ethylphenylisothiocyanate derivative of 6,6’-((1,4,10,13-tetraoxy-7,16-diazacyclooctadecane-7,16-diyl)bis(methylene))dipicolinic acid (MPA) for complexation with radioisotopes ¹⁷⁷Lu or ²²⁵Ac, respectively Methods: The Mil-DOTA and Mil-MPA series of conjugates were prepared by addition of 3, 6, 12, or 22 molar equivalents of the bifunctional DOTA or MPA to aqueous solutions of Mil at room temperature. The number of chelating moieties bound per antibody was determined via MALDI-ToF mass spectrometry and SDS-PAGE. Unconjugated chelating moieties were removed via 15 kDa molecular weight cutoff filters and spin filtration. The Mil-DOTA constructs were radiolabeled with ¹⁷⁷Lu by addition of 1.0-1.1 mCi ¹⁷⁷LuCl₃ in 0.1 M NH₄OAc followed by incubation for 1 h at 45 ^oC. The Mil-MPA constructs were radiolabeled with 8-10 μCi ²²⁵Ac at room temperature in 0.1 M NH₄OAc for ≥ 5 min. Radiolabeling yields were determined by radio-TLC. The immunoreactivity of the Mil-DOTA constructs was assessed in GPC1-expressing DU145 cells using the protocol established by Lindmo et al. [3]. Internalization of select Mil-(DOTA-¹⁷⁷Lu) conjugates was determined at 37 °C in DU145 cells. Results: The conjugation of DOTA and MPA to Mil was successful for all of the molar ratios tested. In general, 1 chelate was bound per antibody for every 2 equivalents of chelate added. Radiolabeling yields of the Mil-DOTA with ¹⁷⁷Lu were ≥ 97 % for all of the conjugates, and all Mil-MPA conjugates were radiolabeled with ²²⁵Ac in ≥ 95% yield within 15 min. All radiolabeled constructs were stable to radiolysis in solution out to 48 h. The immunoreactivity of the Mil-DOTA constructs against DU145 cells ranged from 94.1 ± 1.4 % for the Mil:DOTA = 1:1 construct to 83.3 ± 1.5 % for the 1:13 ratio construct. 60.1 ± 1.9 % of the 1:3 Mil-(DOTA-¹⁷⁷Lu) was internalized in DU145 cells after 45 min at 37 ^oC. Internalization of the 1:13 Mil-(DOTA-¹⁷⁷Lu) construct was 44.7 ± 1.8 %. Conclusions: The GPC-1-targeting antibody Mil can be conjugated with DOTA or MPA at a number of molar ratios and radiolabeled in ≥ 95% yield with either ¹⁷⁷Lu or ²²⁵Ac, respectively. The radiolabeled constructs are stable over 48 h. In addition, the Mil-DOTA conjugates retain > 80% immunoreactivity, which is consistent with other antibody-based constructs currently in clinical trials [4]. These properties indicate great potential for Mil constructs as agents for imaging and treating cancer. Further studies will evaluate the efficacy of these constructs to image and kill xenograft models of prostate cancer. References:[1] Campbell, D. H, et al. Detection of Glypican-1 (GPC-1) Expression in Urine Cell Sediments in Prostate Cancer. PLoS One 2018, 13 (4). [2] Wang, S, et al. The Expression, Regulation, and Biomarker Potential of Glypican-1 in Cancer. Front. Oncol. 2019, 9. [3] Lindmo, T, et al. A. Determination of the Immunoreactive Function of Radiolabeled Monoclonal Antibodies by Linear Extrapolation to Binding at Infinite Antigen Excess. Journal of Immunological Methods 1984, 72 (1), 77-89. [4] Tagawa, S. T, et al. Phase II Study of Lutetium-177-Labeled Anti-Prostate-Specific Membrane Antigen Monoclonal Antibody J591 for Metastatic Castration-Resistant Prostate Cancer. Clin Cancer Res 2013, 19 (18), 5182-5191.