^99m technetium-Tocilizumab Fragments As Molecular Imaging Agent for Multiple Myeloma

Introduction: Multiple myeloma (MM) is a neoplasm of B lymphoid line that is characterized by clonal proliferation of malignant plasma cells in the bone marrow, producing monoclonal paraprotein (M) in blood and/or serum. Interleukin-6 (IL-6) is one of the key molecules related to growth, survival and proliferation of MM cells. Tocilizumab (TCZ) is a humanized monoclonal antibody directed against IL-6 receptor (IL-6R). When radiolabeled and used for tumor imaging, intact IgG exhibits high liver uptake. Antibody fragments (Fab´s) are quickly eliminated from blood and normal tissues (except kidneys), achieving high tumor-to-blood and tumor-to-normal tissue ratios with renal clearance. The aim of our work was to develop a ^99mTc radiolabeled TCZ Fab´s fragment and to perform its chemical and biological evaluation in order to be used as a potential MM imaging agent for staging and restaging.

Methods: Antibody fragmentation was carried out with papain and, once purified, Fab´s(TCZ) fragments were identified and derivatized with NHS-HYNIC-Tfa as bifunctional coupling agent. MALDITOF/TOF was used to confirm all procedures. A mixture of Tricine/SnCl₂.2H₂O was added to Fab´s(TCZ)-Tfa-HYNIC and radiolabeled with ^99mTcO₄^-. Radiochemical purity and in-vitro stability in saline, serum and different concentration of L-cysteine up to 4 h were analyzed by ITLC and HPLC. In-vitro binding assays were performed using U266 and MM1S cell lines up to 120 min. Biodistribution and SPECT/CT images were evaluated on healthy Balb/c mice and MM1S tumor-bearing Balb/c nude mice at 0.5, 2 and 4 h.

Results: Radiolabeling of HYNIC-Tfa-Fab´s(TCZ) was carried out in a fast, reproducible, easy, stable way showing high radiochemical purity and high specific activity. In vitro binding assays confirm that after its derivatization and radiolabeleing, Tfa-HYNIC-Fab`s(TCZ) does not interfere with the epitope recognition. In vivo biodistribution studies on healthy Balb/c mice and MM1S tumor-bearing Balb/c mice showed that ^99mTc-HYNIC-Fab´s (TCZ) has significant renal uptake with neglectable uptake in other organs, indicating renal clearance. Tumor uptake was 12.84±1.80 %ID/g followed by 8.94±0.61 %ID/g and 3.05±1.49 %ID/g at 2 and 4 h, respectively. U266 tumor-to-muscle ratios were 5.79, 8.61 and 2.71 at 0.5, 2 and 4 h, respectively.Tumor uptake for MM1S tumor-bearing Balb/c nude mice was 10.05±1.32 %ID/g, 8.59±2.36 %ID/g and 3.88±0.68 %ID/g at 0.5, 2 and 4 h, respectively. MM1S tumor-to-muscle ratios were 6.32, 4.61 and 3.08 at 0.5, 2 and 4 h, respectively. Biodistribution data of ^99mTc-HYNIC-Fab´s(TCZ) on U266 tumor-bearing Balb/c nude mice showed good tumor uptake and retention 0.5 h after its injection SPECT/CT images on healthy Balb/c mice and MM1S tumor-bearing Bal/c nude mice of ^99mTc-HYNIC-Fab´s(TCZ) showed renal uptake and a discrete tumor uptake at 4 h p.i (Figure 1).

Conclusions: Labeling Fab´s(TCZ) with ^99mTc using HYNIC was performed in an easy, fast, stable and reproducible way preserving its biological activity. Biodistribution and SPECT/CT imaging assays allowed us to observe and evaluate its potential role as a diagnostic molecular imaging agent for MM.