Development of 99mTcHYNIC-Tocilizumab as Imaging Agent In Multiple Myeloma

Multiple myeloma (MM) accounts for 10% of hematological malignancies and 1% of cancer. It represents the second most common hematologic malignancy. The disease is characterised by plasma cell infiltration of the bone marrow, osteolytic bone lesions and the presence of a monoclonal paraprotein in the serum or urine. Interleukin-6 (IL-6) is a key molecule in the pathogenesis of MM. Its secretion occurs from the tumoral plasma cell as well as from the microenvironment. This results in myeloma cell proliferation. Tocilizumab, a humanized anti IL-6 receptor monoclonal antibody that blocks IL-6 cell-to-cell signaling, is currently being studied in MM. Most MM patients experience bone lesions. In spite of the new imaging techniques, the distinction between malignant and benign lesions remains sometimes difficult in clinical practice. The development of a more specific imaging agent may improve the accuracy of this diagnosis.

Development of 99mTcHYNIC-Tocilizumab conjugate. Chemical and biological evaluation as a potential imaging agent in multiple myeloma.

10 mg Tocilizumab (Roche) were added to 20 mg HYNIC in NaHCO3 1M, incubated for 30 min at room temperature and purified by means of PD10 column. 1 ml fractions were measured at 280nm. Conjugation of HYNIC-Tocilizumab was evaluated by mass spectrometry MALDITOF and acrylamide gel electrophoresis. 20 uL Tricine/stannous chloride in a 1:1 mass relation were added to 1 mg HYNIC-Tocilizumab in 100 uL of 0.9% NaCl. 2 mL of a solution containing 370 MBq ^99mTcO₄^- was added. Radiochemical purity was controlled by three chromatographic systems: ITLC/NaCl 0.9%, Whatman 1MM/MEK, ITLC in BSA/EtOH-NH3-H2O (2:1:5) as stationary and mobile phase, respectively. Binding to IL-6 receptor in U266 myeloma cells was evaluated in vitro at 15, 30, 60 and 120 min.

Tocilizumab was derivatized with FTIC and purified by PD10 column. Laser scanning confocal microscopy was done with an excitation/emission wavelength of 488/530 nm. Fluorescent images were obtained. Biodistribution studies were performed at 24 h in CD1 normal mice (n=3). Each mice was injected with 37 MBq of 99mTcHYNIC-Tocilizumab and sacrificed 24 hs after injection. Organs of interest were collected.

Mass spectroscopy MALDITOF (MH+Tocilizumab 147626 Da, M+HYNIC-Tocilizumab 148375 Da) confirmed derivatization of the antibody. This represents 5 molecules of HYNIC per molecule of the antibody. Poliacrylamide gel electrophoresis confirmed that the antibody retained its integrity after derivatization. Radiochemical purity was 88±3 %. In vitro cell binding assays confirmed the binding ability of the radiolabeled antibody to IL-6 receptor. Specificity of binding was supported by competition experiments using unlabelled antibody. Confocal microscopy proved the ability of the fluorescent antibody to recognize the IL-6 receptor in U266 myeloma cells. Biodistribution at 24 h showed blood (8.0±2.0) %act/g, liver (11±1.5) %act/g, kidney (4.0±1.0)%act/g and spleen (6±1.4) %act/g uptake with hepatic and renal elimination. Animal models of MM are being evaluated using xenografted MM cells.

99mTcHYNIC-Tocilizumab labeling was performed. The labeled antibody retained adequate biochemical and biological properties, which included maintenance of specific binding to IL-6 receptor. These findings indicate that this radiolabeled antibody could be used as an imaging agent in multiple myeloma.

OIEA, Laboratorios Roche, Dr Gustavo Arroyo.

No relevant conflicts of interest to declare.