Background: Multiple Myeloma (MM) is a malignant hematologic disorder and the second most
common blood cancer. Interleukin-6 (IL-6) has been identified as a crucial factor for the proliferation and
survival of MM cells and the overexpression of IL-6 receptor is being studied as a molecular target for
therapeutic and diagnostic use in myelomas and other comorbidities. Tocilizumab is a humanized monoclonal
antibody that binds IL-6R.
Objective: We aim to label and evaluate Fab(Tocilizumab) with 99mTechnetium or Cy7 as potential MM imaging
Methods: IL-6R distribution was analyzed by Laser Confocal Microscopy (LCM) in MM cell lines.
Fab(Tocilizumab) was produced by the digestion of Tocilizumab with papain for 24h at 37°C, derivatized with
NHS-HYNIC-Tfa and radiolabeled with 99mTc. Radiochemical stability and in vitro cell assays were evaluated.
Biodistribution and SPECT/CT were performed. Also, Fab(Tocilizumab) was labeled with Cy7 for in vivo
fluorescence imaging up to 72h.
Results: LCM analysis demonstrates IL-6R distribution on MM cell lines. Incubation with papain resulted in
complete digestion of Tocilizumab and exhibited a good purity and homogeneity. Radiolabeling with 99mTc via
NHS-HYNIC-Tfa was found to be fast, easy, reproducible and stable, revealing high radiochemical purity and
without interfering with IL-6R recognition. Biodistribution and SPECT/CT studies showed a quick blood
clearance and significant kidney and MM engrafted tumor uptake. Cy7-Fab(Tocilizumab) fluorescent imaging
allowed MM1S tumor identification up to 72h p.i.
Conclusion: These new molecular imaging agents could potentially be used in the clinical setting for staging and
follow-up of MM through radioactive whole-body IL-6R expression visualization in vivo. The fluorescent
version could be used for tissue sample evaluation and to guide surgical excision, if necessary.