Radiolabeled methotrexate as a diagnostic agent of inflammatory target sites: A proof-of-concept study

Methotrexate (MTX), as a pharmaceutical, is frequently used in tumor chemotherapy and is also a part of the established treatment of a number of autoimmune inflammatory disorders. Radiolabeled MTX has been studied as a tumor‑diagnostic agent in a number of published studies. In the present study, the potential use of technetium‑99m‑labelled MTX (99mTc‑MTX) as a radiotracer was investigated for the identification of inflammatory target sites. The labelling of MTX was carried out via a 99mTc‑gluconate precursor. Evaluation studies included in vitro stability, plasma protein binding assessment, partition‑coefficient estimation, in vivo scintigraphic imaging and ex vivo animal experiments in an animal inflammation model. MTX was successfully labelled with 99mTc, with a radiochemical purity of >95%. Stability was assessed in plasma, where it remained intact up to 85% at 4 h post‑incubation, while protein binding of the radiotracer was observed to be ~50% at 4 h. These preclinical ex vivo and in vivo studies indicated that 99mTc‑MTX accumulates in inflamed tissue, as well as in the spinal cord, joints and bones; all areas with relatively high remodeling activity. The results are promising, and set the stage for further work on the development and application of 99mTc‑MTX as a radiotracer for inflammation associated with rheumatoid arthritis.