Retention and biodistribution of Sn-117m colloid in intra-articular injections

Objectives Radiosynoviorthesis (a.k.a. radiosynovectomy) employing the theranostic isotope, Sn-117m (t½ 14d, γ 159 keV, 86%), is well suited to treat smaller arthritic joints since the therapeutic conversion electrons (~140 keV, 112%) have a range of about 300 µm. This allows for treatment of the synovium without affecting the surrounding tissues. Retention of the colloidal product in the synovial cavity is highly desirable. We describe the joint retention and biodistribution trial measurements of Sn-117m colloid injected into normal canine elbows.

Methods Five normal purpose-bred hounds were injected with ~2.5 mCi of a homogeneous preparation of Sn-117m colloid (~6 μm diameter) in the intra-articular space of the left elbow. The animals were sacrificed after six weeks (3 half-lives) and the amount of radioactivity in the dissected tissues was measured. Urine, blood, feces and synovial fluid samples were taken periodically during the trial and analyzed for radioactive content.

Results Average joint retention of the colloid was 99.1% of the administered dose at three-half-lives. When integrated over the study period the activity in blood, urine and feces was <0.1%. The majority of the extra-synovial activity was found in the liver (<0.7%) and spleen (<0.1%). Activity also was detected in the left prescapular lymph node (<0.03%). Some of the leakage might be attributed to the multiple synovial fluid taps performed over the duration of the trial.

Conclusions These results demonstrate a high degree of retention of the Sn-117m homogeneous colloid delivered intra-articularly. Other than minimal transient leakages possibly due to synovial punctures the results are consistent with a gradual removal of the colloid through the lymphatic system. The high joint retention supports the use of Sn-117m colloid in treating arthritis.