Stability of Yttrium-90-citrate-, Erbium-169-citrate- and Rhenium-186-sulfur-colloid in vitro

Aim: Yttrium-90-citrate- (YMM-1), Erbium-169-citrate- (ERMM-1), and Rhenium-186-sulfur-colloid (Re-186-MM1) were applied in radiosynovectomy (RSO), a local intraarticular injection of radionuclides in colloidal form for radiotherapy. The local metabolism of the colloidal particles is not fully understood; however, knowledge about the leakage is important for risk assessment. Therefore we investigated stability of the radiocolloids by by equilibrium dialysis, which provided information about interaction of the colloids with buffer components, synovia in controlled equilibrium conditions.

Materials & Methods: Carrier added YMM-1, ERMM-1, Re-186-MM1 colloids were dispersed and dialyzed against synovia, different electrolytes and buffers separated by dialysis membranes with a molecular weight cut off (MWCO) of 10,000 Dalton. The activity concentration in the dialysis compartment was studied up to 24 hours. Tl-201-chloride, F-18-FDG, F-18-fluoride, and Er-169-chloride were used as reference tracers.

Results: The equilibria were reached at the latest 10 hours after start of dialysis. The ionic Er-169-chloride (as negative control) and Re-186-perrhenate showed very similar diffusion kinetics compared with reference radiotracers. The colloidal radionuclides, incubated and dialyzed against electrolytes comparable with plasma (MEM Dulbeco medium), human synovia, or phosphate containing buffers were not detected in the dialysis solution (lower 0.5% of applied dose (%ID)), except Re-186-MM1. Significant amounts of compounds were found in the dialysis compartment. Incubation of the colloids with isotonic sodium chloride or 0.1 M hydrochloric acid was followed by an activity release into the dialysis solution YMM-1 (67%ID) and ERMM-1 (62%ID).

Conclusion: The increased stabilities of the carrier added radiocolloids studied in different electrolyte solutions was mainly a result of the presence of phosphate ions. The low solubility of the phosphates of rare earth elements, the binding to synovia proteins, and hydroxylation were the main mechanisms of colloidal particle stabilization or reformation of free or secondary formed Erbium or Yttrium ions. This generally results in formation of particles of low diffusibility, increasing the retention of the radiocolloids inside the joint before the radionuclide-loaded colloidal particles are phagocytozed by macrophages in the inflamed synovial membrane.