The retention and distribution in the rabbit knee of a radionuclide complexed with a lipophilic chelator in liposomes.

The application of radiosynovectomy to patients with rheumatoid arthritis has been severely restricted by the difficulty of preventing leakage of the radioisotope from the joint cavity. We have synthesised a lipophilic chelator, 3-cholesteryl 6-[N'-iminobis(ethylenenitrilo)-tetraacetic acid]hexyl ether (Chol-DTTA) which can complex with a variety of beta-emitting radionuclides and is incorporated into the lipid phase of liposomes. The retention in the synovial cavities of rabbit knees of liposomes containing Chol-DTTA, complexed with the gamma-emitting tracer 51Cr, has been measured over a period of 21 days and compared with colloidal and water-soluble preparations. The distribution of the radionuclide between the tissues of the joint was also examined. Results show retention of 51Cr delivered in chelator liposomes to be greater than 99% after 24 h. At this time, over 93% of the radioactivity had become associated with the synovium. We conclude that chelator liposomes offer considerable promise as vehicles for radioisotopes in radiosynovectomy.