In-vivo evaluation of Lu-177 hydroxyapatite for application in radiation synovectomy of digital joints

Objectives Coupling of Lutetium-177 (Lu-177) with biomolecules of hydroxyapatite (HA) labeled is being supported by many researchers for the use in radiation synovectomy of small synovial joints. The present work describes the bio-evaluation of locally prepared Lu-177 labeled hydroxyapatite particles in small animals.

Methods Natural Lu2O3 target was irradiated at a thermal flux ~8.0×10^13 n/cm^2/s for 12 h to yield ~25.3 mCi/mg of Lu-177. HA particles were synthesized and characterized by FTIR-ATR and X-ray diffractometry. Ca/P ratio was determined by ICP-OES. For animal study 5-10 MBq of Lu-177 HA in 0.1 mL solution was injected into knee joint spaces of two healthy rabbits. Sequential images at 11 time points were acquired over a period of 30 days. They were analyzed visually for any extra-articular activity in the torso. For quantitative analysis counts within appropriate ROIs over the knees were plotted on a semi-log with exponential line-fit to show the disappearance of Lu-177 HA activity from the synovial cavity and compared it with physical decay of Lu-177.

Results The chief inorganic phase on XRD pattern was hydroxyapatite crystal. The Ca/P ratio was 1.68. High radiochemical purity and yield (98%) of Lu-177 HA particles was achieved at pH 7. No extra-articular activity was seen in any image even after masking the injected joint. The half lives of exponential line-fit of knee counts were 154 hours and 158 hours with linear coefficients (r) 0.998 and 0.997 for rabbit 1 and 2 respectively. Minimal downward divergence from the theoretical decay of Lu-177 (T1/2 = 161.5 hours) was seen in both the rabbits. The biological half of HA in the knees calculated from effective half-life (average i.e., 156 hours) and physical half-life was 190 days.

Conclusions Lu-177 HA has adequate biological characteristics for the application of radiation synovectomy of small joints