Iodine-131-labeled diphosphonates for the palliative treatment of bone metastases: I. Organ distribution and kinetics of I-131 BDP3 in rats.

I-131-labeled alpha-amino-(4-hydroxybenzylidene)-diphosphonate (BDP3) was examined in Sprague-Dawley rats for its bone affinity and other biokinetical characteristics. Iodine-131 BDP3 exhibited high bone affinity, with rapid blood clearance and renal excretion. Total-body retention measurements revealed an effective half-life of the activity of t1/2 = 169 hr. Similar kinetics were found in rat femurs, with t1/2 = 158 hr. Rats bearing osteosarcomas showed an enhanced uptake with a slower clearance (t1/2 = 177 hr). The amount of I-131 BDP3 excreted in urine agreed with the total body retention values. Fifty percent was excreted in the urine, as unchanged I-131 BDP3, by 24 hr after application. Almost no activity was found in the feces of the rats. LD50 measurements in rats amounted to 64 mg/kg. Because of these biokinetical characteristics and the simple labeling procedure, I-131 BDP3 is suggested as a radiopharmaceutical for the palliative treatment of pain that often accompanies disseminated bone metastases.