The effect of formulation on radioiodide thyroid uptake in the hyperthyroid cat

This investigation was designed to compare in vitro dissolution profiles fr om sodium iodide capsules with radioiodide thyroid uptake in hyperthyroid c ats using sodium iodide capsules prepared with a formulation exhibiting a c omplete release of radioiodide (I-123) in vitro and a formulation with an i ncomplete release of radioiodide. In vitro dissolution profiles for I-123 s odium iodide capsules with two different formulations were determined using the USP XXIII dissolution test. The two formulations studied in vitro were sodium phosphate dibasic powder with 1% magnesium stearate and calcium pho sphate dibasic powder with 3% magnesium stearate. By 20 min after initiatio n of the dissolution test, over 95% of the I-123 was released from capsules of sodium phosphate dibasic powder. The capsules of calcium phosphate diba sic powder reached 7.5% at 65 min, with no further release occurring therea fter. There was a statistically significant difference in the dissolution p rofiles of the two formulations. The thyroid uptake of I-123 from capsules exhibiting complete release and incomplete release of radioiodide was deter mined in hyper thyroid cats. At 4 hr, the mean percentage thyroid uptake va lue for sodium phosphate dibasic powder with 1% magnesium stearate (complet e release formulation) was 12.0% compared to 9.4% for calcium phosphate dib asic powder with 3% magnesium stearate (incomplete release formulation); at 24 hr; the values were 34.4% compared to 23.7%. The data suggest that the incomplete dissolution profile observed in vitro may correlate with a reduc tion ia the bioavailability of the radioiodide in vivo. However, using the Wilcoxon signed rank test, statistically significant differences did not oc cur between the complete release formulation and incomplete release formula tion at either 4 hr or 24 hr (p >.05). The results of the in vivo study wit h five hyperthyroid cats were not conclusive due to the variability in resp onse between individual cats.