Determination of the ability of 111In Onalta (edotreotide) to be used as an imaging surrogate for the accurate prediction of 90Y Onalta radiation absorbed dose

1411 Objectives: The development of an accurate predictor of beta emitter peptidomimetic radiotherapeutics is an unmet need due to retention in the renal proximal proximal tubules. Methods: 111In Onalta (O) 100 µCi/kg or 90Y O 300 µCi/kg, with O concentration held constant, was administered iv to 4 rabbits per group. At 0.25, 2, 24, 48 and 72 hr pi animals were euthanized and heart, blood, liver, lungs, spleen, large and small intestine, stomach, kidneys, skeletal muscle, adipose tissue, marrow and bone excised and assessed for radioactivity (Bremsstrahlung for 90Y). Radiation absorbed dose and extrapolation into man was performed using OLINDA. Results: Distribution patterns of 111In O and 90Y O were virtually identical in all tissues except for the kidney where significant retention was observed. The maximum kidney levels were 0.24% & 0.23 %ID/g for 111In O and 90Y O. The slopes of the terminal elimination phase were parallel with 111In O levels being consistently greater than 90Y O. 111In O predicatively overestimated 90Y O kidney dose by 12%. Extrapolation of the radiation absorbed dose in man showed a similar pattern where the kidney was 0.8 mSv/MBq for 111In O and 0.6 mSv/MBq for 90Y O resulting in 111In O over estimating the 90Y O radiation absorbed dose by 37%. Conclusions: Tissue distributions of 111In O and 90Y O were virtually identical for all organs except the kidney where 111In O consistently overestimated 90Y O. The systematic overestimation by 111In O of kidney exposure and the comparability of the exposure of all other organs supports the use of 111In O as a surrogate for 90Y O.