DOSIMETRY OF IODINE-123-EPIDEPRIDE - A DOPAMINE D2 RECEPTOR-LIGAND

Substituted benzamides have been shown to have very high affinity and specificity for the dopamine D2 receptor. One of these is radiolabeled epidepride, an iodine-substituted benzamide currently under evaluatio n as a SPECT imaging agent. Detailed estimates of the radiation absorb ed dose to 26 organs and the whole body from [I-123]epidepride have be en calculated. Methods: The dosimetry calculations use a combination o f in vivo uptake and biodistribution data from one rhesus monkey and s even humans to estimate residence times in eight organs. The computer program MIRDOSE2 was used to calculate the dosimetry. Results: Results indicate that 75% of the radioactivity is cleared through the urinary tract while the remaining radioactivity clears through the gallbladde r and intestinal tract. The radiation absorbed dose can be minimized b y administering a high lipid content meal 1.5 hr postinjection to empt y the gallbladder and by giving large volumes of fluids throughout the study to induce increased urinary output. Conclusion: By emptying the gallbladder and urinary bladder, the lower large intestine becomes th e critical organ, 0.102 mGy/MBq (0.38 rad/mCi) followed by the upper l arge intestine, 0.092 mGy/MBq (0.34 rad/mCi). The effective dose equiv alent is 0.025 mSv/MBq (0.092 rem/mCi).