Dosimetric evaluation for radioimmunotherapy of Cu-64-2IT-BAT-LYM-1 radiocontaminant in Cu-67-2IT-BAT-LYM-1 radiopharmaceutical

Cu-67 is considered to be an attractive radionuclide for radioimmunotherapy for cancer because of its physical and biological characteristics. However Cu-67 that is currently available has a radiocontaminant, Cu-64. The activity ratio of Cu-64 to Cu-67 can be as great as 0.25 at the time of administration to the patient. By assuming identical pharmacokinetics for Cu-64 and Cu-67, the biodistribution of Cu-64 in a patient can be assessed using quantitative imaging data for Cu-67, if the activity ratio of Cu-64 to Cu-67 is known. A Cu-67 source contaminated with Cu-64 was used to evaluate the impact of Cu-64 on camera sensitivity, and on activity quantitation for organs and tumors using an Alderson phantom that simulated the abdomen of patients. Quantitative imaging of the Cu-67 source contaminated with Cu-64 indicated that the effect of Cu-64 photons scattered into the Cu-67 energy window set was negligible even if the activity ratio of Cu-64 to Cu-67 was 0.25. MIRD formalism was used to estimate the therapeutic index based on the ratio of radiation dose to the tumor and that to the marrow. The marrow dose from Cu-64 was significantly less than that of Cu-67. However, the tumor radiation dose from Cu-67 per unitmore » of injected activity was about four times greater than that from Cu-64 (160 vs 40 cGy/kBq). For Cu-64 contamination as great as 0.25 for the activity ratio of Cu-64 to Cu-67, the therapeutic index (tumor to marrow radiation doses) was changed from 26.4 for the Cu-67 to 25.9 for the mixture. When pure Cu-64-2IT-BAT-Lym-1 and Cu-67-2IT-BAT-Lym-1 were considered, the therapeutic index of Cu-67 was about 20% higher than that of Cu-64 (20.5). When the biological behavior of the radiocontaminant is identical to that of the intended radionuclide, then the therapeutic index should be used to determine the optimal time for injection of the radiopharmaceutical mixture.« less