DOSIMETRY OF RHENIUM-186-LABELED MONOCLONAL-ANTIBODIES - METHODS, PREDICTION FROM TC-99M-LABELED ANTIBODIES AND RESULTS OF PHASE-I TRIALS

Rhenium-1 86 is a beta-emitting radionuclide that has been studied for applications in radioimmunotherapy. Its 137 keV gamma photon is ideal for imaging the biodistribution of the immunoconjugates and for obtai ning gamma camera data for estimation of dosimetry. Methods used for d etermining radiation absorbed dose are described. We have estimated ab sorbed dose to normal organs and tumors following administration of tw o different Re-186-labeled immunoconjugates, intact NR-LU-10 antibody and the F(ab')2 fragment of N R-CO-02. Tumor dose estimates in 46 pati ents varied over a wide range, 0.4-18.6 rads/mCi, but were similar in both studies. Accuracy of activity estimates in superficial tumors was confirmed by biopsy. Prediction of Re-186 dosimetry from a prior Tc-9 9m imaging study using a tracer dose of antibody was attempted in the NR-CO-02 (Fab')2 study. Although Tc-99m was an accurate predictor of t umor localization and the mean predicted and observed radiation absorb ed doses to normal organs compared favorably, Re-186 dosimetry could n ot be reliably predicted in individual patients. The methods described nevertheless provide adequate estimates of Re-186 dosimetry to tumor and normal organs.