Pretargeted radioimmunotherapy permits the administration of doses of 90Y five times higher than is possible with antibodies directly labeled with 90Yttrium (90Y). These high doses of 90Y introduced new issues for dosimetry that were not encountered in prior studies using conventional radioimmunotherapy. We have addressed these issues here and correlated dosimetry estimates with observed toxicity and tumor responses.
Methods: The pretargeted radioimmunotherapy (PRIT) system employed the antibody NR-LU-10 conjugated with streptavidin, a glycoprotein clearing agent and 90Y-DOTA-biotin. A single dose of 90Y was escalated to 140 mCi/m2. Indium-111(111In) (3-5 mCi) DOTA-biotin was co-injected for gamma camera imaging and dosimetry assessment. The effect of bremsstrahlung radiation from increasing 90Y activity levels with a constant dose of 111In was studied using a phantom. Patient images identified the intestinal tract and the kidneys as potential organs at risk of clinically significant radiation toxicity. A method of measuring the activity localized in the intestinal tract was developed, and S values were calculated to estimate intestinal wall dose from radioactivity present in the intestine. Intestinal, bone marrow and renal toxicity were observed. Coefficients were derived for correlating the relationships between observed intestinal and marrow toxicity and the estimated radiation absorbed doses.
Results: At an 90Y:111In ratio of 50:1, bremsstrahlung radiation accounted for 12% of the counts in the images. Grade IV diarrhea was observed in patients estimated to have received 6850-14,000 cGy to the large intestinal wall. The correlation coefficient of intestinal toxicity with absorbed dose was 0.64. Myelotoxicity (measured as grade of suppression of absolute neutrophil count) correlated better with marrow dose (r = 0.72) than with the whole body dose, (r = 0.44). Delayed renal toxicity was observed in two patients 8 and 11 months following therapy. Tumor response was seen in the two patients with the highest estimated dose to tumor, 4,000-6,000 cGy.
Conclusion: Dosimetry is feasible using 111In as a tracer in the presence of high 90Y activity. The absorbed dose estimates derived in the PRIT schema correlated moderately well with clinically observed toxicity and response.