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Enhanced Therapeutic Index of Radioimmunotherapy (RIT) in Prostate Cancer Patients

Comparison of Radiation Dosimetry for 1,4,7,10-Tetraazacyclododecane-N,N',N'',N'''-Tetraacetic Acid (DOTA)-Peptide versus 2IT-DOTA Monoclonal Antibody Linkage for RIT¹

University of California–Davis School of Medicine, Sacramento, California 95816 [S. J. D., G. L. D., A. Y., C. M. R., R. T. O., P. N. L., A. N.]; University of California–San Francisco School of Medicine, San Francisco, California 94143 [K. R. L.]; Biomira, Inc., Edmonton, Alberta, Canada T6N 1H1; Lawrence Livermore National Laboratory, Livermore, California 94550 [C. L. H. S.]; and Equine Analytical Chemistry Laboratory, University of California–Davis, Davis, California 95617 [D. L. K.]

Purpose: Radioimmunotherapy delivered by radiometal immunoconjugates and followed by marrow support is dose limited by deposition of radioactivity in normal organs. To increase elimination of radioactivity from the liver and body and, thus, minimize hepatic radiation dose, a peptide having a specific cathepsin B cleavage site was placed between the radiometal chelate DOTA (1,4,7,10-tetraazacyclododecane-N,N',N'',N'''-tetraacetic acid) and the monoclonal antibody m170, and the comparative pharmacokinetics was evaluated in prostate cancer patients.

Experimental Design: ¹¹¹In-DOTA-2IT-m170 and ¹¹¹In-DOTA-peptide-(GGGF)-m170, representing the same monoclonal antibody and chelate with and without the cleavable linkage, were studied in comparable groups of prostate cancer patients (17 with In-2IT-BAD-m170 and 8 with In-DOTA-peptide-m170). Pharmacokinetics over 7 days, calculated yttrium-90 radiation dosimetry, therapeutic index, and projected maximum tolerated injected yttrium-90 dose were evaluated.

Results: The radioimmunoconjugates pharmacokinetics and calculated tumor and normal organ absorbed radiation dose (rads/mCi) were similar, except for a significant decrease in the mean dose to the liver (31%; P < 0.01) and lungs (31%; P < 0.01) with the DOTA-peptide immunoconjugates. Because mean tumor dose was not statistically different, this peptide linkage provided a significant increase in the therapeutic index for this tumor targeting radiopharmaceutical. If marrow support is adequate, the radiation dose historically tolerated by normal organs other than marrow would allow a 30% increase in the administered dose, resulting in a mean dose of 9500 rads to metastatic prostate cancer.

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