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Targeted Therapy of Experimental Renal Cell Carcinoma with a Novel Conjugate of Monoclonal Antibody 138H11 and Calicheamicin ^I₁¹

Laboratory of Molecular Biology, Charité Children’s Hospital, Humboldt-University, 10117 Berlin [K. K., W. W., G. G., P. F.], and Medical Department II, Division of Renal Diseases, Hospital München-Harlaching, 81545 München [J. E. S.], Germany

In search for a new therapeutic approach for metastasized renal cell carcinoma (RCC), we evaluated the cytotoxicity of a novel prodrug chemoimmunoconjugate with monoclonal antibody (mAb) 138H11 and the DNA-cleaving enediyne calicheamicin ^I₁ (Cam) in vitro and in vivo. Previously, mAb 138H11, produced against human renal -glutamyltransferase, stained over 99% clear cell and papillary RCC on frozen sections, showing a membranous expression of the target antigen. In contrast, in normal kidneys GT was restricted to the brush-border in the lumen of proximal tubules and not accessible to the circulation. Thus, human tumor-bearing kidneys perfused in an extra-corporeal system with ^99mTc-138H11 revealed a high, specific uptake into the tumor. In this study, fluorescence-activated cell sorting analysis showed binding of mAb 138H11 to RCC cell lines, whereas squamous cell carcinoma lines, fibroblasts, and the murine RENCA were negative. XTT cell proliferation assays revealed efficient killing of the Caki-1 cell line by the 138H11-Cam conjugate using SPDP (EC₅₀ = 5 x 10^-11 M) as a covalent linker. For in vivo testing, five groups of eight nude mice each were injected with 2.5 x 10⁶ Caki-1 cells s.c. and treated with the following: (a) PBS; (b) 138H11; (c) Cam; (d) a mixture of 138H11 and Cam; and (e) 138H11-Cam conjugate. Treatment started on day 1 after tumor induction and was repeated three times. The data show a highly significant inhibition of tumor growth with the 138H11-Cam conjugate versus PBS (P = 0.004). Only mice treated with 138H11-Cam showed a tumor shrinkage to minimal residues. In a second experiment, lower doses of the 138H11-Cam conjugate were compared with an antineuroblastoma mAb (ch14.18), confirming targeted killing of RCC by the 138H11-Cam conjugate at tolerable toxicity in vivo. In conclusion, these combined results encourage further studies for targeted therapy of metastatic RCC with mAb 138H11 conjugates.

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