Targeted therapy of experimental renal cell carcinoma with a novel conjugate of monoclonal antibody 138H11 and calicheamicin thetaI1.

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 thetaI1 (Camtheta) in vitro and in vivo. Previously, mAb 138H11, produced against human renal gamma-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 gammaGT 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-Camtheta 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 10(6) Caki-1 cells s.c. and treated with the following: (a) PBS; (b) 138H11; (c) Camtheta; (d) a mixture of 138H11 and Camtheta; and (e) 138H11-Camtheta 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-Camtheta conjugate versus PBS (P = 0.004). Only mice treated with 138H11-Camtheta showed a tumor shrinkage to minimal residues. In a second experiment, lower doses of the 138H11-Camtheta conjugate were compared with an antineuroblastoma mAb (ch14.18), confirming targeted killing of RCC by the 138H11-Camtheta conjugate at tolerable toxicity in vivo. In conclusion, these combined results encourage further studies for targeted therapy of metastatic RCC with mAb 138H11 conjugates.