IN-VITRO AND IN-VIVO ANTITUMOR-ACTIVITY OF A CHIMERIC ANTI-CD19 ANTIBODY

Mouse monoclonal antibodies to CD19 detect an antigenic determinant ex pressed exclusively on the surface of B lymphocytes and have previousl y been shown to be potentially useful therapeutic reagents for human B cell lymphoma. We report the production and characterization of a mou se/human chimeric antibody cCD19, with potent in vivo antitumour activ ity. The genes encoding the variable domains for heavy (V-H) and light (V-L) chains were subcloned into eukaryotic expression vectors contai ning human constant region genes (IgG1 and kappa) and co-transfected i nto non-secreting Sp2.0 mouse myeloma cells. Intraperitoneal administr ation of cCD19 produced inhibition of growth th of subcutaneous CD19(- ) Sultan human B lymphoma tumours in scid scid mice. When the antibody was administered 18 and 20 days after subcutaneous tumour inoculation . an approximately 30% reduction in tumour size was noted by day 29. c CD19 faithfully mimicked the in vitro binding characteristics of mCD19 as (a) the chimeric antibody was shown by flow cytometry to bind excl usively to cell lines that expressed CD19, (b) cCD19 was able to inhib it the binding of mCD19 on CD19(+) cells completely and (c) the affini ty of binding of the two antibodies was not significantly. different [ K-a = (2.03 +/- 1.5) x 10(8)]. In biodistribution studies, up to 14.8% of the total injected antibody dose per gram of tissue was localized in CD19(+) Sultan tumours at 24 h approximately, 14.4% was present in the tumors at 48 h, and about 13.7% at 72 h. These levels were compara ble to mCD19 administered in the same fashion. cCD19 conjugated to ida rubicin was specifically and strongly cytotoxic to CD19(-) cells cultu red in vitro, and demonstrated an IC50 of 0.17 mu M, similar to that o f mCD19 (0.32 mu M) and approximately 14-fold greater than the IC50 of free idarubicin. The specific cytotoxic capacity of cCD19 and its lik ely reduced immunogenicity suggest that it may potentially be of use i n the treatment of refractory B cell lymphoma in humans.