EXPERIMENTAL STUDIES ON THERAPEUTIC EFFECT OF RAT MONOCLONAL ANTIBODY-BLEOMYCIN A6 CONJUGATE AGAINST HUMAN COLORECTAL CANCER

Bleomycin A6 (A6), a single component of bleomycin complex, is highly active against human colon and cecum cancer cells in vitro and xenografts in nude mice. R19, a rat monoclonal antibody against human cecum cancer Hce-8693 cells, was linked to A6. R19-A6 conjugate retained complete activity of McAb R19 and 10% activity of A6. As determined by clonogenic assay with human cecum cancer Hce-8693 cells for 1 hour exposure, the 50% inhibitory concentration (IC₅₀) values for R19-A6, A6 and M3-A6 (conjugate of irrelevant Mc-A6) were 0.019, 1.05 and 1.00 μmol/L, respectively. The effect of the conjugate R19-A6 was 55-fold stronger than that of free A6 and 53-fold than irrelevant conjugate M3-A6. Clonogenic assay with human colon cancer HT-29 cells showed that the IC₅₀ values were 0.078 μmol/L and 4.0 μmol/L for R19-A6 and free A6, respectively. The cytotoxicity to Hce-8693 and HT-29 cells was markedly blocked by unconjugated McAb R19 but not by irrelevant McAb MARK-3. The R19-A6 conjugate exerted 90% inhibition on the growth of cecum cancer Hce-8693 xenografts in nude mice, whereas equivalent doses of free A6, R19 plus A6 mixture and M3-A6 showed 52%, 34% and 48% inhibition, respectively. Histopathological examination showed no toxic changes in the heart, lung, liver, kidney and bone marrow in the R19-A6 conjugate treated animals. These results suggest that the conjugate of R19 and A6 shows selective cytotoxicity to target human colon and cecum cancer cells and is highly effective against cecum cancer xenografts in nude mice with more remarkable tumor growth inhibition than free A6 at equivalent dose level.