Pretreatment with rituximab does not inhibit the human immune response against the immunogenic protein LMB-1

Purpose: Rituximab, a humanized monoclonal antibody directed to the CD20 antigen present on B lymphocytes, could potentially abrogate the humoral immune response to murine monoclonal antibodies or immunotoxins by depleting antibody-producing B cells. Experimental Design: A Phase II study of LMB-1, an immunotoxin targeting the Lewis Y tumor antigen, in combination with rituximab was conducted to test the hypothesis that rituximab could abolish or diminish the development of human antibodies to LMB-1. Five patients were treated in this study and received 375 mg/m(2) rituximab on days 1 and 7 followed by 45 mug/kg/day LMB-1 on days 10, 12, and 14. The development of human antibodies against LMB-1 was detected using a serum neutralization and ELISA. Results: All five of the patients had a total suppression of circulating CD20/CD19 B-cell population before the administration of the first dose of the immunotoxin. Before rituximab treatment, the mean percentage of CD20/CD19-positive B cells in the five treated patients was 19.8% (range, 4.5-29.8%) of the total peripheral lymphocytes. After two doses of rituximab, CD20/CD19-positive B lymphocytes constituted less than or equal to0.1% of the total peripheral lymphocytes. Despite absent circulating antibody-producing B cells, before and during LMB-1 treatment, all of the patients developed neutralizing antibodies to the immunotoxin by day 21 of drug administration, which prevented retreatment. Conclusions: Even though rituximah caused complete depletion of circulating CD20/CD19-positive B cells, it had no effect in suppressing the human antibody response to LMB-1 and may be of limited utility in suppressing human antibody responses to other immunogenic proteins.