Apoptosis-related gene and protein expression in human lymphoma xenografts(Raji) after low dose rate radiation using Cu-67-2IT-BAT-Lym-1 radioimmunotherapy

Despite low radiation dose rates, radioimmunotherapy (RIT) has proven parti cularly effective in the treatment of malignancies, such as Lymphoma. Apopt osis has been suggested to be a major mechanism for cell death from continu ous low-dose rate radiation from radioimmunotherapy. The goal of this study was to examine Raji lymphoma xenografts for induction of apoptosis and mod ulation of apoptosis-related gene and protein expression in response to Cu- 67-2IT-BAT-Lym-1 RIT. In preclinical and clinical trials, Cu-67-2IT-BAT-Lym -1 has shown an exceptionally long tumor residence time associated with sub stantial cumulated radiation doses. The Raji model mirrors human lymphomas that have mutant p53 and increased BCL2 expression. Untreated athymic BALB/ c nu/nu mice and mice treated with 400 mug Lym-1, or 335-500 mu Ci Cu-67 on less than 400 mug Lym-1 antibody, were observed for toxicity and response over 84 days. Subgroups of 4-5 mice were sacrificed at 3, 6 and 24 h after therapy so that tumors could be examined for poly(ADP-ribose) polymerase (P ARP) and DNA ladder evidence for apoptosis and for BCL2, p53, p21, GADD45, TGF-beta (1) and c-MYC gene and protein expression. Untreated tumors had li ttle evidence of apoptosis and Lym-1 had no effect on apoptosis or gene exp ression. Cu-67-21T-BAT-Lym-1 RIT induced an overall response rate of 50% wi th tolerable toxicity, and 29% of the tumors were cured at cumulated tumor radiation doses of about 1800 cGy. Apoptosis was greatly increased in the R IT treated Raji xenografts as evidenced by cleavage of PARP to the characte ristic 85 kD fragment at 3 and 6 h and by the DNA cleavage pattern. BCL2 ge ne and protein expression were substantially decreased at 3 and 24 h, respe ctively, after Cu-67-21T-BAT-Lym-1 RIT despite only modest cumulated radiat ion doses (56 cCy at 3 h). Evidence for apoptosis preceded tumor regression by 4-6 days. In these therapy-resistant, human lymphoma tumors treated wit h Cu-67-21T-BAT-Lym-1, apoptosis was convincingly demonstrated to be a majo r mechanism for the effectiveness of RIT and occurred by p53-independent me chanisms.