Despite low radiation dose rates,
radioimmunotherapy (RIT) has proven particularly effective in the treatment of
malignancies, such as
lymphoma. Apoptosis has been suggested to be a major mechanism for cell death from continuous low-dose rate radiation from
radioimmunotherapy. The goal of this study was to examine Raji
lymphoma xenografts for induction of apoptosis and modulation of apoptosis-related gene and
protein expression in response to
67Cu-2IT-BAT-Lym-1 RIT. In preclinical and clinical trials,
67Cu-2IT-BAT-Lym-1 has shown an exceptionally long
tumor residence time associated with substantial 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 micrograms Lym-1, or 335-500 microCi 67Cu on less than 400 micrograms 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 (PARP) 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 little evidence of apoptosis and Lym-1 had no effect on apoptosis or gene expression.
67Cu-2IT-BAT-Lym-1 RIT induced an overall response rate of 50% with tolerable toxicity, and 29% of the
tumors were cured at cumulated
tumor radiation doses of about 1800 cGy. Apoptosis was greatly increased in the RIT treated Raji xenografts as evidenced by cleavage of PARP to the characteristic 85 kD fragment at 3 and 6 h and by the DNA cleavage pattern. BCL2 gene and
protein expression were substantially decreased at 3 and 24 h, respectively, after
67Cu-2IT-BAT-Lym-1 RIT despite only modest cumulated radiation doses (56 cGy at 3 h). Evidence for apoptosis preceded
tumor regression by 4-6 days. In these
therapy-resistant, human
lymphoma tumors treated with
67Cu-2IT-BAT-Lym-1, apoptosis was convincingly demonstrated to be a major mechanism for the effectiveness of RIT and occurred by p53-independent mechanisms.