131 I-Lym-1 in Mice Implanted with Human Burkitt's Lymphoma (Raji) Tumors: Loss of Tumor Specificity due to Radiolysis

Preliminary evaluations of ¹²⁵I-labeled Lytn-1, an anti-lymphoma mouse IgG_2a monoclonal antibody, demonstrated favorable tumor uptake in mice bearing human Burkitt's lymphoma (Raji) tumors. In this study, the pharmacokinetics of ¹²⁵I- and ¹³¹I-Lym-1, and the dosimetry, efficacy, and toxicity of ¹³¹I-Lym-1 in Raji-tumored mice were evaluated.

Methods

Lym-1 was radioiodinated by the chloramine-T method and analyzed for monomeric fraction and immunoreactivity (antigen cell binding, relative to unmodified Lym-1). Nude mice bearing Raji tumors (20-500 mm³) received 1.5 MBq (40 μCi) ¹²⁵I-Lym-1, or 1.5, 7.4, 14.8, or 18.5 MBq (40, 200, 400, or 500 μCi) ¹³¹I-Lym-1. Pharmacokinetic data (total body and blood clearance and biodistribution) were used to estimate radiation dosimetry. Mini-thermoluminescent dosimetry (TLD) was also used to measure radiation dosimetry directly for 7 days after injection of ¹³¹I-Lym-1. Tumor size, survival, body weight, and blood counts were monitored for 60 days to evaluate therapeutic efficacy and toxicity of ¹³¹I-Lym-1.

Results

At the time of injection, the mean quality assurance (QA) values for ¹²⁵I-Lym-1 were 100% monomer and 100% relative immunoreactivity; the corresponding values for ¹³¹I-Lym-1 were 73% and 66%, indicating that radiolysis had occurred during the interval between radiolabeling and injection. ¹²⁵I-Lym-1 exhibited high and sustained concentration in tumors relative to normal organs, whereas ¹³¹I-Lym-1 did not. Assuming identical pharmacokinetic behavior to ¹²⁵I-Lym-1, ¹³¹I-Lym-1 would deliver radiation doses of 3.45, 0.83, 1.03, 0.34, and 0.56 Gy per MBq injected (12.8, 3.1, 3.8, 1.3, and 2.1 rad/μCi), to tumor, liver, lungs, total body, and marrow, respectively. When the actual pharmacokinetic data for ¹³¹I-Lym-1 (1.5 MBq) were used to estimate dosimetry, corresponding values of 0.51, 0.72, 0.49, 0.31, and 0.41 Gy/MBq (1.9, 2.7, 1.8, 1.1, and 1.5 rad/μCi) were obtained. Similar values were obtained for mice receiving 7.4 or 14.8 MBq of ¹³¹I-Lym-1. Similarly, TLD data indicated little preferential radiation dosimetry to tumor. Response rates (cure + CR + PR) for mice receiving 0, 7.4, 14.8, and 18.5 MBq of ¹³¹I-Lym-1 were 8%, 7%, 21%, and 45%, respectively. The LD_50/30 dose of ¹³¹I-Lym-1 was 12.7 MBq (343 μCi).

Conclusions

¹²⁵I-Lym-1 exhibited high and sustained concentration in Raji tumors in mice, indicating excellent therapeutic potential for ¹³¹I-Lym-1. However, in vitro QA results for ¹³¹I-Lym-1 indicated that radiolysis had occurred, and ¹³¹I-Lym-1 demonstrated little accumulation in tumor, or preferential radiation dosimetry to tumor in the same model.