The impact of tumor size on the efficacy of monoclonal antibody-targeted radiotherapy: studies using a nude mouse model with human renal cell carcinoma xenografts.

Monoclonal antibody (Mab)-targeted radiotherapy is a unique approach in cancer therapy. Multiple factors affect the success of treatment. Internal radiation dosimetry and mini-dose Mab-targeted radiotherapy studies reveal that tumor size affects the efficacy of treatment. For tumors with calculated weight greater than 400 mg., intravenous administration of 131I-labeled A6H or A6H-C5H combination delivers significantly less tumor radiation dose (2070 +/- 580 cGy/100 microCi) than those for tumors of weight less than 200 mg. (5260 +/- 2460 cGy/100 microCi). In the mini-dose (an average of 73 to 86 microCi) Mab-targeted radiotherapy study of 109 mice with small tumors (six, 12, or 19 days after implantation), tumors 12 days after implantation (approximately 60 mg. in weight) showed regression in all mice and in 62% of mice gross tumor elimination was observed. In contrast, mini-dose therapy at day 19 (tumor weights approximately 170 mg.) resulted in tumor regression and tumor elimination rates of 33% and 17% respectively. These studies suggest that Mab-targeted radiotherapy is more suitable for treating small tumors.