Bispecific Antibody and Iodine-131-Labeled Bivalent Hapten Dosimetry in Patients with Medullary Thyroid or Small-Cell Lung Cancer

The purpose of this study was to estimate the dose delivered to tumor targets and normal tissues after two-step injection of an anti-CEA/anti-DTPA-In (F6-734) bispecific antibody and a 131 I-labeled di-DTPA In-TL bivalent hapten in patients with medullary thyroid carcinoma (MTC) and small-cell lung cancer (SCLC). Methods : Five patients with persistent disease or recurrences of MTC and five patients with primary SCLC or relapse were studied. In a first step, 0.1 to 0.3 mg/kg of F6-734 bispecific antibody was injected intravenously. Four days later, 6 nmole (5.8 to 9.8 mCi) of 131 I-labeled di-DTPA In-TL bivalent hapten were injected. Quantitative imaging was performed during one week after the second injection. Results : All 5 patients with MTC showed positive immunoscintigraphy (IS). In the smallest visualized and resected tumor (0.8 g), the fraction of injected activity per gram (% ID/g) was 0.1% at Day 3. IS was positive in 4 of the 5 patients with SCLC. The volume of the smallest visualized SCLC tumor was estimated at 11 ± 2 ml, and tumor uptake was about 0.009% ID/g. Tumor dose estimates ranged from 4.2 to 174 cGy/mCi in patients with MTC and from 1.7 to 8 cGy/mCi in patients with SCLC. Conclusion : High absorbed dose values were calculated for small MTC recurrences. For SCLC recurrences the values were smaller but in the same range as those obtained by other investigators with the one-step technique in lymphoma.