PET radioimmunodetection of neuroblastoma: Optimizing pharmacokinetics through antibody engineering

Objectives To identify the optimal antibody format for PET detection of neuroblastoma.

Methods Whole IgG ch14.18 (150 kDa) and its ΔC_H2 (120 kDa) and scFv-Fc (100 kDa) derivatives were conjugated to pba-DOTA and labeled with ⁶⁴Cu. The labeled radioimmunoconjugate (RIC) (50 μg, 100-250 μCi each) was injected into mice bearing M21 tumors. MicroPET images were collected at 1, 24 and 48 hours post-injection, followed by a biodistribution study.

Results MicroPET imaging and the biodistribution study demonstrated that these bivalent RICs localized to the tumor. Blood clearance for the smaller molecules was more rapid: ch14.18 (n = 5) 5.9 ±1.4 %ID/g; ΔC_H2 (n = 3) 1.5 ±0.3 %ID/g; scFv-Fc (n = 5) 3.9 ±0.8 %ID/g (Ab and scFv-Fc vs ΔC_H2 P < 0.05, Ab vs scFv-Fc P > 0.05). The normal tissue distribution of the molecules also varied. Liver uptake for the intact antibody was 7.4 ±3.0 %ID/g, but for ΔC_H2: 12.9 ±1.9 %ID/g; and scFv-Fc 9.9 ±0.8 %ID/g. Kidney uptake followed a different pattern, with low intact antibody and ΔC_H2 uptake (3.7 ±1.2 %ID/g and 3.7 ±0.4 %ID/g respectively) but higher scFv-Fc uptake (7.6 ±1.2 %ID/g, all data 48 hours p.i.).

Conclusions Blood levels of the whole antibody and scFv-Fc molecules were probably maintained through recycling by the FcRn molecule binding to intact Fc regions. More rapid clearance from non-target tissues, reducing exposure to the radionuclide, suggests that the Cu-64-labeled ▵C_H2 molecule would be a better format for clinical detection of neuroblastoma lesions.

Research Support National Institutes of Health Grants 5K08CA093554 and 5R01CA094338.