Colitis ImmunoPET: Defining Target Cell Populations and Optimizing Pharmacokinetics

Positron emission tomography combined with a specific probe presents the ability to noninvasively assess inflammatory bowel disease. We previously reported increased intestinal uptake of a ⁶⁴Cu-labeled anti-β₇ integrin antibody (clone FIB504.64) in colitic mice. Here, we evaluated an anti-α₄ β₇ integrin antibody (clone DATK32), and the F(ab′)₂ and Fab fragments of the anti-β₇ antibody, which should have faster blood clearance than the intact antibody, as imaging probes for the detection of colitis in a mouse model. Methods: The immunoproteins were labeled with ⁶⁴Cu, injected into mice with dextran sodium sulphate-induced colitis. Positron emission tomography data were collected between 1 and 48 hours postinjection. Results: Focal uptake of the anti-β₇ fragments was observed in the gut as early as 1 hour postinjection, and they cleared more rapidly from normal tissues than the whole antibody. For example, the blood concentrations at 24 hours postinjection were 23.3 ± 3.0% ID/g for ⁶⁴Cu-labeled DATK32, 12.9 ± 2.1% ID/g for FIB504.64, 4.1 ± 0.4% ID/g for FIB504.64-F(ab′)₂, and 0.62 ± 0.2% ID/g for FIB504.64-Fab (P < 0.0001, analysis of variance). The ratio of uptake of DATK32 between the colitis and control groups in the large intestine (1.38) was lower than for the FIB504.64 fragments (3.15 for F(ab′)₂, 1.84 for Fab) or intact FIB504.64 (1.78). Conclusions: The lower intestinal uptake ratio of the ⁶⁴Cu-labeled anti-α₄ β₇ antibody (DATK32) compared with the anti-β₇ immunoproteins suggests that targeting all β₇-expressing lymphocytes, not just those expressing α₄ β₇, is a more promising route to the development of an inflammatory bowel disease imaging agent. The FIB504.64-F(ab′)₂ fragment demonstrated the greatest differential between colitis and control groups, and is therefore the most promising lead molecule for the development of an inflammatory bowel disease-specific imaging agent.