Room: Karl Dean Ballroom A1
Purpose: Checkpoint immunotherapy treatments are revolutionizing cancer care, including targeting of the CTLA-4 pathway. However, only a subpopulation of patients exhibits durable responses and predictors of response to these treatments are limited.
Methods: Ipilimumab, a commercially-available anti-CTLA-4 antibody for checkpoint immunotherapy treatments, was enzymatically digested to develop ipilimumab[F(abâ€™)2]. Both intact ipilimumab and ipilimumab[F(abâ€™)2] were conjugated with 2-S-(4-Isothiocyanatobenzyl)-1,4,7-triazacyclononane-1,4,7-triacetic acid (p-SCN-Bn-NOTA) for radiolabeling with â?¶â?´Cu (half-life: 12.7 h) for PET imaging. NBSGW mice were inoculated with human peripheral blood mononuclear cells to develop humanized (PBL) mice, in which CTLA-4+ human T-cells were imaged. PET scans were conducted out to 48 h post-injection of the tracers in both PBL and control NBSGW mice, and ex vivo studies were used to verify observations in PET imaging.
Results: Salivary glands in PBL mice were found to contain high concentrations of human T-cells, due to the onset of graft-versus-host disease and subsequent gland dysfunction. Imaging with both â?¶â?´Cu-NOTA-ipilimumab and â?¶â?´Cu-NOTA-ipilimumab[F(abâ€™)2] allowed imaging of activated CTLA-4+ T-cells, as evidenced through specific uptake in the salivary glands of PBL mice and absence of this uptake in NBSGW controls. Higher peak salivary gland-to-blood ratios were observed in PBL mice (1.19 Â± 0.49 for ipilimumab, 1.77 Â± 0.72 for ipilimumab[F(abâ€™)2]) as compared to NBSGW (0.76 Â± 0.19 for ipilimumab, 0.82 Â± 0.11 for ipilimumab[F(abâ€™)2]). Use of the F(abâ€™)2 fragment allowed higher ratios due to the quick background clearance of this tracer. Ex vivo gamma-counting biodistribution studies also verified PET imaging quantification, with notable salivary gland uptake in PBL mice with both tracers (5.74 Â± 2.22 %ID/g for ipilimumab, 1.53 Â± 0.65 %ID/g for ipilimumab[F(abâ€™)2]).
Conclusion: We are able to image human CTLA-4+ T-cells in humanized mice using both â?¶â?´Cu-NOTA-ipilimumab and â?¶â?´Cu-NOTA-ipilimumab[F(abâ€™)2]. These tracers may help stratify patients for CTLA-4 checkpoint immunotherapy treatments in the future.
Funding Support, Disclosures, and Conflict of Interest: This work was supported, in part, by the University of Wisconsin - Madison, the National Institutes of Health (P30CA014520, T32CA009206, T32GM008505), and the American Cancer Society (125246-RSG-13-099-01-CCE).