PET imaging of CD38 expression in non-small cell lung cancer xenografts

Objectives: Daratumumab (Darzalex®, Janssen Biotech) is a clinically-approved antibody targeting CD38 for the treatment of multiple myeloma. However, CD38 is also expressed by other cancer cell types, including lung cancer, where its expression or absence may offer prognostic value. In addition, tracking of CD38 expression by T cells is of great interest for monitoring responses to therapy and potentially predicting response. We therefore developed a PET tracer based upon daratumumab for tracking CD38 expression, utilizing murine models of non-small cell lung cancer to verify its specificity.

Methods: Daratumumab, an anti-CD38 antibody, was prepared for radiolabeling with ⁸⁹Zr (t_1/2 = 78.4 h) through conjugation with desferrioxamine (Df). Western blot, flow cytometry, saturation binding assays, and internalization assays were utilized to characterize CD38 expression of and binding of daratumumab to three non-small cell lung cancer cell lines: A549, H460, and H358. Murine hindlimb xenograft models of the three cancer cell lines were also generated for further in vivo studies. Longitudinal PET imaging was performed following injection of 5-10 MBq ⁸⁹Zr-Df-daratumumab out to 120 h post-injection. Nonspecific uptake was also evaluated through the injection of a radiolabeled isotype control IgG antibody in A549 mice, ⁸⁹Zr-Df-IgG. Ex vivo biodistribution and histological analyses were also performed after the terminal imaging timepoint at 120 h post-injection.

Results: Through cellular studies, A549 cells were found to express higher levels of CD38 than the H460 or H358 cell lines, with a K_d in the micromolar range for the binding of daratumumab to the cells. PET imaging and ex vivo biodistribution studies verified this trend, with A549 tumor uptake peaking at 8.1 ± 1.2 %ID/g at 120 h post-injection according to PET analysis, and H460 and H358 at lower levels at the same timepoint (7.1 ± 0.2 %ID/g and 5.4 ± 0.1 %ID/g, respectively; n = 3-4). Injection of a non-specific radiolabeled IgG into A549 tumor-bearing mice also demonstrated lower tracer uptake of 5.9 ± 2.8 %ID/g at 120 h. Trends found in PET ROI analysis were verified in ex vivo biodistributions, where significantly decreased uptake was found in all cell lines as compared to A549 (p<0.05, all groups). Immunofluorescent staining of tumor tissues showed higher staining levels present in A549 tissues over H460 and H358.

Conclusion: ⁸⁹Zr-Df-daratumumab is able to image CD38-expressing tissues in vivo using PET, as verified through the exploration of non-small cell lung cancer models in this study. Thus, this agent holds potential to image CD38 in other malignancies and aid in patient stratification and elucidation of the biodistribution of CD38. Research Support: NIH, American Cancer Society, University of Wisconsin - Madison

• Download figure
• Open in new tab
• Download powerpoint