Nivolumab-DTPA as a Potential Qualitative and Quantitative Probe Platform for Assessing PD-1 Expression of Colon Cancer via Imaging

Objectives: PD-1 expression is presumed to be a prognostic marker of tumor response to immuno-blocking therapy in clinical trials. Herein, Nivolumab-DTPA was developed as an imaging probe platform for qualitative assessing via Gd-based T1 MR, and for quantitative assessing via Tc-99m-based SPECT, where the integrated functional imagings may contribute to the accurate evaluation of colon cancer on PD-1 expression. Methods: Nivolumab, the PD-1 antibody, was conjugated with DTPA via the condensation reaction with bifunctional chelator p-SCN-Bn-DTPA, which provided the labeling site for Tc-99m and Gd. The mouse models (highly expressed human PD-1 antigen) bearing colorectal carcinoma in situ were prepared using AOM/DSS-based chemical induction. For in vivo imaging, SPECT was performed at 2 and 6 hours post injection (i.v.) of ^99mTc-DTPA-Nivolumab (11.1 MBq/10 μg) to observe the distribution of probes. With interval of 48 hours, T1-weighted MR imaging (T1WI) was observed at 2 hours after Gd-DTPA-Nivolumab injection, and compared with the intestinal morphology and signal of the Gd-free T1WI. Furthermore, PD-1 expression of different intestinal parts, including tumor tissues with varying tracer uptake, were assessed by immunohistochemistry analysis, and corresponding results were compared with the findings from functional SPECT and T1 MR.

Results: DTPA-Nivolumab was synthesized and can be used for further ^99mTc labeling with chemical purity of > 95%, or Gd labeling. A typical SPECT image was shown in Figure 1A, focus with high ^99mTc-Nivolumab uptake was shown in the right colon at 6 hours post injection, correspondingly, a high PD-1 expressed tissue can be detected via IHC. Accordingly, T1WI showed a suspected thickening of the intestinal wall in the right intestine (Figure 1B), and an enhanced scan acquired at 2 hours after the additive injection of Gd-Nivolumab revealed a marked enhancement of the suspected thickening of the intestinal wall (Figure 1C). IHC results manifested that the intestinal tumor was of high PD-1 expression. Most importantly, all these manifestations of SPECT and T1 MR were consistent with the quantitative IHC results and qualitative visual observations. Conclusion: This study demonstrated that Nivolumab-DTPA could be a promising imaging probe platform for displaying PD-1 expression positive lesions in vivo, not only integrating the advantages of IHC but also providing a noninvasive method for monitoring the changes in tumor expression of PD-1. Keywords: PD-1; Nivolumab; SPECT; T1 MR; Colon Cancer

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