Imaging of Human Insulin Secreting Cells with Gd-DOTA-P88, a Paramagnetic Contrast Agent Targeting the Beta Cell Biomarker FXYD2γa

Abstract

Non-invasive imaging and quantification of human beta cell mass remains a major challenge. We performed pre-clinical in vivo validation of a peptide previously discovered by our group, namely, P88 that targets a beta cell specific biomarker, FXYD2γa. We conjugated P88 with DOTA and then complexed it with GdCl3 to obtain the MRI (magnetic resonance imaging) contrast agent (CA) Gd-DOTA-P88. A scrambled peptide was used as a negative control CA, namely Gd-DOTA-Scramble. The CAs were injected in immunodeficient mice implanted with EndoC-βH1 cells, a human beta cell line that expresses FXYD2γa similarly to primary human beta cells. The xenograft-bearing mice were analyzed by MRI. At the end, the mice were euthanized and the CA biodistribution was evaluated on the excised tissues by measuring the Gd concentration with inductively coupled plasma mass spectrometry (ICP-MS). The MRI and biodistribution studies indicated that Gd-DOTA-P88 accumulates in EndoC-βH1 xenografts above the level observed in the background tissue, and that its uptake is significantly higher than that observed for Gd-DOTA-Scramble. In addition, the Gd-DOTA-P88 showed good xenograft-to-muscle and xenograft-to-liver uptake ratios, two potential sites of human islets transplantation. The CA shows good potential for future use to non-invasively image implanted human beta cells.

<p>Islet Transplantation Imaging in vivo</p>

“…They conjugated P88 with Gd-DOTA and successfully monitored the transplanted islets in vivo in mice. 29 However, because of the side effects of nephrogenic systemic fibrosis, the development of Gd and its derivatives is limited. Reports on the long- In vivo MR imaging of recipients having transplanted islets.…”

Beta Cell Imaging—From Pre-Clinical Validation to First in Man Testing

“…During the last decade, several putative beta cell-specific probes have been identified, including monoclonal antibodies (mAbs) [ 42 , 43 ], short or fragments of antibodies [ 44 , 45 ], peptides [ 46 , 47 , 48 ], small molecules such as metabolites [ 49 , 50 , 51 ], and ion-binding probes [ 52 , 53 ]. The different existing tracers have been recently and comprehensively reviewed [ 54 ] and will not be further discussed here.…”

“…The rationale for this is that islets are probably exposed to these stresses during the course of diabetes [ 8 ] and we wish to avoid stress-induced changes in the expression of our biomarkers. So far, we identified two biomarkers by using this or a similar approach: dipeptidyl peptidase 6 (DPP6) [ 45 ] and FXYD domain-containing ion transport regulator 2γ (FXYD2γ) variant a [ 47 , 48 ]. We have developed imaging probes for each of them, based on small camelid antibodies (nanobodies, Nb) [ 45 , 101 ] or small proteins generated by phage display [ 47 ], respectively.…”

“…FXYD2γa expression is restricted to beta cells and appears early during fetal development [ 99 ], which makes it one of the most specific beta cell biomarkers identified so far. To target FXYD2γa for imaging purposes, we generated a short peptide, P88, by using phage display and functionalized it for MRI by conjugation to ultra-small particles of iron oxide (USPIO) [ 47 ] or Gd-DOTA [ 48 ]. By using these probes, we were able to detect tumors constituted of cells overexpressing the target [ 47 , 48 ], or EndoC-βH1 [ 48 ].…”

“…To target FXYD2γa for imaging purposes, we generated a short peptide, P88, by using phage display and functionalized it for MRI by conjugation to ultra-small particles of iron oxide (USPIO) [ 47 ] or Gd-DOTA [ 48 ]. By using these probes, we were able to detect tumors constituted of cells overexpressing the target [ 47 , 48 ], or EndoC-βH1 [ 48 ]. Unfortunately, the limited sensitivity of the probe associated with the use of MRI hinders the possibility of detecting BCM and of quantifying their size.…”