Agonist-antagonist dilemma in molecular imaging: Evaluation of a monomolecular multimodal imaging agent for the somatostatin receptor

The combination of different imaging modalities, each providing information according to its strengths, can be a powerful method for diagnosing diseases. We have synthesized a monomolecular multimodal imaging agent (MOMIA), LS172, containing a subtype-2 somatostatin receptor (SSTr2)-avid peptide (Y3-octreotate or Y3-TATE), a radiometal chelating group (DOTA) and a near-infrared (NIR) fluorescent dye (cypate). In addition to optical methods, radiolabeling LS 172 with Cu-64 and Lu-177 provides a strategy for in vitro evaluation or in vivo multimodal imaging by positron emission tomography (PET) and single photon-emission computed tomography (SPECT), respectively. Determination of the binding affinity of LS 172, Cu-nat- and Lu-nat-LS 172 in SSTr2-transfected A427 cells (A427-7) showed that they all displayed high binding affinity toward SSTr2 with K-i values of 0.234 nM, 11.5 nM, and 2.15 nM respectively. In contrast to cypate-labeled Y3-TATE (cytate), fluorescence microscopy showed that LS172 and (CU)-C-nat-LS172 accumulate modestly in A427-7 cells by SSTr2-mediated endocytosis, in spite of their relatively high binding affinity. In vivo, the biodistribution of the SSTr2 receptor specific Cu-64- and Lu-177-LS 172 in AR42J tumor-bearing rats exhibited low (<= 1% ID/g) accumulation in tumor tissue. Clearance from circulation was predominantly hepatobiliary (> 90% ID/liver). Both optical and radionuclear biodistribution studies showed a similar in vivo distribution profile. Surprisingly, the strong binding of LS172 to SSTr2 did not translate into high SSTr2-mediated endocytosis in cells or uptake in tumor in vivo. Considering that LS172 is a putative antagonist, the poor accumulation of the labeled MOMIAs in SSTr2 positive tumor tissue supports the paradigm that agonists with their concomitant internalization favors appreciable target tissue accumulation of receptor-specific ligands.