PET/CT imaging of head-and-neck and pancreatic cancer in humans by targeting the "Cancer Integrin" αvß6 with Ga-68-Trivehexin.

PURPOSE: To develop a new probe for the αvß6-integrin and assess its potential for PET imaging of carcinomas. METHODS: Ga-68-Trivehexin was synthesized by trimerization of the optimized αvß6-integrin selective cyclic nonapeptide Tyr2 (sequence: c[YRGDLAYp(NMe)K]) on the TRAP chelator core, followed by automated labeling with Ga-68. The tracer was characterized by ELISA for activities towards integrin subtypes αvß6, αvß8, αvß3, and α5ß1, as well as by cell binding assays on H2009 (αvß6-positive) and MDA-MB-231 (αvß6-negative) cells. SCID-mice bearing subcutaneous xenografts of the same cell lines were used for dynamic (90 min) and static (75 min p.i.) µPET imaging, as well as for biodistribution (90 min p.i.). Structure-activity-relationships were established by comparison with the predecessor compound Ga-68-TRAP(AvB6)3. Ga-68-Trivehexin was tested for in-human PET/CT imaging of HNSCC, parotideal adenocarcinoma, and metastatic PDAC. RESULTS: Ga-68-Trivehexin showed a high αvß6-integrin affinity (IC50 = 0.047 nM), selectivity over other subtypes (IC50-based factors: αvß8, 131; αvß3, 57; α5ß1, 468), blockable uptake in H2009 cells, and negligible uptake in MDA-MB-231 cells. Biodistribution and preclinical PET imaging confirmed a high target-specific uptake in tumor and a low non-specific uptake in other organs and tissues except the excretory organs (kidneys and urinary bladder). Preclinical PET corresponded well to in-human results, showing high and persistent uptake in metastatic PDAC and HNSCC (SUVmax = 10-13) as well as in kidneys/urine. Ga-68-Trivehexin enabled PET/CT imaging of small PDAC metastases and showed high uptake in HNSCC but not in tumor-associated inflammation. CONCLUSIONS: Ga-68-Trivehexin is a valuable probe for imaging of αvß6-integrin expression in human cancers.

The importance of tyrosines in multimers of cyclic RGD nonapeptides: towards αvß6-integrin targeted radiotherapeutics.

In a recent paper in this journal (RSC Med. Chem., 2023, 14, 2429), we described an unusually strong impact of regiospecific exchange of phenylalanines by tyrosines in 10 gallium-68-labeled trimers of certain cyclic RGD peptides, c[XRGDLAXp(NMe)K] (X = F or Y), on non-specific organ uptakes. We found that there was, in part, no correlation of liver uptake with established polarity proxies, such as the octanol-water distribution coefficient (log D). Since this observation could not be explained straightforwardly, we suggested that the symmetry of the compounds had resulted in a synergistic interaction of certain components of the macromolecules. In the present work, we investigated whether a comparable effect also occurred for a series of 5 tetramers labeled with lutetium-177. We found that in contrast to the trimers, liver uptake of the tetramers was well correlated to their polarity, indicating that the unusual observations along the trimer series indeed was a unique feature, probably related to their particular symmetry. Since the Lu-177 labeled tetramers are also potential agents for treatment of a variety of αvß6-integrin expressing cancers, these were evaluated in mice bearing human lung adenocarcinoma xenografts. Due to their tumor-specific uptake and retention in biodistribution and SPECT imaging experiments, these compounds are considered a step forward on the way to αvß6-integrin-targeted anticancer agents. Furthermore, we noticed that the presence of tyrosines in general had a positive impact on the in vivo performance of our peptide multimers. In view of the fact that a corresponding rule was already proposed in the context of protein engineering, we argue in favor of considering peptide multimers as a special class of small or medium-sized proteins. In summary, we contend that the performance of peptide multimers is less determined by the in vitro characteristics (particularly, affinity and selectivity) of monomers, but rather by the peptides' suitability for the overall macromolecular design concept, and peptides containing tyrosines are preferred.

There is a world beyond αvß3-integrin: Multimeric ligands for imaging of the integrin subtypes αvß6, αvß8, αvß3, and α5ß1 by positron emission tomography.

BACKGROUND: In the context of nuclear medicine and theranostics, integrin-related research and development was, for most of the time, focused predominantly on 'RGD peptides' and the subtype αvß3-integrin. However, there are no less than 24 known integrins, and peptides without the RGD sequence as well as non-peptidic ligands play an equally important role as selective integrin ligands. On the other hand, multimerization is a well-established method to increase the avidity of binding structures, but multimeric radiopharmaceuticals have not made their way into clinics yet. In this review, we describe how these aspects have been interwoven in the framework of the German Research Foundation's multi-group interdisciplinary funding scheme CRC 824, yielding a series of potent PET imaging agents for selective imaging of various integrin subtypes. RESULTS: The gallium-68 chelator TRAP was utilized to elaborate symmetrical trimers of various peptidic and non-peptidic integrin ligands. Preclinical data suggested a high potential of the resulting Ga-68-tracers for PET-imaging of the integrins α5ß1, αvß8, αvß6, and αvß3. For the first three, we provide some additional immunohistochemistry data in human cancers, which suggest several future clinical applications. Finally, application of αvß3- and αvß6-integrin tracers in pancreatic carcinoma patients revealed that unlike αvß3-targeted PET, αvß6-integrin PET is not characterized by off-target uptake and thus, enables a substantially improved imaging of this type of cancer. CONCLUSIONS: Novel radiopharmaceuticals targeting a number of different integrins, above all, αvß6, have proven their clinical potential and will play an increasingly important role in future theranostics.