Peptides as molecular imaging probes

Peptides as molecular imaging probes

Molecular imaging is a rapidly emerging biomedical research in diagnostics and therapeutic fields due to current indispensable tools in modern diagnostics[1]. Thus the need for highly sensitive and specific molecular imaging probes is still unmet. In order to design diagnostic imaging and radiotherapeutic agents, a high target/background ratio, target uptake and rapid removal of untargeted drug is desirable. Proteins, antibodies and their fragment are perfect candidates due to high specificity affinity and selectivity to the target organs. With the advancement of combinatorial chemistry and phage display methods, peptides are currently receiving considerable attention as molecular imaging probes. In addition, peptides have distinct advantages over proteins and antibodies due to many appealing characteristics:

1. Rapid clearance in the body ensures the radioisotopes are not retained in the body for a prolonged period of time.
2. Have low toxicity and immunogenicity
3. They can easily be synthesized, modified to optimize their binding affinity and improve their stability against proteolytic degradation and in vivo circulation providing a means to improve
4. With the well established peptide synthesis protocols they are easy to scale up and reproduce Phage display is a powerful technique that allows vast sequence space screening, peptide affinity and generate unique peptides that binds any given target [1]. Due to the diversified methods of peptide synthesis, peptides can be directly or indirectly labeled with imaging probes to provide or augment the imaging signals, depending on the modality. For example, several radioisotopes (99mTc, 18F, 64Cu, 111In, 123I, 68Ga) for PET and SPECT, organic near-infrared (NIR) fluorophores or quantum dots (QDs) for optical imaging and magnetic nanoparticles for MRI can be conjugated to peptides via organic linkers, macrocyclic or branched chelators (figure 1), polymers or nanoplatforms[2].

Several classic examples of peptides have been utilized as imaging agents for peptide-receptor interaction due to their high selectivity and affinity to their cognitive receptors as described below.

1.1 Somatostatin Analogs

Somatostatins (SST) are regulatory hormones with inhibitory effects on secretion of growth hormone, insulin, glucagon, gastrin, cholecystokinin, vasoactive intestinal peptide (VIP) and secretin[3]. These cyclopeptides are mainly produced by endocrine, gastrointestinal, immune and neuronal cells as well as by certain tumours[3]. Radionuclide labeled synthetic somatostatin analogs like octreotide have long been investigated for imaging neuroendocrine tumors due to their enhanced metabolic stability and biological activity compared to the native hormone. 111In-DTPA, 68GA-DOTA, 64Cu-DOTA and 18F-FP-Gluc-labeled SST analogues have been developed for imaging various tumors[2].

1.2 αvß3-Integrin Analogs

αvß3 Integrin Analogs are closely related with tumor progression and plays an important role during tumor angiogenesis and formation of new blood vessels. Integrin peptides contain an Arg-Gly-Asp (RGD) motif. αvß3 integrin has received considerable attention in diagnostic cancer imaging due to its overexpression in several forms of tumors including melanomas, ovarian and lung carcinoma, neuroblastomas, glioblastomas and breast cancer[2]. Of particular interest, 18F labeled cyclo(Arg-Gly-Asp-D-Tyr-Lys) analog has been used to identify αvß3 receptor expression in patients with melanoma, sarcoma, breast cancer and glioblastoma[4].

1.3 Bombesin analogs

Bombesin (BBN) is an amphibian homologue of mammalian gastrin-releasing peptide (GRP; pGlu1-Gln2-Arg3-Leu4-Gly5-Asn6-Gln7-Trp8-Ala9-Val10-Gly11-His12-Leu13-Met14-NH2) that exhibits high affinity and specificity to the GRP receptor (GRPr). Several BBN analogs have been screened and developed as imaging probes. 99mTc-EDDA/HYNIC, 111In-DOTA, 64Cu-DOTA and NOTA and 18F-Lys3 labeled BBN have been shown to have specific uptake in PC3-tumors in vivo[2].

1.4 Cholecytoskinin analogs

Cholecytoskinin (CCK) and gastrin are structurally and functionally related peptide hormones that function in the gastrointestinal tract and central nervous system. They mediate their biological functions by interacting with CCK/gastrin receptors belonging to GPCR’s[5]. These receptors have been found to be overexpressed in certain human tumors like medullary thyroid cancer (MTC), astrocytomas, stromal ovarian tumors and gastroenteropancreatic cancers[6]. Several of 111In or 99mTc labeled CCK and minigastrin analogs have shown specificity in cells overexpressing CCK receptors and high tumor uptake. However the metabolic stability and in vivo bioavailability of these peptides needs to be improved for them to be clinically useful[2].

1.5 α-melanocyte stimulating hormone analogs

α-melanocyte stimulating hormone (α-MSH) is a 13 amino acid peptide produced by pituitary gland and is mainly responsible for the regulation of skin pigmentation. Studies have shown that a-MSH receptors are distributed in more than 80% human melanoma metastases[7]. Thus radiolabeled peptides that target these receptors are attractive probes for diagnosis of melanoma. 111In DOTA-conjugated, 18F-labeled a-MSH have been investigated in melanoma-bearing mice[8]. 99mTc labeled dual receptor targeting peptide ligands recognizing both integrin and α-MSH receptors have been designed and they demonstrate enhanced cellular intake in melanoma both in vitro and in vivo[9].

Other peptides like neurotensin (NT), a neurotransmitter is being investigated for ductal pancreatic adenocarcinomas which overexpressed NT receptors. Radiolabeled glucagon-like peptide-1 (GLP-1) and Exendin-4 have been developed to target insulinomas which overexpress the GLP-1 receptors.

All these peptides require custom design to improve their metabolic stability and half life in vivo. BIOSYNTHESIS, INC. a leading manufacturer of custom peptides in small scale research and bulk is now offering peptide conjugates containing metal chelators like DOTA, NOTA, TETA and DTPA.

References:

1. S. L. Deutscher, Chem Rev 2010, 110, 3196-3211.
2. S. Lee, J. Xie and X. Chen, Chem Rev 2010, 110, 3087-3111.
3. G. Weckbecker, I. Lewis, R. Albert, H. A. Schmid, D. Hoyer and C. Bruns, Nat Rev Drug Discov 2003, 2, 999-1017.
4. a) A. J. Beer, A. L. Grosu, J. Carlsen, A. Kolk, M. Sarbia, I. Stangier, P. Watzlowik, H. J. Wester, R. Haubner and M. Schwaiger, Clin Cancer Res 2007, 13, 6610-6616; b) A. J. Beer, M. Niemeyer, J. Carlsen, M. Sarbia, J. Nahrig, P. Watzlowik, H. J. Wester, N. Harbeck and M. Schwaiger, J Nucl Med 2008, 49, 255-259.
5. J. C. Reubi, J. C. Schaer and B. Waser, Cancer Res 1997, 57, 1377-1386.
6. J. C. Reubi and B. Waser, Int J Cancer 1996, 67, 644-647.
7. J. B. Tatro, Z. Wen, M. L. Entwistle, M. B. Atkins, T. J. Smith, S. Reichlin and J. R. Murphy, Cancer Res 1992, 52, 2545-2548.
8. Y. Miao, F. Gallazzi, H. Guo and T. P. Quinn, Bioconjug Chem 2008, 19, 539-547.
9. J. Yang, H. Guo, F. Gallazzi, M. Berwick, R. S. Padilla and Y. Miao, Bioconjug Chem 2009, 20, 1634-1642.