Cancer cells that express or overexpress specific receptors have lead to the development of synthetic peptides that target these receptors. Peptides conjugated to metal chelates (e.g, DOTA, NOTA, etc.) offer an attractive approach for both imaging of cancer tissues and as therapies that deliver cytotoxic cargo to cancer cells. The interleukin-13 receptor alpha 2 (IL13RA2) is a receptor that is abundantly overexpressed in most GBMs but not present in healthy brain cells. Ligands that specifically target this receptor may offer new approaches to the treatment of GBM. Because IL13RA2 is internalized upon binding IL13, this receptor is an attractive target for therapies involving the delivery of cytotoxic cargo to Glioblastoma (GBM) cells. Peptide-1 linear (Pep-1L) has been evaluated for its ability to localize at IL13RA2 and deliver cytotoxic alpha-particle emitters (Actinium-225, Ac-225) to GBM cells (right).

Chemical Structures of Common Chelates

Metal chelates for peptide conjugation

Table of Common Metal Chelates

ACRONYM FULL CHELATE NAME
DOTA 1,4,7,10-tetraazacyclododecane-N,N,N,N-tetraacetic acid
NOTA 1,4,7-triazacyclononane-N,N,N-triacetic acid
TETA 1,4,8,11-tetraazacyclotetradecane-1,4,8,11-tetraacetic acid
DTPA diethylenetriaminepentaacetic acid
IDA Iminodiacetic acid
CB-TE2A 4,11-bis(carboxymethyl)-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane
DiAmSar1,8-Diamino-3,6,10,13,16,19-hexaazabicyclo[6,6,6]-eicosane
TRAP3,3′,3”-(((1,4,7-triazonane-1,4,7-triyl)tris(methylene))tris(hydroxyphosphoryl))tripropanoic acid

PD-L1 Binding Peptides

An immune checkpoint protein called programmed cell death ligand (PD-L1) is a common biomarker for cancer because it is over-expressed in many tumor cells as an adaptive immune response to cytotoxic T-cells. Binding of PD-L1 to programmed cell death protein 1 (PD-1) receptors of infiltrating host immune cells triggers deactivation and immune suppression. Rapid and non-invasive positron emission topography (PET) imaging that can monitor PD-L1 expression levels and distribution would enable more efficient treatment options for cancer patients. Current immunotherapies rely on radiolabeled anti-PD-L1 antibodies to determine PD-L1 expression levels non-invasively in human tumors. Radiolabeled antibody conjugates, however, have long clearance times that limit the amount of imaging agents that can be injected and increases diagnosis time. Peptide-based PET tracers, on the other hand, have lower molecular weights compared to proteins and show enhanced clearance rates.

Chatterjee and co-workers hypothesized that PD-L1 binding peptides would more efficiently and rapidly detect PD-L1 expression levels in tumors. From a library of PD-L1 binding peptides, peptide WL12 (below) was selected to test their hypothesis. The peptide has several structural features that make it more resistant to proteolytic metabolism, namely: (1) macrocyclization (thioether), (2) N-methylated substituents and (3) the incorporation of unnatural amino acids. The single primary amine of the ornithine side chain provides a relatively easy method in which to conjugate a DOTA chelator for radiolabeling with 64Cu.

Peptide Receptor Radionuclide Citations

RGD-DOTA Peptide Conjugates

Cell-targeting peptides (CTPs) have emerged as effective tools for targeting cancer cells that overexpress certain receptor proteins that recognize and internalize CTPs. Inhibition of αvβ3 integrin receptors has been associated with tumor prevention and reduced tumor growth by antagonizing angiogenesis. RGD peptides, ligands which bind αvβ3 integrin receptors, are the most well studied for their importance in tumor angiogenesis and metastasis regulation. One of the most potent and selective of these peptide antagonists, cyclo[Arg-Gly-Asp-D-Phe-Val] (c[RGDfV]), was developed by Kessler and co-workers.[2]

The binding of RGD peptides to αvβ3 integrin receptors has been exploited in positron emission tomography (PET) for early detection and diagnosis of cancer. αvβ3 Integrins are co-localized with MMP-2 (a matrix metalloproteinases) in a variety of cancer cells and the expression level of MMP-2 has been correlated with tumor stage, invasiveness, and metastasis. To study the co-localization αvβ3 Integrin and MMP-2, Mebrahtu (left) designed an RGD-DOTA peptide conjugate that incorporates both integrin targeting and MMP-2 substrate moieties. Combining the peptide with DOTA provides the PET agent: cyclo(RGDfE)K-(DOTA)PLGVRY. The incorporation of a C-terminal tyrosine, for radio-halogenation (123I), enables the peptide to give a SPECT (single photon emission computed tomography) signal. The dually radiolabeled peptide provides a diagnostic tool for simultaneous imaging of cancer cells and monitoring of their pathophysiologic activity.

Selected Peptide Chelate Citations

References

  1. Aumailley, M.; Gurrath, M.; Müller, G.; Calvete, J.; Timpl, R.; Kessler, H. FEBS Lett. 291 (1991): 50.
  2. Chatterjee, Samit, et al. “Rapid PD-L1 detection in tumors with PET using a highly specific peptide.” Biochemical and Biophysical Research Communications 483.1 (2017): 258-263.

CPC Scientific PRRT Citations

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