Tumor-Specific Binding of Radiolabeled PEGylated GIRLRG Peptide: A Novel Agent for Targeting Cancers.Cancer-specific targeting sparing normal tissues would significantly enhance cancer therapy outcomes and reduce cancer-related mortality.  One approach is to target receptors or molecules that are specifically expressed on cancer cells.  Peptides as cancer-specific targeting agents offer advantages such as ease of synthesis, low antigenicity, and enhanced diffusion into tissues.  Glucose-regulated protein 78 (GRP78) is an endoplasmic reticulum stress chaperone that regulates the unfolded protein response and is overexpressed in various cancers.  In this study, we evaluated GIRLRG peptide that specifically targets GRP78 for cancer-specific binding (in vitro) and noninvasive tumor imaging (in vivo).


 GIRLRG peptide was modeled into the GRP78 ATPase domain using computational modeling.  Surface plasmon resonance studies were performed to determine the affinity of GIRLRG peptide to GRP78 protein.  GIRLRG was conjugated with PEG to prolong its circulation in mice.  Tumor binding efficacy of PEG-GIRLRG peptide was evaluated in nude mice bearing heterotopic cervical (HT3), esophageal (OE33), pancreatic (BXPC3), lung (A549), and glioma (D54) tumors.  Nano-SPECT/CT imaging of the mice was performed 48 and 72 h after injection with 111In-labeled PEG-GIRLRG or PEG-control peptide.  Post-SPECT biodistribution studies were performed 96 h after injection of the radiolabeled peptides.


 Using molecular modeling and surface plasmon resonance, we identified that GIRLRG was binding with an affinity constant of 2.16 × 10-3 M in the ATPase domain of GRP78.  GIRLRG peptide specifically bound to cervical, lung, esophageal, and glioma cells.  SPECT imaging revealed that 111In-PEG-GIRLRG specifically bound to cervical, esophageal, pancreatic, lung, and brain tumors.  Post-SPECT biodistribution data also validated the SPECT imaging results.


 CONCLUSION
 GIRLRG peptide specifically binds to the ATPase domain of GRP78.  Radiolabeled PEG-GIRLRG could be used to target various cancers.  Further studies would be required to translate PEG-GIRLRG peptide into the clinic.
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Nano-SPECT/CT imaging of the mice was performed 48 and 72 h after injection with 111In-labeled PEG-GIRLRG or PEG-control peptide. 
Nano-SPECT/CT imaging