Abstract
Objective
The multifocal nature of prostate cancer often leads to partial removal of the tumor lesions during
surgery.1 Molecular imaging with dual-modality imaging probes could greatly improve prostate
cancer management by providing more information about the localization of the primary tumor and
the metastases, and by better defining surgical margins. Such probe bears a radionuclide to
enable diagnosis via positron emission tomography (PET) and a fluorescent dye for image-guided
surgery. Gastrin releasing peptide receptor (GRPR) is upregulated in the majority of the prostate
cancer cells, and it has been identified as a promising target for imaging and treatment of prostate
cancer. NeoB is a GRPR antagonist, which has been successfully labeled with gallium-68 and
lutetium-177 for theranostic applications.2,3 The main objective of this project was to develop a
series of dual-modality imaging probes based on the molecular backbone of NeoB and bearing a
DOTA chelator to enable labeling with a radiometal for preoperative PET imaging and a
fluorescent dye for intraoperative surgical guidance.
Methods
Four NeoB analogs were synthesized by solid-phase peptide synthesis (SPPS). A DOTA chelator
and a trans-cyclooctene (TCO) were selectively introduced at the N-terminus of the peptides. The
final probes were obtained by performing a “click reaction” based on the inverse electron demand
Diels-Alder reaction to couple a tetrazine-modified sulfo-cyanine 5 derivative (Tz-sCy5) to the
TCO-peptides. The final products were purified by semi-preparative HPLC and characterized by
MS. Indium-111 radiolabeling and stability studies were carried out, followed by biological assays
to determine the binding affinity (IC50) of the probes. Biodistribution and imaging studies were
performed in GRPR-positive tumor bearing mice with the best two dual-labeled probes.
Results
The IC50 values of the four dual-labeled probes were 5 to 25 folds lower than the binding affinity
of the parent peptide, which was expected due to the chemical modifications taking place.
SPECT/CT imaging and an ex vivo biodistribution were performed at 2 and approximately 3 h post
injection, respectively. Both probes showed high tumor uptake with 8.47 ± 0.46 for DOTA-K(PEG4-
TCO-Tz-sCy5)-pADA-BD (probe 12) and 6.90 ± 0.81 %ID/g for DOTA-K(TCO-Tz-sCy5)-PEG4-BD
(probe 15). Co-localization of the radioactivity and fluorescence was confirmed by ex-vivo
fluorescent imaging.
Conclusion
Our dual-labeled probes demonstrated very promising biodistribution profile. They are promising
drug candidates for preoperative diagnosis and intraoperative surgical guidance of GRPR-positive
prostate cancer.
The multifocal nature of prostate cancer often leads to partial removal of the tumor lesions during
surgery.1 Molecular imaging with dual-modality imaging probes could greatly improve prostate
cancer management by providing more information about the localization of the primary tumor and
the metastases, and by better defining surgical margins. Such probe bears a radionuclide to
enable diagnosis via positron emission tomography (PET) and a fluorescent dye for image-guided
surgery. Gastrin releasing peptide receptor (GRPR) is upregulated in the majority of the prostate
cancer cells, and it has been identified as a promising target for imaging and treatment of prostate
cancer. NeoB is a GRPR antagonist, which has been successfully labeled with gallium-68 and
lutetium-177 for theranostic applications.2,3 The main objective of this project was to develop a
series of dual-modality imaging probes based on the molecular backbone of NeoB and bearing a
DOTA chelator to enable labeling with a radiometal for preoperative PET imaging and a
fluorescent dye for intraoperative surgical guidance.
Methods
Four NeoB analogs were synthesized by solid-phase peptide synthesis (SPPS). A DOTA chelator
and a trans-cyclooctene (TCO) were selectively introduced at the N-terminus of the peptides. The
final probes were obtained by performing a “click reaction” based on the inverse electron demand
Diels-Alder reaction to couple a tetrazine-modified sulfo-cyanine 5 derivative (Tz-sCy5) to the
TCO-peptides. The final products were purified by semi-preparative HPLC and characterized by
MS. Indium-111 radiolabeling and stability studies were carried out, followed by biological assays
to determine the binding affinity (IC50) of the probes. Biodistribution and imaging studies were
performed in GRPR-positive tumor bearing mice with the best two dual-labeled probes.
Results
The IC50 values of the four dual-labeled probes were 5 to 25 folds lower than the binding affinity
of the parent peptide, which was expected due to the chemical modifications taking place.
SPECT/CT imaging and an ex vivo biodistribution were performed at 2 and approximately 3 h post
injection, respectively. Both probes showed high tumor uptake with 8.47 ± 0.46 for DOTA-K(PEG4-
TCO-Tz-sCy5)-pADA-BD (probe 12) and 6.90 ± 0.81 %ID/g for DOTA-K(TCO-Tz-sCy5)-PEG4-BD
(probe 15). Co-localization of the radioactivity and fluorescence was confirmed by ex-vivo
fluorescent imaging.
Conclusion
Our dual-labeled probes demonstrated very promising biodistribution profile. They are promising
drug candidates for preoperative diagnosis and intraoperative surgical guidance of GRPR-positive
prostate cancer.
| Original language | English |
|---|---|
| Pages (from-to) | S82-S82 |
| Number of pages | 1 |
| Journal | Nuclear Medicine and Biology |
| Volume | 96-97 |
| Publication status | Published - 2021 |
