In vitro and in vivo evaluation of ⁶⁴Cu-radiolabeled KCCYSL peptides for targeting epidermal growth factor receptor-2 in breast carcinomas

Preclinical research progress in HER2-targeted small-molecule probes for breast cancer

Breast cancer is a malignant tumor that has the highest morbidity and mortality in women worldwide. Human epidermal growth factor receptor 2 (HER2) is a key driver of breast cancer development. Therefore, accurate assessment of HER2 expression in cancer patients and timely initiation or termination of anti-HER2 treatment are crucial for the prognosis of breast cancer patients. The emergence of radiolabeled molecular probes targeting HER2 makes this assessment possible. This article describes different types of small-molecule probes that target HER2 and are used in current preclinical applications and summarizes their advantages and disadvantages.

N,N-Alkylation Clarifies the Role of N- and O-Protonated Intermediates in Cyclen-Based 64Cu Radiopharmaceuticals

Radioisotopes of Cu, such as ⁶⁴Cu and ⁶⁷Cu, are alluring targets for imaging (e.g., positron emission tomography, PET) and radiotherapeutic applications. Cyclen-based macrocyclic polyaminocarboxylates are one of the most frequently examined bifunctional chelators in vitro and in vivo, including the FDA-approved ⁶⁴Cu radiopharmaceutical, Cu(DOTATATE) (Detectnet); however, connections between the structure of plausible reactive intermediates and their stability under physiologically relevant conditions remain to be established. In this study, we share the synthesis of a cyclen-based, N,N-alkylated spirocyclic chelate, H₂DO3A^C4H8, which serves as a model for N-protonation. Our combined experimental (in vitro and in vivo) and computational studies unravel complex pH-dependent speciation and enable side-by-side comparison of N- and O-protonated species of relevant ⁶⁴Cu radiopharmaceuticals. Our studies suggest that N-protonated species are not inherently unstable species under physiological conditions and demonstrate the potential of N,N-alkylation as a tool for the rational design of future radiopharmaceuticals.

Homodimer 99mTc-HYNIC-E(SSSLTVPWY)2 peptide improved HER2-overexpressed tumor targeting and imaging

We hypothesized that a novel design of the LTVPWY (LY) peptide might exhibit a great potential for improving binding affinity and targeting HER2-overexpressed tumors. Hence, new dimer construction of ^99mTc-labeled LY [^99mTc-HYNIC-E(SSSLTVPWY)₂] (^99mTc-DLY) was introduced. Afterward, a head-to-head comparison of in vitro and in vivo experiments was performed between ^99mTc-DLY and ^99mTc-HYNIC-SSSLTVPWY as the monomer analog. The blocking dosage of trastuzumab reduced the uptake of the dimer about 20% more efficiently than the monomer in the SKOV-3 cell line. A twofold increase in competitive binding affinity and biological half-life was observed for ^99mTc-DLY. The ovarian-tumor-bearing mice were detected with high contrast where the tumor-to-muscle ratio of ^99mTc-DLY was notably increased about 40% using a gamma camera. The biodistribution experiment revealed an approximately 10% enhancement in tumor/blood, tumor/muscle, and tumor/bone ratios for the dimer. More rapid blood clearance was another achievement of the homodimer design. Overall, ^99mTc-DLY successfully affected the pharmacokinetics and consequently the visualization of HER2-overexpressing tumors.

Synthesis and comparative evaluation of 177Lu-labeled PEG and non-PEG variant peptides as HER2-targeting probes

Highest global cancer incidence of female breast cancer is a matter of great concern. HER2-positive breast cancers have high mortality rate hence detection at an early stage is vital for successful treatment, improved cancer care and survival rate. Radiolabeled peptides have emerged as new alternatives to radiolabeled antibodies to overcome the limitations of slow clearance and uptake in non-target tissues. Herein, DOTA-A9 peptide and its pegylated variant were constructed on solid phase and radiolabeled with [177Lu]LuCl3. [177Lu]DOTA-A9 and [177Lu]DOTA-PEG4-A9 displayed high binding affinity (Kd = 48.4 ± 1.4 and 55.7 ± 12.3 nM respectively) in human breast carcinoma SKBR3 cells. Two radiopeptides exhibited renal excretion and rapid clearance from normal organs. Uptake in SKBR3 tumor and tumor-to-background ratios were significantly higher (p < 0.05) for [177Lu]DOTA-PEG4-A9 at the three time points investigated. Xenografts could be clearly visualized by [177Lu]DOTA-PEG4-A9 in SPECT images at 3, 24 and 48 h p.i. indicating the potential for further exploration as HER2-targeting probe. The encouraging in vivo profile of PEG construct, [177Lu]DOTA-PEG4-A9 incentivizes future studies for clinical applications.

Selection of Cancer Stem Cell-Targeting Agents Using Bacteriophage Display

There is a growing need to develop tumor targeting agents for aggressive cancers. Aggressive cancers frequently relapse and are resistant to various therapies. Cancer stem cells (CSCs) are believed to be the cause of relapse and the aggressive nature of many cancers. Targeting CSCs could lead to novel diagnostic and treatment options. Bacteriophage (phage) display is a powerful tool developed by George Smith in 1985 to aid in the discovery of CSC targeting agents. Phage display selections are typically performed in vitro against an immobilized target. There are inherent disadvantages with this technique that can be circumvented by performing phage display selections in vivo. However, in vivo phage display selections present new challenges. A combination of both in vitro and in vivo selections, however, can take advantage of both selection methods. In this chapter, we discuss in detail how to isolate a CSC like population of cells from an aggressive cancer cell line, perform in vivo and in vitro phage display selections against the CSCs, and then characterize the resulting phage/peptides for further use as a diagnostic and therapeutic tool.

Review: Radionuclide Molecular Imaging Targeting HER2 in Breast Cancer with a Focus on Molecular Probes into Clinical Trials and Small Peptides

As the most frequently occurring cancer worldwide, breast cancer (BC) is the leading cause of cancer-related death in women. The overexpression of HER2 (human epidermal growth factor receptor 2) is found in about 15% of BC patients, and it is often associated with a poor prognosis due to the effect on cell proliferation, migration, invasion, and survival. As a result of the heterogeneity of BC, molecular imaging with HER2 probes can non-invasively, in real time, and quantitatively reflect the expression status of HER2 in tumors. This will provide a new approach for patients to choose treatment options and monitor treatment response. Furthermore, radionuclide molecular imaging has the potential of repetitive measurements, and it can help solve the problem of heterogeneous expression and conversion of HER2 status during disease progression or treatment. Different imaging probes of targeting proteins, such as monoclonal antibodies, antibody fragments, nanobodies, and affibodies, are currently in preclinical and clinical development. Moreover, in recent years, HER2-specific peptides have been widely developed for molecular imaging techniques for HER2-positive cancers. This article summarized different types of molecular probes targeting HER2 used in current clinical applications and the developmental trend of some HER2-specific peptides.

Recent Advances in Therapeutic Peptides for Breast Cancer Treatment

Breast cancer is a heterogeneous malignancy and is the second leading cause of mortality among women around the world. Increasing the resistance to anti-cancer drugs in breast cancer cells persuades researchers to search the novel therapeutic approaches for the treatment of this malignancy. Among the novel methods, therapeutic peptides that target and disrupt tumor cells have been of great interest. Therapeutic peptides are short amino acid monomer chains with high specificity to bind and modulate a protein interaction of interest. Several advantages of peptides, such as specific binding on tumor cells surface, low molecular weight, and low toxicity on normal cells, make the peptides appealing therapeutic agents against solid tumors, particularly breast cancer. Also, the National Institutes of Health (NIH) describes therapeutic peptides as a suitable candidate for the treatment of drug-resistant breast cancer. In this review, we attempt to review the different therapeutic peptides against breast cancer cells that can be used in the treatment and diagnosis of the malignancy. Meanwhile, we presented an overview of peptide vaccines that have been developed for the treatment of breast cancer.

Escape from abluminal LRP1-mediated clearance for boosted nanoparticle brain delivery and brain metastasis treatment

Breast cancer brain metastases (BCBMs) are one of the most difficult malignancies to treat due to the intracranial location and multifocal growth. Chemotherapy and molecular targeted therapy are extremely ineffective for BCBMs due to the inept brain accumulation because of the formidable blood‒brain barrier (BBB). Accumulation studies prove that low density lipoprotein receptor-related protein 1 (LRP1) is promising target for BBB transcytosis. However, as the primary clearance receptor for amyloid beta and tissue plasminogen activator, LRP1 at abluminal side of BBB can clear LRP1-targeting therapeutics. Matrix metalloproteinase-1 (MMP1) is highly enriched in metastatic niche to promote growth of BCBMs. Herein, it is reported that nanoparticles (NPs-K-s-A) tethered with MMP1-sensitive fusion peptide containing HER2-targeting K and LRP1-targeting angiopep-2 (A), can surmount the BBB and escape LRP1-mediated clearance in metastatic niche. NPs-K-s-A revealed infinitely superior brain accumulation to angiopep-2-decorated NPs-A in BCBMs bearing mice, while comparable brain accumulation in normal mice. The delivered doxorubicin and lapatinib synergistically inhibit BCBMs growth and prolongs survival of mice bearing BCBMs. Due to the efficient BBB penetration, special and remarkable clearance escape, and facilitated therapeutic outcome, the fusion peptide-based drug delivery strategy may serve as a potential approach for clinical management of BCBMs.

Radiolabeled Peptide Probes for Liver Cancer Imaging

Liver cancer/Hepatocellular Carcinoma (HCC) is a leading cause of cancer death and represents an important cause of mortality worldwide. Several biomarkers are overexpressed in liver cancer, such as Glypican 3 (GPC3) and Epidermal Growth Factor Receptor (EGFR). These biomarkers play important roles in the progression of tumors and could serve as imaging and therapeutic targets for this disease. Peptides with adequate stability, receptor binding properties, and biokinetic behavior have been intensively studied for liver cancer imaging. A great variety of them have been radiolabeled with clinically relevant radionuclides for liver cancer diagnosis, and many are promising imaging and therapeutic candidates for clinical translation. Herein, we summarize the advancement of radiolabeled peptides for the targeted imaging of liver cancer.

Radiolabeled Peptides for SPECT and PET Imaging in the Detection of Breast Cancer: Preclinical and Clinical Perspectives

Breast cancer is the most common cancer in women worldwide. Due to the heterogeneous nature of breast cancer, the optimal treatment and expected response for each patient may not necessarily be universal. Molecular imaging techniques could play an important role in the early detection and targeted therapy evaluation of breast cancer. This review focuses on the development of peptides labeled with SPECT and PET radionuclides for breast cancer imaging. We summarized the current status of radiolabeled peptides for different receptors in breast cancer. The characteristics of radionuclides and major techniques for peptide labeling are also briefly discussed.

The emerging role of radionuclide molecular imaging of HER2 expression in breast cancer

Targeting of human epidermal growth factor type 2 (HER2) using monoclonal antibodies, antibody-drug conjugates and tyrosine kinase inhibitors extends survival of patients with HER2-expressing metastatic breast cancer. High expression of HER2 is a predictive biomarker for such specific treatment. Accurate determination of HER2 expression level is necessary for stratification of patients to targeted therapy. Non-invasive in vivo radionuclide molecular imaging of HER2 has a potential of repetitive measurements, addressing issues of heterogeneous expression and conversion of HER2 status during disease progression or in response to therapy. Imaging probes based of several classes of targeting proteins are currently in preclinical and early clinical development. Both preclinical and clinical data suggest that the most promising are imaging agents based on small proteins, such as single domain antibodies or engineered scaffold proteins. These agents permit a very specific high-contrast imaging at the day of injection.

Peptide-functionalized liposomes as therapeutic and diagnostic tools for cancer treatment

Clinically efficacious medication in anticancer therapy has been successfully designed with liposome-based nanomedicine. The liposomal formulation in cancer drug delivery can be facilitated with a functionalized peptide that mediates the specific drug delivery opportunities with increased drug penetrability, specific accumulation in the targeted site, and enhanced therapeutic efficacy. This review aims to focus on recent advances in peptide-functionalized liposomal formulation techniques in cancer diagnosis and treatment regarding recently published literature. It also will highlight different aspects of novel liposomal formulation techniques that incorporate surface functionalization with peptides for better anticancer effect and current challenges in peptide-functionalized liposomal drug formulation.

Phage Display Selection, Identification, and Characterization of Novel Pancreatic Cancer Targeting Peptides

Pancreatic cancer is characterized by a 5-year survival rate of 3%, in part due to inadequate detection methods. The small size of peptides offers advantages regarding molecular targeting. Thus, peptides may be used in detection of pancreatic cancer. Here, peptides that target pancreatic cancer cells were selected using phage display technology using a 15-mer fUSE5 library. Phage were pre-cleared against immortalized pancreatic cells (hTERT-HPNE), followed by selections against pancreatic cancer (Mia Paca-2) cells. Next-generation sequencing identified two peptides, MCA1 and MCA2, with a Log2 fold change (Mia Paca-2/ hTERT-HPNE) >1.5. Modified ELISA and fluorescent microscopy showed that both peptides bound significantly higher to Mia Paca-2 cells, and not to hTERT-HPNE, embryonic kidney (HEK 293), ovarian (SKOV-3) and prostate cancer (LNCaP) cell lines. Further characterization of MCA1 and MCA2 revealed EC₅₀ values of 16.11 µM (95% CI [9.69, 26.31 µM]) and 97.01 µM (95% CI [58.64, 166.30 µM]), respectively. Based on these results, MCA1 was selected for further studies. A competitive dose response assay demonstrated specific binding and an IC₅₀ value of 2.15 µM (95% CI [1.28, 3.62 µM]). Taken together, this study suggests that MCA1 may be used as a pancreatic cancer targeting ligand for detection of the disease.

Development of the Tumor-Specific Antigen-Derived Synthetic Peptides as Potential Candidates for Targeting Breast and Other Possible Human Carcinomas

The human epidermal growth factor receptor 2 (HER2) represents one of the most studied tumor-associated antigens for cancer immunotherapy. The receptors for HER2 are overexpressed in various human cancers, such as breast and ovarian cancer. The relatively low expression of this antigen on normal tissues makes it a clinically useful molecular target for tumor imaging and targeted therapy. HER2 overexpression is correlated with aggressive tumor behavior and poor clinical outcomes. Thus, HER2 has become an important prognostic and predictive factor, as well as a potential molecular target. Due to the heterogeneity of breast cancer and possible discordance in HER2 status between primary tumors and distant metastases, assessment of HER2 expression by noninvasive imaging is important. Molecular imaging of HER2 expression may provide essential prognostic and predictive information concerning disseminated cancer and aid in the selection of an optimal therapy. Another tumor-specific antigen is MUC1, which is silent on normal tissues, but overexpressed in almost all human epithelial cell cancers, including >90% of human breast, ovarian, pancreatic, colorectal, lung, prostate, and gastric cancers and is a promising tumor antigen with diagnostic as well as the therapeutic potential of cancer. Radiolabeled small peptide ligands are attractive as probes for molecular imaging, as they reach and bind the target receptor efficiently and clear from blood and non-target organs faster than bulky antibodies. In this study, HER2 and MUC1-based peptides were synthesized and preclinically evaluated in an effort to develop peptide-based SPECT radiopharmaceuticals derived from tumor-associated antigens for the detection of breast cancer. Our findings demonstrate that the tumor antigen peptides radiolabeled efficiently with ^99mTc and showed high metabolic stability in human plasma in vitro. The data from breast tumor cell binding confirmed the high affinity (in low nanomolar range) towards respective breast cancer cell lines. In healthy mice, ^99mTc-labeled peptides displayed favorable pharmacokinetics, with high excretion by the renal system. In tumor xenografts nude mice models, good uptake by the SKBR3, MCF7, and T47D tumors were found, with good tumor-to-blood and tumor to muscle ratios. Additionally, tumor lesions can be seen in γ-camera imaging. Our data suggest that based on its ability to detect HER2- and MUC1-positive breast cancer cells in vivo, ^99mTc-HER2 and ^99mTc-MUC1-targeted peptides may be promising tumor imaging probes and warrant further investigation.

Cancer-targeted delivery systems based on peptides

There is a growing interest for the discovery of new cancer-targeted delivery systems for drug delivery and diagnosis. A synopsis of the bibliographic data will be presented on bombesin, neurotensin, octreotide, Arg-Gly-Asp, luteinizing hormone-releasing hormone and other peptides. Many of them have reached the clinics for therapeutic or diagnostic purposes, and have been utilized as carriers of known cytotoxic agents such as doxorubicin, paclitaxel, cisplatin, methotrexate or dyes and radioisotopes. In our article, recent advances in the development of peptides as carriers of cytotoxic drugs or radiometals will be analyzed.

Self-Assembled Peptide-Based Nanomaterials for Biomedical Imaging and Therapy

Peptide-based materials are one of the most important biomaterials, with diverse structures and functionalities. Over the past few decades, a self-assembly strategy is introduced to construct peptide-based nanomaterials, which can form well-controlled superstructures with high stability and multivalent effect. More recently, peptide-based functional biomaterials are widely utilized in clinical applications. However, there is no comprehensive review article that summarizes this growing area, from fundamental research to clinic translation. In this review, the recent progress of peptide-based materials, from molecular building block peptides and self-assembly driving forces, to biomedical and clinical applications is systematically summarized. Ex situ and in situ constructed nanomaterials based on functional peptides are presented. The advantages of intelligent in situ construction of peptide-based nanomaterials in vivo are emphasized, including construction strategy, nanostructure modulation, and biomedical effects. This review highlights the importance of self-assembled peptide nanostructures for nanomedicine and can facilitate further knowledge and understanding of these nanosystems toward clinical translation.

99mTc-HYNIC-(Ser)3-LTVPWY peptide bearing tricine as co-ligand for targeting and imaging of HER2 overexpression tumor

Human epidermal growth factor receptor 2 (HER2) is overexpressed in several cancers. Today’s tumor targeting is receiving more attention due to its specificity to target receptor-dependent cancers. The aim of this study was to evaluate the 99mTc-HYNIC-(tricine)-(Ser)3-LTVPWY peptide for tumor targeting and imaging with overexpression of HER2. HYNIC-(Ser)3-LTVPWY peptide was labeled with 99mTc using tricine as a co-ligand at room temperature. Specific binding of this radiolabeled peptide was assessed on four cancer cell lines with different levels of HER2 receptor expression. Also the affinity of 99mTc-HYNIC-(tricine)-(Ser)3-LTVPWY peptide to the HER2 receptor was evaluated in the SKOV-3 cell line. Biodistribution study of this radiolabeled peptide was performed in SKOV-3 tumor bearing nude mice. The HYNIC conjugated peptide was simply labeled with 99mTc radionuclide with high labeling efficiency about 98±1% showing favorable stability in normal saline and human serum. In the presence of unlabeled peptide as competitor, the HER2 binding capacity of the radiolabeled peptide reduced (approximately five-fold). The KD and Bmax values were found 2.6±0.5 nM and (2.6±0.1)×106, respectively. The tumor/muscle ratios for this radiotracer were determined 1.17±0.77, 1.15±0.32 and 2.65±0.32 at 1, 2 and 4 h after injection, respectively. Presaturation of HER2 receptors in SKOV-3 xenografted nude mice showed a reduction in the tumor/muscle ratio confirming in vivo specificity of the peptide. According to SPECT imaging, the tumor was visualized in mouse after 4 h postinjection of radiolabeled peptide. 99mTc-HYNIC-(tricine)-(Ser)3-LTVPWY peptide exhibited overexpressed HER2 tumor targeting.

Designing a cancer therapeutic peptide by combining the mitochondrial targeting domain of Noxa and ErbB2-targeting moieties

Many anticancer drugs target epidermal growth factor receptors to inhibit receptor tyrosine kinases and tumor growth. Here, we show that an ErbB2-targeting pronecrotic peptide (KWSY:MTD) selectively kills tumor cells expressing ErbB2 in vitro. An antibody against ErbB2 inhibits KWSY:MTD-induced cell death. KWSY:MTD causes membrane permeability which allows propidium iodide entry into the cytosol and the release of HMGB1 into the media, indicative of necrosis. Mitochondrial swelling occurs in response to KWSY:MTD. Moreover, in vivo analysis using a mouse model shows that KWSY:MTD partially suppressed growth in tumor tissue bearing ErbB2-expressing cells, but did not have obvious toxicity in mouse liver or kidney tissue. Taken together, KWSY:MTD has potential as an ErbB2-targeting anticancer drug.

Targeting aggressive prostate cancer-associated CD44v6 using phage display selected peptides

There is a crucial need to identify new biomarkers associated with aggressive prostate cancer (PCa) including those associated with cancer stem cells (CSCs). CD44v6, generated by alternative splicing of CD44, has been proposed as a CSC biomarker due to its correlation with aggressive PCa disease. We hypothesized that phage display selected peptides that target CD44v6 may serve as theranostic agents for aggressive PCa. Here, a 15 amino acid peptide ("PFT") was identified by affinity selection against a peptide derived from the v6 region of CD44v6. Synthesized PFT exhibited specific binding to CD44v6 with an equilibrium dissociation constant (Kd) of 743.4 nM. PFT also bound CD44v6 highly expressed on human PCa cell lines. Further, an aggressive form of PCa cells (v6A3) was isolated and tagged by a novel CSC reporter vector. The v6A3 cells had a CSC-like phenotype including enriched CD44v6 expression, enhanced clonogenicity, resistance to chemotherapeutics, and generation of heterogeneous offspring. PFT exhibited preferential binding to v6A3 cells compared to parental cells. Immunohistofluorescence studies with human PCa tissue microarrays (TMA) indicated that PFT was highly accurate in detecting CD44v6-positive aggressive PCa cells, and staining positivity was significantly higher in late stage, metastatic and higher-grade samples. Taken together, this study provides for the first time phage display selected peptides that target CD44v6 overexpressed on PCa cells. Peptide PFT may be explored as an aid in the diagnosis and therapy of advanced PCa disease.

New Insights in the Design of Bioactive Peptides and Chelating Agents for Imaging and Therapy in Oncology

Many synthetic peptides have been developed for diagnosis and therapy of human cancers based on their ability to target specific receptors on cancer cell surface or to penetrate the cell membrane. Chemical modifications of amino acid chains have significantly improved the biological activity, the stability and efficacy of peptide analogues currently employed as anticancer drugs or as molecular imaging tracers. The stability of somatostatin, integrins and bombesin analogues in the human body have been significantly increased by cyclization and/or insertion of non-natural amino acids in the peptide sequences. Moreover, the overall pharmacokinetic properties of such analogues and others (including cholecystokinin, vasoactive intestinal peptide and neurotensin analogues) have been improved by PEGylation and glycosylation. Furthermore, conjugation of those peptide analogues to new linkers and bifunctional chelators (such as AAZTA, TETA, TRAP, NOPO etc.), produced radiolabeled moieties with increased half life and higher binding affinity to the cognate receptors. This review describes the most important and recent chemical modifications introduced in the amino acid sequences as well as linkers and new bifunctional chelators which have significantly improved the specificity and sensitivity of peptides used in oncologic diagnosis and therapy.

VPAC1 Targeted 64Cu-TP3805 kit preparation and its evaluation

INTRODUCTION

Previously, our laboratory has shown that ⁶⁴Cu-TP3805 can specifically target VPAC1 receptors and be used for positron emission tomography (PET) imaging of breast (BC) and prostate cancer (PC) in humans. Present work is aimed at the formulation of a freeze-dried diaminedithiol-peptide (N₂S₂-TP3805) kit and it's evaluation for the preparation of ⁶⁴Cu labeled TP3805. Parameters such as pH, temperature and incubation time were examined that influenced the radiolabeling efficiency and stability of the product.

METHODS

Kits were prepared under different conditions and radiolabeling efficiency of TP3805 kit was evaluated for a range of pH3.5-8.5, after addition of ⁶⁴Cu in 30μl, 0.1M HCl. Incubation temperature (37-90°C) and time (30-120min.) were also investigated. Kits were stored at -10°C and their long term stability was determined as a function of their radiolabeling efficiency. Further, stability of ⁶⁴Cu-TP3805 complex was evaluated in presence of fetal bovine serum and bovine serum albumin by using SDS polyacrylamide gel electrophoresis. Kits were then used for PET imaging of BC and PC following eIND (101550) and institutional approvals. Specificity of ⁶⁴Cu-TP3805 for VPAC1 was examined with digital autoradiography (DAR) of prostate tissues obtained after prostatectomy, benign prostatic hyperplasia (BPH) tissue, and benign and malignant lymph nodes. Results were compared with corresponding tissue histology.

RESULTS

Radiolabeling efficiency was ≥95% at final pH ~7.2 when incubated at 50°C for 90min. Kits were stable up to 18months when stored at -10°C, and ⁶⁴Cu-TP3805 complex exhibited excellent stability for up to 4h at room temperature. ⁶⁴Cu-TP3805 complex did not show any transchelation even after 2h incubation at 37°C in 10% FBS as well as in BSA as determined by SDS PAGE analysis. DAR identified ≥95% of malignant lesions 11 new PC lesions, 20 high grade prostatic intraepithelial neoplasia, 2/2 ejaculatory ducts and 5/5 urethra verumontanum not previously identified The malignant lymph nodes were correctly identified by DAR and for 3/3 BPH patients, and 5/5 cysts, DAR was negative. In human BC (n=19) and PC (n=26) were imaged with 100% sensitivity.

CONCLUSION

Availability of ready to use N₂S₂-peptide kits for ⁶⁴Cu labeling is convenient and eliminates possible day to day variation during its routine preparation for clinical use.

Tumor-targeting peptides from combinatorial libraries

Cancer is one of the major and leading causes of death worldwide. Two of the greatest challenges in fighting cancer are early detection and effective treatments with no or minimum side effects. Widespread use of targeted therapies and molecular imaging in clinics requires high affinity, tumor-specific agents as effective targeting vehicles to deliver therapeutics and imaging probes to the primary or metastatic tumor sites. Combinatorial libraries such as phage-display and one-bead one-compound (OBOC) peptide libraries are powerful approaches in discovering tumor-targeting peptides. This review gives an overview of different combinatorial library technologies that have been used for the discovery of tumor-targeting peptides. Examples of tumor-targeting peptides identified from each combinatorial library method will be discussed. Published tumor-targeting peptide ligands and their applications will also be summarized by the combinatorial library methods and their corresponding binding receptors.

Liposomes derivatized with multimeric copies of KCCYSL peptide as targeting agents for HER-2-overexpressing tumor cells

Mixed liposomes, obtained by coaggregation of 1,2-dioleoyl-sn-glycero-3-phosphocholine and of the synthetic monomer containing a gadolinium complex ([C18]₂DTPA[Gd]) have been prepared. Liposomes externally decorated with KCCYSL (P6.1 peptide) sequence in its monomeric, dimeric, and tetrameric forms are studied as target-selective delivery systems toward cancer cells overexpressing human epidermal growth factor receptor-2 (HER-2) receptors. Derivatization of liposomal surface with targeting peptides is achieved using the postmodification method: the alkyne-peptide derivative Pra-KCCYSL reacts, through click chemistry procedures, with a synthetic surfactant modified with 1, 2, or 4 azido moieties previously inserted in liposome formulation. Preliminary in vitro data on MDA-MB-231 and BT-474 cells indicated that liposomes functionalized with P6.1 peptide in its tetrameric form had better binding to and uptake into BT-474 cells compared to liposomes decorated with monomeric or dimeric versions of the P6.1 peptide. BT-474 cells treated with liposomes functionalized with the tetrameric form of P6.1 showed high degree of liposome uptake, which was comparable with the uptake of anti-HER-2 antibodies such as Herceptin. Moreover, magnetic MRI experiments have demonstrated the potential of liposomes to act as MRI contrast agents.

Cyclotron Production of High-Specific Activity 55Co and In Vivo Evaluation of the Stability of 55Co Metal-Chelate-Peptide Complexes

This work describes the production of high-specific activity 55Co and the evaluation of the stability of 55Co-metal-chelate-peptide complexes in vivo. 55Co was produced via the 58Ni(p,α)55Co reaction and purified using anion exchange chromatography with an average recovery of 92% and an average specific activity of 1.96 GBq/μmol. 55Co-DO3A and 55Co-NO2A peptide complexes were radiolabeled at 3.7 MBq/μg and injected into HCT-116 tumor xenografted mice. Positron emission tomography (PET) and biodistribution studies were performed at 24 and 48 hours postinjection and compared to those of 55CoCl2. Both 55Co-metal-chelate complexes demonstrated good in vivo stability by reducing the radiotracers' uptake in the liver by sixfold at 24 hours with ~ 1% ID/g and at 48 hours with ~ 0.5% ID/g and reducing uptake in the heart by fourfold at 24 hours with ~ 0.7% ID/g and sevenfold at 48 hours with ~ 0.35% ID/g. These results support the use of 55Co as a promising new radiotracer for PET imaging of cancer and other diseases.

A fluorescent protein scaffold for presenting structurally constrained peptides provides an effective screening system to identify high affinity target-binding peptides

Peptides that have high affinity for target molecules on the surface of cancer cells are crucial for the development of targeted cancer therapies. However, unstructured peptides often fail to bind their target molecules with high affinity. To efficiently identify high-affinity target-binding peptides, we have constructed a fluorescent protein scaffold, designated gFPS, in which structurally constrained peptides are integrated at residues K131-L137 of superfolder green fluorescent protein. Molecular dynamics simulation supported the suitability of this site for presentation of exogenous peptides with a constrained structure. gFPS can present 4 to 12 exogenous amino acids without a loss of fluorescence. When gFPSs presenting human epidermal growth factor receptor type 2 (HER2)-targeting peptides were added to the culture medium of HER2-expressing cells, we could easily identify the peptides with high HER2-affinity and -specificity based on gFPS fluorescence. In addition, gFPS could be expressed on the yeast cell surface and applied for a high-throughput screening. These results demonstrate that gFPS has the potential to serve as a powerful tool to improve screening of structurally constrained peptides that have a high target affinity, and suggest that it could expedite the one-step identification of clinically applicable cancer cell-binding peptides.

Development of a Radiolabeled Irreversible Peptide Ligand for PET Imaging of Vascular Endothelial Growth Factor

Imaging agents based on peptide probes have desirable pharmacokinetic properties provided that they have high affinities for their target in vivo. An approach to improve a peptide ligand's affinity for its target is to make this interaction covalent and irreversible. For this purpose, we evaluated a (64)Cu-labeled affinity peptide tag, (64)Cu-L19K-(5-fluoro-2,4-dinitrobenzene) ((64)Cu-L19K-FDNB), which binds covalently and irreversibly to vascular endothelial growth factor (VEGF) as a PET imaging agent. We compared the in vivo properties of (64)Cu-L19K-FDNB in VEGF-expressing tumor xenografts with its noncovalent binding analogs, (64)Cu-L19K-(2,4-dinitrophenyl) ((64)Cu-L19K-DNP) and (64)Cu-L19K.

METHODS

The L19K peptide (GGNECDIARMWEWECFERK-CONH2) was constructed with 1,4,7-triazacyclononane-1,4,7-triacetic acid at the N terminus for radiolabeling with (64)Cu with a polyethylene glycol spacer between peptide and chelate. 1,5-difluoro-2,4-dinitrobenzene was conjugated at the C-terminal lysine for cross-linking to VEGF, resulting in L19K-FDNB. (64)Cu-L19K-FDNB was assayed for covalent binding to VEGF in vitro. As a control, L19K was conjugated to 1-fluoro-2,4-dinitrobenzene, resulting in L19K-DNP. PET imaging and biodistribution studies of (64)Cu-L19K-FDNB, (64)Cu-L19K-DNP, and the native (64)Cu-L19K were compared in HCT-116 xenografts. Blocking studies of (64)Cu-L19K-FDNB was performed with a coinjection of excess unlabeled L19K-FDNB.

RESULTS

In vitro binding studies confirmed the covalent and irreversible binding of (64)Cu-L19K-FDNB to VEGF, whereas (64)Cu-L19K-DNP and (64)Cu-L19K did not bind covalently. PET imaging showed higher tumor uptake with (64)Cu-L19K-FDNB than with (64)Cu-L19K-DNP and (64)Cu-L19K, with mean standardized uptake values of 0.62 ± 0.05, 0.18 ± 0.06, and 0.34 ± 0.14, respectively, at 24 h after injection (P < 0.05), and 0.53 ± 0.05, 0.32 ± 0.14, and 0.30 ± 0.09, respectively, at 48 h after injection (P < 0.05). Blocking studies with (64)Cu-L19K-FDNB in the presence of excess unlabeled peptide showed a 53% reduction in tumor uptake at 48 h after injection.

CONCLUSION

In this proof-of-concept study, the use of a covalent binding peptide ligand against VEGF improves tracer accumulation at the tumor site in vivo, compared with its noncovalent binding peptide analogs. This technique is a promising tool to enhance the potency of peptide probes as imaging agents.

Improved PET imaging of uPAR expression using new (64)Cu-labeled cross-bridged peptide ligands: comparative in vitro and in vivo studies

The correlation between uPAR expression, cancer cell invasion and metastases is now well-established and has prompted the development of a number of uPAR PET imaging agents, which could potentially identify cancer patients with invasive and metastatic lesions. In the present study, we synthesized and characterized two new cross-bridged ⁶⁴Cu-labeled peptide conjugates for PET imaging of uPAR and performed a head-to-head comparison with the corresponding and more conventionally used DOTA conjugate. Based on in-source laser-induced reduction of chelated Cu(II) to Cu(I), we now demonstrate the following ranking with respect to the chemical inertness of their complexed Cu ions: DOTA-AE105 << CB-TE2A-AE105 < CB-TE2A-PA-AE105, which is correlated to their corresponding demetallation rate. No penalty in the uPAR receptor binding affinity of the targeting peptide was encountered by conjugation to either of the macrobicyclic chelators (IC₅₀ ~ 5-10 nM) and high yields and radiochemical purities (>95%) were achieved in all cases by incubation at 95ºC. In vivo, they display identical tumor uptake after 1h, but differ significantly after 22 hrs, where the DOTA-AE105 uptake remains surprisingly high. Importantly, the more stable of the new uPAR PET tracers, ⁶⁴Cu-CB-TE2A-PA-AE105, exhibits a significantly reduced liver uptake compared to ⁶⁴Cu-DOTA-AE105 as well as ⁶⁴Cu-CB-TE2A-AE105, (p<0.0001), emphasizing that our new in vitro stability measurements by mass spectrometry predicts in vivo stability in mice. Specificity of the best performing ligand, ⁶⁴Cu-CB-TE2A-PA-AE105 was finally confirmed in vivo using a non-binding ⁶⁴Cu-labeled peptide as control (⁶⁴Cu-CB-TE2A-PA-AE105^mut). This control PET-tracer revealed significantly reduced tumor uptake (p<0.0001), but identical hepatic uptake compared to its active counterpart (⁶⁴Cu-CB-TE2A-PA-AE105) after 1h. In conclusion, our new approach using in-source laser-induced reduction of Cu(II)-chelated PET-ligands provides useful information, which are predictive for the tracer stability in vivo in mice. Furthermore, the increased stability of our new macrobicyclic ⁶⁴Cu-CB-TE2A-PA-AE105 PET ligand is paralleled by an excellent imaging contrast during non-invasive PET scanning of uPAR expression in preclinical mouse cancer models. The translational promises displayed by this PET-tracer for future clinical cancer patient management remains, however, to be investigated.