Receptor-mediated tumor targeting with radiopeptides

Peptides Targeting HER2-Positive Breast Cancer Cells and Applications in Tumor Imaging and Delivery of Chemotherapeutics

Breast cancer represents the most common cancer type and one of the major leading causes of death in the female worldwide population. Overexpression of HER2, a transmembrane glycoprotein related to the epidermal growth factor receptor, results in a biologically and clinically aggressive breast cancer subtype. It is also the primary driver for tumor detection and progression and, in addition to being an important prognostic factor in women diagnosed with breast cancer, HER2 is a widely known therapeutic target for drug development. The aim of this review is to provide an updated overview of the main approaches for the diagnosis and treatment of HER2-positive breast cancer proposed in the literature over the past decade. We focused on the different targeting strategies involving antibodies and peptides that have been explored with their relative outcomes and current limitations that need to be improved. The review also encompasses a discussion on targeted peptides acting as probes for molecular imaging. By using different types of HER2-targeting strategies, nanotechnology promises to overcome some of the current clinical challenges by developing novel HER2-guided nanosystems suitable as powerful tools in breast cancer imaging, targeting, and therapy.

Anti-Cancer Peptides: Status and Future Prospects

The dramatic rise in cancer incidence, alongside treatment deficiencies, has elevated cancer to the second-leading cause of death globally. The increasing morbidity and mortality of this disease can be traced back to a number of causes, including treatment-related side effects, drug resistance, inadequate curative treatment and tumor relapse. Recently, anti-cancer bioactive peptides (ACPs) have emerged as a potential therapeutic choice within the pharmaceutical arsenal due to their high penetration, specificity and fewer side effects. In this contribution, we present a general overview of the literature concerning the conformational structures, modes of action and membrane interaction mechanisms of ACPs, as well as provide recent examples of their successful employment as targeting ligands in cancer treatment. The use of ACPs as a diagnostic tool is summarized, and their advantages in these applications are highlighted. This review expounds on the main approaches for peptide synthesis along with their reconstruction and modification needed to enhance their therapeutic effect. Computational approaches that could predict therapeutic efficacy and suggest ACP candidates for experimental studies are discussed. Future research prospects in this rapidly expanding area are also offered.

Anticancer peptides mechanisms, simple and complex

Peptide therapy has started since 1920s with the advent of insulin application, and now it has emerged as a new approach in treatment of diseases including cancer. Using anti-cancer peptides (ACPs) is a promising way of cancer therapy as ACPs are continuing to be approved and arrived at major pharmaceutical markets. Traditional cancer treatments face different problems like intensive adverse effects to patient's body, cell resistance to conventional chemical drugs and in some worse cases the occurrence of cell multidrug resistance (MDR) of cancerous tissues against chemotherapy. On the other hand, there are some benefits conceived for peptides usage in treatment of diseases specifically cancer, as these compounds present favorable characteristics such as smaller size, high activity, low immunogenicity, good biocompatibility in vivo, convenient and rapid way of synthesis, amenable to sequence modification and revision and there is no limitation for the type of cargo they carry. It is possible to achieve an optimum molecular and functional structure of peptides based on previous experience and bank of peptide motif data which may result in novel peptide design. Bioactive peptides are able to form pores in cell membrane and induce necrosis or apoptosis of abnormal cells. Moreover, recent researches have focused on the tumor recognizing peptide motifs with the ability to permeate to cancerous cells with the aim of cancer treatment at earlier stages. In this strategy the most important factors for addressing cancer are choosing peptides with easy accessibility to tumor cell without cytotoxicity effect towards normal cells. The peptides must also meet acceptable pharmacokinetic requirements. In this review, the characteristics of peptides and cancer cells are discussed. The various mechanisms of peptides' action proposed against cancer cells make the next part of discussion. It will be followed by giving information on peptides application, various methods of peptide designing along with introducing various databases. Future aspects of peptides for employing in area of cancer treatment come as conclusion at the end.

Utilization of nanotechnology in targeted radionuclide cancer therapy: monotherapy, combined therapy and radiosensitization

The rapid progress of nanomedicine field has a great influence on the different tumor therapeutic trends. It achieves a potential targeting of the therapeutic agent to the tumor site with neglectable exposure of the normal tissue. In nuclear medicine, nanocarriers have been employed for targeted delivery of therapeutic radioisotopes to the malignant tissues. This systemic radiotherapy is employed to overcome the external radiation therapy drawbacks. This review overviews studies concerned with investigation of different nanoparticles as promising carriers for targeted radiotherapy. It discusses the employment of different nanovehicles for achievement of the synergistic effect of targeted radiotherapy with other tumor therapeutic modalities such as hyperthermia and photodynamic therapy. Radiosensitization utilizing different nanosensitizer loaded nanoparticles has also been discussed briefly as one of the nanomedicine approach in radiotherapy.

Assessing the pharmacokinetics and toxicology of polymeric micelle conjugated therapeutics

Introduction: Analogous to nanocarriers such as nanoparticles, liposomes, nano lipoidal carriers, niosomes, and ethosomes, polymeric micelles have gained significance in the field of drug delivery. They have attracted scientists worldwide by their nanometric size, wide range of polymers available for building block synthesis, stability and potential to enhance the targeting and safety of drugs. Incorporation of drugs within the interior of polymeric micelles alters the drug pharmacokinetics, which generally results in increased efficiency.Areas covered: This review deals with the pharmacokinetics of various anti-neoplastic drugs loaded into micelles. The structure of polymeric micelles, polymers employed in their development and techniques involved will be discussed. This is followed by discussion on the pharmacokinetics of anti-cancer drugs loaded into polymeric micelles and the toxicity concerns associated.Expert opinion: Polymeric micelles are nanometeric carriers, with higher stability, polymeric flexibility and higher drug loading of poorly water-soluble drugs. These nanosystems help in increasing the bioavailability of drugs by encapsulating them within the hydrophobic core. The proper selection and design of the amphiphilic polymer for micelles is a crucial step as it decides the toxicity and the biocompatibility.

Peptide Ligands Specifically Targeting HER2 Receptor and the Role Played by a Synthetic Model System of the Receptor Extracellular Domain: Hypothesized Future Perspectives

A short (Fab)trastuzumab-derived peptide specific for HER2 receptor was identified. Its affinity for the model system HER2-DIVMP was found in a nanomolar range. The structural determinants responsible for the interaction between this ligand (A9) and HER2-DIVMP were investigated by both computational and NMR analysis. Next, the possibility of using A9 as HER2- specific probe for the nuclear medicine imaging was evaluated by conjugating A9 with the DTPA chelator and radiolabeling it with ¹¹¹In. The developed probe retained a nanomolar affinity to HER2-overexpressing cancer cells, however, some unspecific binding also occurred. The peptide internalization into cells by receptor-mediated endocytosis was also studied. Future perspectives are aimed at using A9 as a probe for molecular imaging diagnostics as well as active targeting of anticancer drugs. Lead structure optimization is needed to minimize the percentage of A9 unspecific binding and to increase the binding affinity to the receptor.

177Lu-labeled cyclic RGD peptide as an imaging and targeted radionuclide therapeutic agent in non-small cell lung cancer: Biological evaluation and preclinical study

Non-small cell lung carcinoma (NSCLC) is among the most lethal lung cancers responsible for 80-85% of death. α_vβ₃ integrin receptor subtype has been identified as a lung cancer biomarker since its expression correlates with tumor progression and metastasis. The extracellular domain of the receptor forms a binding site for RGD-based sequences. Therefore, specific targeting of α_vβ₃ integrin receptors by these short peptides can be an excellent candidate for cancer imaging and therapy. In this research, the radiolabeling of DOTA-E(cRGDfK)₂ with ¹⁷⁷Lu was efficiently implemented. The Log P value, in vivo, in vitro, metabolic stability, cellular uptake and specific binding of the radiopeptide was determined. The tumor targeting capacity and the therapeutic potential of the radiotracer was studied in A549 tumor-bearing mice. Imaging studies at different time intervals were performed by SPECT/CT. Radiochemical purity of more than 99% and Log P of -3.878 was obtained for ¹⁷⁷Lu-labelled peptide. Radiotracer showed favorable in vivo, in vitro and metabolic stability. The radiopeptide dissociation constant (K_d) was 15.07 nM. Radiopeptide specific binding was more than 95%. Biodistribution studies showed high accumulation of the radiopeptide in tumor and rapid excretion by urinary route. Maximum tumor uptake was at 4 h post-injection. Following administration of this radiopeptide to mice, not only tumor growth was suppressed, but significant tumor shrinkage was also observed. In conclusion, this radiopeptide can be employed for staging, follow-up imaging and as peptide receptor radionuclide therapeutic agent allowing efficient therapy for NSCLC and other cancers overexpressing α_vβ₃ integrin receptors.

Radiosynthesis, Biological Evaluation, and Preclinical Study of a 68Ga-Labeled Cyclic RGD Peptide as an Early Diagnostic Agent for Overexpressed α v β 3 Integrin Receptors in Non-Small-Cell Lung Cancer

The α_vβ₃ integrin receptors have high expression on proliferating growing tumor cells of different origins including non-small-cell lung cancer. RGD-containing peptides target the extracellular domain of integrin receptors. This specific targeting makes these short sequences a suitable nominee for theranostic application. DOTA-E(cRGDfK)₂ was radiolabeled with ⁶⁸Ga efficiently. The in vivo and in vitro stability was examined in different buffer systems. Metabolic stability was assessed in mice urine. In vitro specific binding, cellular uptake, and internalization were determined. The tumor-targeting potential of [⁶⁸Ga]Ga-DOTA-E(cRGDfK)₂ in a lung cancer mouse model was studied. Besides, the very early diagnostic potential of the ⁶⁸Ga-labeled RGD peptide was evaluated. The acquisition and reconstruction of the PET-CT image data were also carried out. Radiochemical and radionuclide purity for [⁶⁸Ga]Ga-DOTA-E(cRGDfK)₂ was >%98 and >%99, respectively. Radiotracer showed high in vivo, in vitro, and metabolic stability which was determined by ITLC. The dissociation constant (K_d) of [⁶⁸Ga]Ga-DOTA-E(cRGDfK)₂ was 15.28 nM. On average, more than 95% of the radioactivity was specific binding (internalized + surface-bound) to A549 cells. Biodistribution data showed that radiolabeled peptides were accumulated significantly in A549 tumor and excreted rapidly by the renal system. Tumor uptake peaks were at 1-hour postinjection for [⁶⁸Ga]Ga-DOTA-E(cRGDfK)₂. The tumor was clearly visualized in all images. [⁶⁸Ga]Ga-DOTA-E(cRGDfK)₂ can be used as a peptide-based imaging agent allowing very early detection of different cancers overexpressing α_vβ₃ integrin receptors and can be a potential candidate in clinical peptide-based imaging for lung cancer.

Receptor-Mediated Melanoma Targeting with Radiolabeled α-Melanocyte-Stimulating Hormone: Relevance of the Net Charge of the Ligand

A majority of melanotic and amelanotic melanomas overexpress melanocortin type 1 receptors (MC1Rs) for α-melanocyte-stimulating hormone. Radiolabeled linear or cyclic analogs of α-MSH have a great potential as diagnostic or therapeutic tools for the management of malignant melanoma. Compounds such as [¹¹¹In]DOTA-NAP-amide exhibit high affinity for the MC1R in vitro, good tumor uptake in vivo, but they may suffer from relatively high kidney uptake and retention in vivo. We have shown previously that the introduction of negative charges into radiolabeled DOTA-NAP-amide peptide analogs may enhance their excretion and reduce kidney retention. To address the question of where to place negative charges within the ligand, we have extended these studies by designing two novel peptides, Ac-Nle-Asp-His-d-Phe-Arg-Trp-Gly-Lys(DOTA)-d-Asp-d-Asp-OH (DOTA-NAP-d-Asp-d-Asp) with three negative charges at the C-terminal end (overall net charge of the molecule -2) and DOTA-Gly-Tyr(P)-Nle-Asp-His-d-Phe-Arg-Trp-NH₂ (DOTA-Phospho-MSH_2-9) with two negative charges in the N-terminal region (net charge -1). The former peptide showed markedly reduced receptor affinity and biological activity by >10-fold compared to DOTA-NAP-amide as reference compound, and the latter peptide displayed similar bioactivity and receptor affinity as the reference compound. The uptake by melanoma tumor tissue of [¹¹¹In]DOTA-Phospho-MSH_2-9 was 7.33 ± 0.47 %ID/g 4 h after injection, i.e., almost equally high as with [¹¹¹In]DOTA-NAP-amide. The kidney retention was 2.68 ± 0.18 %ID/g 4 h after injection and hence 44% lower than that of [¹¹¹In]DOTA-NAP-amide. Over an observation period from 4 to 48 h, the tumor-to-kidney ratio of [¹¹¹In]DOTA-Phospho-MSH_2-9 was 35% more favorable than that of the reference compound. In a comparison of DOTA-NAP-d-Asp-d-Asp, DOTA-Phospho-MSH_2-9 and DOTA-NAP-amide with five previously published analogs of DOTA-NAP-amide that altogether cover a range of peptides with an overall net charge between +2 and -2, we now demonstrate that a net charge of -1, with the extra negative charges preferably placed in the N-terminal region, has led to the lowest kidney uptake and retention. Charges of +2 or -2 markedly increased kidney uptake and retention. In conclusion, the novel DOTA-Phospho-MSH_2-9 may represent a new lead compound for negatively charged linear MC1R ligands that can be further developed into a clinically relevant melanoma targeting radiopeptide.

Preclinical validations of [18F]FPyPEGCBT-c(RGDfK): a 18F-labelled RGD peptide prepared by ligation of 2-cyanobenzothiazole and 1,2-aminothiol to image angiogenesis

Background

α_Vβ₃, α_Vβ₅ and α₅β₁ integrins are known to be involved in carcinogenesis and are overexpressed in many types of tumours compared to healthy tissues; thereby they have been selected as promising therapeutic targets. Positron emission tomography (PET) is providing a unique non-invasive screening assay to discriminate which patient is more prone to benefit from antiangiogenic therapies, and extensive research has been carried out to develop a clinical radiopharmaceutical that binds specifically to integrin receptors. We recently reported the synthesis of a new ¹⁸F-labelled RGD peptide prepared by 2-cyanobenzothiazole (CBT)/1,2-aminothiol conjugation. This study aims at characterising the preclinical biologic properties of this new tumour-targeting ligand, named [¹⁸F]FPyPEGCBT-c(RGDfK).

The in vitro binding properties of [¹⁸F]FPyPEGCBT-c(RGDfK) were analysed by standard binding assay in U-87 MG and SKOV-3 cancer models and its selectivity towards integrins by siRNA depletions. Its preclinical potential was studied in mice bearing subcutaneous tumours by ex vivo biodistribution studies and in vivo microPET/CT imaging.

Results

In vitro, FPyPEGCBT-c(RGDfK) efficiently bound RGD-recognising integrins as compared to a control c(RGDfV) peptide (IC₅₀ = 30.8 × 10⁻⁷ M vs. 6.0 × 10⁻⁷ M). [¹⁸F]FPyPEGCBT-c(RGDfK) cell uptake was mediated by an active transport through binding to α_V, β₃ and β₅ but not to β₁ subunits. In vivo, this new tracer demonstrated specific tumour uptake with %ID/g of 2.9 and 2.4 in U-87 MG and SKOV-3 tumours 1 h post injection. Tumour-to-muscle ratios of 4 were obtained 1 h after intravenous administration of the tracer allowing good visualisation of the tumours. However, unfavourable background accumulation and high hepatobiliary excretion were observed.

Conclusion

[¹⁸F]FPyPEGCBT-c(RGDfK) specifically detects tumours expressing RGD-recognising integrin receptors in preclinical studies. Further optimisation of this radioligand may yield candidates with improved imaging properties and would warrant the further use of this efficient labelling technique for alternative targeting vectors.

Electronic supplementary material

The online version of this article (doi:10.1186/s41181-016-0019-z) contains supplementary material, which is available to authorized users.

Nanoparticle ligand presentation for targeting solid tumors

Among the many scientific advances to come from the study of nanoscience, the development of ligand-targeted nanoparticles to eliminate solid tumors is predicted to have a major impact on human health. There are many reports describing novel designs and testing of targeted nanoparticles to treat cancer. While the principles of the technology are well demonstrated in controlled lab experiments, there are still many hurdles to overcome for the science to mature into truly efficacious targeted nanoparticles that join the arsenal of agents currently used to treat cancer in humans. One of these hurdles is overcoming unwanted biodistribution to the liver while maximizing delivery to the tumor. This almost certainly requires advances in both nanoparticle stealth technology and targeting. Currently, it continues to be a challenge to control the loading of ligands onto polyethylene glycol (PEG) to achieve maximal targeting. Nanoparticle cellular uptake and subcellular targeting of genes and siRNA also remain a challenge. This review examines the types of ligands that have been most often used to target nanoparticles to solid tumors. As the science matures over the coming decade, careful control over ligand presentation on nanoparticles of precise size, shape, and charge will likely play a major role in achieving success.

Differentiated thyroid cancer: a new perspective with radiolabeled somatostatin analogues for imaging and treatment of patients

BACKGROUND

The expression of somatostatin receptors (SSTR) in thyroid cells may offer the possibility to identify metastatic lesions and to select patients for peptide receptor radionuclide therapy (PRRT). We investigated (68)Ga-DOTATOC positron emission tomography/computed tomography (PET/CT) to select patients with progressive differentiated thyroid cancer (DTC) for PRRT as well as treatment response and toxicity in treated patients.

METHODS

We enrolled 41 patients with progressive radioiodine-negative DTC (24 women and 17 men; mean age=54.3 years, median=59 years, range=19-78 years). In all patients, [(18)F]FDG-PET/CT was performed to determine recurrent disease with enhanced glucose metabolism, and (68)Ga-DOTATOC PET/CT was used to identify SSTR expression. Dosimetric evaluation was performed with (111)In-DOTATOC scintigraphy. Eleven patients were treated with PRRT receiving a fractionated injection of 1.5-3.7 GBq (90)Y-DOTATOC/administration. Serial (68)Ga-DOTATOC PET/CT scans were performed in all treated patients to evaluate treatment response. Parameters provided by (68)Ga-DOTATOC PET/CT were analyzed as potential therapeutic predictors to differentiate responding from nonresponding. In all treated patients, adverse events and toxicity were recorded.

RESULTS

(68)Ga-DOTATOC PET/CT were positive in 24/41 of radioiodine-negative DTC patients. Based on the high expression of SSTR detected by (68)Ga-DOTATOC PET/CT, 13 patients were suitable for PRRT. Two out of 13 patients were not treated due to the lack of fulfillment of other study inclusion criteria. PRRT induced disease control in 7/11 patients (two partial response and five stabilization) with a duration of response of 3.5-11.5 months. Objective response was associated with symptoms relief. Functional volume (FV) over time obtained by PET/CT was the only parameter demonstrating a significant difference between lesions responding and nonresponding to PRRT (p=0.001). Main PRRT adverse events were nausea, asthenia, and transient hematologic toxicity. One patient experienced permanent renal toxicity.

CONCLUSIONS

In our series, SSTR imaging provided positive results in more than half of the cases with radioiodine-negative DTC, and about one third of patients were eligible for PRRT. (68)Ga-DOTATOC PET/CT seems a reliable tool both for patient selection and evaluation of treatment response. In our experience, FV determination over time seems to represent a reliable parameter to determine tumor response to PRRT, although further investigations are needed to better define its role.

Peptides in receptor-mediated radiotherapy: from design to the clinical application in cancers

Short peptides can show high affinity for specific receptors overexpressed on tumor cells. Some of these are already used in cancerology as diagnostic tools and others are in clinical trials for therapeutic applications. Therefore, peptides exhibit great potential as a diagnostic tool but also as an alternative or an additional antitumoral approach upon the covalent attachment of a therapeutic moiety such as a radionuclide or a cytotoxic drug. The chemistry offers flexibility to graft onto the targeting-peptide either fluorine or iodine directly, or metallic radionuclides through appropriate chelating agent. Since short peptides are straightforward to synthesize, there is an opportunity to further improve existing peptides or to design new ones for clinical applications. However, several considerations have to be taken into account to optimize the recognition properties of the targeting-peptide to its receptor, to improve its stability in the biological fluids and its residence in the body, or to increase its overall therapeutic effect. In this review, we highlight the different aspects which need to be considered for the development of an efficient peptide receptor-mediated radionuclide therapy in different neoplasms.

MSH radiopeptides for targeting melanoma metastases

Radiolabeled peptides have become important tools for preclinical cancer research and in nuclear oncology they serve as diagnostic and more recently also as therapeutic agents. Whereas the development of receptor-mediated targeting for therapy has been confined to some radiolabeled antibodies and somatostatin/SRIF analogs, recent research into radiolabeled α-Melanocyte-stimulating hormone (α-MSH) and its receptor MC1R (over-)expressed by melanoma tumor cells has demonstrated that small metastatic melanoma lesions in experimental animals are specifically targeted by MSH radiopeptides. Thus MSH radiopharmaceuticals will eventually open a new avenue for the treatment of melanoma metastases in man, provided that the targeting efficiency can be further enhanced and nonspecific incorporation into nontarget organs, e.g., the kidneys, minimized. Some novel MSH lead compounds containing a glyco moiety, added negatively charged groups or a cyclic structure show very promising in vivo targeting characteristics.

Rationale for the use of radiolabelled peptides in diagnosis and therapy

Nuclear medicine techniques are becoming more important in imaging oncological and infectious diseases. For metabolic imaging of these diseases, antibody and peptide imaging are currently used. In recent years peptide imaging has become important, therefore the rationale for the use of peptide imaging is described in this article. Criteria for a successful peptide tracer are a high target specificity, a high binding affinity, a long metabolic stability and a high target-to-background ratio. Tracer internalization is also beneficial. For oncological imaging, many tracers are available, most originating from regulatory peptides, but penetrating peptides are also being developed. Peptides for imaging inflammatory and infectious diseases include regulatory peptides, antimicrobial peptides and others. In conclusion, for the imaging of oncological, imflammatory and infectious diseases, many promising peptides are being developed. The ideal peptide probe is characterized by rapid and specific target localization and binding with a high tumour-to-background ratio.

The evolution of imaging in cancer: current state and future challenges

Molecular imaging allows for the remote, noninvasive sensing and measurement of cellular and molecular processes in living subjects. Drawing upon a variety of modalities, molecular imaging provides a window into the biology of cancer from the subcellular level to the patient undergoing a new, experimental therapy. As signal transduction cascades and protein interaction networks become clarified, an increasing number of relevant targets for cancer therapy--and imaging--become available. Although conventional imaging is already critical to the management of patients with cancer, molecular imaging will provide even more relevant information, such as early detection of changes with therapy, identification of patient-specific cellular and metabolic abnormalities, and the disposition of therapeutic, gene-tagged cells throughout the body--all of which will have a considerable impact on morbidity and mortality. This overview discusses molecular imaging in oncology, providing examples from a variety of modalities, with an emphasis on emerging techniques for translational imaging.

Radiometallated peptides targeting guanylate cyclase C and the urokinase-type plasminogen activator receptor

Research is currently underway worldwide into the development of receptor-specific radiopharmaceuticals for the imaging and treatment of cancer. The successful clinical development of radiolabeled somatostatin analogs for imaging and treatment of cancers overexpressing somatostatin receptors has catalyzed further preclinical investigation of other radiolabeled peptides for molecular imaging and peptide-receptor radiotherapy, including such well-studied peptide vectors as cholecystokinin, neurotensin, bombesin and RGD peptides. Within this larger context, this article will focus on the current status of two more recent additions to the list of molecular imaging targets – guanylate cyclase C, a specific marker for colorectal cancer, and the urokinase plasminogen activator receptor, a cell-surface receptor overexpressed in diverse cancer types.