Dimerization of a phage-display selected peptide for imaging of αvβ6- integrin: two approaches to the multivalent effect

A stapled chromogranin A-derived peptide homes in on tumors that express αvβ6 or αvβ8 integrins

Rationale: The αvβ6- and αvβ8-integrins, two cell-adhesion receptors upregulated in many tumors and involved in the activation of the latency associated peptide (LAP)/TGFβ complex, represent potential targets for tumor imaging and therapy. We investigated the tumor-homing properties of a chromogranin A-derived peptide containing an RGDL motif followed by a chemically stapled alpha-helix (called "5a"), which selectively recognizes the LAP/TGFβ complex-binding site of αvβ6 and αvβ8. Methods: Peptide 5a was labeled with IRDye 800CW (a near-infrared fluorescent dye) or with ¹⁸F-NOTA (a label for positron emission tomography (PET)); the integrin-binding properties of free peptide and conjugates were then investigated using purified αvβ6/αvβ8 integrins and various αvβ6/αvβ8 single - or double-positive cancer cells; tumor-homing, biodistribution and imaging properties of the conjugates were investigated in subcutaneous and orthotopic αvβ6-positive carcinomas of the pancreas, and in mice bearing subcutaneous αvβ8-positive prostate tumors. Results: In vitro studies showed that 5a can bind both integrins with high affinity and inhibits cell-mediated TGFβ activation. The 5a-IRDye and 5a-NOTA conjugates could bind purified αvβ6/αvβ8 integrins with no loss of affinity compared to free peptide, and selectively recognized various αvβ6/αvβ8 single- or double-positive cancer cells, including cells from pancreatic carcinoma, melanoma, oral mucosa, bladder and prostate cancer. In vivo static and dynamic optical near-infrared and PET/CT imaging and biodistribution studies, performed in mice with subcutaneous and orthotopic αvβ6-positive carcinomas of the pancreas, showed high target-specific uptake of fluorescence- and radio-labeled peptide by tumors and low non-specific uptake in other organs and tissues, except for excretory organs. Significant target-specific uptake of fluorescence-labeled peptide was also observed in mice bearing αvβ8-positive prostate tumors. Conclusions: The results indicate that 5a can home to αvβ6- and/or αvβ8-positive tumors, suggesting that this peptide can be exploited as a ligand for delivering imaging or anticancer agents to αvβ6/αvβ8 single- or double-positive tumors, or as a tumor-homing inhibitor of these TGFβ activators.

Up-regulation of a stress-responsive endochitinase VvChit-IV in grapevine cell cultures improves in vitro stress tolerance

Chitinases are pathogenesis-related proteins, which play an important role in plant growth regulation, defense mechanism, and stress tolerance. Embryogenic cultures from Vitis vinifera cv. Tempranillo exposed to in vitro stress exhibited the expression of an extracellular class IV endochitinase VvChit-IV. Phylogenetic and conserved motif analyses provided insights into the evolutionary relationships of chitinases. A computation-based investigation showed conserved domains and illustrated a chitin-binding site for chitin cleavage with a catalytic domain of glycoside hydrolase. Interestingly, gene expression pattern showed a differential expression of VvChit-IV associated with embryonic stress response to in vitro conditions. In response to in vitro stress, transcript level of VvChit-IV increased in embryogenic calli and cell suspensions and peaked at 1.5 and 3 folds, respectively, when compared to an internal reference gene. Evidence of tissue culture stress-induced endochitinase was reported here for the first time indicating that in vitro stress could mitigate elicitor application to induce chitinase expression and can stimulate an immune response against abiotic constraints. Data showed that up-regulation of VvChit-IV was associated with a substantial increase of H₂O₂ and proline without significant change in malondialdehyde content suggesting that the H₂O₂ signaling network might trigger a priming effect to boost the defense response against environmental stress. Endochitinase activation in plant stress mitigation was thus highlighted to improve tolerance through attenuation of oxidative stress. This study revealed that the grapevine endochitinase is promising for enhancing coping-oriented adaptation and abiotic stress tolerance, which gives new insights into its feasibility for use in cross-tolerance and crop improvement.

Versatile Synthesis of Multivalent Porphyrin–Peptide Conjugates by Direct Porphyrin Construction on Resin

Multifunctional porphyrin–peptide conjugates with different propensities for self‐assembly into various supramolecular nanoarchitectures play important roles in advanced materials and biomedical research. However, preparing prefunctionalized core porphyrins by traditional low‐yielding statistical synthesis and purifying them after peptide ligation through many rounds of HPLC purification is tedious and unsustainable. Herein, we report a novel integrated solid‐phase synthetic protocol for the construction of porphyrin moieties from simple aldehydes and dipyrromethanes on resin‐bound peptides directly to form mono‐, cis/trans‐di‐, and trivalent porphyrin–peptide conjugates in a highly efficient and controllable manner; moreover, only single final‐stage HPLC purification of the products is needed. This efficient strategy enables the rapid, greener, and substrate‐controlled diversity‐oriented synthesis of multivalent porphyrin–(long) peptide conjugate libraries for multifarious biological and materials applications.

68Ga-Labeled Cystine Knot Peptide Targeting Integrin αvβ6 for Lung Cancer PET Imaging

Integrin α_vβ₆ has been considered as a promising biomarker for lung cancer, and its expression is often related to poor prognosis. An α_vβ₆-binding cystine knot peptide R₀1-MG was previously engineered and validated. Here, we developed a positron emission tomography (PET) probe of R₀1-MG for imaging α_vβ₆-positive lung cancer. Cystine knot peptide R₀1-MG was synthesized through solid-phase peptide synthesis chemistry and radiolabeled with ⁶⁸Ga after being conjugated with 1,4,7,10-tetraazacyclododecane-N,N',N″,N‴-tetraacetic acid (DOTA). The stability of ⁶⁸Ga-DOTA-R₀1-MG was analyzed in phosphate-buffered saline (PBS) (pH 7.4) and fetal bovine serum (FBS). The cell uptake assay of the probe was evaluated using α_vβ₆-positive (A549 and H1975) and α_vβ₆-negative (H1299) lung cancer cell lines. In addition, small animal PET imaging and biodistribution studies of ⁶⁸Ga-DOTA-R₀1-MG were performed in α_vβ₆-positive and α_vβ₆-negative lung cancer models. Our study showed that ⁶⁸Ga-DOTA-R₀1-MG exhibited excellent stability in PBS and FBS. Small animal PET imaging and biodistribution data revealed that ⁶⁸Ga-DOTA-R₀1-MG displayed rapid and good tumor uptake in animal models with α_vβ₆-positive lung cancer, and the probe was rapidly cleared from the normal tissues, resulting in good tumor-to-normal tissue contrasts. Meanwhile, no obvious tumor uptake of ⁶⁸Ga-DOTA-R₀1-MG was observed in animal models with α_vβ₆-negative lung cancer, demonstrating specific binding of the probe to integrin α_vβ₆. In conclusion, ⁶⁸Ga-DOTA-R₀1-MG has great potential to be a promising PET tracer for imaging α_vβ₆-positive lung cancer.

Evaluation of Copper-64-Labeled αvβ6-Targeting Peptides: Addition of an Albumin Binding Moiety to Improve Pharmacokinetics

The incorporation of non-covalent albumin binding moieties (ABMs) into radiotracers results in increased circulation time, leading to a higher uptake in the target tissues such as the tumor, and, in some cases, reduced kidney retention. We previously developed [¹⁸F]AlF NOTA-K(ABM)-α_vβ₆-BP, where α_vβ₆-BP is a peptide with high affinity for the cell surface receptor integrin α_vβ₆ that is overexpressed in several cancers, and the ABM is an iodophenyl-based moiety. [¹⁸F]AlF NOTA-K(ABM)-α_vβ₆-BP demonstrated prolonged blood circulation compared to the non-ABM parent peptide, resulting in high, α_vβ₆-targeted uptake with continuously improving detection of α_vβ₆(+) tumors using PET/CT. To further extend the imaging window beyond that of fluorine-18 (t_1/2 = 110 min) and to investigate the pharmacokinetics at later time points, we radiolabeled the α_vβ₆-BP with copper-64 (t_1/2 = 12.7 h). Two peptides were synthesized without (1) and with (2) the ABM and radiolabeled with copper-64 to yield [⁶⁴Cu]1 and [⁶⁴Cu]2, respectively. The affinity of [^natCu]1 and [^natCu]2 for the integrin α_vβ₆ was assessed by enzyme-linked immunosorbent assay. [⁶⁴Cu]1 and [⁶⁴Cu]2 were evaluated in vitro (cell binding and internalization) using DX3puroβ6 (α_vβ₆(+)), DX3puro (α_vβ₆(-)), and pancreatic BxPC-3 (α_vβ₆(+)) cells, in an albumin binding assay, and for stability in both mouse and human serum. In vivo (PET/CT imaging) and biodistribution studies were done in mouse models bearing either the paired DX3puroβ6/DX3puro or BxPC-3 xenograft tumors. [⁶⁴Cu]1 and [⁶⁴Cu]2 were synthesized in ≥97% radiochemical purity. In vitro, [^natCu]1 and [^natCu]2 maintained low nanomolar affinity for integrin α_vβ₆ (IC₅₀ = 28 ± 3 and 19 ± 5 nM, respectively); [⁶⁴Cu]1 and [⁶⁴Cu]2 showed comparable binding to α_vβ₆(+) cells (DX3puroβ6: ≥70%, ≥42% internalized; BxPC-3: ≥19%, ≥12% internalized) and ≤3% to the α_vβ₆(-) DX3puro cells. Both radiotracers were ≥98% stable in human serum at 24 h, and [⁶⁴Cu]2 showed a 6-fold higher binding to human serum protein than [⁶⁴Cu]1. In vivo, selective uptake in the α_vβ₆(+) tumors was observed with tumor visualization up to 72 h for [⁶⁴Cu]2. A 3-5-fold higher α_vβ₆(+) tumor uptake of [⁶⁴Cu]2 vs [⁶⁴Cu]1 was observed throughout, at least 2.7-fold improved BxPC-3-to-kidney and BxPC-3-to-blood ratios, and 2-fold improved BxPC-3-to-stomach ratios were noted for [⁶⁴Cu]2 at 48 h. Incorporation of an iodophenyl-based ABM into the α_vβ₆-BP ([⁶⁴Cu]2) prolonged circulation time and resulted in improved pharmacokinetics, including increased uptake in α_vβ₆(+) tumors that enabled visualization of α_vβ₆(+) tumors up to 72 h by PET/CT imaging.

It's Time to Shift the Paradigm: Translation and Clinical Application of Non-αvβ3 Integrin Targeting Radiopharmaceuticals

Simple Summary

Cancer cells often present a different set of proteins on their surface than normal cells. This also applies to integrins, a class of 24 cell surface receptors which mainly are responsible for physically anchoring cells in tissues, but also fulfil a plethora of other functions. If a certain integrin is found on tumor cells but not on normal ones, radioactive molecules (named tracers) that specifically bind to this integrin will accumulate in the cancer lesion if injected into the blood stream. The emitted radiation can be detected from outside the body and allows for localization and thus, diagnosis, of cancer. Only one of the 24 integrins, the subtype αvβ3, has hitherto been thoroughly investigated in this context. We herein summarize the most recent, pertinent research on other integrins, and argue that some of these approaches might ultimately improve the clinical management of the most lethal cancers, such as pancreatic carcinoma.

Abstract

For almost the entire period of the last two decades, translational research in the area of integrin-targeting radiopharmaceuticals was strongly focused on the subtype αvβ3, owing to its expression on endothelial cells and its well-established role as a biomarker for, and promoter of, angiogenesis. Despite a large number of translated tracers and clinical studies, a clinical value of αvβ3-integrin imaging could not be defined yet. The focus of research has, thus, been moving slowly but steadily towards other integrin subtypes which are involved in a large variety of tumorigenic pathways. Peptidic and non-peptidic radioligands for the integrins α5β1, αvβ6, αvβ8, α6β1, α6β4, α3β1, α4β1, and αMβ2 were first synthesized and characterized preclinically. Some of these compounds, targeting the subtypes αvβ6, αvβ8, and α6β1/β4, were subsequently translated into humans during the last few years. αvβ6-Integrin has arguably attracted most attention because it is expressed by some of the cancers with the worst prognosis (above all, pancreatic ductal adenocarcinoma), which substantiates a clinical need for the respective theranostic agents. The receptor furthermore represents a biomarker for malignancy and invasiveness of carcinomas, as well as for fibrotic diseases, such as idiopathic pulmonary fibrosis (IPF), and probably even for Sars-CoV-2 (COVID-19) related syndromes. Accordingly, the largest number of recent first-in-human applications has been reported for radiolabeled compounds targeting αvβ6-integrin. The results indicate a substantial clinical value, which might lead to a paradigm change and trigger the replacement of αvβ3 by αvβ6 as the most popular integrin in theranostics.

There is a world beyond αvβ3-integrin: Multimeric ligands for imaging of the integrin subtypes αvβ6, αvβ8, αvβ3, and α5β1 by positron emission tomography

BACKGROUND

In the context of nuclear medicine and theranostics, integrin-related research and development was, for most of the time, focused predominantly on 'RGD peptides' and the subtype αvβ3-integrin. However, there are no less than 24 known integrins, and peptides without the RGD sequence as well as non-peptidic ligands play an equally important role as selective integrin ligands. On the other hand, multimerization is a well-established method to increase the avidity of binding structures, but multimeric radiopharmaceuticals have not made their way into clinics yet. In this review, we describe how these aspects have been interwoven in the framework of the German Research Foundation's multi-group interdisciplinary funding scheme CRC 824, yielding a series of potent PET imaging agents for selective imaging of various integrin subtypes.

RESULTS

The gallium-68 chelator TRAP was utilized to elaborate symmetrical trimers of various peptidic and non-peptidic integrin ligands. Preclinical data suggested a high potential of the resulting Ga-68-tracers for PET-imaging of the integrins α5β1, αvβ8, αvβ6, and αvβ3. For the first three, we provide some additional immunohistochemistry data in human cancers, which suggest several future clinical applications. Finally, application of αvβ3- and αvβ6-integrin tracers in pancreatic carcinoma patients revealed that unlike αvβ3-targeted PET, αvβ6-integrin PET is not characterized by off-target uptake and thus, enables a substantially improved imaging of this type of cancer.

CONCLUSIONS

Novel radiopharmaceuticals targeting a number of different integrins, above all, αvβ6, have proven their clinical potential and will play an increasingly important role in future theranostics.

Insights on the emerging biotechnology of histidine-rich peptides

In the late 70's, the discovery of the restriction enzymes made possible the biological production of functional proteins by recombinant DNA technologies, a fact that largely empowered both biotechnological and pharmaceutical industries. Short peptides or small protein domains, with specific molecular affinities, were developed as purification tags in downstream processes to separate the target protein from the culture media or cell debris, upon breaking the producing cells. Among these tags, and by exploiting the interactivity of the imidazole ring of histidine residues, the hexahistidine peptide (H6) became a gold standard. Although initially used almost exclusively in protein production, H6 and related His-rich peptides are progressively proving a broad applicability in novel utilities including enzymatic processes, advanced drug delivery systems and diagnosis, through a so far unsuspected adaptation of their binding capabilities. In this context, the coordination of histidine residues and metals confers intriguing functionalities to His-rich sequences useable in the forward-thinking design of protein-based nano- and micro-materials and devices, through strategies that are comprehensively presented here.

Evaluation of Two Optical Probes for Imaging the Integrin αvβ6- In Vitro and In Vivo in Tumor-Bearing Mice

PURPOSE

The purpose of this study was to develop and evaluate two α_vβ₆-targeted fluorescent imaging agents. The integrin subtype α_vβ₆ is significantly upregulated in a wide range of epithelial derived cancers, plays a key role in invasion and metastasis, and expression is often located at the invasive edge of tumors. α_vβ₆-targeted fluorescent imaging agents have the potential to guide surgical resection leading to improved patient outcomes. Both imaging agents were based on the bi-PEGylated peptide NH₂-PEG₂₈-A20FMDV2-K16R-PEG₂₈ (1), a peptide that has high affinity and selectivity for the integrin α_vβ₆: (a) 5-FAM-X-PEG₂₈-A20FMDV2-K16R-PEG₂₈ (2), and (b) IRDye800-PEG₂₈-A20FMDV2-K16R-PEG₂₈ (3).

PROCEDURES

Peptides were synthesized using solid-phase peptide synthesis and standard Fmoc chemistry. Affinity for α_vβ₆ was evaluated by ELISA. In vitro binding, internalization, and localization of 2 was monitored using confocal microscopy in DX3puroβ₆ (α_vβ₆+) and DX3puro (α_vβ₆-) cells. The in vivo imaging and ex vivo biodistribution of 3 was evaluated in three preclinical mouse models, DX3puroβ₆/DX3puro and BxPC-3 (α_vβ₆+) tumor xenografts and a BxPC-3 orthotopic pancreatic tumor model.

RESULTS

Peptides were obtained in > 99% purity. IC₅₀ values were 28 nM (2) and 39 nM (3). Rapid α_vβ₆-selective binding and internalization of 2 was observed. Fluorescent intensity (FLI) measurements extracted from the in vivo images and ex vivo biodistribution confirmed uptake and retention of 3 in the α_vβ₆ positive subcutaneous and orthotopic tumors, with negligible uptake in the α_vβ₆-negative tumor. Blocking studies with a known α_vβ₆-targeting peptide demonstrated α_vβ₆-specific binding of 3.

CONCLUSION

Two fluorescence imaging agents were developed. The α_vβ₆-specific uptake, internalization, and endosomal localization of the fluorescence agent 2 demonstrates potential for targeted therapy. The selective uptake and retention of 3 in the α_vβ₆-positive tumors enabled clear delineation of the tumors and surgical resection indicating 3 has the potential to be utilized during image-guided surgery.

Development and characterization of a 68Ga-labeled A20FMDV2 peptide probe for the PET imaging of αvβ6 integrin-positive pancreatic ductal adenocarcinoma

Pancreatic ductal adenocarcinoma (PDAC) is known to be one of the most lethal cancers. Since the majority of patients are diagnosed at an advanced stage, development of a detection method for PDAC at an earlier stage of disease progression is strongly desirable. Integrin αVβ6 is a promising target for early PDAC detection because its expression increases during precancerous changes. The present study aimed to develop an imaging probe for positron emission tomography (PET) which targets αVβ6 integrin-positive PDAC. We selected A20FMDV2 peptide, which binds specifically to αvβ6 integrin, as a probe scaffold, and 68Ga as a radioisotope. A20FMDV2 peptide has not been previously labeled with 68Ga. A cysteine residue was introduced to the N-terminus of the probe at a site-specific conjugation of maleimide-NOTA (mal-NOTA) chelate. Different numbers of glycine residues were also introduced between cysteine and the A20FMDV2 sequence as a spacer in order to reduce the steric hindrance of the mal-NOTA on the binding probe to αVβ6 integrin. In vitro, the competitive binding assay revealed that probes containing a 6-glycine linker ([natGa]CG6 and [natGa]Ac-CG6) showed high affinity to αVβ6 integrin. Both probes could be labeled by 67/68Ga with high radiochemical yield (>50%) and purity (>98%). On biodistribution analysis, [67Ga]Ac-CG6 showed higher tumor accumulation, faster blood clearance, and lower accumulation in the surrounding organs of pancreas than did [67Ga]CG6. The αVβ6 integrin-positive xenografts were clearly visualized by PET imaging with [68Ga]Ac-CG6. The intratumoral distribution of [68Ga]Ac-CG6 coincided with the αVβ6 integrin-positive regions detected by immunohistochemistry. Thus, [68Ga]Ac-CG6 is a useful peptide probe for the imaging of αVβ6 integrin in PDAC.

Development of a novel drug targeting delivery system for cervical cancer therapy

'Targeting peptides' have demonstrated their value in diagnostic imaging and therapy and novel peptide probes specific to cervical cancer were developed. In the M13KE phage dodecapeptide (12-mer) peptide library, the phage clone S7 showed the best binding to the cancer cells as confirmed by immunofluorescence and flow cytometry assays, and was selected for continued studies. Its binding peptide, CSP3, was synthesized from the sequence of S7's 12-mer at the N-terminus of the minor coat protein pIII of this M13KE phage vector. The peptide's binding was analyzed by the same assays used for S7. It was also assessed using competitive inhibition and binding to a tissue chip. The results demonstrated that the CSP3 peptide bound to cervical carcinoma cells with high sensitivity and specificity. The positive results indicated that the peptide CSP3, conjugated with nanomaterials and chemotherapeutics, may be developed as a targeting vehicle for therapeutic drug delivery against cervical cancer, especially cervical cancer with multiple drug resistance. For this aim, we prepared a CSP3 conjugated liposome drug delivery system containing doxorubicin (DOX) and microRNA101 (miR101) expression plasmids (CSP3-Lipo-DOX-miR101), and the primary result showed that the system demonstrated significantly enhanced cytotoxicity to SiHa cells and DOX resistant SiHa cells, SiHa/ADR. Our results showed that CSP3 is a cervical cancer targeting 12aa peptide with high specificity and sensitivity, and the CSP3 conjugated drug delivery system, CSP3-Lipo-DOX-miR101 has promising potential for development as an efficient drug system for the therapy of cervical cancer.

Small-animal SPECT/CT imaging of cancer xenografts and pulmonary fibrosis using a 99mTc-labeled integrin αvβ6-targeting cyclic peptide with improved in vivo stability

Abstract

Integrin αvβ6 is expressed at an undetectable level in normal tissues, but is remarkably upregulated during many pathological processes, especially in cancer and fibrosis. Noninvasive imaging of integrin αvβ6 expression using a radiotracer with favorable in vivo pharmacokinetics would facilitate disease diagnosis and therapy monitoring. Through disulfide-cyclized method, we synthesized in this study, a new integrin αvβ6-targeted cyclic peptide (denoted as cHK), and radiolabeled it with ^99mTc. The ability of the resulting radiotracer ^99mTc–HYNIC–cHK to detect integrin αvβ6 expression in pancreatic cancer xenografts and idiopathic pulmonary fibrosis was evaluated using small-animal single-photon emission computed tomography (SPECT)/computed tomography (CT). ^99mTc–HYNIC–cHK showed significantly improved in vivo metabolic stability compared to the linear peptide-based radiotracer ^99mTc–HYNIC–HK. ^99mTc–HYNIC–cHK exhibited similar biodistribution properties to ^99mTc–HYNIC–HK, but the tumor-to-muscle ratio was significantly increased (2.99 ± 0.87 vs. 1.82 ± 0.27, P < 0.05). High-contrast images of integrin αvβ6-positive tumors and bleomycin-induced fibrotic lungs were obtained by SPECT/CT imaging using ^99mTc–HYNIC–cHK. Overall, our studies demonstrate that ^99mTc–HYNIC–cHK is a promising SPECT radiotracer for the noninvasive imaging of integrin αvβ6 in living subjects.

Graphical Abstract

Peptide-Functionalized Nanoinhibitor Restrains Brain Tumor Growth by Abrogating Mesenchymal-Epithelial Transition Factor (MET) Signaling

Malignant gliomas are the most common primary brain tumors and are associated with aggressive growth, high morbidity, and mortality. Aberrant mesenchymal-epithelial transition factor (MET) activation occurs in approximately 30% of glioma patients and correlates with poor prognosis, elevated invasion, and increased drug resistance. Therefore, MET has emerged as an attractive target for glioma therapy. In this study, we developed a novel nanoinhibitor by conjugating MET-targeting cMBP peptides on the G4 dendrimer. Compared to the binding affinity of the free peptide ( K_D = 3.96 × 10^-7 M), the binding affinity of the nanoinhibitor to MET increased 3 orders of magnitude to 1.32 × 10^-10 M. This nanoinhibitor efficiently reduced the proliferation and invasion of human glioblastoma U87MG cells in vitro by blocking MET signaling with remarkably attenuated levels of phosphorylated MET ( pMET) and its downstream signaling proteins, such as pAKT and pERK1/2. Although no obvious therapeutic effect was observed after treatment with free cBMP peptide, in vivo T2-weighted magnetic resonance imaging (MRI) showed a significant delay in tumor growth after intravenous injection of the nanoinhibitor. The medium survival in mouse models was extended by 59%, which is similar to the effects of PF-04217903, a small molecule MET inhibitor currently in clinical trials. Immunoblotting studies of tumor homogenate verified that the nanoinhibitor restrained glioma growth by blocking MET downstream signaling. pMET and its downstream proteins pAKT and pERK1/2, which are involved in the survival and invasion of cancer cells, decreased in the nanoinhibitor-treated group by 44.2%, 62.2%, and 32.3%, respectively, compared with those in the control group. In summary, we developed a peptide-functionalized MET nanoinhibitor that showed extremely high binding affinity to MET and effectively inhibited glioma growth by blocking MET downstream signaling. To the best of our knowledge, this is the first report of therapeutic inhibition of glioma growth by blocking MET signaling with a novel nanoinhibitor. Compared to antibodies and chemical inhibitors in clinical trials, the nanoinhibitor blocks MET signaling and provides a new approach for the treatment of glioma with the advantages of high efficiency, affordability, and, most importantly, potentially reduced drug resistance.

Therapeutic Radiopharmaceuticals Targeting Integrin αvβ6

The epithelial integrin αvβ6 is expressed by many malignant carcinoma cell types, including pancreatic cancer, and thus represents a promising target for radionuclide therapy. The peptide cyclo(FRGDLAFp(NMe)K) was decorated with different chelators (DOTPI, DOTAGA, and DOTA). The Lu(III) complexes of these conjugates exhibited comparable αvβ6 integrin affinities (IC₅₀ ranging from 0.3 to 0.8 nM) and good selectivities against other integrins (IC₅₀ for αvβ8 >43 nM; for α5β1 >238 nM; and for αvβ3, αvβ5, and αIIbβ3 >1000 nM). Although different formal charges of the Lu(III) chelates (ranging from 0 to 4) resulted in strongly varying degrees of hydrophilicity (log D ranging from -3.0 to -4.1), biodistributions in murine H2009 xenografts of the Lu-177-labeled compounds (except the DOTPI derivative) were quite similar and comparable to our previously reported αvβ6 integrin positron emission tomography tracer Ga-68-avebehexin. Hence, combinations of existing Ga-68- and Lu-177-labeled c(FRGDLAFp(NMe)K) derivatives could be utilized for αvβ6 integrin-targeted theranostics, whereas our data nonetheless suggest that further improvement of pharmacokinetics might be necessary to ensure clinical success.

Exploring the Role of RGD-Recognizing Integrins in Cancer

Integrins are key regulators of communication between cells and with their microenvironment. Eight members of the integrin superfamily recognize the tripeptide motif Arg-Gly-Asp (RGD) within extracelluar matrix (ECM) proteins. These integrins constitute an important subfamily and play a major role in cancer progression and metastasis via their tumor biological functions. Such transmembrane adhesion and signaling receptors are thus recognized as promising and well accessible targets for novel diagnostic and therapeutic applications for directly attacking cancer cells and their fatal microenvironment. Recently, specific small peptidic and peptidomimetic ligands as well as antibodies binding to distinct integrin subtypes have been developed and synthesized as new drug candidates for cancer treatment. Understanding the distinct functions and interplay of integrin subtypes is a prerequisite for selective intervention in integrin-mediated diseases. Integrin subtype-specific ligands labelled with radioisotopes or fluorescent molecules allows the characterization of the integrin patterns in vivo and later the medical intervention via subtype specific drugs. The coating of nanoparticles, larger proteins, or encapsulating agents by integrin ligands are being explored to guide cytotoxic reagents directly to the cancer cell surface. These ligands are currently under investigation in clinical studies for their efficacy in interference with tumor cell adhesion, migration/invasion, proliferation, signaling, and survival, opening new treatment approaches in personalized medicine.

Multifunctional αvβ6 Integrin-Specific Peptide-Pt(IV) Conjugates for Cancer Cell Targeting

Increasing the specificity of cancer therapy, and thereby decreasing damage to normal cells, requires targeting to cancer-cell specific features. The α_vβ₆ integrin is a receptor involved in cell adhesion and is frequently up-regulated in cancer cells compared to normal cells. We have selected a peptide ligand reported to bind specifically to the β₆ integrin and have synthesized a suite of multispecific molecules to explore the potential for targeting of cancer cells. A combination of solid-phase peptide synthesis and chemoselective ligations was used to synthesize multifunctional molecules composed of integrin-targeting peptides, cytotoxic platinum(IV) prodrugs, and fluorescent or affinity probes joined with flexible linkers. The modular synthesis approach facilitates the construction of peptide-drug conjugates with various valencies and properties in a convergent manner. The binding and specificity of the multifunctional peptide conjugates were investigated using a cell line transfected with the β₆ integrin and fluorescence microscopy. This versatile and highly controlled approach to synthesizing labeled peptide-drug conjugates has the potential to target potent cytotoxic drugs specifically to cancer cells, reducing the doses required for effective treatment.

Identification of a Novel ITGαvβ6-Binding Peptide Using Protein Separation and Phage Display

Purpose: Targeted therapies are regarded as promising approaches to increase 5-year survival rate of head and neck squamous cell carcinoma (HNSCC) patients.Experimental design: For the selection of carcinoma-specific peptides membrane proteome of HNO97 tumor cells fractionated by the ProteomeLab PF2D system and corresponding HNO97 cells were deployed for an alternating biopanning using a sunflower trypsin inhibitor1-based phage display (SFTI8Ph) library. Stability, binding properties and affinity of novel candidates were assessed in vitro using radio-HPLC, binding experiments and surface plasmon resonance assay (SPR), respectively. Subsequently, the affinity of the peptide was verified in situ by using peptide histochemistry, in vitro using flow cytometry, and in vivo by positron emissions tomography (PET/CT).Results: We identified a novel ITGα_vβ₆ binding peptide (SFITGv6) containing the amino acid sequence FRGDLMQL. SFITGv6 provides stability over a period of 24 hours and demonstrates high affinity (K_D = 14.8 nmol/L) for ITGα_vβ₆ In HNO97 cells, a maximal uptake and internalization of up to 37.3% and 37.5%, respectively, was measured. Small-animal PET imaging and biodistribution studies of HNO97 xenografted Balb/c nu/nu mice showed tumor-specific accumulation of ⁶⁸Ga- and ¹⁷⁷Lu-labeled DOTA-SFITGv6, respectively, 30 to 60 minutes after injection. Moreover, peptide histochemistry revealed a strong and homogenous binding of biotin-labeled SFITGv6 to HNSCC tumors and breast- and lung cancer-derived brain metastases. Finally, first PET/CT scans of HNSCC and NSCLC patients displayed SFITGv6 accumulation specifically in tumors, but not in inflammatory lesions.Conclusions: Thus, SFITGv6 represents a novel powerful tracer for imaging and possibly for endoradiotherapy of ITGα_vβ₆-positive carcinoma. Clin Cancer Res; 23(15); 4170-80. ©2017 AACR.

Phage Display-Derived Ligand for Mucosal Transcytotic Receptor GP-2 Promotes Antigen Delivery to M Cells and Induces Antigen-Specific Immune Response

Successful oral immunization depends on efficient delivery of antigens (Ags) to the mucosal immune induction site. Glycoprotein-2 (GP-2) is an integral membrane protein that is expressed specifically on M cells within follicle-associated epithelium (FAE) and serves as transcytotic receptor for luminal Ags. In this study, we selected peptide ligands against recombinant human GP-2 by screening a phage display library and evaluated their interaction with GP-2 in vitro and ex vivo. Selected peptides were conjugated to the C-terminal of enhanced green fluorescence protein (EGFP) and evaluated for their ability to induce an immune response in mice. One of our selected peptides, Gb-1, showed high binding affinity to GP-2 and, when fused to EGFP, significantly increased the uptake of EGFP by M cells compared to EGFP alone. After oral administration, the Gb1-EGFP fusion induced efficient mucosal and systemic immune responses in mice measured at the level of antigen-specific serum and fecal antibodies, cytokine secretion, and lymphocyte proliferation. Furthermore, the IgG subclasses and cytokine secretion showed that ligand Gb-1 induced a Th2-type immune response. Collectively, our findings suggest that the ligand we selected through phage library screening is capable of targeting Ags to GP-2 on M cells and can be used as an oral vaccine adjuvant.

Imaging the Tumor Microenvironment

The tumor microenvironment consists of tumor, stromal, and immune cells, as well as extracellular milieu. Changes in numbers of these cell types and their environments have an impact on cancer growth and metastasis. Non-invasive imaging of aspects of the tumor microenvironment can provide important information on the aggressiveness of the cancer, whether or not it is metastatic, and can also help to determine early response to treatment. This chapter provides an overview on non-invasive in vivo imaging in humans and mouse models of various cell types and physiological parameters that are unique to the tumor microenvironment. Current clinical imaging and research investigation are in the areas of nuclear imaging (positron emission tomography (PET) and single photon emission computed tomography (SPECT)), magnetic resonance imaging (MRI) and optical (near infrared (NIR) fluorescence) imaging. Aspects of the tumor microenvironment that have been imaged by PET, MRI and/or optical imaging are tumor associated inflammation (primarily macrophages and T cells), hypoxia, pH changes, as well as enzymes and integrins that are highly prevalent in tumors, stroma and immune cells. Many imaging agents and strategies are currently available for cancer patients; however, the investigation of novel avenues for targeting aspects of the tumor microenvironment in pre-clinical models of cancer provides the cancer researcher with a means to monitor changes and evaluate novel treatments that can be translated into the clinic.

Binding behaviour of a 12-mer peptide and its tandem dimer to gymnospermae and angiospermae lignins

Analysis of conformational changes of lignin-binding dodecapeptide and its tandem dimer on addition of lignin by ATR-FTIR.

Stable Peptides Instead of Stapled Peptides: Highly Potent αvβ6-Selective Integrin Ligands

The αvβ6 integrin binds the RGD-containing peptide of the foot and mouth disease virus with high selectivity. In this study, the long binding helix of this ligand was downsized to an enzymatically stable cyclic peptide endowed with sub-nanomolar binding affinity toward the αvβ6 receptor and remarkable selectivity against other integrins. Computational studies were performed to disclose the molecular bases underlying the high binding affinity and receptor subtype selectivity of this peptide. Finally, the utility of the ligand for use in biomedical studies was also demonstrated here.

The Effect of Bi-Terminal PEGylation of an Integrin αvβ₆-Targeted ¹⁸F Peptide on Pharmacokinetics and Tumor Uptake

Radiotracers based on the peptide A20FMDV2 selectively target the cell surface receptor integrin αvβ6. This integrin has been identified as a prognostic indicator correlating with the severity of disease for several challenging malignancies. In previous studies of A20FMDV2 peptides labeled with 4-(18)F-fluorobenzoic acid ((18)F-FBA), we have shown that the introduction of poly(ethylene glycol) (PEG) improves pharmacokinetics, including increased uptake in αvβ6-expressing tumors. The present study evaluated the effect of site-specific C-terminal or dual (N- and C-terminal) PEGylation, yielding (18)F-FBA-A20FMDV2-PEG28 (4) and (18)F-FBA-PEG28-A20FMDV2-PEG28 (5), on αvβ6-targeted tumor uptake and pharmacokinetics. The results are compared with (18)F-FBA -labeled A20FMDV2 radiotracers (1- 3) bearing either no PEG or different PEG units at the N terminus.

METHODS

The radiotracers were prepared and radiolabeled on solid phase. Using 3 cell lines, DX3puroβ6 (αvβ6+), DX3puro (αvβ6-), and BxPC-3 (αvβ6+), we evaluated the radiotracers in vitro (serum stability; cell binding and internalization) and in vivo in mouse models bearing paired DX3puroβ6-DX3puro and, for 5, BxPC-3 xenografts.

RESULTS

The size and location of the PEG units significantly affected αvβ6 targeting and pharmacokinetics. Although the C-terminally PEGylated 4 showed some improvements over the un-PEGylated (18)F-FBA-A20FMDV2 (1), it was the bi-terminally PEGylated 5 that displayed the more favorable combination of high αvβ6 affinity, selectivity, and pharmacokinetic profile. In vitro, 5 bound to αvβ6-expressing DX3puroβ6 and BxPC-3 cells with 60.5% ± 3.3% and 48.8% ± 8.3%, respectively, with a significant fraction of internalization (37.2% ± 4.0% and 37.6% ± 4.1% of total radioactivity, respectively). By comparison, in the DX3puro control 5: showed only 3.0% ± 0.5% binding and 0.9% ± 0.2% internalization. In vivo, 5: maintained high, αvβ6-directed binding in the paired DX3puroβ6-DX3puro model (1 h: DX3puroβ6, 2.3 ± 0.2 percentage injected dose per gram [%ID/g]; DX3puroβ6/DX3puro ratio, 6.5:1; 4 h: 10.7:1). In the pancreatic BxPC-3 model, uptake was 4.7 ± 0.9 %ID/g (1 h) despite small tumor sizes (20-80 mg).

CONCLUSION

The bi-PEGylated radiotracer 5 showed a greatly improved pharmacokinetic profile, beyond what was predicted from individual N- or C-terminal PEGylation. It appears that the 2 PEG units acted synergistically to result in an improved metabolic profile including high αvβ6+ tumor uptake and retention.

Molecular imaging of integrin αvβ6 expression in living subjects

Integrins, a family of cell adhesion molecules composed of α and β heterodimeric subunits, are involved in a wide range of cell-extracellular matrix and cell-cell interactions. The study of integrin family members as targets for molecular imaging and therapy has been generally limited with the exception of integrin αvβ3. vβ6, a member of the integrin family, is expressed at low or undetectable levels in normal tissues, but is widely upregulated during many pathological and physiological processes, especially cancer and fibrosis, making it a promising target for molecular imaging. Noninvasive and quantitative imaging of integrin vβ6 expression would be very useful for disease diagnosis, treatment monitoring, and prognosis assessment. Although various molecular probes have been developed for positron emission tomography and single-photon emission computed tomography imaging of integrin vβ6 expression in preclinical animal models, further research efforts are required to optimize integrin vβ6-targeting probes for future potential clinical applications in the fields of oncology and beyond.