Variation of Specific Activities of 68Ga-Aquibeprin and 68Ga-Avebetrin Enables Selective PET Imaging of Different Expression Levels of Integrins α5β1 and αvβ3

Importance of integrin transmembrane helical interactions for antagonistic versus agonistic ligand behavior: Consequences for medical applications

Integrins are well-characterized receptors involved in cell adhesion and signaling. With six approved drugs, they are recognized as valuable therapeutic targets. Here, we explore potential activation mechanisms that may clarify the agonist versus antagonist behavior of integrin ligands. The reorganization of the transmembrane domain (TMD) in the integrin receptor, forming homooligomers within focal adhesions, could be key to the understanding of the agonistic properties of integrin ligands at substoichiometric concentrations. This has significant implications for medical applications. While we focus on the RGD peptide-recognizing integrin subfamily, we propose that these mechanistic insights may also apply to other integrin subtypes. For application of integrin ligands in medicine it is essential to consider this mechanism and its consequences for affinity and bioavailability.

Enhanced radiotheranostic targeting of integrin α5β1 with PEGylation-enabled peptide multidisplay platform (PEGibody): A strategy for prolonged tumor retention with fast blood clearance

Peptide-based radiopharmaceuticals targeting integrin α5β1 show promise for precise tumor diagnosis and treatment. However, current peptide-based radioligands that target α5β1 demonstrate inadequate in vivo performance owing to limited tumor retention. The use of PEGylation to enhance the tumor retention of radiopharmaceuticals by prolonging blood circulation time poses a risk of increased blood toxicity. Therefore, a PEGylation strategy that boosts tumor retention while minimizing blood circulation time is urgently needed. Here, we developed a PEGylation-enabled peptide multidisplay platform (PEGibody) for PR_b, an α5β1 targeting peptide. PEGibody generation involved PEGylation and self-assembly. [64Cu]QM-2303 PEGibodies displayed spherical nanoparticles ranging from 100 to 200 nm in diameter. Compared with non-PEGylated radioligands, [64Cu]QM-2303 demonstrated enhanced tumor retention time due to increased binding affinity and stability. Importantly, the biodistribution analysis confirmed rapid clearance of [64Cu]QM-2303 from the bloodstream. Administration of a single dose of [177Lu]QM-2303 led to robust antitumor efficacy. Furthermore, [64Cu]/[177Lu]QM-2303 exhibited low hematological and organ toxicity in both healthy and tumor-bearing mice. Therefore, this study presents a PEGibody-based radiotheranostic approach that enhances tumor retention time and provides long-lasting antitumor effects without prolonging blood circulation lifetime. The PEGibody-based radiopharmaceutical [64Cu]/[177Lu]QM-2303 shows great potential for positron emission tomography imaging-guided targeted radionuclide therapy for α5β1-overexpressing tumors.

Three Is a Magic Number: Tailored Clickable Chelators Used to Determine Optimal RGD-Peptide Multiplicity in αvβ6-Integrin Targeted 177Lu-Labeled Cancer Theranostics

The cellular adhesion receptor αvβ6-integrin is highly expressed in many cancers, e.g., pancreatic, lung, head-and-neck, cervical, bladder, and esophageal carcinoma. Multimerization of αvβ6-integrin-specific RGD peptides increases the target affinity and retention but affects biodistribution and pharmacokinetics. Amide formation of the terminal carboxylic acid moieties of the square-symmetrical bifunctional chelator DOTPI with 3-azidopropylamine yields derivatives with 4, 3, and 2 terminal azides and zero, 1, and 2 remaining carboxylic acids, respectively, whereby formation of the 2-cis-isomer is preferred according to NMR investigation of the Eu(III)-complexes. Cu(II)-catalyzed alkyne-azide cycloaddition (CuAAC) of the alkyne-functionalized αvβ6-integrin binding peptide cyclo[YRGDLAYp(NMe)K(pent-4-ynoic amide)] (Tyr2) yields the respective di-, tri-, and tetrameric conjugates for Lu-177-labeling. In mice bearing αvβ6-integrin-expressing xenografts of H2009 (human lung adenocarcinoma) cells, the Lu-177-labeled trimer's tumor-to-blood ratio of 112 exceeds that of the tetramer (10.4) and the dimer (54). Co-infusion of gelofusine (succinylated gelatin) reduces the renal uptake of the trimer by 89%, resulting in a 10-fold better tumor-to-kidney ratio, while no improvement of that ratio is observed with arginine/lysine, para-aminohippuric acid (PAH), and hydroxyethyl starch (HES) coinfusions. Since the Lu-177-labeled Tyr2-trimer outperforms the dimer and the tetramer, such trimers are considered the best lead structures for the ongoing development of αvβ6-integrin targeted anticancer theranostics.

The importance of tyrosines in multimers of cyclic RGD nonapeptides: towards αvβ6-integrin targeted radiotherapeutics

In a recent paper in this journal (RSC Med. Chem., 2023, 14, 2429), we described an unusually strong impact of regiospecific exchange of phenylalanines by tyrosines in 10 gallium-68-labeled trimers of certain cyclic RGD peptides, c[XRGDLAXp(NMe)K] (X = F or Y), on non-specific organ uptakes. We found that there was, in part, no correlation of liver uptake with established polarity proxies, such as the octanol-water distribution coefficient (log D). Since this observation could not be explained straightforwardly, we suggested that the symmetry of the compounds had resulted in a synergistic interaction of certain components of the macromolecules. In the present work, we investigated whether a comparable effect also occurred for a series of 5 tetramers labeled with lutetium-177. We found that in contrast to the trimers, liver uptake of the tetramers was well correlated to their polarity, indicating that the unusual observations along the trimer series indeed was a unique feature, probably related to their particular symmetry. Since the Lu-177 labeled tetramers are also potential agents for treatment of a variety of αvβ6-integrin expressing cancers, these were evaluated in mice bearing human lung adenocarcinoma xenografts. Due to their tumor-specific uptake and retention in biodistribution and SPECT imaging experiments, these compounds are considered a step forward on the way to αvβ6-integrin-targeted anticancer agents. Furthermore, we noticed that the presence of tyrosines in general had a positive impact on the in vivo performance of our peptide multimers. In view of the fact that a corresponding rule was already proposed in the context of protein engineering, we argue in favor of considering peptide multimers as a special class of small or medium-sized proteins. In summary, we contend that the performance of peptide multimers is less determined by the in vitro characteristics (particularly, affinity and selectivity) of monomers, but rather by the peptides' suitability for the overall macromolecular design concept, and peptides containing tyrosines are preferred.

Sensitive Positron Emission Tomography Imaging of PD-L1 Expression in Human Breast and Lung Carcinoma Xenografts Using the Radiometalated Peptide Ga-68-TRAP-WL12

Noninvasive imaging of the immune checkpoint protein programmed death ligand 1 (PD-L1; synonyms: CD274, B7-H1) holds great promise to improve patient selection and, thus, response rates for immune checkpoint therapy (ICT) with monoclonal antibodies targeting the PD1/PD-L1 axis. The PD-L1 specific peptide WL12 (cyclo(AcY-(NMe)A-N-P-H-L-Hyp-W-S-W(Me)-(NMe)Nle-(NMe)Nle-O-C)-G-NH2) was functionalized with the Gallium-68 chelator TRAP by means of click chemistry (CuAAC). The resulting conjugate TRAP-WL12 was labeled with Gallium-68 within 16 min, with approximately 90% radiochemical yield and 99% radiochemical purity, affording Ga-68-TRAP-WL12 with molar activities typically exceeding 100 MBq/nmol. This radiotracer was characterized by positron emission tomography (PET) imaging and ex vivo biodistribution in murine xenografts of nontransfected PD-L1 expressing tumor cell lines, MDA-MB-231 (human breast carcinoma), and H2009 (human lung adenocarcinoma). It showed a favorable biodistribution profile with rapid renal clearance and low background (tumor-to-blood ratio = 26.6, 3 h p.i.). Conjugation of the Ga-68-TRAP moiety to WL12 successfully mitigated the nonspecific uptake of this peptide in organs, particularly the liver. This was demonstrated by comparing Ga-68-TRAP-WL12 with the archetypical Ga-68-DOTA-WL12, for which tumor-to-liver ratios of 1.4 and 0.5, respectively, were found. Although immunohistochemistry (IHC) revealed a low PD-L1 expression in MDA-MB-213 and H2009 xenografts that corresponds well to the clinical situation, PET showed high tumor uptakes (6.6 and 7.3% injected activity per gram of tissue (iA/g), respectively) for Ga-68-TRAP-WL12. Thus, this tracer has the potential for routine clinical PD-L1 PET imaging because it detects even very low PD-L1 expression densities with high sensitivity and may open an avenue to replace PD-L1 IHC of biopsies as the standard means to select potential responders for ICT.

Complexity of αvβ6-integrin targeting RGD peptide trimers: emergence of non-specific binding by synergistic interaction

Multimerization is an established strategy to design bioactive macromolecules with enhanced avidity, which has been widely employed to increase the target-specific binding and uptake of imaging probes and pharmaceuticals. However, the factors governing the general biodistribution of multimeric probes are less well understood but are nonetheless decisive for their clinical application. We found that regiospecific exchange of phenylalanine by tyrosine (chemically equivalent to addition of single oxygen atoms) can have an unexpected, dramatic impact on the in vivo behavior of gallium-68 labeled αvβ6-integrin binding peptides trimers. For example, introduction of one and two Tyr, equivalent to just 1 and 2 additional oxygens and molecular weight increases of 0.38% and 0.76% for our >4 kDa constructs, reduced non-specific liver uptake by 50% and 72%, respectively. The observed effect did not correlate to established polarity measures such as log D, and generally defies explanation by reductionist approaches. We conclude that multimers should be viewed not just as molecular combinations of peptides whose properties simply add up, but as whole entities with higher intrinsic complexity and thus a strong tendency to exhibit newly emerged properties that, on principle, cannot be predicted from the characteristics of the monomers used.

Gallium-68 Labelled Radiopharmaceuticals for Imaging Inflammatory Disorders

Inflammation is an important component of several chronic and debilitating diseases that result in significant morbidity and mortality. This is best evidenced within the cardiovascular system where it may manifest as atherosclerosis or myocarditis, and at the extreme end of the spectrum as myocardial infarction, ventricular remodeling, or cardiac failure. Early non-invasive detection and monitoring of inflammation in these and other settings may better guide patient management with resultant improved outcomes. Key role players in inflammation pathophysiology include chemokines, macrophages, neutrophils, fibroblasts, integrins, and reactive oxygen species, amongst others. Examples of receptor expression and over-expression include somatostatin receptors, CXCR4-, folate-, mannose-, TSPO- receptors and secretion of various vascular adhesion molecules (such as VCAM and ICAM). Gallium-68-based PET offers imaging possibilities for nearly all the major pathophysiological role players in inflammation, with mounting recent interest in macrophage differentiation, various forms of receptor expression and secretion of chemokines and vascular adhesion molecules. The advantages in terms of logistics and costs of having generator-produced PET probes available is well known, and a ⁶⁸Ga-based tracer provides easily translatable theranostic possibilities to especially Lu-177. Some of the more versatile and better validated Ga-68-based inflammation probes include ⁶⁸Ga-DOTA-TATE/NOC/TOC, ⁶⁸Ga-NOTA-RGD, ⁶⁸Ga-CXCR4, ⁶⁸Ga-citrate and ⁶⁸Ga-FAPI.

RGD Forever!-Past, Present, and Future of a 3-Letter-Code in Radiopharmacy and Life Sciences

"RGD" is frequently pictured as a ligand for αvβ3-integrin and useful for molecular targeting of angiogenesis-which is about as simplistic as the idea that laser beams are green or red and particularly useful for arming spaceships. There is, however, much more to RGD. In particular, targeting angiogenesis is likely not the most significant stronghold of RGD-comprising constructs. RGD is the one-letter code of a very short peptide sequence, arginine-lysine-aspartate, which is recognized by eight different integrins, namely, α(IIb)β3, α5β1, α8β1, and the five dimers that αv forms with β1, β3, β5, β6, and β8. These 8 RGD receptors form an own subset among the entire class of 24 known integrins, which furthermore comprises another three distinct groups (4 collagen receptors, 4 laminin receptors, and 8 leukocyte receptors). However, the 8 RGD-recognizing integrins are far from being alike. They do not even share the same tissue prevalences and functions, but are expressed on fundamentally different cell types and fulfill the most diverse biological tasks. For example, α(IIb)β3 is found on platelets and mediates thrombus formation, whereas αvβ6- and αvβ8-integrin are expressed on epithelial cells, activate TFG-β, and thus may promote cancer progression and invasion as well as fibrosis. Recent non-clinical experiments and clinical findings suggest that the highly specific expression of αvβ6-integrin by some carcinoma types, in combination with the availability of the corresponding small-molecule ligands, may open a multitude of new and promising avenues for improved cancer diagnosis and therapy, including, but not limited to, radiopharmaceutical approaches.

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.

Biological Relevance of RGD-Integrin Subtype-Specific Ligands in Cancer

Integrins are heterodimeric transmembrane proteins able to connect cells with the micro-environment. They represent a family of receptors involved in almost all the hallmarks of cancer. Integrins recognizing the Arg-Gly-Asp (RGD) peptide in their natural extracellular matrix ligands have been particularly investigated as tumoral therapeutic targets. In the last 30 years, intense research has been dedicated to designing specific RGD-like ligands able to discriminate selectively the different RGD-recognizing integrins. Chemists' efforts have led to the proposition of modified peptide or peptidomimetic libraries to be used for tumor targeting and/or tumor imaging. Here we review, from the biological point of view, the rationale underlying the need to clearly delineate each RGD-integrin subtype by selective tools. We describe the complex roles of RGD-integrins (mainly the most studied αvβ3 and α5β1 integrins) in tumors, the steps towards selective ligands and the current usefulness of such ligands. Although the impact of integrins in cancer is well acknowledged, the biological characteristics of each integrin subtype in a specific tumor are far from being completely resolved. Selective ligands might help us to reconsider integrins as therapeutic targets in specific clinical settings.

Radiometal-labeled anti-VCAM-1 nanobodies as molecular tracers for atherosclerosis - impact of radiochemistry on pharmacokinetics

Radiolabeling of nanobodies with radiometals by chelation has the advantage of being simple, fast and easy to implement in clinical routine. In this study, we validated 68Ga/111In-labeled anti-VCAM-1 nanobodies as potential radiometal-based tracers for molecular imaging of atherosclerosis. Both showed specific targeting of atherosclerotic lesions in ApoE-/- mice. Nevertheless, uptake in lesions and constitutively VCAM-1 expressing organs was lower than previously reported for the 99mTc-labeled analog. We further investigated the impact of different radiolabeling strategies on the in vivo biodistribution of nanobody-based tracers. Comparison of the pharmacokinetics between 68Ga-, 18F-, 111In- and 99mTc-labeled anti-VCAM-1 nanobodies showed highest specific uptake for 99mTc-nanobody at all time-points, followed by the 68Ga-, 111In- and 18F-labeled tracer. No correlation was found with the estimated number of radioisotopes per nanobody, and mimicking specific activity of other radiolabeling methods did not result in an analogous biodistribution. We also demonstrated specificity of the tracer using mice with a VCAM-1 knocked-down phenotype, while showing for the first time the in vivo visualization of a protein knock-down using intrabodies. Conclusively, the chosen radiochemistry does have an important impact on the biodistribution of nanobodies, in particular on the specific targeting, but differences are not purely due to the tracer's specific activity.

In vivo imaging of early stages of rheumatoid arthritis by α5β1-integrin-targeted positron emission tomography

Background

Rheumatoid arthritis (RA) is one of the most common rheumatic diseases. Joint inflammation and pathological growth of joint cartilage cause swollen and painful joints, which severely diminishes the patients’ life quality. There is no causal treatment. Symptomatic therapies should start as early as possible to take maximal effect. Hence, diagnostic procedures capable of detecting affected joints before the onset of clinical symptoms are highly desirable. We explored the value of PET imaging of integrin subtypes αvβ3 and α5β1 for early detection of RA foci in collagen-induced arthritis (CIA) mouse models.

Results

Development of RA in CIA mice was monitored by paw scoring, and αvβ3- and α5β1-integrin expression was quantified by μPET using ⁶⁸Ga-Avebetrin and ⁶⁸Ga-Aquibeprin. For consecutive sections of selected decalcified joints (knee, ankle), arthritic degeneration and integrin expression were assessed by MOVAT staining and β3/α5 immunohistochemistry (IHC), respectively. β3- and α5-IHC revealed elevated levels of both αvβ3- and α5β1-integrin in arthritic joints. Unlike αvβ3, α5β1 is strongly expressed in the proliferating synovial lining layer, which suggests that its presence is directly related to RA development. For mice with advanced RA (6 weeks after CIA), PET signals for α5β1-integrin were substantially stronger (> 300% of baseline) than that of αvβ3-integrin (< 200%). A longitudinal PET follow-up revealed that the manifestation of clinical symptoms of RA is preceded by upregulation of α5β1- but not of αvβ3-integrin.

Conclusion

α5β1-integrin PET could add a new functional imaging aspect to the portfolio of RA diagnostics because it appears to be a sensitive biomarker for early RA development. We suggest α5β1-integrin PET as a valuable tool to achieve a higher precision for early diagnosis of RA, including initial staging, monitoring of the disease course, and drug treatment, and for planning of radiosynoviorthesis (RSO).

Evaluating Ga-68 Peptide Conjugates for Targeting VPAC Receptors: Stability and Pharmacokinetics

PURPOSE

In recent years, considerable progress has been made in the use of gallium-68 labeled receptor-specific peptides for imaging oncologic diseases. The objective was to examine the stability and pharmacokinetics of [⁶⁸Ga]NODAGA and DOTA-peptide conjugate targeting VPAC [combined for vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase-activating peptide (PACAP)] receptors on tumor cells.

PROCEDURES

A VPAC receptor-specific peptide was chosen as a model peptide and conjugated to NODAGA and DOTA via solid-phase synthesis. The conjugates were characterized by HPLC and MALDI-TOF. Following Ga-68 chelation, the radiochemical purity of Ga-68 labeled peptide conjugate was determined by radio-HPLC. The stability was tested against transmetallation using 100 nM Fe³⁺/Zn²⁺/Ca²⁺ ionic solution and against transchelation using 200 μM DTPA solution. The ex vivo and in vivo stability of the Ga-68 labeled peptide conjugate was tested in mouse plasma and urine. Receptor specificity was determined ex vivo by cell binding assays using human breast cancer BT474 cells. Positron emission tomography (PET)/X-ray computed tomography (CT) imaging, tissue distribution, and blocking studies were performed in mice bearing BT474 xenografts.

RESULTS

The chemical and radiochemical purity was greater than 95 % and both conjugates were stable against transchelation and transmetallation. Ex vivo stability at 60 min showed that the NODAGA-peptide-bound Ga-68 reduced to 42.1 ± 3.7 % (in plasma) and 37.4 ± 2.9 % (in urine), whereas the DOTA-peptide-bound Ga-68 was reduced to 1.2 ± 0.3 % (in plasma) and 4.2 ± 0.4 % (in urine) at 60 min. Similarly, the in vivo stability for [⁶⁸Ga]NODAGA-peptide was decreased to 2.1 ± 0.2 % (in plasma) and 2.2 ± 0.4 % (in urine). For [⁶⁸Ga]DOTA-peptide, it was decreased to 1.4 ± 0.3 % (in plasma) and 1.2 ± 0.4 % (in urine) at 60 min. The specific BT474 cell binding was 53.9 ± 0.8 % for [⁶⁸Ga]NODAGA-peptide, 25.8 ± 1.4 % for [⁶⁸Ga]-DOTA-peptide, and 18.8 ± 2.5 % for [⁶⁸Ga]GaCl₃ at 60 min. Inveon microPET/CT imaging at 1 h post-injection showed significantly (p < 0.05) higher tumor to muscle (T/M) ratio for [⁶⁸Ga]NODAGA-peptide (3.4 ± 0.3) as compared to [⁶⁸Ga]DOTA-peptide (1.8 ± 0.6). For [⁶⁸Ga]GaCl₃ and blocked mice, their ratios were 1.5 ± 0.6 and 1.5 ± 0.3 respectively. The tissue distributions data were similar to the PET imaging data.

CONCLUSION

NODAGA is superior to DOTA in terms of radiolabeling kinetics. The method of radiolabeling was reproducible and yielded higher specific activity. Although both agents have relatively low in vivo stability, PET/CT imaging studies delineated BC tumors with [⁶⁸Ga]NODAGA-peptide, but not with [⁶⁸Ga]DOTA-peptide.

Increasing molar activity by HPLC purification improves 68Ga-DOTA-NAPamide tumor accumulation in a B16/F1 melanoma xenograft model

Purpose

Melanocortin receptor 1 (MC1R) is overexpressed in melanoma and may be a molecular target for imaging and peptide receptor radionuclide therapy. ⁶⁸Gallium (⁶⁸Ga) labeling of DOTA-conjugated peptides is an established procedure in the clinic for use in positron emission tomography (PET) imaging. Aim of this study was to compare a standard labeling protocol against the ⁶⁸Ga-DOTA peptide purified from the excess of unlabeled peptide.

Procedures

The MC1R ligand DOTA-NAPamide was labeled with ⁶⁸Ga using a standard clinical protocol. Radioactive peptide was separated from the excess of unlabeled DOTA-NAPamide by HPLC. Immediately after the incubation of peptide and ⁶⁸Ga (95°C, 15 min), the reaction was loaded on a C18 column and separated by a water/acetonitrile gradient, allowing fractionation in less than 20 minutes. Radiolabeled products were compared in biodistribution studies and PET imaging using nude mice bearing MC1R-expressing B16/F1 xenograft tumors.

Results

In biodistribution studies, non-purified ⁶⁸Ga-DOTA-NAPamide did not show significant uptake in the tumor at 1 h post injection (0.78% IA/g). By the additional HPLC step, the molar activity was raised around 10,000-fold by completely removing unlabeled peptide. Application of this rapid purification strategy led to a more than 8-fold increase in tumor uptake (7.0% IA/g). The addition of various amounts of unlabeled DOTA-NAPamide to the purified product led to a blocking effect and decreased specific tumor uptake, similar to the result seen with non-purified radiopeptide. PET imaging was performed using the same tracer preparations. Purified ⁶⁸Ga-DOTA-NAPamide, in comparison, showed superior tumor uptake.

Conclusions

We demonstrated that chromatographic separation of radiolabeled from excess unlabeled peptide is technically feasible and beneficial, even for short-lived isotopes such as ⁶⁸Ga. Unlabeled peptide molecules compete with receptor binding sites in the target tissue. Purification of the radiopeptide therefore improved tumor uptake.

Molar Activity of Ga-68 Labeled PSMA Inhibitor Conjugates Determines PET Imaging Results

Radiopharmaceuticals targeting the enzyme prostate-specific membrane antigen (PSMA; synonyms: glutamate carboxypeptidase II, NAALADase; EC 3.4.17.21) have recently emerged as powerful agents for diagnosis and therapy (theranostics) of prostate carcinoma (PCa). The radiation doses for therapeutic application of such compounds are limited by substantial uptakes in kidneys and salivary glands, with excess doses reportedly leading to radiotoxicity-related adverse effects, such as kidney insufficiency or xenostomia. On the basis of the triazacyclononane-triphosphinate (TRAP) chelator, monomeric to trimeric conjugates of the PSMA inhibitor motif lysine-urea-glutamic acid (KuE) were synthesized by means of Cu(I)-mediated (CuAAC) or 5-aza-dibenzocyclooctyne (DBCO)-driven, strain-promoted click chemistry (SPAAC), which were labeled with gallium-68 for application in positron emission tomography (PET), and characterized in terms of PSMA affinity (determined in cellular displacement assays against I-125-BA) and lipophilicity (expressed as log D). Using subcutaneous murine LNCaP (PSMA-positive human prostate carcinoma) xenografts, the influence of ligand multiplicity, affinity, polarity, and molar activity (i.e., mass dose) on the uptakes in tumor, kidney, salivary, and background (muscle) was analyzed by means of region-of-interest (ROI) based quantification of small-animal PET imaging data. As expected, trimerization of the KuE motif resulted in high PSMA affinities (IC₅₀ ranging from 6.0-1.5 nM). Of all parameters, molar activity/cold mass had the most pronounced influence on PET uptakes. Because accumulation in nontumor tissues was effected to a larger extent than tumor uptakes, lower molar activities resulted in substantially better tumor-to-organ ratios. For example, for one trimer, ⁶⁸Ga-AhxKuE₃ (IC₅₀ = 1.5 ± 0.3 nM, log D = -3.8 ± 0.1), a higher overall amount of active compound (12 pmol vs 2 nmol, equivalent to molar activities of 1200 and 8 MBq/nmol) resulted in a remarkable reduction of the kidney-to-tumor ratio from 11.4 to 1.4, respectively, at 60 min p.i. Our study suggests that, for PSMA-targeting radiopharmaceuticals, molar activity has a more pronounced influence on small-animal PET imaging results than structural or in vitro parameters.

Every step of the way: integrins in cancer progression and metastasis

Cell adhesion to the extracellular matrix is fundamental to tissue integrity and human health. Integrins are the main cellular adhesion receptors that through multifaceted roles as signalling molecules, mechanotransducers and key components of the cell migration machinery are implicated in nearly every step of cancer progression from primary tumour development to metastasis. Altered integrin expression is frequently detected in tumours, where integrins have roles in supporting oncogenic growth factor receptor (GFR) signalling and GFR-dependent cancer cell migration and invasion. In addition, integrins determine colonization of metastatic sites and facilitate anchorage-independent survival of circulating tumour cells. Investigations describing integrin engagement with a growing number of versatile cell surface molecules, including channels, receptors and secreted proteins, continue to lead to the identification of novel tumour-promoting pathways. Integrin-mediated sensing, stiffening and remodelling of the tumour stroma are key steps in cancer progression supporting invasion, acquisition of cancer stem cell characteristics and drug resistance. Given the complexity of integrins and their adaptable and sometimes antagonistic roles in cancer cells and the tumour microenvironment, therapeutic targeting of these receptors has been a challenge. However, novel approaches to target integrins and antagonism of specific integrin subunits in stringently stratified patient cohorts are emerging as potential ways forward.

Dual-Nuclide Radiopharmaceuticals for Positron Emission Tomography Based Dosimetry in Radiotherapy

Improvement of the accuracy of dosimetry in radionuclide therapy has the potential to increase patient safety and therapeutic outcomes. Although positron emission tomography (PET) is ideally suited for acquisition of dosimetric data because PET is inherently quantitative and offers high sensitivity and spatial resolution, it is not directly applicable for this purpose because common therapeutic radionuclides lack the necessary positron emission. This work reports on the synthesis of dual-nuclide labeled radiopharmaceuticals with therapeutic and PET functionality, which are based on common and widely available metal radionuclides. Dual-chelator conjugates, featuring interlinked cyclen- and triazacyclononane-based polyphosphinates DOTPI and TRAP, allow for strictly regioselective complexation of therapeutic (e.g., 177 Lu, 90 Y, or 213 Bi) and PET (e.g., 68 Ga) radiometals in the same molecular framework by exploiting the orthogonal metal ion selectivity of these chelators (DOTPI: large cations, such as lanthanide(III) ions; TRAP: small trivalent ions, such as GaIII ). Such DOTPI-TRAP conjugates were decorated with 3 Gly-urea-Lys (KuE) motifs for targeting prostate-specific membrane antigen (PSMA), employing Cu-catalyzed (CuAAC) as well as strain-promoted (SPAAC) click chemistry. These were labeled with 177 Lu or 213 Bi and 68 Ga and used for in vivo imaging of LNCaP (human prostate carcinoma) tumor xenografts in SCID mice by PET, thus proving practical applicability of the concept.

PET and SPECT imaging of melanoma: the state of the art

Melanoma represents the most aggressive form of skin cancer, and its incidence continues to rise worldwide. 18F-FDG PET imaging has transformed diagnostic nuclear medicine and has become an essential component in the management of melanoma, but still has its drawbacks. With the rapid growth in the field of nuclear medicine and molecular imaging, a variety of promising probes that enable early diagnosis and detection of melanoma have been developed. The substantial preclinical success of melanin- and peptide-based probes has recently resulted in the translation of several radiotracers to clinical settings for noninvasive imaging and treatment of melanoma in humans. In this review, we focus on the latest developments in radiolabeled molecular imaging probes for melanoma in preclinical and clinical settings, and discuss the challenges and opportunities for future development.

Targeting Cardiovascular Implant Infection: Multimodality and Molecular Imaging

Owing to their success in clinical practice, the prevalence of cardiovascular implants is continuously increasing. Implant infection is a relevant complication but remains a diagnostic challenge because echocardiography as a first-line test may be limited. Accordingly, a multimodality approach is increasingly used for diagnostic workup and supported by recent guidelines. As reviewed here, computed tomography and nuclear imaging provide incremental diagnostic value and may be combined in a single hybrid imaging session using positron emission tomography/computed tomography or single photon emission computed tomography/computed tomography. Molecular or cellular imaging helps to overcome the limitations of morphological imaging in implants. Larger-scale clinical studies, earlier application in the time course of diagnosis, monitoring of therapy success, technical advances, and novel radiopharmaceuticals will all contribute to sustained growth of advanced infection-targeted imaging in cardiovascular medicine.

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.

Evaluation of agonist and antagonist radioligands for somatostatin receptor imaging of breast cancer using positron emission tomography

Background

The somatostatin receptor subtype 2 (sstr2) is expressed on a majority of luminal breast cancers, however SPECT and scintigraphy imaging with agonistic sstr2 probes has been sub-optimal. High affinity antagonists can access more binding sites on the cell surface, resulting in higher tumor uptake and improved sensitivity. We compared the tumor uptake and biodistribution of the antagonist ⁶⁸Ga-NODAGA-JR11 with two agonists ⁶⁸Ga-DOTA-Tyr³-octreotide (⁶⁸Ga-DOTATOC) and ⁶⁸Ga-DOTA-Tyr³-octreotate (⁶⁸Ga-DOTATATE), in the human, sstr2-positive, luminal breast cancer model: ZR-75-1.

Results

Peptides were assayed for binding affinity using a filtration-based competitive assay to sstr2. ^natGa-DOTATOC and ^natGa-DOTATATE had excellent affinity (inhibition constant K_i: 0.9 ± 0.1 nM and 1.4 ± 0.3 nM respectively) compared to ^natGa-NODAGA-JR11 (25.9 ± 0.2 nM). The number of binding sites on ZR-75-1 cells was determined in vitro by saturation assays. Agonist ^67/natGa-DOTATOC bound to 6.64 ± 0.39 × 10⁴ sites/cells, which was 1.5-fold higher than ^67/natGa-NODAGA-JR11 and 2.3-fold higher than ^67/natGa-DOTATATE. All three ⁶⁸Ga-labeled peptides were obtained in good decay-corrected radiochemical yield (61-68%) and were purified by high performance liquid chromatography to ensure high specific activity (137 – 281 MBq/nmol at the end of synthesis). NOD scid gamma mice bearing ZR-75-1 tumors were injected intravenously with the labeled peptides and used for PET/CT imaging and biodistribution at 1 h post-injection. We found that ⁶⁸Ga-DOTATOC had the highest tumor uptake (18.4 ± 2.9%ID/g), followed by ⁶⁸Ga-DOTATATE (15.2 ± 2.2%ID/g) and ⁶⁸Ga-NODAGA-JR11 (12.2 ± 0.8%ID/g). Tumor-to-blood and tumor-to-muscle ratios were also higher for the agonists (>40 and >150 respectively), compared to the antagonist (15.6 ± 2.2 and 45.2 ± 11.6 respectively).

Conclusions

The antagonist ⁶⁸Ga-NODAGA-JR11 had the lowest tumor uptake and contrast compared to agonists ⁶⁸Ga-DOTATOC and ⁶⁸Ga-DOTATATE in ZR-75-1 xenografts.

The main contributing factor to this result could be the use of an endogenously expressing cell line, which may differ from previously published transfected models in the number of low-affinity, antagonist-specific binding sites. The relative merit of agonists versus antagonists for sstr2 breast cancer imaging warrants further investigation, first in preclinical models with other sstr2-positive breast cancer xenografts, and ultimately in luminal breast cancer patients.

Electronic supplementary material

The online version of this article (doi:10.1186/s41181-017-0023-y) contains supplementary material, which is available to authorized users.

Theranostic Value of Multimers: Lessons Learned from Trimerization of Neurotensin Receptor Ligands and Other Targeting Vectors

Neurotensin receptor 1 (NTS1) is overexpressed on a variety of cancer entities; for example, prostate cancer, ductal pancreatic adenocarcinoma, and breast cancer. Therefore, it represents an interesting target for the diagnosis of these cancers types by positron emission tomography (PET). The metabolically-stabilized neurotensin (NT) derivative peptide Nlys⁸-Lys⁹-Pro¹⁰-Tyr¹¹-Tle¹²-Leu¹³-OH was elongated at the N-terminus with 6-azido norleucine and coupled with the 1,4,7-triazacyclononane-1,4,7-tris[(2-carboxyethyl)methylenephosphinic acid] (TRAP) chelator TRAP(alkyne)₃ in order to synthesize a NT trimer with subnanomolar affinity and high stability. The ⁶⁸Ga-labeled peptide [⁶⁸Ga]Ga-TRAP(NT4)₃ was characterized in vitro using the NTS1-expressing human colorectal adenocarcinoma cell line HT29. It displayed fast and high internalization rates of >90%, but also fast efflux rates of 50% over 15 min. In vivo, [⁶⁸Ga]Ga-TRAP(NT4)₃ showed moderate HT29 tumor uptake values of 1.7 %ID/g at 60 min post-injection (p.i.), but also high uptake and retention in the kidneys and liver. A comparison of data for trimer/monomer pairs of NT ligands and other targeting vectors (peptides and peptoids targeting integrins α_vβ₃, α₅β₁, and α_vβ₆, the PSMA-ligand DUPA (2-[3-(1,3-dicarboxypropyl)-ureido]pentanedioic acid), and nitroimidazoles targeting hypoxia) revealed that multimers always exhibit higher target affinities and tumor uptake, but not necessarily improved tumor-to-tissue ratios. Thus, although in vitro data are not suitable for prediction of in vivo performance, multimers are potentially superior to monomers, particularly for applications where high tumor accumulation is crucial.

Automated, Resin-Based Method to Enhance the Specific Activity of Fluorine-18 Clicked PET Radiotracers

Radiolabeling of substrates with 2-[¹⁸F]fluoroethylazide exploits the rapid kinetics, chemical selectivity, and mild conditions of the copper-catalyzed azide-alkyne cycloaddition reaction. While this methodology has proven to result in near-quantitative labeling of alkyne-tagged precursors, the relatively small size of the fluoroethylazide group makes separation of the ¹⁸F-labeled radiotracer and the unreacted precursor challenging, particularly with precursors >500 Da (e.g., peptides). We have developed an inexpensive azide-functionalized resin to rapidly remove unreacted alkyne precursor following the fluoroethylazide labeling reaction and integrated it into a fully automated radiosynthesis platform. We have carried out 2-[¹⁸F]fluoroethylazide labeling of four different alkynes ranging from <300 Da to >1700 Da and found that >98% of the unreacted alkyne was removed in less than 20 min at room temperature to afford the final radiotracers at >99% radiochemical purity with specific activities up to >200 GBq/μmol. We have applied this technique to label a novel cyclic peptide previously evolved to bind the Her2 receptor with high affinity, and demonstrated tumor-specific uptake and low nonspecific background by PET/CT. This resin-based methodology is automated, rapid, mild, and general allowing peptide-based fluorine-18 radiotracers to be obtained with clinically relevant specific activities without chromatographic separation and with only a minimal increase in total synthesis time.

The effect of purification of Ga-68-labeled exendin on in vivo distribution

BACKGROUND

Ga-labeled radiotracers are increasingly used for PET imaging. During the labeling procedure, formation of (68)Ga-colloid may occur. Upon i.v. injection, (68)Ga-colloid will accumulate rapidly in the liver, spleen, and bone marrow, resulting in reduced target-to-background ratios. In this study, we applied a thin layer chromatography (TLC) method to measure colloid content and we studied the effect of the purification method on the in vivo characteristics of (68)Ga-labeled DOTA-exendin-3. DOTA-exendin-3 was labeled with (68)Ga, and the colloid content was measured by TLC on silica gel ITLC with two mobile phases. The labeling mixture was purified by gel filtration on a 5-ml G25M column, by reversed-phase high-performance liquid chromatography (RP-HPLC) using a C8 column or by solid phase extraction (SPE) on an HLB cartridge. The in vivo characteristics of the preparations were determined in BALB/c nude mice, and PET images were acquired 1 h p.i. using a microPET scanner. In these studies, unpurified (68)Ga-DOTA-exendin-3 and (111)In-DOTA-exendin-3 were used as a reference.

RESULTS

The colloid content of (111)In-DOTA-exendin-3 and unpurified, gel filtration, RP-HPLC- and SPE-purified (68)Ga-DOTA exendin-3 was <3, 7, 9, <3, and <3 %, respectively. Unpurified (68)Ga-DOTA exendin-3 showed high liver and spleen uptake. Gel filtration partly removed (68)Ga-colloid from the preparation, resulting in moderate liver and spleen SPE-purified (68)Ga-DOTA exendin-3 showed very low liver and spleen uptake, that was similar to that of RP-HPLC purified (68)Ga-DOTA exendin-3.

CONCLUSIONS

We showed that the colloid content can be measured by TLC and that solid phase extraction and HPLC completely remove (68)Ga-colloid from (68)Ga-labeled tracer preparations, resulting in very low liver and spleen uptake. This study clearly shows the importance of removal of (68)Ga-colloid from preparations.