Synthesis of Gd and68Ga Complexes in Conjugation with a Conformationally Optimized RGD Sequence as Potential MRI and PET Tumor-Imaging Probes

TRASUTA: The Effect of the Structural Rigidity of a Mesocyclic AAZTA-like Chelating Agent on the Thermodynamic, Kinetic, and Structural Properties of Some Divalent Metal and Ga3+ Complexes

Mesocyclic chelating agents such as AAZTA and its derivatives have been recently reported to overcome the relatively low thermodynamic stability of metal complexes of acyclic chelating agents and the slow complexation kinetics of macrocyclic chelating agents. This work reports the preparation of a spirobicyclic hexadentate AAZTA-like chelating agent (TRASUTA) and the investigation of the thermodynamic, kinetic, and structural properties of the corresponding chelates with the PET-relevant Ga3+ and selected metal ions. A combination of analytical techniques allowed identification of a coordination isomerization process, involving the coordinating side arms and the inversion of a nitrogen atom and leading to lower thermodynamic and kinetic inertness with respect to mononuclear mesocyclic analogues. The bicyclic system of TRASUTA retains significant dynamics despite the conformational constraint imposed by the spiro-fusion, resulting in a lower stability of the corresponding metal chelates.

Novel [68Ga/177Lu]Ga/Lu-AZ-093 as PSMA-Targeting Agent for Diagnosis and Radiotherapy

Prostate-specific membrane antigen (PSMA) overexpressed in prostate cancer cells can serve as a target for imaging and radioligand therapy (RLT). Previously, [68Ga]Ga-P16-093, containing a Ga(III) chelator, N,N'-bis[2-hydroxy-5-(carboxyethyl)benzyl]ethylenediamine-N,N'-diacetic acid (HBED-CC), displayed excellent PSMA-targeting properties and showed a high tumor uptake and retention useful for diagnosis in prostate cancer patients. Recently, [177Lu]Lu-PSMA-617 has been approved by the U.S. food and drug administration (FDA) for the treatment of prostate cancer patients. Derivatives of PSMA-093 using AAZTA (6-amino-6-methylperhydro-1,4-diazepinetetraacetic acid), as the chelator, were designed as alternative agents forming complexes with both diagnostic and therapeutic radiometals, such as gallium-68 (log K = 22.18) or lutetium-177 (log K = 21.85). The aim of this study is to evaluate AAZTA-Gly-O-(methylcarboxy)-Tyr-Phe-Lys-NH-CO-NH-Glu (designated as AZ-093, 1) leading to a gallium-68/lutetium-177 theranostic pair as potential PSMA targeting agents. Synthesis of the desired precursor, AZ-093, 1, was effectively accomplished. Labeling with either [68Ga]GaCl3 or [177Lu]LuCl3 in a sodium acetate buffer solution (pH 4-5) at 50 °C in 5 to 15 min produced either [68Ga]Ga-1 or [177Lu]Lu-1 with high yields and excellent radiochemical purities. Results of in vitro binding studies, cell uptake, and retention (using PSMA-positive prostate carcinoma cells line, 22Rv1-FOLH1-oe) were comparable to that of [68Ga]Ga-P16-093 and [177Lu]Lu-PSMA-617, respectively. Specific cellular uptake was determined with or without the competitive blocking agent (2 μM of "cold" PSMA-11). Cellular binding and internalization showed a time-dependent increase over 2 h at 37 °C in the PSMA-positive cells. The cell uptakes were completely blocked by the "cold" PSMA-11 suggesting that they are competing for the same PSMA binding sites. In the mouse model with implanted PSMA-positive tumor cells, both [68Ga]Ga-1 and [177Lu]Lu-1 displayed excellent uptake and retention in the tumor. Results indicate that [68Ga]Ga/[177Lu]Lu-1 (68Ga]Ga/[177Lu]Lu-AZ-093) is potentially useful as PSMA-targeting agent for both diagnosis and radiotherapy of prostate cancer.

Advances in MRI: Peptide and peptidomimetic-based contrast agents

Magnetic resonance imaging (MRI) is a common medical imaging technique that provides three-dimensional body images. MRI contrast agents improve image contrast by raising the rate of water proton relaxation in specific tissues. Peptides and peptidomimetics act as scaffolds for MRI imaging agents because of their increased size and offer the possibility to engine a higher hydration value within the design. The design of a new Gd-based contrast agent must take into account high stability constants to avoid free Gd(III), with the subsequent nephrotoxicity, and high relaxivity values. This review analyzes various synthetic approaches, reports studies of relaxometric parameters, and focuses on the description and application of Gd(III)-chelates based on peptide and peptidomimetic scaffolds. In addition, the X-ray molecular structures of three DOTA complexes will be reported to emphasize the necessity of using the X-ray diffraction analysis to identify the coordination sphere of the metals and the mechanism of action of the compounds.

Improved synthesis of DA364, an NIR fluorescence RGD probe targeting αvβ3 integrin

Optical imaging (OI) is gaining increasing attention in medicine as a non-invasive diagnostic imaging technology and as a useful tool for image-guided surgery. OI exploits the light emitted in the near-infrared region by fluorescent molecules able to penetrate living tissues. Cyanines are an important class of fluorescent molecules and by their conjugation to peptides it is possible to achieve optical imaging of tumours by selective targeting. We report here the improvements obtained in the synthesis of DA364, a small fluorescent probe (1.5 kDa) prepared by conjugation of pentamethine cyanine Cy5.5 to an RGD peptidomimetic, which can target tumour cells overexpressing integrin αvβ3 receptors.

Peptides as Therapeutic Agents: Challenges and Opportunities in the Green Transition Era

Peptides are at the cutting edge of contemporary research for new potent, selective, and safe therapeutical agents. Their rise has reshaped the pharmaceutical landscape, providing solutions to challenges that traditional small molecules often cannot address. A wide variety of natural and modified peptides have been obtained and studied, and many others are advancing in clinical trials, covering multiple therapeutic areas. As the demand for peptide-based therapies grows, so does the need for sustainable and environmentally friendly synthesis methods. Traditional peptide synthesis, while effective, often involves environmentally draining processes, generating significant waste and consuming vast resources. The integration of green chemistry offers sustainable alternatives, prioritizing eco-friendly processes, waste reduction, and energy conservation. This review delves into the transformative potential of applying green chemistry principles to peptide synthesis by discussing relevant examples of the application of such approaches to the production of active pharmaceutical ingredients (APIs) with a peptide structure and how these efforts are critical for an effective green transition era in the pharmaceutical field.

Small Molecules against Metastatic Tumors: Concrete Perspectives and Shattered Dreams

Metastasis is the main cause of anti-cancer therapy failure, leading to unfavorable prognosis for patients. The true challenge to increase cancer patient life expectancy by making cancer a chronic disease with periodic but manageable relapses relies on the development of efficient therapeutic strategies specifically directed against key targets in the metastatic process. Traditional chemotherapy with classical alkylating agents, microtubule inhibitors, and antimetabolites has demonstrated its limited efficacy against metastatic cells due to their capacity to select chemo-resistant cell populations that undergo epithelial-to-mesenchymal transition (EMT), thus promoting the colonization of distant sites that, in turn, sustain the initial metastatic process. This scenario has prompted efforts aimed at discovering a wide variety of small molecules and biologics as potential anti-metastatic drugs directed against more specific targets known to be involved in the various stages of metastasis. In this short review, we give an overview of the most recent advances related to important families of antimetastatic small molecules: intracellular tyrosine kinase inhibitors, cyclin-dependent kinase inhibitors, KRAS inhibitors, and integrin antagonists. Although the majority of these small molecules are not yet approved and not available in the drug market, any information related to their stage of development could represent a precious and valuable tool to identify new targets in the endless fight against metastasis.

Peptide-based positron emission tomography probes: current strategies for synthesis and radiolabelling

In medical imaging, techniques such as magnetic resonance imaging, contrast-enhanced computerized tomography, and positron emission tomography (PET) are extensively available and routinely used for disease diagnosis and treatment. Peptide-based targeting PET probes are usually small peptides with high affinity and specificity to specific cellular and tissue targets opportunely engineered for acting as PET probes. For instance, either the radioisotope (e.g., 18F, 11C) can be covalently linked to the peptide-probe or another ligand that strongly complexes the radioisotope (e.g., 64Cu, 68Ga) through multiple coordinative bonds can be chemically conjugated to the peptide delivery moiety. The main advantages of these probes are that they are cheaper than classical antibody-based PET tracers and can be efficiently chemically modified to be radiolabelled with virtually any radionuclide making them very attractive for clinical use. The goal of this review is to report and summarize recent technologies in peptide PET-based molecular probes synthesis and radiolabelling with the most used radioisotopes in 2022.

A Chemical Strategy for the Preparation of Multimodified Peptide Imaging Probes

Multimodality probes appear of great interest for innovative imaging applications in disease diagnosis. Herein, we present a chemical strategy enabling site-specific double-modification and cyclization of a peptide probe exploiting native chemical ligation (NCL) and thiol-maleimide addition. The synthetic strategy is straightforward and of general applicability for the development of double-labeled peptide multimodality probes.

[111In]In/[177Lu]Lu-AAZTA5-LM4 SST2R-Antagonists in Cancer Theranostics: From Preclinical Testing to First Patient Results

Aiming to expand the application of the SST2R-antagonist LM4 (DPhe-c[DCys-4Pal-DAph(Cbm)-Lys-Thr-Cys]-DTyr-NH2) beyond [68Ga]Ga-DATA5m-LM4 PET/CT (DATA5m, (6-pentanoic acid)-6-(amino)methy-1,4-diazepinetriacetate), we now introduce AAZTA5-LM4 (AAZTA5, 1,4-bis(carboxymethyl)-6-[bis(carboxymethyl)]amino-6-[pentanoic-acid]perhydro-1,4-diazepine), allowing for the convenient coordination of trivalent radiometals of clinical interest, such as In-111 (for SPECT/CT) or Lu-177 (for radionuclide therapy). After labeling, the preclinical profiles of [111In]In-AAZTA5-LM4 and [177Lu]Lu-AAZTA5-LM4 were compared in HEK293-SST2R cells and double HEK293-SST2R/wtHEK293 tumor-bearing mice using [111In]In-DOTA-LM3 and [177Lu]Lu-DOTA-LM3 as references. The biodistribution of [177Lu]Lu-AAZTA5-LM4 was additionally studied for the first time in a NET patient. Both [111In]In-AAZTA5-LM4 and [177Lu]Lu-AAZTA5-LM4 displayed high and selective targeting of the HEK293-SST2R tumors in mice and fast background clearance via the kidneys and the urinary system. This pattern was reproduced for [177Lu]Lu-AAZTA5-LM4 in the patient according to SPECT/CT results in a monitoring time span of 4–72 h pi. In view of the above, we may conclude that [177Lu]Lu-AAZTA5-LM4 shows promise as a therapeutic radiopharmaceutical candidate for SST2R-expressing human NETs, based on previous [68Ga]Ga-DATA5m-LM4 PET/CT, but further studies are needed to fully assess its clinical value. Furthermore, [111In]In-AAZTA5-LM4 SPECT/CT may represent a legitimate alternative diagnostic option in cases where PET/CT is not available.

Boosting Bismuth(III) Complexation for Targeted α-Therapy (TAT) Applications with the Mesocyclic Chelating Agent AAZTA

Targeted α therapy (TAT) is a promising tool in the therapy of cancer. The radionuclide ²¹³ Bi^III shows favourable physical properties for this application, but the fast and stable chelation of this metal ion remains challenging. Herein, we demonstrate that the mesocyclic chelator AAZTA quickly coordinates Bi^III at room temperature, leading to a robust complex. A comprehensive study of the structural, thermodynamic and kinetic properties of [Bi(AAZTA)]^- is reported, along with bifunctional [Bi(AAZTA-C4-COO^- )]^2- and the targeted agent [Bi(AAZTA-C4-TATE)]^- , which incorporates the SSR agonist Tyr³ -octreotate. An unexpected increase in the stability and kinetic inertness of the metal chelate was observed for the bifunctional derivative and was maintained for the peptide conjugate. A cyclotron-produced ^205/206 Bi mixture was used as a model of ²¹³ Bi in labelling, stability, and biodistribution experiments, allowing the efficiency of [²¹³ Bi(AAZTA-C4-TATE)]^- to be estimated. High accumulation in AR42J tumours and reduced kidney uptake were observed with respect to the macrocyclic chelate [²¹³ Bi(DOTA-TATE)]^- .

Derivatives of GdAAZTA Conjugated to Amino Acids: A Multinuclear and Multifrequency NMR Study

The GdAAZTA (AAZTA = 6-amino-6-methylperhydro-1,4-diazepinetetraacetic acid) complex represents a platform of great interest for the design of innovative MRI probes due to its remarkable magnetic properties, thermodynamic stability, kinetic inertness, and high chemical versatility. Here, we detail the synthesis and characterization of new derivatives functionalized with four amino acids with different molecular weights and charges: l-serine, l-cysteine, l-lysine, and l-glutamic acid. The main reason for conjugating these moieties to the ligand AAZTA is the in-depth study of the chemical properties in aqueous solution of model compounds that mimic complex structures based on polypeptide fragments used in molecular imaging applications. The analysis of the ¹H NMR spectra of the corresponding Eu(III)-complexes indicates the presence of a single isomeric species in solution, and measurements of the luminescence lifetimes show that functionalization with amino acid residues maintains the hydration state of the parent complex unaltered (q = 2). The relaxometric properties of the Gd(III) chelates were analyzed by multinuclear and multifrequency NMR techniques to evaluate the molecular parameters that determine their performance as MRI probes. The relaxivity values of all of the novel chelates are higher than that of GdAAZTA over the entire range of applied magnetic fields because of the slower rotational dynamics. Data obtained in reconstituted human serum indicate the occurrence of weak interactions with the proteins, which result in larger relaxivity values at the typical imaging fields. Finally, all of the new complexes are characterized by excellent chemical stability in biological matrices over time, by the absence of transmetallation processes, or the formation of ternary complexes with oxyanions of biological relevance. In particular, the kinetic stability of the new complexes, measured by monitoring the release of Gd³⁺ in the presence of a large excess of Zn²⁺, is ca. two orders of magnitude higher than that of the clinical MRI contrast agent GdDTPA.

Novel GdAAZTA derivatives conjugated to four amino acids were synthesized and characterized through a multi-technique approach. The complexes maintained the favorable thermodynamic and kinetic properties of the parent compound and showed higher relaxivity values in clinical fields. Therefore, they represent a useful model of more complex bio-conjugated structures used in molecular imaging applications.

AAZTA-Derived Chelators for the Design of Innovative Radiopharmaceuticals with Theranostic Applications

With the development of 68Ga and 177Lu radiochemistry, theranostic approaches in modern nuclear medicine enabling patient-centered personalized medicine applications have been growing in the last decade. In conjunction with the search for new relevant molecular targets, the design of innovative chelating agents to easily form stable complexes with various radiometals for theranostic applications has gained evident momentum. Initially conceived for magnetic resonance imaging applications, the chelating agent AAZTA features a mesocyclic seven-membered diazepane ring, conferring some of the properties of both acyclic and macrocyclic chelating agents. Described in the early 2000s, AAZTA and its derivatives exhibited interesting properties once complexed with metals and radiometals, combining a fast kinetic of formation with a slow kinetic of dissociation. Importantly, the extremely short coordination reaction times allowed by AAZTA derivatives were particularly suitable for short half-life radioelements (i.e., 68Ga). In view of these particular characteristics, the scope of this review is to provide a survey on the design, synthesis, and applications in the nuclear medicine/radiopharmacy field of AAZTA-derived chelators.

A Semi Rigid Novel Hydroxamate AMPED-Based Ligand for 89Zr PET Imaging

In this work, we designed, developed, characterized, and investigated a new chelator and its bifunctional derivative for ⁸⁹Zr labeling and PET-imaging. In a preliminary study, we synthesized two hexadentate chelators named AAZTHAS and AAZTHAG, based on the seven-membered heterocycle AMPED (6-amino-6-methylperhydro-1,4-diazepine) with the aim to increase the rigidity of the ⁸⁹Zr complex by using N-methyl-N-(hydroxy)succinamide or N-methyl-N-(hydroxy)glutaramide pendant arms attached to the cyclic structure. N-methylhydroxamate groups are the donor groups chosen to efficiently coordinate ⁸⁹Zr. After in vitro stability tests, we selected the chelator with longer arms, AAZTHAG, as the best complexing agent for ⁸⁹Zr presenting a stability of 86.4 ± 5.5% in human serum (HS) for at least 72 h. Small animal PET/CT static scans acquired at different time points (up to 24 h) and ex vivo organ distribution studies were then carried out in healthy nude mice (n = 3) to investigate the stability and biodistribution in vivo of this new ⁸⁹Zr-based complex. High stability in vivo, with low accumulation of free ⁸⁹Zr in bones and kidneys, was measured. Furthermore, an activated ester functionalized version of AAZTHAG was synthesized to allow the conjugation with biomolecules such as antibodies. The bifunctional chelator was then conjugated to the human anti-HER2 monoclonal antibody Trastuzumab (Tz) as a proof of principle test of conjugation to biologically active molecules. The final ⁸⁹Zr labeled compound was characterized via radio-HPLC and SDS-PAGE followed by autoradiography, and its stability in different solutions was assessed for at least 4 days.

AAZTA5-squaramide ester competing with DOTA-, DTPA- and CHX-A″-DTPA-analogues: Promising tool for 177Lu-labeling of monoclonal antibodies under mild conditions

BACKGROUND

Combining the advantages of both cyclic and acyclic chelator systems, AAZTA (1,4-bis(carboxymethyl)-6-[bis(carboxymethyl)]amino-6-methylperhydro-1,4-diazepine) is well suited for complexation of various diagnostic and therapeutic radiometals such as gallium-68, scandium-44 and lutetium-177 under mild conditions. Due to its specificity for primary amines and pH dependent binding properties, squaric acid (SA) represents an excellent tool for selective coupling of the appropriate chelator to different target vectors. Therefore, the aim of this study was to evaluate radiolabeling properties of the novel bifunctional AAZTA5-SA being coupled to a model antibody (bevacizumab) in comparison to DOTA-SA, DTPA-p-Bn-SA and CHX-A″-DTPA-p-Bn-SA using the therapeutic nuclide lutetium-177.

METHODS AND RESULTS

As proof-of-concept, bevacizumab was first functionalized with AAZTA5-SA, DOTA-SA, DTPA-p-Bn-SA or CHX-A″-DTPA-p-Bn-SA. After purification via fractionated size exclusion chromatography (SEC), the corresponding immunoconjugates were subsequently radiolabeled with lutetium-177 at pH 7 and room temperature (RT) as well as 37 °C. After 90 min, labeling of AAZTA5-SA-mAb resulted in almost quantitative radiochemical yields (RCY) of >98% and >99%, respectively. Formation of [177Lu]Lu-DTPA-p-Bn-SA-mAb indicated rapid labeling kinetics reaching similar yields at RT already after 30 min. Fast but incomplete radiolabeling of the CHX-A″-analogue could be observed with a yield of 74% after 10 min and no further significant increase. In contrast, 177Lu-labeling of DOTA-SA-mAb showed negligible radiochemical yields of <2% both at room temperature and 37 °C. In vitro complex stability measurements of [177Lu]Lu-AAZTA5-SA-mAb at 37 °C indicated >94% protein bound activity in human serum and >92% in phosphate buffered saline (PBS), respectively within 15 days. [177Lu]Lu-DTPA-p-Bn-SA-mAb and [177Lu]Lu-CHX-A″-DTPA-p-Bn-SA-mAb revealed similar to even slightly higher in vitro stability in both media.

CONCLUSION

Coupling of AAZTA5-SA to the monoclonal antibody bevacizumab allowed for 177Lu-labeling with almost quantitative radiochemical yields both at room temperature and 37 °C. Within 15 days, the resulting radioconjugate indicated very high in vitro complex stability both in human serum and PBS. Therefore, AAZTA5-SA is a promising tool for 177Lu-labeling of sensitive biomolecules such as antibodies for theranostic applications.

Synthesis, radiolabeling, and pre-clinical evaluation of [44Sc]Sc-AAZTA conjugate PSMA inhibitor, a new tracer for high-efficiency imaging of prostate cancer

PURPOSE

The aim of this work was to demonstrate the suitability of AAZTA conjugated to PSMA inhibitor (B28110) labeled with scandium-44 as a new PET tracer for diagnostic imaging of prostate cancer.

BACKGROUND

Nowadays, scandium-44 has received significant attention as a potential radionuclide with favorable characteristics for PET applications. A polyaminopolycarboxylate heptadentate ligand based on a 1,4-diazepine scaffold (AAZTA) has been thoroughly studied as chelator for Gd³⁺ ions for MRI applications. The excellent results of the equilibrium, kinetic, and labeling studies led to a preliminary assessment of the in vitro and in vivo behavior of [⁴⁴Sc][Sc-(AAZTA)]^- and two derivatives, i.e., [⁴⁴Sc][Sc (CNAAZTA-BSA)] and [⁴⁴Sc][Sc (CNAAZTA-cRGDfK)].

RESULTS

B28110 was synthesized by hybrid approach, combining solid-phase peptide synthesis (SPPS) and solution chemistry to obtain high purity (97%) product with an overall yield of 9%. Subsequently, the radioactive labeling was performed with scandium-44 produced from natural calcium target in cyclotron, in good radiochemical yields (RCY) under mild condition (pH 4, 298 K). Stability study in human plasma showed good RCP% of [⁴⁴Sc]Sc-B28110 up to 24 h (94.32%). In vivo PET/MRI imaging on LNCaP tumor-bearing mice showed high tracer accumulation in the tumor regions as early as 20 min post-injection. Ex vivo biodistribution studies confirmed that the accumulation of ⁴⁴Sc-PSMA-617 was two-fold lower than that of the radiolabeled B28110 probes.

CONCLUSIONS

This work demonstrated the suitability of B28110 for the complexation with scandium-44 at room temperature and the high performance of the resulting new tracer based on AAZTA chelator for the diagnosis of prostate cancer using PET.

[Gd(AAZTA)]- Derivatives with n-Alkyl Acid Side Chains Show Improved Properties for Their Application as MRI Contrast Agents*

Herein, the synthesis and an extensive characterization of two novel Gd(AAZTA) (AAZTA=6-amino-6-methylperhydro-1,4-diazepine tetra acetic acid) derivatives functionalized with short (C₂ and C₄ ) n-alkyl acid functions are reported. The carboxylate functionality is the site for further conjugations for the design of more specific contrast agents (CAs). Interestingly, it has been found that the synthesized complexes display enhanced properties for use as MRI contrast agents on their own. The stability constants determined by using potentiometric titration and UV/Vis spectrophotometry were slightly higher than the one reported for the parent Gd(AAZTA) complex. This observation might be accounted for by the larger sigma-electron donation of the acyl substituents with respect to the one provided by the methyl group in the parent complex. As far as concerns the kinetic stability, transmetallation experiments with endogenous ions (e.g. Cu²⁺ ) implied that the Gd³⁺ ions present in these Gd(AAZTA) derivatives show somewhat smaller susceptibility to chemical exchange towards these ions at 25 °C, close to the physiological condition. The ¹ H NMR spectra of the complexes with Eu^III and Yb^III displayed a set of signals consistent with half the number of methylene protons present on each ligand. The number of resonances was invariant over a large range of temperatures, suggesting the occurrence of a fast interconversion between structural isomers. The relaxivity values (298 K, 20 MHz) were consistent with q=2 being equal to 8.8 mm^-1  s^-1 for the C₂ derivative and 9.4 mm^-1  s^-1 for the C₄ one, that is, sensibly larger than the one reported for Gd(AAZTA) (7.1 mm^-1  s^-1 ). Variable-temperature (VT)-T₂ ¹⁷ O NMR measurements showed, for both complexes, the presence of two populations of coordinated water molecules, one in fast and one in slow exchange with the bulk water. As the high-resolution ¹ H NMR spectra of the analogs with Eu^III and Yb^III did not show the occurrence of distinct isomers (as frequently observed in other macrocyclic lanthanide(III)-containing complexes), we surmised the presence of two fast-interconverting isomers in solution. The analysis of the ¹⁷ O NMR VT-T₂ profiles versus temperature allowed their relative molar fraction to be established as 35 % for the isomer with the fast exchanging water and 65 % for the isomer with the water molecules in slower exchange. Finally, ¹ H NMRD profiles over an extended range of applied magnetic field strengths have been satisfactory fitted on the basis of the occurrence of the two interconverting species.

Biodistribution, dosimetry, and temporal signal-to-noise ratio analyses of normal and cancer uptake of [68Ga]Ga-P15-041, a gallium-68 labeled bisphosphonate, from first-in-human studies

INTRODUCTION

[⁶⁸Ga]Ga-P15-041 ([⁶⁸Ga]Ga-HBED-CC-BP) is a novel bone-seeking PET radiotracer that can be generator-produced. We undertook a Phase 0/I clinical trial to assess its potential for imaging bone metastases in prostate cancer including assessment of radiotracer biodistribution and dosimetry.

METHODS

Subjects with prostate cancer and known or suspected osseous metastatic disease were enrolled into one of two arms: dosimetry or dynamic. Dosimetry was performed with 6 whole body PET acquisitions and urine collection spanning 3 h; normal organ dosimetry was calculated using OLINDA/EXM. Dynamic imaging included a 60 min acquisition over a site of known or suspected disease followed by two whole body scans. Bootstrapping and subsampling of the acquired list-mode data were conducted to recommend image acquisition parameters for future clinical trials.

RESULTS

Up to 233 MBq (6.3 mCi) of [⁶⁸Ga]Ga-P15-041 was injected into 12 enrolled volunteers, 8 in dosimetry and 4 in dynamic cohorts. Radiotracer accumulated in known bone lesions and cleared rapidly from blood and soft tissue. The highest individual organ dose was 0.135 mSv/MBq in the urinary bladder wall. The average effective dose was 0.0173 ± 0.0036 mSv/MBq. An average injected activity of 166.5 MBq (4.5 mCi) resulted in absorbed dose estimates of 22.5 mSv to the urinary bladder wall, 8.2 mSv to the kidneys, and an effective dose of 2.9 mSv. Lesion signal to noise ratios on images generated from subsampled data were significantly higher for injected activities above 74 MBq (2 mCi) and were also significantly higher for imaging at 90 min than at 180 min post-injection.

CONCLUSIONS

Dosimetry estimates are acceptable and [⁶⁸Ga]Ga-P15-041 uptake characteristics in patients with confirmed bone metastases support its continued development.

ADVANCES IN KNOWLEDGE AND IMPLICATIONS FOR PATIENT CARE

Use of [⁶⁸Ga]Ga-P15-041 would not require cyclotron infrastructure for manufacturing and distribution, allowing for improved patient access to a promising PET bone imaging agent.

An innovative approach for the synthesis of dual modality peptide imaging probes based on the native chemical ligation approach

Peptide-targeting probes tagged with optical imaging and PET reporters may find applications in innovative diagnostic procedures and image-guided surgeries. The reported synthesis procedure is of general applicability to obtain dual imaging probes using fully unprotected moieties with a selective and rapid chemistry based on native chemical ligation.

Gallium-68 and scandium-44 labelled radiotracers based on curcumin structure linked to bifunctional chelators: Synthesis and characterization of potential PET radiotracers

Curcumin metal complexes showed widespread applications in medicine and can be exploited as a lead structure for developing new tracers for nuclear medicine application. Herein, the synthesis, chemical characterization and radiolabelling with gallium-68 and scandium-44 of two new targeting vectors based on curcumin scaffolds and linked to the chelators 1,4,7-triazacyclononane,1-glutaric acid-4,7-acetic acid (NODAGA) and 1,4-bis(carboxymethyl)-6-[bis(carboxymethyl)]amino-6-methylperhydro-1,4-diazepine (AAZTA) are reported. Synthesis of the precursors could be achieved with a 13% and 11% yield and radiolabelling generally afforded rapid incorporation under mild conditions (>95%). Stability in physiological media (~75% after 2 h in human blood for [⁶⁸Ga]Ga-/[⁴⁴Sc]Sc-AAZTA-PC21 and ~60% for [⁶⁸Ga]Ga-NODAGA-C21, respectively) are generally enhanced if compared to the previously radiolabelled analogues. MSⁿ fragmentation experiments showed high stability of the AAZTA-PC21 structure mainly due to the pyrazole derivatization of the curcumin keto-enol moiety and a more feasible radiolabelling was noticed both with gallium-68 and scandium-44 mainly due to the AAZTA-chelator properties. [⁶⁸Ga]Ga-NODAGA-C21 showed the most favorable lipophilicity value (logD = 1.3). Due to these findings, both compounds appear to be promising candidates for the imaging of colorectal cancer, but further studies such as in vitro uptake and in vivo biodistribution experiments are needed.

PIDAZTA: Structurally Constrained Chelators for the Efficient Formation of Stable Gallium-68 Complexes at Physiological pH

Two structurally constrained chelators based on a fused bicyclic scaffold, 4-amino-4-methylperhydro-pyrido[1,2-a][1,4]diazepin-N,N',N'-triacetic acids [(4R*,10aS*)-PIDAZTA (L1) and (4R*,10aR*)-PIDAZTA (L2)], were designed for the preparation of Ga^III -based radiopharmaceuticals. The stereochemistry of the ligand scaffold has a deep impact on the properties of the complexes, with unexpected [Ga(L2)OH] species being superior in terms of both thermodynamic stability and inertness. This peculiar behavior was rationalized on the basis of molecular modeling and appears to be related to a better fit in size of Ga^III into the cavity of L2. Fast and efficient formation of the Ga^III chelates at room temperature was observed at pH values between 7 and 8, which enables ⁶⁸ Ga radiolabeling under truly physiological conditions (pH 7.4).

AAZTA5/AAZTA5-TOC: synthesis and radiochemical evaluation with 68Ga, 44Sc and 177Lu

PURPOSE

AAZTA (1,4-bis (carboxymethyl)-6-[bis (carboxymethyl)]amino-6-methylperhydro-1,4-diazepine) based chelators were initially developed in the context of magnetic resonance imaging. First radiochemical studies showed the capability of AAZTA to form stable complexes with radiolanthanides and moderately stable complexes with ⁶⁸Ga. For a systematic comparison of the labelling capabilities with current diagnostic and therapeutic trivalent radiometals, AAZTA⁵ (1,4-bis (carboxymethyl)-6-[bis (carboxymethyl)]amino-6-[pentanoic-acid]perhydro-1,4-diazepine) was synthesized representing a bifunctional version with a pentanoic acid at the carbon-6 atom. To evaluate the effect of adding a targeting vector (TV) to the bifunctional chelator on the complex formation, AAZTA⁵-TOC was synthesized, radiolabeled and tested in comparison to the uncoupled AAZTA⁵.

METHODS

AAZTA⁵ was synthesized in a 5-step synthesis. It was coupled to the cyclic peptide TOC (Phe¹-Tyr³ octreotide) via amide bound formation. AAZTA and AAZTA⁵-TOC complex formations with ⁶⁸Ga, ⁴⁴Sc and ¹⁷⁷Lu were investigated at different pH, temperature and precursor amounts. Stability studies against human serum, PBS buffer, EDTA and DTPA were performed.

RESULTS

AAZTA⁵ and AAZTA⁵-TOC achieved quantitative labelling (> 95%) at room temperature in less than 5 min with all three nuclides at pH ranges from 4 to 5.5 with low precursor amounts of 1 to 10 nmol. [⁴⁴Sc]Sc-AAZTA⁵ complexes as well as [⁴⁴Sc]Sc-AAZTA⁵-TOC were completely stable. The ¹⁷⁷Lu complexes of AAZTA⁵ and AAZTA⁵-TOC showed high stability comparable to the ⁴⁴Sc complexes. In contrast, the [⁶⁸Ga]Ga-AAZTA⁵ complex stability was rather low, but interestingly, [⁶⁸Ga]Ga-AAZTA⁵-TOC was completely stable.

CONCLUSION

AAZTA⁵ appears to be a promising bifunctional chelator for ⁶⁸Ga, ⁴⁴Sc and ¹⁷⁷Lu with outstanding labelling capabilities at room temperature. Complex stabilities are high in the case of ⁴⁴Sc and ¹⁷⁷Lu. While [⁶⁸Ga]Ga-AAZTA complexes alone lacking stability, [⁶⁸Ga]Ga-AAZTA⁵-TOC demonstrated high stability. The latter indicates an interesting feature of [⁶⁸Ga]Ga-AAZTA⁵-labelled radiopharmaceuticals.

Synthesis of High Relaxivity Gadolinium AAZTA Tetramers as Building Blocks for Bioconjugation

Molecular imaging requires the specific accumulation of contrast agents at the target. To exploit the superb resolution of MRI for applications in molecular imaging, gadolinium chelates, as the MRI contrast agents (CA), have to be conjugated to a specific vector able to recognize the epitope of interest. Several Gd(III)-chelates can be chemically linked to the same binding vector in order to deliver multiple copies of the CA (multimers) in a single targeting event thus increasing the sensitivity of the molecular probe. Herein three novel bifunctional agents, carrying one functional group for the bioconjugation to targeting vectors and four Gd(III)-AAZTA chelate functions for MRI contrast enhancement (AAZTA = 6-amino-6-methylperhydro-1,4-diazepinetetraacetic acid), are reported. The relaxivity in the tetrameric derivatives is 16.4 ± 0.2 mM_Gd^-1 s^-1 at 21.5 MHz and 25 °C, being 2.4-fold higher than that of parent, monomeric Gd(III)-AAZTA. These compounds can be used as versatile building blocks to insert preformed, high relaxivity, and high density Gd-centers to biological targeting vectors. As an example, we describe the use of these bifunctional Gd(III)-chelates to label a fibrin-targeting peptide.

New anionic carbosilane dendrons functionalized with a DO3A ligand at the focal point for the prevention of HIV-1 infection

Novel third-generation polyanionic carbosilane dendrons with sulfonate or carboxylate end-groups and functionalized with a DO3A ligand at the focal point, and their corresponding copper complexes, have been prepared as antiviral compounds to prevent HIV-1 infection. The topology enables the compound to have an excellent chelating agent, DO3A, while keeping anionic peripheral groups for a therapeutic action. In this study, the cytotoxicity and anti-HIV-1 abilities of carboxylate- (5) or sulfonate-terminated (6) dendrons containing DO3A and their copper complexes (7 or 8) were evaluated. All compounds showed low cytotoxicity and demonstrated potent and broad-spectrum anti-HIV-1 activity in vitro. We also assessed the mode of antiviral action on the inhibition of HIV-1 through a panel of different in vitro antiviral assays. Our results show that copper-free dendron 6 protects the epithelial monolayer from short-term cell disruption. Copper-free dendrons 5 and 6 exert anti-HIV-1 activity at an early stage of the HIV-1 lifecycle by binding to the envelope glycoproteins of HIV-1 and by interacting with the CD4 cell receptor and blocking the binding of gp120 to CD4, and consequently HIV-1 entry. These findings show that copper-free dendrons 5 and 6 have a high potency against HIV-1 infection, confirming their non-specific ability and suggesting that these compounds deserve further study as potential candidate microbicides to prevent HIV-1 transmission.

Equilibrium, Kinetic and Structural Properties of Gallium(III) and Some Divalent Metal Complexes Formed with the New DATAm and DATA5m Ligands

The development of ⁶⁸ Ge/⁶⁸ Ga generators has made the positron-emitting ⁶⁸ Ga isotope widely accessible and raised interest in new chelate complexes of Ga³⁺ . The hexadentate 1,4-di(acetate)-6-methyl[amino(methyl)acetate]perhydro-1,4-diazepane (DATA^m ) ligand and its bifunctional analogue, 1,4-di(acetate)-6-pentanoic acid[amino(methyl)acetate]perhydro-1,4-diazepane (DATA^5m ), rapidly form complexes with ⁶⁸ Ga in high radiochemical yield. The stability constants of DATA^m and DATA^5m complexes formed with Ga³⁺ , Zn²⁺ , Cu²⁺ , Mn²⁺ and Ca²⁺ have been determined by using pH potentiometry, spectrophotometry (Cu²⁺ ) and ¹ H and ⁷¹ Ga NMR spectroscopy (Ga³⁺ ). The stability constants of Ga(DATA^m ) and Ga(DATA^5m ) complexes are slightly higher than those of Ga(AAZTA). The species distribution calculations indicated the predominance of Ga(L)OH mixed-hydroxo complexes at physiological pH. The ¹ H and ⁷¹ Ga NMR spectroscopy studies provided information about the coordinated functional groups of ligands and on the kinetics of exchange between the Ga(L) and Ga(L)OH complexes. The transmetalation reactions between the Ga(L) complexes and Cu²⁺ citrate (6<pH<8.5) occur through both spontaneous and OH^- -assisted dissociation of the Ga(L)OH species. At pH 7.4 and 25 °C, the half-lives of the dissociation of Ga(DATA^m ), Ga(DATA^5m ) and Ga(AAZTA) were 11, 44 and 24 h, respectively. Similar half-lives have been obtained for the ligand-exchange reactions between the Ga(L)OH complexes and transferrin. The equilibrium and kinetic data indicate that the Ga(DATA^5m ) complex is a good ⁶⁸ Ga-based radiodiagnostic candidate.

New potent αvβ3 integrin ligands based on azabicycloalkane (γ,α)-dipeptide mimics

We have designed a new synthetic strategy for the preparation of a new class of cyclic RGD integrin ligands in which the azabicycloalkane scaffold can be envisaged as a (γ,α) dipeptide mimic. The synthesis and in vitro biological evaluation of these RGD derivatives, as well as the computational study of their conformational properties and binding modes to αVβ3 integrin are described. Compound has shown to be a promising candidate as αVβ3 integrin antagonist able to interfere with both cell adhesion and movement on vitronectin with no evidence of cytotoxic effects.

Guidance of Signaling Activations by Cadherins and Integrins in Epithelial Ovarian Cancer Cells

Epithelial ovarian cancer (EOC) is the deadliest tumor among gynecological cancer in the industrialized countries. The EOC incidence and mortality have remained unchanged over the last 30 years, despite the progress in diagnosis and treatment. In order to develop novel and more effective therapeutic approaches, the molecular mechanisms involved in EOC progression have been thoroughly investigated in the last few decades. At the late stage, peritoneal metastases originate from the attachment of small clusters of cancer cells that shed from the primary site and carried by the ascites adhere to the abdominal peritoneum or omentum. This behavior suggests that cell–cell or cell–matrix adhesion mechanisms regulate EOC growth and dissemination. Complex downstream signalings, which might be influenced by functional cross-talk between adhesion molecules and co-expressed and activated signaling proteins, can affect the proliferation/survival and the migration/invasion of EOC cells. This review aimed to define the impact of the mechanisms of cell–cell, through cadherins, and cell–extracellular matrix adhesion, through integrins, on the signaling cascades induced by membrane receptors and cytoplasmic proteins known to have a role in the proliferation, migration and invasion of EOC cells. Finally, some novel approaches using peptidomimetic ligands to cadherin and integrins are summarized.

Design, synthesis and biological evaluation of novel dimeric and tetrameric cRGD-paclitaxel conjugates for integrin-assisted drug delivery

Integrins are associated with tumour cell survival and progression, and their expression has been shown to be increased in tumours. Thus, four novel conjugates of the tripeptide integrin ligand Arg-Gly-Asp (RGD) and the cytotoxic agent paclitaxel (cRGD-PTX) were prepared to investigate the potential of the multivalent presentation of the RGD moiety in improving the antitumor efficacy of PTX by tumour targeting. PTX was conjugated to two or four integrin recognizing ligands. The influence of multivalent presentation on in vitro αvβ3-receptor affinity was confirmed. For all the conjugates compared to the previously synthesized monovalent counterparts, an enhancement of the binding strength was observed; this behaviour was more pronounced when considering the tetravalent presented RGD-conjugate. Cell growth inhibition assays on a panel of human tumour cell lines showed remarkable cytotoxic activity for all conjugates with IC50 values in a nanomolar range. Among the four conjugates, the bivalent derivative 3b was selected for in vivo studies in an ovarian carcinoma cell model xenografted in immunodeficient mice. A marked antitumor activity was observed, similar to that of PTX, but with a much more favourable toxicity profile. Overall, the novel cRGD-PTX conjugates disclosed here represent promising candidates for further advancement in the domain of targeted anti-tumour therapy.

DATATOC: a novel conjugate for kit-type 68Ga labelling of TOC at ambient temperature

Background

The widespread acceptance and application of ⁶⁸Ga-PET depends on our ability to develop radiopharmaceuticals that can be prepared in a convenient and suitable manner. A kit-type labelling protocol provides such characteristics and requires chelators that can be radiolabelled under exceptionally mild conditions. Recently the DATA chelators have been introduced that fulfil these requirements. In continuing their development, the synthesis and radiolabelling of the first DATA bifunctional chelator (BFC) and peptide conjugate are described.

Results

A BFC derived from the DATA ligand (2,2'-(6-((carboxymethyl)amino)-1,4-diazepane-1,4-diyl)diacetic acid) has been synthesised in five steps from simple building blocks, with an overall yield of 8 %. DATA^M5-3^tBu (5-[1,4-Bis-tert-butoxycarbonylmethyl-6-(tert-butoxycarbonylmethyl-methyl-amino)-[1, 4]diazepan-6-yl]-pentanoic acid) has been coupled to [DPhe¹][Tyr³]-octreotide (TOC) and the resulting peptide conjugate (DATATOC) radiolabelled with purified ⁶⁸Ga derived via four different ⁶⁸Ge/⁶⁸Ga generator post-processing (PP) methods. The stability and lipophilicity of the radiotracer have been assessed and a kit-type formulation for radiolabelling evaluated. ⁶⁸Ga-DATATOC has been prepared with a > 95 % radiochemical yield (RCY) within 1 (fractionated and acetone-PP) and 10 min (ethanol- and NaCl-PP) at 23 °C (pH 4.2-4.9, 13 nmol). The radiolabelled peptide is stable in the presence of human serum. Lipophilicity of ⁶⁸Ga-DATATOC was calculated as logP = -3.2 ± 0.3, with a HPLC retention time (t_R = 10.4 min) similar to ⁶⁸Ga-DOTATOC (logP = -2.9 ± 0.4, t_R = 10.3 min). Kit-type labelling from a lyophilised solid using acetone-PP based labelling achieves > 95 % RCY in 10 min at 23 °C.

Conclusions

The favourable labelling properties of the DATA chelators have been retained for DATATOC. High radiochemical purity can be achieved at 23 °C in less than 1 min and from a kit formulation. The speed, reliability, ease, flexibility and simplicity with which ⁶⁸Ga-DATATOC can be prepared makes it a very attractive alternative to current standards.

New (68)Ga-PhenA bisphosphonates as potential bone imaging agents

INTRODUCTION

In vivo positron emission tomography (PET) imaging of the bone using [(68)Ga]bisphosphonates may be a valuable tool for cancer diagnosis and monitoring therapeutic treatment. We have developed new [(68)Ga]bisphosphonates based on the chelating group, AAZTA (6-[bis(hydroxycarbonyl-methyl)amino]-1,4-bis(hydroxycarbonyl methyl)-6-methylperhydro-1,4-diazepine).

METHOD

Phenoxy derivative of AAZTA (2,2'-(6-(bis(carboxymethyl)amino)-6-((4-(2-carboxyethyl)phenoxy)methyl)-1,4-diazepane-1,4-diyl)diacetic acid), PhenA, 2, containing a bisphosphonate group (PhenA-BPAMD, 3, and PhenA-HBP, 4) was prepared. Labeling of these chelating agents with (68)Ga was evaluated.

RESULTS

The ligands reacted rapidly in a sodium acetate buffer with [(68)Ga]GaCl3 eluted from a commercially available (68)Ge/(68)Ga generator (pH4, >95% labeling at room temperature in 5min) to form [(68)Ga]PhenA-BPAMD, 3, and [(68)Ga]PhenA-HBP, 4. The improved labeling condition negates the need for further purification. The (68)Ga bisphosphonate biodistribution and autoradiography of bone sections in normal mice after an iv injection showed excellent bone uptake.

CONCLUSION

New (68)Ga labeled bisphosphonates may be useful as in vivo bone imaging agents in conjunction with positron emission tomography (PET).

Advancement in integrin facilitated drug delivery

The research of integrin-targeted anticancer agents has recorded important advancements in ingenious design of delivery systems, based either on the prodrug approach, or on nanoparticle carriers, but for now, none of these has reached a clinical stage of development. Past work in this area has been extensively reviewed by us and others. Thus, the purpose and scope of the present review is to survey the advancement reported in the last 3years, with focus on innovative delivery systems that appear to afford openings for future developments. These systems exploit the labelling with conventional and novel integrin ligands for targeting the interface of cancer cells and of endothelial cells involved in cancer angiogenesis, with the proteins of the extracellular matrix, in the circulation, in tissues, and in tumour stroma, as the site of progression and metastatic evolution of the disease. Furthermore, these systems implement the expertise in the development of nanomedicines to the purpose of achieving preferential biodistribution and uptake in cancer tissues, internalisation in cancer cells, and release of the transported drugs at intracellular sites. The assessment of the value of controlling these factors, and their combination, for future developments requires support of biological testing in appropriate mechanistic models, but also imperatively demand confirmation in therapeutically relevant in vivo models for biodistribution, efficacy, and lack of off-target effects. Thus, among many studies, we have tried to point out the results supported by relevant in vivo studies, and we have emphasised in specific sections those addressing the medical needs of drug delivery to brain tumours, as well as the delivery of oligonucleotides modulating gene-dependent pathological mechanism. The latter could constitute the basis of a promising third branch in the therapeutic armamentarium against cancer, in addition to antibody-based agents and to cytotoxic agents.

Influence of a novel, versatile bifunctional chelator on theranostic properties of a minigastrin analogue

Background

6-[Bis(carboxymethyl)amino]-1,4-bis(carboxymethyl)-6-methyl-1,4-diazepane (AAZTA ) is a promising chelator with potential advantages over 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) for radiopharmaceutical applications. Its mesocyclic structure enables fast radiolabelling under mild conditions with trivalent metals including not only ⁶⁸Ga for positron emission tomography (PET) but also ¹⁷⁷Lu and ¹¹¹In for single-photon emission computed tomography (SPECT) and radionuclide therapy. Here, we describe the evaluation of a bifunctional AAZTA derivative conjugated to a model minigastrin derivative as a potential theranostic agent.

Methods

An AAZTA derivative with an aliphatic C₉ chain as linker was coupled to a minigastrin, namely [AAZTA⁰, D-Glu¹, desGlu^2–6]-minigastrin (AAZTA-MG), and labelled with ⁶⁸Ga, ¹⁷⁷Lu and ¹¹¹In. The characterisation in vitro included stability studies in different media and determination of logD (octanol/PBS). Affinity determination (IC₅₀) and cell uptake studies were performed in A431-CCK2R cells expressing the human CCK2 receptor. μPET/CT and ex vivo biodistribution studies were performed in CCK2 tumour xenograft-bearing nude mice and normal mice.

Results

AAZTA-MG showed high radiochemical yields for ⁶⁸Ga (>95 %), ¹⁷⁷Lu (>98 %) and ¹¹¹In (>98 %). The logD value of −3.7 for both [⁶⁸Ga]- and [¹⁷⁷Lu]-AAZTA-MG indicates a highly hydrophilic character. Stability tests showed overall high stability in solution with some degradation in human plasma for [⁶⁸Ga]- and transchelation towards DTPA for and [¹⁷⁷Lu]-AAZTA-MG. An IC₅₀ value of 10.0 nM was determined, which indicates a high affinity for the CCK2 receptor. Specific cell uptake after 60 min was >7.5 % for [⁶⁸Ga]-AAZTA-MG and >9.5 % for [¹⁷⁷Lu]-AAZTA-MG, comparable to other DOTA-MG-analogues. μPET/CT studies in CCK2 receptor tumour xenografted mice not only revealed high selective accumulation in A431-CCK2R positive tumours of ⁶⁸Ga-labelled AAZTA-MG (1.5 % ID/g in 1 h post injection) but also higher blood levels as corresponding DOTA-analogues. The ¹¹¹In-labelled peptide had a tumour uptake of 1.7 % ID/g. Biodistribution in normal mice with the [¹⁷⁷Lu]-AAZTA-MG showed a considerable uptake in intestine (7.3 % ID/g) and liver (1.5 % ID/g).

Conclusion

Overall, AAZTA showed interesting properties as bifunctional chelator for peptides providing mild radiolabelling conditions for both ⁶⁸Ga and trivalent metals having advantages over the currently used chelator DOTA. Studies are ongoing to further investigate in vivo targeting properties and stability issues and the influence of spacer length on biodistribution of AAZTA.

Electronic supplementary material

The online version of this article (doi:10.1186/s13550-015-0154-7) contains supplementary material, which is available to authorized users.

Main applications of hybrid PET-MRI contrast agents: a review

In medical imaging, the continuous quest to improve diagnostic performance and optimize treatment strategies has led to the use of combined imaging modalities. Positron emission tomography (PET) and computed tomography (CT) is a hybrid imaging existing already for many years. The high spatial and contrast resolution of magnetic resonance imaging (MRI) and the high sensitivity and molecular information from PET imaging are leading to the development of this new hybrid imaging along with hybrid contrast agents. To create a hybrid contrast agent for PET-MRI device, a PET radiotracer needs to be combined with an MRI contrast agent. The most common approach is to add a radioactive isotope to the surface of a small superparamagnetic iron oxide (SPIO) particle. The resulting agents offer a wide range of applications, such as pH variation monitoring, non-invasive angiography and early imaging diagnosis of atherosclerosis. Oncology is the most promising field with the detection of sentinel lymph nodes and the targeting of tumor neoangiogenesis. Oncology and cardiovascular imaging are thus major areas of development for hybrid PET-MRI imaging systems and hybrid contrast agents. The aim is to combine high spatial resolution, high sensitivity, morphological and functional information. Future prospects include the use of specific antibodies and hybrid multimodal PET-MRI-ultrasound-fluorescence imaging with the potential to provide overall pre-, intra- and postoperative patient care.

Design, synthesis, conformational analysis and application of indolizidin-2-one dipeptide mimics

Growth in the field of peptide mimicry over the past few decades has resulted in the synthesis of many new compounds and the investigation of novel pharmacological agents. Azabicyclo[X.Y.0]alkanone amino acids are among the attractive classes of constrained mimics, because they can create rigid peptide structures for probing the conformation and roles of natural motifs in recognition events important for biological activity. Herein, we review the last ten years of the synthesis, conformational analysis and activity of analogs of the azabicyclo[4.3.0]alkan-2-one amino acid subclass, so-called indolizidin-2-one amino acids, with particular attention on their employment as inputs for biological applications.

Synthesis and preclinical evaluation of a novel, selective 111In-labelled aminoproline-RGD-peptide for non-invasive melanoma tumor imaging

An ¹¹¹In-labelled Amp-based RGD-DOTA conjugate was synthesized and evaluated in preclinical models of human melanoma as a novel integrin-targeted SPECT imaging tracer.

Cyclic RGD Peptides Incorporating Cycloalkanes: Synthesis and Evaluation as PET Radiotracers for Tumor Imaging

Two new bicyclic arginine-glycine-aspartic acid (RGD) peptides, c(RGD-ACP-K) (1a) and c(RGD-ACH-K) (1b), incorporating the aminocyclopentane (ACP) and aminocyclohexane (ACH) carboxylic acids, respectively, were synthesized by grafting the aminocycloalkane carboxylic acids onto the tetra-peptide RGDK sequence. These peptides and their conjugates with DO3A (1,4,7,10-tetraazacyclododecane-1,4,7-trisacetic acid) (2a-b) exhibit high affinity toward U87MG glioblastoma cells. Their affinity is greater than that exhibited by c(RGDyK). Labeling these conjugates with radiometal (64)Cu resulted in high radiochemical yields (>97%) of the corresponding complexes, abbreviated as c(RGD-ACP-K)-DOTA-(64)Cu (3a) and c(RGD-ACH-K)-DOTA-(64)Cu (3b). Both 3a and 3b are stable for 24 h in human and mouse serums and show high tumor uptake, as observed by positron emission tomography (PET). Blocking experiments with 3a and 3b by preinjection of c(RGDyK) confirmed their target specificity and demonstrated their promise as PET radiotracers for imaging ανβ3-positive tumors.

An enzymatic approach to bifunctional chelating agents

Three novel BFCAs are obtained through an original protocol involving an enzymatic hydrolysis as the key step.

U1 Adaptor Oligonucleotides Targeting BCL2 and GRM1 Suppress Growth of Human Melanoma Xenografts In Vivo

U1 Adaptor is a recently discovered oligonucleotide-based gene-silencing technology with a unique mechanism of action that targets nuclear pre-mRNA processing. U1 Adaptors have two distinct functional domains, both of which must be present on the same oligonucleotide to exert their gene-silencing function. Here, we present the first in vivo use of U1 Adaptors by targeting two different human genes implicated in melanomagenesis, B-cell lymphoma 2 (BCL2) and metabotropic glutamate receptor 1 (GRM1), in a human melanoma cell xenograft mouse model system. Using a newly developed dendrimer delivery system, anti-BCL2 U1 Adaptors were very potent and suppressed tumor growth at doses as low as 34 µg/kg with twice weekly intravenous (iv) administration. Anti-GRM1 U1 Adaptors suppressed tumor xenograft growth with similar potency. Mechanism of action was demonstrated by showing target gene suppression in tumors and by observing that negative control U1 Adaptors with just one functional domain show no tumor suppression activity. The anti-BCL2 and anti-GRM1 treatments were equally effective against cell lines harboring either wild-type or a mutant V600E B-RAF allele, the most common mutation in melanoma. Treatment of normal immune-competent mice (C57BL6) indicated no organ toxicity or immune stimulation. These proof-of-concept studies represent an in-depth (over 800 mice in ~108 treatment groups) validation that U1 Adaptors are a highly potent gene-silencing therapeutic and open the way for their further development to treat other human diseases.

RGD-based PET tracers for imaging receptor integrin αv β3 expression

Positron emission tomography (PET) imaging of receptor integrin αv β3 expression may play a key role in the early detection of cancer and cardiovascular diseases, monitoring disease progression, evaluating therapeutic response, and aiding anti-angiogenic drugs discovery and development. The last decade has seen the development of new PET tracers for in vivo imaging of integrin αv β3 expression along with advances in PET chemistry. In this review, we will focus on the radiochemistry development of PET tracers based on arginine-glycine-aspartic acid (RGD) peptide, present an overview of general strategies for preparing RGD-based PET tracers, and review the recent advances in preparations of (18) F-labeled, (64) Cu-labeled, and (68) Ga-labeled RGD tracers, RGD-based PET multivalent probes, and RGD-based PET multimodality probes for imaging receptor integrin αv β3 expression.