Tailored Gallium(III) Chelator NOPO: Synthesis, Characterization, Bioconjugation, and Application in Preclinical Ga-68-PET Imaging

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.

Modern Developments in Bifunctional Chelator Design for Gallium Radiopharmaceuticals

The positron-emitting radionuclide gallium-68 has become increasingly utilised in both preclinical and clinical settings with positron emission tomography (PET). The synthesis of radiochemically pure gallium-68 radiopharmaceuticals relies on careful consideration of the coordination chemistry. The short half-life of 68 min necessitates rapid quantitative radiolabelling (≤10 min). Desirable radiolabelling conditions include near-neutral pH, ambient temperatures, and low chelator concentrations to achieve the desired apparent molar activity. This review presents a broad overview of the requirements of an efficient bifunctional chelator in relation to the aqueous coordination chemistry of gallium. Developments in bifunctional chelator design and application are then presented and grouped according to eight categories of bifunctional chelator: the macrocyclic chelators DOTA and TACN; the acyclic HBED, pyridinecarboxylates, siderophores, tris(hydroxypyridinones), and DTPA; and the mesocyclic diazepines.

, a Chelator for 68Ga Positron Emission Tomography: Hydroxide Coordination Increases Biological Stability of [68Ga][Ga(Bn2DT3A)(OH)]. Inorg Chem 2022;61:17059-17067. [PMID: 36251390 DOI: 10.1021/acs.inorgchem.2c01992] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

The chelator Bn₂DT3A was used to produce a novel ⁶⁸Ga complex for positron emission tomography (PET). Unusually, this system is stabilized by a coordinated hydroxide in aqueous solutions above pH 5, which confers sufficient stability for it to be used for PET. Bn₂DT3A complexes Ga³⁺ in a hexadentate manner, forming a mer-mer complex with log K([Ga(Bn₂DT3A)]) = 18.25. Above pH 5, the hydroxide ion coordinates the Ga³⁺ ion following dissociation of a coordinated amine. Bn₂DT3A radiolabeling displayed a pH-dependent speciation, with [⁶⁸Ga][Ga(Bn₂DT3A)(OH)]^- being formed above pH 5 and efficiently radiolabeled at pH 7.4. Surprisingly, [⁶⁸Ga][Ga(Bn₂DT3A)(OH)]^- was found to show an increased stability in vitro (for over 2 h in fetal bovine serum) compared to [⁶⁸Ga][Ga(Bn₂DT3A)]. The biodistribution of [⁶⁸Ga][Ga(Bn₂DT3A)(OH)]^- in healthy rats showed rapid clearance and excretion via the kidneys, with no uptake seen in the lungs or bones.

Emerging chelators for nuclear imaging

Chelators are necessary in nuclear medicine imaging to direct an inorganic radionuclide, a radiometal, to a desired target; unfortunately, there is no 'one-size-fits-all' chelator. As the toolbox of radiometals is expanding, new chelators are required to prevent off-target side effects. 1,4,7,10-Tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) is the current gold standard chelator for several radiometals, but typically, chelation requires harsh conditions, making it unsuitable to label biological vectors. The ideal chelator would allow labelling under mild conditions (near-neutral pH and low temperatures [∼37 °C]) and be both thermodynamically and kinetically stable. Over the past 2-3 years, several exciting chelators have been developed that have superior properties to make them worth investigating for future clinical applications.

Gallium and indium complexes with new hexadentate bis(semicarbazone) and bis(thiosemicarbazone) chelators

The synthesis of two new hexadentate potentially tetra-anionic acyclic chelators, an N2O4-donor bis(semicarbazone) (H4bsc) and an N2O2S2-donor bis(thiosemicarbazone) (H4btsc), is described. Coordination reactions of the ligands with gallium and indium precursors were investigated and yielded the complexes [Ga(Hbsc)] (1) and [In(Hbtsc)] (2), respectively. Ligands and complexes structures were confirmed by several techniques, including FTIR, NMR (1H, 13C, COSY, HSQC), ESI(+)-MS and single crystal X-ray diffraction analysis. The radioactive congeners [67Ga(Hbsc)] (1*) and [111In(Hbtsc)] (2*) were also synthesized and their radiolabeling yield and radiochemical purity were certified by HPLC and ITLC analyses. Biodistribution assays in groups of CD-1 mice showed a high uptake of both radiocomplexes in liver and intestine where 1* presented higher retention. In vitro and in vivo assays revealed higher stability of 1* compared with 2*, namely in the blood. The results suggest that radiocomplex 1* is a candidate for further investigation as it could be prepared in high yields (>95%), at low temperature (20-25 °C) and at fast reaction time (15 min), which are very desirable synthesis conditions for potential new radiopharmaceuticals.

Click-Chemistry (CuAAC) Trimerization of an αv β6 Integrin Targeting Ga-68-Peptide: Enhanced Contrast for in-Vivo PET Imaging of Human Lung Adenocarcinoma Xenografts

αv β6 Integrin is an epithelial transmembrane protein that recognizes latency-associated peptide (LAP) and primarily activates transforming growth factor beta (TGF-β). It is overexpressed in carcinomas (most notably, pancreatic) and other conditions associated with αv β6 integrin-dependent TGF-β dysregulation, such as fibrosis. We have designed a trimeric Ga-68-labeled TRAP conjugate of the αv β6 -specific cyclic pentapeptide SDM17 (cyclo[RGD-Chg-E]-CONH2 ) to enhance αv β6 integrin affinity as well as target-specific in-vivo uptake. Ga-68-TRAP(SDM17)3 showed a 28-fold higher αv β6 affinity than the corresponding monomer Ga-68-NOTA-SDM17 (IC50 of 0.26 vs. 7.4 nM, respectively), a 13-fold higher IC50 -based selectivity over the related integrin αv β8 (factors of 662 vs. 49), and a threefold higher tumor uptake (2.1 vs. 0.66 %ID/g) in biodistribution experiments with H2009 tumor-bearing SCID mice. The remarkably high tumor/organ ratios (tumor-to-blood 11.2; -to-liver 8.7; -to-pancreas 29.7) enabled high-contrast tumor delineation in PET images. We conclude that Ga-68-TRAP(SDM17)3 holds promise for improved clinical PET diagnostics of carcinomas and fibrosis.

Cross-Bridged Cyclam with Phosphonate and Phosphinate Pendant Arms: Chelators for Copper Radioisotopes with Fast Complexation

Cross-bridged cyclam derivatives bearing two phosphonate (H₄L¹), bis(phosphinate) (H₄L²), or phosphinate (H₂L³) pendant arms were synthesized and studied with respect to their application as copper radioisotope carriers in nuclear medicine. The ligands show high macrocycle basicity (pK₁ > 14) and high Cu(II) complex stability (log K = 20-24). The complexation and dissociation kinetics of the Cu(II) complexes were studied by ultraviolet-visible spectroscopy. Phosphonate Cu(II)-H₄L¹ and bis(phosphinate) Cu(II)-H₄L² complexes form very quickly, reaching quantitative formation within 1 s at pH ∼6 and millimolar concentrations. Conversely, the formation of the phosphinate complex Cu(II)-H₂L³ is much slower (9 min at pH ∼6) due to the low stability of the out-of-cage reaction intermediate. All studied complexes are highly kinetically inert, showing half-lives of 120, 11, and 111 h for Cu(II)-H₄L¹, Cu(II)-H₄L², and Cu(II)-H₂L³ complexes, respectively, in 1 M HClO₄ at 90 °C. The high thermodynamic stability, fast formation, and extreme kinetic inertness of Cu(II) complexes indicate that phosphonate and bis(phosphinate) derivatives are promising ligands for nuclear medicine.

Selective and clean synthesis of aminoalkyl-H-phosphinic acids from hypophosphorous acid by phospha-Mannich reaction

Aminoalkyl-H-phosphinic acids, also called aminoalkylphosphonous acids, are investigated as biologically active analogues of carboxylic amino acids and/or as valuable intermediates for synthesis of other aminoalkylphosphorus acids. Their synthesis has been mostly accomplished by phospha-Mannich reaction of a P–H precursor, an aldehyde and an amine. The reaction is rarely clean and high-yielding. Here, reaction of H₃PO₂ with secondary amines and formaldehyde in wet AcOH led to aminomethyl-H-phosphinic acids in nearly quantitative yields and with almost no by-products. Surprisingly, the reaction outcome depended on the basicity of the amines. Amines with pK_a > 7–8 gave the desired products. For less basic amines, reductive N-methylation coupled with oxidation of H₃PO₂ to H₃PO₃ became a relevant side reaction. Primary amines reacted less clearly and amino-bis(methyl-H-phosphinic acids) were obtained only for very basic amines. Reaction yields with higher aldehydes were lower. Unique carboxylic–phosphinic–phosphonic acids as well as poly(H-phosphinic acids) derived from polyamines were obtained. Synthetic usefulness of the aminoalkyl-H-phosphinic was illustrated in P–H bond oxidation and its addition to double bonds, and in selective amine deprotection. Compounds with an ethylene-diamine fragment, e.g. most common polyazamacrocycles, are not suitable substrates. The X-ray solid-state structures of seventeen aminoalkyl-phosphinic acids were determined. In the reaction mechanism, N-hydroxyalkyl species R₂NCH₂OH and [R₂N(CH₂OH)₂]⁺, probably stabilized as acetate esters, are suggested as the reactive intermediates. This mechanism is an alternative one to the known phospha-Mannich reaction mechanisms. The conditions can be utilized in syntheses of various aminoalkylphosphorus compounds.

Acetic acid was used as a new solvent for phospha-Mannich reaction leading to clear reaction mixtures and high yields of the aminoalkylphosphonous acids (AHPA), and hydroxymethylated species were suggested as key intermediates in the reaction.

Rapid Thermodynamically Stable Complex Formation of [nat/111In]In3+, [nat/90Y]Y3+, and [nat/177Lu]Lu3+ with H6dappa

A phosphinate-bearing picolinic acid-based chelating ligand (H₆dappa) was synthesized and characterized to assess its potential as a bifunctional chelator (BFC) for inorganic radiopharmaceuticals. Nuclear magnetic resonance (NMR) spectroscopy was employed to investigate the chelator coordination chemistry with a variety of nonradioactive trivalent metal ions (In³⁺, Lu³⁺, Y³⁺, Sc³⁺, La³⁺, Bi³⁺). Density functional theory (DFT) calculations explored the coordination environments of aforementioned metal complexes. The thermodynamic stability of H₆dappa with four metal ions (In³⁺, Lu³⁺, Y³⁺, Sc³⁺) was deeply investigated via potentiometric and spectrophotometric (UV-vis) titrations, employing a combination of acidic in-batch, joint potentiometric/spectrophotometric, and ligand-ligand competition titrations; high stability constants and pM values were calculated for all four metal complexes. Radiolabeling conditions for three clinically relevant radiometal ions were optimized ([¹¹¹In]In³⁺, [¹⁷⁷Lu]Lu³⁺, [⁹⁰Y]Y³⁺), and the serum stability of [¹¹¹In][In(dappa)]^3- was studied. Through concentration-, time-, temperature-, and pH-dependent labeling experiments, it was determined that H₆dappa radiolabels most effectively at near-physiological pH for all radiometal ions. Furthermore, very rapid radiolabeling at ambient temperature was observed, as maximal radiolabeling was achieved in less than 1 min. Molar activities of 29.8 GBq/μmol and 28.2 GBq/μmol were achieved for [¹¹¹In]In³⁺ and [¹⁷⁷Lu]Lu³⁺, respectively. For H₆dappa, high thermodynamic stability did not correlate with kinetic inertness-lability was observed in serum stability studies, suggesting that its metal complexes might not be suitable as a BFC in radiopharmaceuticals.

Radiolabelled Peptides for Positron Emission Tomography and Endoradiotherapy in Oncology

This review deals with the development of peptide-based radiopharmaceuticals for the use with positron emission tomography and peptide receptor radiotherapy. It discusses the pros and cons of this class of radiopharmaceuticals as well as the different labelling strategies, and summarises approaches to optimise metabolic stability. Additionally, it presents different target structures and addresses corresponding tracers, which are already used in clinical routine or are being investigated in clinical trials.

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).

Instant kit preparation of 68Ga-radiopharmaceuticals via the hybrid chelator DATA: clinical translation of [68Ga]Ga-DATA-TOC

Purpose

The widespread use of ⁶⁸Ga for positron emission tomography (PET) relies on the development of radiopharmaceutical precursors that can be radiolabelled and dispensed in a simple, quick, and convenient manner. The DATA (6-amino-1,4-diazapine-triacetate) scaffold represents a novel hybrid chelator architecture possessing both cyclic and acyclic character that may allow for facile access to ⁶⁸Ga-labelled tracers in the clinic. We report the first bifunctional DATA chelator conjugated to [Tyr³]octreotide (TOC), a somatostatin subtype 2 receptor (SST₂)-targeting vector for imaging and functional characterisation of SSTR₂ expressing tumours.

Methods

The radiopharmaceutical precursor, DATA-TOC, was synthesised as previously described and used to complex ^natGa(III) and ⁶⁸Ga(III). Competition binding assays of [^natGa]Ga-DATA-TOC or [^natGa]Ga-DOTA-TOC against [¹²⁵I-Tyr²⁵]LTT-SS28 were conducted in membranes of HEK293 cells transfected to stably express one of the hSST_2,3,5 receptor subtypes (HEK293-hSST_2/3/5 cells). First in vivo studies were performed in female NMRI-nude mice bearing SST₂-positive mouse phaeochromocytoma mCherry (MPC-mCherry) tumours to compare the in vivo SST₂-specific tumour-targeting of [⁶⁸Ga]Ga-DATA-TOC and its overall pharmacokinetics versus the [⁶⁸Ga]Ga-DOTA-TOC reference. A direct comparison of [⁶⁸Ga]Ga-DATA-TOC with the well-established PET radiotracer [⁶⁸Ga]Ga-DOTA-TOC was additionally performed in a 46-year-old male patient with a well-differentiated NET (neuroendocrine tumour), representing the first in human administration of [⁶⁸Ga]Ga-DATA-TOC.

Results

DATA-TOC was labelled with ⁶⁸Ga with a radiolabelling efficiency of > 95% in less than 10 min at ambient temperature. A molar activity up to 35 MBq/nmol was achieved. The hSST₂-affinities of [^natGa]Ga-DATA-TOC and [^natGa]Ga-DOTA-TOC were found similar with only sub-nanomolar differences in the respective IC₅₀ values. In mice, [⁶⁸Ga]Ga-DATA-TOC was able to visualise the tumour lesions, showing standardised uptake values (SUVs) similar to [⁶⁸Ga]Ga-DOTA-TOC. Direct comparison of the two PET tracers in a NET patient revealed very similar tumour uptake for the two ⁶⁸Ga-radiotracers, but with a higher tumour-to-liver contrast for [⁶⁸Ga]Ga-DATA-TOC.

Conclusion

[⁶⁸Ga]Ga-DATA-TOC was prepared, to a quality appropriate for in vivo use, following a highly efficient kit type process. Furthermore, the novel radiopharmaceutical was comparable or better than [⁶⁸Ga]Ga-DOTA-TOC in all preclinical tests, achieving a higher tumour-to-liver contrast in a NET-patient. The results illustrate the potential of the DATA-chelator to facilitate the access to and preparation of ⁶⁸Ga-radiotracers in a routine clinical radiopharmacy setting.

Electronic supplementary material

The online version of this article (10.1186/s13550-019-0516-7) contains supplementary material, which is available to authorized users.

Recent developments in the synthesis and applications of phosphinic peptide analogs

Synthetic pseudopeptides that fit well with the active site architecture allow the most effective binding to enzymes, similar to native substrates in high-energy transition states. Phosphinic acid peptide analogs that comprise the tetrahedral phosphorus moiety introduced to replace an internal amide bond exert such an isosteric or isoelectronic resemblance, combined with providing other advantageous features, for example, metal complexing properties. Accordingly, they are capable of inhibiting metal-dependent enzymes involved in biological functions in eukaryotic and prokaryotic cells. These enzymes are associated with notorious human diseases, such as cancer, e.g., matrix metalloproteinases, or are etiological factors of protozoal and bacterial infections, e.g., metalloaminopeptidases. The affinity and selectivity of these compounds can be conveniently adjusted, either by structural modification of dedicated side chains or by backbone elongation to enhance specific interactions with the corresponding binding pockets. Recent approaches to the synthesis of these compounds are illustrated by examples of the preparation of rationally designed structures of inhibitors of particular enzymes. Activity against appealing enzymatic targets is presented, along with the molecular mechanisms of action and therapeutic implications. Innovative aspects of phosphinic peptide application, e.g., as activity-based probes, and ligands of complexes of radioisotopes for nuclear medicine are also outlined.

Radioactive Main Group and Rare Earth Metals for Imaging and Therapy

Radiometals possess an exceptional breadth of decay properties and have been applied to medicine with great success for several decades. The majority of current clinical use involves diagnostic procedures, which use either positron-emission tomography (PET) or single-photon imaging to detect anatomic abnormalities that are difficult to visualize using conventional imaging techniques (e.g., MRI and X-ray). The potential of therapeutic radiometals has more recently been realized and relies on ionizing radiation to induce irreversible DNA damage, resulting in cell death. In both cases, radiopharmaceutical development has been largely geared toward the field of oncology; thus, selective tumor targeting is often essential for efficacious drug use. To this end, the rational design of four-component radiopharmaceuticals has become popularized. This Review introduces fundamental concepts of drug design and applications, with particular emphasis on bifunctional chelators (BFCs), which ensure secure consolidation of the radiometal and targeting vector and are integral for optimal drug performance. Also presented are detailed accounts of production, chelation chemistry, and biological use of selected main group and rare earth radiometals.

A novel galactose-PEG-conjugated biodegradable copolymer is an efficient gene delivery vector for immunotherapy of hepatocellular carcinoma

Successful immunogene therapy depends not only on the therapeutic gene but also on the gene delivery vector. In this study, we synthesized a novel copolymer consisting of low-molecular-weight polyethylenimine (PEI) cross-linked by myo-inositol (INO) and conjugated with a galactose-grafted PEG chain, named LA-PegPI. We characterized the chemical structure and molecular weight of the copolymer and particle properties of LA-PegPI/pDNA. Furthermore, we showed that LA-PegPI/pDNA polyplexes possessed excellent stability in physiological salt solution, low cytotoxicity, and high transfection efficiency in the asialoglycoprotein receptor (ASGPR)-positive liver cells in vitro. Importantly, we also showed that through intraperitoneal injection of LA-PegPI/pDNA nanoparticles, the reporter gene was forcefully expressed in the liver hepatocytes of mice. Finally, we documented that intraperitoneal injection of LA-PegPI/pIL15 nanoparticles effectively suppressed tumor growth and prolonged survival time of tumor-bearing mice via activation of CD8⁺ T cells and NK cells and upregulation of the cytokines IFN-γ, TNF, and IL12 in an orthotopic hepatocellular carcinoma mouse model. Interestingly, LA-PegPI/pluc nanoparticles could effectively stimulate the proliferation of NK cells and inhibit tumor growth in this model. In summary, LA-PegPI is a useful gene vector for immunogene therapy of hepatocellular carcinoma, and its potential for clinical application warrants further study.

Equilibrium Thermodynamics, Formation, and Dissociation Kinetics of Trivalent Iron and Gallium Complexes of Triazacyclononane-Triphosphinate (TRAP) Chelators: Unraveling the Foundations of Highly Selective Ga-68 Labeling

In order to rationalize the influence of Fe^III contamination on labeling with the ⁶⁸Ga eluted from ⁶⁸Ge/⁶⁸Ga-generator, a detailed investigation was carried out on the equilibrium properties, formation and dissociation kinetics of Ga^III- and Fe^III-complexes of 1,4,7-triazacyclononane-1,4,7-tris(methylene[2-carboxyethylphosphinic acid]) (H₆TRAP). The stability and protonation constants of the [Fe(TRAP)]^3- complex were determined by pH-potentiometry and spectrophotometry by following the competition reaction between the TRAP ligand and benzhydroxamic acid (0.15 M NaNO₃, 25°C). The formation rates of [Fe(TRAP)] and [Ga(TRAP)] complexes were determined by spectrophotometry and ³¹P-NMR spectroscopy in the pH range 4.5-6.5 in the presence of 5-40 fold H_xTRAP^(x-6) excess (x = 1 and 2, 0.15 M NaNO₃, 25°C). The kinetic inertness of [Fe(TRAP)]^3- and [Ga(TRAP)]^3- was examined by the trans-chelation reactions with 10 to 20-fold excess of H_xHBED^(x-4) ligand by spectrophotometry at 25°C in 0.15 M NaCl (x = 0,1 and 2). The stability constant of [Fe(TRAP)]^3- (logK_FeL = 26.7) is very similar to that of [Ga(TRAP)]^3- (logK_GaL = 26.2). The rates of ligand exchange reaction of [Fe(TRAP)]^3- and [Ga(TRAP)]^3- with H_xHBED^(x-4) are similar. The reactions take place quite slowly via spontaneous dissociation of [M(TRAP)]^3-, [M(TRAP)OH]^4- and [M(TRAP)(OH)₂]^5- species. Dissociation half-lives (t_1/2) of [Fe(TRAP)]^3- and [Ga(TRAP)]^3- complexes are 1.1 × 10⁵ and 1.4 × 10⁵ h at pH = 7.4 and 25°C. The formation reactions of [Fe(TRAP)]^3- and [Ga(TRAP)]^3- are also slow due to the formation of the unusually stable monoprotonated [^*M(HTRAP)]^2- intermediates [^*logK_Ga(HL) = 10.4 and ^*logK_Fe(HL) = 9.9], which are much more stable than the [^*Ga(HNOTA)]⁺ intermediate [^*logK_Ga(HL) = 4.2]. Deprotonation and transformation of the monoprotonated [^*M(HTRAP)]^2- intermediates into the final complex occur via OH^--assisted reactions. Rate constants (k_OH) characterizing the OH^--driven deprotonation and transformation of [^* Ga(HTRAP)]^2- and [^*Fe(HTRAP)]^2- intermediates are 1.4 × 10⁵ M^-1s^-1 and 3.4 × 10⁴ M^-1s^-1, respectively. In conclusion, the equilibrium and kinetic properties of [Fe(TRAP)] and [Ga(TRAP)] complexes are remarkably similar due to the close physico-chemical properties of Fe^III and Ga^III-ions. However, a slightly faster formation of [Ga(TRAP)] over [Fe(TRAP)] provides a rationale for a previously observed, selective complexation of ⁶⁸Ga^III in presence of excess Fe^III.

Therapeutic Radiopharmaceuticals Targeting Integrin αvβ6

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

Current advances in ligand design for inorganic positron emission tomography tracers 68Ga, 64Cu, 89Zr and 44Sc

A key part of the development of metal based Positron Emission Tomography probes is the chelation of the radiometal. In this review the recent developments in the chelation of four positron emitting radiometals, ⁶⁸Ga, ⁶⁴Cu, ⁸⁹Zr and ⁴⁴Sc, are explored. The factors that effect the chelation of each radio metal and the ideal ligand system will be discussed with regards to high in vivo stability, complexation conditions, conjugation to targeting motifs and complexation kinetics. A series of cyclic, cross-bridged and acyclic ligands will be discussed, such as CP256 which forms stable complexes with ⁶⁸Ga under mild conditions and PCB-TE2A which has been shown to form a highly stable complex with ⁶⁴Cu. ⁸⁹Zr and ⁴⁴Sc have seen significant development in recent years with a number of chelates being applied to each metal - eight coordinate di-macrocyclic terephthalamide ligands were found to rapidly produce more stable complexes with ⁸⁹Zr than the widely used DFO.

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.

Prospective of 68Ga Radionuclide Contribution to the Development of Imaging Agents for Infection and Inflammation

During the last decade, the utilization of ⁶⁸Ga for the development of imaging agents has increased considerably with the leading position in the oncology. The imaging of infection and inflammation is lagging despite strong unmet medical needs. This review presents the potential routes for the development of ⁶⁸Ga-based agents for the imaging and quantification of infection and inflammation in various diseases and connection of the diagnosis to the treatment for the individualized patient management.

Re-thinking the role of radiometal isotopes: Towards a future concept for theranostic radiopharmaceuticals

The potential and future role of certain metal radionuclides, for example, ⁴⁴ Sc, ⁸⁹ Zr, ⁸⁶ Y, ⁶⁴ Cu, ⁶⁸ Ga, ¹⁷⁷ Lu, ²²⁵ Ac, and ²¹³ Bi, and several terbium isotopes has been controversially discussed in the past decades. Furthermore, the possible benefits of "matched pairs" of isotopes for tandem applications of diagnostics and therapeutics (theranostics) have been emphasized, while such approaches still have not made their way into routine clinical practice. Analysis of bibliographical data illustrates how popularity of certain nuclides has been promoted by cycles of availability and applications. We furthermore discuss the different practical requirements for diagnostic and therapeutic radiopharmaceuticals and the resulting consequences for efficient development of clinically useful pairs of radionuclide theranostics, with particular emphasis on the underlying economical factors. Based on an exemplary assessment of overall production costs for ⁶⁸ Ga and ¹⁸ F radiopharmaceuticals, we venture a look into the future of theranostics and predict that high-throughput PET applications, that is, diagnosis of frequent conditions, will ultimately rely on ¹⁸ F tracers. PET radiometals will occupy a niche in the clinical low-throughput sector (diagnosis of rare diseases), but above all, dominate preclinical research and clinical translation. Matched isotope pairs will be of lesser relevance for theranostics but may become important for future PET-based therapeutic dosimetry.

Incorporation of gallium-68 into the crystal structure of Prussian blue to form K(68)GaxFe1-x[Fe(CN)6] nanoparticles: toward a novel bimodal PET/MRI imaging agent

Similarity between the Ga(+) ion and the Fe(3+) ion allows for partial replacement of Fe(3+) ions with Ga(3+) ions in the Fe(iii) crystallographic positions in Prussian blue (PB) to form various solid solutions KGaxFe1-x[Fe(CN)6] (0 < x < 1). Such solid solutions possess very high thermodynamic stability as expected from the parent PB structure. Consequently, a simple one-step (68)Ga-labeling method was developed for preparing a single-phase nanoparticulate bimodal PET/MRI imaging agent based on the PB structural platform. Unlike the typical (68)Ga-labelling reaction based on metal complexation, this novel chelator-free (68)Ga-labeling reaction was shown to be kinetically fast under the acidic conditions. The Ga(3+) ion does not hydrolyze, and affords the (68)Ga-labelled PB nanoparticles, which are easy to purify and have extremely high stability against radionuclidic leaching in aqueous solution.

Microwave gallium-68 radiochemistry for kinetically stable bis(thiosemicarbazone) complexes: structural investigations and cellular uptake under hypoxia

We report the microwave synthesis of several bis(thiosemicarbazones) and the rapid gallium-68 incorporation to give the corresponding metal complexes. These proved kinetically stable under 'cold' and 'hot' biological assays and were investigated using laser scanning confocal microscopy, flow cytometry and radioactive cell retention studies under normoxia and hypoxia. (68)Ga complex retention was found to be 34% higher in hypoxic cells than in normoxic cells over 30 min, further increasing to 53% at 120 min. Our data suggests that this class of gallium complexes show hypoxia selectivity suitable for imaging in living cells and in vivo tests by microPET in nude athymic mice showed that they are excreted within 1 h of their administration.

The influence of the combination of carboxylate and phosphinate pendant arms in 1,4,7-triazacyclononane-based chelators on their 68Ga labelling properties

In order to compare the coordination properties of 1,4,7-triazacyclononane (tacn) derivatives bearing varying numbers of phosphinic/carboxylic acid pendant groups towards 68Ga, 1,4,7-triazacyclononane-7-acetic-1,4-bis(methylenephosphinic) acid (NOPA) and 1,4,7- triazacyclononane-4,7-diacetic-1-[methylene(2-carboxyethyl)phosphinic] acid (NO2AP) were synthesized using Mannich reactions with trivalent or pentavalent forms of H-phosphinic acids as phosphorus components. Stepwise protonation constants logK1-3 12.06, 3.90 and 1.95, and stability constants with GaIII and CuII, logKGaL 24.01 and logKCuL 16.66, were potentiometrically determined for NOPA. Both ligands were labelled with 68Ga and compared with NOTA (tacn-N,N',N″-triacetic acid) and NOPO, a TRAP-type [tacn-N,N',N″- tris(methylenephosphinic acid)] chelator. At pH 3, NOPO and NOPA showed higher labelling efficiency (binding with lower ligand excess) at both room temperature and 95 °C, compared to NO2AP and NOTA. Labelling efficiency at pH = 0-3 correlated with a number of phosphinic acid pendants: NOPO >> NOPA > NO2AP >> NOTA; however, it was more apparent at 95 °C than at room temperature. By contrast, NOTA was found to be labelled more efficiently at pH > 4 compared to the ligands with phosphinic acids. Overall, replacement of a single phosphinate donor with a carboxylate does not challenge 68Ga labelling of TRAP-type chelators. However, the presence of carboxylates facilitates labelling at neutral or weakly acidic pH.

A shortcut to high-affinity Ga-68 and Cu-64 radiopharmaceuticals: one-pot click chemistry trimerisation on the TRAP platform

Due to its 3 carbonic acid groups being available for bioconjugation, the TRAP chelator (1,4,7-triazacyclononane-1,4,7-tris(methylene(2-carboxyethylphosphinic acid))) is chosen for the synthesis of trimeric bioconjugates for radiolabelling. We optimized a protocol for bio-orthogonal TRAP conjugation via Cu(I)-catalyzed Huisgen-cycloaddition of terminal azides and alkynes (CuAAC), including a detailed investigation of kinetic properties of Cu(II)-TRAP complexes. TRAP building blocks for CuAAC, TRAP(alkyne)3 and TRAP(azide)3 were obtained by amide coupling of propargylamine/3-azidopropyl-1-amine, respectively. For Cu(II) complexes of neat and triply amide-functionalized TRAP, the equilibrium properties as well as pseudo-first-order Cu(II)-transchelation, using 10 to 30 eq. of NOTA and EDTA, were studied by UV-spectrophotometry. Dissociation of any Cu(II)-TRAP species was found to be independent on the nature or excess of a competing chelator, confirming a proton-driven two-step mechanism. The respective thermodynamic stability constants (log K(ML): 19.1 and 17.6) and dissociation rates (k: 38 × 10(-6) and 7 × 10(-6) s(-1), 298 K, pH 4) show that the Cu(II) complex of the TRAP-conjugate possesses lower thermodynamic stability but higher kinetic inertness. At pH 2-3, its demetallation with NOTA was complete within several hours/days at room temperature, respectively, enabling facile Cu(II) removal after click coupling by direct addition of NOTA trihydrochloride to the CuAAC reaction mixture. Notwithstanding this, an extrapolated dissociation half life of >100 h at 37 °C and pH 7 confirms the suitability of TRAP-bioconjugates for application in Cu-64 PET (cf. t(1/2)(Cu-64) = 12.7 h). To showcase advantages of the method, TRAP(DUPA-Pep)3, a trimer of the PSMA inhibitor DUPA-Pep, was synthesized using 1 eq. TRAP(alkyne)3, 3.3 eq. DUPA-Pep-azide, 10 eq. Na ascorbate, and 1.2 eq. Cu(II)-acetate. Its PSMA affinity (IC50), determined by the competition assay on LNCaP cells, was 18-times higher than that of the corresponding DOTAGA monomer (IC50: 2 ± 0.1 vs. 36 ± 4 nM), resulting in markedly improved contrast in Ga-68-PET imaging. In conclusion, the kinetic inertness profile of Cu(II)-TRAP conjugates allows for simple Cu(II) removal after click functionalisation by means of transchelation, but also confirms their suitability for Cu-64-PET as demonstrated previously (Dalton Trans., 2012, 41, 13803).

Approaching 'Kit-Type' Labelling with (68)Ga: The DATA Chelators

The DATA chelators are a novel class of tri-anionic ligands based on 6-amino-1,4-diazepine-triacetic acid, which have been introduced recently for the chelation of (68)Ga. Compared with macrocyclic chelators based on the cyclen scaffold (i.e., DOTA, DO3A, and DO2A derivatives), DATA chelators undergo quantitative radiolabelling more rapidly and under milder conditions. In this study, a systematic evaluation of the labelling of four DATA chelators--DATA(M), DATA(P), DATA(Ph), and DATA(PPh)--with (68)Ga is presented. The results highlight the extraordinary potential of this new class of chelators for application in molecular imaging using (68)Ga positron emission tomography (PET).

Fusarinine C, a novel siderophore-based bifunctional chelator for radiolabeling with Gallium-68

Fusarinine C (FSC), a siderophore‐based chelator coupled with the model peptide c(RGDfK) (FSC(succ‐RGD)₃), revealed excellent targeting properties in vivo using positron emission tomography (PET). Here, we report the details of radiolabeling conditions and specific activity as well as selectivity for ⁶⁸Ga.

⁶⁸Ga labeling of FSC(succ‐RGD)₃ was optimized regarding peptide concentration, pH, temperature, reaction time, and buffer system. Specific activity (SA) of [⁶⁸Ga]FSC(succ‐RGD)₃ was compared with ⁶⁸Ga‐1,4,7‐triazacyclononane, 1‐glutaric acid‐4,7 acetic acid RGD ([⁶⁸Ga]NODAGA‐RGD). Stability was evaluated in 1000‐fold ethylenediaminetetraacetic acid (EDTA) solution (pH 7) and phosphate‐buffered saline (PBS). Metal competition tests (Fe, Cu, Zn, Al, and Ni) were carried out using [⁶⁸Ga]‐triacetylfusarinine C.

High radiochemical yield was achieved within 5 min at room temperature, in particular allowing labeling with ⁶⁸Ga up to pH 8 with excellent stability in 1000‐fold EDTA solution and PBS. The 10‐fold to 20‐fold lower concentrations of FSC(succ‐RGD)₃ led to the same radiochemical yield compared with [⁶⁸Ga]NODAGA‐RGD with SA up to 1.8 TBq/µmol. Metal competition tests showed high selective binding of ⁶⁸Ga to FSC.

FSC is a multivalent siderophore‐based bifunctional chelator allowing fast and highly selective labeling with ⁶⁸Ga in a wide pH range and results in stable complexes with high SA. Thus it is exceptionally well suited for the development of new ⁶⁸Ga‐tracers for in vivo molecular imaging with PET.

Drug delivery systems, CNS protection, and the blood brain barrier

Present review highlights various drug delivery systems used for delivery of pharmaceutical agents mainly antibiotics, antineoplastic agents, neuropeptides, and other therapeutic substances through the endothelial capillaries (BBB) for CNS therapeutics. In addition, the use of ultrasound in delivery of therapeutic agents/biomolecules such as proline rich peptides, prodrugs, radiopharmaceuticals, proteins, immunoglobulins, and chimeric peptides to the target sites in deep tissue locations inside tumor sites of brain has been explained. In addition, therapeutic applications of various types of nanoparticles such as chitosan based nanomers, dendrimers, carbon nanotubes, niosomes, beta cyclodextrin carriers, cholesterol mediated cationic solid lipid nanoparticles, colloidal drug carriers, liposomes, and micelles have been discussed with their recent advancements. Emphasis has been given on the need of physiological and therapeutic optimization of existing drug delivery methods and their carriers to deliver therapeutic amount of drug into the brain for treatment of various neurological diseases and disorders. Further, strong recommendations are being made to develop nanosized drug carriers/vehicles and noninvasive therapeutic alternatives of conventional methods for better therapeutics of CNS related diseases. Hence, there is an urgent need to design nontoxic biocompatible drugs and develop noninvasive delivery methods to check posttreatment clinical fatalities in neuropatients which occur due to existing highly toxic invasive drugs and treatment methods.

Benefits of NOPO as chelator in gallium-68 peptides, exemplified by preclinical characterization of (68)Ga-NOPO-c(RGDfK)

The αvβ3-integrin addressing cyclic pentapeptide cyclo(RGDfK) was conjugated to NOPO, 1,4,7-triazacyclononane-1,4-bis[methylene(hydroxymethyl)phosphinic acid]-7-[methylene(2-carboxyethyl)phosphinic acid], a bifunctional chelator with exceptional gallium-68 labeling properties. NOPO-c(RGDfK) and its Ga(III) and Cu(II) complexes showed high affinity to αvβ3 integrin (IC50 = 0.94 ± 0.06, 1.02 ± 0.09, and 0.51 ± 0.06 nM, respectively). (68)Ga labeling of NOPO-c(RGDfK) in an automated GMP-compliant procedure was performed with near-quantitative radiochemical yield, using precursor amounts as low as 0.5 nmol (approximately 0.6 μg). (68)Ga-NOPO-c(RGDfK) was obtained with high purity (>99% by radio-HPLC/TLC) and, optionally, could be produced with specific activities up to 6 TBq/μmol. M21/M21L (human melanoma with high/low αvβ3 integrin expression) xenografted athymic CD-1 nude mice were used for biodistribution, in vivo stability studies, and PET imaging. (68)Ga-NOPO-c(RGDfK) showed rapid and specific uptake in M21 tumor xenografts (2.02 ± 0.34% ID/g at 60 min p.i.) and was found stable in vivo. Its high hydrophilicity is reflected by an octanol-water distribution coefficient (log D = -4.6) which is more than 1 order of magnitude lower compared to respective NOTA or DOTA analogues. As expected, (68)Ga-NOPO-c(RGDfK) thus showed fast renal clearance from nontargeted tissues. We conclude that NOPO might generally prove a useful means to improve renal clearance of corresponding radiopharmaceuticals by increasing the polarity of its bioconjugates. Favorable labeling properties render NOPO conjugates highly recommendable for reliable routine production of (68)Ga-radiopharmaceuticals in a clinical setting.