NMR Studies Reveal Structural Differences between the Gallium and Yttrium Complexes of DOTA-d-Phe-Tyr3-octreotide

Multinuclear 1H/13C/15N chemical shift assignment of therapeutic octreotide acetate performed at natural abundance

Octreotide acetate, the active pharmaceutical ingredient in the long-acting release (LAR) drug product Sandostatin®, is a cyclic octapeptide that mimics the naturally occurring somatostatin peptide hormone. Modern NMR can be a robust analytical method to identify and quantify octreotide molecules. Previous 1H chemical shift assignments were mostly performed in organic solvents, and no assignments for heteronuclear 13C, 15N, and aromatic 1H nuclei are available. Here, using state-of-the-art 1D and 2D homo- and heteronuclear NMR experiments, octreotide was fully assigned, including water exchangeable amide protons, in aqueous buffer except for 13CO and 15NH of F1, 15NH of C2, and 15NζHζ of K5 that were not observed because of water exchange or conformational exchange. The solution NMR spectra were then directly compared with 1D 1H/13C/15N solid-state NMR (SSNMR) spectra showing the potential applicability of 13C/15N SSNMR for octreotide drug product characterization.

Recent Advances in Radiometals for Combined Imaging and Therapy in Cancer

Nuclear medicine is defined as the use of radionuclides for diagnostic and therapeutic applications. The imaging modalities positron emission tomography (PET) and single-photon emission computed tomography (SPECT) are based on γ-emissions of specific energies. The therapeutic technologies are based on β^- -particle-, α-particle-, and Auger electron emitters. In oncology, PET and SPECT are used to detect cancer lesions, to determine dosimetry, and to monitor therapy effectiveness. In contrast, radiotherapy is designed to irreparably damage tumor cells in order to eradicate or control the disease's progression. Radiometals are being explored for the development of diagnostic and therapeutic radiopharmaceuticals. Strategies that combine both modalities (diagnostic and therapeutic), referred to as theranostics, are promising candidates for clinical applications. This review provides an overview of the basic concepts behind therapeutic and diagnostic radiopharmaceuticals and their significance in contemporary oncology. Select radiometals that significantly impact current and upcoming cancer treatment strategies are grouped as clinically suitable theranostics pairs. The most important physical and chemical properties are discussed. Standard production methods and current radionuclide availability are provided to indicate whether a cost-efficient use in a clinical routine is feasible. Recent preclinical and clinical developments and outline perspectives for the radiometals are highlighted in each section.

Iodinated Choline Transport-Targeted Tracers

We present a novel series of radioiodinated tracers and potential theranostics for diseases accompanied by pathological function of proteins involved in choline transport. Unlike choline analogues labeled with ¹¹C or ¹⁸F that are currently used in the clinic, the iodinated compounds described herein are applicable in positron emission tomography, single-photon emission computed tomography, and potentially in therapy, depending on the iodine isotope selection. Moreover, favorable half-lives of iodine isotopes result in much less challenging synthesis by isotope exchange reaction. Six of the described compounds were nanomolar ligands, and the best compound possessed an affinity 100-fold greater than that of choline. Biodistribution data of ¹²⁵I-labeled ligands in human prostate carcinoma bearing (PC-3) mice revealed two compounds with a biodistribution profile superior to that of [¹⁸F]fluorocholine.

Developing scandium and yttrium coordination chemistry to advance theranostic radiopharmaceuticals

The octadentate siderophore analog 3,4,3-LI(1,2-HOPO), denoted 343-HOPO hereafter, is known to have high affinity for both trivalent and tetravalent lanthanide and actinide cations. Here we extend its coordination chemistry to the rare-earth cations Sc³⁺ and Y³⁺ and characterize fundamental metal-chelator binding interactions in solution via UV-Vis spectrophotometry, nuclear magnetic resonance spectroscopy, and spectrofluorimetric metal-competition titrations, as well as in the solid-state via single crystal X-ray diffraction. Sc³⁺ and Y³⁺ binding with 343-HOPO is found to be robust, with both high thermodynamic stability and fast room temperature radiolabeling, indicating that 343-HOPO is likely a promising chelator for in vivo applications with both metals. As a proof of concept, we prepared a ⁸⁶Y-343-HOPO complex for in vivo PET imaging, and the results presented herein highlight the potential of 343-HOPO chelated trivalent metal cations for therapeutic and theranostic applications.

Chelation with a twist: a bifunctional chelator to enable room temperature radiolabeling and targeted PET imaging with scandium-44

Scandium-44 has emerged as an attractive, novel PET radioisotope with ideal emission properties and half-life (t 1/2 = 3.97 h, E mean β+ = 632 keV) well matched to the pharmacokinetics of small molecules, peptides and small biologics. Conjugates of the current gold-standard chelator for 44Sc, 1,4,7,10-tetraaza-cyclododecane-1,4,7,10-tetraacetic acid (DOTA), require heating to achieve radiochemical complexation, limiting application of this isotope in conjunction with temperature-sensitive biologics. To establish Sc(iii) isotopes as broadly applicable tools for nuclear medicine, development of alternative bifunctional chelators is required. To address this need, we characterized the Sc(iii)-chelation properties of the small-cavity triaza-macrocycle-based, picolinate-functionalized chelator H3mpatcn. Spectroscopic and radiochemical studies establish the [Sc(mpatcn)] complex as kinetically inert and appropriate for biological applications. A proof-of-concept bifunctional conjugate targeting the prostate-specific membrane antigen (PSMA), picaga-DUPA, chelates 44Sc to form 44Sc(picaga)-DUPA at room temperature with an apparent molar activity of 60 MBq μmol-1 and formation of inert RRR-Λ and SSS-Δ-twist isomers. Sc(picaga)-DUPA exhibits a K i of 1.6 nM for PSMA, comparable to the 18F-based imaging probe DCFPyL (K i = 1.1 nM) currently in phase 3 clinical trials for imaging prostate cancer. Finally, we successfully employed 44Sc(picaga)-DUPA to image PSMA-expressing tumors in a preclinical mouse model, establishing the picaga bifunctional chelator as an optimal choice for the 44Sc PET nuclide.

Prostate-specific membrane antigen radioligand therapy of prostate cancer

Defining an optimal therapeutic approach in metastatic castration-resistance prostate cancer (mCRPC) patients in advanced stages is still challenging in routine clinical practice. Prostate-specific membrane antigen (PSMA) targeted radionuclide therapy with β- or α-emitters such as 177-Lutethium (177Lu) or 225-Actinium (225A) has been a main focus at multiple academic research centers in the last few years. This review article provides an overview of PSMA characteristics, clinical performance, safety and toxicity of PSMA targeted β- or α-radiation therapy.

Theranostic radiopharmaceuticals: established agents in current use

Although use of the term "theranostic" is relatively recent, the concept goes back to the earliest days of nuclear medicine, with the use of radioiodine for diagnosis and therapy of benign and malignant thyroid disease being arguably the most successful molecular radiotherapy in history. A diagnostic scan with ¹²³I-, ¹²⁴I-, or a low activity of ¹³¹I-iodide is followed by therapy with high activity ¹³¹I-iodide. Similarly, adrenergic tumours such as phaeochromocytoma and neuroblastoma can be imaged with ¹²³I-metaiodobenzylguanidine and treated with ¹³¹I-metaiodobenzylguanidine. Bone scintigraphy can be used to select patients with painful bone metastases from prostate cancer who may benefit from treatment with beta- or alpha-particle emitting bone seeking agents, the most recent and successful of which is ²²³Ra radium chloride. Anti-CD20 monoclonal antibodies can be used to image and treat non-Hodgkins lymphoma, though this has not been as commercially successful as initially predicted. More recently established theranostics include somatostatin receptor targeting peptides for diagnosis and treatment of neuroendocrine tumours with agents such as ⁶⁸Ga-DOTATATE and ¹⁷⁷Lu-DOTATATE, respectively. Finally, agents which target prostate-specific membrane antigen are becoming increasingly widely available, despite the current lack of a commercial product. With the recent licensing of the somatostatin peptides and the rapid adoption of ⁶⁸Ga- and ¹⁷⁷Lu-labelled prostate-specific membrane antigen targeting agents, we have built upon the experience of radioiodine and are already seeing a great expansion in the availability of widely accepted theranostic radiopharmaceuticals.

Targeted alpha therapy of mCRPC: Dosimetry estimate of 213Bismuth-PSMA-617

Purpose

PSMA-617 is a small molecule targeting the prostate-specific membrane antigen (PSMA). In this work, we estimate the radiation dosimetry for this ligand labeled with the alpha-emitter ²¹³Bi.

Methods

Three patients with metastatic prostate cancer underwent PET scans 0.1 h, 1 h, 2 h, 3 h, 4 h and 5 h after injection of ⁶⁸Ga-PSMA-617. Source organs were kidneys, liver, spleen, salivary glands, bladder, red marrow and representative tumor lesions. The imaging nuclide ⁶⁸Ga was extrapolated to the half-life of ²¹³Bi. The residence times of ²¹³Bi were forwarded to the instable daughter nuclides. OLINDA was used for dosimetry calculation. Results are discussed in comparison to literature data for ²²⁵Ac-PSMA-617.

Results

Assuming a relative biological effectiveness of 5 for alpha radiation, the dosimetry estimate revealed equivalent doses of mean 8.1 Sv _RBE5/GBq for salivary glands, 8.1 Sv _RBE5/GBq for kidneys and 0.52 Sv _RBE5/GBq for red marrow. Liver (1.2 Sv _RBE5/GBq), spleen (1.4 Sv _RBE5/GBq), bladder (0.28 Sv _RBE5/GBq) and other organs (0.26 Sv_RBE5/GBq) were not dose-limiting. The effective dose is 0.56 Sv _RBE5/GBq. Tumor lesions were in the range 3.2–9.0 Sv_RBE5/GBq (median 7.6 Sv_RBE5/GBq). Kidneys would limit the cumulative treatment activity to 3.7 GBq; red marrow might limit the maximum single fraction to 2 GBq. Despite promising results, the therapeutic index was inferior compared to ²²⁵Ac-PSMA-617.

Conclusions

Dosimetry of ²¹³Bi-PSMA-617 is in a range traditionally considered reasonable for clinical application. Nevertheless, compared to ²²⁵Ac-PSMA-617, it suffers from higher perfusion-dependent off-target radiation and a longer biological half-life of PSMA-617 in dose-limiting organs than the physical half-life of ²¹³Bi, rendering this nuclide as a second choice radiolabel for targeted alpha therapy of prostate cancer.

Electronic supplementary material

The online version of this article (10.1007/s00259-017-3817-y) contains supplementary material, which is available to authorized users.

Selection of optimal chelator improves the contrast of GRPR imaging using bombesin analogue RM26

Bombesin (BN) analogs bind with high affinity to gastrin-releasing peptide receptors (GRPRs) that are up-regulated in prostate cancer and can be used for the visualization of prostate cancer. The aim of this study was to investigate the influence of radionuclide-chelator complexes on the biodistribution pattern of the 111In-labeled bombesin antagonist PEG2-D-Phe-Gln-Trp-Ala-Val-Gly-His-Sta-Leu-NH2 (PEG2-RM26) and to identify an optimal construct for SPECT imaging. A series of RM26 analogs N-terminally conjugated with NOTA, NODAGA, DOTA and DOTAGA via a PEG2 spacer were radiolabeled with 111In and evaluated both in vitro and in vivo. The conjugates were successfully labeled with 111In with 100% purity and retained binding specificity to GRPR and high stability. The cellular processing of all compounds was characterized by slow internalization. The IC50 values were in the low nanomolar range, with lower IC50 values for positively charged natIn-NOTA-PEG2-RM26 (2.6 ± 0.1 nM) and higher values for negatively charged natIn-DOTAGA-PEG2-RM26 (4.8 ± 0.5 nM). The kinetic binding studies showed KD values in the picomolar range that followed the same pattern as the IC50 data. The biodistribution of all compounds was studied in BALB/c nu/nu mice bearing PC-3 prostate cancer xenografts. Tumor targeting and biodistribution studies displayed rapid clearance of radioactivity from the blood and normal organs via kidney excretion. All conjugates showed similar uptake in tumors at 4 h p.i. The radioactivity accumulation in GRPR-expressing organs was significantly lower for DOTA- and DOTAGA-containing constructs compared to those containing NOTA and NODAGA. 111In-NOTA-PEG2-RM26 with a positively charged complex showed the highest initial uptake and the slowest clearance of radioactivity from the liver. At 4 h p.i., DOTA- and DOTAGA-coupled analogs showed significantly higher tumor-to-organ ratios compared to NOTA- and NODAGA-containing variants. The NODAGA conjugate demonstrated the best retention of radioactivity in tumors, and, at 24 h p.i., had the highest contrast to blood, muscle and bones.

A benzimidazole functionalised DO3A chelator showing pH switchable coordination modes with lanthanide ions

The synthesis of a new macrocyclic chelator incorporating a benzimidazole heterocycle is reported. Lanthanide complexes with macrocyclic chelators based on 1,4,7,10-tetra(carboxymethyl)-1,4,7,10-tetraazacyclododecane (DOTA) and 1,4,7-tris(carboxymethyl)-1,4,7,10-tetraazacyclododecane (DO3A) are of interest in luminescent, radiopharmaceutical and magnetic resonance (MR) biomedical imaging applications. The benzimidazole DO3A chelator allows for sensitisation of europium(iii), terbium(iii) and ytterbium(iii) luminescence by the heterocycle and also shows a pH dependent coordination change due to protonation of the chelator (pKa = 4.1 for the europium(iii) complex). The thermodynamic stability of the complexes has been investigated by potentiometric titration with the gadolinium(iii) complex showing significantly higher stability than the zinc(ii) complex, where log βZnLH = 28.1 and log βGdLH = 32.1.

The diversity of (68)Ga-based imaging agents

Development of new radiopharmaceuticals and their availability are crucial factors influencing the expansion of clinical nuclear medicine. The number of new (68)Ga-based imaging agents for positron emission tomography (PET) is increasing greatly. (68)Ga has been used for labeling of a broad range of molecules (small organic molecules, peptides, proteins, and oligonucleotides) as well as particles, thus demonstrating its potential to become a PET analog of the legendary generator-produced gamma-emitting (99m)Tc but with added value of higher sensitivity and resolution as well as quantitation and dynamic scanning. Further, the availability of technology for GMP-compliant automated tracer production can facilitate the introduction of new radiopharmaceuticals and enable standardized, harmonized multicenter studies to be conducted for regulatory approval. This chapter presents some examples of tracers for targeted, pretargeted, and nontargeted imaging with emphasis on the potential of (68)Ga to facilitate clinically practical PET development and to promote the PET technique worldwide for earlier and better diagnostics, and personalized medicine with the ultimate objective of improved therapeutic outcome.

Somatostatin subtype-2 receptor-targeted metal-based anticancer complexes

Conjugates of a dicarba analogue of octreotide, a potent somatostatin agonist whose receptors are overexpressed on tumor cells, with [PtCl(2)(dap)] (dap = 1-(carboxylic acid)-1,2-diaminoethane) (3), [(η(6)-bip)Os(4-CO(2)-pico)Cl] (bip = biphenyl, pico = picolinate) (4), [(η(6)-p-cym)RuCl(dap)](+) (p-cym = p-cymene) (5), and [(η(6)-p-cym)RuCl(imidazole-CO(2)H)(PPh(3))](+) (6), were synthesized by using a solid-phase approach. Conjugates 3-5 readily underwent hydrolysis and DNA binding, whereas conjugate 6 was inert to ligand substitution. NMR spectroscopy and molecular dynamics calculations showed that conjugate formation does not perturb the overall peptide structure. Only 6 exhibited antiproliferative activity in human tumor cells (IC(50) = 63 ± 2 μM in MCF-7 cells and IC(50) = 26 ± 3 μM in DU-145 cells) with active participation of somatostatin receptors in cellular uptake. Similar cytotoxic activity was found in a normal cell line (IC(50) = 45 ± 2.6 μM in CHO cells), which can be attributed to a similar level of expression of somatostatin subtype-2 receptor. These studies provide new insights into the effect of receptor-binding peptide conjugation on the activity of metal-based anticancer drugs, and demonstrate the potential of such hybrid compounds to target tumor cells specifically.

Reflections on the theory of "silver bullet" octreotide tracers: implications for ligand-receptor interactions in the age of peptides, heterodimers, receptor mosaics, truncated receptors, and multifractal analysis

The classical attitude of Nuclear Medicine practitioners on matters of peptide-receptor interactions has maintained an intrinsic monogamic character since many years. New advances in the field of biochemistry and even in clinical Nuclear Medicine have challenged this type of thinking, which prompted me to work on this review. The central issue of this paper will be the use of somatostatin analogs, i.e., octreotide, in clinical imaging procedures as well as in relation to neuroendocirne tumors. Newly described characteristics of G-protein coupled receptors such as the formation of receptor mosaics will be discussed. A small section will enumerate the regulatory processes found in the cell membrane. Possible new interpretations, other than tumor detection, based on imaging procedures with somatostatin analogs will be presented. The readers will be taken to situations such as inflammation, nociception, mechanosensing, chemosensing, fibrosis, taste, and vascularity where somatostatin is involved. Thyroid-associated orbitopathy will be used as a model for the development of multi-agent therapeutics. The final graphical summary depicts the multifactorial properties of ligand binding.

Gallium(III) complexes of DOTA and DOTA-monoamide: kinetic and thermodynamic studies

Given the practical advantages of the (68)Ga isotope in positron emission tomography applications, gallium complexes are gaining increasing importance in biomedical imaging. However, the strong tendency of Ga(3+) to hydrolyze and the slow formation and very high stability of macrocyclic complexes altogether render Ga(3+) coordination chemistry difficult and explain why stability and kinetic data on Ga(3+) complexes are rather scarce. Here we report solution and solid-state studies of Ga(3+) complexes formed with the macrocyclic ligand 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid, (DOTA)(4-), and its mono(n-butylamide) derivative, (DO3AM(Bu))(3-). Thermodynamic stability constants, log K(GaDOTA) = 26.05 and log K(GaDO3AM(Bu)) = 24.64, were determined by out-of-cell pH-potentiometric titrations. Due to the very slow formation and dissociation of the complexes, equilibration times of up to ∼4 weeks were necessary. The kinetics of complex dissociation were followed by (71)Ga NMR under both acidic and alkaline conditions. The GaDOTA complex is significantly more inert (τ(1/2) ∼12.2 d at pH = 0 and τ(1/2) ∼6.2 h at pH = 10) than the GaDO3AM(Bu) analogue (τ(1/2) ∼2.7 d at pH = 0 and τ(1/2) ∼0.7 h at pH = 10). Nevertheless, the kinetic inertness of both chelates is extremely high and approves the application of Ga(3+) complexes of such DOTA-like ligands in molecular imaging. The solid-state structure of the GaDOTA complex, crystallized from a strongly acidic solution (pH < 1), evidenced a diprotonated form with protons localized on the free carboxylate pendants.

Automated synthesis, characterization and biological evaluation of [(68)Ga]Ga-AMBA, and the synthesis and characterization of (nat)Ga-AMBA and [(67)Ga]Ga-AMBA

Ga-AMBA (Ga-DO3A-CH(2)CO-G-[4-aminobenzoyl]-QWAVGHLM-NH(2)) is a bombesin-like agonist with high affinity for gastrin releasing peptide receptors (GRP-R). Syntheses for (nat)Ga-AMBA, [(67)Ga]Ga-AMBA and [(68)Ga]Ga-AMBA were developed. The preparation of HPLC-purified and Sep-Pak purified [(68)Ga]Ga-AMBA were fully automated, using the built-in radiodetector of the Tracerlab FX F-N synthesizer to monitor fractionated (68)Ge/(68)Ga generator elution and purification. The total synthesis time, including the fractional elution of the generator, was 20 min for Sep-Pak purified material and 40 min for HPLC-purified [(68)Ga]Ga-AMBA. Both [(67)Ga]Ga-AMBA and [(177)Lu]Lu-AMBA showed comparable high affinity for GRP-R in the human prostate cancer cell line PC-3 in vitro (k(D)=0.46+/-0.07; 0.44+/-0.08 nM), high internalization (78; 77%) and low efflux from cells at 2 h (2.4+/-0.7; 2.9+/-1.8%). Biodistribution results in PC-3 tumor-bearing male nude mice showed comparable uptake for [(177)Lu]Lu-, [(111)In]In-, [(67)Ga]Ga- and [(68)Ga]Ga-AMBA.

Three-dimensional consensus structure of sst2-selective somatostatin (SRIF) antagonists by NMR

The three-dimensional NMR structures of seven octapeptide analogs of somatostatin (SRIF), based on octreotide, with the basic sequence H-Cpa/Phe2-c[DCys3-Xxx7-DTrp/DAph(Cbm)8-Lys9-Thr10-Cys14]-Yyy-NH2 (the numbering refers to the position in native SRIF), with Xxx7 being Aph(Cbm)/Tyr/Agl(NMe,benzoyl) and Yyy being Nal/DTyr/Thr, are presented here. Most of these analogs exhibit potent and highly selective binding to sst2 receptors, and all of the analogs are antagonists inhibiting receptor signaling. Based on their consensus 3D structure, the pharmacophore of the sst2-selective antagonist has been defined. The pharmacophore involves the side chains of Cpa2, DTrp/DAph(Cbm)8, and Lys9, with the backbone for most of the sst2-selective antagonists comprised a Type-II' beta-turn. Hence, the sst2-selective antagonist pharmacophore is very similar to the sst2-selective agonist pharmacophore previously described.

Agonist-antagonist dilemma in molecular imaging: evaluation of a monomolecular multimodal imaging agent for the somatostatin receptor

The combination of different imaging modalities, each providing information according to its strengths, can be a powerful method for diagnosing diseases. We have synthesized a monomolecular multimodal imaging agent (MOMIA), LS172, containing a subtype-2 somatostatin receptor (SSTr2)-avid peptide (Y3-octreotate or Y3-TATE), a radiometal chelating group (DOTA) and a near-infrared (NIR) fluorescent dye (cypate). In addition to optical methods, radiolabeling LS172 with 64Cu and 177Lu provides a strategy for in vitro evaluation or in vivo multimodal imaging by positron emission tomography (PET) and single photon emission computed tomography (SPECT), respectively. Determination of the binding affinity of LS172, nat Cu- and nat Lu-LS172 in SSTr2-transfected A427 cells (A427-7) showed that they all displayed high binding affinity toward SSTr2 with K i values of 0.234 nM, 11.5 nM, and 2.15 nM respectively. In contrast to cypate-labeled Y3-TATE (cytate), fluorescence microscopy showed that LS172 and nat Cu-LS172 accumulate modestly in A427-7 cells by SSTr2-mediated endocytosis, in spite of their relatively high binding affinity. In vivo, the biodistribution of the SSTr2 receptor specific 64Cu- and 177Lu-LS172 in AR42J tumor-bearing rats exhibited low (<or=1% ID/g) accumulation in tumor tissue. Clearance from circulation was predominantly hepatobiliary (>90% ID/liver). Both optical and radionuclear biodistribution studies showed a similar in vivo distribution profile. Surprisingly, the strong binding of LS172 to SSTr2 did not translate into high SSTr2-mediated endocytosis in cells or uptake in tumor in vivo. Considering that LS172 is a putative antagonist, the poor accumulation of the labeled MOMIAs in SSTr2 positive tumor tissue supports the paradigm that agonists with their concomitant internalization favors appreciable target tissue accumulation of receptor-specific ligands.

Isolation, characterization, and biological evaluation of syn and anti diastereomers of [(99m)Tc]technetium depreotide: a somatostatin receptor binding tumor imaging agent

The early and later eluting [(99m)TcO]depreotide products on RP-HPLC were confirmed to be the anti and syn diastereomers, respectively, based on proton NMR and circular dichroism spectroscopy. NMR provided evidence of a folded, conformationally constrained structure for the syn diastereomer. The syn diastereomer is predominant (anti/syn approximately 10:90) in the [(99m)TcO]depreotide preparation and shows a slightly higher affinity (IC50 = 0.15 nM) for the somatostatin receptor than the anti diastereomer (IC50 = 0.89 nM). Both diastereomers showed higher binding affinities than the free peptide (IC(50) = 7.4 nM). Biodistribution studies in AR42J tumor xenograft nude mice also showed higher tumor uptake for syn [(99m)TcO]depreotide (6.58% ID/g) than for the anti [(99m)TcO]depreotide (3.38% ID/g). Despite the differences in biological efficacy, the favorable binding affinity, tumor uptake, and tumor-to-background ratio results for both diastereomeric species predict that both are effective for imaging somatostatin receptor-positive tumors in vivo.

TiO2 Sphere-Tube-Fiber Transition Induced by Oligovaline Concentration Variation

L-Valine-based oligopeptides with the general structure Z-(L-Val)(n)-OMe or -OH (n = 1-4) form stable organogels in a variety of solvents, including the inorganic liquid tetraethylorthosilicate. The acid form Z-(L-Val)(n)-OH is a less efficient gelator than the methyl ester, but forms stable organogels in aromatic solvents and di- and trichloromethane. In all cases the peptides form micrometer long helical fibers with a beta-sheet structure. IR and X-ray diffraction show that the peptides have closely related structures in the crystalline state and the fibers in the organogels. The gels are efficient templates for the fabrication of complex titania architectures on a (sub)micron length scale: at low peptide concentrations titania spheres form and at higher concentrations one-dimensional shapes like hollow titania tubes or titania fibers are observed. The tubes are stable towards calcination whereas the fibers (partially) transform into spherical or even bulk particles.

68Ga-PET radiopharmacy: A generator-based alternative to 18F-radiopharmacy

Positron emission tomography (PET) is becoming a dominating method in the field of molecular imaging. Most commonly used radionuclides are accelerator produced 11C and 18F. An alternative method to label biomolecules is the use of metallic positron emitters; among them 68Ga is the most promising as it can be produced from a generator system consisting of an inorganic or organic matrix immobilizing the parent radionuclide 68Ge. Germanium-68 has a long half-life of 271 days which allows the production of long-lived, potentially very cost-effective generator systems. A commercial generator from Obninsk, Russia, is available which uses TiO2 as an inorganic matrix to immobilize 68Ge in the oxidation state IV+. 68Ge(IV) is chemically sufficiently different to allow efficient separation from 68Ga(III). Ga3+ is redox-inert; its coordination chemistry is dominated by its hard acid character. A variety of mono- and bifunctional chelators were developed which allow immobilization of 68Ga3+ and convenient coupling to biomolecules. Especially peptides targeting G-protein coupled receptors overexpressed on human tumour cells have been studied preclinically and in patient studies showing high and specific tumour uptake and specific localization. 68Ga-radiopharmacy may indeed be an alternative to 18F-based radiopharmacy. Freeze-dried, kit-formulated precursors along with the generator may be provided, similar to the 99Mo/99mTc-based radiopharmacy, still the mainstay of nuclear medicine.

Thermodynamic study of lanthanide(iii) complexes with bifunctional monophosphinic acid analogues of H4dota and comparative kinetic study of yttrium(iii) complexes

New bifunctional H(4)dota-like ligands with three acetic acid and one phosphinic acid pendant arms and propionate (H(5)do3ap(PrA)) or 4-aminobenzyl (H(4)do3ap(ABn)) reactive groups bound to the phosphorus atom were investigated. Potentiometric studies showed that the ligands have a similar basicity to the parent H(4)dota and the stability constants of their complexes with sodium(i) and selected lanthanide(III) ions are also similar. Formation and acid-assisted decomplexation kinetics of yttrium(III) complexes with a series of H(4)dota-like ligands (H(4)dota and its phosphinic/phosphonic acid analogues) were studied and the reactions are sensitive to a slight modification of the ligand structure. The (2-carboxyethyl)phosphinic acid derivative H(5)do3ap(PrA) and the phosphonic acid ligand H(5)do3ap form complexes faster than H(4)dota. The most kinetically inert complex is that with H(4)do3ap(ABn). Rates of complexation and decomplexation can depend on the ability to transfer proton(s) outside/inside the complex cavity and, therefore, on the hydrophobicity of the ligands. The results demonstrate that the new bifunctional ligands are suitable for labelling biomolecules with yttrium(iii) radioisotopes for utilization in nuclear medicine.

Influence of Different Spacers on the Biological Profile of a DOTA−Somatostatin Analogue

Radiolabeled somatostatin analogues have been successfully used for targeted radiotherapy and for imaging of somatostatin receptor (sst1-5)-positive tumors. Nevertheless, these analogues are subject to improving their tumor-to-nontarget ratio to enhance their diagnostic or therapeutic properties, preventing nephrotoxicity. In order to understand the influence of lipophilicity and charge on the pharmacokinetic profile of [1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA)]-somatostatin-based radioligands such as [DOTA,1-Nal3]-octreotide (DOTA-NOC), different spacers (X) based on 8-amino-3,6-dioxaoctanoic acid (PEG2), 15-amino-4,7,10,13-tetraoxapentadecanoic acid (PEG4), N-acetyl glucosamine (GlcNAc), triglycine, beta-alanine, aspartic acid, and lysine were introduced between the chelator DOTA and the peptide NOC. All DOTA-X-NOC conjugates were synthesized by Fmoc solid-phase synthesis. The partition coefficient (log D) at pH = 7.4 indicated that higher hydrophilicity than [111In-DOTA]-NOC was achieved with the introduction of the mentioned spacers, except with triglycine and beta-alanine. The high affinity of [InIII-DOTA]-NOC for human sst2 (hsst2) was preserved with the structural modifications, while an overall drop for hsst3 affinity was observed, except in the case of [InIII-DOTA]-beta-Ala-NOC. The new conjugates preserved the good affinity for hsst5, except for [InIII-DOTA]-Asn(GlcNAc)-NOC, which showed decreased affinity. A significant 1.2-fold improvement in the specific internalization rate in AR4-2J rat pancreatic tumor cells (sst2 receptor expression) at 4 h was achieved with the introduction of Asp as a spacer in the parent compound. In sst3-expressing HEK cells, the specific internalization rate at 4 h for [111In-DOTA]-NOC (13.1% +/- 0.3%) was maintained with [111In-DOTA]-beta-Ala-NOC (14.0% +/- 1.8%), but the remaining derivatives showed <2% specific internalization. Biodistribution studies were performed with Lewis rats bearing the AR4-2J rat pancreatic tumor. In comparison to [111In-DOTA]-NOC (2.96% +/- 0.48% IA/g), the specific uptake in the tumor at 4 h p.i. was significantly improved for the 111In-labeled sugar analogue (4.17% +/- 0.46% IA/g), which among all the new derivatives presented the best tumor-to-kidney ratio (1.9).

Design, Synthesis, and Biological Evaluation of Somatostatin-Based Radiopeptides

The prototypes for tumor targeting with radiolabeled peptides are derivatives of somatostatin. Usually, they primarily have high affinity for somatostatin receptor subtype 2 (sst2), and they have moderate affinity for sst5. We aimed at developing analogs that recognize different somatostatin receptor subtypes for internal radiotherapy in order to extend the present range of accessible tumors. We synthesized DOTA-octapeptides based on octreotide by replacing Phe3 mainly with unnatural amino acids. The affinity profile was determined by using cell lines transfected with sst1-5. Internalization was determined by using AR42J, HEK-sst3, and HEK-sst5 cell lines, and biodistribution was studied in rat tumor models. Two of the derivatives thus obtained showed an improved binding affinity profile, enhanced internalization into cells expressing sst2 and sst3, respectively, and better tumor:kidney ratios in animals.

Validation of a Novel CHX-A‘ ‘ Derivative Suitable for Peptide Conjugation: Small Animal PET/CT Imaging Using Yttrium-86-CHX-A‘ ‘-Octreotide

A versatile bifunctional chelating reagent based on a preorganized cyclohexyl derivative of DTPA (CHX-A'') has been developed for the convenient N-terminal labeling of peptides with metal ion radionuclides of Bi(III), In(III), Lu(III), or Y(III). This was achieved via the synthesis of a mono-N-hydroxysuccinimidyl penta-tert-butyl ester derivative of CHX-A'' (trans-cyclohexyldiethylenetriaminepenta-acetic acid) featuring a glutaric acid spacer. Commercially obtained octreotide was modified at its N-terminus by this reagent in the solution phase, and its subsequent radiolabeling with (111)In (T(1/2) = 2.8 d) and (86)Y (T(1/2) = 14.7 h) demonstrated. Small animal PET/CT imaging results of (86)Y-CHX-A''-octreotide in a somatostatin receptor-positive tumor-bearing rat model are presented for the validation of the novel agent.

Metal Ion Binding by a Bicyclic Diamide: Deep UV Raman Spectroscopic Characterization

Deep UV resonance Raman spectroscopy was used for characterizing ligand-metal ion complexes. The obtained results demonstrated a strong intrinsic sensitivity and selectivity of a Raman spectroscopic signature of a bicyclic diamide, a novel chelating agent for lanthanides and actinides (Lumetta, G. J.; Rapko, B. M.; Garza, P. A.; Hay, B. P.; Gilbertson, R. D.; Weakley, T. J. R.; Hutchison, J. E. J. Am. Chem. Soc. 2002, 124, 5644). Molecular modeling, which included structure optimization and calculation of Raman frequencies and resonance intensities, allowed for assigning all strong Raman bands of the bicyclic diamide as well as predicting the band shifts observed because of complex formation with metal ions. A comparative analysis of Raman spectra and the results of the molecular modeling could be used for elucidating the structure of complexes in solution.