Investigation of Two Zr-p-NO2Bn-DOTA Isomers via NMR and Quantum Chemical Studies.

A combination of NMR studies and quantum chemical calculations were employed to investigate the structure and energetics of Zr4+ chelates of pNO2Bn-DOTA. We have demonstrated that two discrete regioisomeric chelates are generated during the complex formation. The nitrobenzyl substituent can adopt either an equatorial corner or side position on the macrocyclic ring. These regioisomers are incapable of interconversion and were isolated by HPLC. The corner isomer is more stable than the side, and the SAP conformer of both regioisomers is energetically more favorable than the corresponding TSAP conformer.

Origin of the Isomer Stability of Polymethylated DOTA Chelates Complexed with Ln3+ ions.

DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid)-based chelates that give only a single isomer in solution when complexed with lanthanide (Ln3+) ions is of value for studying protein dynamics and interactions via NMR. Herein, we have investigated the geometries, energetics, and electrostatic potentials of Lu complexed with DOTA (1), ring methylated M4DOTA (2), and arm methylated R-DOTMA (3) and S-DOTMA (4), as well as, both ring and arm methylated 4S-4S-M4DOTMA (5) and 4S-4R-M4DOTMA (6) at the level of M06-L/6-31+G(d)-SDD, to elucidate the origin of the isomer stability. These analyses indicate that the electrostatic repulsion between the arm methyl and the neighboring carboxylate significantly destabilizes the square antiprism (SAP) isomer of Lu-5 and the twisted square antiprism (TSAP) isomer of Lu-6, while the steric repulsion between the ring and arm methyl groups attenuates the stability of both TSAP of Lu-5 and SAP of Lu-6. To rationalize the variable temperature proton NMR spectra, the energy barriers for the inter-conversion in Lu-5 and Lu-6 via arm rotation were also calculated. The modulation of the stability and rigidity of Ln complexes via a modification of DOTA is also discussed. Our investigation will aid to design better chelates for the Ln3+ ions for its use in molecular medicine.

The Intriguing Effects of Substituents in the N-Phenethyl Moiety of Norhydromorphone: A Bifunctional Opioid from a Set of "Tail Wags Dog" Experiments.

(-)-N-Phenethyl analogs of optically pure N-norhydromorphone were synthesized and pharmacologically evaluated in several in vitro assays (opioid receptor binding, stimulation of [35S]GTPγS binding, forskolin-induced cAMP accumulation assay, and MOR-mediated ß-arrestin recruitment assays). "Body" and "tail" interactions with opioid receptors (a subset of Portoghese's message-address theory) were used for molecular modeling and simulations, where the "address" can be considered the "body" of the hydromorphone molecule and the "message" delivered by the substituent (tail) on the aromatic ring of the N-phenethyl moiety. One compound, N-p-chloro-phenethynorhydromorphone ((7aR,12bS)-3-(4-chlorophenethyl)-9-hydroxy-2,3,4,4a,5,6-hexahydro-1H-4,12-methanobenzofuro[3,2-e]isoquinolin-7(7aH)-one, 2i), was found to have nanomolar binding affinity at MOR and DOR. It was a potent partial agonist at MOR and a full potent agonist at DOR with a δ/µ potency ratio of 1.2 in the ([35S]GTPγS) assay. Bifunctional opioids that interact with MOR and DOR, the latter as agonists or antagonists, have been reported to have fewer side-effects than MOR agonists. The p-chlorophenethyl compound 2i was evaluated for its effect on respiration in both mice and squirrel monkeys. Compound 2i did not depress respiration (using normal air) in mice or squirrel monkeys. However, under conditions of hypercapnia (using air mixed with 5% CO2), respiration was depressed in squirrel monkeys.

Comparison of Solution Properties of Polymethylated DOTA-like Lanthanide Complexes with Opposite Chirality of the Pendant Arms.

Polymethylated lanthanide 4S4R-M4DOTMA complexes, bearing the ring methyl groups oriented in the SSSS position and the arm methyl groups in the RRRR configuration, exist exclusively as the SAP [Λ(δδδδ)] isomer in solution throughout the lanthanide series. This observation is in contrast to Ln-8S-M4DOTMA, which was recently reported to adopt the SAP [Λ(δδδδ)] isomer in the early lanthanides, while the late lanthanides adopt the TSAP [Δ(δδδδ)] isomer. The methyl groups on the ring and the arm are both oriented in the SSSS configuration for Ln-8S-M4DOTMA ( Dalton Trans. 2016 , 45 , 4673 - 4687 , DOI: 10.1039/C5DT03210E ). Quantum chemical calculations for Pr- and Yb-4S4R-M4DOTMA indicate that the SAP isomer is significantly more stable. The luminescence profiles of Eu-8S-M4DOTMA and Eu-4S4R-M4DOTMA showed similar profiles signifying identical coordination environments. The hydration state, q, of the Eu(III) complexes was q = 0.91-0.95, while Tb-8S-M4DOTMA had q = 0.86. A much lower q value was obtained for Tb-4S4R-M4DOTMA (q = 0.67), which indicates an elongation of the Ln-Ow bond. At 400 MHz, the relaxivity of Gd-8S-M4DOTMA is 5.1 ± 0.1 mM-1 s-1 and 3.9 ± 0.1 mM-1 s-1 at 25 and 37 °C, respectively, whereas the relaxivity of Gd-4S4R-M4DOTMA is 4.6 ± 0.1 mM-1 s-1 at 25 °C and 3.6 ± 0.1 mM-1 s-1 at 37 °C. At 45 MHz, the relaxivity of Gd-8S-M4DOTMA is 5.4 ± 0.1 mM-1 s-1, and the relaxivity of Gd-4S4R-M4DOTMA is 4.5 ± 0.1 mM-1 s-1 at 25 °C. The temperature dependence of the 17O NMR transverse relaxation rate of the Gd complexes revealed a 7-fold increase in the bound water residence lifetime of Gd-8S-M4DOTMA (1/kex = τM = 9.0 ± 0.5 ns and 1/kex = τM = 60 ± 3 ns). The Pr(III) complex of 8S-M4DOTMA crystallized as TSAP isomer with an apical water. The presence of the apical water for the TSAP of Pr-8S-M4DOTMA was further confirmed with the observation that the fluoride ion replaces the bound water from the TSAP isomer of Pr-8S-M4DOTMA. This was shown by the disappearance of the TSAP peaks and appearance of a new set of less shifted resonances, which exchange with the SAP isomer as confirmed by NMR exchange spectroscopy (EXSY).

Crystal Structures of Diaryliodonium Fluorides and Their Implications for Fluorination Mechanisms.

The radiofluorination of diaryliodonium salts is of value for producing radiotracers for positron emission tomography. We report crystal structures for two diaryliodonium fluorides. Whereas diphenyliodonium fluoride (1 a) exists as a tetramer bridged by four fluoride ions, 2-methylphenyl(phenyl)iodonium fluoride (2 a) forms a fluoride-bridged dimer that is further halogen bonded to two other monomers. We discuss the topological relationships between the two and their implications for fluorination in solution. Both radiofluorination and NMR spectroscopy show that thermolysis of 2 a gives 2-fluorotoluene and fluorobenzene in a 2 to 1 ratio that is in good agreement with the ratio observed from the radiofluorination of 2-methylphenyl(phenyl)iodonium chloride (2 b). The constancy of the product ratio affirms that the fluorinations occur via the same two rapidly interconverting transition states whose energy difference dictates chemoselectivity. From quantum chemical studies with density functional theory we attribute the "ortho-effect" to the favorable electrostatic interaction between the incoming fluoride and the o-methyl in the transition state. By utilizing the crystal structures of 1 a and 2 a, the mechanisms of fluoroarene formation from diaryliodonium fluorides in their monomeric, homodimeric, heterodimeric, and tetrameric states were also investigated. We propose that oligomerization energy dictates whether the fluorination occurs through a monomeric or an oligomeric pathway.

Modulation of opioid receptor affinity and efficacy via N-substitution of 9ß-hydroxy-5-(3-hydroxyphenyl)morphan: Synthesis and computer simulation study.

The enantiomers of a variety of N-alkyl-, N-aralkyl-, and N-cyclopropylalkyl-9ß-hydroxy-5-(3-hydroxyphenyl)morphans were synthesized employing cyanogen bromide and K2CO3 to improve the original N-demethylation procedure. Their binding affinity to the µ-, δ-, and κ-opioid receptors (ORs) was determined and functional (GTPγ35S) assays were carried out on those with reasonable affinity. The 1R,5R,9S-enantiomers (1R,5R,9S)-(-)-5-(3-hydroxyphenyl)-2-(4-nitrophenethyl)-2-azabicyclo[3.3.1]nonan-9-ol (1R,5R,9S-16), (1R,5R,9S)-(-) 2-cinnamyl-5-(3-hydroxyphenyl)-2-azabicyclo[3.3.1]nonan-9-ol (1R,5R,9S-20), and (1R,5R,9S)-(-)-5-(3-hydroxyphenyl)-2-(4-(trifluoromethyl)phenethyl)-2-azabicyclo[3.3.1]nonan-9-ol (1R,5R,9S-15), had high affinity for the µ-opioid receptor (e.g., 1R,5R,9S-16: Ki=0.073, 0.74, and 1.99nM, respectively). The 1R,5R,9S-16 and 1R,5R,9S-15 were full, high efficacy µ-agonists (EC50=0.74 and 18.5nM, respectively) and the former was found to be a partial agonist at δ-OR and an antagonist at κ-OR, while the latter was a partial agonist at δ-OR and κ-OR in the GTPγ35S assay. The enantiomer of 1R,5R,9S-16, (+)-1S,5S,9R-16 was unusual, it had good affinity for the µ-OR (Ki=26.5nM) and was an efficacious µ-antagonist (Ke=29.1nM). Molecular dynamics simulations of the µ-OR were carried out with the 1R,5R,9S-16 µ-agonist and the previously synthesized (1R,5R,9S)-(-)-5-(9-hydroxy-5-(3-hydroxyphenyl-2-phenylethyl)-2-azabicyclo[3.3.1]nonane (1R,5R,9S-(-)-NIH 11289) to provide a structural basis for the observed high affinities and efficacies. The critical roles of both the 9ß-OH and the p-nitro group are elucidated, with the latter forming direct, persistent hydrogen bonds with residues deep in the binding cavity, and the former interacting with specific residues via highly structured water bridges.

Unexpected Behavior of the Heaviest Halogen Astatine in the Nucleophilic Substitution of Aryliodonium Salts.

Aryliodonium salts have become precursors of choice for the synthesis of (18) F-labeled tracers for nuclear imaging. However, little is known on the reactivity of these compounds with heavy halides, that is, radioiodide and astatide, at the radiotracer scale. In the first comparative study of radiohalogenation of aryliodonium salts with (125) I(-) and (211) At(-) , initial experiments on a model compound highlight the higher reactivity of astatide compared to iodide, which could not be anticipated from the trends previously observed within the halogen series. Kinetic studies indicate a significant difference in activation energy (Ea =23.5 and 17.1 kcal mol(-1) with (125) I(-) and (211) At(-) , respectively). Quantum chemical calculations suggest that astatination occurs via the monomeric form of an iodonium complex whereas iodination occurs via a heterodimeric iodonium intermediate. The good to excellent regioselectivity of halogenation and high yields achieved with diversely substituted aryliodonium salts indicate that this class of compounds is a promising alternative to the stannane chemistry currently used for heavy radiohalogen labeling of tracers in nuclear medicine.

An Investigation of (Diacetoxyiodo)arenes as Precursors for Preparing No-Carrier-Added [(18)F]Fluoroarenes from Cyclotron-Produced [(18)F]Fluoride Ion.

Treatment of (diacetoxyiodo)arenes (1a-1u) with cyclotron-produced [(18)F]fluoride ion rapidly affords no-carrier-added [(18)F]fluoroarenes (2a-2u) in useful yields and constitutes a new method for converting substituted iodoarenes into substituted [(18)F]fluoroarenes in just two steps.

Structural Basis of Species-Dependent Differential Affinity of 6-Alkoxy-5-Aryl-3-Pyridinecarboxamide Cannabinoid-1 Receptor Antagonists.

6-Alkoxy-5-aryl-3-pyridincarboxamides, including the brain-penetrant compound 14G: [5-(4-chlorophenyl)-6-(cyclopropylmethoxy)-N-[(1R,2R)-2-hydroxy-cyclohexyl]-3-pyridinecarboxamide] and its peripherally restricted analog 14H: [5-(4-chlorophenyl)-N-[(1R,2R)-2-hydroxycyclohexyl]-6-(2-methoxyethoxy)-3-pyridinecarboxamide], have been recently introduced as selective, high-affinity antagonists of the human cannabinoid-1 receptor (hCB1R). Binding analyses revealed two orders of magnitude lower affinity of these compounds for mouse and rat versus human CB1R, whereas the affinity of rimonabant is comparable for all three CB1Rs. Modeling of ligand binding to CB1R and binding assays with native and mutant (Ile105Met) hCB1Rs indicate that the Ile105 to Met mutation in rodent CB1Rs accounts for the species-dependent affinity of 14G: and 14H: . Our work identifies Ile105 as a new pharmacophore component for developing better hCB1R antagonists and invalidates rodent models for assessing the antiobesity efficacy of 14G: and 14H: .

Rational design, synthesis, and evaluation of tetrahydroxamic acid chelators for stable complexation of zirconium(IV).

Metals of interest for biomedical applications often need to be complexed and associated in a stable manner with a targeting agent before use. Whereas the fundamentals of most transition-metal complexation processes have been thoroughly studied, the complexation of Zr(IV) has been somewhat neglected. This metal has received growing attention in recent years, especially in nuclear medicine, with the use of (89) Zr, which a ß(+) -emitter with near ideal characteristics for cancer imaging. However, the best chelating agent known for this radionuclide is the trishydroxamate desferrioxamine B (DFB), the Zr(IV) complex of which exhibits suboptimal stability, resulting in the progressive release of (89) Zr in vivo. Based on a recent report demonstrating the higher thermodynamic stability of the tetrahydroxamate complexes of Zr(IV) compared with the trishydroxamate complexes analogues to DFB, we designed a series of tetrahydroxamic acids of varying geometries for improved complexation of this metal. Three macrocycles differing in their cavity size (28 to 36-membered rings) were synthesized by using a ring-closing metathesis strategy, as well as their acyclic analogues. A solution study with (89) Zr showed the complexation to be more effective with increasing cavity size. Evaluation of the kinetic inertness of these new complexes in ethylenediaminetetraacetic acid (EDTA) solution showed significantly improved stabilities of the larger chelates compared with (89) Zr-DFB, whereas the smaller complexes suffered from insufficient stabilities. These results were rationalized by a quantum chemical study. The lower stability of the smaller chelates was attributed to ring strain, whereas the better stability of the larger cyclic complexes was explained by the macrocyclic effect and by the structural rigidity. Overall, these new chelating agents open new perspectives for the safe and efficient use of (89) Zr in nuclear imaging, with the best chelators providing dramatically improved stabilities compared with the reference DFB.

A Toxoplasma gondii O-glycosyltransferase that modulates bradyzoite cyst wall rigidity is distinct from host homologues.

Infection with the apicomplexan protozoan Toxoplasma gondii can be life-threatening in immunocompromised hosts. Transmission frequently occurs through the oral ingestion of T. gondii bradyzoite cysts, which transition to tachyzoites, disseminate, and then form cysts containing bradyzoites in the central nervous system, resulting in latent infection. Encapsulation of bradyzoites by a cyst wall is critical for immune evasion, survival, and transmission. O-glycosylation of the protein CST1 by the mucin-type O-glycosyltransferase T. gondii (Txg) GalNAc-T3 influences cyst wall rigidity and stability. Here, we report X-ray crystal structures of TxgGalNAc-T3, revealing multiple features that are strictly conserved among its apicomplexan homologues. This includes a unique 2nd metal that is coupled to substrate binding and enzymatic activity in vitro and cyst wall O-glycosylation in T. gondii. The study illustrates the divergence of pathogenic protozoan GalNAc-Ts from their host homologues and lays the groundwork for studying apicomplexan GalNAc-Ts as therapeutic targets in disease.

Z and E rotamers of N-formyl-1-bromo-4-hydroxy-3-methoxymorphinan-6-one and their interconversion as studied by 1H/13C NMR spectroscopy and quantum chemical calculations.

N-Formyl-1-bromo-4-hydroxy-3-methoxymorphinan-6-one (compound 2), an important intermediate in the NIH Opiate Total Synthesis, presumably exists as a mixture of two rotamers (Z and E) in both CHCl(3) and DMSO at room temperature due to the hindered rotation of its N-C18 bond in the amide moiety. By comparing the experimental (1)H and (13)C chemical shifts of a single rotamer and the mixture of compound 2 in CDCl(3) with the calculated chemical shifts of the geometry optimized Z and E rotamers utilizing density functional theory, the crystalline rotamer of compound 2 was characterized as having the E configuration. The energy barrier between the two rotamers was also determined with the temperature dependence of (1)H and (13)C NMR coalescence experiments, and then compared with that from the reaction path for the interconversion of the two rotamers calculated at the level of B3LYP/6-31G*. Detailed geometry of the ground state and the transition states of both rotamers are given and discussed.

Cannabinoformins: Designing Biguanide-Embedded, Orally Available, Peripherally Selective Cannabinoid-1 Receptor Antagonists for Metabolic Syndrome Disorders.

We have designed orally bioavailable, non-brain-penetrant antagonists of the cannabinoid-1 receptor (CB1R) with a built-in biguanide sensor to mimic 5'-adenosine monophosphate kinase (AMPK) activation for treating obesity-associated co-morbidities. A series of 3,4-diarylpyrazolines bearing rational pharmacophoric pendants designed to limit brain penetration were synthesized and evaluated in CB1R ligand binding assays and recombinant AMPK assays. The compounds displayed high CB1R binding affinity and potent CB1R antagonist activities and acted as AMPK activators. Select compounds showed good oral exposure, with compounds 36, 38-S, and 39-S showing <5% brain penetrance, attesting to peripheral restriction. In vivo studies of 38-S revealed decreased food intake and body weight reduction in diet-induced obese mice as well as oral in vivo efficacy of 38-S in ameliorating glucose tolerance and insulin resistance. The designed "cannabinoformin" four-arm CB1R antagonists could serve as potential leads for treatment of metabolic syndrome disorders with negligible neuropsychiatric side effects.

Synthesis, Biological Evaluation, and Molecular Modeling Studies of 3,4-Diarylpyrazoline Series of Compounds as Potent, Nonbrain Penetrant Antagonists of Cannabinoid-1 (CB1R) Receptor with Reduced Lipophilicity.

In the present report, we describe the synthesis and structure-activity relationships of novel "four-arm" dihydropyrazoline compounds designed as peripherally restricted antagonists of cannabinoid-1 receptor (CB1R). A series of racemic 3,4-diarylpyrazolines were synthesized and evaluated initially in CB1 receptor binding assays. The novel compounds, designed to limit brain penetrance and decreased lipophilicity, showed high affinity for CB1R and potent in vitro CB1R antagonist activities. Promising compounds with potent CB1R activity were evaluated in tissue distribution studies. Compounds 6a, 6f, and 7c showed limited brain penetrance attesting to its peripheral restriction. The 4S-enantiomer of these compounds further showed a stereoselective affinity for the CB1 receptor and behaved as inverse agonists. In vivo studies on food intake and body weight reduction in diet-induced obese (DIO) mice showed that these compounds could serve as potential leads for the development of selective CB1R antagonists with improved potency and peripheral restriction.

Functional Selectivity of a Biased Cannabinoid-1 Receptor (CB1R) Antagonist.

Seven-transmembrane receptors signal via G-protein- and ß-arrestin-dependent pathways. We describe a peripheral CB1R antagonist (MRI-1891) highly biased toward inhibiting CB1R-induced ß-arrestin-2 (ßArr2) recruitment over G-protein activation. In obese wild-type and ßArr2-knockout (KO) mice, MRI-1891 treatment reduces food intake and body weight without eliciting anxiety even at a high dose causing partial brain CB1R occupancy. By contrast, the unbiased global CB1R antagonist rimonabant elicits anxiety in both strains, indicating no ßArr2 involvement. Interestingly, obesity-induced muscle insulin resistance is improved by MRI-1891 in wild-type but not in ßArr2-KO mice. In C2C12 myoblasts, CB1R activation suppresses insulin-induced akt-2 phosphorylation, preventable by MRI-1891, ßArr2 knockdown or overexpression of CB1R-interacting protein. MRI-1891, but not rimonabant, interacts with nonpolar residues on the N-terminal loop, including F108, and on transmembrane helix-1, including S123, a combination that facilitates ßArr2 bias. Thus, CB1R promotes muscle insulin resistance via ßArr2 signaling, selectively mitigated by a biased CB1R antagonist at reduced risk of central nervous system (CNS) side effects.

Conformational structure and energetics of 2-methylphenyl(2'-methoxyphenyl)iodonium chloride: evidence for solution clusters.

Diaryliodonium salts allow the efficient incorporation of cyclotron-produced [(18)F]fluoride ions into electron-rich and electron-deficient arenes to provide potential radiotracers for molecular imaging in vivo with positron emission tomography (PET). This process (ArI(+)Ar'+(18)F(-)→Ar(18)F+Ar'I) is still not well understood mechanistically. To better understand this and similar reactions, it would be valuable to understand the structures of diaryliodonium salts in organic media, where the reactions are typically conducted. In this endeavor, the X-ray structure of a representative iodonium salt, 2-methylphenyl(2'-methoxyphenyl)iodonium chloride (1), was determined. Our X-ray structure analysis showed 1 to have the conformational M-P dimer as the unit cell with hypervalent iodine as a stereogenic center in each conformer. With the ab initio replica path method we constructed the inversion path between the two enantiomers of 1, thereby revealing two additional pairs of enantiomers that are likely to undergo fast interconversion in solution. Also LC-MS of 1 showed the presence of dimeric and tetrameric anion-bridged clusters in weak organic solution. This observation is consistent with the energetics of 1, both as monomeric and dimeric forms in MeCN, calculated at the B3LYP/DGDZVP level. These evidences of the existence of dimeric and higher order clusters of 1 in solution are relevant to achieve a deeper general understanding of the mechanism and outcome of reactions of diaryliodonium salts in organic media with nucleophiles, such as the [(18)F]fluoride ion.

Fast and high-yield microreactor syntheses of ortho-substituted [(18)F]fluoroarenes from reactions of [(18)F]fluoride ion with diaryliodonium salts.

A microreactor was applied to produce ortho-substituted [(18)F]fluoroarenes from the reactions of cyclotron-produced [(18)F]fluoride ion (t(1/2) = 109.7 min) with diaryliodonium salts. The microreactor provided a very convenient means for running sequential reactions rapidly with small amounts of reagents under well-controlled conditions, thereby allowing reaction kinetics to be followed and Arrhenius activation energies (E(a)) to be measured. Prepared symmetrical iodonium chlorides (Ar(2)I(+)Cl(-)) rapidly (<4 min) gave moderate (Ar = 2-MeOC(6)H(4), 51%) to high (Ar = Ph or 2-MeC(6)H(4), 85%) decay-corrected radiochemical yields (RCYs) of a single radioactive product (Ar(18)F). Reaction velocity with respect to Ar group was 2-MeOC(6)H(4) < Ph < 2-MeC(6)H(4). Activation energies were in the range 18-28 kcal/mol. Prepared unsymmetrical salts (e.g., 2-RC(6)H(4)I(+)2'-R'C(6)H(4)X(-); X = Cl or OTs) also rapidly gave two products (2-RC(6)H(4)(18)F and 2-R'C(6)H(4)(18)F) in generally high total RCYs (79-93%). Selectivity for product [(18)F]fluoroarene was controlled by the nature of the ortho substituents. The power of ortho substituents to impart an ortho-effect was in the following order:, 2,6-di-Me > 2,4,6-tri-Me > Br > Me > Et approximately (i)Pr >> H > OMe. For (2-methyphenyl)(phenyl)iodonium chloride, the time-course of reaction product selectivity was constant and consistent with the operation of the Curtin-Hammett principle. These results will aid in the design of diaryliodonium salt precursors to (18)F-labeled tracers for molecular imaging.

Probes for narcotic receptor mediated phenomena. 40. N-substituted cis-4a-ethyl-1,2,3,4,4a,9a-hexahydrobenzofuro[2,3-c]pyridin-8-ols.

A series of N-substituted rac-cis-4a-ethyl-1,2,3,4,4a,9a-hexahydrobenzofuro[2,3-c]pyridin-8-ols have been prepared using a simple synthetic route previously designed for synthesis of related cis-2-methyl-4a-alkyl-1,2,3,4,4a,9a-hexahydrobenzofuro[2,3-c]pyridin-6-ols. The new phenolic compounds, where the aromatic hydroxy moiety is situated ortho to the oxygen atom in the oxide-bridged ring, do not interact as well as the pyridin-6-ols with opioid receptors. The N-para-fluorophenethyl derivative had the highest mu-opioid receptor affinity of the examined compounds (K(i)=0.35 microM).

Reply to the 'Comment on "Investigation of Zr(iv) and 89Zr(iv) complexation with hydroxamates: progress towards designing a better chelator than desferrioxamine B for immuno-PET imaging"' by A. Bianchi and M. Savastano, Chem. Commun., 2020, 56, D0CC01189D.

The alternative analysis of A. Bianchi and M. Savastano is a valuable contribution to the understanding of the complex systems at stake in the complexation chemistry of Zr4+ by considering polynuclear species. Placed in the context of nuclear medicine where such aggregates are unlikely and considering recent literature data, this however points out that no clear agreement exists to describe such complex formation.