A comparison of the selectivity of carazolol with that of other beta 2-selective adrenoceptor antagonists

The β-Blocker Carvedilol and Related Aryloxypropanolamines Promote ERK1/2 Phosphorylation in HEK293 Cells with K A Values Distinct From Their Equilibrium Dissociation Constants as β 2-Adrenoceptor Antagonists: Evidence for Functional Affinity

The determination of affinity by using functional assays is important in drug discovery because it provides a more relevant estimate of the strength of interaction of a ligand to its cognate receptor than radioligand binding. However, empirical evidence for so-called, "functional affinity" is limited. Herein, we determined whether the affinity of carvedilol, a β-adrenoceptor antagonist used to treat heart failure that also promotes extracellular signal-regulated kinases 1 and 2 (ERK1/2) phosphorylation, differed between these two pharmacological activities. Four structurally related β-adrenoceptor antagonists (alprenolol, carazolol, pindolol, propranolol) that also activated ERK1/2 were included as comparators to enhance our understanding of how these drugs work in the clinical setting. In HEK293 cells stably expressing the human β 2-adrenoceptor carvedilol and related aryloxypropanolamines were partial agonists of ERK1/2 phosphorylation with potencies ([A]50s) that were lower than their equilibrium dissociation constants (K Bs) as β 2-adrenoceptor antagonists. As the [A]50 of a partial agonist is a good approximation of its K B, then these data indicated that the affinities of carvedilol and related ligands for these two activities were distinct. Moreover, there was a significant negative rank order correlation between the [A]50 of each ligand to activate ERK1/2 and their intrinsic activities (i.e., as intrinsic activity for ERK1/2 phosphorylation increased, so did affinity). Genome editing revealed that the transducer that coupled the β 2-adrenoceptor to ERK1/2 phosphorylation in response to carvedilol and other β 2-adrenoceptor antagonists was Gαs. Collectively, these data support the concept of "functional affinity" and indicate that the ability of the β 2-adrenoceptor to recruit Gαs may influence the affinity of the activating ligand. SIGNIFICANCE STATEMENT: In HEK293 cells overexpressing the human β2-adrenoceptor carvedilol and four related aryloxypropanolamines behaved as β2-adrenoceptor antagonists and partial agonists of ERK1/2 phosphorylation with rank orders of affinity that were distinct. These data imply that carvedilol and other β-blockers can stabilize the β2-adrenoceptor in different affinity conformations that are revealed when functionally distinct responses are measured. This is the basis for the pharmacological concept of "functional affinity."

High-Affinity Functional Fluorescent Ligands for Human β-Adrenoceptors

Visualization of the G-protein coupled receptor (GPCR) is of great importance for studying its function in a native cell. We have synthesized a series of red-emitting fluorescent probes targeting β-adrenergic receptor (βAR) that are compatible with confocal and Stimulated Emission Depletion (STED) microscopy as well as with Time-Resolved Fluorescence Resonance Energy Transfer (TR-FRET) binding assay in living cells. The probe based on the agonist BI-167107 and fluorescent dye KK114 demonstrates nanomolar binding affinity and up to nine-fold β₂AR selectivity over β₁AR. Carazolol-derived probes are fluorogenic and allow no-wash imaging experiments. STED microscopy of β₂ARs stained at the native expression level on pancreatic CAPAN cells provides two-fold improvement in lateral optical resolution over confocal mode and reveals the formation of receptor microdomains. These probes retain their functional (agonist or antagonist) properties, allowing simultaneous modulation of cyclic adenosine monophosphate (cAMP) levels and receptor internalization as well as imaging receptor localization.

Direct synthesis of N-H carbazoles via iridium(III)-catalyzed intramolecular C-H amination

The iridium-catalyzed dehydrogenative cyclization of 2-aminobiphenyls proceeds smoothly in the presence of a copper cocatalyst under air as a terminal oxidant through intramolecular direct C-H amination to produce N-H carbazoles. A similar iridium/copper system can also catalyze the unprecedented dimerization reaction of 2-aminobiphenyl involving 2-fold C-H/N-H couplings.

Palladium-catalyzed method for the synthesis of carbazoles via tandem C-H functionalization and C-N bond formation

The development of a new method for the assembly of unsymmetrical carbazoles is reported. The strategy involves the selective intramolecular functionalization of an arene C-H bond and the formation of a new arene C-N bond. The substitution pattern of the carbazole product can be controlled by the design of the biaryl amide substrate, and the method is compatible with a variety of functional groups. The utility of the new protocol was demonstrated by the concise synthesis of three natural products from commercially available materials.

Synthesis and in vitro and in vivo characteristics of an iodinated analogue of the beta-adrenoceptor antagonist carazolol

A new (radio)iodinated, beta-adrenoceptor ligand, (S)-(-)-4-[3-[(1,1-dimethyl-3-iodo-(2E)-propenyl)-amino]-2- hydroxypropoxy]carbazole (CYBL8E, 1), was prepared. 1 is an iodinated analogue of the high-affinity beta-adrenoceptor antagonist carazolol (2). The asymmetric synthesis was achieved in four steps starting from 4-hydroxycarbazole. The iodine-123-labeled form was obtained by an iododestannylation reaction with [123I]NaI in the presence of H2O2. Using classical in vitro displacement experiments with membrane fractions of cardiac left ventricular muscle, 1 proved to have a high affinity for the receptor (Ki = 0.31 +/- 0.03). Biodistribution studies performed in New Zealand white rabbits demonstrated the specificity of the binding in vivo to the receptor. Uptake of [123I]1 was reduced significantly in both atrial muscle, left ventricular muscle, frontal cortex, cerebellum, and striatum, by the pretreatment of the animals with different beta-adrenoceptor antagonists. In conclusion, 1 is a potent nonselective beta-adrenoceptor antagonist, which binds specifically to the beta-adrenoceptor in vivo, and is therefore a promising radioligand for the imaging of beta-adrenoceptors using single photon emission computerized tomography.

Quantification of the beta-adrenoceptor ligand S-1'-[18F]fluorocarazolol in plasma of humans, rats and sheep

Myocardial and pulmonary beta-adrenoceptors can be imaged with 2-(S)-(-)-(9H-carbazol-4-yl-oxy)-3-[1-(fluoromethyl)ethyl]amino-2- propanol (S-1'-[18F]fluorocarazolol, I). Quantification of unmodified fluorocarazolol in plasma is necessary for analysis of PET images in terms of receptor densities. We have determined I and its radioactive metabolites in rat, sheep and human plasma, using (1) solid-phase extraction (C18) followed by reversed-phase HPLC and (2) direct injection of untreated plasma samples on an internal-surface reversed-phase (ISRP) column. The two methods were in good agreement. Unmodified I decreased from over 99% initially to less than 5%, 5-10% and 20% at 60 min post-injection in rats, sheep and human volunteers, respectively. Protein binding in sheep and human plasma was determined by ultrafiltration. The fraction of total plasma radioactivity bound to protein and the fraction representing unmodified radioligand were linearly correlated, suggesting that fluorocarazolol was more than 70% protein-bound, whereas its metabolites showed negligible protein binding. Direct injection of plasma on an ISRP column seems a convenient method for quantification of lipophilic radioligands such as fluorocarazolol.

(S,S)- and (S,R)-1'-[18F]fluorocarazolol, ligands for the visualization of pulmonary beta-adrenergic receptors with PET

The beta-adrenoceptor antagonist carazolol has been labelled with fluorine-18 in the isopropyl group via a reductive alkylation by [18F]-fluoroacetone of the corresponding (S)-desisopropyl compound according to a known procedure. The introduction of fluorine in the isopropyl group creates a new stereogenic centre resulting in the formation of (S,S)- and (S,R)-1'-[18F]fluorocarazolol, which were separated by HPLC. Tissue distribution studies were performed in male Wistar rats. Both the (S,S)- and (S,R)-diastereomers (S.A. 500-2000 Ci/mmol; 18.5-74 TBq/mmol) showed high uptake in lung and heart, which could be blocked by pretreatment of the animals with (+/-)-propranolol. No significant differences were observed between the biodistribution of the two diastereomers. Metabolite analysis showed a rapid appearance of polar metabolites in plasma, while at 60 min postinjection 92% and 82% of the total radioactivity in lung and heart was unmetabolized 1'-[18F]fluorocarazolol. In a PET-study with male Wistar rats, the lungs were clearly visualized and the pulmonary uptake was decreased after pretreatment of the animals with (+/-)-propranolol. The heart could not be visualized. Similar results were obtained in PET-studies with lambs.

Myocardial and pulmonary uptake of S-1'-[18F]fluorocarazolol in intact rats reflects radioligand binding to beta-adrenoceptors

The biodistribution of S-(-)-4-(2-hydroxy-3-(1'-[18F]fluoroisopropyl)- aminopropoxy)carbazole ([18F]S-fluorocarazolol, a non-selective beta-adrenoceptor antagonist) was studied in rats (60 min after 18F injection when specific binding in peripheral organs was maximal). 18F uptake in brain, erythrocytes, heart and lung appeared to be linked to beta-adrenoceptors. CGP-20712A and ICI-89,406 inhibited 18F uptake in heart (predominantly beta 1-adrenoceptors) more potently than in lungs (predominantly beta 2-adrenoceptors). In contrast, ICI-118,551 and procaterol were more potent in the lungs than in the heart. ICI-118,551 inhibited 18F uptake in cerebellum (predominantly beta 2-adrenoceptors) more potently than in cerebral cortex (predominantly beta 1-adrenoceptors). Stereoselectivity of the in vivo binding was demonstrated since S-(-)-propranolol inhibited uptake in target tissues more effectively than R-(+)-propranolol. Myocardial and cerebral imaging may be hampered by poor heart-to-lung contrast and low signal-to-noise ratios, but [18F]S-fluorocarazolol seems suitable for positron emission tomography (PET) of pulmonary beta-adrenoceptors.

Preparation and in vivo binding of [11C]carazolol, a radiotracer for the beta-adrenergic receptor

Carazolol is a high affinity beta-adrenergic receptor antagonist which is relatively non-specific for the receptor subtypes. The labeling of the two enantiomers of this compound with carbon-11, including the synthesis of the required labeling precursors, is reported. The yield and specific activity are sufficient for use in positron tomography. Biodistribution and specific receptor binding studies show the labeled material to be of interest for further investigation as a radiopharmaceutical for positron tomography.