Molecular recognition of amiloride analogs: a molecular electrostatic potential analysis. 1. Pyrazine ring modifications

Discovery and development of BI 1265162, an ENaC inhibitor for the treatment of cystic fibrosis

Lung selective inhibition of the endothelial sodium channel (ENaC) is a potential mutation agnostic treatment of Cystic Fibrosis (CF). We describe the discovery and development of BI 1265162, the first ENaC inhibitor devoid of the amiloride structural motif that entered clinical trials. The design of BI 1265162 focused on its suitability for inhalation via the Respimat® Soft Mist™ Inhaler and a long duration of action. A convergent and scalable route for the synthesis of BI 1265162 as dihydrogen phosphate salt is presented, that was applied to support clinical trials. A phase 2 study with BI 1265162 did not provide a clear sign of clinical benefit. Whether ENaC inhibition will be able to hold its promise for CF patients remains an open question.

Different electrostatic descriptors in comparative molecular field analysis: A comparison of molecular electrostatic and coulomb potentials

Comparative molecular field analysis (CoMFA) is a three-dimensional quantitative structure-activity relationship (3D-QSAR) method which correlates precalculated fields surrounding a set of molecules with some target property. Among others, the electrostatic fields are commonly used. These are usually generated by calculating the Coulomb potential between a probe and the molecules bearing atom-centered point charges. The present study was performed in order to investigate up to which extent different methods to calculate electrostatic potentials can influence the results of a CoMFA. Therefore, a variety of charge calculation methods was applied to a data set consisting of 37 ligands of the benzodiazepine receptor inverse agonist-antagonist active site. These methods included Gasteiger-Marsili, semiempirical (MNDO, AM1, and PM3), and ab initio (HF/STO-3G, HF/3-21G*, and HF/6-31G*) charges. Semiempirical as well as ab initio electron populations were derived both from the Mulliken population analysis (MPA) or from fitting the charges to the molecular electrostatic potential (ESPFIT charges). In addition, the molecular electrostatic potentials (MEPs) resulting from ab initio calculations were mapped directly onto the CoMFA grid. With regard to the cross-validated r(2) values (r(2) cv ) of the resulting QSAR models, the ESPFIT-derived potentials yielded generally higher r(2) cv values than those resulting from MPA charges. For example, at the HF/3-21G* level the r(2) cv rose from 0.61 (MPA-derived potentials) to 0.76 (ESPFIT fields). The MEPs mapped directly onto the CoMFA grid were not superior to the corresponding ESPFIT-derived potentials. Semiempirical ESPFIT charges appeared to be of similar quality compared with ab initio ESPFIT electron populations in the CoMFAs. When no scaling between the steric and electrostatic descriptor matrices was applied, the electrostatic contributions were influenced to a high degree by the magnitude of the corresponding field values. © 1996 by John Wiley & Sons, Inc.

Design, synthesis, and structure-activity relationships of novel 2-substituted pyrazinoylguanidine epithelial sodium channel blockers: drugs for cystic fibrosis and chronic bronchitis

Amiloride (1), the prototypical epithelial sodium channel (ENaC) blocker, has been administered with limited success as aerosol therapy for improving pulmonary function in patients with the genetic disorder cystic fibrosis. This study was conducted to synthesize and identify more potent, less reversible ENaC blockers, targeted for aerosol therapy and possessing minimal systemic renal activity. A series of novel 2-substituted acylguanidine analogues of amiloride were synthesized and evaluated for potency and reversibility on bronchial ENaC. All compounds tested were more potent and less reversible at blocking sodium-dependent short-circuit current than amiloride. Compounds 30-34 showed the greatest potency on ENaC with IC(50) values below 10 nM. A regioselective difference in potency was found (compounds 30, 39, and 40), whereas no stereospecific (compounds 33, 34) difference in potency on ENaC was displayed. Lead compound 32 was 102-fold more potent and 5-fold less reversible than amiloride and displayed the lowest IC(50) value ever reported for an ENaC blocker.

Altering airway surface liquid volume: inhalation therapy with amiloride and hyperosmotic agents

The thin layer of liquid lining the entire respiratory tract is the first line of defense against the continuous insult of inhaled bacteria and noxious chemicals. Many chronic obstructive diseases of the airway may reflect decreased airway surface liquid, which results from imbalances in ion transport and mucus production. Reduction in the thickness of airway surface liquid leads to reduced mucociliary clearance rates, causing mucus accumulation and infection in the airway. In this chapter, two inhalation therapies to replenish airway surface liquid and enhance mucociliary clearance are discussed: (1) aerosolized hyperosmotic agents; and (2) aerosolized sodium channel blockers. The advantages and disadvantages of each therapy are discussed, as well as future directions for improving airway surface liquid volume by inhalation pharmacotherapy.

Trypanosomal nucleoside hydrolase. Resonance Raman spectroscopy of a transition-state inhibitor complex

The transition state for hydrolysis of the N-ribosidic bond of inosine by nucleoside hydrolase has oxocarbenium character and a protonated leaving group hypoxanthine with an sp2-hybridized C1' of the ribosyl [Horenstein, B. A., Parkin, D. W., Estupinan, B., & Schramm, V. L. (1991) Biochemistry 30, 10788-10795]. These features are incorporated into N-(p-nitrophenyl)-D-riboamidrazone, a transition state analogue which binds with a dissociation constant of 2 nM [Boutellier, M., Horenstein, B. A., Semenyaka, A., Schramm, V. L., & Ganem, B. (1994) Biochemistry 33, 3994-4000]. Resonance Raman and ultraviolet-visible absorbance spectroscopy has established that the inhibitor binds as the neutral, zwitterionic species. The enzyme stabilizes a specific resonance state characterized by the quinonoid form of the p-nitrophenyl group with evidence for ion pairing at the nitro group. Incorporation of 15N into a specific position of the amidrazone reveals that the exo-ribosyl nitrogen bonded to the C1' position carries the proton while that bonded to the p-nitrophenyl carbon is unprotonated. This tautomer carries a distributed positive charge centered at the position analogous to C1' of the ribosyl group at the transition state. The molecular electrostatic potentials for the substrate inosine, the transition state, and the transition state inhibitor are compared at the van der Waals surface of the molecules. The tautomer of the inhibitor bound to the enzyme bears a striking electrostatic resemblance to the transition state determined by kinetic isotope effect analysis. The spectral and resonance Raman properties of free and enzyme-bound inhibitor have permitted tautomeric assignment of these species and establish that the enzyme substantially changes the electronic distribution of the bound inhibitor toward that of the enzyme-stabilized transition state.

The application of stereolithography to the fabrication of accurate molecular models

The process of stereolithography, which automatically fabricates plastic models from designs created in certain computer-aided design programs, has been applied to the production of accurate plastic molecular models. Atomic coordinates obtained from quantum mechanical calculations and from neutron diffraction data were used to locate spheres in the I-DEAS CAD program with radii proportional to the appropriate van der Waals radii. The sterolithography apparatus was used to build the models using a photosensitive liquid resin, resulting in hard plastic models that accurately represent the computed or experimental input structures. Three examples are given to illustrate how the models can be used to interpret experimental structure-activity data for systems of biological importance or host-guest chemistry: (1) Interpretation of kinetic data for the formation of a stable blocking complex between amiloride analogs and the epithelial sodium channel, (2) interpretation of binding and neural activity data for the interaction of certain amino acids and their analogs at the L-alanine taste receptor of the channel catfish, and (3) interpretation of shape selectivity and rate acceleration in cyclodextrin catalysis using models of the neutron diffraction structure of beta-cyclodextrin and of the transition state for the cleavage of phenyl acetate by the secondary hydroxyl oxygen of beta-cyclodextrin.

Active-site mutations of the diphtheria toxin catalytic domain: role of histidine-21 in nicotinamide adenine dinucleotide binding and ADP-ribosylation of elongation factor 2

Diphtheria toxin (DT) has been studied as a model for understanding active-site structure and function in the ADP-ribosyltransferases. Earlier evidence suggested that histidine-21 of DT is important for the ADP-ribosylation of eukaryotic elongation factor 2 (EF-2). We have generated substitutions of this residue by cassette mutagenesis of a synthetic gene encoding the catalytic A fragment (DTA) of DT, and have characterized purified mutant forms of this domain. Changing histidine-21 to alanine, aspartic acid, leucine, glutamine, or arginine diminished ADP-ribosylation activity by 70-fold or greater. In contrast, asparagine proved to be a functionally conservative substitution, which reduced ADP-ribosylation activity by < 3-fold. The asparagine mutant was approximately 50-fold-attenuated in NAD glycohydrolase activity, however. Dissociation constants (Kd) for NAD binding, determined by quenching of the intrinsic protein fluorescence, were 15 microM for wild-type DTA, 160 microM for the asparagine mutant, and greater than 500 microM NAD for the alanine, leucine, glutamine, and arginine mutants. These and previous results support a model of the ADP-ribosylation of EF-2 in which histidine-21 serves primarily a hydrogen-bonding function. We propose that the pi-imidazole nitrogen of His-21 hydrogen-bonds to the nicotinamide carboxamide, orienting the N-glycosidic bond of NAD for attack by the incoming nucleophile in a direct displacement mechanism, and then stabilizing the transition-state intermediate of this reaction.