Variations on the Bergman Cyclization Theme: Electrocyclizations of Ionic Penta-, Hepta-, and Octadiynes.

The Bergman cyclization of (Z)-hexa-3-ene-1,5-diyne to form the aromatic diradical p-benzyne has garnered attention as a potential antitumor agent due to its relatively low cyclization barrier and the stability of the resulting diradical. Here, we present a theoretical investigation of several ionic extensions of the fundamental Bergman cyclization: electrocyclizations of the penta-1,4-diyne anion, hepta-1,6-diyne cation, and octa-1,7-diyne dication, leveraging the spin-flip formulation of the equation-of-motion coupled cluster theory with single and double substitutions (EOM-SF-CCSD). Though the penta-1,4-diyne anion exhibits a large cyclization barrier of +66 kcal mol-1, cyclization of both the hepta-1,6-diyne cation and octa-1,7-diyne dication along a previously unreported triplet pathway requires relatively low energy. We also identified the presence of significant aromaticity in the triplet diradical products of these two cationic cyclizations.

Actin filament curvature biases branching direction.

Mechanical cues affect many important biological processes in metazoan cells, such as migration, proliferation, and differentiation. Such cues are thought to be detected by specialized mechanosensing molecules linked to the cytoskeleton, an intracellular network of protein filaments that provide mechanical rigidity to the cell and drive cellular shape change. The most abundant such filament, actin, forms branched networks nucleated by the actin-related protein (Arp) 2/3 complex that support or induce membrane protrusions and display adaptive behavior in response to compressive forces. Here we show that filamentous actin serves in a mechanosensitive capacity itself, by biasing the location of actin branch nucleation in response to filament bending. Using an in vitro assay to measure branching from curved sections of immobilized actin filaments, we observed preferential branch formation by the Arp2/3 complex on the convex face of the curved filament. To explain this behavior, we propose a fluctuation gating model in which filament binding or branch nucleation by Arp2/3 occur only when a sufficiently large, transient, local curvature fluctuation causes a favorable conformational change in the filament, and we show with Monte Carlo simulations that this model can quantitatively account for our experimental data. We also show how the branching bias can reinforce actin networks in response to compressive forces. These results demonstrate how filament curvature can alter the interaction of cytoskeletal filaments with regulatory proteins, suggesting that direct mechanotransduction by actin may serve as a general mechanism for organizing the cytoskeleton in response to force.

Conformational analysis of a model for the trans-fused FGH ether rings in brevetoxin A.

We have applied the Low Mode:Monte Carlo (LM:MC) conformational search method to an investigation of the low energy structures for 3R,4aS,7aR,11aS,12aR)-11a-methyl-3-phenyldecahydro-1H-[1,3]dioxino[5,4-b]pyrano[2,3-g]oxocine, 3, a model for the FGH rings in brevetoxin A, a potent marine toxin. Searches performed using a variety of classical force field-solvent combinations yielded ensembles containing between 30 and 60 structures with the central G-ring adopting the crown, twist-crown, boat-chair, and boat conformations. The lowest energy structure depended on the force field-solvent treatment, and the twist-crown and boat-chair conformers were typically lowest in energy. B3LYP/6-31G** geometry optimizations using the SCRF continuum solvent model confirmed these structures but indicated that the crown was the energetically preferred conformer with the boat-chair lying within 1.4 kcal/mol.

Revisiting the hERG safety margin after 20 years of routine hERG screening.

INTRODUCTION: It has been two decades since screening new molecules and potential clinical drug candidates against the hERG potassium channel became a routine part of safety pharmacology. The earliest heuristic for what was an adequate safety margin to separate molecules with a potential liability to cause the arrhythmia torsade de pointes (TdP) from those with no such liability emerged in 2002 and was determined to be a hERG IC50 value 30-fold above the therapeutic free plasma concentration (Webster, Leishman, & Walker, 2002). In the intervening years nonclinical and clinical ICH guidance has been introduced and intense scrutiny has been applied to the QT interval of the electrocardiogram in animals and man. Has the 30-fold heuristic stood the test of time? METHODS: The hERG margins between the IC50 value and the therapeutic unbound plasma concentrations were examined for 367 compounds. These margins were compared against the categories used by www.CredibleMeds.com to classify a drug's TdP risk. A subset of 336 of these drugs were compared against their US product labels with respect to black box warnings on QTc prolongation or TdP, warnings and precautions on QTc or TdP, and QTc language in the clinical pharmacology section. RESULTS: Against the CredibleMeds classification the means of the margins for Known, Conditional, or Possible Risk of TdP, and Not Listed (presumably no TdP liability) were 4.8, 28, 71 and 339, respectively. Against the US label language the means of margins for black boxes and warnings were 3.1 and 26, respectively. The average margins associated with, positive QTc outcome, negative QTc outcome and no QTc language were 16, 479 and 204, respectively. Based on ROC curves the optimal hERG margin thresholds to separate "Known risk of TdP" from "Not Listed" and, QTc prolongation positive from QTc negative were 37- and 50-fold, respectively. CONCLUSIONS: The observed optimal margin of 50-fold in the current study is not appreciably different from a previously reported 45-fold optimal margin (Gintant, 2011). The margin falls between the margins for negative (QTc outcome or no QTc language) and positive (positive QTc outcome, warnings or black boxes) compounds. The observed optimal margin of 37-fold in the current study is not appreciably different from the commonly used 30-fold optimal margin (Webster et al., 2002). This margin falls between those for drugs with a known or conditional TdP risk and those where it is at best a possible risk, and from the 240 drugs not listed on www.CredibleMeds.com. It is expected that there would be a small numerical difference (e.g. 37 vs. 50, or as previously published 30 vs. 45) between optimal cut-offs for the TdP liability and QTc prolongation predictions since some QTc positive drugs are described on CredibleMeds.com as having only a "Possible Risk of TdP" as they are not associated with TdP when used as directed. The fact that the margins in each category form distributions is also expected given biologic variability. However, we argue that a more consistent manner of assessing hERG potency and evaluating relevant exposures would be likely to reduce the spread in these distributions and make margins even more useful as a decision-making data point.

An extended multireference study of the electronic states of para-benzyne.

A state-averaged, multireference complete active space (CAS) approach was used for the determination of the vertical excitation energies of valence and Rydberg states of para-benzyne. Orbitals were generated with a 10- and 32-state averaged multiconfigurational self-consistent field approach. Electron correlation was included using multireference configuration interaction with singles and doubles, including the Pople correction for size extensivity, multireference averaged quadratic coupled cluster (MR-AQCC), and MR-AQCC based on linear response theory. There is a very high density of electronic states in this diradical system-there are more than 17 states within 7 eV of the ground state including two 3s Rydberg states. All excitations, except 2 (1)A(g), are from the pi system to the sigmasigma(*) system. Of the 32 states characterized, 15 were multiconfigurational, including the ground (1)A(g) state, providing further evidence for the necessity of a multireference approach for p-benzyne. The vertical singlet-triplet splitting was also characterized using a two-state averaged approach. A CAS(2,2) calculation was shown to be inadequate due to interaction with the pi orbitals.

Population Pharmacokinetics of LY2623091 in Patients With Hypertension and Chronic Kidney Disease.

LY2623091 is a selective, orally active, nonsteroidal, competitive mineralocorticoid receptor antagonist that blocks the actions of aldosterone and other mineralocorticoid receptor ligands at the receptor level. The aim of this work was to explore and establish a population pharmacokinetic model, quantify the degree of interindividual variability, and identify significant disease-, patient-, and study-specific covariates that alter the disposition of LY2623091. The data included concentrations from 294 healthy subjects and patients with hypertension and/or chronic kidney disease (CKD), sampled in 5 phase 1 and 2 studies. The pharmacokinetics of LY2623091 was well described by a 2-compartment model with first-order absorption and elimination. Formulation (on oral bioavailability) as well as weight and age (both on apparent central volume of distribution) were found to be significant covariates. The relative bioavailability of the capsule formulation was 68.4% compared to that of the solution. Hypertension and CKD status were not significant covariates. The pharmacokinetic model suggests that given the same dose, patients with hypertension and/or CKD would receive a similar exposure compared to subjects without these disease conditions.