Hepatitis C virus (HCV) infection is a serious cause of chronic liver disease worldwide with more than 170 million infected individuals at risk of developing significant morbidity and mortality. Current interferon-based therapies are suboptimal especially in patients infected with HCV genotype 1, and they are poorly tolerated, highlighting the unmet medical need for new therapeutics. The HCV-encoded NS3 protease is essential for viral replication and has long been considered an attractive target for therapeutic intervention in HCV-infected patients. Here we identify a class of specific and potent NS3 protease inhibitors and report the evaluation of BILN 2061, a small molecule inhibitor biologically available through oral ingestion and the first of its class in human trials. Administration of BILN 2061 to patients infected with HCV genotype 1 for 2 days resulted in an impressive reduction of HCV RNA plasma levels, and established proof-of-concept in humans for an HCV NS3 protease inhibitor. Our results further illustrate the potential of the viral-enzyme-targeted drug discovery approach for the development of new HCV therapeutics.

An underlying goal of drug discovery is to develop safe and stable substances that specifically target essential elements that cause disease. Molecular chirality adds an additional level of specificity and complexity in achieving this objective, as mirror image molecules are distinct substances and must be treated as such. Classical chiral-center enantiomers ( Figure 1A) have been shown to differ significantly in biological activity, pharmacodynamics, pharmacokinetics, and toxicity. 1 The cases of thalidomide 2 and perhexiline, 3 whose enantiomers differ dramatically with respect to toxicity and metabolic properties, emphasize the importance of addressing stereochemistry in drug development.In this Perspective, we address the pharmaceutical implications of a largely overlooked alternative source of drug chirality, atropisomerism, 4 which has the distinct feature of creating molecular chirality as a result of hindered rotation about a bond axis ( Figure 1B). Figure 1C shows space-filling models where it is evident that rotation about the vertical axis is hindered because of steric clashes between the bulky R1 and R2 groups with R3 and R4.Unlike compounds with classical chiral centers, which are often stable and which racemize via a bond breaking and making process, atropisomers racemize via an intramolecular dynamic process that only involves bond rotation. As bond rotation is time-dependent, racemization half-lives for atropisomers can vary dramatically between minutes to years, depending on the degree of steric hindrance, electronic influences, temperature, solvent, etc. Because of this time-dependent feature, drug discovery campaigns can become more complex, or may even be abandoned, when atropisomeric properties are observed. Atropisomerism frequently results as researchers strive to design more compact and conformationally constrained inhibitors. Even for courageous design and synthetic campaigns that attempt to develop atropisomeric compounds, important differences in properties have been reported for enantiomeric pairs, such as in vitro inhibition, crystallization, in vivo racemization rates, and absorption, distribution, metabolism, excretion, and toxicity (ADMET) properties. There are also examples of compounds that were unknowingly developed as a racemic mixture of atropisomers and required chiral detection experiments to finally reveal their existence. Overall, many view atropisomer chirality as a lurking menace with the potential to increase the cost of pharmaceutical research and development and to derail drug Figure 1. (A) Mirror-image enantiomers S and R arise from a classical chiral center (atom). (B) Other enantiomers S a and R a can arise from hindered rotation that creates a chiral axis. (C) Atropisomeric enantiomers S a and R a are shown as space-filling models. Reproduced with permission from ChemMedChem (LaPlante, S. R.; Edwards, P. J.; Fader, L. D.; Jakalian, A.; Hucke, O. Revealing atropisomer axial chirality in drug discovery. 2011, 6, 505À513,

A twist in the tale: Recent reports have highlighted solutions to the problems encountered when drug candidates exist as slowly interconverting conformers or atropisomers (see scheme). This Minireview brings together the various strategies that have been adopted and proposes a general approach to handling an aspect of stereochemistry which has received little attention from drug regulatory agencies.

An often overlooked source of chirality is atropisomerism, which results from slow rotation along a bond axis due to steric hindrance and/or electronic factors. If undetected or not managed properly, this time-dependent chirality has the potential to lead to serious consequences, because atropisomers can be present as distinct enantiomers or diastereoisomers with their attendant different properties. Herein we introduce a strategy to reveal and classify compounds that have atropisomeric chirality. Energy barriers to axial rotation were calculated using quantum mechanics, from which predicted high barriers could be experimentally validated. A calculated rotational energy barrier of 20 kcal mol(-1) was established as a suitable threshold to distinguish between atropisomers and non-atropisomers with a prediction accuracy of 86%. This methodology was applied to subsets of drug databases in the course of which atropisomeric drugs were identified. In addition, some drugs were exposed that were not yet known to have this chiral attribute. The most valuable utility of this tool will be to predict atropisomerism along the drug discovery pathway. When used in concert with our compound classification scheme, decisions can be made during early discovery stages such as "hit-to-lead" and "lead optimization," to foresee and validate the presence of atropisomers and to exercise options of removing, further stabilizing, or rendering the chiral axis of interest more freely rotatable via SAR design, thereby decreasing this potential liability within a compound series. The strategy can also improve drug development plans, such as determining whether a drug or series should be developed as a racemic mixture or as an isolated single compound. Moreover, the work described herein can be extended to other chemical fields that require the assessment of potential chiral axes.

An assay recapitulating the 3′ processing activity of HIV-1 integrase (IN) was used to screen the Boehringer Ingelheim compound collection. Hit-to-lead and lead optimization beginning with compound 1 established the importance of the C3 and C4 substituent to antiviral potency against viruses with different aa124/ aa125 variants of IN. The importance of the C7 position on the serum shifted potency was established. Introduction of a quinoline substituent at the C4 position provided a balance of potency and metabolic stability. Combination of these findings ultimately led to the discovery of compound 26 (BI 224436), the first NCINI to advance into a phase Ia clinical trial.

The pharmaceutical industry has recognized that many drug-like molecules can self-aggregate in aqueous media and have physicochemical properties that skew experimental results and decisions. Herein, we introduce the use of a simple NMR strategy for detecting the formation of aggregates using dilution experiments that can be performed on equipment prevalent in most synthetic chemistry departments. We show that (1)H NMR resonances are sensitive to large molecular-size entities and to smaller multimers and mixtures of species. Practical details are provided for sample preparation and for determining the concentrations of single molecule, aggregate entities, and precipitate. The critical concentrations above which aggregation begins can be found and were corroborated by comparisons with light scattering techniques. Disaggregation can also be monitored using detergents. This NMR assay should serve as a practical and readily available tool for medicinal chemists to better characterize how their compounds behave in aqueous media and influence drug design decisions.

CommunicationsPotent and selective macrocyclic inhibitors of the hepatitis C virus NS3 serine protease based on the conformation of a enzyme-bound substratelike hexapeptide demonstrate many of the desirable properties of a druglike archetype, which could lead to an antiviral agent for the treatment of hepatitis C in man. For more details see the following communication by Tsantrizos et al.

BI 224436 is an HIV-1 integrase inhibitor with effective antiviral activity that acts through a mechanism that is distinct from that of integrase strand transfer inhibitors (INSTIs). This 3-quinolineacetic acid derivative series was identified using an enzymatic integrase long terminal repeat (LTR) DNA 3=-processing assay. A combination of medicinal chemistry, parallel synthesis, and structure-guided drug design led to the identification of BI 224436 as a candidate for preclinical profiling. It has antiviral 50% effective concentrations (