No abstract

Novel chemotherapeutics for treating multidrug-resistant (MDR) strains of Mycobacterium tuberculosis (MTB) are required to combat the spread of tuberculosis, a disease that kills more than 2 million people annually. Using structure-based drug design, we have developed a series of alkyl diphenyl ethers that are uncompetitive inhibitors of InhA, the enoyl reductase enzyme in the MTB fatty acid biosynthesis pathway. The most potent compound has a Ki' value of 1 nM for InhA and MIC99 values of 2-3 microg mL(-1) (6-10 microM) for both drug-sensitive and drug-resistant strains of MTB. Overexpression of InhA in MTB results in a 9-12-fold increase in MIC99, consistent with the belief that these compounds target InhA within the cell. In addition, transcriptional response studies reveal that the alkyl diphenyl ethers fail to upregulate a putative efflux pump and aromatic dioxygenase, detoxification mechanisms that are triggered by the lead compound triclosan. These diphenyl ether-based InhA inhibitors do not require activation by the mycobacterial KatG enzyme, thereby circumventing the normal mechanism of resistance to the front line drug isoniazid (INH) and thus accounting for their activity against INH-resistant strains of MTB.

We present Raman spectra, obtained using 752 nm excitation, on wild-type GFP and the S65T mutant of this intrinsically fluorescent protein together with data on a model chromophore, ethyl 4-(4-hydroxyphenyl)methylidene-2-methyl-5-oxoimidazolacetate . In the pH range 1-14, the model compound has two macroscopic pK(a)s of 1.8 and 8.2 attributed to ionization of the imidazolinone ring nitrogen and the phenolic hydroxyl group, respectively. Comparison of the model chromophore with the chromophore in wild-type GFP and the S65T mutant reveals that the cationic form, with both the imidazolinone ring nitrogen and the phenolic oxygen protonated, is not present in these particular GFP proteins. Our results do not provide any evidence for the zwitterionic form of the chromophore, with the phenolic group deprotonated and the imidazolinone ring nitrogen protonated, being present in the GFP proteins. In addition, since the position of the Raman bands is a property exclusively of the ground state structure, the data enable us to investigate how protein-chromophore interactions affect the ground state structure of the chromophore without contributions from excited state effects. It is found that the ground state structure of the anionic form of the chromophore, which is most relevant to the fluorescent properties, is strongly dependent on the chromophore environment whereas the neutral form seems to be insensitive. A linear correlation between the absorption properties and the ground state structure is demonstrated by plotting the absorption maxima versus the wavenumber of a Raman band found in the range 1610-1655 cm(-1).

Isoniazid (INH), a frontline antitubercular drug, inhibits InhA, the enoyl reductase from Mycobacterium tuberculosis, by forming a covalent adduct with the NAD cofactor. Here, we report that the INH-NAD adduct is a slow, tight-binding competitive inhibitor of InhA. Demonstration that the adduct binds to WT InhA by a two-step enzyme inhibition mechanism, with initial, weak binding (K ؊1 ‫؍‬ 16 ؎ 11 nM) followed by slow conversion to a final inhibited complex (EI*) with overall K i ‫؍‬ 0. 75 A ttempts to treat tuberculosis, a disease that kills more than two million people every year, are hindered by the spread of multidrug-resistant strains of Mycobacterium tuberculosis (MDRTB), the causative agent, and by the increased susceptibility of HIV-positive individuals to this disease (1-4). Although isoniazid (INH; Scheme 1) has been the most effective and widely used drug for the treatment of tuberculosis since the 1950s, the mode of action of this compound is still not completely understood. INH is a prodrug that is activated by the mycobacterial catalase-peroxidase enzyme KatG (Scheme 1) (5-8) and a substantial fraction of all clinical isolates that are resistant to INH result from KatG mutations (2, 9-11). Consequently, compounds that inhibit the ultimate molecular target(s) of INH, but that do not require activation by KatG, have tremendous promise as novel drugs for combating MDRTB.Two enzymes, InhA and KasA, have been proposed as targets for INH. Both are members of the type II dissociated fatty acid biosynthesis pathway (FASII) in M. tuberculosis (Scheme 2), consistent with the observation that INH interferes with the biosynthesis of mycolic acids, very long chain fatty acid components of the mycobacterial cell wall. InhA, an enoyl reductase that catalyzes the NADH-dependent reduction of long chain trans-2-enoyl-acyl carrier proteins (ACPs), was first identified as a target by Jacobs and coworkers (6, 12) who observed mutations in the inhA gene in INH-resistant clinical isolates and identified a point mutant (S94A) that conferred resistance to INH and ethionamide in Mycobacterium smegmatis and in Mycobacterium bovis. Subsequently, Blanchard, Sacchettini, and coworkers (13-15) demonstrated that InhA was inhibited in vitro by a covalent adduct formed between activated INH and the nicotinamide head group of NAD (Scheme 1). InhA mutations observed in INH-resistant clinical isolates were found to be localized to the cofactor binding site and were shown to result in decreased affinity of the purified enzyme for NADH (12,16), consistent with the hypothesis that binding of NADH to the enzyme precedes adduct formation. In addition, Vilcheze et al. (17) used a temperature-sensitive mutation in the inhA gene to show that the phenotypic response to InhA inactivation in M. smegmatis was identical to that caused by treatment with INH, thereby validating InhA as a target for drug discovery. However, although there is convincing evidence that InhA is inhibited by INH, Barry and coworkers (18) have also proposed that KasA, on...

The photodynamics of wtGFP have been studied by ultrafast time-resolved infrared spectroscopy (TIR). In addition to the expected bleaching and transient infrared absorption of bands associated with the chromophore, we observe the dynamics of the proton relay reaction in the protein. Protonation of a protein carboxylate group occurs on the tens of picoseconds time scale following photoexcitation. Comparison with data for mutant GFPs, in which excited-state proton transfer has been disabled, supports the assignment of the carboxylate to the side chain of E222, a component of the hydrogen bonding network that links the two ends of the chromophore. The TIR data show that the rate-limiting step in the proton relay is deprotonation of the chromophore.

No abstract

The development of therapies for the treatment of neurological cancer faces a number of major challenges including the synthesis of small molecule agents that can penetrate the blood brain barrier (BBB). Given the likelihood that in many cases drug exposure will be lower in the CNS than in systemic circulation, it follows that strategies should be employed that can sustain target engagement at low drug concentration. Time dependent target occupancy is a function of both the drug and target concentration as well as the thermodynamic and kinetic parameters that describe the binding reaction coordinate, and sustained target occupancy can be achieved through structural modifications that increase target (re)binding and/or that decrease the rate of drug dissociation. The discovery and deployment of compounds with optimized kinetic effects requires information on the structure-kinetic relationships that modulate the kinetics of binding, and the molecular factors that control the translation of drug-target kinetics to time-dependent drug activity in the disease state. This review first introduces the potential benefits of drug-target kinetics, such as the ability to delineate both thermodynamic and kinetic selectivity, and then describes factors, such as target vulnerability, that impact the utility of kinetic selectivity. The review concludes with a description of a mechanistic PK/PD model that integrates drug-target kinetics into predictions of drug activity.

The rise of multi-drug resistant (MDR) and extensively drug resistant (XDR) tuberculosis around the world, including in industrialized nations, poses a great threat to human health and defines a need to develop new, effective and inexpensive anti-tubercular agents. Previously we developed a chemical systems biology approach to identify off-targets of major pharmaceuticals on a proteome-wide scale. In this paper we further demonstrate the value of this approach through the discovery that existing commercially available drugs, prescribed for the treatment of Parkinson's disease, have the potential to treat MDR and XDR tuberculosis. These drugs, entacapone and tolcapone, are predicted to bind to the enzyme InhA and directly inhibit substrate binding. The prediction is validated by in vitro and InhA kinetic assays using tablets of Comtan, whose active component is entacapone. The minimal inhibition concentration (MIC99) of entacapone for Mycobacterium tuberculosis (M.tuberculosis) is approximately 260.0 µM, well below the toxicity concentration determined by an in vitro cytotoxicity model using a human neuroblastoma cell line. Moreover, kinetic assays indicate that Comtan inhibits InhA activity by 47.0% at an entacapone concentration of approximately 80 µM. Thus the active component in Comtan represents a promising lead compound for developing a new class of anti-tubercular therapeutics with excellent safety profiles. More generally, the protocol described in this paper can be included in a drug discovery pipeline in an effort to discover novel drug leads with desired safety profiles, and therefore accelerate the development of new drugs.