Current Topics in Medicinal Chemistry - Volume 13, Issue 18, 2013

There is a major controversy in cytochrome P450 chemistry regarding the nature of the active oxidant responsible for substrate monoxygenation. Part of this controversy originates from the fact that the later stages in the catalytic cycle of P450 enzymes proceed so fast that little experimental evidence is available. Early studies suggested an iron(IV)- oxo heme cation radical ([heme+˙)-FeIV=O] or Compound I) as the active species able to abstract a hydrogen atom from a substrate and rebind the hydroxyl group to form an alcohol product. Such simplistic early models involving a single active species have subsequently been invalidated by several experimental studies which clearly indicates that there must be at least two active species of some description. Based on these and other data, a two-oxidant hypothesis was put forward where Compound I and its precursor in the catalytic cycle ([heme-FeIII-OOH]– or Compound 0) are competitive oxidants. Density functional theory studies, however, suggest an alternative hypothesis involving a two-state-reactivity scenario where Compound I has two close-lying spin states that react differently with substrates and masquerade as two distinct oxidants. These theoretical studies show that the two spin states of Compound I react with substrates via aliphatic and aromatic C–H hydroxylation, C=C epoxidation and sulfoxidation reactions, and explain experimentally observed product distributions and kinetic isotope effects. This review will give an overview of recent studies on the two-oxidant versus two-state-reactivity hypotheses and how theory contributes to the understanding of enzymatic reaction processes.

In vitro validation of a protein homology model is critical for determining the predictivity of a computationally generated structure. Here we discuss the generation, validation, and application of a homology model for CYP1A1. Validation of the CYP1A1 homology model, generated using the highly homologous crystal template of human CYP1A2 (pdb 2HI4), was achieved using the prototypic substrate 7-ethoxyresorufin (Eres). The model was subsequently applied to generate CYP1A1 mutants with increased catalytic efficiency (Vmax/Km) towards the anticancer prodrug dacarbazine (DTIC). Thirty-three directed CYP1A1 mutants were generated and expressed in E. coli; six of these were generated to rationalise docking data obtained from in silico experiments using Eres. DTIC N-demethylation by the CYP1A1 E161K, E256K, and I458V mutants exhibited Michaelis-Menten kinetics, with decreases in Km that doubled the catalytic efficiency relative to wild-type (P < 0.05). As a chemotherapeutic agent, DTIC has relatively poor clinical activity in human malignancies and exhibits numerous adverse effects, which presumably arise from bioactivation in the liver and other tissues resulting in systemic exposure to the cytotoxic metabolite. The successful generation of CYP1A1 enzymes with catalytically enhanced DTIC activation highlights their potential use as a strategy for P450-based gene directed enzyme prodrug therapy (GDEPT) in the treatment of metastatic malignant melanoma. Moreover, the combination of in vitro kinetic analyses with in silico docking data from a validated homology model has allowed interpretation of the structure-activity relationships of this enzyme-substrate pair.

CYP 2C8, which carries out the oxidative metabolism of at least 5% of clinical drugs, has attracted increasing attention in recent years. New drugs (substances), inducers and inhibitors of CYP 2C8 have been developed and the drug metabolism has been investigated to understand the clinical role of CYP2C8. The cases of CYP2C8 genetic polymorphisms linked to diseases have increased and have been investigated. Herein, important progress in these areas has been reviewed with an emphasis on drug metabolism. Polymorphisms, diseases related to CYP2C8, some important drugs (substances) and inhibitors are reviewed and discussed.

Engineered biocatalysts offer the opportunity to introduce modifications into complex lead molecules and drug candidates in a chemo-, regio- and stereoselective manner that is difficult to accomplish using traditional synthetic organic chemistry. As candidate biocatalysts, the cytochrome P450 enzymes that metabolize drugs and other xenobiotics are amongst the most versatile agents known. Not only can they mediate an exceptional range of biotransformation reactions, but they act on an unparalleled diversity of substrates. However, this versatility comes at the cost of relatively poor catalytic efficiency and low rates of coupling of cofactor consumption to product formation. Directed evolution is being used to redefine the substrate specificity of P450 enzymes while at the same time improving their efficiency, thermostability and other properties. This review will outline the key successes with bacterial P450s used as biocatalysts, examine the studies done to date with mammalian forms, and assess the prospects for exploiting xenobiotic-metabolizing P450s for applications in medicinal chemistry.

Stroke is a devastating disease associated with high morbidity and mortality. Despite the approved indication of systemic thrombolytic therapy in the United States for the acute management of ischemic stroke, its use is limited given a strict eligibility criteria and a risk for hemorrhagic transformation as a feared adverse effect. Many agents have been studied without success for neuroprotection in patients with stroke to reduce vascular injury and improve long-term functional outcomes. Minocycline is a tetracycline antibiotic that shows promise for its neuroprotective effects in multiple animal models and three human trials. It affects multiple pathways to reduce apoptosis, neuroinflammation, infarct size, and vascular injury. The aim of this review is to discuss current evidence for minocycline from pre-clinical and early clinical trials and its potential role in neuroprotection in patients with acute ischemic stroke.

Emergence of new and medically resistant pathogenic microbes continues to escalate toward worldwide public health, wild habitat, and commercial crop and livestock catastrophes. Attempts at solving this problem with sophisticated modern biotechnologies, such as smart vaccines and microbicidal and microbistatic drugs that precisely target parasitic bacteria, fungi, and protozoa, remain promising without major clinical and industrial successes. However, discovery of a more immediate, broad spectrum prophylaxis beyond conventional epidemiological approaches might take no longer than the time required to fill a prescription at your neighborhood pharmacy. Findings from a growing body of research suggest calcium antagonists, long approved and marketed for various human cardiovascular and neurological indications, may produce safe, efficacious antimicrobial effects. As a general category of drugs, calcium antagonists include compounds that disrupt passage of Ca&2+ molecules across cell membranes and walls, sequestration and mobilization of free intracellular Ca2+, and downstream binding proteins and sensors of Ca2+-dependent regulatory pathways important for proper cell function. Administration of calcium antagonists alone at current therapeutically relevant doses and schedules, or with synergistic compounds and additional antimicrobial medications, figures to enhance host immunoprotection by directly altering pathogen infection sequences, life cycles, homeostasis, antibiotic tolerances, and numerous other infective, survival, and reproductive processes. Short of being miracle drugs, calcium antagonists are welcome old drugs with new tricks capable of controlling some of the most virulent and pervasive global infectious diseases of plants, animals, and humans, including Chagas’ disease, malaria, and tuberculosis.

Alzheimer's disease (AD) is the most common cause of dementia and a major cause of morbidity and mortality. The greatest risk factor for AD is age and as many countries are experiencing an aging population, the expected rise in AD threatens to have serious medical and socioeconomic impact in the coming decades. The only approved medications for AD are symptomatic and there are no currently available disease modifying treatments. Hence, a disease modifying treatment is desperately needed for AD not only for proper care and management of affected patients, but also to reduce society's socioeconomic burden. Developing novel compounds for any indication is a time, effort, and money consuming endeavor and most treatments never make it to market. Other research and development strategies are needed, especially for the treatment of AD. We provide a review of the current literature in assessing possibilities of repurposing medications currently used for non-AD indications. Many different compounds from many different pharmacological classes have already been studied in an AD context. We provide a “pragmatic drug repurposing score” for each of these compounds based on type of studies conducted, number of possible mechanisms of action, efficacy in AD and other neurodegenerative disease studies, tolerability profile, and their ability to cross the blood brain barrier. The current data suggest several compounds worthy of further study as treatments for AD. Compounds with the highest scores include lithium, minocycline, exenatide, valproic acid, methylene blue, and nicotine.

The process of discovering a pharmacological compound that elicits a desired clinical effect with minimal side effects is a challenge. Prior to the advent of high-performance computing and large-scale screening technologies, drug discovery was largely a serendipitous endeavor, as in the case of thalidomide for erythema nodosum leprosum or cancer drugs in general derived from flora located in far-reaching geographic locations. More recently, de novo drug discovery has become a more rationalized process where drug-target-effect hypotheses are formulated on the basis of already known compounds/protein targets and their structures. Although this approach is hypothesis-driven, the actual success has been very low, contributing to the soaring costs of research and development as well as the diminished pharmaceutical pipeline in the United States. In this review, we discuss the evolution in computational pharmacology as the next generation of successful drug discovery and implementation in the clinic where high-performance computing (HPC) is used to generate and validate drug-target-effect hypotheses completely in silico. The use of HPC would decrease development time and errors while increasing productivity prior to in vitro, animal and human testing. We highlight approaches in chemoinformatics, bioinformatics as well as network biopharmacology to illustrate potential avenues from which to design clinically efficacious drugs. We further discuss the implications of combining these approaches into an integrative methodology for high-accuracy computational predictions within the context of drug repositioning for the efficient streamlining of currently approved drugs back into clinical trials for possible new indications.

Movement disorders are a heterogeneous group of both common and rare neurological conditions characterized by abnormalities of motor functions and movement patterns. This work overviews recent successes and ongoing studies of repositioning relating to this disease group, which underscores the challenge of integrating the voluminous and heterogeneous findings required for making suitable drug repositioning decisions. In silico drug repositioning methods hold the promise of automated fusion of heterogeneous information sources, but the controllable, flexible and transparent incorporation of the expertise of medicinal chemists throughout the repositioning process remains an open challenge. In support of a more systematic approach toward repositioning, we summarize the application of a computational repurposing method based on statistically rooted knowledge fusion. To foster the spread of this technique, we provide a step-by-step guide to the complete workflow, together with a case study in Parkinson's disease.

Characteristic symptoms of schizophrenia and bipolar disorders have been described and classified about a century ago. Each of these disorders may cause considerable impairment reflecting substantial alterations in cognition, perception, and mood. Though both disease concepts are well established, psychopharmacological treatment strategies, involving first- and second-generation antipsychotics, benzodiazepines and mood stabilizing drugs, often fail to keep their purported alleviating effects on respective characteristic symptom spectra, producing unsatisfactory patient responses. While drug profiles may differ concerning the underlying mechanism of action, the breadth of treatment options remains limited. Besides developing new drugs with different mechanisms of action, side-effect profile and efficacy, it has to be emphasized that repurposed drugs might serve as alternative or adjuvant treatment options for patients, who continue to poorly respond to standard treatment algorithms. Here, we review the current evidence of selected drugs whose repurposed use might expand the range of treatment options for schizophrenia and bipolar disorders.

Drug repurposing (drug repositioning, drug reprofiling, drug retasking) gains increasing importance as the development of new drugs becomes increasingly expensive. Though only a few compounds have been approved for new indications in the field of metabolic disorders, there are a number of substances which have the potential to become reprofiled in a new indication. Generally, reprofiled drugs for metabolic disorders can be classified in three groups. Group A contains those of which both, the original and repurposed indication, concern metabolic disorders. Group B comprises drugs, which were originally approved for non-metabolic disorders but show beneficial effects for metabolic disorders after repurposing. Group C comprises drugs, which were originally approved for metabolic disorders and are effective for non-metabolic disorders in their repurposed indication. Repurposed drugs in the field of metabolic disorders of group A include tetra-hydrobiopterin, originally indicated for phenylketonuria and now also approved for tetrahydrobiopterindeficiency, coenzyme-Q, originally approved for primary coenzyme-Q deficiency and reprofiled for statin-myopathy, and colesevelam, originally approved to reduce elevated low-density lipoprotein (LDL)-cholesterol (LDL-C) and now being approved for type-2-diabetes. An example of group C is phenylbutyrate, which was originally approved for urea-cycle disorders and meanwhile gained approval for progressive familial intrahepatic cholestasis type 2 due to mutations in the ABCB11 gene. Still additional compounds used to treat metabolic (non-metabolic) disorders show promising effects in non-metabolic (metabolic disorders) after repurposing in cell and tissue models. Future investigations will need to identify which candidate drugs may leave the pipeline status to acquire approval for new indications.