Targeting enzyme inhibitors in drug discovery

Drugs that function as enzyme inhibitors constitute a significant portion of the orally bioavailable therapeutic agents that are in clinical use today. Likewise, much of drug discovery and development efforts at present are focused on identifying and optimizing drug candidates that act through inhibition of specific enzyme targets. The attractiveness of enzymes as targets for drug discovery stems from the high levels of disease association (target validation) and druggability (target tractability) that typically characterize this class of proteins. In this expert opinion the authors describe the existing practices and future directions in drug discovery enzymology, with emphasis on how a detailed understanding of the catalytic mechanism of specific targets can be used to identify and optimize small-molecule compounds that interact with conformationally distinct forms of the enzyme, thus resulting in high potency, high selectivity inhibitors.     

Targeting cancer using fragment based drug discovery

Over the past decade, fragment-based drug discovery has developed significantly and has gained increasing popularity in the pharmaceutical industry as a powerful alternative and complement to traditional high-throughput screening approaches for hit identification. Fragment-based methods are capable of rapidly identifying starting points for structure-based drug design from relatively small libraries of low molecular weight compounds. The main constraints are the need for sensitive methods that can reliably detect the typically weak interactions between fragments and the target protein, and strategies for transforming fragments into higher molecular weight drug candidates. This approach has recently been validated as series of compounds from various programs have entered clinical trials.     

Targeting GABAB receptors for anti-abuse drug discovery

Introduction: There is increasing evidence encouraging the development of drugs that positively modulate the γ-aminobutyric acid type B (GABA_B) receptor for combating addiction. Compounds that target GABA_B receptors are unique as anti-abuse therapies because of their impact against multiple addictive drugs.

Areas covered: The authors present the basic information concerning the drug actions of GABA and GABA_B receptor orthosteric agonists and positive allosteric modulators (PAM). Furthermore, they discuss several recent excellent reviews and newer results pertaining to GABA_B receptor drug effects on responses to and self-administration of: alcohol (ethanol), nicotine, cocaine, (meth)amphetamine, and opioids. Preclinical and clinical data are considered.

Expert opinion: Clinical data exist only for baclofen and mostly for alcohol use disorders. Additional trials are needed, but effects are promising. Whether PAMs, given alone or in combination with a direct GABA_B receptor agonist, will be clinically effective and have fewer side effects requires investigation. The sedative effects of baclofen, a Food and Drug Administration (FDA)-approved drug, become less severe over time. Based on existing data, baclofen is well-tolerated. However, genetic and physiological differences are likely to contribute to individual responses to different therapeutic agents. The more immediate development of baclofen as a therapeutic for alcohol use disorders may be of significant benefit to some individuals.     

Targeting C5a: recent advances in drug discovery

Activation of complement via the innate and adaptive immune system is vital to the body's defences in fighting infections. Unregulated complement activation is likely to play a crucial role in the pathogenesis of several diseases including psoriasis, adult (acute) respiratory distress syndrome (ARDS), bullous pemphigoid (BP), rheumatoid arthritis (RA) and ischemia-reperfusion (I/R) injury. The 74 amino acid peptide C5a is released after complement activation at sites of inflammation and is a potent chemoattractant for neutrophils, basophils, eosinophils, leukocytes, monocytes and macrophages. Recombinant proteins and humanized anti-C5 antibodies have been recently developed for blocking specific proteins of the complement system bringing renewed attention towards complement inhibition. Pharmacological studies have been highlighting the complement fragment C5a as an interesting target for the management of complement mediated diseases. Specific inhibition of C5a biological activity could gain therapeutic benefit without affecting the protective immune response. In the last few years several peptide and non-peptide antagonists of C5a have been discovered and tested in relevant pharmacological models; the availability of orally active compounds is rapidly helping to delineate the precise role of C5a in immunoinflammatory disorders. Moreover, mutagenesis data for the C5a/C5a receptor (C5aR) couple make C5aR a valuable model for mechanistic studies of peptidergic G-protein coupled receptors (GPCRs). The aim of this review is to outline the recent advances in C5a inhibition, especially highlighting the value of a multidisciplinary integrated approach in drug discovery.     

Targeting the vitamin D receptor: advances in drug discovery

The vitamin D receptor (VDR), a member of the nuclear receptor superfamily, mediates the biological action of 1α,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃], the active form of vitamin D. VDR regulates bone and calcium homeostasis, immunity, cellular differentiation and other physiological processes. Mutations in the VDR gene were identified in hereditary vitamin D-resistant rickets (HVDRR), and VDR-null mice exhibit the HVDRR phenotype, characterised by rickets and hypocalcaemia. In addition to the treatment of rickets, vitamin D analogues are important therapeutics in osteoporosis and psoriasis. Vitamin D analogues are effective drugs in experimental models of immune disorders and malignancies, such as breast cancer, prostate cancer and leukaemia. The development of functionally selective VDR-targeted drugs is leading to an enhanced understanding and novel therapies for these VDR-related diseases.     

Advances in structure-based drug discovery of carbonic anhydrase inhibitors

Introduction: The enzyme carbonic anhydrase (CA, EC 4.2.1.1) is found in numerous organisms across the tree of life, with seven distinct classes known to date. CA inhibition can be exploited for the treatment of edema, glaucoma, seizures, obesity, cancer and infectious diseases. A myriad of CA inhibitor (CAI) classes and inhibition mechanisms have been identified over the past decade, mainly through structure-based drug design approaches. Five different CA inhibition mechanisms are presently known.

Areas covered: Recent advances in structure-based CAI design are reviewed, with periodic table-based organization of inhibitor classes.

Expert opinion: Various structure-based drug design studies have led to deep understanding of factors governing tight binding and selectivity for the various isoforms. Carboxylic acids, phenols, polyamines, diols, borols, boronic acids, coumarins and sulfonamides represent successful stories which led to an anti-tumor sulfonamide in Phase I clinical trials (SLC-0111). For many inhibitor classes, no detailed crystallographic data are available. Detailed structural characterization of all CAI classes may lead to further advances in the field with potential therapeutic implications in the management of indications including neuropathic pain, cerebral ischemia, arthritis and tumor imaging.     

Antimalarial drug discovery: development of inhibitors of dihydrofolate reductase active in drug resistance

In view of widespread drug resistance in Plasmodium falciparum, new antimalarials are needed. Modelling techniques are being applied with some success to the design of drugs targeting dihydrofolate reductase (DHFR), and screening systems using recombinant enzyme are producing valuable data. The author reviews the interaction of inhibitors with probable active-site residues and examines new approaches. As the interaction of straight rigid drugs, like pyrimethamine, with the binding site of plasmodial DHFR is inhibited by the bulky Ser108→Asn resistance mutation, more flexible drugs, such as trimethoprim, may have a role in areas where this mutation is found.     

Targeting information-processing deficit in schizophrenia: a novel approach to psychotherapeutic drug discovery

Current observations indicate that dysfunction of neuronal circuitry dynamics contributes to the abnormal information processing in the brain in schizophrenia. It is presumed that disrupted auditory gating, abnormal P300-evoked potentials and deficits in mismatch negativity in schizophrenic patients indicate impaired processing of information. Recently, abnormalities in neuronal synchrony and oscillatory activity have been postulated as the mechanisms that underlie the distorted perception and cognitive dysfunction associated with schizophrenia. These novel observations might reveal the pathophysiology of the disorder, and indicate potential targets for antipsychotic drug therapy. Neuronal circuitry dynamics, such as network oscillations and sensory-gating processes, are conserved phylogenetically, which provides excellent opportunities for designing translational biomarkers. Whether preclinical, experimental compounds that impact on network oscillations and sensory processing (such as agonists and modulators of alpha7 nicotinic acetylcholine receptors) elicit the same neurophysiological events in schizophrenic patients and, subsequently, improve perception and cognitive functions will be determined when these drug candidates are available clinically.     

Recent progress in the drug discovery of non-peptidic BACE1 inhibitors

Background: β-Secretase, also called BACE1, is a promising molecular target for developing anti-Alzheimer's disease drugs. Several approaches of drug discovery for this therapy have been attempted, for example, substrate-based and structure-based designs, high-throughput screening, fragment-based lead generation and in silico screening. Method: In this review, we describe the design of non-peptidic BACE1 inhibitors from a historical perspective and not by the inhibitor's category. Conclusion: The respective methods have both merits and demerits and should be used in a mutually complementary manner.     

Multi-targeted hybrids based on HDAC inhibitors for anti-cancer drug discovery

Multi-targeted hybrids combine two drugs in a single molecule to have greater medicinal effects than its individual components. Recently, a number of anti-cancer drug candidates such as CUDC-101 (Curis) have been designed based on linking properly two selected pharmacophores endowed with activity against different therapeutic targets.     

Targeting protein kinases for bone disease: discovery and development of Src inhibitors

The dynamic and highly regulated processes of bone remodeling involve two major cells, osteoclasts and osteoblasts, both of which command a multitude of cellular signaling pathways involving protein kinases. Of the possible kinases in these cells, Src tyrosine kinase stands out as a promising therapeutic target for bone disease as validated by Src knockout mouse studies and in vitro cellular experiments, suggesting a regulatory role for Src in both osteoclasts (positive) and osteoblasts (negative). Advances in structural studies involving both Src and non-Src family kinases, in activated and unactivated protein states, have uncovered key binding site interactions that have led to the design of potent Src inhibitors. The lead compounds originate from a variety of synthetic templates and have demonstrated nM potency in enzymatic/binding assays and efficacy in animal models of bone disease. This review will provide a current understanding of critical Src signalling pathways in osteoclasts and osteoblasts, while detailing the structure-based design and screening-based lead discovery of Src inhibitors to be developed as therapeutic agents for bone disease.     

Novel TACE inhibitors in drug discovery: a review of patented compounds

TNF-α converting enzyme (TACE), a pro-inflammatory cytokine, catalyzes the formation of TNF-α from membrane bound TNF-α precursor protein. TNF-α is believed to play pathophysiological roles in inflammation, anorexia, cachexia, septic shock, viral replication and so on. TNF-α is a key player in inflammation and joint damage in rheumatoid arthritis. While a variety of TACE inhibitors have been reported in the literature, a vast majority of these compounds are peptidic and peptide-like compounds that are expected to have bioavailability and pharmacokinetic problems, common to such compounds, limiting their clinical effectiveness. Low molecular mass, long acting, orally bioavailable inhibitors of TACE are, therefore, highly desirable for the treatment of potential chronic diseases mentioned above. A review of patented compounds as TACE inhibitors in drug discovery is given. A selection of interesting patents recorded from 2001 to 2009 is presented. Various novel TACE inhibitors developed by different companies have been discussed.     

Targeting FtsZ for antituberculosis drug discovery: noncytotoxic taxanes as novel antituberculosis agents

Screening of 120 taxanes identified a number of compounds that exhibited significant antituberculosis activity. Rational optimization of selected compounds led to the discovery that the C-seco-taxane-multidrug-resistance (MDR) reversal agents (C-seco-TRAs) are noncytotoxic at the upper limit of solubility and detection (>80 microM), while maintaining MIC(99) values of 1.25-2.5 microM against drug-resistant and drug-sensitive strains of Mycobacterium tuberculosis (MTB). Treatment of MTB cells with TRA 3aa and 10a at the MIC caused filamentation and prolongation of the cells, a phenotypic response to FtsZ inactivation.     

Structure-based drug discovery of ApoE4 inhibitors from the plant compounds

Apolipoprotein E4 (ApoE4) is a potential target for developing new therapeutics for Alzheimer’s disease (AD). Till now there is no drug available to inhibit this protein and cholinesterase inhibitor was given for almost all the AD patients. In this study, we have approached to identify the potential ApoE4 inhibitor from the plant compounds. Rigid docking study was performed for 18 plant compounds and 11 cholinesterase inhibitors. Based on the docking score, binding energy and number of hydrogen bonding curcumin posses the best scoring function. For further validation induce fit docking was performed and it also shows that curcumin binds to the same binding pocket of ApoE4 protein. Biological activity prediction reveals that curcumin has a potential therapeutic activity against AD. Pharmacokinetic properties of this compound are under the acceptable range. From the results we concluded that the plant compound curcumin could be a potential inhibitor of ApoE4 and it can control the AD.     

Structure based drug design of angiotensin-I converting enzyme inhibitors

Cardiovascular disease (CVD) is responsible for ～27% of deaths worldwide, with 80% of these occuring in developing countries. Hypertension is one of the most important treatable factors in the prevention of CVD. Angiotensin-I converting enzyme (ACE) is a two-domain dipeptidylcarboxypeptidase that is a key regulator of blood pressure as a result of its critical role in the reninangiotensin- aldosterone and kallikrien-kinin systems. Consequently, ACE is an important drug target in the treatment of CVD. ACE is primarily known for its ability to cleave angiotensin-I to the vasoactive octapeptide angiotensin-II, but is also able to cleave a number of other substrates including the vasodilator bradykinin and N-acetyl-seryl-aspartyl-lysyl-proline (acetyl-SDKP), a physiological modulator of hematopoiesis. Numerous ACE inhibiors are available clinically, and these are generally effective in treating hypertension. However some adverse effects are associated with ACE inhibition, such as the persistent dry cough and the potentially fatal angioedema. The solution of ACE crystal structures over the last decade has facilitated rational drug design which has contributed to the development of domain-selective ACE inhibitors, the most notable of which include RXP407 (N-domain) and RXPA380 (C-domain), which in principle may herald new therapeutic approaches for ACE inhibition. Additionally, dual inhibitors to ACE and other targets such as neprilysin, endothelin converting enzyme and chymase have been developed. The success of ACE inhibitors has also led to the search for novel inhibitors in food and natural products and the structure guided screening of such libraries may well reveal a number of new ACE inhibitors.     

A guide to drug discovery: Target selection in drug discovery

Target selection in drug discovery--defined here as the decision to focus on finding an agent with a particular biological action that is anticipated to have therapeutic utility--is influenced by a complex balance of scientific, medical and strategic considerations. In this article, we provide an introduction to the key issues in target selection and discuss the rationale for decision making.