Current Medicinal Chemistry - Volume 18, Issue 20, 2011

Targeting cancer with small molecule irreversible inhibitors of kinases represents an emerging challenge in drug discovery. Irreversible inhibitors bind to kinase active site in a covalent and irreversible form, most frequently by reacting with a nucleophilic cysteine residue, located near the ATP binding pocket. The most common mechanism is the Michael reaction, that refers to the addition of a nucleophile, such as cysteine, to an α,β-unsaturated carbonyl. The nucleophile reacts at the electrophilic β-position to form an adduct; as a result the inhibitor irreversibly blocks binding of ATP to the kinase, rendering the kinase inactive. Different cysteine-reactive groups have been evaluated, an acrylamide or a substituted acrylamide moiety are the Michael acceptors of choice. There are some advantages for the irreversible kinase inhibition. These compounds are highly selective because they target a specific cysteine and only a limited number of kinases has a cysteine at the corresponding position. Another advantage is that covalent bond formation can overcome competition with the high endogenous concentration of ATP. A further motivation for designing irreversible inhibitors is their longer duration of action respect to conventional inhibitors. In fact, once bound to enzyme, these compounds do not readily dissociate and the inhibition continues even after the inhibitor leaves the circulation. Moreover, these inhibitors have the potential to overcome and prevent the emergence of acquired resistance conferred by mutations. In this review examples of irreversible inhibitors are reported, focusing on chemical structures, SAR and biological activities. The great potential of these compounds could open new and promising perspectives for a broader application of this approach

mTOR (mammalian target of rapamycin) is a serine-threonine kinase belonging to the PI3K/Akt/mTOR signalling pathway that is involved in several cell functions, including growth, proliferation, apoptosis and autophagy. mTOR hyperactivation has been detected in several human cancers, thus representing, together with its upstream effectors, an important target for cancer therapy. mTOR exists in two different complexes in cells, mTORC1 and mTORC2 which could both be targeted by potential anticancer agents. Rapamycin, the selective and allosteric inhibitor of mTOR, inhibits the enzyme in mTORC1, but not in mTORC2. In the last few years a number of mTOR ATP-competitive inhibitors has been reported acting on mTOR in both complexes and possessing a more complete anticancer activity in comparison with that of rapamycin and its derivatives. mTOR shares high sequence homology in the hinge-region with PI3K that is a lipid kinase upstream to mTOR in the same signalling pathway; for this reason some compounds originally developed as PI3K inhibitors later showed to also target mTOR. As indicated by preclinical and clinical studies, compounds acting on more than one target could result in a better biological response and in enhanced therapeutic potential and also dual PI3K/mTOR inhibitors result of great interest as potential antitumor agents. This review mainly reports the recently discovered mTOR ATP-competitive inhibitors in terms of medicinal chemistry, classified by their chemical structures, focusing on SAR and modelling studies that led to the discovery of very potent and selective agents, such as AZD- 8055, OSI-027 and INK128, already entered clinical trials, or WYE-132, Torin1 and others in preclinical studies. Also some examples of dual PI3K/mTOR inhibitors, including PI-103, GNE477, WJD008 and GSK2126458 are reported together with their biological and clinical data.

Compounds currently used for the treatment of HIV-1 Infections are targeted to viral proteins. However, the high intrinsic mutation and replication rates of HIV-1 often lead to the emergence of drug resistant strains and consequent therapeutic failure. On this basis, cellular cofactors represent attractive new targets for HIV-1 chemotherapy, since targeting a cellular factor that is required for viral replication should help to overcome the problem of viral resistance. We and others have recently reported the identification of compounds suppressing HIV-1 replication by targeting the cellular DEAD-box helicase DDX3. These results provide a proof-of-principle for the feasibility of blocking HIV-1 infection by rendering the host cell environment less favorable for the virus. The rationale for such an approach and its implications in potentially overcoming the problem of drug resistance related to drugs targeting viral proteins will be discussed in the context of the known cellular functions of the DEAD-box helicase DDX3.

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disease that affects motor neurons. Lately, this disease has often been related to the protein kinase called glycogen synthase kinase 3 (GSK-3), through the experimental evidence of alterations of this enzyme on ALS patients. Therefore, there have been several experimental studies using GSK-3 inhibitors, in cellular and animal models and also in clinical studies that showed the potential of the therapeutic role of these molecules. GSK-3 inhibitors might play a pivotal role in the pharmacology of ALS disease with no curative treatment nowadays. In this review we give an overview of the current research in the area, showing all the evidences of the implication of dysfunctional GSK-3 in this disease on one hand, and on the other presenting the potential role of the GSK-3 inhibitors as a future pharmacological ALS therapy.

Tumor blood vessels are an important emerging target for anti-cancer therapy. The antimitotic agent combretastatin A-4 (CA- 4), a cis-stilbene natural product isolated from the South African tree Combretum caffrum Kuntze, is the lead compound of a new class of anti-cancer drugs that target tumor vasculature. CA-4 inhibits tubulin polymerization by interacting at the colchicine binding site on tubulin. This alters the morphology of endothelial cells and causes vascular shutdown and regression of tumor vasculature. Some tubulin-binding vascular-disrupting agents (VDAs) are currently in clinical trials for cancer therapy. As a consequence of the potential favorable applications of these compounds, several analogs projected to induce rapid and selective vascular shutdown in tumors have been synthesized during the last few years. Many of these molecules have already been tested for their effects on tubulin polymerization as well as for their antiproliferative activity and other biological properties, and possible mechanisms of action have been investigated. The aim of the present review is to offer an overview of most recently developed combretastatin derivatives, focusing on biological effects exerted by these compounds. The published data about new analogs are presented and compared, and a detailed investigation of structure-activity relationships is described.

After years of viewing cirrhosis as the irreversible end-stage of liver fibrosis, it has been shown recently that the possibility of its reversal is no longer a dream. Several studies on experimental animal models showed possible spontaneous resolution of fibrosis after the removal of fibrogenic stimulus. Similar results were also observed in human patients with liver fibrosis due to autoimmune hepatitis and biliary etiology. However, the need for other means of treatment is urgent, especially when the removal of the causative factor is unlikely. Recent antifibrotic strategies were designed to target one or more of the three stages involved in the process of fibrosis. These are the triggering stage, fibrogenesis, and extracellular matrix accumulation. In this review, the classification of the current drugs or agents that showed inhibition of one or more of fibrosis stages with their chemical synthesis are presented.

Conventional treatment for metastatic bone pain requires a multidisciplinary approach (medical therapy, surgery, and radiation), but is primarily palliative. Biphosphonates introduced the concept of disease-modifying therapy, by effectively reducing bone pain and skeletal related events in patients suffering from bone metastatic cancer. In the past decade, the growing knowledge of bone biology and our understanding of the molecular mechanisms at the basis of the interaction between cancer cells and bone matrix led to the identification of new therapeutic targets for innovative “smart drugs”. The most investigated is the RANK/RANKL/OPG pathway, and denosumab, among novel targeted therapies, is the molecule that is in the most advanced development phase. Additional targets have been identified and potential novel therapeutic interventions, classified as inhibitors of bone resorption or stimulators of bone formation, are under preclinical and clinical evaluation. These promising targets include cathepsin K, the Src tyrosine kinases, integrins, chloride channels, the parathyroid hormone-related peptide, endotelin-1, sclerostin, and TGF-beta. Other pathways or molecules expressed by bone cells and cancer cells, such as CXCR4, GPNMB, EGF-family ligands, Wnt/DKK1, and MIP-1 alpha have recently emerged as potential targets. The aim of this review is to discuss the molecular mechanisms behind these emerging therapeutic targets in bone metastases and to give an overview of results from those in advanced clinical phases.

Bioactivation of xenobiotics can, in certain circumstances, result in the formation of reactive electrophilic species. These reactive metabolites may covalently modify proteins and macromolecules and it has been suggested that protein modification is a key initial step in provoking idiosyncratic adverse drug reactions. Understanding these bioactivation pathways is critical in order to rationally design drug candidates with a lower propensity to form reactive intermediates. Herein, we provide an overview of the importance of Structural Alerts and bioactivation pathways and describe the creation of an in-house database as a tool aimed at informing medicinal chemists about these potential liabilities.