Current Medicinal Chemistry (v.18, #18)

The enzyme phosphoinositide 3-kinase (PI3K) plays a central role in the cellular response to growth factors and receptor activationthrough activation of downstream effectors such as Akt and mTOR [1]. PI3K is involved in protein synthesis, cell proliferation, survival andmultiple drug resistance mechanisms in cancer cells [2,3]. Several signalling molecules in the PI3K/Akt pathways are frequently mutated,deleted or amplified in human cancer including PI3K, PTEN, and Akt [3]. Treatment with PI3K inhibitors as single agents can inhibit cancercell proliferation and induce apoptosis and cell death. The combination of PI3K inhibitors with other therapeutic agents has often synergisticeffects on tumor growth inhibition in experimental models and in some clinical trials [4]. The inhibition of PI3K signalling therefore providesa strong lead to improve cancer treatment. Currently, there are promising PI3K inhibitors in clinical trials to treat human cancer [5]. Theimmediate future challenge is to determine whether or how these PI3K inhibitors can be applied in a highly tumour-specific way with littleadverse effects and how to select the most sensitive patients among different genetic backgrounds.The increasing importance of PI3K-dependent signalling as a cancer target has generated a large number of therapeutics targeting thePI3K/Akt pathways. Despite the surge of expectations elicited by their promising preclinical data, preliminary clinical results obtained so farwith PI3K pathway inhibitors have not fulfilled their promises especially if considered as single-agent.Among the different explanations it is possible that this is due to insufficient inhibition of the target or to the fact that PI3K inhibition issimply not sufficient to stop tumor growth.Nevertheless, the clinical testing of PI3K pathway inhibitors is still in its infancy and requires a concerted tuning of strategies andapproaches.On this basis, novel aspects of pharmacology and drug development of PI3K pathway inhibitors are the focus of this Hot Topic issue ofCurrent Medicinal Chemistry. This is a timely topic, as there is tremendous interest in the therapeutic development of PI3K inhibitors.As a consequence of the increasing interest around PI3K pathways, during the last few years, there have been several reviews on PI3Ksignaling and on PI3K pathway inhibitors. Therefore, to avoid inevitable overlapping and repetitions I decided to concentrate the attention ofthis Hot Topic on the ultimate challenges that lay ahead in this scientific field. In order to distinguish themselves, Ciraolo et al., have focusedon describing the pharmacology of the developmental compounds, preliminary clinical findings and potential side effects based on researchmainly from genetic studies [6].Shuttleworth et al. provide a comprehensive review of the recent development of compounds targeting the class I PI3Ks and alsocompounds that target both the class I PI3-Ks and mTOR [7]. Much of the data that the authors summarize is from patents and meetingabstracts and therefore absent from the general scientific literature. The cancer stem cell hypothesis is generating a great deal of interestbecause of its potential clinical implications, as it indicates that the route for cancer eradication will require the use of strategies whichexpunge the root cause of the tumour [8]. The findings reviewed in one article of this Hot Topic, strongly suggest that increasedPI3K/Akt/mTOR signalling activity is important to regulate some of the cancer stem cells properties, including resistance to chemotherapyand radiotherapy [9].Finally, three articles focused their attention on key downstream targets of PI3K, namely Akt, mTOR and PDK1, discussing strategiesused to develop novel kinase inhibitors [10-12].Overall, I hope the articles of this Hot Topic can represent a useful instrument for the broad readership of Current Medicinal Chemistry tobecome acquainted with signal transduction pathways that have emerged as important therapeutic targets in the context of cancer and theexciting and promising small molecule discovery research in the PI3K field.

Phosphoinositide 3-kinases (PI3Ks) control key signaling pathways in cancer cells, leading to cell proliferation, survival,motility and angiogenesis. In several human cancers, activation of PI3Ks results from gain-of-function or over-expression of PI3Ksand/or hyperactivity of up- or downstream players in the pathway. As inhibition of PI3Ks and downstream targets such as mammaliantarget of rapamycin (mTOR) has been shown to reduce tumor growth in vitro and in preclinical models, several small molecule inhibitorsof PI3Ks are currently undergoing clinical trial as novel agents in cancer therapy. These drugs include inhibitors targeting all class IPI3Ks (α, β, γ, δ isoforms), compounds blocking selective PI3K isoforms and dual inhibitors active on both PI3Ks and mTOR. Herein,we summarize the pharmacology and preliminary clinical data of the main PI3K inhibitors undergoing clinical trial. We will also reviewthe preclinical studies documenting the major effects of systemic PI3K inhibition on non-cancer tissues, which have shed light onpotential side effects, caveats and limitations for PI3K blockade in patients.

Progress in the Preclinical Discovery and Clinical Development of Class I and Dual Class I/IV Phosphoinositide 3-Kinase (PI3K) Inhibitors by S.J. Shuttleworth, F.A. Silva, A.R.L. Cecil, C.D. Tomassi, T.J. Hill, F.I. Raynaud, P.A. Clarke, P. Workman (2686-2714).
The phosphoinositide 3-kinases (PI3Ks) constitute an important family of lipid kinase enzymes that control a range of cellularprocesses through their regulation of a network of signal transduction pathways, and have emerged as important therapeutic targets in thecontext of cancer, inflammation and cardiovascular diseases. Since the mid-late 1990s, considerable progress has been made in thediscovery and development of small molecule ATP-competitive PI3K inhibitors, a number of which have entered early phase humantrials over recent years from which key clinical results are now being disclosed. This review summarizes progress made to date, primarilyon the discovery and characterization of class I and dual class I/IV subtype inhibitors, together with advances that have been made intranslational and clinical research, notably in cancer.

Targeting the Phosphatidylinositol 3-Kinase/Akt/Mammalian Target of Rapamycin Signaling Network in Cancer Stem Cells by A.M. Martelli, C. Evangelisti, M.Y. Follo, G. Ramazzotti, M. Fini, R. Giardino, L. Manzoli, J.A. McCubrey, L. Cocco (2715-2726).
Cancer stem cells (CSCs) comprise a subset of hierarchically organized, rare cancer cells with the ability to initiate cancer inxenografts of genetically modified murine models. CSCs are thought to be responsible for tumor onset, self-renewal/maintenance,mutation accumulation, and metastasis. The existence of CSCs could explain the high frequency of neoplasia relapse and resistance to allof currently available therapies, including chemotherapy. The phosphatidylinositol 3-kinase (PI3K)/Akt/mammalian target of rapamycin(mTOR) signaling pathway is a key regulator of physiological cell processes which include proliferation, differentiation, apoptosis,motility, metabolism, and autophagy. Nevertheless, aberrantly upregulated PI3K/Akt/mTOR signaling characterizes many types ofcancers where it negatively influences prognosis. Several lines of evidence indicate that this signaling system plays a key role also inCSC biology. Of note, CSCs are more sensitive to pathway inhibition with small molecules when compared to healthy stem cells. Thisobservation provides the proof-of-principle that functional differences in signaling transduction pathways between CSCs and healthystem cells can be identified. Here, we review the evidence which links the signals deriving from the PI3K/Akt/mTOR network with CSCbiology, both in hematological and solid tumors. We then highlight how therapeutic targeting of PI3K/Akt/mTOR signaling with smallmolecule inhibitors could improve cancer patient outcome, by eliminating CSCs.

Protein kinase B/AKT plays a central role in cancer. The serine/threonine kinase is over-expressed or constitutively active inmany cancers and has been validated as a therapeutic target for cancer treatment. However, targeting the kinase activity has revealeditself to be a challenge due to non-selectivity of the compounds towards other kinases. This review summarizes other approachesscientists have developed to inhibit the activity and function of AKT. They consist of targeting the pleckstrin homology (PH) domain ofAKT. Indeed, upon the generation of 3-phosphorylated phosphatidylinositol phosphates (PI3Ps) by PI3-kinase (PI3K), AKT translocatesfrom the cytosol to the plasma membrane and binds to the PI3Ps via its PH domain. Thus, several analogs of PI3Ps (PI Analogs or PIAs),alkylphospholipids (APLs), such as edelfosine, and inositol phosphates (IPs) have been described that inhibit the binding of the PHdomain to PI3Ps. Recently allosteric inhibitors and small molecules that do not bind the kinase domain but affect the kinase activity ofAKT, presumably by interacting with the PH domain, have also been identified. Finally, several drug screening studies spawned novelchemical scaffolds that bind the PH domain of AKT. Together, these approaches have been more or less successful in vitro and to someextent translated in preclinical studies. Several of these new AKT PH domain inhibitors exhibit promising anti-tumor activity in mousemodels and some of them show synergy with ionizing radiation and chemotherapy. Early clinical trials have started and results will attestto the validity and efficacy of such approaches in the near future.

mTOR Inhibitors: Facing New Challenges Ahead by I. Mavrommati, T. Maffucci (2743-2762).
The enzyme mammalian target of rapamycin (mTOR) is a master kinase that regulates several critical intracellular processes.It is now well established that this enzyme has a key role in cancer and its inhibition as therapeutic anti-cancer strategy is wellrecognised. Several clinical trials using mTOR inhibitors have been and are currently being performed. A huge scientific literature existsnot only reporting the results of these trials but also discussing the reasons for the limited efficacy of strategies used so far and the needfor new strategies to overcome the problem of resistance. The aim of this review is mainly to reflect on how the complexity of themTOR-dependent signalling pathway and our difficulty to untangle it can ultimately affect the development of proper strategies to fullyexploit the potential benefits of mTOR inhibition as anti-cancer strategy.

Targeting PDK1 in Cancer by C. Raimondi, M. Falasca (2763-2769).
Abnormal activation of phosphoinositide 3-kinase (PI3K) signalling is very common in cancer, leading to deregulation ofseveral intracellular processes normally controlled by this enzyme, including cell survival, growth, proliferation and migration. Mutationsin the gene encoding the tumour suppressor phosphatase and tensin homologue deleted on chromosome 10 (PTEN), which leads touncontrolled activation of the PI3K pathway, are reported in different cancers. Among the downstream effectors of PI3Ks, 3-phosphoinositide-dependent protein kinase 1 (PDK1) and protein kinase B (PKB)/Akt have a key role in several cancer types. Morerecent data indicate that alteration of PDK1 is a critical component of oncogenic PI3K signalling in breast cancer, suggesting thatinhibition of PDK1 can inhibit breast cancer progression. PDK1 has an essential role in regulating cell migration especially in the contextof PTEN deficiency. Downregulation of PDK1 levels inhibits migration and experimental metastasis of human breast cancer cells. PDK1activates a large number of proteins, including Akt, some PKC isoforms, S6K and SGK. Data also reveal that PDK1 is oncogenic and thisis dependent on PI3K pathway. Therefore, accumulating evidence demonstrates that PDK1 is a valid therapeutic target and suggests thatPDK1 inhibitors may be useful to prevent cancer progression and abnormal tissue dissemination. This review will focus on publisheddata on the role of PDK1 in cancer and approaches used to inhibit PDK1.

Background: Subtilisin Kexin Isozyme-1 (SKI-1)/Site1Protease (S1P) is a Ca+2-dependent membrane bound pyrolysin-typeserine protease of mammalian subtilase super family Proprotein Convertases (PCs)/Proprotein Convertase Subtilisin Kexins (PCSKs). Itcleaves precursor proteins at the carboxy terminus of a non basic amino acid characterized by the sequence Arg/Lys-..-..-Leu/Ser/Thr..,where .. = any amino acid except Cys, .. = the alkyl side chain containing hydrophobic amino acid. SKI-1 cleaves pro-BDNF, pro-SREBP2, pro-ATF6, pro-somatostatin and viral glycoproteins to generate their active forms. As a result SKI-1 plays important roles incartilage development, bone mineralization, cholesterol metabolism, fatty acid synthesis and infections caused by Arina viruses ofhemorrhagic type. Interest has grown to develop inhibitors of SKI-1 that may find useful therapeutic and biochemical applications.Objective: Our objective is to develop small molecule inhibitors of SKI-1/S1P and study their kinetic and biochemical properties.Results: Peptide analogs were designed by inserting a protease resistant methylene-oxy (-CH2-O-) pseudoamide function at the cleavagesite of 251Asp-Ile-Tyr-Ile-Ser-Arg-Arg-Leu-Leu..Gly-Thr-Phe-Thr263, derived from SKI-1 processing site of Lassa virus glycoprotein. Thesynthesis was conducted by substituting Leu-Gly with previously made Leu-CH2-O-Gly. Flexible linear and conformationallyconstrained circular and disulphide bridged cyclic peptides were prepared by solid phase method. Circular and cyclic peptides inhibitedSKI-1 more potently (Ki~14-20 μM) than the corresponding acyclic peptide (Ki~51 μM). They also blocked SKI-1-mediated processingof pro-h(human)SREBP2 into its mature form in HepG2 cells. Circular pseudopeptides designed from hATF6 and hSREBP2 alsoinhibited SKI-1. This is the first report of circular and cyclic ..(CH2-O) containing peptides as SKI-1 inhibitors with potential therapeuticapplications in cholesterol synthesis.

Serotonin Receptors of Type 6 (5-HT6): What can we Expect from them? by D. Marazziti, S. Baroni, M. Catena Dell'Osso, F. Bordi, F. Borsini (2783-2790).
The serotonin (5-HT) receptors of type 6 (5-HT6) are relatively new. They are quite different from all other 5-HT receptors, asthey are characterized by a short third cytoplasmatic loop and a long C-terminal tail, and contain one intron located in the middle of thethird cytoplasmatic loop. After some initial controversies, the available findings are now apparently more congruent. Nevertheless,discrepancies still exist, such as those in binding affinity, effects of 5-HT6 ligands on brain catecholamines and behavioral syndromesmediated by them. Much interest in 5-HT6 receptors was triggered by the evidence that some antipsychotics could bind to them.Subsequently, despite the lack of complete information on metabolic patterns of the various compounds, some of 5-HT6 receptor ligandsentered the clinical development as potential anti-dementia, antipsychotic and anti-obese drugs. In any case, the available information onthe pharmacology of 5-HT6 receptors is still quite scant. Therefore, with the present paper we aimed at reporting a comprehensive reviewon the status of art of the 5-HT6 receptors, while highlighting the potential clinical applications of 5-HT6 receptor agonists/antagonists.

Adenosine in the Inflamed Gut: A Janus Faced Compound by A.B. Estrela, W.-R. Abraham (2791-2815).
The purine ribonucleoside adenosine (Ado) has been recognized for its regulatory functions in situations of cellular stress likeischemia, hypoxia and inflammation. The importance of extracellular Ado as a modulator in the immune system is a theme of greatappreciation and the focus of recent increasing interest in the field of gastrointestinal inflammation. In this review, the different aspects ofAdo signaling during inflammatory responses in the gut are discussed, considering the contribution of the four known Ado receptors(ARs; A1, A2A, A2B, and A3), their mechanisms and expression patterns. Activation of these receptors in epithelial cells as well as inimmune cells recruited to the inflamed intestinal mucosa determines the overall effect, ranging from a protective, anti-inflammatorymodulation to a strong pro-inflammatory induction. Here we present the current advances in agonists and antagonists development andtheir potential therapeutic application studied in animal models of intestinal inflammation. In addition, alternative complementaryapproaches to manipulate such a complex signaling system are discussed, for example, the use of AR allosteric modulators orinterference with Ado metabolism. Special features of the gut environment are taken into account: the contribution of diet components;the involvement of Ado in intestinal infections; the interactions with the gut microbiome, particularly, the recent exciting finding that anintestinal bacterium can directly produce extracellular Ado in response to host defense mechanisms in an inflammation scenario.Understanding each component of this dynamic system will broaden the possibilities for applying Ado signaling as a therapeutic target ingut inflammation.

Molecular Chaperone Hsp90 as a Target for Oxidant-Based Anticancer Therapies by R. Beck, N. Dejeans, C. Glorieux, R.C. Pedrosa, D. Vasquez, J.A. Valderrama, P.B. Calderon, J. Verrax (2816-2825).
Hsp90 is a molecular chaperone involved in the stabilization of many oncoproteins that are required for the acquisition andmaintenance of the so-called six major hallmarks of cancer cells. Various strategies have, therefore, been developed to inhibit thechaperone activity of Hsp90 and induce cancer cell death through the destabilization of its client proteins. Among these strategies, wehave shown that generation of oxidative stress leads to the cleavage and deactivation of Hsp90. Because cancer cells are often deficient inantioxidant enzymes and exhibit higher basal levels of reactive oxygen species (ROS) than their normal counterparts, inducing a selectiveoxidative stress may be a promising approach for cancer treatment. Thus, many redox-modulating agents have, therefore, been developedor are undergoing clinical trials and Hsp90 represents a new target for oxidative stress-generating agents. The purpose of this article is toreview the current state of knowledge about Hsp90 and the use of oxidative stress-generating agents in cancer treatment. We willillustrate the review with some of our results concerning the effects of oxidative stress on Hsp90 using various oxidative stress-generatingsystems based on different quinones in combination with a well-known reducing agent (i.e., ascorbate). Our results show that oxidativestress provokes the cleavage of Hsp90 in CML cells, as well as the degradation of its client protein Bcr-Abl and the deactivation of itsdownstream signaling pathways, namely MAPK and STAT5. Overall, these results highlight the potential interest of using oxidativestress to target Hsp90.