Studies on the mechanism of phosphatidylinositol 3-kinase inhibition by wortmannin and related analogs

Emerging and Re-emerging Warheads for Targeted Covalent Inhibitors: An Update

Covalent inhibitors and other types of covalent modalities have seen a revival in the past two decades, with a variety of new targeted covalent drugs having been approved in recent years. A key feature of such molecules is an intrinsically reactive group, typically a weak electrophile, which enables the irreversible or reversible formation of a covalent bond with a specific amino acid of the target protein. This reactive group, often called the "warhead", is a critical determinant of the ligand's activity, selectivity, and general biological properties. In 2019, we summarized emerging and re-emerging warhead chemistries to target cysteine and other amino acids (Gehringer, M.; Laufer, S. A. J. Med. Chem. 2019, 62, 5673-5724; DOI: 10.1021/acs.jmedchem.8b01153). Since then, the field has rapidly evolved. Here we discuss the progress on covalent warheads made since our last Perspective and their application in medicinal chemistry and chemical biology.

Intracellular remodeling associated with endoplasmic reticulum stress modifies biomechanical compliance of bladder cells

Bladder cells face a challenging biophysical environment: mechanical cues originating from urine flow and regular contraction to enable the filling voiding of the organ. To ensure functional adaption, bladder cells rely on high biomechanical compliance, nevertheless aging or chronic pathological conditions can modify this plasticity. Obviously the cytoskeletal network plays an essential role, however the contribution of other, closely entangled, intracellular organelles is currently underappreciated. The endoplasmic reticulum (ER) lies at a crucial crossroads, connected to both nucleus and cytoskeleton. Yet, its role in the maintenance of cell mechanical stability is less investigated. To start exploring these aspects, T24 bladder cancer cells were treated with the ER stress inducers brefeldin A (10-40nM BFA, 24 h) and thapsigargin (0.1-100nM TG, 24 h). Without impairment of cell motility and viability, BFA and TG triggered a significant subcellular redistribution of the ER; this was associated with a rearrangement of actin cytoskeleton. Additional inhibition of actin polymerization with cytochalasin D (100nM CytD) contributed to the spread of the ER toward cell periphery, and was accompanied by an increase of cellular stiffness (Young´s modulus) in the cytoplasmic compartment. Shrinking of the ER toward the nucleus (100nM TG, 2 h) was related to an increased stiffness in the nuclear and perinuclear areas. A similar short-term response profile was observed also in normal human primary bladder fibroblasts. In sum, the ER and its subcellular rearrangement seem to contribute to the mechanical properties of bladder cells opening new perspectives in the study of the related stress signaling cascades. Video Abstract.

Semisynthetic LC3 Probes for Autophagy Pathways Reveal a Noncanonical LC3 Interacting Region Motif Crucial for the Enzymatic Activity of Human ATG3

Macroautophagy is one of two major degradation systems in eukaryotic cells. Regulation and control of autophagy are often achieved through the presence of short peptide sequences called LC3 interacting regions (LIR) in autophagy-involved proteins. Using a combination of new protein-derived activity-based probes prepared from recombinant LC3 proteins, along with protein modeling and X-ray crystallography of the ATG3-LIR peptide complex, we identified a noncanonical LIR motif in the human E2 enzyme responsible for LC3 lipidation, ATG3. The LIR motif is present in the flexible region of ATG3 and adopts an uncommon β-sheet structure binding to the backside of LC3. We show that the β-sheet conformation is crucial for its interaction with LC3 and used this insight to design synthetic macrocyclic peptide-binders to ATG3. CRISPR-enabled in cellulo studies provide evidence that LIR^ATG3 is required for LC3 lipidation and ATG3∼LC3 thioester formation. Removal of LIR^ATG3 negatively impacts the rate of thioester transfer from ATG7 to ATG3.

Astaxanthin Inhibits Matrix Metalloproteinase Expression by Suppressing PI3K/AKT/mTOR Activation in Helicobacter pylori-Infected Gastric Epithelial Cells

Helicobacter pylori (H. pylori) increases production of reactive oxygen species (ROS) and activates signaling pathways associated with gastric cell invasion, which are mediated by matrix metalloproteinases (MMPs). We previously demonstrated that H. pylori activated mitogen-activated protein kinase (MAPK) and increased expression of MMP-10 in gastric epithelial cells. MMPs degrade the extracellular matrix, enhancing tumor invasion and cancer progression. The signaling pathway of phosphatidylinositol 3-kinase (PI3K)/serine/threonine protein kinase B (AKT)/mammalian target of rapamycin (mTOR) is associated with MMP expression. ROS activates PIK3/AKT/mTOR signaling in cancer. Astaxanthin, a xanthophyll carotenoid, shows antioxidant activity by reducing ROS levels in gastric epithelial cells infected with H. pylori. This study aimed to determine whether astaxanthin inhibits MMP expression, cell invasion, and migration by reducing the PI3K/AKT/mTOR signaling in H. pylori-infected gastric epithelial AGS cells. H. pylori induced PIK3/AKT/mTOR and NF-κB activation, decreased IκBα, and induced MMP (MMP-7 and -10) expression, the invasive phenotype, and migration in AGS cells. Astaxanthin suppressed these H. pylori-induced alterations in AGS cells. Specific inhibitors of PI3K, AKT, and mTOR reversed the H. pylori-stimulated NF-κB activation and decreased IκBα levels in the cells. In conclusion, astaxanthin suppressed MMP expression, cell invasion, and migration via inhibition of PI3K/AKT/mTOR/NF-κB signaling in H. pylori-stimulated gastric epithelial AGS cells.

Mechanisms of Resistance to PI3K Inhibitors in Cancer: Adaptive Responses, Drug Tolerance and Cellular Plasticity

The phosphatidylinositol-3-kinase (PI3K) pathway plays a central role in the regulation of several signalling cascades which regulate biological processes such as cellular growth, survival, proliferation, motility and angiogenesis. The hyperactivation of this pathway is linked to tumour progression and is one of the most common events in human cancers. Additionally, aberrant activation of the PI3K pathway has been demonstrated to limit the effectiveness of a number of anti-tumour agents paving the way for the development and implementation of PI3K inhibitors in the clinic. However, the overall effectiveness of these compounds has been greatly limited by inadequate target engagement due to reactivation of the pathway by compensatory mechanisms. Herein, we review the common adaptive responses that lead to reactivation of the PI3K pathway, therapy resistance and potential strategies to overcome these mechanisms of resistance. Furthermore, we highlight the potential role in changes in cellular plasticity and PI3K inhibitor resistance.

Fisetin Attenuates Myocardial Ischemia-Reperfusion Injury by Activating the Reperfusion Injury Salvage Kinase (RISK) Signaling Pathway

Ischemia-reperfusion (I/R) injury is an unavoidable injury that occurs during revascularization procedures. In the previous study, we reported that fisetin is a natural flavonoid that attenuates I/R injury by suppressing mitochondrial oxidative stress and mitochondrial dysfunction. Though fisetin is reported as a GSK3β inhibitor, it remains unclear whether it attenuates myocardial ischemia by activating the phosphoinositide 3-kinase/protein kinase B (PI3K/Akt) pathway, thereby inhibiting the downstream GSK3β, or by directly interacting with GSK3β while rendering its cardioprotection. In this study, the research team investigates the possible mechanism of action of fisetin while rendering its cardioprotective effect against myocardial I/R injury in rats. For this investigation, the team utilized two myocardial I/R models: Ligation of the left anterior descending artery and Langendorff isolated heart perfusion system. The latter has no neurohormonal influences. The PI3K inhibitor (Wortmannin, 0.015 mg/kg), GSK3β inhibitor (SB216763, 0.7 mg/kg), and fisetin (20 mg/kg) were administered intraperitoneally before inducing myocardial I/R. The result of this study reveals that the administration of fisetin decreases the myocardial infarct size, apoptosis, lactate dehydrogenase, and creatine kinase in serum\perfusate of the rat hearts subjected to I/R. However, the inhibition of PI3K with Wortmannin significantly reduced the cardioprotective effect of fisetin both in the ex vivo and vivo models. The administration of GSK3β inhibitor after the administration of fisetin and Wortmannin, re-establishing the cardioprotection, indicates the major role of PI3K in fisetin action. Changes in myocardial oxidative stress (level) and mitochondrial functional preservation of interfibrillar and subsarcolemmal mitochondria support the above findings. Hence, the team here reports that fisetin conferred its cardioprotection against I/R injury by activating the PI3K/Akt/GSK3β signaling pathway in rat hearts.

Recent progress in the synthesis of the furanosteroid family of natural products

This review focuses on an overview of recent advances in the synthesis of furanosteroids and illustrates their applications in medicinal chemistry over the period of 2005–present.

Rapid Access to Tetracyclic Core of Wortmannin via an Intramolecular Reductive Olefin Coupling Strategy

A convergent approach to assemble the fused BCDE tetracyclic framework of wortmannin is presented. This route features a very challenging Suzuki-Miyaura coupling to prepare the fully functionalized furan intermediate, a Negishi-type acylation to unite the two enantio-enriched fragments, and a subsequent hydrogen-atom-transfer-initiated 6-endo radical cyclization to install the central cyclohexadienone moiety, which establishes the C10 all-carbon quaternary stereocenter.

Synaptically driven phosphorylation of ribosomal protein S6 is differentially regulated at active synapses versus dendrites and cell bodies by MAPK and PI3K/mTOR signaling pathways

High-frequency stimulation of the medial perforant path triggers robust phosphorylation of ribosomal protein S6 (rpS6) in activated dendritic domains and granule cell bodies. Here we dissect the signaling pathways responsible for synaptically driven rpS6 phosphorylation in the dentate gyrus using pharmacological agents to inhibit PI3-kinase/mTOR and MAPK/ERK-dependent kinases. Using phospho-specific antibodies for rpS6 at different sites (ser235/236 versus ser240/244), we show that delivery of the PI3-kinase inhibitor, wortmannin, decreased rpS6 phosphorylation throughout the somatodendritic compartment (granule cell layer, inner molecular layer, outer molecular layer), especially in granule cell bodies while sparing phosphorylation at activated synapses (middle molecular layer). In contrast, delivery of U0126, an MEK inhibitor, attenuated rpS6 phosphorylation specifically in the dendritic laminae leaving phosphorylation in the granule cell bodies intact. Delivery of the mTOR inhibitor, rapamycin, abolished activation of rpS6 phosphorylation in granule cell bodies and dendrites, whereas delivery of a selective S6K1 inhibitor, PF4708671, or RSK inhibitor, SL0101-1, attenuated rpS6 phosphorylation throughout the postsynaptic cell. These results reveal that MAPK/ERK-dependent signaling is predominately responsible for the selective induction of rpS6 phosphorylation at active synapses. In contrast, PI3-kinase/mTOR-dependent signaling induces rpS6 phosphorylation throughout the somatodendritic compartment but plays a minimal role at active synapses. Collectively, these results suggest a potential mechanism by which PI3-kinase/mTOR and MAPK/ERK pathways regulate translation at specific subcellular compartments in response to synaptic activity.

Strategies for the Synthesis of the Halenaquinol and Xestoquinol Families of Natural Products

The halenaquinol family of naphthoquinol natural products includes a few closely related polycyclic compounds that feature an activated, electrophilic furan ring. This motif is likely responsible for the rich biological activity attributed to these secondary metabolites. Their interesting structures-related via their electrophilic furan to the biologically important furanosteroids-and their activities prompted significant efforts by organic chemists that resulted in many strategically compelling laboratory syntheses of these targets. These different strategies are compared and contrasted in this Microreview, and the authors' recent work on the structurally different but biogenetically related natural product exiguaquinol is put into the context of the previous studies on halenaquinol-type targets.

Covalent Modifiers: A Chemical Perspective on the Reactivity of α,β-Unsaturated Carbonyls with Thiols via Hetero-Michael Addition Reactions

Although Michael acceptors display a potent and broad spectrum of bioactivity, they have largely been ignored in drug discovery because of their presumed indiscriminate reactivity. As such, a dearth of information exists relevant to the thiol reactivity of natural products and their analogues possessing this moiety. In the midst of recently approved acrylamide-containing drugs, it is clear that a good understanding of the hetero-Michael addition reaction and the relative reactivities of biological thiols with Michael acceptors under physiological conditions is needed for the design and use of these compounds as biological tools and potential therapeutics. This Perspective provides information that will contribute to this understanding, such as kinetics of thiol addition reactions, bioactivities, as well as steric and electronic factors that influence the electrophilicity and reversibility of Michael acceptors. This Perspective is focused on α,β-unsaturated carbonyls given their preponderance in bioactive natural products.

A role for the metalloprotease invadolysin in insulin signaling and adipogenesis

Invadolysin is a novel metalloprotease conserved amongst metazoans that is essential for life in Drosophila. We previously showed that invadolysin was essential for the cell cycle and cell migration, linking to metabolism through a role in lipid storage and interaction with mitochondrial proteins. In this study we demonstrate that invadolysin mutants exhibit increased autophagy and decreased glycogen storage - suggestive of a role for invadolysin in insulin signaling in Drosophila. Consistent with this, effectors of insulin signaling were decreased in invadolysin mutants. In addition, we discovered that invadolysin was deposited on newly synthesized lipid droplets in a PKC-dependent manner. We examined two in vitro models of adipogenesis for the expression and localization of invadolysin. The level of invadolysin increased during both murine 3T3-L1 and human Simpson-Golabi-Behmel syndrome (SGBS), adipogenesis. Invadolysin displayed a dynamic localization to lipid droplets over the course of adipogenesis, which may be due to the differential expression of distinct invadolysin variants. Pharmacological inhibition of adipogenesis abrogated the increase in invadolysin. In summary, our results on in vivo and in vitro systems highlight an important role for invadolysin in insulin signaling and adipogenesis.

Thiol Probes To Detect Electrophilic Natural Products Based on Their Mechanism of Action

New methods are urgently needed to find novel natural products as structural leads for the development of new drugs against emerging diseases such as cancer and multiresistant bacterial infections. Here we introduce a reactivity-guided drug discovery approach for electrophilic natural products, a therapeutically relevant class of natural products that covalently modify their cellular targets, in crude extracts. Using carefully designed halogenated aromatic reagents, the process furnishes derivatives that are UV-active and highly conspicuous via mass spectrometry by virtue of an isotopically unique bromine or chlorine tag. In addition to the identification of high-value metabolites, the process facilitates the difficult task of structure elucidation by providing derivatives that are primed for X-ray crystallographic analysis. We show that a cysteine probe efficiently and chemoselectively labels enone-, β-lactam-, and β-lactone-based electrophilic natural products (parthenolide, andrographolide, wortmannin, penicillin G, salinosporamide), while a thiophenol probe preferentially labels epoxide-based electrophilic natural products (triptolide, epoxomicin, eponemycin, cyclomarin, salinamide). Using the optimized method, we were able to detect and isolate the epoxide-bearing natural product tirandalydigin from Salinispora and thereby link an orphan gene cluster to its gene product.

Selective anticancer agents suppress aging in Drosophila

Mutations of the PI3K, TOR, iNOS, and NF-κB genes increase lifespan of model organisms and reduce the risk of some aging-associated diseases. We studied the effects of inhibitors of PI3K (wortmannin), TOR (rapamycin), iNOS (1400W), NF-κB (pyrrolidin dithiocarbamate and QNZ), and the combined effects of inhibitors: PI3K (wortmannin) and TOR (rapamycin), NF-κB (pyrrolidin dithiocarbamates) and PI3K (wortmannin), NF-κB (pyrrolidine dithiocarbamates) and TOR (rapamycin) on Drosophila melanogaster lifespan and quality of life (locomotor activity and fertility). Our data demonstrate that pharmacological inhibition of PI3K, TOR, NF-κB, and iNOS increases lifespan of Drosophila without decreasing quality of life. The greatest lifespan expanding effect was achieved by a combination of rapamycin (5 μM) and wortmannin (5 μM) (by 23.4%). The bioinformatic analysis (KEGG, REACTOME.PATH, DOLite, and GO.BP) showed the greatest aging-suppressor activity of rapamycin, consistent with experimental data.

Rationale-based therapeutic combinations with PI3K inhibitors in cancer treatment

The PI3K/AKT/mTOR signaling is important for cell proliferation, survival, and metabolism. Hyperactivation of this pathway is one of the most common signaling abnormalities observed in cancer and a substantial effort has recently been made to develop molecules targeting this signaling cascade. However, it is becoming evident that PI3K inhibitors used as single agents do not elicit dramatic or durable responses. Given the numerous mechanisms mediating intrinsic and acquired resistance to these agents, hypothesis-based combinatorial strategies are probably needed to fully exploit their antitumor activity. In the first part of this review, we briefly dissect the PI3K/AKT/mTOR axis and list the most advanced compounds targeting different nodes of this cascade. The second part focuses on what we believe to be the most promising rationale-based therapeutic combinations with PI3K/AKT/mTOR inhibitors in solid tumors, with special emphasis on breast cancer.

Improved synthesis of 17β-hydroxy-16α-iodo-wortmannin, 17β-hydroxy-16α-iodoPX866, and the [(131)I] analogue as useful PET tracers for PI3-kinase

An improved method for the synthesis of 17β-hydroxy-16α-iodo-wortmannin along with the first synthesis of 17β-hydroxy-16α-iodoPX866 and [(131)I] radiolabeled 17β-hydroxy-16α-[(131)I]iodo-wortmannin, as potential PET tracers for PI3K was also described. The differences between wortmannin and its iodo analogue were compared by covalently docking each structure to L833 in PI3K.

Viridin analogs derived from steroidal building blocks

Naturally occurring furanosteroids such as viridin and wortmannin have long been known as potent inhibitors of the lipid kinase PI-3K. We have been interested in directly accessing analogs of these complex natural products from abundant steroid feedstock materials. In this communication, we describe the synthesis of viridin/wortmannin hybrid molecules from readily available building blocks that function as PI-3K inhibitors and maintain their electrophilic properties. The compounds also show anti-proliferative effects against a breast cancer line.

Design, synthesis, and evaluation of inhibitors of pyruvate phosphate dikinase

Pyruvate phosphate dikinase (PPDK) catalyzes the phosphorylation reaction of pyruvate that forms phosphoenolpyruvate (PEP) via two partial reactions: PPDK + ATP + P(i) → PPDK-P + AMP + PP(i) and PPDK-P + pyruvate → PEP + PPDK. Based on its role in the metabolism of microbial human pathogens, PPDK is a potential drug target. A screen of substances that bind to the PPDK ATP-grasp domain active site revealed that flavone analogues are potent inhibitors of the Clostridium symbiosum PPDK. In silico modeling studies suggested that placement of a 3–6 carbon-tethered ammonium substituent at the 3′- or 4′-positions of 5,7-dihydroxyflavones would result in favorable electrostatic interactions with the PPDK Mg-ATP binding site. As a result, polymethylene-tethered amine derivatives of 5,7-dihydroxyflavones were prepared. Steady-state kinetic analysis of these substances demonstrates that the 4′-aminohexyl-5,7-dyhydroxyflavone 10 is a potent competitive PPDK inhibitor (K(i) = 1.6 ± 0.1 μM). Single turnover experiments were conducted using 4′-aminopropyl-5,7-dihydroxyflavone 7 to show that this flavone specifically targets the ATP binding site and inhibits catalysis of only the PPDK + ATP + P(i) → PPDK-P + AMP PP(i) partial reaction. Finally, the 4′-aminopbutyl-5,7-dihydroxyflavone 8 displays selectivity for inhibition of PPDK versus other enzymes that utilize ATP and NAD.

Inhibition of autophagy as a strategy to augment radiosensitization by the dual phosphatidylinositol 3-kinase/mammalian target of rapamycin inhibitor NVP-BEZ235

We investigated the effect of 2-methyl-2-{4-[3-methyl-2-oxo-8-(quinolin-3-yl)-2,3-dihydro-1H-imidazo[4,5-c]quinolin-1-yl]phenyl} propanenitrile (NVP-BEZ235) (Novartis, Basel Switzerland), a dual phosphatidylinositol 3-kinase (PI3K)/mammalian target of rapamycin (mTOR) inhibitor currently being tested in phase I clinical trials, in radiosensitization. NVP-BEZ235 radiosensitized a variety of cancer cell lines, including SQ20B head and neck carcinoma cells and U251 glioblastoma cells. NVP-BEZ235 also increased in vivo radiation response in SQ20B xenografts. Knockdown of Akt1, p110α, or mTOR resulted in radiosensitization, but not to the same degree as with NVP-BEZ235. NVP-BEZ235 interfered with DNA damage repair after radiation as measured by the CometAssay and resolution of phosphorylated H2A histone family member X foci. NVP-BEZ235 abrogated the radiation-induced phosphorylation of both DNA-dependent protein kinase catalytic subunit (DNA-PKcs) and ataxia telangiectasia mutated. Knockdown of either p110α or mTOR failed to decrease the phosphorylation of DNA-PKcs, suggesting that the effect of the drug was direct rather than mediated via p110α or mTOR. The treatment of cells with NVP-BEZ235 also promoted autophagy. To assess the importance of this process in radiosensitization, we used the autophagy inhibitors 3-methyladenine and chloroquine and found that either drug increased cell killing after NVP-BEZ235 treatment and radiation. Knocking down the essential autophagy proteins autophagy related 5 (ATG5) and beclin1 increased NVP-BEZ235-mediated radiosensitization. Furthermore, NVP-BEZ235 radiosensitized autophagy-deficient ATG5(-/-) fibroblasts to a greater extent than ATG5(+/+) cells. We conclude that NVP-BEZ235 radiosensitizes cells and induces autophagy by apparently distinct mechanisms. Inhibiting autophagy via pharmacologic or genetic means increases radiation killing after NVP-BEZ235 treatment; hence, autophagy seems to be cytoprotective in this situation. Our data offer a rationale for combining NVP-BEZ235 along with an autophagy inhibitor (i.e., chloroquine) and radiation in future clinical trials.

Hepatocyte entry leads to degradation of autoreactive CD8 T cells

Although most self-reactive T cells are eliminated in the thymus, mechanisms to inactivate or control T cells specific for extrathymic antigens are required and exist in the periphery. By investigating the site in which autoreactive T cells are tolerized, we identify a unique mechanism of peripheral deletion in which naïve autoreactive CD8 T cells are rapidly eliminated in the liver after intrahepatic activation. T cells actively invade hepatocytes, enter endosomal/lysosomal compartments, and are degraded. Blockade of this process leads to accumulation of autoreactive CD8 T cells in the liver and breach of tolerance, with the development of autoimmune hepatitis. Cell into cell invasion, or emperipolesis, is a long-observed phenomenon for which a physiological role has not been previously demonstrated. We propose that this "suicidal emperipolesis" is a unique mechanism of autoreactive T-cell deletion, a process critical for the maintenance of tolerance.

Discovery of novel anticancer therapeutics targeting the PI3K/Akt/mTOR pathway

BACKGROUND

Among promising targeted therapies for cancer treatment, phosphatidylinositol 3-kinase pathway inhibitors have in the last 3 years continued to retain the attention of both academic institutions and pharmaceutical companies. The large amount of published clinical and preclinical data has indeed confirmed the preponderant role of this so-called survival pathway for tumor maintenance.

DISCUSSION

Global efforts have, therefore, been deployed that have led to the genesis of a panoply of small molecule inhibitors. This review will focus on updating the reader on the current medicinal chemistry efforts targeting this pathway.

CONCLUSIONS

Recent discoveries important for patient stratification, quantification of target modulation in humans and combination therapies will be presented and discussed.

PI3Kα inhibitors that inhibit metastasis

Previous genetic analyses have suggested that mutations of the genes encoding PI3Kα facilitate invasion and metastasis but have less effect on primary tumor growth. These findings have major implications for therapeutics but have not been factored into pre-clinical drug development designs. Here we show that the inhibition of PI3Kα by newly designed small molecule inhibitors prevented metastasis formation in mice but had much less effect on the growth of subcutaneous xenografts or primary intra-abdominal tumors. These data support the idea that PI3Kα plays an important role in the metastatic process and suggest a more informed strategy for selecting drugs worthy of further development for clinical application.

Docking studies on isoform-specific inhibition of phosphoinositide-3-kinases

Phosphatidylinositol 3-kinase α (PI3Kα) is a promising target for anticancer drug design. Oncogenic mutation H1047R in the catalytic domain is observed in many tumors and may enhance PI3Kα kinase activity by affecting loop confirmations as well as membrane binding. We applied docking methods to 33 PI3K inhibitors against the wild type (wt) PI3Kα, the H1047R mutant of PI3Kα and the γ isoform of PI3K (PI3Kγ). We also investigated the effect of protein flexibility on ligand binding by docking the same set of ligands to conformations of the wt and mutant PI3Kα generated by molecular dynamics simulations. Our data suggests that conformational differences in Gln859, Ser854, Tyr836, and Ser774 between the PI3Kα wt and H1047R mutant may be used to design ligands that are active against both the wt and H1047R mutant isoforms. Gln859, Ser854 and Ser774 may play critical roles in ligand binding to the α isoform H1047R mutant while formation of H-bonds with Ser806 of PI3Kγ may enhance γ-isoform-specific inhibition. In addition to H-bond interactions, structural and size differences in the activation and hydrophobic domains of PI3Kα, PI3Kγ, and the PI3Kα H1047R mutant could be exploited to direct the design of isoform- and/or mutant-specific PI3K inhibitors. Our data provide a reasonable explanation for the activity and selectivity of small molecular PI3K inhibitors and are in good agreement with available experimental and computational data.

Synthesis and SAR of novel 4-morpholinopyrrolopyrimidine derivatives as potent phosphatidylinositol 3-kinase inhibitors

Significant evidence suggests that deregulation of the PI3K/Akt pathway is important in tumor progression. Mechanisms include loss of function of the tumor suppressor PTEN and high frequency of mutation of the PI3K p110alpha isoform in human malignancies. This connection between PI3K and tumor genesis makes PI3K a promising target for cancer treatment. A series of 4-morpholinopyrrolopyrimidine derivatives were synthesized and evaluated as inhibitors of PI3Kalpha and mTOR, leading to the discovery of PI3Kalpha selective inhibitors (e.g., 9) and dual PI3Kalpha/mTOR kinase inhibitors (e.g., 46 and 48). PI3Kalpha/mTOR dual inhibitors demonstrated inhibition of tumor cell growth in vitro and in vivo and caused suppression of the pathway specific biomarkers [e.g., the phosphorylation of Akt at Thr308 (T308) and Ser473 (S473)] in the human breast cancer cell line MDA361. In addition, compound 46 demonstrated good in vivo efficacy in the MDA361 human breast tumor xenograft model.

Inhibitors of phosphatidylinositol-3-kinase in cancer therapy

The phosphatidylinositol-3-kinase (PI3K) signaling pathway is implicated in multiple aspects of tumorigenesis and tumor maintenance, and recent years have seen significant efforts towards developing agents to inhibit the pathway. However, the development of such agents raises issues such as what specific member or members in the PI3K family should be inhibited to achieve maximal therapeutic benefit, and can specific inhibitors be developed with the necessary pharmacologic properties to allow them to proceed to clinical trials? The number of PI3K inhibitors has gone from a handful of archetypal inhibitors which largely determined how the pathway was initially defined through their inhibition of PI3K, but also due to their off target properties, to a much larger number of inhibitors of not only PI3K but also other members of the PI3K family. The question remains to be answered whether greater therapeutic efficacy will be obtained through the use of inhibitors with increased specificity, or through inhibitors that target a spectrum of targets within the pathway. This review will cover the development of agents targeting the pathway, and will discuss current issues surrounding the development of such agents.

Akt signaling pathway: a target for radiosensitizing human malignant glioma

Radiation therapy plays a central role in the treatment of glioblastoma, but it is not curative due to the high tumor radioresistance. Phosphatidyl-inositol 3-kinase/protein kinase B (Akt) and Janus kinase (JAK)/signal transducer and activator of transcription 3 (STAT3) pathways serve to block the apoptosis process, keeping cells alive in very toxic environments such as chemotherapy or ionizing radiation. In the present study, from a panel of 8 human malignant glioma cell lines, investigations on the relationship between intrinsic radioresistance and Akt or STAT3 basal activation were done. Secondly, the impact of down-modulation of Akt or STAT3 signaling on in vitro intrinsic radiosensitivity was evaluated. Using a clonogenic cell survival assay, our results revealed a significant correlation between the basal Akt activation and the surviving fraction at 2 Gy (SF2). In contrast, no correlation was found between STAT3 activation and SF2. According to this, down-modulation of Akt with a specific chemical inhibitor (Akt inhibitor IV) demonstrated a significant enhancement of radiation sensitivity on glioma cells in a clonogenic survival assay. On the contrary, down-modulation of STAT3 signaling with a specific chemical inhibitor (JSI-124) or a neutralizing gp130 antibody failed to radiosensitize glioma cells. These data indicate that the Akt intercept node could be a more relevant therapeutic target than STAT3 for radiosensitizing human malignant glioma.

Hybrid inhibitors of phosphatidylinositol 3-kinase (PI3K) and the mammalian target of rapamycin (mTOR): design, synthesis, and superior antitumor activity of novel wortmannin-rapamycin conjugates

Hyperactivation of the PI3K/AKT/mTOR signaling pathway is common in cancer, and PI3K and mTOR act synergistically in promoting tumor growth, survival, and resistance to chemotherapy. Thus, combined targeting of PI3K and mTOR presents an opportunity for robust and synergistic anticancer efficacy. 17-Hydroxywortmannin (2a) analogues conjugated to rapamycin (3a) analogues via a prodrug linker are uniquely positioned for this approach. Our efforts led to the discovery of diester-linked conjugates that, upon in vivo hydrolysis, released two highly potent inhibitors. Conjugate 7c provided enhanced solubility relative to 3a and to an equivalent mixture of 3a and 9a and demonstrated profound activity in U87MG mouse xenografts, achieving an MED of 1.5 mg/kg, following weekly intravenous dosing. At 15 mg/kg, 7c completely inhibited the growth of HT29 tumors, whereas an equivalent mixture of the inhibitors was poorly tolerated. In the A498 renal tumor model, 7c exhibited superior efficacy over 3a or 9a when administered as a single agent or in combination with bevacizumab. Thus, we have uncovered a novel approach to target both PI3K and mTOR via hybrid inhibitors, leading to a broader and more robust anticancer efficacy.

Somatic mutations in PI3Kalpha: structural basis for enzyme activation and drug design

The PI3K pathway is a communication hub coordinating critical cell functions including cell survival, cell growth, proliferation, motility and metabolism. Because PI3Kalpha harbors recurrent somatic mutations resulting in gains of function in human cancers, it has emerged as an important drug target for many types of solid tumors. Various PI3K isoforms are also being evaluated as potential therapeutic targets for inflammation, heart disease, and hematological malignancies. Structural biology is providing insights into the flexibility of the PI3Ks, and providing basis for understanding the effects of mutations, drug resistance and specificity.

The antiproliferative cytostatic effects of a self-activating viridin prodrug

Although viridins like wortmannin (Wm) have long been examined as anticancer agents, their ability to self-activate has only recently been recognized. Here, we describe the cytostatic effects of a self-activating viridin (SAV), which is an inactive, polymeric prodrug. SAV self-activates to generate a bioactive, fluorescent viridin NBD-Wm with a half-time of 9.2 hours. With cultured A549 cells, 10 micromol/L SAV caused growth arrest without inducing apoptosis or cell death, a cytostatic action markedly different from other chemotherapeutic agents (vinblastine, camptothecin, and paclitaxel). In vivo, a SAV dosing of 1 mg/kg once in 48 hours (i.p.) resulted in growth arrest of an A549 tumor xenograft, with growth resuming when dosing ceased. With a peak serum concentration of SAV of 2.36 micromol/L (at 2 hours post i.p. injection), the concentration of bioactive NBD-Wm was 41 nmol/L based on the partial inhibition of neutrophil respiratory burst. Therefore, SAV was present as an inactive prodrug in serum (peak = 2.36 micromol/L), which generated low concentrations of active viridin (41 nmol/L). SAV is a prodrug, the slow release and cytostatic activities of which suggest that it might be useful as a component of metronomic-based chemotherapeutic strategies.

Take your PIK: phosphatidylinositol 3-kinase inhibitors race through the clinic and toward cancer therapy

The phosphatidylinositol 3-kinase (PI3K)/Akt signaling pathway is currently one of the most exciting drug targets in oncology. However, only a short time ago, the paradigm existed that drugs targeted to the four PI3K class I isoforms would be too toxic for use in cancer therapy due to effects on physiologic signaling. Since that time, studies have delineated the roles of these four isoforms in nonpathologic signaling as well as their roles in cancer. An extensive effort has gone into developing agents that inhibit one or more PI3K isoforms, as well as closely related proteins implicated in cancer. These agents have proved to be tolerable and therapeutically beneficial in animal studies, and a number are in clinical testing. The agents, their properties, and their molecular targets are discussed in this review.

Inhibitors of phosphoinositide-3-kinase: a structure-based approach to understanding potency and selectivity

Phosphoinositide-3-kinase is a pivotal protein involved in a wide variety of signaling cascades and there has been a great deal of interest in the development of potent and selective inhibitors of this enzyme. In this review, the potency and selectivity of the known inhibitors is presented along with key structural information that helps rationalize the observed trends.