Differential regulatory functions of three classes of phosphatidylinositol and phosphoinositide 3-kinases in autophagy

Autophagy is an evolutionarily conserved and exquisitely regulated self-eating cellular process with important biological functions. Phosphatidylinositol 3-kinases (PtdIns3Ks) and phosphoinositide 3-kinases (PI3Ks) are involved in the autophagic process. Here we aim to recapitulate how 3 classes of these lipid kinases differentially regulate autophagy. Generally, activation of the class I PI3K suppresses autophagy, via the well-established PI3K-AKT-MTOR (mechanistic target of rapamycin) complex 1 (MTORC1) pathway. In contrast, the class III PtdIns3K catalytic subunit PIK3C3/Vps34 forms a protein complex with BECN1 and PIK3R4 and produces phosphatidylinositol 3-phosphate (PtdIns3P), which is required for the initiation and progression of autophagy. The class II enzyme emerged only recently as an alternative source of PtdIns3P and autophagic initiator. However, the orthodox paradigm is challenged by findings that the PIK3CB catalytic subunit of class I PI3K acts as a positive regulator of autophagy, and PIK3C3 was thought to be an amino acid sensor for MTOR, which curbs autophagy. At present, a number of PtdIns3K and PI3K inhibitors, including specific PIK3C3 inhibitors, have been developed for suppression of autophagy and for clinical applications in autophagy-related human diseases.

Acquired PIK3CA amplification causes resistance to selective phosphoinositide 3-kinase inhibitors in breast cancer

Agents targeting the PI3K/mTOR signaling axis have shown promise in early-phase clinical trials and are currently being studied in later stages of clinical development in multiple indications. Experience with other targeted agents suggests that clinical responses may be short-lived because of acquired resistance to therapy. Here, we report preclinical modeling of acquired resistance in a HER2-positive, PIK3CA mutant breast cancer cell line, KPL-4. We identified a heretofore-unreported mechanism of resistance, specifically high-level amplification of the mutant allele of PIK3CA, which resulted in a marked upregulation of PI3K signaling, enabling resistant cells to regain proliferative capacity at clinically relevant concentrations of the PI3K inhibitor, GDC-0941. We show that knockdown of the amplified PIK3CA mutant allele in these cells by small interfering RNA restored pathway signaling and sensitivity to PI3K inhibition at levels comparable to parental cells. These novel preclinical findings suggest that, in addition to assessment of other previously reported mechanisms of resistance, evaluation of PI3K copy number variation should be integrated into the exploratory analysis of biopsies obtained at disease progression.

The Biological Role of PI3K Pathway in Lung Cancer

Lung cancer is the primary cause of cancer-related mortality worldwide and although improvements in treatment have been achieved over the last few years, long-term survival rates for lung cancer patients remain poor. Therefore, there is an imperative need for molecularly targeted agents that will achieve long-term disease control. Numerous downstream molecular pathways, such as EGF/RAS/RAF/MEK/ERK and PI3K/AKT/mTOR are identified as having a key role in the pathogenesis of various forms of human cancer, including lung cancer. PI3K/AKT/mTOR signal pathway is an important intracellular signal transduction pathway with a significant role in cell proliferation, growth, survival, vesicle trafficking, glucose transport, and cytoskeletal organization. Aberrations in many primary and secondary messenger molecules of this pathway, including mutations and amplifications, are accounted for tumor cell proliferation, inhibition of apoptosis, angiogenesis, metastasis and resistance to chemotherapy-radiotherapy. In this review article, we investigate thoroughly the biological role of PI3K pathway in lung cancer and its contribution in the development of future therapeutic strategies.

Preclinical pharmacokinetics of the novel PI3K inhibitor GDC-0941 and prediction of its pharmacokinetics and efficacy in human

The phosphatidylinositol 3-kinase (PI3K) pathway is a major determinant of cell cycling and proliferation. Its deregulation is associated with the development of many cancers. GDC-0941, a potent and selective inhibitor of PI3K, was characterised preclinically in in vitro and in vivo studies. Plasma protein binding was extensive, with free fraction less than 7%, and blood-to-plasma ratio ranged from 0.6 to 1.2 among the species tested. GDC-0941 human hepatic clearance was predicted to be moderate by liver microsomal incubations. GDC-0941 had high permeability in Madin-Darby canine kidney cells. The clearance of GDC-0941 was high in mouse (63.7 mL/min/kg), rat (49.3 mL/min/kg) and cynomolgus monkey (58.6 mL/min/kg), and moderate in dog (11.9 mL/min/kg). The volume of distribution ranged from 2.52 L/kg in rat to 2.94 L/kg in monkey. Oral bioavailability ranged from 18.6% in monkey to 77.9% in mouse. Predicted human clearance and volume of distribution using allometry were 6 mL/min/kg and 2.9 L/kg, respectively. The human efficacious doses were predicted based on results from preclinical pharmacokinetic studies and xenograft models. GDC-0941 preclinical characterisation and predictions of its properties in human supported its progression towards clinical development. GDC-0941 is currently in phase II clinical trials.

The phosphatidyl inositol 3-kinase/AKT signaling pathway in breast cancer

The phosphatidyl inositol 3-kinase (PI3K)/Akt pathway mediates the effects of a variety of extracellular signals in a number of cellular processes including cell growth, proliferation, and survival. The alteration of integrants of this pathway through mutation of its coding genes increases the activation status of the signaling and can thus lead to cellular transformation. The frequent dysregulation of the PI3K/Akt pathway in breast cancer (BC) and the mediation of this pathway in different processes characteristically implicated in tumorigenesis have attracted the interest of this pathway in BC; however, a more comprehensive understanding of the signaling intricacies is necessary to develop clinical applications of the modulation of this pathway in this pathology. We review a series of experiments examining the contribution of alteration of integrants of this signaling network to human BC and we make an update of the information about the effect of the modulation of this pathway in this cancer.

Development of phosphoinositide-3 kinase pathway inhibitors for advanced cancer

The phosphoinositide-3 kinase (PI3K) pathway plays a critical role in cancer cell growth and survival. PI3K is activated in human cancers by elevated receptor tyrosine kinase activity, RAS mutation, as well as by mutation, amplification, and deletion of genes encoding components of the pathway. Additionally, PI3K pathway activation plays an important role in acquired resistance to both chemotherapy and targeted agents. The essential role of PI3K in human cancer has led to the development of PI3K pathway inhibitors that have shown promise in preclinical models and have entered phase 1 clinical trials. This article reviews preclinical and clinical data on members of this novel drug class, as well as data justifying the combination of PI3K inhibitors with other anticancer agents.

Drugging the PI3 kinome: from chemical tools to drugs in the clinic

The phosphatidylinositide 3-kinase (PI3K) pathway is very commonly activated in a wide range of human cancers and is a major driving force in oncogenesis. One of the class I lipid kinase members of the PI3K family, p110alpha, is probably the most commonly mutated kinase in the human genome. Alongside genetic, molecular biological, and biochemical studies, chemical inhibitors have been extremely helpful tools in understanding the role of PI3K enzymes in signal transduction and downstream physiological and pathological processes, and also in validating PI3Ks as therapeutic targets. Although they have been valuable in the past, the early and still frequently employed inhibitors, wortmannin and LY294002, have significant limitations as chemical tools. Here, we discuss the case history of the discovery and properties of an increasingly used chemical probe, the pan-class I PI3K and mammalian target of rapamycin (mTOR) inhibitor PI-103 (a pyridofuropyrimidine), and its very recent evolution into the thienopyrimidine drug GDC-0941, which exhibits excellent oral anticancer activity in preclinical models and is now undergoing phase I clinical trials in cancer patients. We also illustrate the impact of structural biology on the design of PI3K inhibitors and on the interpretation of their effects. The challenges and outlook for drugging the PI3 kinome are discussed in the more general context of the role of structural biology and chemical biology in innovative drug discovery.

From the bench to the bed side: PI3K pathway inhibitors in clinical development

A number of intracellular kinase components of the PI3K/Akt/mTOR pathway have been targeted over the past few years, leading to a new generation of anticancer agents that effectively and specifically disrupt this pathway in tumor cells. Here, progress in the identification and clinical evaluation of compounds designed to modulate the enzymatic activity of PI3K, Akt, mTOR, and Hsp90 is reviewed.

Inhibiting PI3K as a therapeutic strategy against cancer

Class I PI3K is composed of heterodimeric lipid kinases regulating essential cellular functions including proliferation, apoptosis and metabolism. Class I PI3K isoforms are commonly amplified in different cancer types and the PI3Kalpha catalytic subunit, PIK3CA, has been found mutated in a variable proportion of tumours of different origin. Furthermore, PI3K has been shown to mediate oncogenic signalling induced by several oncogenes such as HER2 or Ras. These facts suggest that PI3K might be a good target for anticancer drug discovery. Today, the rise of PI3K inhibitors and their first in vivo results have cleared much of the path for the development of PI3K inhibitors for anticancer therapy. Here we will review the PI3K pathway and the pharmacological results of PI3K inhibition.

Targeting the PI3K/AKT pathway for the treatment of prostate cancer

Despite recent advances in our understanding of the biological basis of prostate cancer, the management of the disease, especially in the castration-resistant phase, remains a significant challenge. Deregulation of the phosphatidylinositol 3-kinase pathway is increasingly implicated in prostate carcinogenesis. In this review, we detail the role of this pathway in the pathogenesis of prostate cancer and the rapidly evolving therapeutic implications of targeting it. In particular, we highlight the importance of the appropriate selection of agents and combinations, and the critical role of predictive and pharmocodynamic biomarkers.