Pathway backgroundAbstract | The phosphoinositide 3‑kinase (PI3K) pathway is a key signal transduction system that links oncogenes and multiple receptor classes to many essential cellular functions, and is perhaps the most commonly activated signalling pathway in human cancer. This pathway therefore presents both an opportunity and a challenge for cancer therapy. Even as inhibitors that target PI3K isoforms and other major nodes in the pathway, including AKT and mammalian target of rapamycin (mTOR), reach clinical trials, major issues remain. Here, we highlight recent progress that has been made in our understanding of the PI3K pathway and discuss the potential of and challenges for the development of therapeutic agents that target this pathway in cancer.Figure 1 | The class I PI3K signalling pathway. Following growth factor stimulation and subsequent activation of receptor tyrosine kinases (RTKs), class IA phosphoinositide 3‑kinases (PI3Ks), consisting of p110α–p85, p110β–p85 and p110δ–p85, are recruited to the membrane by direct interaction of the p85 subunit with the activated receptors (for example, platelet-derived growth factor receptor) or by interaction with adaptor proteins associated with the receptors (for example, insulin receptor substrate 1). The activated p110 catalytic subunit converts phosphatidylinositol‑4,5-bis­phosphate (PtdIns(4,5)P2) to phosphatidylinositol‑3,4,5-trisphosphate (PtdIns(3,4,5)P3) at the membrane, providing docking sites for signalling proteins that have pleckstrin homology domains, including putative 3-phosphoinositide-dependent kinase 1 (PDPK1) and serine–threonine protein kinase AKT (also known as protein kinase B). PDPK1 phosphorylates and activates AKT, which elicits a broad range of downstream signalling events. The class IB PI3K (p110γ–p101) can be activated directly by G protein-coupled receptors (GPCRs) through interaction with the Gβγ subunit of trimeric G proteins. The p110β and p110δ subunits can also be activated by GPCRs. PTEN (phosphatase and tensin homologue) antagonizes the PI3K action by dephosphorylating PtdIns(3,4,5)P3. BAD, BCL2-associated agonist of cell death; FOXO1, forkhead box O1 (also known as FKHR); GSK3β, glycogen synthase kinase 3β; mTOR, mammalian target of rapamycin; NF-κB, nuclear factor-κB; PKC, protein kinase C; RAC1, RAS-related C3 botulinum toxin substrate 1; SGK, serum and glucocorticoid-regulated kinase; S6K, ribosomal protein S6 kinase; LPA, lysophosphatidic acid.Figure 2 | The PI3K family and phosphatidylinositol-3,4,5-trisphosphate generation. a | Phosphoinositide 3‑kinases (PI3Ks) are divided into three classes according to their structural characteristics and substrate specificity. Class IA PI3Ks are heterodimers consisting of a p110 catalytic subunit and a p85 regulatory subunit. There are three p110 catalytic isoforms: p110α, p110β and p110δ. Whereas the expression of p110δ is largely restricted to the immune system, p110α and p110β are ubiquitously expressed3,8. The p110 catalytic isoforms are highly homologous and share five distinct domains: an amino‑terminal p85-binding domain (p85 BD), which interacts with the p85 regulatory subunit; a RAS-binding domain (RAS BD), which mediates activation by members of the RAS family of small GTPases; a putative membrane-binding domain, C2; the helical domain; and the carboxy‑terminal kinase catalytic domain. There are also three p85 isoforms: p85α (and its splice variants p55α and p50α), p85β and p55γ. They share three core domains, including a p110-binding domain called the inter-Src homology 2 (iSH2) domain, flanked by two SH2 domains. The two longer isoforms, p85α and p85β, have an SH3 domain and a BCR homology domain (BHD) located in their extended N‑terminal regions. In the basal state, p85 binds to the N‑terminus of the p110 subunit through its iSH2 domain, inhibiting its catalytic activity7,8. Class IB PI3K is a heterodimer composed of the catalytic subunit p110γ and the regulatory subunit p101. p110γ is mainly expressed in leukocytes and can be activated directly by G protein-coupled receptors8. Class II PI3Ks are monomers with a single catalytic subunit. There are three class II PI3K isoforms: PI3KC2α, PI3KC2β and PI3KC2γ, each of which has a divergent N‑terminus followed by a RAS-binding domain, a C2 domain, a helical domain and a catalytic domain, with PX (Phox homology) and C2 domains at the C‑termini (reviewed in Refs 2,3). The class III PI3K consists of a single catalytic subunit, VPS34 (homologue of the yeast vacuolar protein sorting-associated protein 34). b | Phosphatidylinositol-3,4,5-trisphosphate (PtdIns(3,4,5)P3) is an important lipid second messenger that regulates many cellular processes. Class I PI3Ks phosphorylate the inositol ring of phosphatidylinositol‑4,5-trisphosphate (PtdIns(4,5)P2) on the 3 position, to generate PtdIns(3,4,5)P3. PTEN (phosphatase and tensin homologue) is a lipid phosphatase that removes phosphate on the 3 position of PtdIns(3,4,5)P3 and converts it back to PtdIns(4,5)P2. Gβγ BD, G protein βγ subunit-binding domain.Table 1 | Incidence of genetic alterations in the PI3K pathway in cancerLinking the PI3K pathway to human cancersTable 1 (cont.) | Incidence of genetic alterations in the PI3K pathway in cancerCurrent targeting of nodes in the PI3K pathwayFigure 3 | Targeting the PI3K pathway in cancer. a | Inhibitors that target key nodes in the phosphoinositide 3‑kinase (PI3K) signalling pathway, including receptor tyrosine kinases (RTKs), PI3K, AKT and mammalian target of rapamycin (mTOR), have reached clinical trials. Dual inhibitors that target both PI3K and RTK or PI3K and mTOR may provide more potent therapeutic effects in suppressing the PI3K signalling. Combinations of PI3K and RAF–mitogen-activated protein kinase (MAPK) inhibitors may achieve more effective clinical results. b | Inhibitors in clinical development that target the PI3K or related pathways are shown. EGFR, epidermal growth factor receptor; ERK, extracellular signal-regulated kinase; HER2, human epidermal growth factor receptor 2 (also known as ERBB2); MEK, mitogen-activated protein kinase kinase; VEGFR, vascular endothelial growth factor receptor. *Bevacizumab targets VEGFA instead of VEGFR directly. ‡Both AZD8055 and OSI-027 are ATP-competitive mTOR inhibitors that target the mTOR complexes mTORC1 and mTORC2.Figure 4 | Selected chemical structures of inhibitors of