Development of PI3 kinase inhibitors for cancer treatment
Authors: Paul Workman
Journal: Cancer Research (2005)

Proc Amer Assoc Cancer Res, Volume 46, 2005

SY13-4 

The phosphoinositide 3-kinase (PI3 kinase) enzymes are a family of lipid kinases that regulate cell signalling pathways that are involved in controlling growth, proliferation, survival, motility, invasion, angiogenesis and other processes ([1][1],[2][2]). Small molecule inhibitors of PI3 kinases have therapeutic potential in a range of pathological conditions including cancer, diabetes, inflammation, autoimmunity and cardiovascular disease ([3][3]–[6][4]). In particular, there is accumulating evidence that the PI3 kinase pathway is deregulated in cancer and plays a major role in the malignant phenotype and tumor progression ([7][5],[8][6]). For example, increased expression and more recently mutation of the p110α catalytic subunit of PI3 kinase has been shown to be common in various human cancers ([9][7],[10][8]). In addition, the upstream receptor tyrosine kinases such as ErbB2 and the downstream serine-threonine kinase Akt/PKB are frequently overexpressed in malignancy. Furthermore, PTEN is the second most commonly mutated tumor suppressor in human cancer and the product of this gene is a lipid phosphatase that reverses the conversion of PIP2 to PIP3 that is catalysed by PI3 kinase. Taken together this series of abnormalities provide strong genetic and biochemical validation for pathological activation of the PI3 kinase pathway and for the development of PI3 kinase inhibitors for cancer treatment. Until recently, only a small number of PI3 kinase inhibitors have been identified and these are limited by factors such as low potency, poor isoform or kinase selectivity, limited stability and unacceptable pharmacological and pharmacokinetic properties. However, PI3 kinase inhibitors with more promising drug-like properties have now begun to emerge that show promise for the treatment of cancer and other disease indications ([3][3],[5][9],[6][4]). In cancer the evidence suggests that inhibition of the class 1A PI3 kinases p110α and p110β appear to be the most appropriate to target initially. The majority of the validation relates to p110α, however siRNA knockdown data also potentially implicate p110β. In a collaboration initially involving Ludwig Institute for Cancer Research at University College, London UK, Cancer Research UK London Research Institute, The Institute of Cancer Research, Sutton UK and the Yamanouchi Pharmaceutical Company, Japan we previously identified two series of compounds that inhibit class PI 3 kinase with a high degree of selectivity. These inhibitors are termed imidazopyridines and the pyridofuropyrimidines (PFPs). These inhibitors are now being developed under the auspices of PIramed Ltd. Detailed characterisation was carried out on the lead PFP compound PI103 ([11][10]–[14][11]). This compound exhibited IC50 values of around 2-3nM against recombinant class 1A isoforms p110α, β and δ compared to 15nM for the class 1B isoform p110γ and 3.85μM for the class III isoform Vps34. Growth inhibitory activity was seen against a range of human cancer cell lines in vitro. For example PI103 exhibited an IC50 value of 100nM against the PTEN negative PC3 human prostate cancer line. A very rapid and concentration-dependent decrease in Akt/PKB phosphorylation was observed at the serine 473 site and this was associated with a G1 cell cycle arrest and also with apoptosis in some cancer cell lines. Similar results were seen in a panel of high grade glioblastoma cell lines with defined molecular characteristics and a high level of constitutive Akt/PKB phosphorylation. In addition to its direct effects on tumor cell proliferation, PI103 also inhibited the motility and invasion of both tumor and endothelial cells. Pharmacokinetic properties of PIU103 were characterized: Vz 150ml, CL 120ml/hr. Significant growth inhibitory activity was seen in human tumor xenografts in nude mice, including the U87MG glioblastoma model. Inhibition of the orthotopic OVCAR3 human ovarian cancer xenograft was seen and was associated with a corresponding decrease in Akt/PKB phorsphorylation. Based on PI103 as a starting point, a medicinal chemistry effort at PIramed has identified a series of second generation PI3 kinase inhibitors with high potency and selectivity for the class IA isoforms, promising functional activity and markedly improved physicochemical properties, including high aqueous solubility at physiological pH. Profiling of representative compounds showed nanomolar potency against p110α, β and δ but no inhibitory activity against a panel of 72 serine/threonine and tyrosine protein kinases when tested at 500nM. PI540 had an aqueous solubility of >100μM compared to 3.5μM for PI103 and a mouse microsomal stability of 91% at 30 minutes compared to 13% for PI103. Good permeability was predicted from CaCo-2 cell studies and the lead compounds did not show activity against the major forms of human cytochrome P450 and did not block the hERG channel as determined by the rubidium efflux assay. Good antitumor activity was seen in several different human tumor xenografts, including those with molecular pathologies indicative of activation of the PI3 kinase pathway, using doses in the range of around 25-50 mg/kg once or twice a day which were well tolerated. In summary, a series of potent and selective inhibitors of class 1A PI3 kinases have been identified. Optimised members of the series have improved physicochemical and pharmacological properties. Antitumor activity has been demonstrated at well tolerated doses in a range of human tumor xenografts, including those with molecular pathologies predictive of likely oncogene addiction to the PI3 kinase pathway. Moreover, effects of the inhibitors are seen on several of the multiple hallmark traits of cancer ([15][12]), including proliferation, survival, invasion, metastasis and angiogenesis. Thus these agents show considerable potential for development as a novel class of molecular cancer therapeutics.

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