Woo-Jeong Lee

A drug targeting only p110α can block phosphoinositide 3-kinase signalling and tumour growth in certain cell types

Genetic alterations in PI3K (phosphoinositide 3-kinase) signalling are common in cancer and include deletions in PTEN (phosphatase and tensin homologue deleted on chromosome 10), amplifications of PIK3CA and mutations in two distinct regions of the PIK3CA gene. This suggests drugs targeting PI3K, and p110α in particular, might be useful in treating cancers. Broad-spectrum inhibition of PI3K is effective in preventing growth factor signalling and tumour growth, but suitable inhibitors of p110α have not been available to study the effects of inhibiting this isoform alone. In the present study we characterize a novel small molecule, A66, showing the S-enantiomer to be a highly specific and selective p110α inhibitor. Using molecular modelling and biochemical studies, we explain the basis of this selectivity. Using a panel of isoform-selective inhibitors, we show that insulin signalling to Akt/PKB (protein kinase B) is attenuated by the additive effects of inhibiting p110α/p110β/p110δ in all cell lines tested. However, inhibition of p110α alone was sufficient to block insulin signalling to Akt/PKB in certain cell lines. The responsive cell lines all harboured H1047R mutations in PIK3CA and have high levels of p110α and class-Ia PI3K activity. This may explain the increased sensitivity of these cells to p110α inhibitors. We assessed the activation of Akt/PKB and tumour growth in xenograft models and found that tumours derived from two of the responsive cell lines were also responsive to A66 in vivo. These results show that inhibition of p110α alone has the potential to block growth factor signalling and reduce growth in a subset of tumours.

Synthesis and Biological Evaluation of Novel Analogues of the Pan Class I Phosphatidylinositol 3-Kinase (PI3K) Inhibitor 2-(Difluoromethyl)-1-[4,6-di(4-morpholinyl)-1,3,5-triazin-2-yl]-1H-benzimidazole (ZSTK474)

A structure-activity relationship (SAR) study of the pan class I PI 3-kinase inhibitor 2-(difluoromethyl)-1-[4,6-di(4-morpholinyl)-1,3,5-triazin-2-yl]-1H-benzimidazole (ZSTK474) identified substitution at the 4 and 6 positions of the benzimidazole ring as having significant effects on the potency of substituted derivatives. The 6-amino-4-methoxy analogue displayed a greater than 1000-fold potency enhancement over the corresponding 6-aza-4-methoxy analogue against all three class Ia PI 3-kinase enzymes (p110α, p110β, and p110δ) and also displayed significant potency against two mutant forms of the p110α isoform (H1047R and E545K). This compound was also evaluated in vivo against a U87MG human glioblastoma tumor xenograft model in Rag1(-/-) mice, and at a dose of 50 mg/kg given by ip injection at a qd × 10 dosing schedule it dramatically reduced cancer growth by 81% compared to untreated controls.

Novel pyrazolo[1,5-a]pyridines as p110α-selective PI3 kinase inhibitors: Exploring the benzenesulfonohydrazide SAR

Structure-activity relationship studies of the pyrazolo[1,5-a]pyridine class of PI3 kinase inhibitors show that substitution off the hydrazone nitrogen and replacement of the sulfonyl both gave a loss of p110α selectivity, with the exception of an N-hydroxyethyl analogue. Limited substitutions were tolerated around the phenyl ring; in particular the 2,5-substitution pattern was important for PI3 kinase activity. The N-hydroxyethyl compound also showed good inhibition of cell proliferation and inhibition of phosphorylation of Akt/PKB, a downstream marker of PI3 kinase activity. It had suitable pharmacokinetics for evaluation in vivo, and showed tumour growth inhibition in two human tumour cell lines in xenograft studies. This work has provided suggestions for the design of more soluble analogues.

Discovery of pyrazolo[1,5-a]pyridines as p110α-selective PI3 kinase inhibitors

We have made a novel series of pyrazolo[1,5-a]pyridines as PI3 kinase inhibitors, and demonstrated their selectivity for the p110α isoform over the other Class Ia PI3 kinases. We investigated the SAR around the pyrazolo[1,5-a]pyridine ring system, and found compound 5x to be a particularly potent example (p110α IC(50) 0.9nM). This compound inhibits cell proliferation and phosphorylation of Akt/PKB, a downstream marker of PI3 kinase activity, and showed in vivo activity in an HCT-116 human xenograft model.

Enzyme activity effects of N-terminal His-tag attached to catalytic sub-unit of phosphoinositide-3-kinase

NTT (N-terminal tags) on the catalytic (p110) sub-unit of PI 3-K (phosphoinositol 3-kinase) have previously been shown to increase cell signalling and oncogenic transformation. Here we test the impact of an NT (N-terminal) His-tag on in vitro lipid and protein kinase activity of all class-1 PI 3-K isoforms and two representative oncogenic mutant forms (E545K and H1047R), in order to elucidate the mechanisms behind this elevated signalling and transformation observed in vivo. Our results show that an NT His-tag has no impact on lipid kinase activity as measured by enzyme titration, kinetics and inhibitor susceptibility. Conversely, the NT His-tag did result in a differential effect on protein kinase activity, further potentiating the elevated protein kinase activity of both the helical domain and catalytic domain oncogenic mutants with relation to p110 phosphorylation. All other isoforms also showed elevated p110 phosphorylation (although not statistically significant). We conclude that the previously reported increase in cell signalling and oncogenic-like transformation in response to p110 NTT is not mediated via an increase in the lipid kinase activity of PI 3-K, but may be mediated by increased p110 autophosphorylation and/or other, as yet unidentified, intracellular protein/protein interactions. We further observe that tagged recombinant protein is suitable for use in in vitro lipid kinase screens to identify PI 3-K inhibitors; however, we recommend that in vivo (including intracellular) experiments and investigations into the protein kinase activity of PI 3-K should be conducted with untagged constructs.

Oncogenic Mutations of p110α Isoform of PI 3-Kinase Upregulate Its Protein Kinase Activity

In addition to lipid kinase activity, the class-I PI 3-kinases also function as protein kinases targeting regulatory autophosphorylation sites and exogenous substrates. The latter include a recently identified regulatory phosphorylation of the GM-CSF/IL-3 βc receptor contributing to survival of acute myeloid leukaemia cells. Previous studies suggested differences in the protein kinase activity of the 4 isoforms of class-I PI 3-kinase so we compared the ability of all class-I PI 3-kinases and 2 common oncogenic mutants to autophosphorylate, and to phosphorylate an intracellular fragment of the GM-CSF/IL-3 βc receptor (βic). We find p110α, p110β and p110γ all phosphorylate βic but p110δ is much less effective. The two most common oncogenic mutants of p110α, H1047R and E545K have stronger protein kinase activity than wildtype p110α, both in terms of autophosphorylation and towards βic. Importantly, the lipid kinase activity of the oncogenic mutants is still inhibited by autophosphorylation to a similar extent as wildtype p110α. Previous evidence indicates the protein kinase activity of p110α is Mn2+ dependent, casting doubt over its role in vivo. However, we show that the oncogenic mutants of p110α plus p110β and p110γ all display significant activity in the presence of Mg2+. Furthermore we demonstrate that some small molecule inhibitors of p110α lipid kinase activity (PIK-75 and A66) are equally effective against the protein kinase activity, but other inhibitors (e.g. wortmannin and TGX221) show different patterns of inhibition against the lipid and protein kinases activities. These findings have implications for the function of PI 3-kinase, especially in tumours carrying p110α mutations.

Synthesis and biological evaluation of novel phosphatidylinositol 3-kinase inhibitors: Solubilized 4-substituted benzimidazole analogs of 2-(difluoromethyl)-1-(4,6-di(4-morpholinyl)-1,3,5-triazin-2-yl)- 1H-benzimidazole (ZSTK474)

A range of 4-substituted derivatives of the pan class I PI 3-kinase inhibitor 2-(difluoromethyl)-1-[4,6-di-(4-morpholinyl)-1,3,5-triazin-2-yl]-1H-benzimidazole (ZSTK474) were prepared in a search for more soluble analogs. 4-Aminoalkoxy substituents provided the most potent derivatives, with the 4-O(CH2)3NMe2 analog (compound 14) being identified as displaying the best overall activity in combination with good aqueous solubility (25 mg/mL for the hydrochloride salt). This compound was tested in a U87MG xenograft model, but displayed less potency than ZSTK474 as a result of an unfavorable pharmacokinetic profile.

Novel pyrazolo[1,5-a]pyridines as PI3K inhibitors: variation of the central linker group

As part of our investigation into the pyrazolo[1,5-a]pyridines as novel PI3K inhibitors, we report a range of analogues where the central linker portion of the molecule was varied while retaining the pyrazolo[1,5-a]pyridine and arylsulfonyl or arylcarbonyl groups. Isostere generating software BROOD was used to assist with producing ideas. The isoform selectivity of the compounds varied from pan-PI3K for compound 41 to p110α-selective for compound 58 or p110δ-selective for compound 57. The latter two compounds varied only in their sulphur oxidation state.

Biological characterization of SN32976, a selective inhibitor of PI3K and mTOR with preferential activity to PI3Kα, in comparison to established pan PI3K inhibitors

Multiple therapeutic agents have been developed to target the phosphatidylinositol 3-kinase (PI3K) signaling pathway, which is frequently dysregulated in cancer promoting tumor growth and survival. These include pan PI3K inhibitors, which target class Ia PI3K isoforms and have largely shown limited single agent activity with narrow therapeutic windows in clinical trials. Here, we characterize SN32976, a novel pan PI3K inhibitor, for its biochemical potency against PI3K isoforms and mTOR, kinase selectivity, cellular activity, pharmacokinetics, pharmacodynamics and antitumor efficacy relative to five clinically-evaluated pan PI3K inhibitors: buparlisib, dactolisib, pictilisib, omipalisib and ZSTK474. SN32976 potently inhibited PI3K isoforms and mTOR, displaying preferential activity for PI3Kα and sparing of PI3Kδ relative to the other inhibitors, while showing less off-target activity than the clinical inhibitors in a panel of 442 kinases. The major metabolites of SN32976 were also potent PI3K inhibitors with similar selectivity for PI3Kα as the parent compound. SN32976 compared favorably with the clinically-evaluated PI3K inhibitors in cellular assays, inhibiting pAKT expression and cell proliferation at nM concentrations, and in animal models, inducing a greater extent and duration of pAKT inhibition in tumors than pictilisib, dactolisib and omipalisib at similarly tolerated dose levels and inhibiting tumor growth to a greater extent than dactolisib and ZSTK474 and with similar efficacy to pictilisib and omipalisib. These results suggest that SN32976 is a promising clinical candidate for cancer therapy with enhanced kinase selectivity and preferential inhibition of PI3Kα compared to first generation pan PI3K inhibitors, while retaining comparable anticancer activity.

Novel pyrazolo[1,5-a]pyridines with improved aqueous solubility as p110α-selective PI3 kinase inhibitors

As part of our investigation into pyrazolo[1,5-a]pyridines as novel p110α selective PI3 kinase inhibitors, we report a range of analogues with improved aqueous solubility by the addition of a basic amine. The compounds demonstrated comparable p110α potency and selectivity to earlier compounds but with up to 1000× greater aqueous solubility, as the hydrochloride salts. The compounds also displayed good activity in a cellular assay of PI3 kinase activity.