Tuesday, June 9, 2015

Drugs in Clinical Pipeline: AT7867

AT7867 [4-(4-(1H-pyrazol-4-yl)phenyl)-4-(4-chlorophenyl)piperidine] is an ATP-competitive small molecule inhibitor of all three AKT isoforms with nanomolar potency. The inhibition values were confirmed using isolated enzyme assays (IC50 AKT1, AKT2, AKT3 = 32, 17, 47 nM, respectively). AT7867 also displayed potent activity against the structurally related AGC kinases p70S6K (IC50 = 85 nM) and PKA (IC50 = 20 nM) but showed a clear selectivity against kinases from other kinase subfamilies (CDK2, EGFR, SRC, etc all have IC50’s greater than 1000 nM). AT7867 was discovered using fragment-based screening combined with structure-based design and was previously called compound 8a by Saxty et. al [1]. The molecule is a pyrazole, linked via the 4-position to a geminally substituted 4,4-biaryl piperidine, with the terminal aromatic group incorporating a para-chloro substituent (Fig. 1A). The compound is further distinguished by its relatively low molecular weight of 337 Da [2].

AT7867 potently inhibits both AKT and p70S6K activity at the cellular level, as measured by inhibition of GSK3β and S6 ribosomal protein phosphorylation, and also causes growth inhibition in a range of human cancer cell lines as a single agent. The novel strategy of AKT and p70S6K blockade may have therapeutic value and supports further evaluation of AT7867 as a single-agent anticancer strategy.

The activity of AT7867 is as follows:

IC50 (AKT1 enzyme assay) = 32 nM
IC50 (AKT2 enzyme assay) = 17 nM
IC50 (AKT3 enzyme assay) = 47 nM

Common Name: AT7867
Synonyms:  AT7867; AT-7867; AT 7867
IUPAC Name: 4-(4-(1H-pyrazol-4-yl)phenyl)-4-(4-chlorophenyl)piperidine
CAS Number: 857531-00-1
Mechanism of Action: Kinase Inhibitor; pan-AKT Inhibitor
Indication: Various Cancers; Anti-tumor Therapy
Development Stage: Pre-Clinical
Company: Astex Therapeutics

AKT (also known as protein kinase B) is a serine/threonine kinase that lies downstream of phosphatidylinositol 3-kinase (PI3K) and plays a key role in a range of cellular functions, including cell growth, proliferation, metabolism, and survival. Three closely related isoforms of AKT with overlapping cellular functions have been identified, termed AKT1, AKT2, and AKT3.The activation of AKT is antagonized by the tumor suppressor PTEN (phosphatase and tensin homologue on chromosome 10) through the dephosphorylation of phosphatidylinositol (3,4,5)-trisphosphate. There are numerous proteins downstream of AKT which, when phosphorylated, participate in the regulation of critical cellular processes, including growth, proliferation, metabolism, and survival. Those involved in cell growth include p70 S6 kinase (p70S6K), S6 ribosomal protein (S6RP), and the mammalian target of rapamycin complex 1, whereas cell proliferation and metabolism are regulated through GSK3β phosphorylation. A number of proapoptotic proteins, including BAD, caspase-9, and the forkhead family of transcription factors, enable AKT to regulate cell survival. Importantly, negative feedback loops have been described, which link proteins downstream of AKT with those upstream or with AKT itself [2].

Aberrations along the PI3K/AKT pathway have been shown to drive a range of malignancies through mechanisms including activation of upstream receptor tyrosine kinases, PIK3CA mutations, PTEN mutations, AKT amplifications and mutations, and overexpression and hyperactivation of AKT proteins themselves. Thus, the pharmacologic ablation of AKT activity represents a rational approach to anticancer therapy. Moreover, PI3K/AKT pathway activation is a frequent hallmark of tumors resistant to treatment with chemotherapy or targeted therapies, such as growth factor inhibitors. Therefore, AKT inhibition in these tumor types may also have therapeutic value either as monotherapy or in rational combinations with other antitumor agents.

The inhibition of AKT2 by AT7867 was shown to be ATP-competitive with a Ki of 18 nM. Binding at the ATP site was confirmed by determining the three-dimensional structure of the AT7867-AKT2 complex using X-ray crystallography. The structure revealed that AT7867 fulfills a three-point pharmacophore required for potent binding to AKT2, forming hydrogen bonding interactions with the kinase hinge region, electrostatic interactions with the ribose site, and hydrophobic contacts with a lipophilic pocket in the glycine-rich loop [2]. Moreover, pharmacokinetic studies showed that potentially active concentrations of AT7867 could be achieved in plasma and tumor after 20 mg/kg i.p. or 90 mg/kg p.o. reflecting the relatively high oral bioavailability of AT7867.

AT7867 inhibits the proliferation of a range of human tumor cell lines. It is interesting to note that, whereas the IC50 for growth inhibition varied 12-fold (1-12 µM), the IC50 for GSK3β phosphorylation was much more consistent (2-4.5 µM) across the same cell line panel. Further detailed studies in the PTEN-deficient human glioblastoma U87MG cell line showed that AT7867 caused both a concentration-dependent and time-dependent reduction in the phosphorylation of proteins downstream of AKT, including the AKT substrate GSK3β and p70S6K target S6RP. It is an intriguing possibility that the combined effects of targeting p70S6K and AKT with this compound may serve to improve its therapeutic potential by delivering a double vertical blockade to the pathway.

1.    Saxty, G.; et. al. Identification of inhibitors of protein kinase B using fragment-based lead discovery. J Med Chem 2007, 50(10), 2293-2296.
2.    Grimshaw, K. M.; et. al. AT7867 is a potent and oral inhibitor of AKT and p70 S6 kinase that induces pharmacodynamic changes and inhibits human tumor xenograft growth. Mol Cancer Ther 2010, 9(5), 1100-1110.