In vitro activity: ON123300 is a novel, low molecular weight and potent multikinase inhibitor identified through a series of screens that supported further analyses for brain tumor chemotherapy. It inhibits CDK4 with IC50 of 3.8 nM and has little inhibitory activity against other CDKs such as 1,2,5 and 8. Biochemical assays indicated that ON123300 was a strong inhibitor of Ark5 and CDK4, as well as growth factor receptor tyrosine kinases such as β-type platelet-derived growth factor receptor (PDGFRβ). ON123300 inhibited U87 glioma cell proliferation with an IC(50) 3.4 ± 0.1 μmol/L and reduced phosphorylation of Akt, yet it also unexpectedly induced Erk activation, both in a dose- and time-dependent manner that subsequently was attributed to relieving Akt-mediated C-Raf S259 inactivation and activating a p70S6K-initiated PI3K-negative feedback loop. ON123300 demonstrated favorable pharmacokinetic characteristics, and future development for brain tumor therapy would require use of combinations, such as gefitinib, that mitigate its Erk activation and enhance its activity.

Kinase Assay: ON123300 is a novel kinase inhibitor, inhibits CDK4/6 and PI3K-δ and exhibits potent activity against mantle cell lymphomas (MCLs). ON123300 is a low molecular weight multi-kinase inhibitor identified through a series of screens that supported further analyses for brain tumor chemotherapy. Biochemical assays indicated ON123300 was a strong inhibitor of Ark5 and CDK4 as well as growth factor receptor tyrosine kinases such as Beta-type platelet-derived growth factor receptor [PDGFRβ]. ON123300 inhibited U87 glioma cell proliferation with an IC50 = 3.4 ± 0.1 μM and reduced phosphorylation of Akt, yet it also unexpectedly induced Erk activation; both in a dose- and time-dependent manner that subsequently was attributed to relieving Akt-mediated C-Raf S259 inactivation and activating a p70S6K initiated PI3K negative feedback loop.

Cell Assay: As a substantial number of multiple myeloma cell lines and primary samples were found to express AMPK-related protein kinase 5(ARK5), a member of the AMPK family associated with tumor growth and invasion, we examined whether dual inhibition of CDK4 and ARK5 kinases using ON123300 results in a better therapeutic outcome. Treatment of multiple myeloma cell lines and primary samples with ON123300 in vitro resulted in rapid induction of cell-cycle arrest followed by apoptosis. ON123300-mediated ARK5 inhibition or ARK5-specific siRNAs resulted in the inhibition of the mTOR/S6K pathway and upregulation of the AMPK kinase cascade. AMPK upregulation resulted in increased SIRT1 levels and destabilization of steady-state MYC protein. Furthermore, ON123300 was very effective in inhibiting tumor growth in mouse xenograft assays. In addition, multiple myeloma cells sensitive to ON123300 were found to have a unique genomic signature that can guide the clinical development of ON123300. Our study provides preclinical evidence that ON123300 is unique in simultaneously inhibiting key oncogenic pathways in multiple myeloma and supports further development of ARK5 inhibition as a therapeutic approach in multiple myeloma. ON123300 inhibited U87 glioma cell proliferation with an IC(50) 3.4 ± 0.1 μmol/L and reduced phosphorylation of Akt.