ON123300

ON123300 inhibits U87 glioma cell proliferation with an IC50 3.4±0.1 μmol/L and reduces phosphorylation of Akt, yet it also unexpectedly induces Erk activation, both in a dose- and time-dependent manner that subsequently is attributed to relieving Akt mediated C-Raf S259 inactivation and activating a p70S6K-initiated PI3K-negative feedback loop^[1]. ON123300 also inhibits CDK4/6 and PI3K-δ and exhibits potent activity against mantle cell lymphomas (MCLs). ON123300 is a potent inhibitor of CDK4, with an IC50 of 3.8 nM, with little inhibitory activity against CDKs 1,2,5 and 8. MCL cell lines treated with ON123300 accumulate in the G1 phase at lower concentrations (0.1-1.0μM), at higher concentrations of the compound, a large proportion of the cells progress through the S and G2/M phases of the cell cycle and eventually accumulate in the sub-G1 phase, suggesting an induction of apoptosis. ON123300 also inhibits the phosphorylation of pRb and p130 in a dose-dependent manner. ON123300 treatment results in inhibition of FOXO1 phosphorylation, a target of mTOR^[3].

The cytotoxicity of ON123300 is determined using a colorimetric sulforhodamine B (SRB)-based assay. Suspensions of glioma cells (100 mL containing 2×10³ cells) are seeded in 96-well plates and allowed to attach to the surface by overnight incubation. The cells are then treated with increasing concentrations of ON123300 for 72 hour. At the end of the treatment, cells are fixed with 10% (v/v) trichloroacetic acid (TCA) and stained with 0.4% SRB. The optical densities are measured at a wavelength of 570 nm. 

In a preclinical brain tumor model (U87MG), ON123300 shows high brain and brain tumor accumulation. Consistent with the in vitro studies, single agent ON123300 causes a dose-dependent suppression of phosphorylation of Akt as well as activation of Erk in brain tumors^[1]. ON123300 is highly bound (99.4%) to plasma proteins in mice and rapidly penetrates into brain. It has proficient BBB penetration and accumulates in normal brain^[2]. The pharmacokinetic profiles of plasma ON123300 concentration are multiexponential and overall declines fairly rapidly with terminal elimination half-lives of 1.5 hours^[1]. Mouse xenograft assays show a strong inhibition of MCL tumor growth in ON123300-treated animals. Safety studies in mice suggest that ON123300 is orally bio-available and is minimally toxic when administered orally or intraperitoneally^[3].