In vitro activity: PCI-34051 possesses promising potency for HDAC8 with a Ki of 10 nM. PCI-34051 has high selectivity (approximately fivefold) for HDAC8 relative to the other class I HDACs including HDAC1. PCI-34051 reveals greater than 200-fold selectivity over HDAC1 and HDAC6, and greater than 1000-fold selectivity over HDAC2, HDAC3 and HDAC10. PCI-34051 inhibits ovarian tumor line OVCAR-3 with a GI50 of 6 μM and 15% cell death. Neither significant tubulin nor histone acetylation is observed in the sensitive cell lines treated with PCI-34051 at concentrations less than 25 μM at 24 hours nor at earlier timepoints. PCI-34051 induces a selective cytotoxic effect in cell lines derived only from T-cell malignancies. PCI-34051 induces caspase-dependent apoptosis. When caspase-3 activity is measured at various times after treatment with 5 μM PCI-34051, increasing levels of activity are observed from 12 to 24 to 48 hours, another hallmark of apoptosis, consistent with the higher levels of caspase activity at this timepoint. PCI-34051 does not stimulate Bid cleavage, a characteristic effect of the extrinsic apoptotic pathway. While P116 and J.RT3-T.5 are sensitive to PCI-34051, the PLCγ1-deficient J.gamma1 line reveals a marked decrease in the extent of PCI-34051-induced apoptosis. In addition, steady-state calcium levels strongly influence the apoptosis induced by PCI-34051. PCI-34051 induces cytochrome c release from mitochondria.

Kinase Assay: For PCI-34051 characterization, measurements are perfomed in a reaction volume of 100 μL using 96-well assay plates in a fluorescence plate reader. For each isozyme. The HDAC protein in reaction buffer (50 mM HEPES, 100 mM KCl, 0.001% Tween-20, 5% dimethyl sulfoxide, pH7.4, supplemented with bovine serum albumin at concentrations of 0-0.05%) is mixed with PCI-34051 at various concentrations and allowed to incubate for 15 min. Trysin is added to a final concentration of 50 nM, and acetyl-gly-Ala-(N-acetyl-Lys)-amino-4-methylcoumarin is added to a final concentration of 25-100 μM to initiate the reaction. After a 30 min lag time, the fluorescence is measured over a 30 min time frame using an excitation wavelength of 335 nm and a detection wavelength of 460 nm. The increase in fluorescence wih time is used as the measure of the reaction rate.

Cell Assay: Tumor cell lines and human umbilical vein endothelial cells are cultured for at least two doubling times, and growth is monitored at the end of PCI-34051 exposure using an Alamar Blue fluorometric cell proliferation assay as recommended by the manufacturer. PCI-34051 is assayed in triplicate wells in 96-well plates. The concentration required to inhibit cell growth by 50% (GI50) and 95% confidence intervals are estimated from nonlinear regression using a four-parameter logistic equation.