Plasma and cerebrospinal fluid pharmacokinetics of the histone deacetylase inhibitor, belinostat (PXD101), in non-human primates.

PURPOSE: Histone deacetylases (HDAC) are involved in the regulation of gene transcription. Aberrant HDAC activity has been associated with tumorigenesis, and, therefore, HDACs are potential targets for the treatment of cancers, including tumors of the central nervous system (CNS). Belinostat is a novel, potent, pan-HDAC inhibitor with antiproliferative activity on a wide variety of tumor cell lines. We studied the cerebrospinal fluid (CSF) penetration of intravenous (IV) belinostat in a non-human primate model as a surrogate for blood:brain barrier penetration. DESIGN: Five adult rhesus monkeys received increasing doses of belinostat (10-60 mg/kg) as a 30-min IV infusion. Serial blood and CSF samples were collected over 48 h. Plasma and CSF concentrations of belinostat were quantified with an LC/MS/MS assay. Pharmacokinetic parameters were calculated using non-compartmental methods, and CSF penetration is expressed as the ratio of the area under the concentration-time curve (AUC) in CSF to the AUC in plasma. RESULTS: Belinostat was cleared rapidly from plasma with a half-life of 1.0 h, a mean residence time of 0.47 h, and a clearance of 425 ml/min/m(2). CSF penetration of belinostat was limited. CSF drug exposure was <1% of plasma drug exposure and <10% of free (non-protein bound) plasma drug exposure. CONCLUSION: IV belinostat is rapidly cleared from plasma and has limited penetration into the CSF.

Substituted purine analogues define a novel structural class of catalytic topoisomerase II inhibitors.

By screening 1,990 compounds from the National Cancer Institute diversity set library against human topoisomerase IIalpha, we identified a novel catalytic topoisomerase II inhibitor NSC35866, a S6-substituted analogue of thioguanine. In addition to inhibiting the DNA strand passage reaction of human topoisomerase IIalpha, NSC35866 also inhibited its ATPase reaction. NSC35866 primarily inhibited DNA-stimulated ATPase activity, whereas DNA-independent ATPase activity was less sensitive to inhibition. We compared the mode of topoisomerase II ATPase inhibition induced by NSC35866 with that of 12 other substituted purine analogues of different chemical classes. The ability of thiopurines with free SH functionalities to inhibit topoisomerase II ATPase activity was completely abolished by DTT, suggesting that these thiopurines inhibit topoisomerase II ATPase activity by covalently modifying free cysteine residues. In contrast, NSC35866 as well as two O6-substituted guanine analogues, O6-benzylguanine and NU2058, could inhibit topoisomerase II ATPase activity in the presence of DTT, indicating that they have a different mechanism of inhibition. NSC35866 did not increase the level of topoisomerase II covalent cleavable complexes with DNA, indicating that it is a catalytic inhibitor and not a poison. NSC35866 was also capable of inducing a salt-stable complex of topoisomerase II on closed circular DNA. In accordance with these biochemical data, NSC35866 could antagonize etoposide-induced cytotoxicity and DNA breaks in human and murine cancer cells, confirming that NSC35866 also functions as a catalytic topoisomerase II inhibitor in cells.