Purpose: Vorinostat [suberoylanilide hydroxamic acid (SAHA)] is a potent histone deacetylase inhibitor with promising clinical efficacy as an anticancer agent. In this preclinical study, we evaluated combining cytosine arabinoside [1-beta-D-arabinofuranosylcytosine (ara-C)] and/or etoposide with vorinostat for use in the treatment of acute leukemias.
Experimental design: Cell survival was examined in vitro in HL-60 human myeloid leukemia cells and K562 myeloid blast crisis chronic myelogenous leukemia cells, using the 2,3-bis[2-methoxy-4-nitro-5-sulfophenyl]-2H-tetrazolium-5-carboxanilide inner salt and/or fluorescein diacetate/propidium iodide assays. Drug interactions were analyzed by the combination index method (CalcuSyn) and by a novel statistical method that we developed (SynStat). Cell cycle phase distribution was measured by flow cytometry.
Results: Cytotoxic antagonism resulted when vorinostat was combined concomitantly with ara-C; however, when vorinostat was given first followed by a drug-free interval before ara-C treatment, this sequential combination was mostly synergistic. Etoposide combined with vorinostat was additive to synergistic, and the synergism became more pronounced when etoposide was given after vorinostat. Cell cycle analyses revealed that the sequence-dependent interaction of vorinostat and ara-C or etoposide reflected the arrest of cells in G1 or G2 phase during vorinostat treatment and recovery into S phase after removal of vorinostat.
Conclusions: These findings using two independent methods to assess drug combination effects provide a preclinical rationale for phase I trials of the sequential combination of vorinostat followed by ara-C and etoposide in patients with advanced or refractory leukemias. CalcuSyn findings were concordant with those of SynStat, validating the use of the latter in analyzing drug interactions.