Induction of apoptosis in malignant B cells by phenylbutyrate or phenylacetate in combination with chemotherapeutic agents

Phenylacetate (PA) and phenylbutyrate (PB) are aromatic fatty acids that ar e presently undergoing evaluation as potential antineoplastic agents. In vi tro, PA and PB cause differentiation or growth inhibition of malignant cell s, Clinical trials of these drugs as single agents indicate that they are n ot myelosuppressive; therefore, combinations with other chemotherapy agents may be possible. The goals of this study were to determine whether PA and PB (a) are cytotoxic to malignant B cells from patients with non-Hodgkin's lymphoma and B-cell chronic lymphocytic leukemia and (b) exhibit additive o r synergistic induction of apoptosis when administered to myeloma cell line s in combination with conventional drugs. In the clinical specimens, cytoto xicity was measured by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazo lium bromide assay, and percent apoptosis was measured using 7-aminoactinom ycin D and now cytometry, Viability was decreased by >50% in 7% (1/15) of n on-Hodgkin's lymphoma samples treated with 5 mM PA, 27% treated with I mM P B, and 60% treated with 2 mM PB, Likewise, viability was decreased by >50% in 44% (4/9) of chronic lymphocytic leukemia samples treated with 5 mM PA, 67% treated with 1 mm PB, and 100% treated with 2 mh PB, Studies in the mye loma cell lines demonstrated that PB treatment induced activation of caspas es 3, 7, and 9 accompanied by cleavage of their substrates and internucleos omal DNA degradation. Combinations of PA or PB with conventional drugs (cyt arabine, topotecan, doxorubicin, etoposide, chlorambucil, melphalan, fludar abine, carboplatin, and cisplatin) mere examined for synergism (combination index <1 in median effect analysis) in inducing apoptosis of both the MY5 and 8226 human myeloma cell lines, At concentrations that killed >50% of ce lls, most combinations were additive; however, PB was synergistic with cyta rabine, etoposide, and topotecan, with the combination index <1 at each of the 50, 75, and 95% apoptosis levels. These observations indicate that PA a nd PB can induce apoptosis in malignant B cells and enhance the cytotoxicit y of agents used in the treatment of these malignancies.