Cytodifferentiation therapy by all-trans beta-retinoic acid (RA) for APL patients is encouraging although there are several limitations preventing better clinical outcomes. Most patients in complete remission induced by RA relapse within a few months and resist further treatment with RA. This resistance is due to an increased systemic catabolism of RA and, possibly, a change in the leukemia cell to a less RA-sensitive phenotype. Whatever the reasons for relapse, treatment with combinations of RA and other differentiation agents may lower the levels of RA needed for clinical effects without causing resistance. In these studies we used the human acute promyelocytic leukemia cell line NB4 as a model system. The characteristics reported for NB4 cell growth and differentiation differ markedly. Based on our results, the initial cell density and the stage of growth of the cells used to inoculate experimental cultures were critical parameters for obtaining reproducible growth and differentiation responses of NB4 cells. We found that though butyric acid, tributyrin (a prodrug of butyric acid), or hexamethylene bisacetamide were inactive as sole agents they potentiated RA-induced differentiation of NB4 cells. A measure of the effectiveness of these combinations was that the concentration of MA alone required to induce half-maximal differentiation was reduced 20- to 40-fold in combination with concentrations of butyric acid and hexamethylene bisacetamide that correspond to plasma levels that are or are anticipated to be achievable in the clinic. Therefore, these combinations may have utility for differentiation inducing therapy.
