Antitumoral activity of 13-demethyl or 13-substituted analogues of all-trans retinoic acid and 9-cis retinoic acid in the human myeloid leukemia cell line HL-60

The antitumoral activity of 13-demethyl or 13-substituted all-trans retinoic acid (ATRA) and 9-cis retinoic acid (9CRA) was tested using the myeloid leukemia cell line HL-60. Cell proliferation, differentiation and apoptosis were evaluated by flow cytometry and DNA fragmentation assay. The ability to bind to human RXR alpha and to activate either human retinoic acid response element (RARE)-mediated gene expression or rat CRABPII retinoid X response element (RXRa)-mediated gene expression were determined using luciferase reporter plasmids. In terms of the magnitude of the regulatory activity for the proliferation and differentiation of HL-60 cells, the compounds ranked as follows: ATRA > 13-ethyl ATRA > 13-demethyl ATRA > 13-phenylethyl ATRA > 13propyl ATRA > 13-butyl ATRA (ATRA analogues) and 9CRA > 13-ethyl 9CRA > 13-demethyl 9CRA > 13-propyl 9CRA > 13-phenetyl 9CRA > 13-butyl 9CRA (9CRA analogues). Regarding the magnitude of the apoptosis-inducing activity, the order was: 9CRA > 13-ethyl 9CRA > 13-demethyl 9CRA, with ATRA and its analogues and the other 9CRA analogues virtually inactive. Similar trends were observed in binding affinity for RXRa and transactivation activity toward RARIE- or RXRE-mediated gene expression. The results clearly indicate that the presence of a methyl group at C-13 is essential for the antitumoral activity of ATRA and 9CRA, and that bulky substituents exceeding two carbon atoms or the absence of substitution at position 13 significantly reduce the binding affinity for RAR and RXR, leading to a decreased RAR/RARE and/or RXR/RXRE-mediated gene expression.