tokens: The purpose of this study was to evaluate the in vitro activity of the principal metabolites of ATRA [ 4-hydroxy-retinoic acid ( 4-OH-RA ) , 18-hydroxy-retinoic acid ( 18-OH-RA ) , 4-oxo-retinoic acid ( 4-oxo-RA ) , and 5 , 6-epoxy-retinoic acid ( 5 , 6-epoxy-RA ) ] in NB4 , a human promyelocytic leukemia cell line that exhibits the APL diagnostic t ( 15 ; 17 ) chromosomal translocation and expresses the PML-RAR alpha fusion protein .