NO-donating
aspirin (
NO-ASA) is a potentially important chemopreventive agent against
cancer. Since positional isomerism affects strongly its potency in inhibiting
colon cancer cell growth, we studied the metabolic transformations of its ortho-, meta-, and para-isomers in rat liver and colon cytosolic, microsomal, and mitochondrial fractions as well as in intact HT-29 human
colon cancer cells.
NO-ASA and metabolites were determined by high-performance liquid chromatography and products identified by mass spectroscopy, as required. For all three isomers, the acetyl group on the ASA moiety was hydrolyzed rapidly. This was followed by hydrolysis of the
ester bond linking the
salicylate anion to the spacer. The ortho- and para-isomers produced
salicylic acid and a putative intermediate consisting of the remainder of the molecule, which via a rapid step generated
nitrate, (hydroxymethyl)
phenol, and a conjugate of spacer with
glutathione. The meta-isomer, in contrast, generated
salicylic acid and (nitroxymethyl)
phenol, the latter leading to (hydroxymethyl)
phenol and the
glutathione-spacer conjugate. This metabolic pathway takes place in its entirety only in the cytosolic fraction of the tissues tested and in intact human
colon cancer cells, perhaps reflecting exposure to the cytosolic
glutathione S-transferase, which catalyzes the formation of the spacer-
glutathione conjugate. Thus, the three positional isomers of
NO-ASA differ in their metabolism and these differences correlate with their differential effects on
cancer cell growth, underscoring the importance of positional isomerism in modulating
drug effects.