Abstract 5560: All three positional isomers of acetyl salicylic acids are equally potent in inhibiting colon cancer cell growth: Differences in mode of COX inhibition

Proceedings: AACR 102nd Annual Meeting 2011‐‐ Apr 2‐6, 2011; Orlando, FL Introduction: Aspirin, 2-(acetyloxy)-benzoic acid or acetyl salicylic acid (o-ASA) is the prototypic chemopreventive agent. The mode of action of o-ASA is irreversible inhibition of cyclo-oxygenases (COX-1 and COX-2). o-ASA has potentially serious gastrointestinal side effects ranging from dyspepsia to gastrointestinal bleeding, obstruction, and perforation. There are 2 other positional isomers of acetyl salicylic acid, 3-(acetyloxy)-benzoic acid (m-ASA) and 4-(acetyloxy)-benzoic acid (p-ASA). In this study we did a head-to-head comparison between these 3 structural isomers in terms of their ulcerogenic properties, mode of COX inhibition, effects on colon cancer growth and cell kinetics. Methods: Cell line: HT-29 human colon adenocarcimoa; Cell growth: MTT; Cell cycle phase distribution: Flow cytometry; Apoptosis: subdiploid (sub-G/G1) peak in DNA content histograms; Proliferation: PCNA. COX-1, -2 enzyme activity: colorimetric kit; Enzyme inhibition reversibility: dialysis. In vivo: 6 hrs after drug administration rats were euthanized, stomachs removed, cut along the greater curvature, rinsed with water, and observed (with magnifying lenses) to count the numbers and measure the lengths (in mm) of all hemorrhagic lesions (ulcer index). Tissue samples were immediately frozen in liquid nitrogen for PGE2, SOD, and MDA determination. Results: All 3 positional isomers inhibited the growth of HT-29 cells with IC50s >3 mM. At 1 mM and 3 mM, the 3 positional isomers showed similar but dose-dependent: (i) increases in apoptosis; (ii) reductions in proliferation; (iii) increases in G/G1 phase, decreases in S and G2/M phases, suggesting a G/G1 block. COX-1 inhibition before and after dialysis with 1 mM concentrations were: o-ASA 72 ± 3% and 72 ± 4%; m-ASA 54 ± 2% and 38 ± 2%; p-ASA 48 ± 2% and 29 ± 1%. COX-2 inhibition before and after dialysis with 3 mM concentrations were: o-ASA 83 ± 2% and 81 ± 2%; m-ASA 80 ± 2% and 56 ± 3%; p-ASA 80 ± 3% and 38 ± 2%. For indomethacine (positive control) at 1 µM the corresponding values were: COX-1 69 ± 2% and 24 ± 1%; COX-2 67 ± 2% and 32 ± 2%. In the stomachs all 3 positional isomers reduced PGE2 and SOD levels and increased MDA to the same extent. Ulcer index in o-ASA treated animals was 38 whereas in m-ASA and p-ASA treated animals it was 12 and 8 respectively. However, in the stomachs of the m-ASA and p-ASA treated animals there were significant numbers of micro-hemorrhagic lesions (m-ASA = 45, p-ASA = 37) representing pre-ulcers. Conclusion: m-ASA and p-ASA are reversible inhibitors of COX-1 and COX-2 whereas o-ASA in an irreversible inhibitor. Gastric side effects were qualitatively and quantitatively different possibly because o-ASA inhibits COX-1 to a greater extent than m-ASA or p-ASA. All 3 positional isomers had similar effects on HT-29 cell kinetics. m-ASA and p-ASA may be alternatives to traditional ASA. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 5560. doi:10.1158/1538-7445.AM2011-5560