Evaluation of low-dose pharmacological combinations for inhibition of the effect of Escherichia coli heat-stable enterotoxin in suckling mice

Several pharmacological agents have been reported to block E coli ST effects in suckling mice [1]. We examined combinations of some of these agents in order to evaluate any additive or possibly synergistic inhibitory effect on E coli ST-induced intestinal-fluid secretion in suckling mice. The agents selected have been reported to block E coli ST effects when given in doses higher than those used in the present study. All drugs were mixed into solution with 50 mM of Tris buffer (pH, 7.4). A dose of 0.1 ml of the agent mixed with a second agent and/or 6.4 units of E coli ST was given intragastrically except in the cases of chlorpromazine and quinacrine, which were given sc at a dose of 0.1 ml. The standard 3-hr suckling-mouse assay was performed [2]. Drug concentrations selected were ~1 log lower than reported effective doses. The 6.4 units of partially purified E coli ST used in the experiment resulted in an intestinal-fluid secretion:body weight ratio of 0.127 ± 0.002 (n = 110). WR-2721 (S-2-[3-aminopropyl-amino]ethyl phosphorothioic acid; provided by Dr M.H. Heiffer, Walter Reed Army Institute of Research, Bethesda, MD) is a compound that has properties similar to a sulfhydryl compound [3]. We have found that WR-2721 inhibits E coli ST effects in suckling mice at 0.16 mg/mouse (authors' unpublished observations). Statistical analysis used the data acquired on the day of actual testing, whereas the table incorporates pooled data from all experiments with a given agent. The two-tailed Student's t-test was used to test the significance of the differences between the mean values. The results identify cysteamine and either lanthanum chloride or indomethacin or berberine, chlorpromazine and berberine, lanthanum chloride and lidamadine, indomethacin and quinacrine, and lidamidine and berberine as combinations which are at least additive in their inhibitory effect on E coli ST-induced intestinal-fluid secretion in suckling mice. Perhaps such combinations may eventually be found useful in the treatment of enterotoxic diarrheal illness.