Structure-activity relationship studies on 1-[2-(4-Phenylphenoxy)ethyl]pyrrolidine (SC-22716), a potent inhibitor of leukotriene A(4) (LTA(4)) hydrolase.

Leukotriene B(4) (LTB(4)) is a pro-inflammatory mediator that has been implicated in the pathogenesis of a number of diseases including inflammatory bowel disease (IBD) and psoriasis. Since the action of LTA(4) hydrolase is the rate-limiting step for LTB(4) production, this enzyme represents an attractive pharmacological target for the suppression of LTB(4) production. From an in-house screening program, SC-22716 (1, 1-[2-(4-phenylphenoxy)ethyl]pyrrolidine) was identified as a potent inhibitor of LTA(4) hydrolase. Structure-activity relationship (SAR) studies around this structural class resulted in the identification of a number of novel, potent inhibitors of LTA(4) hydrolase, several of which demonstrated good oral activity in a mouse ex vivo whole blood assay.

Eicosanoids and histamine mediate C5a-induced electrolyte secretion in guinea pig ileal mucosa.

C5a is a biologically active polypeptide formed during the course of complement activation and is known to possess histamine-releasing and neutrophil chemotactic properties. In the present study, we have demonstrated that C5a can regulate electrolyte transport across guinea pig ileum, and we have investigated its mechanism of action. Segments of ileum stripped of longitudinal muscle were mounted in Ussing chambers (Krebs' buffer, 37 degrees C, 95% O2/5% CO2) for monitoring short-circuit current (Isc). Serosal application of C5a evoked a transient increase in Isc with an EC50 value of 5.0 nM indicating a potent effect. The C5a-induced increase in Isc was abolished by elimination of both Cl- and HCO3- from the Krebs' solution. Pretreatment with the cyclooxygenase inhibitor indomethacin (5 microM), the neurotoxin tetrodotoxin (0.5 microM) and the H1 receptor antagonist pyrilamine (0.5 microM) reduced the effect of C5a, but the muscarinic antagonist atropine (0.5 microM) was without effect. C5a (100 nM) also evoked the release of histamine (measured by radioimmunoassay in the serosal bathing fluid) by 282% of the control value. In conclusion, in the guinea pig ileum C5a stimulates mucosal anion secretion by releasing histamine and cyclooxygenase products of arachidonic acid. The response is also mediated, in part, via non-chloinergic enteric nerves.

Colitis reduces short-circuit current response to inflammatory mediators in rat colonic mucosa.

Inflammatory mediators may contribute to the diarrhea associated with colitis. Although the secretory action of such mediators is reported in normal tissue, there is little information regarding their effects on inflamed tissue. We examined the short-circuit current response (Isc) to these mediators, in mitomycin-C (MC)-induced colitis, a model with histological similarities to colitis in man. Rats were injected once with MC (3.25 mg/kg, intraperitoneally) or vehicle. The colons were removed three and seven days later and mounted, devoid of muscularis, in Ussing chambers for measurement of Isc, potential difference (PD), and resistance (Rt). MC-treated rats had diarrhea after three days, and microscopic studies revealed colonic inflammation. There were no significant differences in Rt, PD, and Isc between control and MC-treated tissues at three and seven days. Maximal increases in Isc to bradykinin, prostaglandin E1, carbachol, substance P, and serotonin were depressed at three and/or seven days after MC. The Isc response to theophylline was not affected. Theophylline activates secretion through an intracellular mechanism; the other agonists act by interaction with epithelial cell membranes. Therefore, the mechanism for the decreased Isc may result from uncoupling of receptors to second-messenger systems or desensitization of receptor-linked secretory mechanisms.

Second-generation leukotriene B4 receptor antagonists related to SC-41930: heterocyclic replacement of the methyl ketone pharmacophore.

Our previous reports have highlighted the first-generation leukotriene B4 (LTB4) receptor antagonist SC-41930 (7-[3-(4-acetyl-3-methoxy-2-propylphenoxy)propoxy]3,4- dihydro-8-propyl-2H-1-benzopyran-2-carboxylic acid) which has potent oral, topical, and intracolonic activity in various animal models of inflammation. Extensive structure-activity relationship studies, in which a series of heterocyclic replacements for the methyl ketone functional group of SC-41930 was explored, identified SC-50605 (7-[3-[2-(cyclopropylmethyl)-3-methoxy-4- (4-thiazolyl)phenoxy]propoxy]-3,4-dihydro-8-propyl-2H-1-benzopyran-2- carboxylic acid) as an optimized analog within a series of thiazoles. SC-50605 was found to be significantly more potent than SC-41930 in LTB4 receptor binding, chemotaxis, and degranulation assays. It also displayed very good activity in animal models of colitis and epidermal inflammation by oral, topical, intravenous, and intracolonic routes of administration. The resolved enantiomers of SC-50605 were obtained by chiral chromatography and both demonstrated good in vitro and in vivo activity. The (+)-isomer (SC-52798) is currently being evaluated as a potential clinical candidate for psoriasis and ulcerative colitis therapy.

The in vitro pharmacology of SC-51146: a potent antagonist of leukotriene B4 receptors.

Previously, we reported that SC-41930 is a potent leukotriene B4 (LTB4) receptor antagonist. An analog of SC-41930, SC-51146, was evaluated as an antagonist of LTB4 receptors. SC-51146 was shown to bind to LTB4 high affinity binding sites on human neutrophils (PMN) with a dissociation constant (KD) value of 1.5 +/- 0.1 nM, compared to 19 +/- 1.3 nM for SC-41930. PMN chemotaxis studies and Scatchard analyses of [3H]LTB4 binding in PMN membranes indicated that SC-51146 acted as a competitive antagonist. The IC50 value of SC-51146 for the inhibition of PMN chemotaxis induced by 30 nM LTB4 was 38 +/- 12 nM. SC-51146 inhibited PMN degranulation induced by 50 nM LTB4 with an IC50 value of 29 +/- 7 nM. The antagonism by SC-51146 of LTB4-induced PMN degranulation appeared to be noncompetitive. The specificity of SC-51146 for LTB4 receptors vs. fMLP receptors was improved approximately 29 and 44 times over SC-41930. SC-51146 showed relatively weak inhibitory activity on the production of superoxide, LTB4 and/or prostaglandin E2 by human PMN or HL-60 cells. SC-51146 had little activity on ram seminal vesicle cyclooxygenase, and no activity on porcine pancreatic phospholipase A2. SC-51146 is a racemate comprised of the (+) enantiomer, SC-53228, and the (-) enantiomer, SC-53229. Both stereoisomers exhibited pharmacological profiles similar to SC-51146 in these aforementioned in vitro systems. The highly potent and specific antagonistic action of SC-51146 on LTB4 receptors should be particularly useful in elucidating the role of LTB4 in the pathogenesis of inflammatory diseases where excessive levels of LTB4 have been reported.

Characterization of human uroguanylin: a member of the guanylin peptide family.

Guanylin, a peptide homologue of the bacterial heat-stable enterotoxins (ST), is an endogenous activator of guanylate cyclase C (GC-C). We have initiated a search for other members of the guanylin peptide family and in the current study describe a "guanylin-like peptide" from human urine. Bioactivity was monitored by determining the effect of urine extracts on T84 cell guanosine 3',5'-cyclic monophosphate (cGMP) levels. Purification yielded two bioactive peaks of peptides that, when sequenced by NH2-terminal analysis, possessed 15 and 16 amino acids. The sequence of the smaller peptide represented an NH2-terminal truncation of the larger peptide. We have termed the larger peptide human uroguanylin; it has the following amino acid sequence: NDDCELCVNVACTGCL. Human uroguanylin shares amino acid sequence homology with guanylin and ST. Synthetic uroguanylin increased cGMP levels in T84 cells, competed with 125I-labeled ST for receptors, and stimulated Cl- secretion as reflected by an increased short-circuit current. Thus we report the isolation from human urine of a unique peptide, uroguanylin, that behaves in a manner similar to guanylin and appears to be a new member of this peptide family.

Stereospecific actions of misoprostol on rat colonic electrolyte transport.

Misoprostol (Miso) produces a mild, transient diarrhea in some patients, which is believed to be partly due to intraluminal fluid accumulation. To better understand this diarrheagenic action, we compared the effects of Miso, its 4 stereoisomers (11R16R, 11R16S, 11S16S, 11S16R), misoprostol free acid (Miso-FA), and 16,16-dimethyl PGE2 (dmPGE2) on rat colonic electrolyte transport in vitro. Increases in short-circuit current (Isc) were measured (after serosal addition) in segments of mucosa stripped of muscularis and mounted in Ussing chambers. The rank order of apparent potencies, in terms of threshold, were (microM): 11R, 16S (1.2) approximately dmPGE2 (1.0) > Miso-FA (10.0) approximately Miso >> 11R, 16R; 11S, 16R; 11S, 16S (all inactive at 100 microM). The response to dmPGE2 and Miso was attenuated in the presence of the Na+/K+/Cl- co-transport inhibitor bumetanide (100 microM). Pretreatment with atropine (0.1 microM) did not affect the Isc response to Miso, Miso-FA, or dmPGE2. Tetrodotoxin partially attenuated (39 +/- 9% inhibition) the response to Miso-FA, but did not affect Miso or dmPGE2. In conclusion, Miso increases Cl- secretion across rat colonic mucosa through a direct action on epithelial cells. The activity resides in the 11R,16S isomer, thus implying a stereospecific interaction at PGE receptors. The effect of Miso to stimulate epithelial Cl- secretion might contribute to its diarrheagenic action in vivo.

Effect of the thiol-oxidizing agent diamide on NH2Cl-induced rat colonic electrolyte secretion.

The granulocyte-derived oxidant, monochloramine (NH2Cl), is known to stimulate chloride ion secretion in rat distal colonic mucosa mounted in Ussing chambers, through mechanisms that are sensitive and insensitive to tetrodotoxin (TTX). The possible role of intracellular thiols, in the mechanism of action of NH2Cl as a secretagogue, was evaluated with the thiol-oxidizing agent diamide and by measuring tissue sulfhydryl levels in response to NH2Cl. Serosal exposure to the antioxidant glutathione (0.25 mM), 5 min before NH2Cl (50 microM) addition, decreased the maximal effect of 50 microM NH2Cl on short-circuit current (Isc). The NH2Cl-stimulated increase in Isc was not affected by mucosal amiloride (5 microM). Pretreatment with 0.1 mM diamide shortened the lag period before the increase in Isc in response to NH2Cl, but it did not affect the maximal increase in Isc. Although TTX (0.5 microM) increased the lag time for achievement of the maximal Isc response to NH2Cl, the neurotoxin did not inhibit the effect of diamide, suggesting that diamide acts primarily on the nonneural component of NH2Cl-stimulated secretion. Incubation of colonic mucosa with NH2Cl, with or without diamide, decreased cellular acid-soluble sulfhydryl concentrations. Taken together, the results support a role for epithelial cell thiols in NH2Cl-stimulated electrolyte secretion by the rat colon.

An in vitro system to screen for diarrheagenic chemicals.

We examined an in vitro system to screen for diarrheagenic chemicals using an established intestinal cell line (T84 human colonic carcinoma). The cells were grown on Millicell-PCF (polycarbonate membrane) wells. The cells were seeded at approximately 5 x 10(6) cells/30mm well and incubated for 9-11 days in a 5% CO2 incubator saturated with water at 37 degrees C. The culture medium was a 1:1 mixture of Ham's F12 and Dulbecco's MEM with 5% fetal bovine serum and 25 micrograms/ml gentamicin sulfate. The well containing cells was removed from the incubator and mounted in a modified Ussing chamber for measurement of short-circuit current (ISC). Chemical-induced increases in ISC are usually indicative of electrogenic epithelial Cl- secretion, which is associated with diarrheagenic effects in animals and humans. T84 cells grown on Millicell-PCF membrane responded with an increase in ISC after basolateral addition of the cholinergic (muscarinic) agonist carbachol, prostaglandin E2, 16,16-dimethylprostaglandin E2, and forskolin, while non-diarrheagenic prostaglandin D2 did not affect ISC. Based on our results, this in vitro system has the potential to be adapted as a rapid screen for detecting diarrheagenic chemicals.

The design and synthesis of second generation leukotriene B4 (LTB4) receptor antagonists related to SC-41930.

SC-41930, 7-[3-(4-acetyl-3-methoxy-2-propylphenoxy)propoxy]-3,4- dihydro-8-propyl-2H-1-benzopyran-2-carboxylic acid, is a selective, orally active, LTB4 receptor antagonist currently in clinical trials for psoriasis and ulcerative colitis. Exhaustive SAR studies found a potential backup compound, SC-50605, which was 7-16 times more potent than SC-41930 in LTB4 receptor binding, chemotaxis and degranulation assays. SC-50605 also inhibited LTB4-induced intradermal chemotaxis in cavine skin at an oral dose of 0.10 mg/kg and displayed good activity in animal models of colitis and epidermal inflammation both orally and topically.

Human guanylin: cDNA isolation, structure, and activity.

Guanylin is a mammalian peptide homologue of heat-stable enterotoxins that acts on intestinal guanylate cyclase to elicit an increase in cyclic GMP. We have isolated a cDNA encoding an apparent precursor of guanylin from a human intestinal cDNA library. The mRNA is expressed at high levels in human ileum and colon. Human guanylin stimulated increases in T84 cell cyclic GMP levels, displaced 125I-labelled heat-stable enterotoxin (STa) binding to this cell line, and stimulated increases in short-circuit current (Isc) of isolated rat proximal colonic mucosa. This peptide may play a role in regulating fluid and electrolyte absorption in human intestines.

Endothelin-1 stimulates contraction and ion transport in the rat colon: different mechanisms of action.

Endothelin-like immunoreactivity has been detected in all regions of the rat gastrointestinal tract. In the present study, we studied the effect of endothelin-1 (ET-1) on muscle contraction and ion transport in the rat colon. Isometric tension was recorded in colonic muscle strips oriented along their longitudinal axis. The effect of ET-1 on ion transport was investigated by assessing changes in short-circuit current in segments of muscle-stripped rat colon in Ussing chambers. ET-1 induced concentration-dependent contraction of the colon (EC50, 3 nM). The concentration-response curve to ET-1 was not modified by the neuronal blocker tetrodotoxin (0.1 microM) or by atropine (1 microM). Pretreatment of colon muscle strips with the calcium channel blockers diltiazem (0.1 microM) or nicardipine (1 microM) had no effect on the contractile response to ET-1. Furthermore, the response was not affected by removal of extracellular calcium. In the ion transport studies, serosal addition of ET-1 produced a transient, bumetanide (chloride secretion inhibitor) -sensitive, increase in transepithelial short-circuit current. The maximal increase was 107 +/- 13 microA/sq. cm, with an EC50 of 2.5 nM. The increase in short-circuit current evoked by ET-1 was not significantly affected by 1 microM atropine, but was reduced by 50% (P less than .05) by 1 microM tetrodotoxin, or removal of extracellular calcium. We conclude that ET-1 stimulates smooth muscle directly, whereas its effect on epithelial chloride secretion is mediated in part via the enteric nerves. Moreover, the effect of ET-1 in these two systems can be differentiated on the basis of sensitivity to extracellular calcium.

Multiple actions of the leukotriene B4 receptor antagonist SC-41930.

7-[3-(4-acetyl-3-methoxy-2-propylphenoxy)-propoxy]-3,4-dihydro-8- propyl-2H-1-benzopyran-2-carboxylic acid (SC-41930), a leukotriene B4 (LTB4) receptor antagonist with anti-inflammatory activity in animal models of colitis, was evaluated for effects on superoxide, LTB4 and prostaglandin E2 production. SC-41930 inhibited human neutrophil (PMN) superoxide generation maximally stimulated by f-Met-Leu-Phe (IC50 4 microM) and C5a (IC50 approximately 12 microM). Moreover, postreceptor stimulation of superoxide production by NaF (a G protein activator), but not by phorbol myristate acetate, was significantly inhibited by SC-41930, indicating that SC-41930 may act via attenuation of a G protein-mediated signal transduction. SC-41930 also inhibited A23187-stimulated LTB4 production (IC50 5.3 microM) in human PMN as well as LTB4 (IC50 2.1 microM) and prostaglandin E2 (IC50 2.9 microM) production in HL-60 cells. When coinjected intradermally (400 micrograms/site), SC-41930 inhibited A23187-stimulated increases in LTB4 levels in guinea pig skin. SC-41930 inhibited human synovial phospholipase A2 (IC50 72 microM), A23187-stimulated 5-hydroxy-eicosatetranoic acid production in human PMN (IC50 8.5 microM), and rat peritoneal leukotriene A4 hydrolase (IC50 20 microM), but not ram seminal vesical cyclooxygenase. The results suggest that the anti-inflammatory activity of SC-41930 could be attributed to postreceptor inhibition of inflammatory mediator production by PMN and other cells in addition to antagonism of PMN LTB4 receptors.

Ca-mediated stimulation of Cl secretion by reactive oxygen metabolites in human colonic T84 cells.

Monochloramine (NH2Cl), a granulocyte-derived reactive oxygen metabolite (ROM), increases short-circuit current (Isc) in cultured T84 monolayers in a concentration-dependent manner up to nonlethal concentrations of 75 microM. Isc increases slowly after NH2Cl, reaching a peak value of 18 +/- 2 microA/cm2 20 min after addition. The Isc changes are persistent (lasting over 20-30 min), depend on medium Cl, and are inhibitable with bumetanide. 36Cl flux studies demonstrated that NH2Cl increases serosa-to-mucosa flux of Cl without changing mucosa-to-serosa flux, consistent with stimulation of electrogenic Cl secretion. Isc responses to NH2Cl, but not PGE2, are dependent on medium calcium. As demonstrated in fura-2-loaded T84 cells, NH2Cl increases free cytosolic calcium by influx of extracellular Ca2+ and by release of Ca2+ from endogenous stores. However, NH2Cl had no effect on phosphatidylinositol metabolism or cyclic nucleotide levels. We conclude that ROM directly stimulate electrolyte secretion, an effect in part mediated by increases in cytosolic Ca2+, possibly through increasing Ca2+ permeability of cellular membranes.

Cholecystokinin-mediated ileal electrolyte transport in the guinea pig. Characterization of receptor subtype.

Cholecystokinin (CCK) receptors are currently divided into at least two subtypes: a CCK-A subtype, responsive to the sulfated form of cholecystokinin octapeptide (CCK-8) and selectively antagonized by L-364,718, and a CCK-B subtype, which shares equal affinities for gastrin and CCK-8. In the present study the receptor subtype that mediates guinea pig ileal secretion by evaluating the potencies of CCK- and gastrin-related peptides to evoke increases in transmucosal short-circuit current was characterized. The antagonist potencies of L-365,260 (CCK-B selective) and L-364,718 (CCK-A selective) against CCK-8 were also determined. Both CCK-8 and cerulein, when added to the serosal side of the tissue, evoked increases in the short-circuit current, having EC50 values of 0.8 and 0.2 nmol/L, respectively. Desulfated (SO3) CCK-8, CCK-4, gastrin17-I, pentagastrin, gastrin17-II, and gastrin13-I were relatively weak agonists (EC50 greater than 1000 nmol/L. Cholecystokinin octapeptide-induced short-current responses were competitively antagonized by L-364,718 (pA2, 10.3) and L-365,260 (pA2, 7.4). The high selectivity of the tissue for sulfated CCK-8 suggests that the secretory effect of CCK-8 on guinea pig ileal electrolyte transport is mediated by a CCK-A receptor. The potent effect of L-364,718 against CCK-8 is also consistent with an action at the A-subtype receptor.

Neurokinin A increases short-circuit current across rat colonic mucosa: a role for vasoactive intestinal polypeptide.

1. Neurokinin A (NKA) is a mammalian tachykinin distributed principally in the nervous system, including the myenteric innervation of the gut. 2. NKA may be involved in neurogenic inflammation and as a modulatory factor in the diarrhoea associated with mucosal inflammation of inflammatory bowel disease (ulcerative colitis). 3. We evaluated the effect of NKA on the short-circuit current ISC, assumed to reflect electrogenic chloride secretion, across muscle-stripped rat colonic mucosa mounted in Ussing chambers. 4. Serosal addition of NKA produced a concentration-dependent (0.1-100 nM) increase in ISC with an EC50 (half-maximal effective concentration) value of 7.5 nM. The maximum (mean +/- S.E.M.) increase in ISC (microA/cm2) for NKA was 111 +/- 10. 5. Tetrodotoxin (0.5 microM) and bumetanide (10 microM), but not atropine (1.0 microM), hexamethonium (100 microM) or pyrilamine (10 microM), significantly inhibited NKA-induced increases in ISC. 6. The response to NKA was attenuated by 45 min pre-treatment with antisera raised against vasoactive intestinal polypeptide (VIP). Moreover, prior desensitization to VIP attenuated the effect of NKA. 7. These studies suggest that NKA increases ISC in rat colon, in part, through a non-cholinergic neural mechanism involving VIP.

Monochloramine, a neutrophil-derived oxidant, stimulates rat colonic secretion.

Neutrophil-derived oxidants may be involved in inflammatory bowel disease. Stable oxidants formed by halogenation of primary amines (e.g., monochloramine, NH2Cl) are extremely potent and may be of major importance in the pathophysiological response in inflammatory bowel disease. We tested the effects of NH2Cl relative to other oxidants on muscle-stripped rat colon under short-circuit conditions. Serosal addition of the oxidants evoked a concentration-dependent increase in short-circuit current (Isc). The EC50 values were 3.2 microM for NH2Cl, 6.5 microM for HOCl and 6.5 microM for H2O2. Responses to NH2Cl and H2O2 were abolished by removal of Cl- or Ca++. Unidirectional 36Cl- flux measurements showed that both oxidants (50 microM) evoked significant decreases in net chloride absorption. Tetrodotoxin (0.5 microM) and atropine (0.5 microM) partially inhibited the initial phase of the response to 50 microM NH2Cl; tetrodotoxin completely inhibited and atropine partially inhibited the response to 50 microM H2O2. Piroxicam (5 microM) inhibited the increase in Isc to 50 microM NH2Cl and H2O2 by 20-45% and 90%, respectively. Serosal prostaglandin E2 levels were significantly increased after the addition of 50 microM NH2Cl and H2O2. The morphological response to NH2Cl consisted of changes in mucosal depth and crypt architecture. In conclusion, at concentrations found in inflamed tissue, both NH2Cl and H2O2 increase Isc probably by stimulating release of arachidonate metabolites and neurotransmitter(s). NH2Cl also may act directly on the epithelial cell to stimulate Isc and evoke Cl- secretion.

Aminoalkynyldithianes. A new class of calcium channel blockers.

Several dithiane derivatives, prepared as intermediates for compounds structurally related to the therapeutically useful antimuscarinic agent oxybutynin, were effective inhibitors of calcium ion induced contraction of guinea pig ileal strips and of KCl-induced calcium entry into neuronal cells. Although the first member of this series, 2-[5-(diethylamino)-3-pentynyl]-1,3-dithiane (2a), was only marginally effective, its condensation product with diphenyl ketone, i.e. 2-[5-(diethylamino)-3-pentynyl]-2-(a,a-diphenyl-a- hydroxymethyl)-1,3-dithiane (3a), demonstrated weak, but significant, calcium channel antagonist activity. As part of a structure-activity relationship (SAR) study, various structural analogues of 2a and 3a were prepared and examined for calcium antagonist properties. In addition to these structural types, ring bridged (tricyclic) congeners of 3, i.e. 4, related bicyclic compounds 5, dehydroxylated derivatives 6, some homologous 2-[[[(N,N-disubstituted-amino)methyl]2- phenyl-1,3-dithianes (7), and a series of 2-[6-[N,N-disubstituted-amino)methyl]-1-hydroxy-1-phenyl- 4-hexynyl]-1,3-dithianes (8) were prepared and studied for calcium channel blocking activity. In general, greatest potency was noted in the tricyclic series 4; however, a definitive SAR could not be established. A structural similarity between several potent calcium antagonists having the structures 7c, 8b, and 8d and the well-known calcium channel blockers verapamil and tiapamil suggests these compounds may act at the same site. Compounds in the other classes (2-6) failed to show clearly defined SAR and their potency differed markedly in two tests for calcium channel antagonist activity. These results may indicate that the dithiane derivatives 2-6 produce their effects in a manner differing from that of the calcium channel antagonists diltiazem, verapamil, and nitrendepine.

Scavenging effect of 5-aminosalicylic acid on neutrophil-derived oxidants. Possible contribution to the mechanism of action in inflammatory bowel disease.

Inflammatory phagocytic leukocytes produce superoxide and hydrogen peroxide and secrete myeloperoxidase (MPO) into the extracellular medium. MPO catalyzes the oxidation of Cl- by H2O2 to yield chlorinated oxidants (e.g. HOCl and NH2Cl), which have been shown to induce pathologic changes in mucosal function. We examined the ability of 5-aminosalicylic acid (5-ASA), a drug used to treat inflammatory bowel disease (IBD), to inhibit oxidation of L-cysteine by NH2Cl, HOCl and H2O2. NH2Cl and HOCl were especially strong oxidants against L-cysteine. 5-ASA prevented L-cysteine oxidation by NH2Cl and HOCl; an interaction associated with the formation of characteristic absorption spectra due to the oxidation of 5-ASA was observed. NH2Cl and HOCl evoked characteristic increases in short-circuit current (Isc), indicative of net electrolyte transport, when added to the serosal side of stripped rat colon mounted in Ussing chambers. Premixing of NH2Cl with 5-ASA 10 min before addition to the tissue markedly reduced the secretory response to NH2Cl. In contrast, 5-ASA immediately reduced the response to HOCl. The reduction in the functional response to NH2Cl and HOCl by 5-ASA may contribute to its mechanism of action in the treatment of the symptoms of IBD.