4-Aminosalicylic acid, in contrast to 5-aminosalicylic acid, has no effect on arachidonic acid metabolism in human neutrophils, or on the free radical 1,1-diphenyl-2-picrylhydrazyl

Colon-specific prodrugs of 4-aminosalicylic acid for inflammatory bowel disease

Despite the advent of biological products, such as anti-tumor necrosis factor-α monoclonal antibodies (infliximab and adalimumab), for treatment of moderate to severe cases of inflammatory bowel disease (IBD), most patients depend upon aminosalicylates as the conventional treatment option. In recent years, the increased knowledge of complex pathophysiological processes underlying IBD has resulted in development of a number of newer pharmaceutical agents like low-molecular-weight heparin, omega-3 fatty acids, probiotics and innovative formulations such as high-dose, once-daily multi-matrix mesalamine, which are designed to minimize the inflammatory process through inhibition of different targets. Optimization of delivery of existing drugs to the colon using the prodrug approach is another attractive alternative that has been utilized and commercialized for 5-aminosalicylic acid (ASA) in the form of sulfasalazine, balsalazide, olsalazine and ipsalazine, but rarely for its positional isomer 4-ASA - a well-established antitubercular drug that is twice as potent as 5-ASA against IBD, and more specifically, ulcerative colitis. The present review focuses on the complete profile of 4-ASA and its advantages over 5-ASA and colon-targeting prodrugs reported so far for the management of IBD. The review also emphasizes the need for reappraisal of this promising but unexplored entity as a potential treatment option for IBD.

Mechanism of action of 5-arninosalicylic acid

5-Aminosalicylic Acid (5-ASA) has been used for over 50 years in the treatment of inflammatory bowel disease in the pro-drug form sulphasalazine (SASP). SASP is also used to treat rheumatoid arthritis. However whether the therapeutic properties of SASP are due to the intact molecule, the 5-ASA or sulphapyridine components is unknown. Several mechanisms of action have been proposed for 5-ASA and SASP including interference in the metabolism of arachidonic acid to prostaglandins and leukotrienes, scavenging,of reactive oxygen species, effects on leucocyte function and production of cytokines. However, it is unlikely that the anti-inflammatory properties of SASP and 5-ASA are due to several different properties but more likely that a single property of 5-ASA explains the theraapeutic effects of 5-ASA and SASP. Reactive oxygen species (ROS) are involved in the metabolism of prostaglandins and leukotrienes and can act as second messengers, and so the scavenging of ROS may be the single mechanism of action of 5-ASA that gives rise to its antiinflammatory effects in both inflammatory bowel disease and rheumatoid arthritis.

Inhibition by aminosalicylates of phosphatidic acid formation induced by superoxide, calcium or spermine in enterocyte mitochondria

Inflammation is associated with oxidative stress and altered cellular calcium homeostasis. Our earlier studies have shown that, increased phosphatidic acid (PA) formation occurred in enterocyte mitochondria when exposed to superoxide, divalent metal ions or polyamines resulting in altered lipid composition. Since aminosalicylates are the drug of choice for gut inflammation, we have tested the effect of aminosalicylates on PA formation by enterocyte mitochondria. When stimulated by superoxide, Ca2+ or spermine, phosphatidyleth-anolamine (PE) degradation and PA formation occurred in enterocyte mitochondria which can be inhibited by aminosalicylates. The inhibition was 50-60% at 0.5-mM concentration and at 1- or 2-mM final concentration, complete inhibition was observed. Both 5-aminosalicylate (5-ASA) and 4-aminosalicylate (4-ASA) showed similar effects. The stimulation of PA formation by calcium or spermine was not due to increased generation of superoxide by mitochondria which was confirmed by measurement of superoxide production by the mitochondria. These studies suggest that in addition to other cellular effects, aminosalicylates may prevent the enterocyte mitochondrial damage by inhibition of PA formation and PE degradation and alteration of mitochondrial lipid composition.

The effect of 5-aminosalicylate and para-aminosalicylate on the synthesis of prostaglandin E2 and leukotriene B4 in isolated colonic mucosal cells

The effect of 5-aminosalicylate (5-ASA) and para-aminosalicylate (PAS) on the synthesis of prostaglandin E2 (PGE2) and leukotriene B4 (LTB4) was studied in 24 healthy volunteers. Both drugs are successfully used in the treatment of chronic inflammatory bowel disease, but the biochemical pathway of their anti-inflammatory action is still unknown. Biopsies were taken from the descending colon and were isolated biochemically. 5-ASA, PAS and a control were added to the suspension of isolated colonic mucosal cells and incubated over 0-45 min. Both 5-ASA and PAS did not alter the PGE2 production but, compared with PAS and the control, 5-ASA decreased the LTB4 synthesis in a dose-related fashion. As a result, the LTB4/PGE2 ratio was significantly diminished by 10(-4) mol/l 5-ASA. These findings are consistent with those of other authors, indicating that 5-ASA, at least in part, modulates the colonic eicosanoid synthesis. In contrast, PAS did not influence the mucosal production of PGE2 and LTB4 and therefore must exert some other biochemical action in order to explain its therapeutic effects in the treatment of Crohn's disease or ulcerative colitis.

Salicylates for ulcerative colitis--their mode of action

Delivery of 5-aminosalicylic acid to the colon by sulphasalazine, other azo-bonded compounds and controlled-release preparations is introduced in the context of metabolism by epithelial cells and therapeutic efficacy in ulcerative colitis. Potential modes of action are then reviewed, including actions on luminal bacteria, epithelial cell surface receptors, cellular events (such as nitric oxide release or butyrate oxidation), electrolyte transport and epithelial permeability. Evidence for an influence of salicylates on circulating and lamina propria inflammatory cells is presented, as well as actions on adhesion molecules, chemotactic peptides and inflammatory mediators, such as eicosanoids, platelet-activating factor, cytokines or reactive oxygen metabolites. The precise mechanism will remain uncertain as long as the aetiology of ulcerative colitis is unknown, but a pluripotential mode of action of salicylates is an advantage when influencing the network of events that constitute chronic inflammation.

Placebo-controlled trial of ulcerative colitis with oral 4-aminosalicylic acid

Forty patients with active ulcerative colitis were randomly assigned to receive either 4 g of oral enterically coated 4-aminosalicylic acid (para-aminosalicylic acid) or placebo. The duration of treatment was 12 weeks. Disease activity was assessed by grading clinical symptoms of blood, mucus, urgency, sigmoidoscopic findings, and degree of histological inflammation in rectal biopsy specimens. At 12 weeks, 11 of 20 patients (55%) who received 4-aminosalicylic acid showed improvement in clinical and sigmoidoscopic variables. In contrast, only 1 of 20 patients (5%) who had received placebo showed improvement (P less than 0.005). Eighteen of the 19 patients in the placebo group who showed no improvement were treated subsequently with open-label 4-aminosalicylic acid. Of the 18, 11 showed clinical and sigmoidoscopic improvement. Patients allergic or intolerant to sulfasalazine with extensive disease were more likely to respond to 4-aminosalicylic acid.

Superoxide, hydroxyl and fatty acid radical scavenging by aminosalicylates. Direct evaluation with electron spin resonance spectroscopy

Reactive oxygen radicals such as superoxide and hydroxyl radicals, as well as intermediate unsaturated fatty acid radicals, have been proposed as playing an important role in various diseases including inflammatory bowel disease (IBD). In this study we evaluated radical scavenger properties of aminosalicylates used in the therapy of IBD using spin trapped electron spin resonance spectroscopy. 5-Aminosalicylic acid (5-ASA), 4-aminosalicylic acid and olsalazine had superoxide radical scavenger properties (IC50 = 0.4, 0.4 and 1.0 mM, respectively). 5-ASA and benzalazine also inhibited hydroxyl radicals (IC50 = 6.5 mM). Fatty acid radicals were not inhibited by aminosalicylates. Our results support the hypothesis that therapeutically active compounds may be oxygen radical scavengers and that fatty acid radical scavenging has to be performed by drugs other than aminosalicylates.

Free radicals and other reactive oxygen metabolites in inflammatory bowel disease: cause, consequence or epiphenomenon?

Oxygen-derived free radicals and other reactive oxygen metabolites have emerged as a common pathway of tissue injury in a wide variety of otherwise disparate disease processes. This has given rise to the hope that efforts directed towards the pharmacologic control of free radical-mediated tissue injury (Reilly, P.M., Schiller, H. J. and Bulkley, G. B. (1991) Pharmacologic approach to tissue injury mediated by free radicals and other reactive oxygen metabolites. Am. J. Surg. 161: 488-503) may have particular application to patients suffering from Crohn's disease and/or ulcerative colitis. However, because tissue injury by any mechanism, even direct mechanical trauma, can elicit an inflammatory response which entails the secondary generation of toxic oxidants by neutrophils and tissue macrophages, it is important that the evidence for this association be examined critically, so as to discriminate the possibility of an etiologic role for these toxic compounds from their presence as a reflection of injury caused primarily by other agents. Similarly, in considering the therapeutic potential of free radical ablation for the treatment of patients with IBD it is important to distinguish between interventions that might specifically block the fundamental injury mechanism from those which would act in a more nonspecific, anti-inflammatory role.

Olsalazine. A review of its pharmacodynamic and pharmacokinetic properties, and therapeutic potential in inflammatory bowel disease

Olsalazine (sodium azodisalicylate; azodisal sodium) is an anti-inflammatory agent designed to deliver its active moiety, mesalazine (5-aminosalicylic acid; mesalamine), to the colon while avoiding the adverse effects associated with the use of a sulfapyridine carrier. As a prodrug, olsalazine is an effective oral treatment for both active ulcerative colitis and for maintenance of disease remission and may possibly be of benefit in patients with Crohn's colitis. Findings from both short and long term noncomparative and comparative studies demonstrate that olsalazine 1 to 3g daily in divided doses improves clinical signs and symptoms of colitis in approximately 60 to 80% of patients with acute ulcerative colitis of mild to moderate severity. This improvement rate was similar to that obtained with sulfasalazine. Lower doses of olsalazine, usually 1g daily in divided doses, also maintained remission in patients with chronic ulcerative colitis. While olsalazine effectively delivers mesalazine to the colon, the prodrug itself increases net luminal water secretion and accelerates gastrointestinal transit of a meal. The resulting diarrhoea (occurring in approximately 17% of patients and resulting in withdrawal from therapy in 6% of patients) is distinguishable from that associated with inflammatory bowel disease by the high water content and the absence of blood. Olsalazine-induced diarrhoea usually occurred soon after initiation of olsalazine therapy or dosage increase, was more frequent with higher doses and was usually transient. Dosage reduction, increases in frequency of dosing and concomitant administration with food reduced the severity in many patients with persistent olsalazine-induced diarrhoea. With the exception of diarrhoea, olsalazine was generally well tolerated. Fewer than 14% of patients allergic to or intolerant of sulfasalazine had similar reactions to olsalazine. Olsalazine appears to be a suitable therapy for the treatment of first attacks as well as acute exacerbation of mild to moderate acute ulcerative colitis, and for the maintenance of remission in patients with chronic ulcerative colitis.

New salicylate therapies for ulcerative colitis

Since the development of sulfasalazine about a half century ago, the prognosis for patients with ulcerative colitis has improved significantly. Very recently, the active moiety of oral sulfasalazine, 5-aminosalicylic acid, has been made available as a topical (enema) preparation. In this article, Dr. Bruckstein reviews the advantages of the enemas in the treatment of ulcerative colitis and briefly examines the potential benefits of the new oral salicylate preparations.

Mesalazine. A review of its pharmacodynamic and pharmacokinetic properties, and therapeutic potential in chronic inflammatory bowel disease

Mesalazine (5-aminosalicylic acid; mesalamine), the active moiety of sulphasalazine (salazosulfapyridine), is available in specially formulated oral and rectal forms for the treatment of active ulcerative colitis of mild to moderate severity and for maintenance therapy during disease remission. Tablets or capsules coated with acrylic-based resin and tablets containing microgranules coated with ethylcellulose deliver mesalazine to the distal small intestine and colon, thus avoiding the need for the carrier, sulphapyridine, which is responsible for many of the adverse effects associated with sulphasalazine. Since mesalazine is released in the small intestine from some coated preparations in contrast to sulphasalazine, these oral formulations have therapeutic potential in Crohn's disease. A limited number of therapeutic trials suggest that orally administered mesalazine 1.5 to 2.4g daily is of similar efficacy to sulphasalazine 2 to 3g daily in patients with mild to moderate ulcerative colitis. The efficacy of mesalazine enemas has been more widely investigated, a dose of 1 to 4g once daily being consistently more effective than placebo and apparently similar to enemas of prednisone 25mg or oral sulphasalazine 3g. Initial results suggest that mesalazine 4g enemas may be more effective than those containing hydrocortisone 100mg. Mesalazine and sulphasalazine in approximately equivalent oral dosages are similarly effective in maintaining remission in ulcerative colitis. Orally administered coated mesalazine is generally well tolerated by about 85% of patients allergic to or intolerant of sulphasalazine, the remainder experiencing similar reactions to both drugs. Adverse effects of mesalazine enemas are confined to local irritation and effects resulting from enema-tip insertion. Thus, orally administered coated mesalazine is a suitable alternative to sulphasalazine in the treatment of patients with mild to moderate active distal ulcerative colitis and for maintaining remission particularly in patients allergic to or intolerant of sulphasalazine. In patients who find enema therapy acceptable, mesalazine enemas are effective and well tolerated.