The Effect of Anti-Inflammatory Drugs on Recovery of Ischemic-Injured Equine Intestine
Vanessa L. Cook, VetMB, MS, DACVS, DACVECC
Ideally, all ischemic-injured intestine is resected surgically. However, this is not always possible because the injured intestine may be inaccessible or difficult to identify. Recovery of this remaining intestine in the post operative period is critical to prevent absorption of endotoxin which is a major cause of short term mortality. There has been much work evaluating different treatments for endotoxemia, but ultimately it is rapid recovery of the intestinal barrier which is important. Therefore we have focused on evaluating how drugs which are commonly administered post operatively influence intestinal recovery after ischemic injury.
Laboratory Measurement of Mucosal Barrier Function
To objectively assess the effect of different drugs on the recovery of the mucosal barrier after ischemic injury, in vitro techniques are used. In our laboratory this is performed at 18 hours after ischemic injury to coincide with the time of peak intestinal inflammation. The two principal measurements used are transepithelial electrical resistance (TER) and permeability to labeled substrates. TER is measured by mounting the mucosa in Ussing chambers and bathing it with warmed, oxygenated equine Ringer's solution. The voltage and current across the mucosa is measured and then the resistance is calculated using Ohms law. The resistance basically consists of a measurement of the impedance of Na+ movement from the apical to the basolateral side of the cell. Therefore, it is comprised of both transcellular and paracellular resistance. A high resistance is indicative of an intact mucosal barrier. The resistance is decreased if the paracellular resistance is low, or if epithelial cells are lost. Therefore histologic examination of the mucosa to evaluate the epithelial covering of the villi is usually performed as well. The permeability of the paracellular pathway can be evaluated by placing probes in the mucosal bathing fluid and monitoring their appearance on the serosal side. These probes are usually labeled with a radioactive tag, such as 3H-mannitol or 14C-inulin, or fluorescence, such as FITC-LPS, to allow their detection.
Effect of NSAIDS on Recovery of Barrier Function
Non-steriodal anti-inflammatory drugs (NSAIDs) are the most commonly used analgesic in horses. Tissue injury and inflammation causes upregulation of the cyclooxygenase (COX) enzyme and the production of prostaglandins. These in turn activate sensory nerve endings and cause pain.1 NSAIDs inhibit the COX enzyme and prevent the production of prostaglandins. There are at least 2 isoforms of COX, COX-1 and COX-2, with a splice variant of COX-1 being termed COX-3. The most commonly used NSAIDs in horses, phenylbutazone and flunixin meglumine prevent inflammation by inhibiting COX-2, but also have side effects due to their inhibition of constitutive COX-1. The effect of different NSAIDs on mucosal recovery has been assessed by using an established model of 2 hours of jejunal ischemia followed by 18 hours of recovery. This has shown that treatment with flunixin meglumine inhibits recovery of the mucosal barrier after ischemic injury, as indicated by a lower TER and increased permeability to mannitol and LPS.2,3 This is likely due to inhibition of COX-1 by flunixin meglumine and hence a critical reduction in prostaglandins which are required for mucosal repair. If this theory is correct, treatment with a COX-2 selective drug to provide analgesia and anti-inflammatory effects while allowing the production of beneficial prostaglandins would be preferable in horses recovering from ischemic intestinal injury. Therefore, both etodolac and deracoxib were evaluated using the same model and laboratory methods. However, both of these drugs were determined not to be COX-2 selective in horses, and their administration retarded mucosal recovery. This underscores the importance of evaluating COX selectivity in the species in which the drug is to be used, and not extrapolating results from other species. Subsequently, meloxicam has been evaluated at a dose of 0.6mg/Kg every 24 hours in the same model and was determined to permit recovery of transepithelial barrier function, whilst providing effective analgesia. Unfortunately this drug is not available for equine use in the United States, although a 20mg/ml IV formulation is increasingly being used in Europe for treatment of colic.
Firocoxib has been shown to be COX-2 selective in horses and was released last summer as an oral paste for the treatment of osteoarthritis. Evaluation of its use in 253 horses with osteoarthritis proved that it was an effective musculoskeletal analgesic and anti-inflammatory with no documented adverse effects.4 We recently evaluated the effect of an intravenous formulation of firocoxib in our model of jejunal ischemia. Analgesic effects were determined by using an established behavioral pain scoring system. Intravenous treatment with firocoxib was determined to be an effective visceral analgesic, and comparable to flunixin meglumine. Additionally, it permitted recovery of mucosal barrier function after ischemic injury as evidenced by a higher TER and reduced permeability to fluorescent labeled LPS. The COX-2 selectiveness of this drug in horses was confirmed by the fact that it did not reduce plasma concentrations of COX-1 associated thromboxane B2, but did prevent an increase in the production of COX-2 associated prostaglandin E2 metabolites after ischemia.
Effect of Lidocaine on Recovery of Barrier Function
Lidocaine has become widely used in equine hospitals as a treatment for postoperative ileus.5 However its effectiveness is unlikely to be due to a direct prokinetic effect,6 but is more likely related to novel anti-inflammatory properties.7 Most of these anti-inflammatory effects are achieved through the effect of lidocaine on neutrophils as it has been shown to reduce neutrophil activation and migration. These effects are likely responsible for its benefits in treatment of cardiac and cerebral ischemia reperfusion injury. Influx of neutrophils also occurs after ischemic intestinal injury and causes significant damage by physically disrupting the epithelial barrier as they migrate and by releasing reactive oxygen metabolites. Therefore we elected to evaluate the effect of systemic lidocaine on ischemic-injured intestine, either when used alone, or in combination with flunixin meglumine. Lidocaine alone was no different to treatment with saline on recovery of the mucosal barrier. However, when treatment with lidocaine was combined with flunixin meglumine, the negative effects of flunixin meglumine on intestinal recovery were avoided. Systemic treatment with lidocaine was not found to be an effective visceral analgesic, confirming previously reported results.9 Lidocaine did not affect the COX-1 associated production of thromboxane B2. However, systemic treatment with lidocaine did reduce the COX-2 associated production of prostaglandin E2 metabolites, suggesting an anti-inflammatory effect. We have previously documented that treatment with NSAIDs, even the COX-2 selective drug meloxicam, actually cause a significant increase in the number of mucosal neutrophils 18 hours after ischemic injury. Mucosal neutrophil counts in horses treated with lidocaine were lower than in horses treated with flunixin, and this effect was sustained when the two treatments were combined. Therefore it appears that lidocaine ameliorates the negative effects of flunixin meglumine by reducing mucosal neutrophil influx after ischemia and hence reducing mucosal inflammation.
The clinical use of flunixin meglumine in post operative colics may delay the recovery ischemic-injured intestine and result in increased absorption of endotoxin. The ideal drug for use in these horses would be a COX-2 selective NSAID that can be administered intravenously to provide analgesic and anti-inflammatory effects while allowing mucosal repair. However, such a drug is not currently available in the United States. If flunixin meglumine is used in a post operative colic with residual injured intestine, concurrent administration of systemic lidocaine may reduce mucosal inflammation and hasten recovery of the mucosal barrier.
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