Horses renal clearance

There are no pharmakinetic data on the ACE inhibitors in the horse. In humans, enalapril is 60% absorbed following oral administration and is de-esterified in the liver to its active form enalapri-lat it undergoes renal clearance. Both the liver and kidneys eliminate the active forms of benazepril (benazeprilat) and ramipril (ramiprilat). 

The physiological mechanisms by which phenylbutazone is cleared from the body in the horse have not been well described. In one study, renal clearance accounted for only 25% of the total drug administered (Lees et al 1985). It has been hypothesized that biliary excretion with subsequent fecal elimination represents the primary clearance mechanism of phenylbutazone in the horse (Lees et al 1985). Renal excretion, of an as yet unidentified phenylbutazone metabolite, may also account for a proportion of total body clearance of the compound. 

On the other hand, acidic drugs tend to ionize under conditions of alkaline pH and so are unable to permeate the renal tubular epithelium and are preferentially excreted. Tire converse applies to conditions of acidic urinary pH. This has been demonstrated experimentally for many drugs weak bases are excreted more rapidly in acidic urine, whereas weak acids are excreted more rapidly in alkaline urine. In the horse, phenylbutazone, which is a weak organic acid with a pKa of 4.6, has a more delayed clearance time under conditions of aciduria than under conditions of alkaline urine. 

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