High‐pressure liquid chromatographic assay of theophylline in dog feces following oral administration of sustained‐release products

A solid‐phase‐extraction reversed‐phase HPLC assay is described for the determination of theophylline embedded in dog feces as powder, sustained‐release tablets, or capsules. The feces is extracted with 5% isopropyl alcohol in chloroform in the presence of β‐hydroxy‐propyl‐theophylline as the internal standard. Separation and quantitation are achieved with a C₁₈ analytical column. UV absorbance is monitored at 280 nm. Recovery of theophylline was >50%. The assay is linear between 10 and 400 mg amounts of theophylline in 50 g of feces. Inter‐and intraday coefficients of variation of the chromatographic assay were <3%, and the extraction procedure was highly reproducible with coefficients of variation of <10% at amounts of drug from 10 to 400 mg. By keeping the stool/solvent extraction ratio constant, the method is equally effective in extracting theophylline from different sizes of stool samples (50 versus 200 g of stool). The assay was applied to evaluate the theophylline content in feces following oral administration of the drug to dogs as tablet (Theo‐Dur) and capsule (Slo‐Bid) dosage forms. The resulting fecal recovery values of each product were inversely related to the corresponding bioavailability values obtained from the literature.