Liquid-chromatographic determination of cimetidine, its known metabolites, and creatinine in serum and urine.

We describe a liquid-chromatographic procedure for determination of cimetidine, its hydroxymethyl-, sulfoxide-, and guanyl urea metabolites, and creatinine in patients serum and urine. SKF 92374 is used as the internal standard. Protein in 0.5 mL of serum or diluted urine is precipitated with 2 mL of acetonitrile, the organic and aqueous phases are separated by adding 0.3-0.5 g of anhydrous K2HPO4. The organic phase is evaporated, and 0.5 mL of 50 mmol/L HCl is added. This solution is washed with 3 mL of water-saturated isoamyl alcohol, the aqueous phase is extracted with 3 mL of methylene chloride and enough K2HPO4 to saturate the solution. The methylene chloride is evaporated, the residue reconstituted with 100 microL of mobile phase, and a 25-microL aliquot injected onto the chromatographic column (Dupont Sil). The mobile phase is acetonitrile/methanol/water/ammonium hydroxide (1000/50/50/2, by vol). The column effluent is monitored at 228 nm. Lower limits of detection ranged from 0.05 mg/L for cimetidine to 0.2 mg/L for guanyl urea. We determined cimetidine and its principal metabolites in the serum of a patient receiving cimetidine for the treatment of Zollinger-Ellison syndrome, and have assessed use of the assay in a clinical setting.