High-performance liquid chromatographic determination of omeprazole and lansoprazole in human plasma for therapeutic drug monitoring purposes

Omeprazole (6-methoxy-2-[[(4-methoxy-3,5-dimethyl-2-pyridinyl)methyl]sulfinyl]-1H-benzimidazo-le, OMP) and lansoprazole (2-[[[3-methyl-4-(2,2,2-trifluoroethoxy)-2-pyridinyl]methyl]sulfinyl]-1H-benzimidazole, LNP) are two of the most widely used anti-ulcer agents which act on the gastric H+-K+-ATPase pump, irreversibly inhibiting it and thus effectively reducing the acid secretion. They are used in the treatment of peptic ulcer of any origin and also for the treatment of gastro-esophageal reflux disease and as gastroprotectives during prolonged anti-inflammatory therapy cycles. OMP is administered at doses ranging from 10 to 40 mg/day, while LNP usual daily doses are in the 15-30 mg/day range. The typical therapy cycle lasts 8 weeks, but OMP and LNP are often prescribed by physicians for the occasional, acute treatment of gastric pain. However, clear dose/plasma level correlations have not been established. The main side effects are cephalea, abdominal pain, diarrhea, nausea, vomit, dry mouth, dermatitis, insomnia and rarely hepatitis, leukopenia, agitation, broncoconstriction. For these reasons, it is clear that a suitable therapeutic drug monitoring can be helpful in establishing chemical-clinical correlations leading to a safer and more effective use of these drugs. Feasible and relatively inexpensive analytical methods are obviously needed for the determination of OMP and LNP in human plasma. The methods should be capable of discriminating the two drugs: physicians often switch from OMP to LNP and vice versa, perceiving them as mostly equivalent, thus their simultaneous analysis is necessary. The aim of this study is the development of a simple and reliable HPLC method coupled to UV detection for the separation and the simultaneous determination of OMP and LNP in plasma. The developed method uses a C8 column (150×4.6 mm I.D., 5 μm) as the stationary phase and a mixture of acetonitrile and a pH 6.0 phosphate buffer containing triethylamine (30/70, v/v) as the mobile phase. UV detection is carried out at 220 nm. Indomethacin was chosen as the Internal Standard (IS). The sample pre-treatment is carried out by solid-phase extraction (SPE) using C8 cartridges; 250 µL of human plasma are sufficient for a complete analysis. The SPE procedure allows concentration of the analytes, while eliminating most endogenous and hexogenous interference. Extraction yield assays gave good results, with absolute recovery values always higher than 92%. Studies are in progress to complete the method validation.