A sensitive liquid chromatographic-single quadrupole mass spectrometric method was developed and validated for the determination of lapatinib in human plasma. Following a liquid-liquid extraction with methyl t-butyl ether, lapatinib and isotope labelled lapatinib, used as an internal standard (IS), were separated from the endogenous compounds on a Zorbax SB-C18 (150 x 3 mm, 3.5 μm) column. An isocratic elution with the mobile phase consisting of formic buffer and the mixture of acetonitrile,

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... hanol and formic acid was used. Mass spectrometry with positive electrospray ionization in a single ion monitoring mode was applied. The proposed method provides the satisfactory recovery of lapatinib from human plasma and a sensitivity comparable to numerous tandem mass spectrometric methods, with a lower limit of quantification of 5 ng/mL. The validated method may be applied to pharmacokinetic studies in humans following a single 250 mg oral dose.