The samples were centrifuged at 3000 rpm for 10 min. Plasma was stored at -20°C

until the measurement of 5-FU and GEM concentrations. Figure 1 Drug administration and blood sampling schedule. GEM assay The high-performance liquid chromatography (HPLC) system consisted of a Waters 2690 liquid chromatograph separation module and a Waters SMH column heater (all from Waters (MA, USA). The AtlantisR dC18 column (150 × 4.6 mm; particle size, 5 μm; Waters) was used for the peak separation of GEM. The HPLC mobile phase was a solution of 5 mM phosphate buffer (pH 7.2). The ultraviolet detector was a Waters 2487 (Waters), and was used at 272 nm. Plasma samples were deproteinized with 20% TCA, and the supernatants were filtered using Ultrafree-MC

Lazertinib concentration (Nihon Millipore, Tokyo, Japan) with pore diameters of 0.20 μm. Aliquots of 20 μl were injected into the HPLC system. The quantitative range of this method was 50-40000 ng/ml. 5-FU assay The high-performance liquid chromatographic-mass spectrometry (LC/MS) system consisted of a Micromass ZQ-2000 mass spectrometer, a Waters 2695 liquid chromatograph separation module and a Waters SMH column heater (all from Waters). The AtlantisR dC18 column (150 × 2.1 mm; particle size, 5 μm; Waters) was used for the peak separation of 5-FU. The HPLC mobile phase was a solution mixed purified water and MK-8776 acetonitrile. The mass spectrometer was used in the negative ESI mode. The detector was used in SIR mode with a selected ion recording procedure at m/z = 128.9 for 5-FU and at m/z = 130.9 for 5-FU-15N2. To plasma samples, internal standard solution (including 5-FU-15N2) was added, and was then extracted with ethyl acetate. The organic layer was evaporated to dryness under a stream of nitrogen. The residue

was dissolved in purified water, and after vortex mixing, the mixture was filtered using Ultrafree-MC (Nihon Millipore) with pore diameters of 0.20 μm. Aliquots of 20 μl were injected into the LC/MS system. The quantitative range of this method was 5-500 ng/ml. Statistical analysis The area under the curve from the drug (S-1 or GEM) administration to the infinite time (AUCinf) was calculated according to the trapezoidal rule using the WinNonlin Avelestat (AZD9668) program (Ver. 5.2; Pharsight Co., Mountain View, CA, USA). Two-sided paired Student’s t-test on log-transformed plasma concentration data was used to compare the maximum concentration (Cmax) and AUCinf between single administration and co-administration. The two-sided paired Student’s t-test was conducted on the elimination half-life (T 1/2) and time required to reach Cmax (T max) in order to compare data for single administration and co-administration. A P value of < 0.05 was considered to be statistically significant. Results Clinical outcome Five of six patients were treated by GEM+S-1 for 5 to 16 courses (median, 8 courses).