Nakata et al. Journal of Experimental & Clinical Cancer Research 2010, 29:15 http://www.jeccr.com/content/29/1/15 Page 5 of 7 Table 2 Comparison of pharmacokinetic parameters of 5-fluorouracil in plasma between administration of S-1 alone and gemcitabine (GEM)+S1 Day 3 (S-1 alone) Day 15 (GEM+S-1) P-value Cmax (ng/ml) 162 ± 46 AUCinf (hXng/ml) 853 ± 329 T1/2 (h) Tmax (h) 1.96 ± 0.73 3.16 ± 0.81 135 ± 56 682 ± 256 2.22 ± 0.84 3.07 ± 0.53 0.8644 0.2063 0.604 0.1683 GEM was intravenously given at a dose of 800 mg/m2 S-1 was orally given at a dose of 30 mg/m2. Cmax, maximum plasma concentration; AUCinf, area under plasma concentration-time curve from time zero to infnite time; T1/2, elimination half-life; Tmax, the time required to reach Cmax. Titers were expressed as means ± SD (n = 6). P-values were examined by two- sided paired t-test after log-transformation. Figure 3 Plasma concentrations of 5-fluorouracil after administration of S-1 30 mg/m2 alone (open circles) and GEM 800 mg/m2 + S-1 30 mg/m2 (closed circles). The sample size affects the statistical accuracy, how- ever, the ethical matters limit the sample size. There have been some reports statistically comparing the PK parameters between two groups composed of five or six patients [13,14]. In our study on six patients, the statisti- cal analysis was done to detect the relative change of the PK parameters in individual patients using the paired Student’s t-test. In this analysis, the statistical power depends on the intra-individual variance and not on the inter-individual variance. Correale et al. reported that pre-administration of GEM had an effect on the plasma PK of 5-FU [15]. In their study, 20 patients with metastatic gastroenteric car- cinomas were treated with 30 min i.v. of 5-FU 400 mg/ m2 and folinic acid (FA) 100 mg/m2 at 1 h after 30 min i. v. of GEM 1000 mg/m2. The control group (5-FU/FA group) consisted of 16 patients with gastroenteric carci- nomas receiving 30 min i.v. of 5-FU 400 mg/m2 and FA 100 mg/m2. The AUC of plasma 5-FU in GEM+5-FU/FA group was approximately twice as high as that in 5-FU/ FA group. The Cmax and T1/2 of 5-FU in GEM+5-FU/ FA group were higher than those in 5-FU/FA group. The enhanced 5-FU systemic exposure in the presence of GEM may induce severe adverse events as well as high levels of antitumor activity. In fact, a clinical phase I/II trial testing GEM+5-FU/FA for 51 patients with gastro- enteric cancers reported frequent grade 4 gastroenteric toxicity and two treatment-related deaths [15]. In contrast to the study by Correale et al., in our examination, the plasma Cmax, AUCinf and T1/2 of 5- FU after co-administration of S-1 with GEM showed no increases when compared to those after S-1 single- administration (Table 2, Figure 3). Although significant differences were not shown, the mean values of Cmax and AUCinf of 5-FU at day 15 were lower than those at day 3 (Table 2). The reason is obscure, however, contin- uous administration of S-1 might affect 5-FU pharmacokinetics. In the catabolic pathways, 5-FU is degraded by DPD. As S-1 contains a very strong DPD inhibitor, CDHP, the Cmax and AUCinf of plasma 5-FU after S-1 administra- tion may reach the limit expected by the amount of tega- fur present in S-1. The affects of GEM metabolites on Cmax and AUC of plasma 5-FU after S-1 administration may be little lower than expected based on the presence of CDHP in plasma. The above-mentioned mechanism may explain our results that PK parameters of plasma 5- FU after S-1 administration did not differ with and with- out GEM administration. Moreover, no enhancement of 5-FU systemic exposure after S-1 administration in the presence of GEM may be an advantage in reducing the frequency of adverse events [16]. The synergistic effects of S-1 and GEM may be explained by the following mechanism occurring in