Pharmacokinetic analysis of metronomic capecitabine in refractory metastatic colorectal cancer patients

The aim of the present study was to assess the pharmacokinetics (PK) of metronomic capecitabine and its metabolites in a population of refractory metastatic colorectal cancer (mCRC) patients. Thirty-four patients (M/F, 22/12) with a diagnosis of mCRC received capecitabine 800 mg p.o. twice a day and cyclophosphamide 50 mg/day p.o. Blood samples were collected at baseline, 15 min, 30 min, 1 h, 1.5 h, 2 h, 3 h and 5 h at day 1 after capecitabine administration. Plasma concentrations of capecitabine and its metabolites were measured by high performance liquid chromatography and the main PK parameters were calculated. Maximum plasma concentrations (Cmax) of capecitabine (11.51 ± 9.73 μg/ml) occurred at 0.5 h, whereas the Cmaxof 5'-deoxy-5-fluorocytidine (5'-DFCR; 2.45 ± 2.93 μg/ml), 5'-deoxy-5-fluorouridine (5'-DFUR; 6.43 ± 8.2 μg/ml), and 5-fluorouracil (5-FU; 0.24 ± 0.16 μg/ml) were found at 1 h, 1.5 h and 1 h, respectively. Capecitabine, 5'-DFCR, 5'-DFUR and 5-FU AUCs at day 1 were 21.30 ± 10.78, 5.2 ± 4.6, 19.59 ± 3.83 and 0.66 ± 0.77 hxμg/ml, respectively. In conclusion, low doses of capecitabine were rapidly absorbed and extensively metabolized, achieving measurable plasma concentrations in a heavily pretreated population of patients.