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Vitamin B12 and folic acid in advanced oesophageal and gastric cancer694reduction, the repeated occurrence of grade 3 or 4 non-haematological toxicity or drug-induced pneumonitis %u00b3 grade 2 or according to investigator%u2019s judgement. Tumor assessments by CT scan of chest and abdomen were performed every 6 weeks until disease progression according to RECIST[17]. A baseline scan was done within 4 weeks before initiation of study therapy. Disease status, date of progression, date of death and subsequent lines of therapy were collected during regular follow-up visits. TTP was defined as the time from randomization to progression. OS was defined as the time from randomization to death.Assessment of potential predictive parametersPlasma pharmacokinetics of gemcitabine, its metabolite dFdU and cisplatin were measured during treatment in the first 20 patients in order to assess whether vitamin supplementation would affect either gemcitabine or cisplatin pharmacokinetics. We also determined the concentration of dFdCTP in WBC and the homocysteine concentration in these 20 patients. The other patients were monitored for homocysteine before randomization and at the beginning of each 3rd chemotherapy cycle (one week after vitamin B12 administration). In order to assess these parameters, blood was collected in heparinized tubes containing tetrahydrouridine to prevent conversion of gemcitabine to dFdU. After centrifugation the plasma was taken off and stored at -20%u00ba C until analysis. The intermediate layer between plasma and red blood cells containing the WBC was layered on Ficoll-Hypaque, centrifuged and the buffy coat with the WBC was washed, counted and the pellet was frozen in liquid nitrogen until analysis for dFdCTP. Gemcitabine, dFdU and dFdCTP were measured with validated HPLC assays[16]. Homocysteine was measured as described earlier[19]. In order to determine the amount of total and free (non-protein) bound platinum species, one part was immediately frozen at -20%u00ba C until analysis (total platinum), and the other part was mixed with ethanol, incubated overnight at -20%u00ba C, and centrifuged. The supernatant contained free platinum[20]. Free plasma platinum and total plasma platinum (free and protein-bound platinum) were determined using flameless atomic absorption spectroscopy[20,16]. The 79A>C (rs2072671) CDA and 667C>T MTHFR polymorphism were analyzed in respectively 37 and 20 patients in this study to asses a possible association with response, survival and toxicity.