The present invention provides methods for increasing plasma dUrd levels comprising the administration of 6R-MTHF. The methods of increasing plasma dUrd increase dUrd levels compared to equimolar concentrations of LV. The present invention also provides methods for increasing TS inhibition comprising the administration of 6R-MTHF.本發明提供用於增加血漿dUrd含量之方法,其包含投與6R-MTHF。與等莫耳濃度LV相比增加血漿dUrd之方法增加dUrd含量。本發明亦提供用於增加TS抑制之方法,其包含投與6R-MTHF。