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. The present invention also provides methods for increasing TS inhibition comprising the administration of 6R-MTHF.