Complex sentence: In this case, folinic acid is not used for "rescue" purposes; rather, it enhances the effect of 5-fluorouracil on inhibiting thymidylate synthase.
Split into simpler sentences: In this case, folinic acid is not used for "rescue" purposes. Rather, folinic acid enhances the effect of 5-fluorouracil on inhibiting thymidylate synthase.