If a drug is particularly valuable, an important strategy for evergreening is for the manufacturer to develop a slightly modified follow-on drug, in anticipation of patent expiry and generic competition. All of the following pairs are examples of drugs with their respective slightly modified follow-on drugs (of the same drug class), except for:
A. Citalopram and escitalopram
B. Venlafaxine and desvenlafaxine
C. Omeprazole and esomeprazole
D. Dexamphetamine and lisdexamfetamine
E. Perindopril and irbesartan