Frank A. Bures: A bit of a chemistry lesson | Health, Medicine and Fitness

An antihistamine called levocetirizine (LCT) is currently being released for over the counter use in the U.S. It is yet another histamine H-1 receptor blocker like its daddy, cetirizine. The brand name is Xyzal (ZY-zal—no idea how this moniker was manufactured). The chemical lineage of LCT is historically interesting. It is made by the Union Chimique Belge or UCB, a company founded in 1928 in Brussels, Belgium, and now a multinational, multibillion-dollar pharmaceutical company.

UCB made the granddaddy of LCT, hydroxyzine in 1956. It was one of their best formulations ever. The brand is Atarax, and has been used extensively worldwide for decades. Called an antihistamine, it has activity in several receptor systems in the brain because it crosses the “blood brain barrier”. It has been used also for itching, increased sensation, sleeping medicine, anxiety relief, and on occasion for opioid withdrawal. It is very sedating for some people. At the time, its main competition was Benadryl.

All drugs are metabolized, changed, or “broken down” by liver, usually to forms that are the active ones. It turns out that the most active form or metabolite of hydroxyzine is cetirizine (CT), brand Zyrtec. CT does not cross the blood brain barrier much, like its daddy, hydroxyzine. That made it a more pure antihistamine with minor sedative effects and much better tolerated. It is a second-generation antihistamine, son of hydroxyzine. It went over the counter in 2008, and became the highest grossing non-food product in the U.S. Generics since have proliferated like rabbits.

In the early 2000’s the company UCB studied CT. It contains 2 chemical mirror images of a metabolite called levocetirizine, LCT. The left-handed version or L-isomer is the most active form. This was extracted and made into its current formulation, Xyzal. It has been…

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