Wednesday, May 26, 2010
Xyzal and Zyrtec, same or different?
We see many new drugs on the market today, being just variants of old drugs. They are sometimes called purified drugs, yet the original drugs are certainly not impure. The new drugs' names are variations on the old drugs' names, often with a prefixes like, "lev," or "dextro" added (for example, cetirizine and levocetirizine; amphetamine and dextroamphetamine). What are their differences, and why are the new drugs viewed as better than their predecessors? Cetirizine is a racemic mixture of levocetirizine (now available under the trademark Xyzal®) and dextrocetirizine. Xyzal and Zyrtec are both used in the treatment of nasal congestion and have anti-histaminic properties. Let’s look into some biological properties of cetirizine and levocetirizine, namely enantioselectivity in pharmacological activity, the possibility of racemization, and the compared behavior of the two enantiomers. Recent data demonstrate that the antihistaminergic activity of the racemate is primarily due to levocetirizine. Levocetirizine is rapidly and extensively absorbed, poorly metabolized, and not subject to racemization. Its pharmacological characteristics are comparable after administration alone or in the racemate (equal mixture). Levocetirizine has an apparent volume of distribution smaller than that of dextrocetirizine (0.41 L kg−1 vs. 0.60 L kg−1). Moreover, the non-renal (kidney), mostly hepatic (liver), clearance of levocetirizine is also notably slower than that of dextrocetirizine (11.8 mL min−1 vs. 29.2 mL min−1). Levocetirizine inhibited histamine-induced healthy subjects response to the same degree as cetirizine, with a maximum result at 6 hr post-dose and a duration of action greater than 24 hr. In contrast, dextrocetirizine was much less powerful at the same dose. Levocetirizine (5 mg) and cetirizine (10 mg) also inhibited histamine-induced increases in nasal airway resistance while dextrocetirizine failed to demonstrate any significant antihistaminergic activity. Thus, the H1-antagonist activity of cetirizine is primarily due to levocetirizine, which is considered to be the most active enantiomer (i.e. the eutomer), while dextrocetirizine is the least active enantiomer (the distomer). Levocetirizine is considerably stable to racemization, and incubation of cetirizine enantiomers in buffer solutions of pH 7.4 at room temperature did not reveal the slightest occurrence of racemization even after days. However, not all medications benefit from being enantiopure. For example, ibuprofen is a racemic drug, but the body has an enzyme that converts one enantiomer to the other, so there is no benefit to purifying one enantiomer. Labels: Anti-histamine, Sensitive Nose
Cetirizine (Zyrtec®) is a second-generation antihistamine indicated for the treatment of seasonal allergic rhinitis, perennial allergic rhinitis and chronic idiopathic urticaria, in adults and children of 6 or more years. It is also used for the treatment of allergic rhinitis (relieves nasal congestion), causes little sedation and has no cardiac adverse effects unlike highly lipophilic antihistamines. 
Levo-enantiomer or (-) rotates in the anti-clockwise direction.
