Monday, June 14, 2010

Lexapro and Prestiq: Nothing New Under the Sun

Remember a couple years ago when television was blanketed with ads for Nexium, the purple pill that represented the next "big step" in the treatment of gastric reflux esophagitis? Was this medicine something new? Well, sort of. To understand what the manufacturer did, first you have to know a little simple chemistry.

Molecules, as most of know, are made of atoms of various elements in a fixed ratio, like H20 is two hydrogen atoms and one oxygen atom combined to make water. If a molecule is large enough, two molecules with exactly the same chemical formula and structure can have slightly different geometry. The two molecules can be mirror images of one another, creating what are called enantiomers. The R-enantiomer is the right handed version and the L-enantiomer is the left handed version.

In particular, some drugs can have both L and R versions, but only one of the two versions may be active in the body, while the other is either completely inert (the usual case) or, in a few instances, creates different side effects. A mixture of the L and R versions is called a racemic mixture.The reflux drug prilosec is such a racemic misture. One enantiomer is active and the other is inert.

When prilosec, a highly profitable blockbuster drug, was about to go generic, the manuafacturer did something to preserve its profits. It created Nexium, which was not a racemic mixture but the single active enantiomer. Of course, the inactive enantiomer in Prilosec is inert, so in order to ingest an equivalent amount of "Nexium," all you have to do is take double the dose of prilosec. There would be no difference at all. And there is no evidence that Nexium is any more effective or has fewer side effects than Prilosec!

Lexapro, one of only three anti-depressants that have not already gone generic, is the R-enantiomer of Celexa (Citalopram) and is a lot more expensive than generic Citalopram, but in all other respects is nearly identical. Some people claim to have more side effects on the original drug, but this is debatable. They are equally effective, so why pay more?

Another trick the pharmaceutical companies use to extend sales of expensive, brand-named drugs when they are about to go off patent is to release what is called the active metabolite of the original drug as a new drug. For example, when the old antidepressant Elavil (amitriptylene) was broken down in the body, the first new molecule created was nortriptylene (Pamelor). Nortriptylene was the active drug for depression, not Elavil.

Now, in that case Pamelor was an improvement because the parent drug, which was not psychiatrically active, did have a lot of side effects that its metabolite did not. Elavil also had to be prescribed in much higher doses, which made its overdose potential more of problem.

When the antidepressant Effexor (venlafaxine) was about to go off patent, the drug company came up with Pristiq, which is desvenlafaxine, Effexor's active metabolite. The effectiveness of the two drugs is once again absolutely the same if the proper dosage is prescribed, and in this case, side effect differences are minimal. Pristiq's only advantage is that it has a narrower effective dosage range, so the doctor does not have to slowly increase the dose of the medication to see what the minimal effective dose will be in a given patient.

Still, unless money is no object, a patient might want to go with the cheaper generic alternative. I have no sympathy for insurance companies, but their interests are finally beginning to diverge from those of big PhARMA in a good way. They are starting to push for use of the generic alternatives for their subscribers in order to save money.

1 comment:

  1. Stuff like this is incredible to me--I wish people would keep it in mind every time they hear about some new "wonder drug".