Posted by moxy1000 on September 2, 2002, at 15:23:11
In reply to Re: Cheap, Generic Celexa, posted by oracle on September 2, 2002, at 4:01:17
Seems to be a lot of people saying that "lexapro is half of Celexa." Okay...that's true, I guess. But that seems to be over simplifying things a bit.
Let's look at science.
Celexa is a racimic compound that contains the s and r enantiomers in equal proportion (50:50). These means essentially that they are stereoisomers.
There are many stereoisomers in nature - look for instance at carvone - s-carvone it is responsible for the taste and smell of caraway, while r-carvone is responsible for the taste and smell of spearmint.
Not really the same thing at all, is it?
Another example, both lemons and oranges contain a compound called limonene - however Lemons have the enantiomer s-limonene, while oranges have the enantiomer r-limonene. As a result, lemons smell and taste sour, while oranges smell and taste sweet.
Just because isomers are mirror images of eachother, does not mean they are the same thing at all. Science proves that.
In the case of Lexapro, the reason it is more potent comes down to receptor sites in the body.
Ready for more science? Receptor sites are very selective - the correct key is required for binding. However, they are not 100% selective. Similar molecules, or similar "keys" may be able to fit into or bind with the same receptor. The binding may be only partial, but it may be enough to keep another receptor binder from latching on.
This is the case with Celexa. The r-enantiomer was not a perfect fit for blocking the reuptake of serotonin. It fit "partially" with the cells. The s-enantiomer IS a perfect fit, but the presence of the r-enantiomer actually interfered with the ability of the s-enantiomer to increase serotonin levels in the brain.
I have tried to stay away from getting into isomer science, as I found it initially difficult to understand and I didn't want to bore anyone with the specifics, but this discussion seems to warrant a bit more depth in discussion regarding the science behind this isomer called Lexapro.
The point is, like caraway and spearmint, like oranges and lemons, Celexa and Lexapro are not the same. They act totally differently at the receptor sites - one is a perfect fit, and the other is not. The result is Lexapro being over 100 times (or in other words, 100%) more potent then the r-enantiomer in the inhibition of serotonin reuptake.
Do the math. Does logic tell you that one should work more efficiently then the other??
P.S. Dr. Dave, I'm not a scientist or a doctor, but I have done my homework, and contrary to your assumption, I do understand the pharmacodynamics.
poster:moxy1000
thread:109458
URL: http://www.dr-bob.org/babble/20020829/msgs/118562.html