L-methamphetamine is a fine example of how molecules that vary only in their chirality can have very different biological effects. Chiral molecules have non-superimposable mirror images. L-methamphetamine, at left, below, is found in Vicks Vapor Inhalers. (Notice how they obfuscate the name a little bit by calling it “levmetamfetamine”). This is the mirror image of D-methamphetamine – the street drug. As you’ll see, the two compounds are very different medically despite having only a subtle structural difference.
If you’re having trouble working out the difference, the dashed bond is “going into the plane of the page (monitor).” That is, in 3-D, it’s around the back side of the molecule. If you’re having trouble figuring out the significance of this, you’re not alone. A good chunk of chemists only get the hang of this after playing with “molecular models” – pretty much chemistry tinkertoy. Just think of the two molecules as left and right feet and you’re on the right track.
The thing is, L-methamphetamine isn’t really anything like the D-methamphetamine isomer that is found in street drugs. D-methamphetamine is psychoactive, while L-methamphetamine isn’t very psychoactive at all. In certain receptor and enzyme pockets where D-methamphetamine fits, L-methamphetamine fits like a left foot in a right shoe.
Because of this, the L-isomer only induces some of the same effects as the illicit D-isomer (like increased blood pressure, which is responsible for its efficacy against nasal congestion), while not effectively inducing others (such as the euphoria and stimulant effects).
This is the only case I know of where the opposite stereoisomer of an illegal drug is actually over the counter!
For some more talk about chiral drugs, take a look at Derek Lowe’s blog.
That’s the end of weird drug week. See you Monday.