I had a question recently about why some chemical elements don't appear much in pharmaceuticals. Boron was one example - the first boron-containing drug (Velcade, from Millennium) was approved just recently.

But it hasn't been for lack of trying. Starting in the 1980s, several drug companies took a crack at boronic acids as head groups for protease inhibitors. Big, long, expensive programs against enzymes like elastase and thrombin went on year after year, but no one could get the things to quite work well enough. In vitro they ruled - a good boronic acid is about as good as an enzyme inhibitor can be. But in vivo they had their problems, with oral absorption and cell penetration leading the way.

As far as I'm aware, there's no particular tox liability for boron. Things like boric acid certainly don't have a reputation for trouble, and we don't take any special precautions with the air-stable boron compounds in the lab. It'll be hard to make any case, one way one another, based on the Velcade data, since the drug's mechanism of action (proteosome inhibition) has a lot of intrinsic toxicity anyway. (There's the anticancer field for you - there aren't many other areas where a target like that would even be considered.)

I think self-censorship is why there aren't more boron-containing structures out there. We don't spend much time looking at the compounds seriously, because everyone knows the problems with boronic acids, and no one wants to be the first to develop a different boron-containing functional group, either. "Why be the first to find a new kind of trouble?", goes the thinking. "Don't we have enough to worry about already?"