Now here's a subject that most medicinal chemists have thought of at one point or another: why don't I put a silicon into my compounds? Pretty much like carbon, at least when there's only one of them, right? I've done it myself - I was working on a series of compounds years ago that had a preferred t-butyl group coming off an aryl ring (not anyone's favorite, to be sure). So I made the trimethylsilyl variation, and it worked fine. We had some patent worries in that series, and I pointed out that a silicon would certainly take care of that little problem, but it was still a bit too "out there" for most people's comfort. (And to be fair, it didn't have any particular other advantages; if it had stood out more in activity, things might have been different).
I wrote a bit about this subject a few years ago here, and mentioned a company in the UK, Amedis, that was giving silicon-for-carbon a go. This idea did not, in the end, set much on fire. Amedis was bought by Paradigm Therapeutics in 2005, and a couple of years later, Paradigm was bought out by Takeda. I'm not sure if there's any remnant of the Amedis silicon era left in Takeda's innards by now; if there is, I haven't come across it.
There's a new paper on medicinally active silicon compounds, though, which might get people thinking about this whole idea once more. It's a roundup of what's known about the biological properties and behavior of these things, and will serve as a handy one-stop source for all the reported drug-like molecules in the class. As far as I can tell, the most advanced silane ever in humans has been Karenitecin, a camptothecin analogue that went into Phase III back in 2008 (and does not seem to have been heard from since).
All silicon needs is one success, and then people will take it more seriously. So far, the right combination of activity, interest, and need hasn't quite come together. If you're thinking of giving it a try, though, this new paper is the first place to start reading.