As some of you may know, there's a big patent dispute between Novartis and the government of India. The issue is Gleevec (imatinib, sold as Glivec in most of the rest of the world - Novartis must have figured that it would have been pronounced "Gly-veck" over here). The product is sold as a mesylate salt, and in fact, as a particular polymorph of that mesylate salt, and there's the problem.
For those outside the business, most drugs have either acidic or basic groups on them, and you can make a salt of them by combining them with a corresponding base or acid. Basic drugs - amines, mostly - are often sold as hydrochloride, mesylate, citrate, etc. salts, and acidic drugs are often sodium, potassium, calcium, etc. salts. These changes are usually done to make a compound absorb better when it's dosed and/or to make it easier to handle or more stable during manufacturing and storage.
Polymorphs, meanwhile, are different crystalline forms of the same compound. That's something that you don't encounter much outside a chemistry lab. The closest everyday analog is to think of table salt vs. kosher salt vs. sea salt, but those are still the same crystal-packing form when you get right down to it. A real polymorph is quite a different beast; it's as if you could dissolve up regular salt, cool it down in some tricky way, and have it crystallize out as needles or prisms instead of tiny cubes. And those needles or prisms might then, as it happens, refuse to dissolve if you added them to your soup. That's a polymorph, and it's a pretty common occurrence with drug substances. A key step in a real manufacturing process is making sure that you have the best one, and that you can always be sure that it's the one being produced. The wrong one will do things like refuse to dissolve into the bloodstream, which can be most unfortunate.
So Gleevec is a particular polymorph of a particular salt, and Novartis has patents on just that form in many countries. But not India, or not yet. As this post from a lawyer there details, the dispute is (to a large extent) about whether this form of the drug should be compared to another polymorph, to another salt, or to the original free base compound when time comes to judge its novelty and patentability. Another question is whether Novartis's previous patent filings disclose or anticipate the particular salt and polymorph form of the final compound. These arguments are complicated by the fact that India didn't even allow patents on pharmaceutical substances until a few years ago. For more on recent drug company patent disputes there, see this from the WSJ.
So I'd like to throw a question out to the readership: how many examples can people think of where a particular salt or polymorph was a key to getting good efficacy or properties for a drug? I realize that a lot of these stories never see the light of day - I've seen polymorph problems give people fits during development, as have many readers, I'm sure, but most of these things never get published. So I'm not asking for anything from the inside, just the publicly known examples.
Update: if you want a good indicator of how serious the IP issues are around these things, check out this conference. . .