After yesterday's post, several people brought up the issue of inverted screening cascades. What happens when your compound works better in the mice than it did in the cells? Worse, what if it would have worked in the mice, but you never put it in there because it was so weak in the cell assays?
These kinds of questions are worth worrying about, because we almost never get a chance to answer them. For obvious reasons, the vast majority of compounds that make it into animal models are ones that looked good in the earlier steps. You'd have to think that the hit rate in vivo would be much lower for the dud compounds, but you'd have to be pretty arrogant to think that it would be zero, too.
As I recall (and I was just down the hall when it happened), the discovery of Schering-Plough's cholesterol absorption inhibitor came out of a compound that made it into an animal model and worked well, even though it turned out later to be a loser at the project's original target. (I believe that the in vitro assay was down that week for some reason, but one of my former colleagues will probably set me straight if I'm wrong about that). This sort of thing is food for thought, all right, extreme example though it might be. Even if your compounds don't suddenly hit a new target, there's still room for plenty of surprises in pharmacokinetics and the like.
But it would be unethical just to shove everything into animals, tempting though it is sometimes. And it would cost an insane amount, too - let's not forget that. But I do advocate getting as close to the real disease as quickly as possible. You can really waste time and effort by over-optimizing in vitro, all the time convincing yourself that you're doing the right thing.
Then there's the ultimate question in this line: how many compounds are there that don't work well in the animal models, but would be good in humans? I've wondered about this for many years, and I'm going to go on wondering, because data points are mighty scarce. Human biomarkers might eventually lead to some companies crossing their fingers and going into man with a compound that they expect to outdo the animal models. But it's going to take a lot of nerve. (And here's another complication - those upside surprises that might show up in the animals? How many of those are going to translate to humans, do you think? Not all of them, clearly. . .)
I have no doubt that there are many potentially useful drugs that are abandoned early. False negatives are probably on the shelves all around us. I don't see that as a strong argument against animal use (what, after all, is the alternative?), but it sure isn't a big argument for it, either. It's just, for now, the way things are.