Why do we test drugs on animals, anyway? This question showed up in the comments section from a lay reader. It's definitely a fair thing to ask, and you'd expect that we in the business would have a good answer. So here it is: because for all we know about biochemistry, about physiology and about biology in general, living systems are still far too complex for us to model. We're more ignorant than we seem to be. The only way we can find out what will happen if we give a new compound to a living creature is to give it to some of them and watch carefully.

That sounds primitive, and I suppose it is. We don't do it in a primitive manner, though. We watch with all the tools of our trade - remote-control physiological radio transmitters, motion-sensing software hooked up to video cameras, sensitive mass spectrometry analysis of blood, of urine, and whatever else, painstaking microscopic inspection of tissue samples, whatever we can bring to bear. But in the end, it all comes down to dosing animals and waiting to see what happens. That principle hasn't changed in decades, just the technology we use to do it.

No isolated enzymes can yet serve as a model for what can happen in a single real cell. And no culture of cells can recapitulate what goes on in a real organism. The signaling, the feedback loops, the interconnectedness of these systems is (so far) too much for us to handle. We keep discovering new pathways all the time, things that no model would have included because we didn't even know that they were there. The end is not yet in sight, occasional newspaper headlines to the contrary.

We do use all those things as filters before a compound even sees its first rodent. In a target-driven approach, which is the great majority of the industry, if a compound doesn't work on an isolated protein, it doesn't go on to the cell assay. If it doesn't work on the cells, it doesn't go on to animals. (And if it kills cells, it most certainly doesn't go on to the animals, unless it's some blunderbuss oncology agent of the old school). The great majority of compounds made in this business have never been given to so much as one mouse, and never will.

So what are we looking for when we finally do dose animals? We're waiting to see if the compound has the effect we're hoping for, first off. Does it lower blood pressure, slow or stop the growth of tumors, or cure viral infections? Doing these things requires having sick animals, of course. But we also give the drug candidates to healthy ones, at higher doses and for longer periods of time, in order to see what else the compounds might do that we don't expect. Most of those effects are bad - I'd casually estimate 99% of the time, anyway - and many of them will stop a drug candidate from ever being developed. The more severe the toxic effect, the greater the chance that it's based on some fundamental mechanism that will be common to all animals. In some cases we can identify what's causing the trouble, once we've seen it, and once in a great while we can use that information to argue that we can keep going, that humans wouldn't be at the same risk. But this is very rare - we generally don't know enough to make a persuasive case. If your compound kills mice or kills rats, your compound is dead, too.

I've lost count of the number of compounds I've worked on that have been pulled due to toxicity concerns; suffice it to say that it's a very common thing. Every time it's been something different, and it's often not for any of the reasons I feared beforehand. I've often said here that if you don't hold your breath when your drug candidate goes into its first two-week tox testing, then you haven't been doing this stuff long enough.

Here's the problem: giving new chemicals to animals to see if they get sick (and making animals sick so that we can see if they get better) are not things that are directly compatible with trying to keep animals from suffering. Ideally, we would want to do neither of those things. Fortunately, several factors all line up in the same direction to keep things moving toward that.

For one thing, animal testing is quite expensive. Only human testing is costlier. In this case, ethical concerns and capitalist principles manage to line up very well indeed. Doing assays in vitro is almost invariably faster and cheaper, so whenever we can confidently replace a direct animal observation with an assay on a dish, plate, or chip, we do. All that equipment I mentioned above has also cut down on the number of animals needed, and that trend is expected to continue as our measurements become more sensitive.

So things are lined up in the right direction. Any company that found a reliable way to eliminate any significant part of its animal testing would immediately find itself in a better competitive position.

And for the existing tests, it's also fortunate that unhappy animals give poor data. We want to observe them under the most normal conditions possible, not with stress hormones running through their systems, and a great deal of time and trouble (and money) goes toward that end. (In this case, it's scientific principles that line up with ethical ones). Diseased animals are clearly going to be in worse shape than normal ones, but in these situation, too, we try to minimize all the other factors so we're getting as clear a read as possible on changes in the disease itself.

So that's my answer: we use animals because we have (as yet) no alternative. And our animal assays prove that to us over and over by surprising us with things we didn't know, and that we would have had no other opportunity to learn. We'd very much like to be able to do things differently, since "differently" would surely mean "faster and more cheaply". None of us enjoy it when our compounds sicken healthy animals, or have no effect on sick ones. Just the wasted time and effort alone is enough to make any drug discoverer think so. There are billions of dollars waiting to be picked up by anyone who finds a better way.