One of the drug targets for obesity that’s been kicking around for years now is a serotonin-receptor based idea, a 5-HT2c agonist. There are several lines of evidence that make this a plausible way to affect appetite – well, as plausible as any of the appetite-based obesity targets are. I’ve long been wary of these, since we’ve found (over and over) that human feeding behavior is protected by multiple, overlapping redundant pathways. We are the descendants of a long line of creatures that have made eating and reproducing their absolute priorities in life, and neither of those behaviors are going to be altered lightly. The animals that can be convinced to voluntarily eat so little that they actually lose weight, just through modifying a single biochemical pathway, are all dead. Our ancestors were the other guys.
Arena Pharmaceuticals is the latest company to give us more evidence for this point of view. Many drug discovery organizations have taken a crack at 5-HT2c compounds, as a look at the patent literature will make clear. But Arena got theirs, Locaserin, well into the clinic, and yesterday they announced the results. And. . .well, it depends on how you spin it. If you’re a glass-half-full sort of person, you could say that twice as many people in the drug treatment group lost at least the FDA’s target of their body mass, as compared to placebo.
Unfortunately, the glass-half-empty people are probably going to win this one. The FDA wants to see 5% weight loss (versus placebo) with a drug therapy, arguing (correctly, I think) that showing less than that really doesn’t give you much risk/benefit over just plain old diet and exercise. Arena’s compound averages out at 3.6%, and I don’t see how that’s going to cut it, especially with a new central nervous system mechanism. By “new”, I don’t mean “new to science” – as mentioned above, this idea has been around for years. But it would be a new thing to try out in millions of patients if you let a drug through, that’s for sure. I think it’s safe to say that a certain fraction of those are going to react in ways that you didn’t expect. 5-HT2 receptors are involved in a lot of different things, and there's bound to be a lot about any agent in this class that we don't know. Locaserin seems to have been well tolerated in trials, but I personally would be jumpy if I were taking something like this out into the broad population.
That’s not why I think this compound won’t make it, though. The FDA doesn’t even have to talk safety; they can reject it just on the grounds of efficacy. And it’s hard to imagine a lot of insurance plans picking up the tab for something with only those levels of clinical support, too. Arena's CEO says that he's pleased with the results of the trial. No, he isn't. Of course, he also says that he's convinced that the company will get Locaserin approved and find a partner to market it with, too. But then, that's his job.