Leptin Insensitives Anonymous

In one of my BIOL classes we were discussing cells and how fat cells as they begin to multiply produce a hormone, called Leptin, which decreases the body's need or desire for food. Homeostasis stuff, right?
Well, than one of my classmates raised their hand and asked, "why haven't they made a weight-loss drug from Leptin than?" Seems like the obvious right? The "golden cure" for all the "leptin producing people" out there...
Our prof answered with an ironic smile, "no, as the cells produce more and more leptin, the body becomes desensitized to it and it no longer works to stave off weight gain". DARN!
So there we have it - people are not fat, they are simply "Leptin Insensitive"!!
Yeah....