Found a study looking at leptin supplementation in dieters. We have leptin, a different little-known hormone (Pramlintide) and both (no placebo group, unfortunately). They see an average 18 lb weight loss in 20 weeks on either Pramlintide or metreleptin and 26 lb weight loss on a combination of both. Seems pretty remarkable for a counseling based diet.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2754219/
As in the FDA approval request above, both drugs here are administered intravenously. I'm guessing that, being proteins, they are badly degraded by the stomach acid.