PT-141 was developed from MT-II, which was tested as a sunless tanning agent. PT-141 is a metabolite of MT-II that lacks the C-terminal amide function-stripping away tanning properties. Bremelanotide belongs to a class called melanocortins. In initial testing, MT-II did induce tanning but additionally caused as unexpected effects in test subjects. PT-141 does not act upon the vascular system, but the nervous system.