Why the chain had to be changed

Natural alpha-MSH is destroyed in the bloodstream within minutes, which makes it useless as a given molecule. Replacing methionine with norleucine removes the soft spot enzymes attack, and flipping phenylalanine to its mirror-image D form makes the chain unreadable to the proteases that would cut it. After injection under the skin the molecule reaches the blood essentially completely, with a working half-life of about one to two hours, and nothing meaningful is absorbed when it is swallowed. Those facts are why it is given under the skin and why the slow-release implant exists.