First article: by Francois Guiliano, a major researcher: (LINK HERE) [Note: For some reason, the author retracted this article…not sure why.]
Second article: Also by Dr. Guiliano; you can download a pdf of the article, which is from Journal of Andrology (2004): (LINK HERE) It is pretty complicated and not for the non-medical person, but it suggests this class of drugs will be useful in the future.
Third article: This study goes back to 2000. There are some side effects with MT-II (eg, severe nausea), which may be why they haven’t developed it. (LINK HERE)