In the Pipeline has a nice history of efforts to develop a drug to block Substance P signaling in the treatment of pain. Substance P is a neurotransmitter peptide which is associated with intense and chronic pain, or so-called "bad pain" (as opposed to "good," necessary, pain which tells us to pull our hand away from the hot stove). Because the pain from several diseases seems to come from excessive substance P signaling, it would be a big help to control it. Merck has had a compound for some time, but unfortunately it does not seem to live up to its potential.
The story shows how difficult it really is to bring something from the laboratory to the hospital.
More background on Substance P is at Medscape.