NK1 (substance P) receptor antagonists--why are they not analgesic in humans?

Tachykinin NK1 receptor antagonists have failed to exhibit efficacy in clinical trials of a variety of clinical pain states. By contrast, in preclinical studies in animals NK1 receptor antagonists have been shown to attenuate nociceptive responses sensitized by inflammation or nerve damage, although they exhibit little effect on baseline nociception. Other agents with this profile of activity in animal tests, typically nonsteroidal anti-inflammatory drugs (NSAIDs), are analgesic in humans. Thus, NK1 receptor antagonists appear able to block behavioural responses to noxious and other stressful sensory stimuli at a level detectable in animal tests but fail to provide the level of sensory blockade required to produce clinical analgesia in humans.