Opiate analgesics can act through three different types of opioid receptors, called mu, delta, and kappa. Morphine, the most widely used opiate analgesic, acts primarily via activation of the mu opioid receptor located in the central nervous system (CNS). This CNS action induces pain relief but is also associated with a wide array of CNS-mediated side effects including sedation, respiratory depression, and abuse liability. As a way to avoid these undesirable CNS effects, there has been an effort to develop opioids which activate peripheral opioid receptors present on sensory nerves, but are also largely excluded from the brain. Such compounds are thought to have the potential to provide pain relief (peripheral opioid analgesia) without producing significant CNS side effects. CR665 belongs to this pharmacological class of compounds; it is a potent peripheral kappa opioid receptor agonist with high selectivity over other opioid receptors.