sentences:  The compound CP-99994 was synthesized by replacing the quinuclidine ring with a piperidine ring and benzhydryl moiety by a benzyl group (figure 2).CP-99994 had a high affinity for the human NK1 receptor and it started a great amount of structure–activity studies, each intending to identify the structural requirements for high-affinity interaction with the NK1 receptor, and to make the molecule even simpler and improve its chemico-physical and pharmacological properties.CP-99994 eased dental pain in humans and entered phase II clinical trials; these were discontinued because of poor bioavailability.