Ex vivo delta opioid receptor autoradiography: CNS receptor occupancy of two novel compounds over their antihyperalgesic dose range

Discovered as part of an effort to identify delta opioid (DOPr or DOR) agonist analgesics, JNJ-20788560 and JNJ-39204880 exhibited high DOR affinity, with K i values of 1.7 and 2.0 nM, respectively, and were selective for DOR over the mu opioid receptor (MOPr or MOR), with 596- and 122-fold selectivity, respectively. Both compounds stimulated DOR but not MOR induced GTPγS binding and were effective antihyperalgesic agents in the complete Freund's adjuvant model of thermal hyperalgesia in the rat, with oral ED 50 values of 13.5 and 35 mg/kg, corresponding to plasma levels of 1 and 9 µM, respectively. Autoradiographic analysis of DOR and MOR occupancy in sections of brain (striatum) and lumbar spinal cord (L4–L6) was determined ex vivo, using radiolabeled naltrindole or DAMGO. Quantitative image analysis resulted in striatal DOR ED 50 values of 6.9 and 10.7 mg/kg, for JNJ-20788560 and JNJ-39204880 respectively, and spinal cord values of 6.4 and 3.2 mg/kg, respectively. Neither compound dose-dependently occupied MOR within the dose range studied. Thus, this study confirmed the DOR selectively over MOR of both compounds following their oral administration, and further demonstrated dose-dependent DOR occupancy by each compound across its antihyperalgesic dose range. Importantly, these in vitro, in vivo, and ex vivo data revealed that the greater in vitro potency of JNJ-20788560 was paralleled by its greater in vivo potency, although JNJ-39204880 achieved higher plasma levels following its oral administration. The receptor occupancy levels observed at the pharmacologic ED 50 doses of these compounds suggest the need for greater target engagement by JNJ-39204880 than by JNJ-20788560 to elicit a similar therapeutic response.