Antinociceptive profile of biphalin, a dimeric enkephalin analog

The dimeric enkephalin biphalin (Tyr-D-Ala-Gly-Phe-NH)₂ was evaluated in mice using antinociceptive, gastrointestinal and physical dependence paradigms and compared with that of morphine (reference μ agonist) and etorphine (ultrapotent opioid agonist). Intracerebroventricular biphalin was 6.7- and 257-fold more potent than etorphine or morphine in eliciting antinociception. When administered i.t., biphalin produced only a 60% maximal antinociceptive effect in the tail-flick test even when given at doses up to 3 orders of magnitude higher than those effective i.c.v.; morphine was equipotent in this assay when given i.c.v. or i.t. Both morphine and biphalin were equipotent after i.p. administration. In spite of its antinociceptive effectiveness after i.p. administration, only a small fraction of [¹²⁵I]biphalin was shown to penetrate to the brain (0.051 ± 0.011%, at 20 min). After i.c.v. administration, biphalin antinociception was antagonized by receptor selective doses of β-funaltrexamine (μ antagonist), naloxonazine (μ₁ antagonist), ICI 174,864 (δ antagonist) and [D- Ala²,Cys⁴]deltorphin (δ₂ antagonist), but not by [D- Ala²,Leu⁵,Cys⁶]enkephalin (δ₁ antagonist) or nor-binaltorphimine (κ antagonist), whereas etorphine antinociception was significantly antagonized only by β-funaltrexamine and naloxonazine. Intracerebroventricular biphalin inhibited gastrointestinal propulsion at doses 8-fold higher than those producing i.c.v. antinociception; i.c.v. morphine showed a similar antinociceptive and gastrointestinal propulsion A₅₀. Intraperitoneal biphalin, but not i.p. morphine, showed little, if any, physical dependence, but both biphalin and morphine produced significant physical dependence when equiantinociceptive doses were infused i.c.v. These results demonstrate an unusual profile for biphalin which suggests a potentially novel mechanism which may involve, in part, the putative opioid receptor complex of physically or functionally interacting μ and δ₂ opioid receptors. Biphalin may thus represent the first in a series of such compounds which may lead to significant therapeutic advantages.