PHARMACOLOGICAL CHARACTERIZATION OF A NOVEL CHIMERIC ANALGESIC PEPTIDE AA 501 (1999)

S361 INTRODUCTION: Many physiological and pharmacological studies have indicated that substance P and opioids are functionally antagonistic in the mediation of nociception and antinociception in vivo. We have previously demonstrated that biphalin, a novel opioid tetrapeptide dimer (Tyr-D-Ala-Gly-Phe-NH-)2, given intrathecally with a peptide substance P antagonist produces potent spinal antinociception in rats (Misterek et al., 1994) [1]. AA 501, a novel compound, binds both to opioid and substance P receptors. Because this novel peptide construct consists of the opioid sequence Tyr-D-Ala-Gly-Phe- covalently linked through a hydrazide bridge to a Z-Trp (N-alpha-Carbobenzoxy-Trp) moiety, and this moiety has putative neurokinin 1 (NK-1) receptor blocking effects, we have now evaluated the activity of AA 501. MATERIAL AND METHODS: After Animal Research Committee approval, adult male Sprague-Dawley rats (225-250 g) were implanted with chronic indwelling IT catheters with tips at the T13-L1 spinal level. Animals had one week to recover from surgery, during which they were habituated daily to the laboratory environment and analgesic testing apparatus. For evaluation of antinociceptive properties of AA 501 we used tail-flick test and formalin test. For measurement of thermal antinociception, a tail-flick apparatus was utilized (baseline latency approximately 3 sec, cutoff 10 sec). Responses were expressed as % maximum possible effect (%MPE): [%MPE=(post-treatment latency-baseline latency)/(cut off time-baseline latency) x100]. In the formalin test 50 [micro sign]l of 5% formalin solution was injected s.c. into the dorsal surface of the rat hind paw. Pain behavior was quantified by periodically counting the occurrence of spontaneous flinching/shaking of the injected paw. AA501 was administered IT 15 minutes before or 9 min after formalin injection. RESULTS: IT administration of AA501 produced dose-dependent antinociception in tail-flick and formalin tests. The time course of antinociceptive responses to lower doses of AA501 (0.25 and 0.5 [micro sign]g) peaked at approximately 50% MPE at 15-30 min and declined to 20-30% MPE at 60 min. Intermediate doses (1 and 4 [micro sign]g) produced an antinociceptive effect that reached 60-80% MPE at 5-45 min. Higher doses (8 and 25 [micro sign]g) produced a maximal antinociceptive response (100% MPE) of long duration (30-75 min). Antinociceptive effects of AA501 were reversed by 10 min pretreatment with naloxone IT 10 [micro sign]g and diminished by SP IT 150 ng. AA501 administered IT in dose 1 [micro sign]g 15 min before formalin injection into the rat hind paw completely blocked the biphasic response to formalin. AA501 administered 9 min after formalin injection also blocked occurrence of the second phase of the response to formalin. DISCUSSION: We have previously shown that a hybrid peptide containing both beta-casomorphin-like and substance P-like structural characteristics possessed an antinociceptive effect in the mouse hot plate test after intrathecal administration (Lipkowski et al., 1994) [2]. AA501 was designed to concurrently activate opioid receptors and block NK-1 receptors. The antinociceptive effects of IT AA501 in the tail-flick and formalin tests are highly significant and dose-dependent, suggesting its value as a potential therapeutic agent or useful tool for study on pain modulation. [Department of Anesthesia funds, NIDA Grant 04128 and The Richard Saltonstall Charitable Foundation.]