Ellen E. Codd

Oral activity of the growth hormone releasing peptide His-D-Trp-Ala-Trp-D-Phe-Lys-NH2 in rats, dogs and monkeys

The purpose of this study was to evaluate the growth hormone (GH) releasing activity of orally administered His-D-Trp-Ala-Trp-D-Phe-Lys-NH2 (GHRP-6, SK&F 110679) in rats, dogs and monkeys. Rats were administered GHRP-6 orally by gavage or parenterally through femoral artery catheters. Blood was collected before and after GHRP-6 administration for estimation of plasma GH and comparison of GH changes resulting from enteral and parenteral administration of the peptide. GHRP-6 was administered to dogs intravenously (i.v.) through cephalic vein catheters, intragastrically (i.g.) through esophagostomy tubes or intraduodenally (i.d.) through vascular access ports, and blood was collected before and after peptide administration for estimation of plasma GH. Cynomolgus monkeys were administered GHRP-6 i.g., and blood was collected from abdominal aorta for estimation of changes in plasma GH. Enteral activity of GHRP-6 was observed in all 3 species tested. In rats, ED50s for enteral and parenteral administration of GHRP-6 were 4 mg/kg and 28 micrograms/kg, respectively. Thus in rats, enterally administered GHRP-6 was 0.7% as bioactive as the parenterally administered peptide. In dogs GHRP-6 was slightly less potent than in rats, with ED50s for i.g. and i.v. administration approximately 15 mg/kg and 125 micrograms/kg, respectively. However, enteral potency of GHRP-6 in dogs was 0.8% of parenteral potency, and thus, comparable to that in rats. Additionally, comparison of plasma GH levels following i.g. vs i.d. administration in dogs suggested greater activity by the i.d. route. Monkeys were the species most sensitive to enterally administered GHRP-6, with plasma GH increased in those receiving i.g. doses as low as 0.3 mg/kg and an ED50 of 0.75 mg/kg compared to 4 and 15 mg/kg in rats and dogs, respectively. The results of this study demonstrate that GHRP-6 releases GH when administered directly into the gastrointestinal tract. Although enteral activity is approximately 1% of parenteral activity, GHRP-6 is potent, especially in primates which require relatively low doses to provoke GH release. These data suggest that orally active GHRP-6 may provide a practical therapeutic alternative to parenterally administered peptides such as GHRH, especially if enteral activity is enhanced with appropriate formulation.

Low affinity of FMRFamide and four FaRPs (FMRFamide-related peptides), including the mammalian-derived FaRPs F-8-Famide (NPFF) and A-18-Famide, for opioid μ, δ, κ1, κ2a, or κ2b receptors

Abstract The binding affinities at opioid receptor subtypes in rat or guinea pig brain membranes were determined for the neuropeptide FMRFamide (Phe-Met-Arg-Phe-NH2), the two mammalian-derived FMRFamide-related peptides F-8-Famide (NPFF; Phe-Leu-Phe-Gln-Pro-Gln-Arg-Phe-NH2) and A-18-Famide (Ala-Gly-Glu-Gly-Leu-Ser-Ser-Pro-Phe-Trp-Ser-Leu-Ala-Ala-Pro-Gln-Arg-Phe-NH2), and the two other FMRFamide-related peptides Tyr-Phe-Met-Arg-Phe-NH2 (Tyr-FMRFamide) and Pro-Gln-Arg-Phe-NH2 (Pro-Gln-RFamide). The μ and δ sites were labeled in rat brain membranes using tritiated [ d -Ala2, N- MePhe 4 ,Gly-ol5]enkephalin ([3H]DAMGO) and [ d -Ala2, d -Leu5]enkephalin ([3H]DADLE), respectively. The κ sites were labeled in guinea pig brain using [3H]U-69,593 after treatment with BIT and FIT for κ 1 and [3H]bremazocine after pretreatment with BIT and FIT for κ 2 . The κ 2 a binding sites were assayed using [Leu5]enkephalin to block κ 2 b sites and the κ 2 b sites were assayed using (−)-(1S,2S)-U50,488 to block κ 2 a sites. Neither FMRFamide nor any of the FMRFamide-related peptides (up to 61.0 μM) displayed significant affinity at any of the subtypes of opioid receptor. Hence, the known ability of FMRFamide and FaRPs to interact with the opioid system does not appear to be related to direct binding to these opioid receptors.

JNJ-20788560 [9-(8-Azabicyclo[3.2.1]oct-3-ylidene)-9H-xanthene-3-carboxylic Acid Diethylamide], a Selective Delta Opioid Receptor Agonist, Is a Potent and Efficacious Antihyperalgesic Agent That Does Not Produce Respiratory Depression, Pharmacologic Tolerance, or Physical Dependence

μ-Opioid analgesics are a mainstay in the treatment of acute and chronic pain of multiple origins, but their side effects, such as constipation, respiratory depression, and abuse liability, adversely affect patients. The recent demonstration of the up-regulation and membrane targeting of the δ-opioid receptor (DOR) following inflammation and the consequent enhanced therapeutic effect of δ-opioid agonists have enlivened the search for δ-opioid analgesic agents. JNJ-20788560 [9-(8-azabicyclo-[3.2.1]oct-3-ylidene)-9H-xanthene-3-carboxylic acid diethylamide] had an affinity of 2.0 nM for DOR (rat brain cortex binding assay) and a naltrindole sensitive DOR potency of 5.6 nM (5′-O-(3-[35S]thio)triphosphate assay). The compound had a potency of 7.6 mg/kg p.o. in a rat zymosan radiant heat test and of 13.5 mg/kg p.o. in a rat Complete Freunds adjuvant RH test but was virtually inactive in an uninflamed radiant heat test. In limited studies, tolerance was not observed to the antihyperalgesic or antinociceptive effects of the compound. Unlike ibuprofen, JNJ-20788560 did not produce gastrointestinal (GI) erosion. Although morphine reduced GI motility at all doses tested and reached nearly full effect at the highest dose, JNJ-20788560 did not retard transit at the lowest dose and reached only 11% reduction at the highest dose administered. Unlike morphine, JNJ-20788560 did not exhibit respiratory depression (blood gas analysis), and no withdrawal signs were precipitated by the administration of opioid (μ or δ) antagonists. Coupled with the previously published lack of self-administration behavior of the compound by alfentanil-trained primates, these findings strongly recommend δ-opioid agonists such as JNJ-20788560 for the relief of inflammatory hyperalgesia.

Tramadol and several anticonvulsants synergize in attenuating nerve injury-induced allodynia

&NA; Neuropathic pain results from injury or dysfunction of the central or peripheral nervous system. The treatment of neuropathic pain is challenging, in part because of its multiple etiologies. The present study explores combinations of the analgesic tramadol and each of four anticonvulsants in the treatment of surgically induced (ligation of the L5 spinal nerve) allodynia in rats. Each of the five drugs studied exhibited a dose‐dependent antiallodynic effect. When studied in combination, tramadol and each of two of the anticonvulsants (topiramate and RWJ‐333369) interacted synergistically at all three ratios studied, whereas tramadol and each of the other two anticonvulsants (gabapentin and lamotrigine) exhibited a synergistic antiallodynic effect at only one of three ratios investigated. In addition, tramadol and topiramate were found to interact synergistically in a nociceptive pain model, the mouse hot‐plate test. These studies suggest the benefit of using combinations of analgesics and anticonvulsants in the relief of neuropathic pain.

Gas chromatographic method using nitrogen-phosphorus detection for the measurement of tramadol and its O-desmethyl metabolite in plasma and brain tissue of mice and rats

A method that allows the measurement of plasma and brain levels of the centrally-acting analgesic tramadol and its major metabolite (O-desmethyl tramadol) in mice and rats was developed using gas chromatography equipped with nitrogen-phosphorus detection (GC-NPD). Plasma samples were extracted with methyl tert.-butyl ether (MTBE) and were injected directly into the GC system. Brain tissue homogenates were precipitated with methanol, the resulting supernatant was dried then acidified with hydrochloric acid. The aqueous solution was washed with MTBE twice, alkalinized, and extracted with MTBE. The MTBE layer was dried, reconstituted and injected into the GC system. The GC assay used a DB-1 capillary column with an oven temperature ramp (135 to 179 degrees C at 4 degrees C/min). Dextromethorphan was used as the internal standard. The calibration curves for tramadol and O-desmethyl tramadol in plasma and brain tissue were linear in the range of 10 to 10000 ng/ml (plasma) and ng/g (brain). Assay accuracy and precision of back calculated standards were within +/- 15%.

Lack of binding of acetaminophen to 5-HT receptor or uptake sites (or eleven other binding/uptake assays)

The mechanism of analgesic action of acetaminophen (paracetamol) remains unknown. However, a central component distinct from that of the NSAIDs (non-steroidal antiinflammatory drugs) seems likely. A recent report (NeuroReport 6:1546-1548, 1995) suggests the involvement of 5-HT3 receptors. In the present study, we measured the affinity of acetaminophen at 5-HT3, as well as 5-HT1A, 5-HT1B, 5-HT1D, 5-HT2, 5-HT2C, 5-HT4, 5-HT6, 5-HT7 and eleven other receptor sites and at serotonin and norepinephrine reuptake sites. At 10 microM, acetaminophen inhibited less than 10% specific radioligand binding at any site. These findings: (i) suggest that acetaminophens effect on serotonergic pathways is indirect, and (ii) circumscribe acetaminophens possible central analgesic mechanism(s).

Alpha2-adrenoceptors vs. imidazoline receptors: Implications for α2-mediated analgesia and other non-cardiovascular therapeutic uses

The multiple clinical actions of clonidine have historically been linked to the same receptor (alpha 2-adrenoceptor) due to the belief that clonidine was a selective alpha 2-agonist. However, it is now recognized that clonidine binds with a similar affinity to alpha 2-adrenoceptors and to non-adrenergic imidazoline receptors. These two pharmacological targets (and subtypes of each alpha 2 and imidazoline receptors) provide the basis for a possible separation of cardiovascular and other targeted effects, such as analgesia. Consequently, the design of selective alpha 2-adrenoceptor (subtype) agonists as analgesics devoid of the cardiovascular effects associated with clonidine appears to be a rational approach to novel therapeutic agents. The present review focuses on alpha 2-adrenoceptor subtype/imidazoline diversity as a target for analgesic (and other CNS) drug discovery.

HIGH AFFINITY IBOGAINE BINDING TO A MU OPIOID AGONIST SITE

The naturally occurring indole alkaloid ibogaine is of interest because of its reported ability to block drug seeking behavior for extended periods. The compound also potentiates morphine-induced analgesia in mice and reduces certain naltrexone-precipitated withdrawal signs in morphine-dependent rats. Although these results might suggest ibogaine interaction with opioid receptors, previous receptor binding studies (Brain Res. 571:242-247, 1980) found that ibogaine had a Ki value of only 2 microM for the kappa opioid receptor and was virtually inactive in blocking mu and delta receptor binding (Ki > 100 microM). The present investigation of ibogaine interaction with the mu opioid receptor from mouse forebrain labeled with [3H]-naloxone, however, yielded significantly more potent mu opioid Ki values. LIGAND analysis indicated that the data were best fit by a two site binding model, with Ki values of about 130 nM and 4 microM, reflecting ibogaine recognition of different agonist affinity states of the receptor. Inclusion of 100 mM NaCl in the assay to induce the agonist low affinity state of the receptor, reduced ibogaines inhibition of [3H]-naloxone binding. These results suggest that ibogaine is an agonist at the mu opioid receptor with a Ki value of about 130 nM, potentially explaining ibogaines antinociceptive effects as well as its reported reduction of opioid withdrawal symptoms and attenuation of drug seeking behavior.

Lack of glibenclamide or TEA affinity for opioid receptors: further evidence for in vivo modulation of antinociception at K+ channels

Radioligand binding of the opioid receptor subtype selective ligands, [3H][D-Ala2,N-Me-Phe4, Gly-ol5]-enkephalin (DAMGO) (mu), [3H][D-Pen2,D-Pen5]-enkephalin (DPDPE) (delta) and [3H]U-69,593 (kappa), to rat brain particulate preparations was virtually unaffected by 1-100 microM glibenclamide (Glib) or tetraethylammonium bromide (TEA). These results argue against opioid receptor antagonism by Glib or TEA and support the hypothesis that antagonism of opioid-induced antinociception by Glib or TEA occurs at the level of K+ (possibly ATP-sensitive KATP) channels.

Determination of the adsorption of tramadol hydrochloride by activated charcoal in vitro and in vivo.

Although tramadol is one of the most widely used centrally acting analgesics worldwide, no literature is available regarding adsorption of tramadol HCl powder or tablets (Ultram; 50 mg tramadol HCl per tablet) by activated charcoal (AC) for use as potential adjunct treatment of overdose. The present study incorporated a novel combination of in vitro and in vivo methods to investigate this question. Based on a binding curve of tramadol UV absorbance (UV(a); 225 nm) plotted against the amount of AC, the ratio of amount of tramadol completely adsorbed by AC was 0.05 mg/mg. Also based on UV(a), no tramadol was detected in filtrate of slurries in which up to 62 tablets of Ultram were mixed with 50 g AC; 4.6% of unbound tramadol was detected when 100 tablets of Ultram were mixed with AC. The ratio of amount of tramadol completely adsorbed by AC in this test was 0.10. In vivo, co-administration of 0.1 g/ml of AC produced a 13- to 14-fold rightward shift in tramadols antinociceptive dose-response curve and a 1.6-fold rightward shift in tramadols lethality dose-response curve.