ABSTRACT(Ϫ)-(1R,2R)-3-(3-Dimethylamino-1-ethyl-2-methyl-propyl)-phenol hydrochloride (tapentadol HCl) is a novel -opioid receptor (MOR) agonist (K i ϭ 0.1 M; relative efficacy compared with morphine 88% in a [35 S]guanosine 5Ј-3-O-(thio)triphosphate binding assay) and NE reuptake inhibitor (K i ϭ 0.5 M for synaptosomal reuptake inhibition). In vivo intracerebral microdialysis showed that tapentadol, in contrast to morphine, produces large increases in extracellular levels of NE (ϩ450% at 10 mg/kg i.p.). Tapentadol exhibited analgesic effects in a wide range of animal models of acute and chronic pain [hot plate, tail-flick, writhing, Randall-Selitto, mustard oil colitis, chronic constriction injury (CCI), and spinal nerve ligation (SNL)], with ED 50 values ranging from 8.2 to 13 mg/kg after i.p. administration in rats. Despite a 50-fold lower binding affinity to MOR, the analgesic potency of tapentadol was only two to three times lower than that of morphine, suggesting that the dual mode of action of tapentadol may result in an opiate-sparing effect. A role of NE in the analgesic efficacy of tapentadol was directly demonstrated in the SNL model, where the analgesic effect of tapentadol was strongly reduced by the ␣ 2 -adrenoceptor antagonist yohimbine but only moderately attenuated by the MOR antagonist naloxone, whereas the opposite was seen for morphine. Tolerance development to the analgesic effect of tapentadol in the CCI model was twice as slow as that of morphine. It is suggested that the broad analgesic profile of tapentadol and its relative resistance to tolerance development may be due to a dual mode of action consisting of both MOR activation and NE reuptake inhibition.

Cebranopadol (trans-6'-fluoro-4',9'-dihydro-N,N-dimethyl-4-phenyl-spiro[cyclohexane-1,1'(3'H)-pyrano[3,4-b]indol]-4-amine) is a novel analgesic nociceptin/orphanin FQ peptide (NOP) and opioid receptor agonist [Ki (nM)/EC50 (nM)/relative efficacy (%): human NOP receptor 0.9/13.0/89; human mu-opioid peptide (MOP) receptor 0.7/1.2/104; human kappa-opioid peptide receptor 2.6/17/67; human delta-opioid peptide receptor 18/110/105]. Cebranopadol exhibits highly potent and efficacious antinociceptive and antihypersensitive effects in several rat models of acute and chronic pain (tail-flick, rheumatoid arthritis, bone cancer, spinal nerve ligation, diabetic neuropathy) with ED50 values of 0.5-5.6 µg/kg after intravenous and 25.1 µg/kg after oral administration. In comparison with selective MOP receptor agonists, cebranopadol was more potent in models of chronic neuropathic than acute nociceptive pain. Cebranopadol's duration of action is long (up to 7 hours after intravenous 12 µg/kg; >9 hours after oral 55 µg/kg in the rat tail-flick test). The antihypersensitive activity of cebranopadol in the spinal nerve ligation model was partially reversed by pretreatment with the selective NOP receptor antagonist J-113397[1-[(3R,4R)-1-cyclooctylmethyl-3-hydroxymethyl-4-piperidyl]-3-ethyl-1,3-dihydro-2H-benzimidazol-2-one] or the opioid receptor antagonist naloxone, indicating that both NOP and opioid receptor agonism are involved in this activity. Development of analgesic tolerance in the chronic constriction injury model was clearly delayed compared with that from an equianalgesic dose of morphine (complete tolerance on day 26 versus day 11, respectively). Unlike morphine, cebranopadol did not disrupt motor coordination and respiration at doses within and exceeding the analgesic dose range. Cebranopadol, by its combination of agonism at NOP and opioid receptors, affords highly potent and efficacious analgesia in various pain models with a favorable side effect profile.

The distinct properties of tapentadol and tramadol generate different CNS functional activities, making each drug the prototype of different classes of opioid/nonopioid analgesics. Tramadol's analgesia derives from relatively weak µ-opioid receptor (MOR) agonism, plus norepinephrine and serotonin reuptake inhibition, provided collectively by the enantiomers of the parent drug and a metabolite that is a stronger MOR agonist, but has lower CNS penetration. Tapentadol's MOR agonist activity is several-fold greater than tramadol's, with prominent norepinephrine reuptake inhibition and minimal serotonin effect. Accordingly, tramadol is well-suited for pain conditions for which a strong opioid component is not needed-and it has the benefit of a low abuse potential; whereas tapentadol, a schedule-II controlled substance, is well-suited for pain conditions requiring a strong opioid component-and it has the benefit of greater gastrointestinal tolerability compared to classical strong opioids. Both drugs offer distinct and complementary clinical options.

In a previous communication, our efforts leading from 1 to the identification of spiro [cyclohexanedihydropyrano[3,4-b]indole]-amine 2a as analgesic NOP and opioid receptor agonist were disclosed and their favorable in vitro and in vivo pharmacological properties revealed. We herein report our efforts to further optimize lead 2a, toward trans-6′-, which is currently in clinical development for the treatment of severe chronic nociceptive and neuropathic pain. KEYWORDS: NOP receptor agonists, MOP receptor agonists, cebranopadol, analgesics R ecent publications indicate that small molecules activating both nociceptin/orphanin FQ peptide (NOP) and mu opioid peptide (MOP) receptors may potentiate opiate analgesia and at the same time display an improved side effects profile. 1,2 We have recently reported the discovery of a series of small molecules, characterized by their high NOP and opioid receptor agonistic activity. 3 This series included uncyclized (e.g., 1) as well as spirocyclic examples (e.g., 2a). The discovery of spirocyclic 2a originated from the respective uncyclized analogues, which were potent NOP and MOP receptor binders but sometimes hampered by only partial agonistic NOP and MOP receptor activity. In particular, the spiroindole derivates sparked our interest due to their structural novelty and favorable in vitro and in vivo properties. The leading spiroether 2a exhibited strong efficacy in preclinical models of acute (ED 50 rat tail-flick: 3.63 nmol/kg i.v.) and neuropathic pain (ED 50 rat spinal nerve ligation: 1.05 nmol/kg i.v.) but was hampered by poor pharmacokinetic (PK) properties in rats with high clearance, large volume of distribution, moderate half-life (Cl = 4.0 L/h·kg; V ss = 7.52 L/kg; t 1/2 = 1.6 h), and a critically very low oral bioavailability (F = 4%).We herein report our efforts to further optimize the spiroindole lead 2a, which eventually led to the discovery of trans-6′-fluoro-4′,9′-dihydro-N,N-dimethyl-4-phenyl-spiro-[cyclohexane-1,1′(3′H)-pyrano [3,4-b]indol]-4-amine (3a, cebranopadol), a novel potent analgesic NOP and opioid receptor agonist, currently in clinical development for the treatment of severe chronic nociceptive and neuropathic pain.The structure−activity relationship (SAR) established around the uncyclized scaffolds (e.g., 1) suggested that a broad variety of linkers such as alcohols, ethers, and amines are tolerated, showing high NOP and MOP receptor binding affinities. 3 As a result, we applied this knowledge through analogous structural variations to lead structure 2a (region A, Figure 1). These changes were also combined with a targeted approach to improve the poor PK profile, in particular by addressing metabolically liable regions B and C.With this in mind, the transformation of the oxacyclic spiro moiety into a carba-, aza-, or thio-cyclic moiety was investigated. Advancing from the oxacyclic spiro 2a to the azacyclic moiety 5a led to equally potent NOP and MOP receptor binders, as well as the introduction of the N-methyl subunit 6a. Similarly, pot...

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