Pre-clinical properties of the alpha4beta2 nicotinic acetylcholine receptor partial agonists varenicline, cytisine and dianicline translate to clinical efficacy for nicotine dependence.

BACKGROUND AND PURPOSE: Smoking cessation trials with three high-affinity partial agonists of alpha4beta2 neuronal nicotinic acetylcholine receptors (nAChRs) have demonstrated differences in their clinical efficacy. This work examines the origin of the differences by taking into account brain exposure and pharmacological effects at human alpha4beta2 nAChRs. EXPERIMENTAL APPROACH: Rat plasma and brain pharmacokinetics were characterized and used to predict human steady-state plasma and brain concentrations following recommended doses of each of the three compounds. The pharmacological characterization included in vitro affinities at different nAChR subtypes, functional efficacies and potencies at the human alpha4beta2 nAChR, as well as in vivo effects on rat mesolimbic dopamine turn-over. KEY RESULTS: A comparison of predicted human brain concentrations following therapeutic doses demonstrated that varenicline and nicotine, but not dianicline and cytisine, can extensively desensitize and, to a lesser extent, activate alpha4beta2 nAChRs. The limited clinical efficacy of dianicline may be accounted for by a combination of weak functional potency at alpha4beta2 nAChRs and moderate brain penetration, while recommended doses of cytisine, despite its high in vitro potency, are predicted to result in brain concentrations that are insufficient to affect alpha4beta2 nAChRs. CONCLUSIONS AND IMPLICATIONS: The data provide a plausible explanation for the higher abstinence rate in smoking cessation trials following treatment with varenicline than with the two other alpha4beta2 nAChR partial agonists. In addition, this retrospective analysis demonstrates the usefulness of combining in vitro and in vivo parameters with estimated therapeutic human brain concentrations for translation to clinical efficacy.

Preclinical characterization of selective phosphodiesterase 10A inhibitors: a new therapeutic approach to the treatment of schizophrenia.

We have recently proposed the hypothesis that inhibition of the cyclic nucleotide phosphodiesterase (PDE) 10A may represent a new pharmacological approach to the treatment of schizophrenia (Curr Opin Invest Drug 8:54-59, 2007). PDE10A is highly expressed in the medium spiny neurons of the mammalian striatum (Brain Res 985:113-126, 2003; J Histochem Cytochem 54:1205-1213, 2006; Neuroscience 139:597-607, 2006), where the enzyme is hypothesized to regulate both cAMP and cGMP signaling cascades to impact early signal processing in the corticostriatothalamic circuit (Neuropharmacology 51:374-385, 2006; Neuropharmacology 51:386-396, 2006). Our current understanding of the physiological role of PDE10A and the therapeutic utility of PDE10A inhibitors derives in part from studies with papaverine, the only pharmacological tool for this target extensively profiled to date. However, this agent has significant limitations in this regard, namely, relatively poor potency and selectivity and a very short exposure half-life after systemic administration. In the present report, we describe the discovery of a new class of PDE10A inhibitors exemplified by TP-10 (2-{4-[-pyridin-4-yl-1-(2,2,2-trifluoro-ethyl)-1H-pyrazol-3-yl]-phenoxymethyl}-quinoline succinic acid), an agent with greatly improved potency, selectivity, and pharmaceutical properties. These new pharmacological tools enabled studies that provide further evidence that inhibition of PDE10A represents an important new target for the treatment of schizophrenia and related disorders of basal ganglia function.

The neurokinin-1 antagonist activity of maropitant, an antiemetic drug for dogs, in a gerbil model.

Maropitant is a novel synthetic nonpeptide neurokinin type 1 (NK1) selective receptor antagonist, recently developed for use in the dog as an antiemetic. The in vivo functional activity of maropitant was investigated in the gerbil foot-tapping model, to determine the ability of maropitant to penetrate the central nervous system and inhibit foot-tapping induced by the selective NK1 agonist GR73632. In comparison with CP-122,721, a previously characterized NK1 receptor antagonist, maropitant (1 mg/kg by s.c. injection) was found to inhibit foot-tapping for significantly longer (P < 0.01). Inhibition of foot-tapping by maropitant was 100% at 2 h and approximately 50% at 8 h postdosing. The mean brain:plasma concentration ratio at 8 h post-treatment was 3.59. These data demonstrate the central functional action of maropitant as a selective and potent NK1 receptor antagonist and help to support and explain its clinical potential as a broad-spectrum antiemetic agent.

Pharmacological profile of the alpha4beta2 nicotinic acetylcholine receptor partial agonist varenicline, an effective smoking cessation aid.

The preclinical pharmacology of the alpha4beta2 nicotinic acetylcholine receptor (nAChR) partial agonist varenicline, a novel smoking cessation agent is described. Varenicline binds with subnanomolar affinity only to alpha4beta2 nAChRs and in vitro functional patch clamp studies in HEK cells expressing nAChRs show that varenicline is a partial agonist with 45% of nicotine's maximal efficacy at alpha4beta2 nAChRs. In neurochemical models varenicline has significantly lower (40-60%) efficacy than nicotine in stimulating [(3)H]-dopamine release from rat brain slices in vitro and in increasing dopamine release from rat nucleus accumbens in vivo, while it is more potent than nicotine. In addition, when combined with nicotine, varenicline effectively attenuates the nicotine-induced dopamine release to the level of the effect of varenicline alone, consistent with partial agonism. Finally, varenicline reduces nicotine self-administration in rats and supports lower self-administration break points than nicotine. These data suggest that varenicline can reproduce to some extent the subjective effects of smoking by partially activating alpha4beta2 nAChRs, while preventing full activation of these receptors by nicotine. Based on these findings, varenicline was advanced into clinical development and recently shown to be an effective and safe aid for smoking cessation treatment.

P-glycoprotein efflux at the blood-brain barrier mediates differences in brain disposition and pharmacodynamics between two structurally related neurokinin-1 receptor antagonists.

CP-122721 and CP-141938 are potent and selective neurokinin-1 (NK(1)) receptor antagonists with very different brain disposition and potency in models of centrally mediated activity. These investigations sought to determine whether differences in potency were related to differences in P-glycoprotein (P-gp) transport at the blood-brain barrier. Both compounds stimulated ATPase activity of human recombinant MDR1 with similar kinetic parameters. Cell-associated drug concentrations of CP-141938 were 9.4-fold lower in KBV1 cells expressing P-gp compared with KB3.1 control cells. In Madin-Darby canine kidney (MDCK) cells expressing human MDR1, asymmetric transport of CP-141938 was 5-fold higher than in wild-type MDCK cells, whereas no asymmetry was observed with CP-122721. In agreement with these differences in cellular transport, the differences in brain/plasma ratio between mdr1a/b(-/-) and FVB mice 1 h following a 3 mg/kg s.c. dose were 3- and 50-fold for CP-122721 and CP-141938, respectively. The effect of inhibiting P-gp efflux on the effects of these agents was evaluated using GR73632-induced foot tapping in gerbils as a model to measure centrally mediated NK(1) antagonism. When gerbils were pretreated with the P-gp inhibitor MS-073 (50 mg/kg s.c.), there was no effect on the activity of CP-122721 (0.05 mg/kg), whereas the percent reversal for CP-141938 (10 mg/kg) increased from 60 to 100%. In gerbils, the brain/plasma ratio for CP-122721 was unaffected by MS-073 pretreatment, whereas the brain/plasma ratio for CP-141938 brain concentrations increased 13-fold. This suggested that P-gp efflux influences the brain disposition and pharmacologic activity of CP-141938, but not CP-122721. Complete response curves for CP-141938 were then determined with respect to dose, and drug concentration in the plasma and brain in the presence and absence of MS-073 pretreatment. The dose and plasma concentration-response curves of CP-141938 were shifted to the left in the presence of MS-073, yet brain concentrations associated with the response were unchanged. This suggested that once in the brain the interaction of CP-141938 with the NK(1) receptor was not affected by P-gp transport. In conclusion, these studies show that brain disposition and centrally mediated in vivo activity of NK(1) antagonists can be profoundly affected by P-gp transport and that such transport should be considered during the design of new agents.

CP-154,526: a potent and selective nonpeptide antagonist of corticotropin releasing factor receptors.

Here we describe the properties of CP-154,526, a potent and selective nonpeptide antagonist of corticotropin (ACTH) releasing factor (CRF) receptors. CP-154,526 binds with high affinity to CRF receptors (Ki < 10 nM) and blocks CRF-stimulated adenylate cyclase activity in membranes prepared from rat cortex and pituitary. Systemically administered CP-154,526 antagonizes the stimulatory effects of exogenous CRF on plasma ACTH, locus coeruleus neuronal firing and startle response amplitude. Potential anxiolytic activity of CP-154,526 was revealed in a fearpotentiated startle paradigm. These data are presented in the context of clinical findings, which suggest that CRF is hypersecreted in certain pathological states. We propose that a CRF antagonist such as CP-154,526 could affirm the role of CRF in certain psychiatric diseases and may be of significant value in the treatment of these disorders.

Evidence for clonidine presynaptically modulating amino acid release in the rostral ventral medulla: role in hypertension.

Reports suggested that the predominant site of action for the antihypertensive effects of clonidine is the rostral ventrolateral medulla (RVL), the presumed tonic vasomotor center. This study examined whether clonidine directly interacts with nerve terminal alpha 2-adrenergic receptors in the RVL to inhibit the release of sympathoexcitatory transmitters like glutamate (Glu) and aspartate (Asp), and/or facilitate the release of sympathoinhibitory transmitters like gamma-aminobutyric acid (GABA). Release of GABA and Glu was measured from synaptosomes prepared from the rostral ventral medulla of spontaneously hypertensive rats (SHR), a genetic model of hypertension, and normotensive Wistar-Kyoto rats (WKY). Quantification of neurotransmitter release was performed by high-performance liquid chromatography. Depolarization with 35 mM K+ significantly increased by 58-110% the release of GABA, Glu and Asp; however, no strain differences were observed. In contrast, spontaneous release of GABA and Asp was significantly lower in SHR than that of WKY (-36 and -41%, respectively); this effect was not observed for Glu. Clonidine (1 and 10 microM) enhanced the spontaneous release of GABA (+44%), Asp (+50%) and Glu (+70%) in SHR, but not WKY; this effect was prevented by yohimbine (1 microM). These data, together with previous findings, support the presence of facilitory alpha 2-adrenergic receptors on nerve terminals of GABAergic, glutamatergic and aspartatergic neurons in the rostral ventral medulla. These findings also suggest the existence of another inhibitory transmitter that may mediate the actions of clonidine to decrease sympathetic outflow from the RVL.