REVIEWS From ligand design to therapeutic efficacy: the challenge for nicotinic receptor research Bruce K.Cassels,Isabel Bermúdez,Federico Dajas,J.Andrés Abin-Carriquiry and Susan Wonnacott S-Nicotine, the principal psychoactive constituent of Nicotiana tabacum, underpins addiction to tobacco smoking. Although tobacco consumption is a leading cause of death worldwide, nicotine itself is also proposed to have potential therapeutic benefits for a diverse range of conditions. Nicotine interacts with its cognate receptors in the central nervous system to exert a predominantly modulatory influence, making neuronal nicotinic receptors attractive therapeutic targets. Here, we focus on three natural products as lead compounds for drug discovery programs, nicotine, epibatidine and cytisine, and consider the aims and limitations that shape these drug discovery endeavors. Heterogeneity of nicotinic acetylcholine their high relative permeability to Ca2+[2]. In contrast receptor: which subtype to target? to the well-defined roles of nAChRs in mediating Bruce K. Cassels Nicotine (Figure 1) is the classical agonist of a fam- synaptic transmission at muscle end plates and in Department of Chemistry, Faculty of Sciences, ily of ligand-gated cation channels that also respond autonomic ganglia, neuronal nAChRs in the brain University of Chile, to acetylcholine (ACh, Figure 2) as the endogenous have rarely been shown to fulfill this function, instead Casilla 653, Santiago, ligand, hence defined as nicotinic acetylcholine being found on presynaptic terminals or at extrasy- Chile receptors (nAChRs). These pentameric receptors are naptic locations on soma and dendrites. Together e-mail:[email protected] generated from different sets of subunits expressed with their capacity for interfacing with cellular sig- Isabel Bermúdez School of Biological and in mammalian skeletal muscle (α1, β1, γ, δ, ε), neurons naling pathways, this has generated the view that Molecular Sciences, (α2–α7, β2–β4) and sensory epithelia (α9, α10). In neuronal nAChRs exert a predominantly modulatory Oxford Brookes University, Gipsy Lane, autonomic neurons, nAChRs responsible for synaptic influence in the CNS [3]. Oxford OX3 0BP, transmission comprise α3 and β4 subunits, with the A modulatory action offers attractive therapeutic UK possible addition of α5 and/or β2 subunits (hence opportunities and interest has focused on emulating Federico Dajas J. Andrés Abin-Carriquiry α3β4*, where * indicates the presence of one or more nicotine’s effects by generating agonist molecules. Instituto de Investigaciones additional types of subunit [1]). α3β4* nAChRs are However, nicotine activates all subtypes of nAChRs Biológicas Clemente Estable, α α Avenida Italia 3318, also expressed in the central nervous system (CNS) (except those comprising 9 and 10 subunits [4]), C.P.11600 Montevideo, to a limited extent, whereas the α4 subunit is exclu- albeit with different potencies. Issues for drug discov- Uruguay sively expressed in the CNS. In combination with the ery include nAChR subtype selectivity and addiction Susan Wonnacott β α β Department of Biology and 2 subunit, 4 2* nAChRs are the most abundant liability. In addition to generating agonists with low Biochemistry, and widespread nAChR subtypes in the brain, potency at muscle and ganglionic nAChRs (to min- University of Bath, exhibiting high affinity for nicotine. Homomeric α7 imize peripheral side effects), is it advantageous to tar- Bath BA2 7AY, UK nAChRs are also prevalent and are distinguished by get specific neuronal nAChR subtypes (e.g. α7 versus REVIEWS self-administration, accompanied by plenty of associated H3C H3C cues and Pavlovian reinforcement; nicotine replacement N N therapies demonstrate the low liability associated with alternative routes of delivery. H3C O Reviews • N N Agonist,antagonist or partial agonist? Nicotine ABT-418 To achieve clinical efficacy, is activation or inhibition of DRUG DISCOVERY TODAY DRUG DISCOVERY OCH H 3 nicotinic signaling required? This is a challenging question O N and the answer is likely to depend on the particular nico- NH tinic contribution to each clinical target. Nicotine, the OCH H3C 3 prototype with credentials for improving several conditions, N Cl O is regarded as an agonist of muscle and neuronal nAChRs PNU-120596 but it is also a powerful desensitizing ligand that can pro- duce functional deactivation of nAChR after a few sec- onds or minutes of exposure (depending on nAChR sub- FIGURE 1 type and nicotine concentration) [8]. Functionally, nAChR Chemical structures of nicotine, its isoxazole analogue ABT-418 and α7-specific potentiator PNU-120596. Reversing the heteroatom can exist in four distinct conformations: resting, open and positions in the isoxazole ring and introducing a hydrophobic two desensitized states. The latter are refractory to activa- extension of the basic sidechain changes an agonist into a positive tion on a time scale of milliseconds or minutes depending allosteric modulator. on the desensitized state but have high affinity (pM–nM) for agonists. Chronic nicotine exposure can induce a long- Cl lasting functional deactivation as a result of rapid and per- CH3 sistent desensitization. Clinical prescription of drugs will + H3C N H N involve chronic administration and the interplay of acti- H3C CH3 vation and desensitization is likely to be crucial, as well as O NH O O the contribution of potentially bioactive metabolites. At Acetylcholine PNU-282987 present, modeling this relationship for human brain Cl nAChRs is simplistic, even for nicotine in relation to S N N cigarette smoking [9], and impossible to predict for novel HN N ligands. However, because cognitive enhancing and neu- O O roprotective effects of nicotine are abolished by coad- O O ministration of nAChR antagonists [5,6,10], activation of AR-R17779 Mitsubishi compound nAChRs is generally sought and attention has focused on generating small agonist ligands. Tourette’s syndrome, on FIGURE 2 the other hand, might present a case for nAChR antagonists The acetylcholine pharmacophore, held in a rigid scaffold, (see below). produces selective α7 nAChR agonists. Partial agonists provide a compromise solution that could have particular utility, for example, for smoking ces- α4β2*)? Although the answer will differ for different clin- sation. In this regard, cytisine was noted for its selective ical scenarios, some studies have implied that α7 and β2* partial agonism at α4β2* nAChR: this subtype plays a subtypes conspire together to effect neuroprotection [5,6], major role in governing mesolimbic dopamine release in in which case selectivity would be inappropriate unless response to nicotine, which underlies the reinforcing combination therapy with complementary subtype-se- properties of the drug. In fact, cytisine (Tabex®) is marketed lective drugs could be achieved. And which nAChR sub- in Eastern Europe as an aid for smokers wishing to break types to target? To date, most effort has focused on tar- their habit. It has been argued that binding of a partial geting the most prevalent subtypes, α7 or α4β2 nAChRs agonist at this receptor would provide a modicum of [7]. Less abundant subtypes with more restrictedpatterns dopamine release to compensate for the absence of nico- of localization could offer more-selective therapeutic tar- tine (providing some relief from withdrawal symptoms), gets, but the potential for nAChR heterogeneity arising from whereas occupancy of the nAChR prevents nicotine from different subunit combinations, with subtle differences in binding [11,12]. The molecular interactions that result in properties or regulation, far exceeds our current knowledge partial agonism are not understood and it is problematic of native nAChR subtypes, making informed decisions to predict the activity of any derivative. about more-complex combinations difficult or impossi- Although beyond the remit of this review, it is worth ble at present. Addiction liability is probably not a major mentioning the current interest in another class of nico- concern, unless novel compounds are likely to be smoked: tinic ligand, nAChR potentiators [13]. Such molecules cigarettes provide the optimum delivery device for nicotine produce no obvious nicotinic response alone but enhance REVIEWS responses to conventional agonists (including endoge- [22,23]. The latter appear to reflect autonomic stimulation nous ACh). The Alzheimer’s disease (AD) drug galan- and emphasize the need for better subtype-selective thamine is credited with allosteric potentiation of nicotinic nAChR agonists. The restricted localization of α6* nAChRs responses, in addition to its anti-cholinesterase activity to catecholamine systems in the brain, where they enhance [13,14]. Whereas galanthamine does not discriminate transmitter release, might make this subtype a credible between nAChR subtypes, an α7-specific potentiator target for PD, if selective agonists could be found [22]. PNU-120596 (Figure 1) has recently been described [15]. A search for a subtype-selective negative modulator, to Schizophrenia depress nAChR responses, has been advocated for treating α7 nAChRs are reduced in number in the post-mortem familial epilepsy [16] (see below). brains of schizophrenic patients and nicotine is beneficial in transiently normalizing the sensory gating deficit, as Clinical targets for nicotinic drugs well as improving some cognitive deficits [24,18]. The DRUG DISCOVERY TODAY DRUG DISCOVERY The development of potential nicotinic therapeutics was association