Epiboxidine and novel-related analogues: A convenient synthetic approach and estimation of their affinity at neuronal nicotinic acetylcholine receptor subtypes

Abstract

a b s t r a c tRacemic exo-epiboxidine 3, endo-epiboxidine 6, and the two unsaturated epiboxidine-related derivatives 7 and 8 were efficiently prepared taking advantage of a palladium-catalyzed Stille coupling as the key step in the reaction sequence. The target compounds were assayed for their binding affinity at neuronal a4b2 and a7 nicotinic acetylcholine receptors. Epiboxidine 3 behaved as a high affinity a4b2 ligand (K i = 0.4 nM) and, interestingly, evidenced a relevant affinity also for the a7 subtype (K i = 6 nM). Derivative 7, the closest analogue of 3 in this group, bound with lower affinity at both receptor subtypes (K i = 50 nM for a4b2 and K i = 1.6 lM for a7) evidenced a gain in the a4b2 versus a7 selectivity when compared with the model compound.Ó 2008 Elsevier Ltd. All rights reserved.Neuronal nicotinic acetylcholine receptors (nAChRs) make up a family of pentameric ligand-gated ion channels, which are formed by combinations of alpha and beta subunits 1,2 or exist as homopentamers, in the cases of a7, a8, and a9 receptors, which are inhibited by a-bungarotoxin. 3 To date, nine a (a2-a10) and three b (b2-b4) isoforms have been characterized, though only a relatively small subset of combinations generates functionally and physiologically relevant channels. 4 Nicotinic receptors are widely distributed in the brain, where they primarily modulate the release of other neurotransmitters and, to a lesser extent, mediate synaptic transmission. 5 Neuronal nAChRs, selectively activated by (S)-nicotine 1 (Fig. 1), are involved in various processes such as cognition, learning and memory, cerebral blood flow and metabolism, as well as an array of pathological conditions such as Alzheimer's and Parkinson's diseases, mild cognitive impairment (MCI), schizophrenia, epilepsy, Tourette's syndrome, anxiety, depression, attention-deficit hyperactivity disorder (ADHD), and nicotine addiction.1,2,6,7The heteromeric a4b2 receptors, the most abundant subtype in the mammalian CNS, and the homomeric a7 channels are privileged biological targets in the search for selective nAChR ligands with therapeutic potential. 2,8 As far as the number of nicotinic agonists isolated from natural sources is taken into consideration, 9 (À)-epibatidine 2 (Fig. 1), a highly toxic alkaloid identified in the skin of the Ecuadorian frog Epipedobates tricolor, 10 has inspired a huge amount of research aimed at designing subtype selective nicotinic agonists. 1,2,9,11,12 Although the pharmacological effects of (À)-2 are mediated by a variety of nAChRs, 13 which preclude any therapeutic potential for epibatidine, the analgesic potency, roughly 100 times higher than that of morphine and 30 times higher than that of nicotine, has been attributed to its high affinity for the a4b2 subtype. Worth mentioning, (À)-2 and (+)-2 are characterized by similar affinities for nAChRs and almost identical ED 50 values in the mouse tail-flick antinociception assay. 13d,14,15 Among the synthetic epibatidine-related compounds, (±)-epiboxidine 3 (Fig. 1), in which ...

Determination of Acid Dissociation Constants of Compounds Active at Neuronal Nicotinic Acetylcholine Receptors by Means of Electrophoretic and Potentiometric Techniques

“…8,10 Phosphoric acid, formic acid, acetic acid, boric acid, 3-(cyclohexylamino)-1-propanesulfonic acid (CAPS), sodium hydroxide, CO2 free potassium hydroxide, potassium chloride, potassium hydrogen phthalate (KHP), hydrochloric acid (0.5 M), acetone and methanol were purchased from Sigma-Aldrich. All reagents used were of analytical grade and all reagents and buffers were prepared with water obtained from a Milli-Q water purification system (Millipore).…”

“…[2][3][4][5] Efforts in this research area have mainly focused on the discovery and development of agonists selectively interacting with α4β2 and α7 subtypes, 6 which are the two main populations of nicotinic receptors in the human brain and thus the most relevant biological targets for potential therapeutic applications. In this frame, epibatidine 1, a highly toxic alkaloid identified in the skin of the Ecuadorian frog Epipedobates tricolor, 7 epiboxidine 2, 8 an epibatidine analogue in which the chloropyridinyl ring has been replaced by the 3-methylisoxazolyl moiety, and ARR1779 3 9 are endowed with high affinity for both the α4β2 and α7 nAChR subtype (Fig. 1).…”

“…2), were synthesized and tested at rat α4β2 and α7 nAChR subtypes. 8,10 In addition to the assessment of pharmacodynamic features, compounds of potential therapeutic significance should be evaluated for properties which will contribute to their pharmacokinetic profile in vivo, such as brain penetration, solubility, lipophilicity, hydrogen bonding capacity and charge. The latter parameter can be easily estimated once the acidic dissociation constant is known.…”

“…An intermediate mobility is a function of its dissociation equilibrium; pKa values can be determined by regression analysis. 14 In this study we aimed at measuring the pKa values of epiboxidine 2 and those of its structurally related compounds 4 -8, 8 as well as of epibatidine analogues 9 -11 and derivatives 12 -16, the latter belonging to a group of recently patented 2010 © The Japan Society for Analytical Chemistry † To whom correspondence should be addressed. E-mail: gabriella.roda@unimi.it analogues of ARR1779 10 ( Fig.…”

Affinity Chromatography as a Tool for Quantification of Interactions Between Drug Molecules and Their Protein Targets

“…In addition, for DM and LM there was no significant difference in the ΔS° values, while the ΔH° value for the DMnAChR interaction was significantly lower than respective value for LM-nAChR interaction showing that the enantioselectivity of 3 4 nAChR is mostly enthalpy-based (Jozwiak et al, 2003).The results of those studies demonstrated that non-linear chromatography approach of investigations of immobilized nAChRs can be useful in characterizing ligand-receptor binding interactions and predicting of properties of drugs and drug candidates. Additional NLC studies with this system were used to construct QSAR models of non-competitive inhibitors of the nAChR (Jozwiak et al, 2004), to develop a molecular model of these interactions (Jozwiak et al, 2004) and to predict IC 50 values ] -bungarotoxin as marker radioligands specific for 3 4 and 7 nAChRs subtypes respectively were applied to determine K i values and check subtype selectivity of newly synthesized derivatives of epiboxidine, synthetic epibatidine-related compounds (Rizzi et al, 2008).…”