Cytisine’s lower potency at 5-HT3 receptors may explain its lower incidence of nausea and vomiting than varenicline Lummis, SCR1, Price, KL1, Clarke A2. 1 Department of Biochemistry, University of Cambridge, United Kingdom 2 Achieve Life Sciences, Inc., Seattle, United States Disclosures ▪ Study was part of a larger project to examine the 5-HT3 receptor binding of cytisine and some semi- synthetic analogues generated in a project managed by Achieve Life Sciences ▪ Performed at Department of Biochemistry, University of Cambridge by Prof Sarah Lummis and Dr Kerry Price ▪ Prof Lummis and Dr Price were not paid to conduct this work ▪ Dr Clarke is a full-time, paid employee and shareholder of Achieve Life Sciences 2 (-)-Cytisine ▪ (-)-Cytisine is a naturally-occurring substance extracted from Cytisus laburnum (Golden chain) and other botanical sources ▪ USAN (U.S. Approved Name) is cytisinicline ▪ Used as an aid to smoking cessation in Central & Eastern Europe for many years ▪ Traditional dosing of 1.5 mg tablets using a downward titration over 25 days ▪ Recent trial results suggest a simplified regimen of a 3 mg tablet 3 times daily is effective and well tolerated 3 Nausea in Clinical Trials Latest Cochrane review reported 32 randomised trials comparing varenicline (Champix/Chantix®) with placebo Nausea Subjects Placebo 596 7,034 8.5% Varenicline 2,207 7,929 27.8% RR = 3.1 Routine use of cytisine is associated with a low incidence of nausea. Analysis based on 4 randomised trials comparing cytisine with placebo Nausea Subjects Placebo 16 582 2.7% RR = 1.2 Cytisine 30 733 4.1% 4 Neuronal Nicotinic Acetylcholine Receptors ▪ Neuronal nicotinic acetylcholine receptors (nACHRs) • Composed on 5 sub-units • Belong to the Cys-loop family of ligand gated ion channel receptors ▪ Cytisine and varenicline act primarily as partial agonists at a4b2 nAChRs ▪ Both drugs also have activity at other nAChRs to varying degrees 5 5-HT3 Receptors ▪ 5-HT3 receptor also belongs to the Cys-loop family of ligand gated ion channel receptors ▪ 5-HT3 receptor activation causes nausea and vomiting ▪ Drugs that block 5-HT3 receptors ameliorate nausea and vomiting - especially chemotherapy and post-operative induced - by binding to the 5-HT binding site Walstab 2010 6 5-HT3 Receptors ▪ Previous study demonstrated that varenicline is a potent and full agonist at 5-HT3 receptors and binds to the 5-HT site ▪ This study was to determine the activity of cytisine compared with varenicline at 5-HT3 receptors 7 Methods ▪ Human embryonic kidney (HEK) 293 cells were maintained on 90 mm tissue culture plates at 37°C and 7% CO2 in a humidified atmosphere ▪ Transfected with 5-HT3A receptor subunit cDNA ▪ Cells were incubated 2 to 3 days before harvesting ▪ Transfected HEK293 cell membranes were incubated for 1 h at 4°C in 0.5 mL of HEPES pH 7.4 buffer containing 3 ▪ 5-HT3 receptor antagonist [ H]-GR65630 (0.1 nM) ▪ ± Cytisine / ± Varenicline ▪ Nonspecific binding was determined using 1 mM quipazine ▪ Data were analyzed by iterative curve fitting using Prism software 8 Results – Displacement Curves % Specific Binding % Specific Varenicline log [varenicline] M 9 Results – Displacement Curves Cytisine % Specific Binding % Specific Varenicline log [compound] M 10 Results – IC50 Mean displacement of [3H]-GR65630 Human 5-HT3A Varenicline Cytisine IC50 0.25 mM 0.50 mM Ki 83 nM 170,000 nM 11 Ki (nM) Varenicline Cytisine Ratio Human 5-HT3A 83 170,000 2,048 Present study a4b2 nAChR 0.06 0.17 2.8 a3b4 nAChR 240 840 3.5 Coe et al, 2005 a7 nAChR 322 2,400 7.5 ▪ Displacement of varenicline and cytisine were in a different order of magnitude for 5-HT3 but are similar for nAChR ▪ Ki ratio cytisine:varenicline at 5-HT3 receptor is approximately 2000 ▪ Cytisine is a very weak agonist at 5-HT3 receptors 12 Head-to-head clinical trial comparing varenicline and cytisine ▪ See Natalie Walker’s presentation this afternoon ▪ Session: “Tobacco dependence and specialist groups: health inequalities and disadvantage” ▪ “Is cytisine at least as effective as varenicline for smoking cessation? Findings from a non-inferiority trial in indigenous New Zealanders and their extended family” ▪ Details of the incidence of nausea associated with varenicline and cytisine will be presented ▪ Results are consistent with the previously reported clinical trial results ▪ Results also consistent with the current in vitro binding outcomes 13 ▪ The incidence of nausea associated with varenicline is high ▪ The incidence for cytisine is much lower ▪ Cytisine’s agonist activity is 2000-fold less potent than varenicline at the human 5-HT3 receptor ▪ Differences in the incidence of nausea and vomiting for varenicline and cytisine might be explained by the differences in human 5-HT3 receptor agonist activity 14 Summary Thank you 15.