ACHE LIFE : ACHV SRNT-E Presentation

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

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(-)-Cytisine

• (-)-Cytisineis 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

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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%
				RR = 3.1
Varenicline	2,207	7,929	27.8%

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%

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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 α4β2 nAChRs
• Both drugs also have activity at other nAChRs to varying degrees

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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

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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
• • 5-HT3 receptor antagonist [3H]-GR65630 (0.1 nM)
  • ± Cytisine / ± Varenicline
• Nonspecific binding was determined using 1 μM quipazine
• Data were analyzed by iterative curve fitting using Prism software

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Results - Displacement Curves

% Specific Binding

Varenicline

log [varenicline] M

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Results - Displacement Curves

% Specific Binding

Varenicline

Cytisine

log [compound] M

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Results - IC50

Mean displacement of [3H]-GR65630

	Human 5-HT3A
	Varenicline	Cytisine
IC50	0.25 μM	0.50 mM
Ki	83 nM	170,000 nM

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		Ki (nM)
		Varenicline	Cytisine	Ratio
Human	5-HT3A	83	170,000	2,048	Present study
α4β2	nAChR	0.06	0.17	2.8
α3β4	nAChR	240	840	3.5	Coe et al, 2005
α7	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

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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

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• 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

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Summary

Thank you

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