Trapping of Nicotinic Acetylcholine Receptor Ligands Assayed by In Vitro Cellular Studies and In Vivo PET Imaging

Protein networking: nicotinic acetylcholine receptors and their protein-protein-associations

Nicotinic acetylcholine receptors (nAChR) are complex transmembrane proteins involved in neurotransmission in the nervous system and at the neuromuscular junction. nAChR disorders may lead to severe, potentially fatal pathophysiological states. To date, the receptor has been the focus of basic and applied research to provide novel therapeutic interventions. Since most studies have investigated only the nAChR itself, it is necessary to consider the receptor as part of its protein network to understand or elucidate-specific pathways. On its way through the secretory pathway, the receptor interacts with several chaperones and proteins. This review takes a closer look at these molecular interactions and focuses especially on endoplasmic reticulum biogenesis, secretory pathway sorting, Golgi maturation, plasma membrane presentation, retrograde internalization, and recycling. Additional knowledge regarding the nAChR protein network may lead to a more detailed comprehension of the fundamental pathomechanisms of diseases or may lead to the discovery of novel therapeutic drug targets.

Varenicline and Nicotine Replacement Therapy for Smokers Admitted to Hospitals: A Randomized Clinical Trial

Importance

Varenicline is the most effective sole pharmacotherapy for smoking cessation. If used in combination with nicotine replacement therapy (NRT), cessation rates may be further improved, but the efficacy and safety of the combination need to be evaluated.

Objective

To examine whether hospitalized smokers treated with varenicline and NRT lozenges achieve higher prolonged smoking abstinence rates compared with those treated with varenicline alone.

Design, Setting, and Participants

A double-blind, placebo-controlled randomized clinical trial was conducted in adult medical or surgical inpatients of 5 Australian public hospitals with a history of smoking 10 cigarettes or more per day, interested in quitting, and available for 12-month follow-up between May 1, 2019, and May 1, 2021 (final 12-month data collection in May 2022). Data analysis was performed from June 1 to August 30, 2023.

Interventions

A 12-week varenicline regimen was initiated during hospitalization at standard doses in all participants. Participants were randomized to additionally use NRT (2 mg) or placebo lozenges if there was an urge to smoke. Behavioral support (Quitline) was offered to all participants.

Main Outcomes and Measures

The primary outcome was biochemically verified sustained abstinence at 6 months. Secondary outcomes included self-reported prolonged abstinence, 7-day point prevalence abstinence (3, 6, and 12 months), and medicine-related adverse events.

Results

A total of 320 participants (mean [SD] age, 52.5 [12.1] years; 183 [57.2%] male) were randomized. The conduct of biochemical verification was affected by COVID-19 restrictions; consequently, the biochemically verified abstinence in the intervention vs control arms (18 [11.4%] vs 16 [10.1%]; odds ratio [OR], 1.14; 95% CI, 0.56-2.33) did not support the combination therapy. The secondary outcomes in the intervention vs control arms of 7-day point prevalence abstinence at 6 months (54 [34.2%] vs 37 [23.4%]; OR, 1.71; 95% CI, 1.04-2.80), prolonged abstinence at 12 months (47 [29.9%] vs 30 [19.1%]; OR, 1.77; 95% CI, 1.05-3.00), and 7-day point prevalence abstinence at 12-months (48 [30.6%] vs 31 [19.7%]; OR, 1.79; 95% CI, 1.07-2.99) significantly improved with the combination therapy. The self-reported 6-month prolonged abstinence (61 [38.6%] vs 47 [29.7%]; OR, 1.49; 95% CI, 0.93-2.39) favored the combination therapy but was not statistically significant. Medicine-related adverse events were similar in the 2 groups (102 [74.5%] in the intervention group vs 86 [68.3%] in the control group).

Conclusions and Relevance

In this randomized clinical trial of the combination of varenicline and NRT lozenges in hospitalized adult daily smokers, the combination treatment improved self-reported abstinence compared with varenicline alone, without compromising safety, but it did not improve biochemically validated abstinence.

Trial Registration

anzctr.org.au Identifier: ACTRN12618001792213.

How is Membrane Permeation of Small Ionizable Molecules Affected by Protonation Kinetics?

According to the pH-partition hypothesis, the aqueous solution adjacent to a membrane is a mixture of the ionization states of the permeating molecule at fixed Henderson-Hasselbalch concentrations, such that each state passes through the membrane in parallel with its own specific permeability. An alternative view, based on the assumption that the rate of switching ionization states is instantaneous, represents the permeation of ionizable molecules via an effective Boltzmann-weighted average potential (BWAP). Such an assumption is used in constant-pH molecular dynamics simulations. The inhomogeneous solubility-diffusion framework can be used to compute the pH-dependent membrane permeability for each of these two limiting treatments. With biased WTM-eABF molecular dynamics simulations, we computed the potential of mean force and diffusivity of each ionization state of two weakly basic small molecules: nicotine, an addictive drug, and varenicline, a therapeutic for treating nicotine addiction. At pH = 7, the BWAP effective permeability is greater than that determined by pH-partitioning by a factor of 2.5 for nicotine and 5 for varenicline. To assess the importance of ionization kinetics, we present a Smoluchowski master equation that includes explicitly the protonation and deprotonation processes coupled with the diffusive motion across the membrane. At pH = 7, the increase in permeability due to the explicit ionization kinetics is negligible for both nicotine and varenicline. This finding is reaffirmed by combined Brownian dynamics and Markov state model simulations for estimating the permeability of nicotine while allowing changes in its ionization state. We conclude that for these molecules the pH-partition hypothesis correctly captures the physics of the permeation process. The small free energy barriers for the permeation of nicotine and varenicline in their deprotonated neutral forms play a crucial role in establishing the validity of the pH-partitioning mechanism. Essentially, BWAP fails because ionization kinetics are too slow on the time scale of membrane crossing to affect the permeation of small ionizable molecules such as nicotine and varenicline. For the singly protonated state of nicotine, the computational results agree well with experimental measurements (P1 = 1.29 × 10-7 cm/s), but the agreement for neutral (P0 = 6.12 cm/s) and doubly protonated nicotine (P2 = 3.70 × 10-13 cm/s) is slightly worse, likely due to factors associated with the aqueous boundary layer (neutral form) or leaks through paracellular pathways (doubly protonated form).

Evaluation of an Image-Derived Input Function for Kinetic Modeling of Nicotinic Acetylcholine Receptor-Binding PET Ligands in Mice

Positron emission tomography (PET) radioligands that bind with high-affinity to α4β2-type nicotinic receptors (α4β2Rs) allow for in vivo investigations of the mechanisms underlying nicotine addiction and smoking cessation. Here, we investigate the use of an image-derived arterial input function and the cerebellum for kinetic analysis of radioligand binding in mice. Two radioligands were explored: 2-[18F]FA85380 (2-FA), displaying similar pKa and binding affinity to the smoking cessation drug varenicline (Chantix), and [18F]Nifene, displaying similar pKa and binding affinity to nicotine. Time-activity curves of the left ventricle of the heart displayed similar distribution across wild type mice, mice lacking the β2-subunit for ligand binding, and acute nicotine-treated mice, whereas reference tissue binding displayed high variation between groups. Binding potential estimated from a two-tissue compartment model fit of the data with the image-derived input function were higher than estimates from reference tissue-based estimations. Rate constants of radioligand dissociation were very slow for 2-FA and very fast for Nifene. We conclude that using an image-derived input function for kinetic modeling of nicotinic PET ligands provides suitable results compared to reference tissue-based methods and that the chemical properties of 2-FA and Nifene are suitable to study receptor response to nicotine addiction and smoking cessation therapies.