Reevaluating the nicotine delivery kinetics hypothesis

Differential effects of nicotine delivery rate on subjective drug effects, urges to smoke, heart rate and blood pressure in tobacco smokers

RATIONALE

The nicotine delivery rate is a key feature of tobacco product design, yet there have been limited human studies examining the effects of nicotine as a function of delivery rate.

OBJECTIVE

We developed an intravenous nicotine infusion protocol to evaluate differential effects of nicotine delivery rate on subjective drug effects, smoking urges, abstinence symptoms, heart rate, and blood pressure.

METHODS

Eighteen non-treatment seeking, overnight abstinent male and female smokers (18 to 30 years old), who smoked ≥ 5 cigarettes per day for the past year completed four sessions, in which they were randomly assigned to a saline infusion, or a 1 mg per 70-kg body weight dose of nicotine delivered over 1, 5, or 10 min at rates of 0.24, 0.048, or 0.024 μg/kg/s, respectively.

RESULTS

Smoking urges, as assessed by the Brief Questionnaire of Smoking Urges, were reduced relative to placebo for the 1- and 5-min infusion, but not the 10-min infusion. Although the 1- and 5-min infusions reduced smoking urges to a similar extent, the 1-min infusion induced a greater heart rate and blood pressure increase. Changes to subjective drug effects, heart rate, and blood pressure delineate the differential effects of nicotine delivery rate for these outcomes.

CONCLUSIONS

We have characterized the delivery rate-response curve for a nicotine dose that is roughly the amount of nicotine (~ 1 mg) delivered by smoking a standard tobacco cigarette. Our findings reinforce the importance of nicotine delivery rate when evaluating the potential effects of nicotine from tobacco products.

How Intravenous Nicotine Administration in Smokers Can Inform Tobacco Regulatory Science

Reducing the negative health effects caused by tobacco products continues to be a public health priority. The Family Smoking Prevention and Tobacco Control Act of 2009 gives the Food Drug Administration authority to pursue several new strategies, including regulating levels of nicotine and other ingredients in tobacco products. A nicotine reduction strategy proposed by Benowitz and Henningfield aims to reduce the nicotine content of tobacco products to an amount below a threshold that supports neither the development nor maintenance of addiction. Many factors must be considered to determine the viability and efficacy of this approach. For example, the policy should be based on precise information on the dose-dependent effects of nicotine on reinforcement and factors that contribute to individual differences in these effects. However, there have been few studies on these topics in humans. Here, we briefly review nicotine pharmacology and reinforcement then present several studies illustrating the application of intravenous (IV) nicotine delivery to study nicotine reinforcement in humans. We discuss how nicotine delivery by IV infusion may be uniquely suited for studying nicotine's dose-dependent effects, and how this can inform tobacco regulatory science to facilitate the development of effective tobacco control policies.

Nicotine reinforcement in never-smokers

RATIONALE

Global tobacco-related mortality dwarfs that of all other drugs. Nicotine is believed to be the primary agent responsible for tobacco use and addiction. However, nicotine is a relatively weak and inconsistent reinforcer in nonhumans and nicotine reinforcement has not been demonstrated in never-smokers.

OBJECTIVES

This study investigated the discriminative, subjective, and reinforcing effects of nicotine in never-smokers.

METHODS

Eighteen never-smokers (< 50 lifetime nicotine exposures) participated in a double-blind study. During a drug discrimination phase, volunteers ingested oral nicotine and placebo capsules (quasi-random order) at least 2 h apart and rated subjective effects repeatedly for 2 h after ingestion in daily sessions. Blocks of 10 sessions were continued until significant discrimination was achieved (p ≤ 0.05, binomial test; ≥ 8 of 10). Following discrimination, nicotine choice was tested by having volunteers choose which capsule set to ingest on each daily session. Successive blocks of 10 sessions were conducted until choice for nicotine or placebo met significance within each volunteer (≥ 8 of 10 sessions).

RESULTS

All 18 volunteers significantly discriminated nicotine from placebo; the lowest dose discriminated ranged from 1.0 to 4.0 mg/70 kg. Nine volunteers significantly chose nicotine (choosers) and nine significantly chose placebo (nicotine avoiders). The choosers reported predominately positive nicotine subjective effects (e.g., alert/attentive, good effects, liking), while avoiders tended to report negative effects (e.g., dizzy, upset stomach, disliking). Both choosers and avoiders attributed their choice to the qualitative nature of drug effects.

CONCLUSIONS

These results provide the first evidence that nicotine can function as a reinforcer in some never-smokers.

Hooked on the nicotine addiction thesis: a response to DiFranza

DiFranza’s rebuttal to our critique of the “Hooked on Nicotine” research program misconstrues our arguments beyond recognition. The grossest misrepresentation of our critique by DiFranza is that we devise (by thwarting science) to rescue “the conventional wisdom” of the “threshold model of nicotine addiction.” In fact, the difference between our positions lies elsewhere: We believe that nicotine is not an addictive drug and that its contribution to the smoking habit is secondary; DiFranza believes that nicotine is so powerfully addictive that novice smokers can lose autonomy over their smoking behavior after one cigarette or even following a single puff. Our review aimed to critically examine the empirical basis of this extreme version of the nicotine “addiction” model. In this brief commentary we illustrate how the commitment to the nicotine “addiction” theory has biased the methodology and the interpretation of the data in “Hooked on Nicotine” research program.

A systematic review of nicotine by inhalation: is there a role for the inhaled route?

INTRODUCTION

A considerable minority of adults remain addicted to smoking cigarettes despite substantial education and public health efforts. Nicotine replacement therapies have only modest long-term quit rates. The pulmonary route of nicotine delivery has advantages over other routes. However, there are regulatory and technical barriers to the development of pulmonary nicotine delivery devices, and hence, none are commercially available. Current knowledge about pulmonary nicotine delivery is scattered throughout the literature and other sources such as patent applications. This review draws together what is currently known about pulmonary nicotine delivery and identifies potential ways that deep lung delivery can be achieved with a simple portable device.

AIMS

To systematically review clinical trials of nicotine inhalers, determine whether they delivered nicotine via the lung, and identify ways that pulmonary delivery of medicinal nicotine might be achieved and the technical issues involved.

METHODS

Systematic search of Medline and Embase.

RESULTS

Thirty-eight trials met the inclusion criteria. Cough, reflex interruption of smooth inspiration, and throat scratch limited the usefulness of nicotine inhalers. The pharmacokinetic profiles of portable nicotine inhalers were inferior to smoking, but among commercially available products, electronic cigarettes are currently the most promising.

CONCLUSIONS

Pulmonary nicotine delivery might be maximized by use of nicotine salts, which have a more physiological pH than pure nicotine, by ensuring the mass of the particles is optimal for alveolar absorption, and by adding flavoring agents. Metered-dose inhalers potentially can deliver nicotine more efficiently than other nicotine products, facilitating smoking cessation and improving smokers' lives.

Genetic variation in CYP2A6 predicts neural reactivity to smoking cues as measured using fMRI

Smoking cues trigger craving for cigarettes and relapse. Nicotine metabolism, mediated by the enzyme CYP2A6, also influences smoking behavior. In this study, we investigated how nicotine metabolism and genetic variation in CYP2A6 influence the neural response to smoking cues in humans using functional magnetic resonance imaging (fMRI). We hypothesized that individuals with faster rates of nicotine metabolism would have stronger conditioned responses to smoking cues because of closer coupling in everyday life between exposure to cigarettes and surges in blood nicotine concentration. In contrast, individuals with reduced rates of metabolism, who have relatively constant nicotine blood levels throughout the day, should be less likely to develop conditioned responses to cues. We screened 169 smokers for their rate of nicotine metabolism and CYP2A6 genotype, and selected 31 smokers with the fastest and slowest rates for fMRI, matched for daily cigarette intake. We measured their neural response to visual smoking and non-smoking cues using fMRI. As predicted, fast metabolizers, by phenotype or genotype, had significantly greater responses to visual cigarette cues than slow metabolizers in the amygdala, hippocampus, striatum, insula, and cingulate cortex. These results support the theory that drug cues are conditioned stimuli, and explain why fast metabolizers who smoke have lower cessation rates. They also provide insight into how genetics can shape human vulnerability to addiction, and have implications for tailoring smoking cessation programs based on individual genetics.

Thwarting science by protecting the received wisdom on tobacco addiction from the scientific method

In their commentary, Dar and Frenk call into question the validity of all published data that describe the onset of nicotine addiction. They argue that the data that describe the early onset of nicotine addiction is so different from the conventional wisdom that it is irrelevant. In this rebuttal, the author argues that the conventional wisdom cannot withstand an application of the scientific method that requires that theories be tested and discarded when they are contradicted by data. The author examines the origins of the threshold theory that has represented the conventional wisdom concerning the onset of nicotine addiction for 4 decades. The major tenets of the threshold theory are presented as hypotheses followed by an examination of the relevant literature. Every tenet of the threshold theory is contradicted by all available relevant data and yet it remains the conventional wisdom. The author provides an evidence-based account of the natural history of nicotine addiction, including its onset and development as revealed by case histories, focus groups, and surveys involving tens of thousands of smokers. These peer-reviewed and replicated studies are the work of independent researchers from around the world using a variety of measures, and they provide a consistent and coherent clinical picture. The author argues that the scientific method demands that the fanciful conventional wisdom be discarded and replaced with the evidence-based description of nicotine addiction that is backed by data. The author charges that in their attempt to defend the conventional wisdom in the face of overwhelming data to the contrary, Dar and Frenk attempt to destroy the credibility of all who have produced these data. Dar and Frenk accuse other researchers of committing methodological errors and showing bias in the analysis of data when in fact Dar and Frenk commit several errors and reveal their bias by using a few outlying data points to misrepresent an entire body of research, and by grossly and consistently mischaracterizing the claims of those whose research they attack.

The effects of nicotine, denicotinized tobacco, and nicotine-containing tobacco on cigarette craving, withdrawal, and self-administration in male and female smokers

The effects of the acute administration of nicotine [through nicotine inhalers (NI) and placebo inhalers (PI)], nicotine-containing tobacco (NT), and denicotinized tobacco (DT), on smokers' subjective responses and motivation to smoke were examined in 22 smokers (12 male, 10 female; 11 low dependent, 11 high dependent). During four randomized blinded sessions, participants self-administered NI, PI, NT, or DT, and assessed their effects using Visual Analogue Scales and the Brief Questionnaire of Smoking Urges. They could then self-administer their preferred brand of cigarettes using a progressive ratio task. NT and DT were each associated with increased satisfaction and relaxation as well as decreased craving relative to the inhalers and NT increased ratings of stimulation relative to each of the other products. Both NT and DT delayed the onset of preferred tobacco self-administration relative to NI and PI but only NT reduced the total amount self-administered. Sex differences were evident in the effects of DT on withdrawal-related cravings with women experiencing greater DT-induced craving relief than men. Findings suggest that DT is effective in acutely reducing many smoking abstinence symptoms, especially in women, but a combination of nicotine and non-nicotine tobacco ingredients may be necessary to suppress smoking behavior.

Rats self-administer intravenous nicotine delivered in a novel smoking-relevant procedure: effects of dopamine antagonists

Attempts to explain tobacco addiction have relied heavily on the assumption that each cigarette puff delivers a bolus of nicotine to the brain within seconds. However, nicotine transits from lungs to brain much more gradually than once thought. Nevertheless, animal self-administration studies continue to use rapid (e.g., <3-s) infusions, as well as high unit doses of nicotine (e.g., 15-30 microg/kg/infusion), each equivalent to one to two cigarettes. Here, we report that nicotine is self-administered across a range of infusion durations (3, 30, 60, and 120 s) in rats. Slow (30-s) infusions were preferred over fast (nominal 3-s) infusions and were self-administered across several reinforcement schedules, at doses as low as 3 microg/kg/infusion, equivalent to one to two puffs. A conventional "fast/high" self-administration procedure (3 s-30 microg/kg/infusion) was then compared with our new "slow/low" procedure (30 s-3 microg/kg/infusion) in rats trained on a progressive ratio schedule and acutely challenged with dopamine receptor antagonists. The D(1) antagonist R-(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine (SCH 23390) (6-25 microg/kg s.c.) reduced intake in both procedures and in rats self-administering cocaine (0.5 mg/kg/infusion). The D(2) antagonists spiperone (3-30 microg/kg s.c.) and sulpiride (5-20 mg/kg i.p.) increased intake of fast/high nicotine and cocaine, but markedly reduced intake of slow/low nicotine. In a final test, in which only infusion speed was varied, an acute spiperone challenge produced the same differential effect on nicotine self-administration. In conclusion, our new slow/low nicotine self-administration procedure, designed to better mimic smoking-associated nicotine intake, is pharmacologically distinct from the conventional fast delivery/high-dose procedure.