Evidence for reduced arterial plasma input, prolonged lung retention and reduced lung monoamine oxidase in smokers

Plasma Protein Binding and Tissue Retention Kinetics Influence the Rate and Extent of Nicotine Delivery to the Brain

Nicotine from inhaled combustible cigarette smoke is delivered rapidly to the brain, and sufficient unbound nicotine concentrations exert pharmacological effects. In addition to nicotine, combustible cigarette smoke also contains a significant number of toxicants that trigger perturbations, leading to an altered steady state due to differential expression of proteins. In this study, a physiologically based pharmacokinetic (PBPK) model for inhaled nicotine was used to simulate the influence of lysosomal change-driven tissue retention and plasma protein binding levels on nicotine pharmacokinetics (PK). A 3× increase in tissue lysosomal volumes lowered the nicotine brain maximum concentration (C_max) by 20.8%. Similarly, a 50% increase in plasma protein binding also lowered the unbound plasma arterial nicotine C_max by 39.4%. Such fundamental changes in nicotine disposition due to physiological changes in combustible cigarette smokers will lead to altered nicotine consumption and exposure-responses of other weakly basic drugs. Literature reports indicate that nicotine consumed from non-combustible products do not alter drug exposures, indicating fewer or less severe toxicant-driven perturbations with the use of these products. Although several other parameters influence nicotine PK, this PBPK modeling study shows that increased tissue retention and plasma protein binding reduce nicotine delivery to the brain and could lead to differential consumption of combustible cigarettes. These differences in physiological states among combustible cigarette smokers need to be evaluated and should be considered during therapeutic interventions. DATA AVAILABILITY: The model code is provided in supplementary information.

Positive association between the plasma levels of 5-hydroxyindoleacetic acid and the severity of depression in patients with chronic obstructive pulmonary disease

BACKGROUND

The role of plasma monoamines in patients with chronic obstructive pulmonary disease (COPD) with depression is unclear. To investigate monoamines in 20 depressed patients with COPD, the plasma concentrations of serotonin, 5-hydroxyindoleacetic acid (5-HIAA), homovanillic acid, and 3-methoxy-4-hydroxyphenylglycol (MHPG) were measured and compared with those in 50 non-depressed COPD patients, and also with 23 age- and gender-matched non-smokers and 13 smokers as non-depressed healthy controls.

METHODS

Diagnosis of depression was assessed using the Centre for Epidemiologic Studies Depression Scale. Plasma concentrations of monoamines were measured by high-performance liquid chromatography.

RESULTS

None of the depressed COPD patients had suicidal ideation. The plasma 5-HIAA level [median, (25% and 75% quartiles)] in depressed COPD patients [6.8 ng/mL, (4.9 and 13.1)] was significantly higher than in non-depressed COPD patients [5.4, (4.2 and 7.5)] (p=0.022) and non-smokers [5.1 (3.8 and 7.2)] (p=0.041), but not smokers [4.7, (4.0 and 6.7)] (p>0.05). The plasma 5-HIAA level (r=0.24, p=0.049) was significantly associated with the severity of depression in patients with COPD. The plasma MHPG level was significantly higher in depressed COPD patients (p=0.043) than in smokers, but was not higher than that in non-depressed COPD patients or non-smokers, although the level of MHPG was not associated with the severity of depression.

CONCLUSION

The plasma 5-HIAA level is increased in depressed COPD patients. Plasma monoamines may be a good biomarker for detection of depression in patients with COPD.

Kinetics of brain nicotine accumulation in dependent and nondependent smokers assessed with PET and cigarettes containing 11C-nicotine

Tobacco smoking is a chronic, relapsing disorder that constitutes one of the primary preventable causes of death in developed countries. Two of the popular hypotheses to explain the development and maintenance of strong nicotine dependence in cigarette smokers posit (i) a rapid brain nicotine accumulation during cigarette smoking and/or (ii) puff-associated spikes in brain nicotine concentration. To address these hypotheses, we investigated the dynamics of nicotine accumulation in the smoker's brain during actual cigarette smoking using PET with 3-s temporal resolution and (11)C-nicotine loaded into cigarettes. The results of the study, performed in 13 dependent smokers (DS) and 10 nondependent smokers (NDS), suggest that puff-associated spikes in the brain nicotine concentration do not occur during habitual cigarette smoking. Despite the presence of a puff-associated oscillation in the rate of nicotine accumulation, brain nicotine concentration gradually increases during cigarette smoking. The results further suggest that DS have a slower process of brain nicotine accumulation than NDS because they have slower nicotine washout from the lungs and that DS have a tendency to compensate for their slower rate of brain nicotine accumulation compared with NDS by inhaling a larger volume of smoke. For these reasons, smokers' dependence on cigarette smoking, or the resistance of NDS to becoming dependent, cannot be explained solely by a faster brain nicotine accumulation.

Translational neuroimaging: positron emission tomography studies of monoamine oxidase

Positron emission tomography (PET) using radiotracers with high molecular specificity is an important scientific tool in studies of monoamine oxidase (MAO), an important enzyme in the regulation of the neurotransmitters dopamine, norepinephrine, and serotonin as well as the dietary amine, tyramine. MAO occurs in two different subtypes, MAO A and MAO B, which have different substrate and inhibitor specificity and which are different gene products. The highly variable subtype distribution with different species makes human studies of special value. MAO A and B can be imaged in the human brain and certain peripheral organs using PET and carbon-11 (half-life 20.4 minutes) labeled mechanism-based irreversible inhibitors, clorgyline and L -deprenyl, respectively. In this article we introduce MAO and describe the development of these radiotracers and their translation from preclinical studies to the investigation of variables affecting MAO in the human brain and peripheral organs.