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. Author manuscript; available in PMC: 2018 May 1.
Published in final edited form as: J Neurosci Methods. 2017 Mar 25;283:55–61. doi: 10.1016/j.jneumeth.2017.03.016

A Programmable Smoke Delivery Device for PET Imaging with Cigarettes Containing 11C-nicotine

Yantao Zuo a, Pradeep K Garg b,§, Rachid Nazih b,§, Sudha Garg b,§, Jed E Rose a, Thangaraju Murugesan a, Alexey G Mukhin a,*
PMCID: PMC5484172  NIHMSID: NIHMS864971  PMID: 28347784

Abstract

Introduction

PET imaging with 11C-nicotine-loaded cigarettes is a valuable tool to directly assess fast nicotine kinetics and its neuropharmacological role in tobacco dependence. To eliminate variations among puffs inhaled by subjects, this work aimed to develop a programmable smoke delivery device (SDD) to produce highly reproducible and adjustable puffs of cigarette smoke for PET experiments.

New method

The SDD was built around a programmable syringe pump as a smoking machine to draw a puff of smoke from a 11C-nicotine-loaded cigarette and make it available for a subject to take the smoke into the mouth and then inhale it during PET data acquisition. Brain nicotine time activity curves and total body absorbed 11C-nicotine doses (TAD) were measured in smokers who inhaled a single puff of smoke via the SDD from a 11C-nicotine-loaded cigarette.

Results

Nearly identical brain nicotine kinetics were observed between participants who inhaled a puff of smoke through the SDD and those who inhaled directly from a cigarette.

Comparison with existing methods

This new device minimizes puff variations that exist with earlier smoke delivery apparatuses which could introduce confounding factors.

Conclusions

The SDD is effective in delivering 11C-nicotine from the study cigarettes. Despite a 2-sec increase in aging of smoke delivered through the SDD versus smoke taken directly from a cigarette, the difference in brain nicotine kinetics after 11C-nicotine delivery with and without use of the SDD is negligible. This refined device may be useful for future research on the deposition and pharmacokinetics of nicotine inhaled with tobacco smoke.

Keywords: Brain nicotine kinetics, smoking, PET, 11C-nicotine, smoke delivery device

1. Introduction

Recent studies have demonstrated that positron emission tomography (PET) with 11C-nicotine-loaded cigarettes is a valuable in vivo imaging tool to directly assess the role of fast brain nicotine accumulation in tobacco dependence. Nicotine intake is detectable in the brain within seconds after a smoker inhales a puff of cigarette smoke (Berridge et al., 2010; Rose et al., 2010). The rapid kinetics of brain nicotine and its immediate subjective effects permit smokers to titrate the level of nicotine intake by adjusting puff-to-puff smoking behavior, which contributes to highly reinforcing and addictive effects of cigarette smoking (Benowitz, 1990; Henningfield and Keegan, 1993). The PET imaging technique may also be helpful in elucidating the role of a number of factors which may contribute to nicotine addiction through their impact on nicotine deposition and absorption in the respiratory tract and ultimately on brain nicotine kinetics. Examples of such factors include sex differences (Zuo et al., 2015), smoking history associated changes in nicotine washout from the lungs (Rose et al., 2010), the impact of various neuropsychiatric conditions on smoking topography (Dome et al., 2010), and anti-nicotine vaccination (Keyler et al., 2005; Maurer et al., 2005; Pentel et al., 2000). In addition, in view of recent findings on the effects of nicotine in the promotion of tumor growth and lung cancer proliferation (Davis et al., 2009; Brown et al., 2013), the technique may also be used to characterize the patterns of pulmonary nicotine deposition and intake and their possible associations with the development of different types of lung cancer. Taken together, PET imaging with 11C-nicotine-loaded cigarettes may have a number of important applications in research areas including nicotine addiction and respiratory pathophysiology. The essential part of implementation of this PET technique is a smoking apparatus for administration of smoke from a cigarette containing 11C-nicotine.

In one of the earlier studies (Berridge et al., 2010), subjects were instructed to take a puff via a tube connected to a holder where a shortened 11C-nicotine-containing cigarette was inserted and then to inhale in their normal fashion. Therefore, this device did not allow any control of variations in nicotine kinetics attributable to varying puffing and inhalational parameters between study sessions and individual subjects. Given that puffing (e.g., puff flow rates and volume) and likely inhalational parameters (e.g., volume and duration) affect nicotine retention and smoke exposure in the respiratory system (Baker and Dixon, 2006; Kane et al., 2010; Zacny et al., 1987), the lack of control of puffing and inhalational parameters between study sessions and individual subjects could confound the effects of variables of interest. In another study (Rose et al., 2010), the participants took a puff directly from a 11C-nicotine-containing cigarette, using a controlled puff apparatus (CPA) which limited the puff volume. However, this apparatus did not allow a control of puff flow-rate, which could introduce variations in particle size distribution of mainstream smoke and consequently the deposition efficiency of smoke particles in the respiratory tract. Previous research has found that greater flow rates, which represent shorter residence time and less coagulation of smoke particles inside a cigarette, are associated with smaller median particle sizes which in turn are predictive of reduced total deposition of smoke particulate matter (Kane et al., 2010). Thus, greater flow rates likely result in slower brain nicotine kinetics. Although the extent of this impact of puff flow rates on nicotine kinetics remains unclear, it is desirable to eliminate this likely source of variance. Inasmuch as puff volume is often correlated with flow rate, puff volume is likely to have an impact on nicotine kinetics as well. Additionally, because nicotine delivery is correlated with puff volume (e.g., Zacny et al., 1987), the control of puff volume would permit an adequate but not excessive amount of 11C-nicotine to be delivered for achieving a good signal-to-noise ratio for PET imaging, while keeping the radiation exposure to the subject at a minimum level. Taken together, these considerations suggest that there is a need to design a refined smoke delivery apparatus.

In this report we described the development of a new smoke delivery device (SDD) for PET imaging research with 11C-nicotine-loaded cigarettes. The device was built around a programmable syringe pump which served as a smoking machine to produce highly reproducible and adjustable puffs of smoke from the cigarettes. Immediately after a puff was produced and taken into the mouth, the inhalation parameters could be monitored in real-time through a laptop computer. Optional features of the device included sensors that permit recording the time when each event (e.g., puffing or inhaling) happened during the smoke delivery process. Besides the details of the design and construction of the device, preliminary PET data were also presented with respect to the delivery efficiency and brain kinetics of 11C-nicotine assessed in human subjects using the SDD.

2. Materials and methods

2.1 Design and operation of the SDD

A schematic diagram illustrating the design of the SDD and two pictures of the actual device set-up are shown in Figure 1. For durability, easy operation and transportation, the SDD was assembled and fixed to a plastic board. To control puff volume and puff duration or flow rate, the new SDD was built around a smoking-machine approach. Specifically, as illustrated in Figure 1, a syringe (a) and a New Era NE-1060 pump (syringepump.com), electronically connected with a 3-way solenoid valve (b), were used to draw a controlled puff from a research cigarette into a piece of Teflon tubing (f; I.D. 6.4 mm, O.D. 7.9 mm, 1.25 m length). Immediately after that, the smoke was withdrawn by a subject through a mouthpiece assembly (d; e). By pulling and pushing the syringe of 140 mL capacity, the programmable pump could draw a single or multiple puffs of a specified volume at a selected rate. The pull-push air flow in and out of the syringe was controlled through the solenoid valve (b) whose operation was electronically coupled with the pump in such a way that prior to the onset of the pump's withdrawal movement, the valve would connect the syringe to a piece of Tygon tubing (c; I.D. 3.2 mm, O.D. 6.4 mm) connected to the mouthpiece valve (d). Thus, when the pump pulled (Fig. 1A) the plunger of the syringe (a), air would flow through the mouthpiece valve (d) and the air tubing (c), and the smoke would be drawn from a lit cigarette (j) into the Teflon tubing (f). The left panel of Figure 1 (1A) illustrates the air flow and the positions of various valves during the process in which one puff of smoke is produced. Upon the completion of the pump withdrawal, the solenoid valve (b) switched and connected the syringe (a) to the port where the air could exit to a smoke exhaust pipe when the pump pushed back to the starting position. With a repetition of this sequence, the next puff of cigarette smoke could be generated.

Figure 1.

Figure 1

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A schematic diagram of the smoke delivery device (SDD) and pictures of the actual device setup. Panel (A) illustrates the valve status and air or smoke flow during the process of generating a puff of smoke; panel (B) illustrates the process of a subject taking the puff generated using the SDD; panels C and D show the actual SDD setup for a PET scanning session. In panels A and B, arrows indicate the air or smoke flow direction.

In all panels: a - New Era pump and syringe; b- 3-way solenoid valve; c - Tygon tubing; d - mouthpiece valve; e - mouthpiece tubing; f - Teflon tubing; g - push-pull valve for control of puff generation and puff-taking; h - push-pull valve for control of inhalation; i - airflow sensor of the CReSS; j - cigarette; k - airflow resister; m - the combustion canister; n - cigarette lighter; o - air pump; p - Cambridge filter; q - bowl-shaped plastic hood; r - vacuum hose.

The mouthpiece assembly consisted of a custom-made push-pull valve made primarily of Teflon materials (d) and a disposable Teflon mouthpiece tube (e). A control module, which included two push-pull valves (g and h), was built into the SDD to control smoke and air flow during the process of the subject's puffing and inhalation of cigarette smoke. The push-pull valve (g), while in its “up” position, allowed a puff of smoke to be drawn from the lit cigarette into the Teflon tubing (Fig. 1A). Upon completion of the pump's drawing of the puff, pressing the push-pull valve (g) would block additional smoke from entering the Tygon tubing (f). At this stage, the Tygon tubing (f) would be connected to the push-pull valve (h), which was open at its “up” position to the ambient air through an air flow resister (k). Next, upon verbal instruction of “puff,” the subject would press the push button on the mouthpiece valve (d) and take a puff from the mouthpiece tubing (e), holding it in his/her mouth. Two seconds after the “puff instruction, the technician pressed the second push-pull valve (h) which would open this valve directly to the ambient air (Fig. 1B), and gave the subject the “inhale” instruction. While the subject inhaled air through the mouth tubing (e) and Tygon tubing (f), the technician monitored the inhalational air flow as shown in a real-time plot via the CReSS interface (Borgwaldt KC, Richmond, VA) running on a laptop computer (Figure 1C). This was made possible by an airflow sensor (i) of the CReSS installed between the two push-pull valves (g and h). These two valves were reset to “up” position prior to the delivery of the next puff of smoke. The air flow resister (k) connected with the push-pull valve (h) was made of a Teflon rod of 22 mm length with a 2 mm diameter hole, providing an air flow resistance similar to that of a cigarette.

Cigarette combustion took place in a clear acrylic canister (11.8 cm I.D., 22.5 cm height) with a lid (see m in Fig. 2). An ashtray was placed at the bottom of the canister to collect cigarette ashes. Three holes were made on the canister wall. One hole would allow a research cigarette, inserted into a cigarette holder, to connect with the push-pull valve (g) of the aforementioned two-valve control module. Through a second hole on the other side of the cigarette entrance, a Coleman type cigarette lighter (n) could be inserted to light the cigarette. The hole also served as an entrance for ambient air which is necessary for cigarette combustion. To prevent cigarette smoke from leaking into the room and to minimize smoke accumulation in the combustion canister, slight negative pressure was maintained inside the canister. This was done through a piece of Tygon air tubing which connected the canister to the inlet of a Top Fin air pump (o; Model: Air-8000) via the third hole positioned close to the top of the canister. A Cambridge filter (p) was placed between the combustion canister (m) and the air pump (o) to trap smoke particles and prevent them from entering the air pump. The airflow from the pump was directed to the smoke exhaust pipe (see Section 2.2 below) via Tygon tubing.

Figure 2.

Figure 2

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Whole body 11C-nicotine retention. (A), a representative image of the radioactivity distribution at 15 minutes after inhalation of a single puff of smoke generated via the smoke delivery device (SDD) from cigarette loaded with 11C-nicotine. The SUV image was superimposed on a CT image from the same subject. SUV - Standard Uptake Value = (radioactivity/g tissue)/(radioactivity/g body weight). (B), efficiency of 11C-nicotine delivery through the SDD (n = 19). TAD0 – total absorbed dose. (C), the fraction of TAD detected in the body fragment from the top of the head to the knee (HTKD; n = 19). The dotted-line boxes in (B) and (C) show ± SD.

As shown in Fig. 1B, the combustion canister (m), the two-valve control module (g, h) and the attachable cigarette holder were lined with lead bricks to minimize radiation exposure to the study subject and staff as well as to reduce the radiation shine to the PET scanner from 11C-nicotine present in this chamber.

Following the use of the SDD for one or more experimental sessions in a day, the tubing and valve modules of the SDD were easily cleaned with ethanol, rinsed with water and dried. The mouth piece tubing was replaced prior to each PET session for sanitary reasons.

2.2 Smoke exhaustion module

To prevent any smoke that was not absorbed by the subject from contaminating the PET scanner or escaping to the room, a smoke exhaustion module was constructed. This module consisted of a hood (q), a hose (r) and a shop-vac machine. The vacuum hose (r) was attached to the large bowl-shaped plastic hood (q) placed over the subject's face when the subject was lying in the gantry of the scanner (Fig. 1 C). When the vacuum was on, this setup could draw in the residue smoke exhaled by the subject after taking a puff of smoke from the SDD. As mentioned above, the air flowing out of the Top Fin air pump and from the solenoid valve of the syringe pump was also collected through the hose of the exhaust module. The vacuum machine was placed in a room adjacent to the scanning room to minimize noise interference. The air flow from the vacuum machine was directed to a radioisotope fume hood. Using a remote control, the SDD operator could turn on the vacuum machine prior to the lighting of a research cigarette and subsequently turn it off after the subject completed puffing.

2.3 Accessory features of the SDD

Additional features were added to the main modules of the SDD to further enhance the utility of this device for certain other research applications. As mentioned in Section 2.1, the installation of an airflow sensor between the two valves of the puffing/inhalation control module allowed the CReSS smoking topography system to be integrated into the SDD, which would allow the monitoring of the subject's inhalational volume and other parameters in real time as the subject inhaled a puff of smoke until a certain volume is reached. A prefilled airbag was also attached to the port, otherwise open to the ambient air, of the push-pull valve (h) of the control module so that a fixed volume of air could be supplied for the subject to inhale after taking a puff of smoke (Zuo et al., 2015). To record the time of puffing, a motion sensor was affixed to the control module so that whenever the technician pressed the push-pull valve (g) while instructing the subject to take a puff, the time of the pressing could be captured through the sensor which was connected to a laptop computer running an Excel macro that was programmed to timestamp the event.

2.4 Assessment of nicotine deposition in the Teflon tubing of the SDD

Prior to the use of the SDD for PET with human subjects, we measured the amount of nicotine deposition in the tubing between the cigarette holder and the mouthpiece in six experiments. During each experiment, a 35 mL puff of smoke was produced from a research cigarette without 11C-nicotine at 2 sec duration using the SDD and then drawn through a Cambridge filter. The residual smoke in SDD's Teflon tubing was flushed through the same filter with 500 mL air. Nicotine deposited in the SDD's Teflon tubing and on the Cambridge filter was extracted in 10 mL of 0.1 N sodium hydroxide solution. Nicotine concentration was assessed using a modification of a previously described approach (Jacob et al. 1981). Briefly, 1 mL of the extract was mixed with 0.1 mL internal standard (5-methyl nicotine at 100 ng/mL final concentration) and 0.5 mL of toluene and butanol mixture (90:10). The final mixture was vortexed, centrifuged and the aqueous layer was frozen to obtain clear solution of the organic layer. About 0.1 mL of the organic layer was transferred into a GC vial and quantified using Gas Chromatography (Agilent GC-HP6890 series with Nitrogen Phosphorus Detector).

2.5 Delivery of 11C-nicotine-containing smoke through the SDD

2.5.1 Research cigarettes

Capri Magenta 120s slim cigarettes (R.J. Reynolds, Winston Salem, NC, USA; FTC yield of nicotine 1.1 mg, tar 12 mg, and CO 9 mg, according to the most recent published data) were chosen for these experiments. Due to their slim design and medium nicotine yield, these cigarettes delivered 11C-nicotine with relatively high efficiency without producing high nicotine concentration-related harsh sensory stimulation for most subjects. Both the length of the cigarette filter and tobacco rod were also shortened (to 5 mm and 10 mm, respectively) to further enhance the efficiency of radioactivity delivery while minimizing radiation exposure.

2.5.2 PET assessment of 11C-nicotine delivery through the SDD

11C-nicotine in the cigarette smoke delivered through the SDD was evaluated through PET imaging in 19 subjects. The participants comprised of 7 female and 12 male smokers, mean age 42.7 (± 9, SD) years; 12 (63.2%) of them were Caucasian and 7 (36.8%) were African-American. Their average reported number of cigarettes smoked per day (CPD) was 21 ± 4 at screening. The subjects were recruited from local communities through newspaper and television advertisements. Inclusion criteria consisted of being 18-65 years old, generally healthy, smoking 10 or more cigarettes per day and with afternoon expired CO 10 ppm or higher (to confirm smoke inhalation). Exclusion criteria included lung and cardiovascular diseases, brain abnormality or neurological disorders, diagnosis of psychiatric conditions, addiction to drugs other than nicotine, pregnancy and lactation (for females), and contraindicated conditions for PET scan (e.g., back pain or claustrophobia). Participants provided written consent prior to participation in the study. The study protocol was approved by the Institutional Review Boards of the Duke University Health System and Wake Forest University School of Medicine.

Each participant went through a PET scanning session during which the head of the participant was scanned after he or she inhaled a single puff of smoke of a Capri cigarette containing 11C-nicotine using the SDD. The 30 mL puff of smoke was generated by the SDD over 2 seconds. Prior to the PET session, participants practiced taking puffs from a cigarette without 11C-nicotine until they were comfortable with taking puffs through the SDD while in a supine position. The PET scans were conducted using a GE Discovery VCT PET/CT scanner (GE Healthcare, Waukesha, WI, USA). After the participant was positioned in the scanner, the SDD was switched to operation mode and the subject was instructed to take a 30 mL puff of smoke from the cigarette containing 11C-nicotine following the procedure described earlier (Section 2.1 and 2.2). The scanning was initiated 2-3 s before the participant took the puff and inhaled it with 500 – 700 mL air. The head of each participant was scanned for just over 12 min in a sequence of twenty-five 1-s, thirty 2-s, one hundred and twenty 3-s, and seventy 4-s frames (field of view: 24.6 × 24.6 × 15.4 cm3, matrix size: 128 × 128 × 47). After the dynamic head scanning (i.e., 15 min after inhalation), a full-body scan was conducted to measure total absorbed dose of radioactivity (TAD). The full-body scanning (2 min per bed position) included a ten bed position scan from the head to the knee and a five bed position scan from the knee to the big toe.

11C-nicotine was synthesized following an established protocol (Halldin et al., 1992). Approximately 740 MBq 11C-nicotine dissolved in 10 μl ethanol was applied to the tip of the tobacco rod of the shortened Capri cigarette and the ethanol was subsequently evaporated with a stream of air. After the radioactivity in the cigarette decayed to below 600 MBq, the cigarette was placed in the combustion canister of the SDD and lit for smoke delivery at 2 min after the last dose assessment of cigarette radioactivity. Mean radioactivity in the cigarettes at the time of puff delivery across the 19 scan sessions was 507 ± 73 (SD) MBq. PET image processing was conducted using PMOD (Version 3.17, PMOD Technologies, Adliswil, Switzerland). Individual whole-brain volume of interest (VOI) was drawn on the time-averaged images from the dynamic head scanning and then applied to images of each time frame. Two cylinder-shaped VOIs were placed to cover the entire body image of each subject, one for the segment from the head to the knee and the other from the knee to the big toe. After decay correction to the brain scan start time, the total radioactivity within these two VOIs was taken as TAD. Whole-brain 11C-nicotine radioactivity over time was calculated as a percentage of TAD per kg of brain tissue. For the estimation of Cmax and T1/2 of brain nicotine accumulation, the individual brain time activity curves were subjected to three-exponential curve fitting (Zuo et al., 2015). The group mean time activity curve of brain nicotine accumulation was then compared to that measured following smokers' inhalation of a single puff of smoke directly from a research cigarette containing 11C-nicotine (Rose et al., 2010). The two subject samples were comparable in demographical and baseline smoking characteristics (Table 1).

Table 1. Description of subjects inhaling smoke via the SDD or directly from cigarettes containing 11C-nicotine.
Inhaling via SDD (n = 19) Inhaling Directly (n = 13)
Male gender n (%) 12 (63.2%) 9 (69.2%)
Race
 Caucasian n (%) 12 (63.2%) 9 (69.2%)
 African-American n (%) 7 (36.8%) 4 (30.8%)
Age 42.7 (9.4) 40.9 (8.5)
CPD 21.5 (4.2) 23.6 (4.7)
Years of Smoking 22.7 (9.6) 22.7 (9.1)
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Note. Mean (± SD) is reported for continuous variables. CPD, cigarettes smoked per day as reported at screening. No significant differences were found between these two groups in any of the variables (all ps > .10; χ2 tests gender and ethnicity, t-tests for age, CPD, and years of smoking).

3. Results

3.1 Body distribution of radioactivity and efficiency of 11C-nicotine delivery through the SDD

Figure 2A shows a representative PET image of body distribution of 11C-nicotine and potentially its radioactive metabolites at 15 min after a subject inhaled a puff of smoke via the SDD. The radioactivity was widely spread in the body. Relatively high concentrations of 11C-radioactivity were observed in the respiratory tract, including the oral cavity, larynx, trachea, bronchi and lungs, and other areas such as the stomach, urinary bladder, kidney, liver, and brain.

The amount of 11C-nicotine retained in the subjects' body, i.e., the total absorbed dose (TAD), averaged 115 ± 28 MBq. Efficiency of 11C-nicotine delivery through the SDD was calculated for each scan session by dividing decay-corrected TAD as measured with whole-body scan by the dose of radioactivity applied to the cigarette from which the subject inhaled a puff of smoke delivered via the SDD. The efficiency (Fig. 2B) ranged from 16.0% to 30.1% (mean ± SD, 22.8% ± 4.2%) across the 19 studied subjects. It should be noted that radioactivity observed in the body segment from the top of the head to the knee (HTKD) accounted for 93.5% to 96.8% (mean ± SD, 95.6% ± 0.9%) of the total absorbed dose (Fig 2C).

Prior to the use of the SDD for PET with human subjects, we measured the amount of nicotine deposition in the tubing between the cigarette holder and the mouthpiece during the generation of a single puff of cigarette smoke in six experiments. The mean (± SD) fraction of nicotine deposited in the tubing was 5.9% (± 1.4%) of the total nicotine in the puff of smoke, i.e., the sum of nicotine deposited in the tubing and on the Cambridge filter.

3.2 Brain nicotine accumulation following inhalation of 11C-nicotine-containing smoke delivered through the SDD

The average time activity curve (TAC, mean ± SD) of the total brain nicotine accumulation after inhalation of a single puff of 11C-nicotine-containing smoke delivered through the SDD is shown in Figure 3. On average the radioactivity steadily increased over the first 6 minutes after inhalation and reached the maximal value (Cmax). The average Cmax values (± SD) obtained from individual TACs measured in the 19 participants were 5.0% (± 0.7%) of TAD/kg brain tissue. The average T1/2 value (time of half maximal accumulation) was 33 (± 12) sec.

Figure 3.

Figure 3

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Mean (± SD) brain time activity curve after smokers (n = 19) inhaled a single puff of smoke via the smoke delivery device (SDD).

To evaluate the potential impact of the use of SDD on the kinetics of brain nicotine accumulation, we compared mean brain time activity curves after smokers inhaled a single puff of smoke via the SDD (Fig. 3; n = 19) with that when smoke was directly inhaled from a cigarette containing 11C-nicotine (n = 13). The latter data were excerpted from an earlier study by us (Rose et al., 2010). These two subject samples were comparable in demographical and baseline smoking characteristics (Table 1). As shown in Figure 4A, the mean time activity curve of brain nicotine concentration measured during PET scans after subjects' inhalation of a single puff of smoke through the present SDD was nearly identical to that measured in the previous PET imaging study where dependent smokers inhaled a puff directly from the 11C-nicotine-containing cigarettes. No statistically significant differences in T1/2 and Cmax values of brain nicotine accumulation were found between the two groups (p =0.998 and p =0.590, respectively; two-sample t-tests; Fig. 4B and 4C). It should be noted that in our previous study (without SDD) the brain nicotine concentration was measured as a fraction of the total inhaled dose (%ID), but in the current study (with SDD) it was measured as a fraction of the total absorbed dose (%TAD). Since the systemic absorption of nicotine is about 95% of the inhaled dose after cigarette smoke inhalation (Armitage et al., 2004), it is likely that the minor increase in brain nicotine Cmax values observed in the present study compared to the previous merely reflects the fact that the former values were expressed relative to the slightly lower estimates of total body radioactivity dose (i.e., TAD < ID).

Figure 4.

Figure 4

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Brain nicotine kinetics after smokers inhaled a single puff of smoke via the smoke delivery device (SDD; n = 19) or directly from a cigarette containing 11C-nicotine (n = 13). (A) Mean brain time activity curves in smokers who inhaled the smoke via SDD (black dots) and in those who inhaled directly from a cigarette (open circles). Data for the direct smoking condition were excerpted from a published study (Rose et al., 2010). A different scan procedure was used in that study (twenty 3-s frames, six 10-s frames, three 20-s frames, two 60-s frames, one 120-s frame, and one 240-s frame). Mean brain time activity curves were derived from individual curves fitted with a three-exponential function. Time zero represented the initial time when 11C-nicotine was detected in the brain. TAD and ID are total absorbed dose and inhaled dose, respectively. (B) Mean (± SE) T1/2 values of brain nicotine accumulation. (C) Mean (± SE) Cmax values. SDD – smoke was taken using SDD. Direct – smoke was taken directly from the cigarette. It should be noted that the maximal brain nicotine concentration as depicted in each of the two mean brain time activity curves (A) appeared to be slightly lower than mean Cmax value shown here in the corresponding bar graph. This is due to the fact that a mean time activity curve averages out the peak (Cmax) of individual time activity curves which occur at varying times, while a mean Cmax value is calculated from individual peak values rather than being derived from the mean time activity curve.

4. Discussion

The new smoke delivery device described here was designed to allow researchers to deliver smoke from cigarettes containing 11C-nicotine with a precise and convenient control of puffing parameters (e.g., puff volume, puff duration and inter puff interval) with enhanced safety features. Both the efficiency of 11C-nicotine delivery by this device and its use for PET imaging of brain nicotine kinetics after inhalation of cigarette smoke were evaluated. The data suggest that the device has desirable technical properties and might be useful for in vivo PET studies of nicotine kinetics during smoking.

The SDD was adequately efficient in delivery of 11C-nicotine from the study cigarettes. Using the non-radioactive cigarettes, we observed that only small fractions (5.9 ± 1.4%, mean ± SD) of nicotine in single puffs of smoke produced via the SDD were deposited in the tubing between the cigarette and the mouthpiece. The fractions of 11C-nicotine deposition in the tubing during the PET experiments are expected to be of similar magnitudes. The amount of 11C-nicotine retained in the subjects' body, i.e., the total absorbed dose, averaged 22.8 ± 4.2% of that loaded into the cigarettes. Despite our best efforts to standardize our experimental procedures, the fraction of the total 11C-nicotine loaded into the cigarettes that was absorbed in the body varied across 19 participants from 16.0% to 30.1%. This variation was likely due to inevitable variations in lighting and combustion of the cigarettes and how the individuals inhaled a puff and then exhaled. Within this reasonable efficiency range, we were able to acquire usable dynamic brain scan data from each session. These results demonstrate the suitability of the SDD to deliver an adequate amount of 11C-nicotine to subjects for high-temporal resolution imaging of nicotine kinetics using PET.

Considerable variability (coefficient variation 18%) in the fraction of 11C-nicotine loaded on the cigarette that was retained in the subjects' body suggests the need to carefully assess TAD for each study session. In the present study, we assessed TAD using the full-body scan (2 min per bed position) that included ten bed positions from the head to the knee and five bed positions from the knee to the big toe. Since 95.6% (± 0.9%, SD) of TAD was detected in the body segment from the top of the head to the knee, the scanning of the 5 bed positions from the knee to the big toe can be omitted for routine studies in which the total scanning time is kept at a minimum duration in order to reduce the participant's discomfort and to lower the requirement for effective tracer doses. In that case, sensible estimates of TAD can be obtained by multiplication of the measured radioactivity in the body segment from the top of the head to the knee by 1.05 (1/95.6%).

Since generating a puff of smoke with the SDD requires some time (in this study the time was set as 2 sec), the use of SDD may lead to an increase in smoke aging time (i.e., time interval between smoke generation and inhalation) and consequently in smoke particle sizes compared to when a subject takes a puff of smoke directly from a cigarette. Using previously reported data (Chen et al., 1990), it can be calculated that increasing the aging time by 2 sec may result in a 20 – 40%) increase in the mass median aerodynamic diameter (MMAD) of smoke particles. The increase in the particle size may in turn result in changes in particle deposition in the lungs and respiratory tract and eventually in the kinetics of brain nicotine accumulation. To evaluate this potential impact, we compared mean brain time activity curves after smokers inhaled a single puff of smoke via the SDD with that when smoke was directly inhaled from a cigarette containing 11C-nicotine (Rose et al., 2010). As shown in Figure 4, nearly identical brain nicotine kinetics were observed between these two conditions. These results suggest that the use of this device for smoke delivery has a negligible impact on brain nicotine kinetics after smoke inhalation. To the extent that the rate of brain nicotine accumulation is closely related to nicotine deposition and clearance in the lungs during smoking (Benowitz, 1990; Rose et al., 2010), it is unlikely that the delivery of smoke via the SDD could have altered/affected pulmonary nicotine deposition relative to inhaling directly from the cigarette. One plausible explanation for the absence of impact of the SDD on nicotine kinetics in the lungs may be that the effects of changes in smoke particles resulting from the use of the SDD are minimal in comparison to those from other important factors that can influence nicotine deposition in the respiratory tract and lungs.

As pointed out by numerous researchers (e.g., Baker and Dixon, 2006; Robinson and Yu, 2001), the typical sub-micrometer smoke particles exiting the cigarette predict that only less than 30% of the fresh mainstream smoke entering the respiratory tract would be retained. This cannot account for the much higher levels of retention of smoke particulate matter (60-80%) as measured in clinical studies with smokers. Instead, the overall high retention and the preferential proximal deposition of cigarette smoke in the human airways are attributed to a combination of smoke particle growth processes, such as coagulation and hygroscopic growth, and the cloud behavior of smoke, which causes smoke particles to deposit as if they have a much larger effective size (Baker and Dixon, 2006; Broday and Robinson, 2003). The higher levels of retention for nicotine (> 90%) are likely due to an additional deposition mechanism by which, as the smoke is diluted when drawn into the large effective space in the lungs, nicotine evaporates out of the smoke particles and is readily absorbed in the humid surface of the alveoli and small airways (Baker and Dixon, 2006). Thus, the particle growth and cloud behavior of inhaled smoke in the respiratory tract may have minimized the impact of the initial particle size differences between the smoke delivered via the SDD versus that taken directly from the cigarette, resulting in nearly identical brain nicotine kinetics as measured in these two conditions. Pertaining to this possibility, the extent to which puff volume and flow rate may influence brain nicotine kinetics should also be empirically determined in future research. For such efforts, this SDD can be a suitable tool for controlling the puff parameters.

Apart from its use for PET imaging of brain nicotine kinetics during smoking, the SDD can also be employed to study nicotine kinetics in the respiratory tract. For instance, at 15 min after smoke inhalation, residual 11C-nicotine was seen in the upper airways at relatively high concentrations, indicating slower clearance rates of nicotine deposited in these areas compared to the distal regions of the lungs (Fig. 2). Dynamic scans of nicotine deposition and clearance in different segments of the respiratory tract following smoke inhalation would yield more information with regards to the specific pattern of nicotine kinetics in these regions, which will contribute to a better understanding of nicotine and smoke deposition in human airways and their relevance to nicotine addiction and carcinoma development (Robinson and Yu, 2001). It remains to be elucidated how factors such as various tobacco/nicotine products (e.g., conventional cigarettes versus electronic cigarettes), cigarette additives (e.g., menthol; Zuo et al., 2015), and individual differences (e.g., sex and heaviness of smoking; Zuo et al., 2015; Rose et al., 2010) can modulate nicotine and smoke deposition in the respiratory tract and ultimately brain nicotine kinetics.

It is worth noting that the SDD's shielding of the radioactive study cigarette away from the subject and the PET scanner's gantry not only reduced unnecessary radiation exposure for the subject and the staff, but it also prevented the interference of this radioactivity with PET image quality. This feature was not present in the smoke delivery apparatuses used in previous studies (Berridge et al., 2010; Rose et al., 2010). Because the CReSS smoking topography system was connected to the SDD, the operator could monitor and instruct the subject to inhale the smoke with no less than 500 mL of air and then exhale without breath holding. Thus, the use of the SDD also helped ensure consistent smoke inhalation across subjects.

In summary, use of the SDD eliminates the puff-to-puff variations encountered in generating single or multiple puffs of cigarette smoke, and it also allows a precise and adjustable control of puff volume and flow rate for delivery of smoke containing 11C-nicotine for PET studies. The device also provides enhanced features for monitoring inhalational processes and minimizes unnecessary radiation exposure. This refined smoke delivery device could be useful in conducting non-invasive studies of nicotine deposition and kinetics in the human body.

Highlights.

  • A new smoke delivery device is developed for PET studies of nicotine kinetics

  • The device minimizes puff variations existing with earlier smoking apparatuses

  • Use of the device does not alter brain nicotine kinetics in smokers

  • The device introduces enhanced behavior monitoring and safety features

Acknowledgments

This research was supported by National Institutes of Health (NIH)/National Institute on Drug Abuse grant RC2 DA028948. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institute on Drug Abuse or the National Institutes of Health. We thank Al Salley, Tanaia Loeback, and Wendy Roberts for subject recruitment, Dena Hill-Cairnes and Holly Smith for assistance in data collection, and Alana Cataldo and Upasana Chandra for assistance in manuscript preparation.

Footnotes

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