Application of carbon-11 labelled nicotine in the measurement of human cerebral blood flow and other physiological parameters

Comparison of brain nicotine accumulation from traditional combustible cigarettes and electronic cigarettes with different formulations

Rapid brain accumulation is critical for the acute reinforcing effects of nicotine. Although nicotine formulation (free-base vs. protonated or salt) in electronic cigarette (E-cig) liquid affects user satisfaction, its impact on brain nicotine accumulation (BNA) from E-cig use has not been evaluated in comparison with traditional combustible cigarettes (C-cigs) using a within-subjects design. BNA was directly assessed with 29 adult dual users (13 females) of E-cigs and C-cigs, using [11C]nicotine and positron emission tomography (PET). Participants underwent two 15-min upper body (from chest to head) scanning sessions during which they inhaled a single puff of [11C]nicotine-labeled vapor from E-cigs with free-base nicotine or C-cig smoke in a randomized order. Seventeen of them also went through another session during which they inhaled from E-cigs with nicotine salt. A full-body scan was also conducted at each session to measure total absorbed dose of [11C]nicotine. Mean maximum nicotine concentration (Cmax) in brain following inhalation of free-base nicotine E-cig vapor was 19% and 15% lower relative to C-cig smoke and nicotine salt E-cig vapor (ps = 0.014 and 0.043, respectively). The Cmax values did not differ significantly between the C-cig and nicotine salt E-cig. Mean values of time to the maximum concentration (Tmax) were not significantly different between the two types of E-cig, but they were 64% and 40% longer than that for C-cig smoking (ps = 0.0005 and 0.004, respectively). Mean Cmax with C-cigs and free-base nicotine E-cigs were greater in females relative to males and correlated with T1/2 of lung nicotine clearance and participants' pack-years. These results suggest that while E-cigs with free-base nicotine formulation can deliver nicotine rapidly to the brain, those with nicotine salt formulation are capable of even more efficient brain nicotine delivery closely resembling combustible cigarettes. Therefore, nicotine formulation or pH in E-liquid should be considered in evaluation of E-cigs in terms of abuse liability and potential in substituting for combustible cigarettes.

Rapid Brain Nicotine Uptake from Electronic Cigarettes

This study sought to determine brain nicotine kinetics from use of the increasingly popular electronic cigarette (E-cig). Methods: In 17 E-cig users (9 men and 8 women), brain uptake of nicotine after inhalation from E-cigs was directly assessed using ¹¹C-nicotine PET. The brain nicotine kinetics were compared with those from smoking combustible cigarettes (C-cigs). Results: A single puff of E-cig vapor caused the nicotine concentration in the brain to rise quickly (mean time to reach 50% of maximum brain nicotine concentration, 27 s), with a peak amplitude 25% higher in women than men, resembling previous observations with C-cigs. Nonetheless, the accumulation from E-cigs (24%) was less than that from C-cigs (32%) in both men and women. Conclusion: E-cigs can deliver nicotine to the brain with a rapidity similar to that of C-cigs. Therefore, to the extent that rapid brain uptake promotes smoking reward, E-cigs might maintain a degree of nicotine dependence and also serve as a noncombustible substitute for cigarettes.

Development and optimization of a novel automated loop method for production of [11C]nicotine

A novel, rapid, and automated loop method for the synthesis of [¹¹C]nicotine was developed and optimized. The method involves, a reaction of the precursor, (+) nornicotine or (-) nornicotine, with a gas-phase produced [¹¹C]CH₃I in an 800 µL loop at 75 °C for 5 min followed by a semi-preparatory Reversed-Phase High-Performance Liquid Chromatography (RP-HPLC) purification. The optimized synthesis and purification process was complete in < 30 min and produced [¹¹C]nicotine with > 99.9% Radiochemical Purity (RCP), no [¹¹C]CH₃I, no (+) nornicotine, 105 mCi/µmole specific activity, 7.0 - 7.2 pH, and 16.6% ethanol. The current method can be optimized, to reduce the ethanol content (<10%), and can be translated to a cGMP production of [¹¹C]nicotine for human clinical trials.

6-[18F]Fluoro-A-85380: an in vivo tracer for the nicotinic acetylcholine receptor

6-[18F]Fluoro-3-(2(S)-azetidinylmethoxy)pyridine (6-[18F]fluoro-A-85380 or 6-[18F]FA), a new tracer for positron emission tomography, was synthesized by no-carrier-added [18F] fluorination of 6-iodo-3-((1-tert-butoxycarbonyl-2(S)-azetidinyl)methoxy)pyridine followed by acidic deprotection. 6-[18F]FA followed the regional densities of brain nicotinic acetylcholine receptors (nAChRs) reported in the literature. Evidence of binding to nAChRs and high specificity of the binding in vivo was demonstrated by inhibition with nAChR selective ligands as well as with unlabeled 6-FA. A preliminary toxicology study of the 6-FA showed a relatively low biological effect.

Pharmacokinetics of radiotracers in human plasma during positron emission tomography

Many radiopharmaceuticals for positron emission tomography (PET) are substantially metabolized in peripheral organs. Pharmacological treatments intended to alter cerebral metabolism might also alter radiotracer metabolism, consequently altering the cerebral uptake. First-order rate constants for the metabolism of PET tracers can be calculated by a linear graphical method from the precursor and metabolite concentrations measured in plasma extracts fractionated by HPLC. We tested the effects of specific pharmacological challenges on the plasma kinetics of six tracers used for PET studies of neurotransmission. The rate of O-methylation of circulating [(18)F]fluorodopa, a tracer of dopa decarboxylase activity in brain, was unaffected by pretreatment with amantadine, an antagonist of glutamate receptors. [(11)C]Deprenyl, a tracer of monoamine oxidase activity, was rapidly metabolized to [(11)C]methamphetamine and polar metabolites in healthy volunteers. The net rate constant of this metabolism was three times higher in a group of subjects under treatment for epilepsy, consistent with induction of hepatic microsomal enzymes by antiepileptic drugs. [(11)C]Sch 23390, a ligand for dopamine D1 receptors, was rapidly metabolized to polar metabolites. The net rate constant of metabolism was unaffected by pretreatment with lorazepam. [(11)C]-(S)-Nicotine, a ligand for nicotinic receptors, was rapidly metabolized to [(11)C]-(S)-cotenine, which is less polar than the parent compound. Pretreatment with mazindol, a dopamine uptake inhibitor, was without effect on peripheral metabolism of [(11)C]-(S)-nicotine. [(11)C]WIN 35,428, a tropane derivative which labels dopamine uptake sites, was metabolized to a nonpolar metabolite, but so slowly that the rate constant of this process could not be calculated. [(11)C]Raclopride, a ligand for dopamine D2 receptors, was first metabolized to a nonpolar metabolite, which then yielded two hydrophilic metabolites. The initial metabolic step was substantially blocked by pretreatment with amphetamine, possibly indicative of competitive inhibition of microsomal oxidation. Together, these results indicate that the linear graphic method is useful for estimating the kinetics of the plasma metabolism of many widely used PET tracers. Drug-drug interactions were revealed in subjects treated with specific pharmacological agents prior to tracer administration.

Cholinergic neurotransmission studied in vivo using positron emission tomography or single photon emission computerized tomography

During the past decade, considerable efforts have been made in the development of radiopharmaceuticals for the in vivo study of the cholinergic neurotransmission using positron emission tomography or single photon emission computerized tomography. The main cholinergic radioligands, labelled with positron- or gamma-photon-emitting radionuclides, are reviewed with respect to use as in vivo markers of either acetylcholinesterase, vesicular acetylcholine transporter, brain and heart muscarinic receptors, or cholinergic nicotinic receptors. The main results obtained in the in vivo study of the physiology, pharmacology or pathology of the different steps of the cholinergic neurotransmission using single photon emission computerized tomography and positron emission tomography are discussed.

PET studies of the uptake of (S)- and (R)-[11C]nicotine in the human brain: difficulties in visualizing specific receptor binding in vivo

(S)- and (R)-[11C]nicotine were synthesized by methylation of (S)- and (R)-nornicotine using [11C]methyl iodide. Following their intravenous injection in tracer doses to smoking and nonsmoking healthy males the radioactivity in arterial blood showed a sharp peak at about 1 min followed by a plateau level for the remaining 50 min of recording. Uptake in the brain, as measured by positron emission tomography (PET), was rapid with a peak at 5 min followed by a steady decline towards the end of the measurement. The regional distribution of radioactivity followed essentially the distribution of gray matter with high uptake in the cortex, the thalamus and the basal ganglia and low uptake in the pons, cerebellum and white matter. Levels of the labelled natural enantiomer, (S)-[11C]nicotine, were higher than those of the synthetic enantiomer, (R)-[11C]nicotine, particularly in the smokers. The time-activity curves of (S)-[11C]nicotine uptake were not changed by co-administration of 1.0 mg of unlabelled nicotine with the labelled nicotine. Similarly administration of unlabelled nicotine at the peak of radioactivity, 6 min following (S)-[11C]nicotine, had no effect on the time-activity curves. Thus essential criteria for visualizing receptor binding with the PET technique could not be fulfilled. Calculation of kinetic constants using a two-compartment model gave values indicating that the brain uptake of [11C]nicotine is mainly determined by the cerebral blood flow, extraction of the tracer over the blood-brain barrier and unspecific binding. Thus 11C-labelled nicotine does not seem to be a suitable tracer for PET studies of nicotinic cholinergic receptors in the human brain.