Synthetic nicotine analogues to evade rules? The case of 6-methylnicotine| Tabaccologia

Abstract

(S)-6-Methylnicotine (6MN), a synthetic nicotine analogue with a methyl group at the 6-position of the pyridine ring, has recently gained attention due to its use in e-cigarettes marketed as alternatives to conventional nicotine products. This strategy has allowed manufacturers to circumvent regulatory oversight, prompting scrutiny from the Food and Drug Administration and other health authorities. About pharmacological and toxicological profile, a limited number of studies suggest that 6MN exhibits greater potency and toxicity than (S)-nicotine, with potential public health implications, particularly for younger populations. While manufacturers claim safety and regulatory exemptions for products containing 6MN, such as SPREE BAR, independent evaluations are lacking. Preliminary data indicate similar efficacy and safety patterns between 6MN and nicotine, though concerns about chronic inhalation risks persist. The emergence and spread of nicotine analogues highlight an urgent need for comprehensive regulatory frameworks and independent and in-depth toxicological studies to safeguard public health.

Introduction

(S)-6-Methylnicotine (6MN), also known as 2-methyl-5-[(2S)-1-methylpyrrolidin-2-yl]pyridine according to IUPAC nomenclature [1], is the S-isomer of a synthetic nicotine analogue featuring a methyl group at the 6-position of the pyridine ring (Figure 1).

The Food and Drug Administration (FDA) has recently become concerned with this substance because some e-cigarette producers are promoting it as an alternative to synthetic nicotine, bypassing laws that regulate nicotine-containing products [2]. The limited pharmacological and toxicological data on 6MN suggest further research to assess potential risks for public health.

From naturally derived nicotine to synthetic nicotine

Efforts to synthesize nicotine rather than extracting it from tobacco date back to 1904 [3], reflecting industry’s interest in developing efficient protocols [4] to reduce costs and processing times associated with extraction. However, these initial attempts were hindered by difficulties in isolating the S-isomer of nicotine, the predominant form in tobacco (> 99%) [5]. For a long period, extracted S-nicotine remained the most feasible and cost-effective option.

Since then, developments in high-yield and stereoselective synthesis techniques [6,7] have increased productivity and decreased costs. In 2015, Next Generation Labs (NGL) began marketing synthetic nicotine in the United States under the trademark TFN (Tobacco Free Nicotine). Consequently, synthetic nicotine-containing e-cigarettes have become widespread.

Synthetic nicotine and analogues: regulatory framework

The European Union first Tobacco Products Directive 2001/37/EC (TPD) was introduced in 2001 to regulate the production, sale, and presentation of tobacco products. In 2014, reflecting new health insights and technological advancements, TPD was revised as Directive 2014/40/EU (TPD II), introducing specific limits on nicotine concentration and regulating e-liquid ingredients [8].

In April 2022, following the 2021 “Puff bar case” [9] involving the sale of TFN-containing e-cigarettes, the FDA was authorized to regulate all nicotine products regardless of origin, including synthetic sources. As a result, companies in the United States must now submit a Premarket Tobacco Product Application (PMTA), accompanied by safety data, for such products [10].

Despite these stricter regulations, a recent World Health Organization (WHO) report [11] warned that non-nicotine tobacco alkaloids or other synthetic nicotine analogues may be exploited by manufacturers to circumvent regulations focused solely on nicotine.

The Spree Bar case

In the United States, as of October 2023, several vape stores have begun selling a new product named Spree Bar, advertised as “PMTA exempt” and marketed with appealing flavours and designs targeting young consumers (Figure 2) [2]. Spree Bar contains Metatine, a registered trade name for 6MN.

The Spree Bar website [12] presents Metatine as a synthetic molecule structurally similar but chemically distinct from nicotine and not derived from tobacco. Consequently, products containing Metatine are not subject to FDA premarket authorization requirements.

According to its manufacturers, Metatine, delivered via Spree Bar e-cigarettes, provides the same satisfaction and enjoyment as traditional nicotine-containing tobacco products and e-cigarettes. However, they acknowledge that while Metatine is chemically distinct from nicotine, it remains addictive and exhibits a similar toxicity profile, warranting use exclusively by adult tobacco or e-cigarette consumers aged 21 or older.

Safety testing reported by the manufacturers indicates that Metatine has been assessed for tobacco-specific nitrosamines (TSNAs), degradation products, chemical impurities, and volatile organic compounds. Compared to additives currently found in traditional vaping goods, the results apparently indicate no additional risks. Nonetheless, these claims have not been evaluated by the FDA. Since Metatine is not intended for diagnosing, curing, mitigating, treating, or preventing diseases and does not affect body structure or functions, it is not classified as a drug under the U.S. Pharmacopeia.

The “Additional Warnings” section on the Spree Bar website [13] lists potential side effects, including nausea, coughing, headaches, oral or throat irritation, nasal congestion, heart palpitations, vomiting, abnormal heart rate, stomach issues, dizziness, rash, respiratory difficulties, and chest pain. Users are advised to discontinue use and seek medical attention if symptoms persist.

Precautionary statements recommend avoiding product use by individuals under 21, pregnant or breastfeeding women, those allergic to propylene glycol or glycerine, and individuals with conditions such as asthma, heart disease, high blood pressure, diabetes, or gastric ulcers. A warning is also issued regarding exposure to chemicals like formaldehyde, known to cause cancer under California law.

6-methylnicotine: pharmacological and toxicological profile

What is known about the pharmacokinetics, pharmacodynamics, and toxicology of 6MN? Limited data are available. There are no human toxicological data, while in vivo studies in rats indicate that the LD50 of 6MN is 1.5 to 3 times lower than that of nicotine, suggesting greater toxicity [14]. A structural affinity study analysing various nicotine analogues (6MN included) reported minimal impact on nicotinic receptor binding when a halogen or methyl group was introduced [15].

Pharmacological research indicates that 6MN is three times more potent than [3H]nicotine in displacing it from rat brain membranes and five times more potent in inducing prostration behaviour in animals [16].

A recent in vitro toxicology study, sponsored by Shenzhen Zinwi Biotech and conducted at Guangdong University, found that 6MN showed greater cytotoxicity than nicotine in human bronchial epithelial cells (BEAS-2B) but had lesser effects on the up-regulation of lung cancer-related oncoproteins [17]. The researchers concluded that e-cigarettes containing 6MN might offer certain advantages over conventional e-cigarettes, potentially encouraging their use.

Cheetham A.G. et al., during the Smoke Science and Product Technology Joint Study Groups Conference (Cancún, 2023), presented a poster titled “Chemical, Pharmacological, and Toxicological Assessment of 6-Methylnicotine” [18]. Computational analyses demonstrated similar potency and receptor binding affinity between 6MN and (S)-nicotine in vivo and ex vivo models. Conventional in vitro toxicology assays (Neutral Red and Ames) revealed comparable cellular cytotoxicity and mutagenicity between 6MN and (S)-nicotine in e-liquid formulations. Collectively, these findings suggest that “6-methylnicotine exhibits chemical, pharmacological, and toxicological properties comparable to nicotine” [18].

Shanghai Lingnuo Biotech conducted clinical studies evaluating the “throat hit” and other psychophysical characteristics of aerosols containing 6MN. In one study, 1 mg/ml of 6MN provided a satisfaction level comparable to 3 mg/ml nicotine, producing a similar “throat hit.” However, this study limitations included a small sample size (n = 10), lack of inhalation protocol description, and absence of statistical analysis [19].

Despite the manufacturers claims, data on toxicological and safety assessments have not been published in peer-reviewed literature, hindering evaluation of the methodologies and reproducibility of these studies.

Furthermore, a significant concern arises regarding the discrepancy between the effective concentrations of 6MN in these products. A notable example is provided by Erythropel H.C. et al. [20]: in samples of e-liquids from Spree Bar electronic cigarettes, the measured concentrations of 6MN were 87-88% lower than the values declared by the products labels (0.58-0.63% detected versus the claimed 5%). These discrepancies call into question the integrity of industrial processes that can be exempt from stricter regulations.

Conclusions

Given potential risks for public health associated with these novel synthetic chemicals, FDA must immediately establish adequate restrictions. The 2014/40/EU Directive in the European Union relates the term "nicotine" to "nicotinic alkaloids", implying that the regulation could also be applicable to substances such as 6MN, which is an analogue of nicotine and, thus, a member of the nicotinic alkaloid class.

Nevertheless, in 2024, "NoNIC" goods – some of which include 6MN – began to appear in Europe. These products are advertised as being free from nicotine and tobacco and seemed to be easily accessible online in the UK and the EU [21].

Similarly, e-cigarettes with nicotine substitutes are becoming more and more common in Australia, even though the country has strict regulations governing the sale of these goods. A new Research Letter by Caitlin Jenkins et al. supports this tendency [22].

It is crucial to deepen the understanding of the effects of this chemical, as well as other nicotine analogues, whose use in smoking products may expand despite regulatory restrictions. The possible health hazards of long-term inhalation must be carefully considered, particularly for younger people, such as adolescents or preadolescents. To assess whether 6MN may have deleterious short- or long-term pulmonary, neurological, or systemic effects, there is currently not sufficient data available.

Figures and tables

Figure 1.A: molecular structure of (S)-nicotine; B: molecular structure of (S)-6-methylnicotine.

Figure 2.Screenshot from the Spree Bar website taken on 01/24/2025: disposable pod device with 6,000 puffs, assorted flavours, and youth-targeted design.

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