High nicotine exposure in rodents is unlikely to inform about its toxicity in humans

To the Editor:

We read with interest the research letter by Reinikovaite et al. [1], in which the authors reported same level of damage in the lungs of Sprague Dawley rats subjected to whole body chronic exposure of e-cigarette vapour emissions and cigarette smoke, as well as to subcutaneous injection of nicotine. Their conclusion is that e-cigarette use and long-time consumption of nicotine are just as toxic as tobacco cigarettes. We appreciate the authors' intention to address concerns related to the potential long-term health effects of e-cigarette use and nicotine exposure, but there are a number of methodological considerations that lessen the impact of their findings.

First, in spite of the authors' effort to establish a meaningful puff/puff comparison between e-cigarette vapour and cigarette smoke, rats weighting about 200 g were being exposed to 600 puffs of vapour or smoke in just over 4 h in small gas chambers of <1 m³ volume. Humans weigh, on average, 400 times more than Sprague Dawley rats and yet they usually inhale the same 600 puffs in just over 3 days. Under these specific experimental conditions, animals' overexposure to aerosol mass and its contents is highly likely. This is not uncommon, as there is proliferation of toxicological studies that are often performed with high-dose exposure protocols that are not applicable to normal human consumption [2]. When calculating the dose to be used, particularly if this is based upon human data, allometric scaling should be used, to take into account the large differences in metabolic rate between animals of different body mass [3, 4]. Moreover, in the work of Reinikovaite et al. [1], rats were exposed daily for 4 h for a total of 5 weeks; such exposure protocol is designed to maximise harm for the induction of pathological status (e.g. emphysema or chronic obstructive pulmonary disease (COPD)), which is used to study the characteristics of the disease in animal models (inflammation, basic mechanisms, response to therapies, etc.) [5]. Also, we noted that the authors did not mention animal health during the study, and standard toxic endpoints detailed in Organisation for Economic Cooperation and Development test number 413 (organ weights, gross pathology, etc.) were not assessed [6].

Second, in trying to equate similar level of plasma nicotine of adult smokers in the much smaller animal model (weighing approximately 400 times less than humans), the authors end up injecting a highly toxic dose. In histopathological studies of rats, it has been found that subcutaneous administration of nicotine doses similar to those used by Reinikovaite et al. [1] can cause extensive tissue damage, not only in the lung, but also in other organs including the liver, kidneys and brain [7, 8]. Thus, the dose used in the study is toxic and far from being informative of the nicotine effects in humans under the normal condition of consumption. It would have been ideal to include a rat group exposed to e-cigarette without nicotine to reach more revealing conclusions. Moreover, given that it is well established that the protease/anti-protease ratio is the central component of our understanding of emphysema, it is incongruous to advance the alternative hypothesis that it is nicotine causing lung destruction in smokers. This hypothesis is also at variance with the epidemiological evidence; for example, biomass fuel combustion is a well-known causative factor for COPD not dependent on nicotine [9, 10].

Third, in stark contrast with the concerns raised by Reinikovaite et al. [1] that e-cigarettes are as toxic as tobacco cigarettes and can cause lung damage, there is emerging evidence from real-life surveys [11] and the prospective evaluation of lung function and high-resolution computed tomography of regular vapers who have never smoked in their life [12], indicating that long term e-cigarette use is unlikely to raise significant lung health concerns. Moreover, despite their being millions of regular e-cigarette users worldwide, there has been no evidence of emerging respiratory disease outbreaks in recent years.

• Received June 8, 2018.
• Accepted June 16, 2019.

• Copyright ©ERS 2019