Carcinogenicity of smokeless tobacco: Evidence from studies in humans & experimental animals

Introduction

The Monographs programme of the International Agency for Research on Cancer (IARC) seeks to identify the causes of human cancer. The objective of the programme is to systematically review and evaluate the published scientific literature on any agent suspected to be carcinogenic to humans. The carcinogenicity of smokeless tobacco (ST) was evaluated by four Working Groups convened by the IARC during the period from 1984 to 2009. The term smokeless tobacco implies the use of unburned tobacco either cured for chewing as tobacco leaves or as packaged commercial products for oral and nasal use. Products available for human use are listed in detail for each WHO Region in the IARC Monograph1. Based on the IARC evaluations there is sufficient evidence in humans and in experimental animals for carcinogenicity of smokeless tobacco whether used alone or with betel quid1234, leading to the overall evaluation that smokeless tobacco is carcinogenic to humans (Group 1). We present here the supportive global data on experimental studies and will focus the human cancer data on smokeless tobacco products as used in the SEARO Region.

Carcinogenic compounds in smokeless tobacco

The majority of ST products available in the markets are made from two species of the tobacco plant Nicotiana tabacum and Nicotiana rustica. ST is a heterogeneous product including a variety of chemicals and multiple carcinogens have been identified in ST; their broad groups are listed: (i) Tobacco-specific nitrosamines (TSNA) (from tobacco alkaloids during curing, fermentation and ageing); (ii) N-nitrosamine acids (from amino acids present in tobacco leaves amenable to N-nitrosation); (iii) Volatile N-nitrosamines; (iv) Polycyclic aromatic hydrocarbons; (v) Aldehydes (formaldehyde, acetaldehyde, acrolein, crotonaldehyde); and (vi) Other carcinogenic compounds (mostly heavy metals: cadmium, uranium and polonium).

The TSNA are the most powerful and most abundant carcinogens in chewing tobacco, snuff and ST products. Both N’-nitrosonornicotine (NNN) and 4(methynitrosamino)-1-(3-pyridyl)-1- butanone (NNK) have strong carcinogenic effects, and there are dramatic variations in nitrosamine levels in ST globally5. From biochemical studies, it is clear that traditional products consumed in the Sudan6 and most available commercial ST products in India have a high concentration of TSNA, particularly NNN and NNK7. A comparison of mean levels of carcinogens in traditional ST products obtained from India against some European products is shown in Table I7. Normal oral mucosa expresses all P450 cytochromes (1A2, 2A13, 3A4, 2A6, 2E1) that metabolize tobacco-associated nitrosamines, located in microsomes in the basal epithelium5.

Nicotine absorption from chewing tobacco

Studies that have measured plasma nicotine levels after administration of ST or cigarette smoking in volunteers have demonstrated that after a single exposure (7.9 g of ST) maximum plasma levels reached were equivalent for a single cigarette smoked and chewing a quid of ST8. Due to ST remaining in contact with oral mucosa for prolonged periods the plasma nicotine levels are sustained for prolonged periods, and the overall amount of nicotine absorbed was twice as high as that of a single cigarette8. Metabolites of tobacco, cotinine and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL) have been demonstrated to be significantly (P<0.001) higher in blood and urine of ST users than in smokers9.

Human studies on smokeless tobacco and oral cancer

The majority of the studies assessing the carcinogenicity of smokeless tobacco in humans have been conducted in USA, Europe, India and Pakistan; one case-control study was conducted in Africa, in the Sudan10. The evidence for carcinogenicity of ST in humans arises from well conducted cohort and case-control studies. These studies have been comprehensively evaluated by the IARC14. A systematic review and meta-analysis examined all epidemiological studies conducted in South Asia published from 1989 to 2013 on smokeless tobacco and risk of oral cancer11. The search by these authors yielded 14 publications on smokeless tobacco and oral cancer11. Smokeless tobacco types used included chewing tobacco, gutka and naswar. In five publications, odds ratios (OR) for oral cancer were adjusted for tobacco smoking, alcohol consumption and other potential confounders. Adjusted OR ranged from 3.6 [confidence interval (CI), 2.5-5.6] to 8.3 (CI, 5.4-13). The meta-analytic OR for ST and oral cancer from all studies was 4.7 (CI, 3.1-7.1) and for adjusted studies, the OR was 4.3 (CI, 3.1-5.8). For Indian studies, the pooled estimate was slightly higher: OR was 4.8 (CI, 3.2-7.4). By examining the frequency and duration of use of the products, a dose-response was demonstrated. These data provide strong evidence of carcinogenicity of ST products used in India and other part of Asia11.

Tobacco added to betel quid

Adding tobacco to betel quid is an age-old tradition in South Asia. Guha et al12 in a meta-analysis examined the risk of betel quid with and without tobacco. In the Indian subcontinent the meta-relative risk for oral/oropharyngeal cancer was 2.56 (95% CI, 2.00-3.28; 15 studies) for chewing betel quid without tobacco and 7.74 (95% CI, 5.38-11.13; 31 studies) for chewing betel quid with tobacco12. The estimated population attributable fraction for oral cancers attributable to betel quid chewing with tobacco was 49.5 per cent.

Smokeless tobacco and other cancers

There is evidence for increased risk of oesophageal and pancreatic cancer from smokeless tobacco, mainly from studies conducted in the USA and Nordic countries. For oesophageal cancer, five studies (1 from US and 4 from Nordic countries) reported an overall OR 1.8 (CI, 1.1-2.9). For pancreatic cancer, six studies (4 from US and 2 from Nordic countries) reported an overall OR 1.6 (CI, 1.1-2.2)1. There are no reported studies to estimate the risk of ST on these cancers in Asia.

Smokeless tobacco and oral potentially malignant disorders

Oral potentially malignant disorders (OPMDs) are conditions that precede the incidence of invasive cancers of the oral cavity. Among various OPMDs described in the literature13, leukoplakia and erythroplakia are two conditions that are known to be associated with tobacco use. A recent systematic review14 reported the meta OR (mOR) for any OPMD with the use of any ST product as 15.5 (95% CI, 9.9-24.2). Women had a higher risk, mOR=22.2 (95% CI, 9.1-54.1) compared to men, mOR=8.7 (95% CI, 2.1-34.8)14. Gupta et al15 in an Indian cohort with nodular leukoplakia (mostly associated with ST use) followed up to 10 yr reported a malignant transformation rate of 16.2 per cent, with a relative risk of 3243.2.

Experimental studies in animals

In general, in vivo experimental studies in animals were conducted in mouse, rat and hamster, with ST applied topically on the skin, oral mucosa or vaginal mucosa. Some studies reported oral administration of ST in drinking water or by gavage, by subcutaneous administration, injected in to surgically created canals in the lower lip or by implantation of ST pellets. Hamster pouch provides a means of retaining ST in contact with the pouch (oral) mucosa for longer periods. Early studies reported before 1985 had various limitations2, but new in vivo studies4 had improved methodology and substantial improvements with rigorous analysis of data. Two reviews on these studies are also available1617 in the literature.

ST preparations used in experimental studies included Indian smokeless tobacco mixtures, US and Scandinavian snuff, bidi tobacco, mishri and naswar. Having revisited a large volume of experimental studies included in the IARC evaluation in 20074, studies with small group sizes and those with inconclusive data were excluded. Studies conducted using the US or Swedish snuff were also excluded. Several studies reporting on Indian ST that produced positive data are summarized below.

Tobacco and mishri

Conclusion

There is no safe form of tobacco, and both smoked and smokeless tobacco are carcinogenic to humans. As summarized here, the evidence for increased risk for oral cancer among people consuming ST products in South Asia is based on case-control and cohort studies mostly conducted in India and Pakistan. Dose-response data demonstrate a higher risk with increased frequency and duration of ST use and further support causal inference. This evidence is further corroborated by studies conducted in the USA reporting increased risks of mouth cancer1.

Based on a meta-analysis of regional cancer epidemiological studies and the prevalence of use of ST it has been estimated that approximately 50 per cent of oral cancers in India are attributable to ST use12 (Table III). Based on the estimated annual incidence of oral cancer in India, this would amount to approximately 35,000 cancers each year2627. The use of ST is the major cause of oral cancer in South Asia, and therefore, oral cancer is largely preventable. This demands heightened public health action in the Region to increase the public awareness of the dangers of ST use.

Governments and professional bodies should consider the translation of this knowledge on ST to public health action. In this context, it is important to consider how ST products can be regulated in the WHO member countries using the existing Framework Convention on Tobacco Control (FCTC) recommendations. Overall, of the 181 member countries, 52 Parties have adopted and implemented policy or regulation specific to ST28. Several articles of the FCTC are of relevance for regulation of ST in the WHO member countries in the SEARO Region. In India and neighbouring regions, gutka (ST mixed with areca nut) is the most abundantly consumed commercially packaged smokeless tobacco product. Gutka has now been banned in some States in India, but a more vigorous implementation is necessary29. It is essential to include programmes that create awareness about effects of smokeless tobacco on health and sustain surveillance levels on ST use in the Region.