NNK

Synthesis

NNK is a compound that is naturally synthesized in tobacco leaves that are exposed to light: the pyrrolidine ring in the nicotine opens and turns the nicotine into NNK. Most of the NNK found in tobacco smoke originates from nicotine burning.

It can also be formed synthetically by taking the following steps:

The potent carcinogen 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) is present in tobacco and tobacco smoke. [Carbonyl14C]NNK (6) was synthesized in 27% overall yield. [Carboxyl-14C] nicotinic acid was esterified with benzyl alcohol and the ester was alkylated by 3-lithio-N-methylpyrrolidin-2-one. The resulting keto-lactam was hydrolyzed and decarboxylated by treatment with boiling hydrochloric acid. Nitrosation at pH 4.0 gave [carbonyl-14C]NNK. Carbonyl reduction of [carbonyl-14C]NNK with either sodium borohydride or cultured rat liver slices gave [carbinol-14C] 4-(methylnitrosamino)-1-(3-pyridyl) butan-1-ol.

Sun-cured tobaccos (aka "Oriental") contain very little NNK and other TSNAs due to low-nitrate soil, lack of nitrate fertilizer, and sun-curing. Flue-cured tobacco (aka "Virginia" tobacco), especially when using an open flame, contains most of the NNK in American blended tobaccos although Marlboro's "virginia blend" had the lowest levels of NNK per nicotine out of many tested with the exception of Natural American Spirit.

Amount in cigarettes and e-cigarettes

NNK was found in 89% of Korean e-cigarette liquids tested in concentrations from 0.22 to 9.84 µg/L. For the product that had the highest amount, if 1 ml is equival to 20 cigarettes, there would be 9.84/20 = 0.5 ng NNK per e-cig cigarette dose. Cigarettes with 1 gram of tobacco average about 350 ng. It is believed that none of the nicotine in an e-cigarette is converted to NNK due to the lower vaporization temperature. The amount of NNK delivered in cigarette smoke ranged from 30 to 280 ng/cigarette in one study and 12 to 110 ng/cigarette in another. The amount of NNK delivered by e-cigarettes was found to range from non-detectable to 2.8 ng per 15 puffs (approximately 1 cigarette).

Metabolism

NNK is initially a procarcinogen that needs activation to exert its effects. The activation of NNK is done by enzymes of the cytochrome pigment (CYP) multigene family. These enzymes catalyze hydroxylation reactions. Beside the CYP family NNK can also be activated by metabolic genes, like myeloperoxidase (MPO) and epoxide hydrolase (EPHX1). NNK can be activated by two different routes, the oxidative path and the reductive path. In the oxidative metabolism NNK undergoes an α-hydroxylation catalyzed by cytochrome P450. This reaction can be done by two pathways namely by α-methylhydoxylation or by α-methylenehydroxylation. Both pathways produce the carcinogenic metabolized isoform of NNK, NNAL.

In the reductive metabolism NNK undergoes either a carbonyl reduction or a pyridine N-oxidation, both producing NNAL.

NNAL can be detoxified by glucuronidation producing an non-carcinogenic compounds known as NNAL-Glucs. The glucuronidation can take place on the oxygen next to the ring (NNAL-O-Gluc), or it takes place on the nitrogen inside the ring(NNAL-N-Gluc). The NNAL-Glucs are then excreted by the kidneys into the urine.

Signaling pathways

Once NNK is activated, NNK initiates a cascade of signaling pathways (for example ERK1/2, NFκB, PI3K/Akt, MAPK, FasL, K-ras), resulting in uncontrolled cellular proliferation and tumorigenesis.

NNK activates µ en m-calpain kinase which induce lung metastasis via the ERK1/2 pathway. This pathway upregulate cellular myelocytomatosis (c-Myc) and B cell leukemia/lymphoma 2 (Bcl2) in which the two oncoprotein are involved in cellular proliferation, transformation and apoptosis. Also does NNK promotes cell survival via phosphorylation with cooperation of c-Myc and Bcl2 causing cellular migration, invasion and uncontrolled proliferation.

The ERK1/2 pathway also phosphorylate NFκB causing a upregulation of cyclin D1, a G1 phase regulator protein. When NNK is present it directly involves cellular survival dependent on NFκB. Further studies are needed to better understand NNK cellular pathyways of NFκB.

The phosphoinositide 3-kinase (PI3K/Akt) pathway is also an important contributor to NNK-induced cellular transformations and metastasis. This process ensures the proliferation and survival of tumorigenic cells. The ERK1/2 and Akt pathways show consequential changes in levels of protein expression as a result of NNK-activation in the cells, but further research is needed to fully understand the mechanism of NNK-activated pathways.

Toxicity

NNK is known as a mutagen, which means it causes polymorphisms in the human genome. Studies showed that NNK induced gene polymorphisms in cells that involve in cell growth, proliferation and differentiation. There are multiple NNK dependent routes that involve cell proliferation. One example is the cell route that coordinates the downregulation of retinoic acid receptor beta (RAR-β). Studies showed that with a 100 mg/kg dose of NNK, several point mutations were formed in the RAR-β gene, inducing tumorigenesis in the lungs.

Other genes affected by NNK include sulfotransferase 1A1 (SULT1A1), transforming growth factor beta (TGF-β), and angiotensin II (AT2).

NNK plays a very important role in gene silencing, modification, and functional disruption which induce carcinogenesis.

Inhibition

Chemical compounds derived from cruciferous vegetables and EGCG inhibit lung tumorigenesis by NNK in animal models. Whether these effects have any relevance to human health is unknown and is a subject of ongoing research.