Cannabis blow back

Using a highly sensitive new test (online column-switching valve liquid chromatography tandem mass spectrometry (LC-MS/MS) selected reaction monitoring (SRM) method), European scientists have found "convincing evidence" that marijuana smoke damages DNA in ways that could increase the risk of cancer.

Parts of the plant Cannabis sativa, also known as marijuana, ganja, and various street names, is are commonly smoked as a recreational drug, although its use for such purposes is illegal in many countries. As far back as 3000 BCE, plants containing mind-altering cannabinoids have been used for various therapeutic purposes. Modern anecdotal evidence and some medical research has, however, suggested that there might be health benefits from smoking cannabis because the active components, cannabinoid compounds including delta-9-tetrahydrocannabinol (THC), seem to alleviate some symptoms in sufferers of various chronic conditions.

Advocates for sufferers of multiple sclerosis, neuropathic pain, glaucoma, wasting caused by AIDS, and nausea caused by cancer chemotherapy, have suggested that cannabis itself or THC and its chemical cousins should be legalised for medical use in these conditions. There are other factors to take into consideration in any changes in the laws surrounding cannabis use

Writing today in the journal Chemical Research in Toxicology, Rajinder Singh of the University of Leicester, UK, and colleagues describe the development of an MS technique that provides a clear indication that marijuana smoke damages DNA, under laboratory conditions.

It is well known that toxic substances in tobacco smoke can damage DNA and increase the risk of lung and other cancers. Scientists were unsure though whether marijuana smoke would have the same effect. Researchers had focused on the toxicity of acetaldehyde, which is present in both tobacco and marijuana. It is difficult to measure DNA damaged caused by acetaldehyde with conventional tests.

Now, Singh and colleagues Jatinderpal Sandhu, Balvinder Kaur, and Peter Farmer of Leicester's Cancer Biomarkers and Prevention Group working with Tina Juren and Dan Segerbaeck of the Karolinska Institute in Huddinge, Sweden, and William Steward of Leicester Royal Infirmary offer a sharp intake of breath for cannabis users.

"Acetaldehyde, they say, "is a ubiquitous genotoxic compound that has been classified as a possible carcinogen to humans." The compound reacts with DNA forming a so-called Schiff base adduct, N²-ethylidene-2'-deoxyguanosine (also known as N²-ethylidene-dG). In order to determine the presence of this adduct after DNA exposure to cannabis smoke, Singh and colleagues have developed an online column-switching valve liquid chromatography tandem mass spectrometry (LC-MS/MS) selected reaction monitoring (SRM) method.

They used an internal isotopically labelled standard containing [¹⁵N₅]N²-ethyl-dG before enzymatically hydrolysing the putatively damaged DNA samples to 2'-deoxynucleosides. Any labelled N²-ethylidene-dG adduct present in the sample is then reduced by buffered sodium cyanoborohydride (NaBH₃CN), which leads to the chemically stable N²-ethyl-2'-deoxyguanosine (N²-ethyl-dG) adduct.

Just 50 micrograms of hydrolysed DNA is necessary and the limit of detection, explain the researchers, for N²-ethyl-dG is 2.0 femtomoles "on column". They carried out several parallel experiments on calf thymus DNA. One set of samples they treated in vitro with acetaldehyde at a concentration range of 0.5 to 100 millimolar. A second sample set was exposed to smoke generated from 1, 5, and 10 cannabis cigarettes. A third sample set was smoked with tobacco.

They demonstrated a linear dose-dependent increases in the level of N²-ethyl-dG adducts. Similar levels of adduct were found after exposure to 10 tobacco or 10 cannabis cigarettes. "It should be noted that the N²-ethyl-dG adduct is not the only adduct formed following exposure to DNA to acetaldehyde," the team explains, "other adducts formed include 1,N²-propanoguanine and N²-dimethyldioxaneguanine.

The team points out that there have been many studies of tobacco smoke. It is known that tobacco smoke contains 4000 chemicals of which 60 are classed as carcinogens. Cannabis in contrast has not been so well studied. It is less combustible than tobacco and is often mixed with tobacco in use. Cannabis smoke contains 400 compounds including 60 canabinoids. However, because of its lower combustibility it contains 50% more carcinogenic polycyclic aromatic hydrocarbons (PAHs) including naphthalene, benzanthracene, and benzopyrene, than tobacco smoke.

The ability of cannabis smoke to damage DNA has significant human health implications especially as users tend to inhale more deeply than cigarette smokers, which increases respiratory burden. "The smoking of 3-4 cannabis cigarettes a day is associated with same degree of damage to bronchial mucus membranes as 20 or more tobacco cigarettes a day," the team adds.

"These results provide evidence for the DNA damaging potential of cannabis [marijuana] smoke," the researchers conclude, "implying that the consumption of cannabis cigarettes may be detrimental to human health with the possibility to initiate cancer development."