By Penny Stern, MD, MPH
NEW YORK, Jun 22 (Reuters Health) - There's one more potential problem to
add to those already known about marijuana: the ingredient that gives it its
kick, also seems to influence tumor growth. According to researchers at the
University of California, Los Angeles, tetrahydrocannabinol (THC) "promotes
tumor growth by inhibiting antitumor immunity."
In their National Institute on Drug Abuse-supported study, Dr. Steven
Dubinett and colleagues looked at how THC affects lung tumors implanted in mice.
When exposed to THC, tumor growth increased in mice with strong, functioning
immune systems. In a second group of mice that lacked an immune system, THC had
no effect on tumor growth, according to a report in the July issue of the
Journal of Immunology.
"I think what we are finding is that we need to approach THC and marijuana
with some element of caution," Dubinett told Reuters Health. "Certainly THC and
marijuana are not the benign substances that some would like to believe."
The current findings could have implications for cancer patients who may use
marijuana to fight chemotherapy-induced nausea. Dubinett and his colleagues
suggest that increased tumor growth may be a downside to medicinal marijuana.
Dubinett pointed out a clear distinction between his group's study and
others that have described positive effects from THC. Those studies "used
intra-tumoral injections of extraordinarily high levels of THC and suggested
that they would have therapeutic benefit," he said. "We used doses (of THC) that
would end in levels considered by those working in the field to be
physiologically relevant."
THC appears to spur tumor growth by increasing levels of two immune
system-inhibiting factors--interleukin-10 (IL-10) and transforming growth
factor-beta (TGF-b). When THC is made available, the levels of IL-10 and TGF-b
increase, leading to enhanced tumor growth in the mice.
Dubinett's team then gave the mice substances that would neutralize the
effect of IL-10 and TGF-b. This "led to the resolution of the tumor
growth-promoting effects of THC," they report.
"THC appears to have a very profound effect on cell-mediated immunity.
However, more studies will need to be performed to fully understand the
implications of this study," Dubinett notes.
Other characteristics of marijuana worry the investigators. They point out
that "the tar phase of marijuana smoke, compared with that of tobacco, contains
higher concentrations of (carcinogenic substances) including benzapyrene, which
has been shown to be a key factor promoting human lung cancer."
And if that were not already enough, the authors add, "four times as much
tar is deposited in the respiratory tract from the smoke of marijuana than from
that of a comparable amount of tobacco, thus amplifying respiratory exposure to
the (cancer-causing substances) in marijuana smoke."
