Anti-obesity treatments such as Sanofi-Aventis’ Acomplia, which block brain molecules similar to those in marijuana, could cause brain damage to young children, according to a new study.

The study, authored by the Massachusetts Institute of Technology's Picower Institute for Learning and Memory and published in the journal Neuron, which was done using mice, found that blocking cannabinoid receptors “could also suppress the adaptive rewiring of the brain necessary for neural development in children”. Acomplia (rimonabant) is the first in a new class of drugs that blocks cannabinoid receptors in the brain, which cause people to get the “munchies” when smoking marijuana.

The Picower team did not use Acomplia but rather a chemical analogue called AM 251 to explore plasticity, ie the brain's ability to change in response to experience. They did this by temporarily depriving newborn mice of vision in one eye soon after birth, which induces a long-lasting loss of synapses that causes blindness in the covered eye, while synapses shift to the uncovered eye.

Lead author Mark Bear said that how and where this synaptic shift occurs in the primary visual cortex has remained controversial but understanding the mechanism is key because the same brain mechanisms are used for normal development “and may go awry in conditions that cause developmental delays in humans”. They may reappear in old age and contribute to synaptic loss during Alzheimer's disease, he added.

In mice, the MIT researchers found, even one day of deprivation from one eye starts the shift to dominance of the uncovered one. However injecting them with a cannabinoid receptor blocker halted the shift in certain brain regions, indicating that cannabinoids play a key role in early synaptic development.

The study concludes that blocking cannabinoids receptors could thwart this developmental process, and Prof Bear said that “our finding of a profound disruption of cortical plasticity in juvenile mice suggests caution is advised in the use of such compounds in children".

Acomplia was approved in Europe in 2006 as an adjunct to diet and exercise for the treatment of obese or overweight patients with associated risk factors such as type 2 diabetes or dyslipidaemia, and was once hailed as a potential blockbuster for Sanofi. However, its marketing application was pulled in the USA after a Food and Drug Administration advisory panel made it clear that approval would not be granted because of potential psychiatric side effects, notably suicidal thoughts. European regulators then added stronger warnings against using the drug in patients with depression.

Merck & Co is also developing a new cannabinoid-based obesity therapy, called taranabant. Recent Phase II data showed that the Merck drug may aid weight loss though there are some concerns that it has similar problematic side effects to Sanofi’s treatment.