Phase II Trial of Droxidopa

The drug company, Chelsea Therapeutics International, are currently conducting a phase II trial to investigate whether the drug droxidopa is effective and safe in treating the pain of fibromyalgia. Droxidopa is a man-made amino acid which is converted into the neurotransmitter norepinephrine in the body.

Norepinephrine is known to dampen down pain signals in the central nervous system (the spinal cord and brain) and many clinical studies have demonstrated that people with fibromyalgia have lower levels of norepinephrine than normal. Increasing levels of norepinephrine in the central nervous system should therefore decrease pain levels.

The problem with norepinephrine is that it cannot penetrate the blood-brain barrier itself to get into the central nervous system because it is a charged molecule; therefore treating people with additional norepinephrine itself would not work. Droxidopa however is a neutral amino acid which can cross the blood brain barrier and be converted to norepinephrine within the central nervous system, thus increasing norepinephrine levels where they are needed to reduce pain.

The other problem though is that norepinephrine also plays a role in the peripheral nervous system (the nervous system that serves other areas of the body beyond the spinal cord) and there is an enzyme which converts amino acids into norepinephrine in the peripheral as well as the central nervous system. So, when droxidopa enters the body the majority of it could be converted to norepinephrine in the peripheral nervous system before it has had the chance to cross the blood brain barrier into the central nervous system where people with fibromyalgia need it most.

A possible solution to this problem is a drug called carbidopa which blocks the enzyme that converts droxidopa to norepinephrine in the peripheral nervous system, allowing more of the droxidopa to reach the central nervous system before it is converted to norepinephrine. This solution also has the advantage of reducing any possible unwanted side-effects from the actions of droxidopa in the peripheral nervous system. The researchers are therefore investigating whether a combination of droxidopa and carbidopa is more effective than droxidopa alone.

Chelsea Therapeutics International have enrolled 120 fibromyalgia patients on to the trial and these patients have been randomised to receive either droxidopa alone, carbidopa alone, droxidopa and carbidopa in combination or a placebo over a 9-week treatment period. The researchers will measure the average reduction in pain using the Short Form McGill Pain Questionnaire and compare the results between the different groups. Preliminary results are looking promising and the researchers hope to publish the results of the trial later this year.

By Kathy Longley