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Inhibiting The Inhibitors Chapter III of IV The Entacapone Story Monoamine oxidase (MAO): Partly to regulate production of the powerful stimulants, the body has an enzyme called monoamine oxidase (MAO). There are two classes of MAO: -A, which is widely distributed, and -B, found mostly in the brain. There, MAO-B catabolizes (neutralizes) norepinephrine and epinephrine, but also dopamine and serotonin. To treat depression, some drugs act by inhibiting MAO, thereby increasing the activity of serotonin in the brain. They may be nonselective, acting on both MAO-A and MAO-B. Inhibiting MAO-A cancels the restraint on epinephrine from the adrenal glands, so taking a nonselective MAO inhibitor together with tyramine-rich foods may risk a "hypertensive crisis" similar to that caused by an overdose of Adrenalin, the so-called "cheese reaction". But in PD, inhibiting MAO-B also helps prevent the loss of dopamine that has been supplied with such difficulty, so a selective inhibition of MAO-B, but not MAO-A, is desirable. The drug selegiline (Eldepryl) is a selective MAO-B inhibitor, in dosage less than the recommended maximum of 10mg/day. Usually, it's safe to ignore the dietary restriction when taking Eldepryl, but in at least one case a hypertensive crisis occurred anyway, so caution is wise. Catechol-O-Methyl Transferase (COMT): The third major hazard to dopamine introduced as therapy is Catechol-O-Methyl Transferase (COMT). COMT is found in various organs of animals, mainly liver and kidneys but also the brain. The function of COMT is to eliminate active catechols such as norepinephrine and epinephrine and some other hydroxylated metabolites. In the presence of a DCI (e.g., Carbidopa or Benserazide), COMT is the major antagonist of levodopa in both brain and peripheral system. Inhibiting COMT therefore should conserve therapeutic levodopa, thereby enabling smaller dosage to accomplish a given degree of symptomatic relief in PD, and smoothing out fluctuation of levodopa concentration during the dosage interval. Two COMT inhibitors are presently available: tolcapone (Tasmar) and entacapone (Comtan). Tasmar vs Comtan: Tasmar and Comtan are chemically unrelated. Neither enters the CNS to any great extent, yet both inhibit COMT there in animals. Tasmar about doubles area under curve (AUC) of a levodopa dose, while Comtan raises it only about 35%. Tasmar is restricted due to fear (not asolutely proven) of liver failure, while Comtan is not. The type of liver failure associated with Tasmar led to only 3 deaths out of 60,000 or more users, and has several other known causes, but the cases were so poorly documented that a relation cannot be confirmed. Tasmar is still available in the U.S., but only to special PD patients and with strict requirements for periodic liver function testing and informed patient consent. The warning value of blood tests for liver function is admittedly a bit dubious, but authorities believe it will be effective. The only liver problem with Comtan seems to be that users having preexisting liver impairment metabolize it for excretion more slowly, so that its plasma half-life is longer. Both drugs have reported serious but very rare incidents of NMS-like syndrome on abrupt withdrawal, and rhabdomyolysis, including 1 death related to Tasmar withdrawal. Data are too scarce for firm conclusions in these areas. Tasmar and Comtan are both compatible with Eldepryl. -- J. R. Bruman (818) 789-3694 3527 Cody Road Sherman Oaks, CA 91403-5013