Sentry drugs---synergism

For example, clavulanic acid inhibits the enzyme 0-lactamase and is therefore able to protect penicillins which are labile to that particular enzyme (Chapter 10). 

Another example is to be found in the drug therapy of Parkinson s disease. The use of L-dopa (levodopa) as a prodrug for dopamine has already been described. However, to be effective, large doses of L-dopa (3-8 g per day) are required, and over a period of time these dose levels lead to side-effects such as nausea and vomiting. L-Dopa is susceptible to the enzyme dopa decarboxylase and as a result, much of the L-dopa administered is decarboxylated to L-dopamine before it reaches the central nervous system (Fig. 8.21). 

As stated earlier, dopamine is unable to cross the blood-brain barrier. As a result, an excess of dopamine builds up in the peripheral blood supply and this is what leads to the nausea and vomiting side-effects. 

If an antagonist was administered to dopa decarboxylase, then it would inhibit the decarboxylation of L-dopa and less would be required. The drug carbidopa has been 

Adrenaline is an example of a sentry drug which acts on a receptor rather than an enzyme. This drug is used along with the injectable local anaesthetic procaine to prolong its action (Fig. 8.22). Adrenaline constricts the blood vessels in the vicinity of the injection and so prevents procaine being washed away by the blood supply. 

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