Drug molecules may or may not have physiological counterparts
The vasoconstricting action of angiotensin can also be countered at the membrane receptor directly. One such inhibitor that has been around for quite a while is saralasin (Figure 1.6c).
Saralasin illustrates that the structure of the physiological mediator or substrate is a logical starting point for the syn-thesis of inhibitors. However, it is not a completely satisfac-tory drug, because it cannot be orally applied – can you see why? The more recently developed drug valsartan (Figure 1.6d) is orally applicable, but has very limited similarity to the physiological agonist.
Enalapril and valsartan represent the two practically most important functional groups of drugs, respectively – en-zyme inhibitors, and hormone or neurotransmitter receptor blockers. Another important group of drugs that act on hormone and neutotransmittor receptors are `mimetic' or agonistic drugs. However, there is no clinically useful ex-ample in the reninangiotensin pathway; we will see examples later.
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