Pharmacology of aminoglutethimide: structure/activity relationships and receptor interactions

The structure/activity relationships for inhibition of aromatase and cholesterol side-chain cleavage enzyme (CSCC) by aminoglutethimide and some of its analogues are reviewed. Although more effective against aromatase than CSCC, aminoglutethimide markedly inhibits both enzymes. Optimal competitive antagonism of aromatase is obtained with a free amino or basic group at the 4' position of the phenyl ring substituent. This is important because two major metabolites of aminoglutethimide where the amino group is conjugated have low or no inhibitory effect on aromatase. Relocation of the free amino group to the nitrogen atom of the piperidinedione ring enhances CSCC inhibition. The piperidinedione ring is not essential for inhibitory activity, several 4'-aminophenyl pyrrolidinediones being as potent as aminoglutethimide for aromatase but less effective against CSCC. In this series 3-(4'-aminophenyl)-pyrrolidine-2,5-dione is a selective aromatase inhibitor. Inactivity of 4'-aminophenyl derivatives of imidazolidinedione and pyrimidinetrione indicate that the 4'-aminophenyl moiety alone is insufficient for antagonist activity against aromatase. Replacement of the aminophenyl group by the stronger basic 4-pyridyl moiety in aminoglutethimide yields a selective aromatase inhibitor. The structure/activity data suggest that there is a critical distance between the basic centre and the dione moiety for effective aromatase inhibition. Aminoglutethimide and its analogues possessing a free basic centre elicit a Type II difference spectrum with various cytochrome P-450 isozymes. The spectrally-determined binding affinity correlates with inhibitory activity of these compounds and indicates binding of the amino group to the iron centre of this haem enzyme. In reviewing the postulated mechanisms for the enzyme-mediated aromatisation of substrates it is seen that the initial oxidative attack is at the C-19 position of the steroid. It is proposed that two binding or association sites on the protohaem moiety of this P-450 enzyme (iron and propionic acid residues respectively) are required for the orientation of the substrate. Crystallographic evidence indicates there is a distance of about 6.3 A between the C-17 oxygen and the C-19 methyl of the aromatase substrates (these points interacting with the association sites). There is a similar distance between the basic function and the C-6 keto group for aminoglutethimide (and the C-5 keto group for the pyrrolidinediones). It is proposed that aminoglutethimide and its active analogues interact with aromatase at these binding sites competitively inhibiting the aromatisation of androstenedione and testosterone.