To confirm that concomitant administration of aminoglutethimide (AG) reduces plasma levels of medroxypro-gesterone acetate (MPA), MPA levels were assayed in six patients with advanced breast cancer receiving the two agents. Patients had disease resistant to AG and were studied during conversion to therapy with MPA. Hydrocortisone was discontinued at the commencement of study and MPA introduced and given at a constant dose of 800 mg daily while AG was reduced in dose from 250 mg b.d. to 125 mg b.d. and then discontinued. MPA levels were measured after two weeks at each dose of AG and after two weeks on MPA alone. Mean MPA levels showed a progressive and significant (P<0.01) rise as the AG dose was reduced — 180 ng ml-1, 250 ng ml-1and-740 ng ml-1respectively. MPA levels were always in excess of the accepted therapeutic level of 100 ng ml-1and plasma cortisol levels fell in parallel with the rise in MPA level
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