A SENSITIVE RADIOIMMUNOASSAY FOR FLUDROCORTISONE IN HUMAN PLASMA

Fludrocortisone has been a mainstay of therapy for orthostatic hypoten sion for many years. Clinical experience suggests that there exists a substantial interindividual variation in responsiveness to the drug. T o assess this, we have developed an assay that permits measurement of the low concentrations of fludrocortisone found in human plasma. Fludr ocortisone was detected by radioimmunoassay. A polyclonal rabbit antib ody, raised against dexamethasone which cross-reacts strongly with flu drocortisone, was reacted with either standard or unknown samples in t he presence of [I-125]fludrocortisone-3-TyrNH(2) (synthesized by coupl ing tyrosine amide to fludrocortisone-3-oxime and iodinating with chlo ramine T oxidation). The ED(10), ED(50), and ED(80) were 0.34, 5.0, an d 30 ng/mL of plasma, respectively. The cross reactivity with other 9- fluorinated steroids was found as follows: dexamethasone, 340%; betame thasone, 230%; and triamcinolone, 8%. To preclude an erroneous result, subjects who were pregnant or receiving any steroid medication were e xcluded from the study. The percent cross-reactivity with the main nat urally occurring steroids was as follows: 11-desoxycortisol 3.2%, cort isol 1.1%, DOC 0.3%, pregnenolone 0.1%, corticosterone 0.06%, progeste rone 0.05%, and aldosterone <0.05%. The only compound with potential f or interference, because of its high level in the circulation in the e arly morning, was cortisol. In plasma samples obtained before administ ration of fludrocortisone, levels of fludrocortisone were undetectable even though the plasma cortisol ranged from 40-175 ng/mL. In prelimin ary studies, fludrocortisone was given orally in doses of 0.2 and 2 mg . Timed plasma samples were collected. With doses of 0.2 mg, fludrocor tisone levels peaked at about 2.4 ng/mL after 1-2 h and were undetecta ble by 12 h. With 2 ms, the levels peaked at 17.6-24.5 ng/mL at 1 h an d was undetectable by 24 h. With the 2 mg dose, plasma cortisol fell a t 10 h to 71-87% (14-23 ng/mL) of baseline. This decline was greater t han would have been expected from the diurnal variation in cortisol. T he estimated T-1/2 for fludrocortisone was 1.6 h for the 2 mg dose and 2.4 h for the 0.3 mg dose. This new fludrocortisone assay can now be used to define the pharmacokinetics of this agent.