The impact of dexamethasone pharmacokinetics on the DST: a review

Even though the DST has not proved successful as a marker for depression, it has stimulated a considerable amount of research into the interaction between neuroendocrine function and mood states. With the objective of perfecting the DST methodology, investigators have explored the interaction between dexamethasone plasma concentrations and cortisol response, and have found that there is a significant inverse correlation between dexamethasone concentrations and cortisol concentrations. Although this relationship is one of the factors that affects cortisol response in depressed patients, it usually explains less than 20 percent of the variance of cortisol response. One can only conclude that the affective state explains a certain amount of the remaining variance. Dexamethasone plasma concentrations may be altered by a variety of drug and disease interactions. Many enzyme inducers, including phenytoin, carbamazepine, and phenobarbital, increase dexamethasone CL, but some drugs that might be expected to alter dexamethasone CL, such as cimetidine and tobacco smoke, do not affect it. Any disease that causes hepatic dysfunction could be expected to decrease dexamethasone CL, whereas renal failure may increase dexamethasone CL. Neither Cushing's syndrome nor congenital adrenal hyperplasia appear to alter dexamethasone CL. Alcoholism has a dual effect on the DST. Chronic alcohol abuse may cause a cushingoid state, which could interfere with the DST interpretation. Also, chronic alcohol use may result in hepatic dysfunction, or an induction of P-450 enzymes. As a result of these different actions, alcohol could result in either an increase or decrease in dexamethasone CL. Studies of dexamethasone pharmacokinetics conducted in depressed patients are few, but they generally agree that DST nonsuppressors exhibit an increased dexamethasone CL when compared with suppressors. The only two studies to investigate this population longitudinally report somewhat contradictory results; one study reports an increase in dexamethasone CL following recovery from depression, and the other a decrease. Since only one of the studies was conducted using intravenous dexamethasone, differences in bioavailability might explain some of the differences in results between the two studies. In spite of the unresolved questions, these studies have stimulated research into an entirely new area: the possibility that affective diseases may alter the pharmacokinetics of some drugs.