Effect of diphenylhydantoin on the metabolism of dexamethasone

Immune Mechanisms in Epileptogenesis: Update on Diagnosis and Treatment of Autoimmune Epilepsy Syndromes

Seizures and epilepsy can result from various aetiologies, yet the underlying cause of several epileptic syndromes remains unclear. In that regard, autoimmune-mediated pathophysiological mechanisms have been gaining attention in the past years and were included as one of the six aetiologies of seizures in the most recent classification of the International League Against Epilepsy. The increasing number of anti-neuronal antibodies identified in patients with encephalitic disorders has contributed to the establishment of an immune-mediated pathophysiology in many cases of unclear aetiology of epileptic syndromes. Yet only a small number of patients with autoimmune encephalitis develop epilepsy in the proper sense where the brain transforms into a state where it will acquire the enduring propensity to produce seizures if it is not hindered by interventions. Hence, the term autoimmune epilepsy is often wrongfully used in the context of autoimmune encephalitis since most of the seizures are acute encephalitis-associated and will abate as soon as the encephalitis is in remission. Given the overlapping clinical presentation of immune-mediated seizures originating from different aetiologies, a clear distinction among the aetiological entities is crucial when it comes to discussing pathophysiological mechanisms, therapeutic options, and long-term prognosis of patients. Moreover, a rapid and accurate identification of patients with immune-mediated epilepsy syndromes is required to ensure an early targeted treatment and, thereby, improve clinical outcome. In this article, we review our current understanding of pathogenesis and critically discuss current and potential novel treatment options for seizures and epilepsy syndromes of underlying or suspected immune-mediated origin. We further outline the challenges in proper terminology.

Addisonian crisis in a dog treated with phenobarbitone

BACKGROUND

A 2-year-old intact female Irish Setter was presented with a 1-week history of anorexia, lethargy, vomiting and diarrhoea. Previous medical therapy included a 3-week treatment with phenobarbitone for suspected idiopathic epilepsy. In humans, phenobarbitone accelerates metabolism of both exogenous and endogenous steroids.

CASE REPORT

Based on history, the physical examination showing abnormal mentation and laboratory abnormalities including azotaemia, hyponatraemia and hyperkalaemia, Addisonian crisis was suspected. An adrenocorticotropic hormone stimulation test was performed and confirmed the diagnosis. Treatment with intravenous fluid therapy, glucocorticoids and mineralocorticoids led to a resolution of clinical signs in 3 days.

CONCLUSION

To the authors' current knowledge, this is the first reported case of Addisonian crisis in a dog most probably related to phenobarbitone administration. As Addisonian crisis can be life-threatening, clinicians should be aware of this adverse effect of phenobarbitone and use it cautiously in dogs with borderline hypoadrenocorticism.

Effect of continuous positive airway pressure therapy on hypothalamic-pituitary-adrenal axis function and 24-h blood pressure profile in obese men with obstructive sleep apnea syndrome

Obstructive sleep apnea syndrome (OSAS) increases the risk of cardiovascular events. Sympathetic nervous system and hypothalamic-pituitary-adrenal (HPA) axis activation may be the mechanism of this relationship. The aim of this study was to evaluate HPA axis and ambulatory blood pressure monitoring in obese men with and without OSAS and to determine whether nasal continuous positive airway pressure therapy (nCPAP) influenced responses. Twenty-four-hour ambulatory blood pressure monitoring and overnight cortisol suppression test with 0.25 mg of dexamethasone were performed in 16 obese men with OSAS and 13 obese men controls. Nine men with severe apnea were reevaluated 3 mo after nCPAP therapy. Body mass index and blood pressure of OSAS patients and obese controls were similar. In OSAS patients, the percentage of fall in systolic blood pressure at night (P = 0.027) and salivary cortisol suppression postdexamethasone (P = 0.038) were lower, whereas heart rate (P = 0.022) was higher compared with obese controls. After nCPAP therapy, patients showed a reduction in heart rate (P = 0.036) and a greater cortisol suppression after dexamethasone (P = 0.001). No difference in arterial blood pressure (P = 0.183) was observed after 3 mo of nCPAP therapy. Improvement in cortisol suppression was positively correlated with an improvement in apnea-hypopnea index during nCPAP therapy (r = 0.799, P = 0.010). In conclusion, men with OSAS present increased postdexamethasone cortisol levels and heart rate, which were recovered by nCPAP.

Alterations in the distribution and orexigenic effects of dexamethasone in CAR-null mice

The constitutive androstane receptor (CAR, NR1I3) has emerged as an important regulator of drug metabolism. CAR responds to a wide spectrum of xenobiotics by inducing expression of cytochrome P450 (CYP) enzymes and a number of other proteins responsible for drug metabolism in the liver. The xenosensor function of CAR overlaps with that of the pregnane X receptor (PXR), another xenobiotic receptor that belongs to the nuclear hormone superfamily. We observed that injection of dexamethasone (Dex), a ligand for the glucocorticoid receptor (GR) and PXR but not CAR, results in an unexpected twofold increase in the stomach weight of CAR-null animals relative to wild-type animals. Here, we show that CAR knockout mice have elevated levels of Dex in the brain, resulting in a more rapid and robust increase in the hypothalamic expression of the GR-responsive target genes encoding neuropeptide Y (NPY) and neuropeptide Y receptor subtype 1 (NPY-R1). As expected, this is accompanied by a higher increase in the food intake of the CAR-null animals. The data described here highlight the complexity of the overlapping functions of CAR and PXR.

Troglitazone induces CYP3A4 activity leading to falsely abnormal dexamethasone suppression test

After evaluating a patient who appeared to have a falsely abnormal response to the dexamethasone suppression test while taking troglitazone, we examined the effects of troglitazone on the activity of hepatic CYP3A4 and the screening tests for Cushing's syndrome. We studied five healthy women and three healthy men, aged 25 +/- 2 yr, before and after treatment with troglitazone (600 mg daily) for 28 d. Baseline 0800 h cortisol and corticosterone were similar before and after troglitazone treatment. Before troglitazone treatment, all subjects suppressed 0800 h cortisol below 1.8 micro g/dl (mean, 0.66 +/- 0.08 micro g/dl) during the 1-mg overnight dexamethasone suppression test (DST), whereas during troglitazone treatment none of the subjects suppressed 0800 h cortisol below 1.8 micro g/dl (mean, 9.0 +/- 1.8 micro g/dl). Serum dexamethasone levels decreased by 66 +/- 4%, and the erythromycin breath test measurements increased by 27 +/- 8%, indicating increased CYP3A4 activity during troglitazone treatment. The hydrocortisone suppression test (HST) was performed by administering 50 mg hydrocortisone at 2300 h. Using the criterion of suppression of 0800 h plasma corticosterone by more than 50%, the specificity of the HST was 100% both before and after troglitazone treatment. In conclusion, troglitazone induced the activity of CYP3A4 leading to falsely abnormal DST. HST is a useful alternative to the DST in patients taking medications that increase the activity of CYP3A4.

Topical corticosteroid-induced adrenocortical insufficiency: clinical implications

Topical corticosteroids are often prescribed for the treatment of dermatological disorders. However, systemic adrenal insufficiency may result from their overuse. Current literature detailing both laboratory and clinical findings was analyzed, in the hopes of evaluating the health-risk potential of topical corticosteroids in producing adrenal insufficiency. Risk factors identified in this report included use of high potency corticosteroids, occlusive or prolonged treatment application, and use in thin-skinned areas. Other patients at risk for development of adrenal suppression include infants and those with damaged skin barriers. Several diagnostic tests may be utilized to measure adrenal function, though each has its limitations. The most common tests include measurement of plasma total cortisol, 24-hour steroid, adrenocorticotrophin hormone stimulation and insulin tolerance. The report finds strong laboratory evidence of adrenal hypofunction. Additionally, clinical reversible adrenal insufficiency has been observed on rare occasions. Therefore, topical corticosteroids should be used with an increased awareness of the potential for systemic adrenal suppressive effects. However, since nonreversible clinical secondary adrenocortical-insufficiency disease has not been clearly documented with even class I topical corticosteroids, native adrenal supplementation in periods of stress appears unnecessary; rare exceptions cannot be excluded.

Effect of phenobarbitone on the low-dose dexamethasone suppression test and the urinary corticoid: creatinine ratio in dogs

OBJECTIVES

To investigate potential effects of phenobarbitone on the low-dose dexamethasone suppression (LDDS) test and urinary corticoid to creatinine ratio in dogs in a controlled prospective study and in a clinical setting.

ANIMALS

Ten crossbreed experimental dogs and 10 client-owned dogs of mixed breeds treated chronically with phenobarbitone to control seizures.

PROCEDURES

Experimental dogs were allocated to treatment (6 mg/kg oral phenobarbitone, n = 6) and control (n = 4) groups. LDDS tests (dexamethasone 0.01 mg/kg intravenously, cortisol concentration determined at 0, 2, 4, 6 and 8 h) were conducted repeatedly over a 3-month period. Urinary corticoid to creatinine ratios were measured before LDDS tests. A single LDDS test was performed on 10 epileptic dogs.

RESULTS

LDDS and urinary corticoid to creatinine ratios in dogs were not affected by treatment with phenobarbitone.

CONCLUSIONS

Phenobarbitone does not interfere with LDDS testing regardless of dosage or treatment time. Urinary corticoid to creatinine ratios are also unaffected.

The effects of therapy with rifampicin and isoniazid on basic investigations for Cushing's syndrome

Rifampicin(R) is a potent enzyme inducer which is now widely used to treat many conditions. We have investigated its effect on adrenal function tests in 18 adults with tuberculosis on rifampicin (R) and isoniazid (INH) as in-patients. Midnight cortisol was above 250 nmol/l in 11 patients with a mean +/- (SD) of 340 +/- 193 nmol/l. The morning serum cortisol following 1 and 4 mg dexamethasone overnight was abnormal in 88.9 per cent and 83.3 per cent respectively. The respective mean values +/- (SD) were 350 +/- 179 and 336 +/- 279 nmol/l. The mean 24-hour urinary free cortisol +/- SD was 513 +/- 375 nmol and was above normal in 6 (33.3 per cent) patients. These results show that midnight cortisol, dexamethasone suppression tests and 24-hour urinary free cortisol are misleading in patients with tuberculosis on (R) and should not be employed for investigating such patients for Cushing's syndrome.

The hypothalamic-pituitary-adrenal axis in obesity

The hypothalamic-pituitary-adrenal (HPA) axis normally maintains the concentration of cortisol within a narrow range with a diurnal variation characterized by higher cortisol concentrations in the morning and reduced levels in the evening. Excessive or deficient secretion of cortisol is associated with pathologic changes. Obesity has been linked with age, sex and racial alterations in the functioning of the HPA axis which are reviewed. The possible relationship of altered HPA axis activity with the long-term complications of obesity are considered.

Advances in the diagnosis and treatment of Cushing's syndrome

Excess endogenous glucocorticoid production, whether ACTH-dependent or ACTH-independent, results in the classic clinical and biochemical picture of Cushing's syndrome. The diagnosis requires demonstration of an increased cortisol secretion rate, best achieved using determination of urinary free cortisol as an index. In mild cases, distinction from the hypercortisolism of pseudo-Cushing states may prove difficult. If the physician is in doubt, a dexamethasone/CRH test should be performed. Primary adrenal Cushing's syndrome can be diagnosed on the basis of undetectable plasma ACTH and the results of adrenal imaging procedures. ACTH-dependent Cushing's syndrome can be differentiated using an oCRH test and imaging procedures. In the presence of a discrete pituitary lesion on imaging, a standard oCRH test with results consistent with such a lesion is sufficient to proceed to transsphenoidal surgery. In the absence of such a lesion or if the oCRH test is equivocal, simultaneous BIPSS with oCRH administration should be performed to distinguish between a pituitary or ectopic source. Surgical ablation is the treatment of choice for all types of Cushing's syndrome. In the 5% of cases with Cushing's disease in whom transsphenoidal surgery fails and in the 5% of cases in whom the disease recurs, repeat transsphenoidal surgery or radiation therapy in association with mitotane treatment are reasonable alternatives. Bilateral adrenalectomy effectively cures hypercortisolism if resection of the ACTH-secreting tumour is unsuccessful and radiation/medical therapy fails.

Antiepileptic drugs. A review of clinically significant drug interactions

Approximately 20 to 30% of patients with active intractable epilepsy are commonly treated with polytherapy antiepileptic drug regimens, and these patients may experience complicated drug interactions. Furthermore, because of the long term nature of treatment, the possibility of drug interactions with drugs used for the treatment of concomitant disease is high. Classically, clinically significant drug interactions, both pharmacokinetic and pharmacodynamic, have been considered to be detrimental to the patient, necessitating dosage adjustment. However, this need not always be the case. With the introduction of new drugs (e.g. vigabatrin and lamotrigine) with known mechanisms of action, the possibility exists that these can be used synergistically. The most commonly observed clinically significant pharmacokinetic interactions can be attributed to interactions at the metabolic and serum protein binding levels. The best known examples relate to induction (e.g. phenobarbital, phenytoin, carbamazepine and primidone) or inhibition [e.g. valproic acid (sodium valproate)] of hepatic monoxygenase enzymes. The extent and direction of interactions between the different antiepileptic drugs are varied and unpredictable. Interactions in which the metabolism of phenobarbital, phenytoin or carbamazepine is inhibited are particularly important since these are commonly associated with toxicity. Some inhibitory drugs include macrolide antibiotics, chloramphenicol, cimetidine, isoniazid and numerous sulphonamides. A reduction in efficacy of antibiotic, cardiovascular, corticosteroid, oral anticoagulant and oral contraceptive drugs occurs during combination therapy with enzyme-inducing antiepileptic drugs. Discontinuation of the enzyme inducer or inhibitor will influence the concentrations of the remaining drug(s) and may necessitate dosage readjustment.

Effects of antiepileptic drugs on hormones

A hormone is an intrinsic substance carried via the blood to a target organ which is then functionally stimulated. Similar to extrinsically administered medications, the metabolism and function of the hormones may be altered by antiepileptic drugs (AEDs). The proposed mechanisms are (a) enhanced metabolism (natural steroids, synthetic steroids, e.g., decadron and birth control pills, thyroxine, and vitamin D3), (b) altered protein bonding (thyroxine, sex hormones), (c) impaired release into the systemic circulation (calcitonin, insulin, vitamin K clotting factors) and (d) altered end-organ effect. The AEDs most likely to interact with hormones are barbiturates, carbamazepine, and phenytoin.

The overnight dexamethasone test is a worthwhile screening procedure

The overnight low-dose dexamethasone test is a convenient screening procedure for Cushing's syndrome. Claims that the test is associated with a high incidence of 'false positives', rendering it of little value particularly in obese and hospital in-patients, have been investigated in the present study. The data from 100 consecutive subjects undergoing overnight low-dose dexamethasone tests to examine for the possibility of Cushing's syndrome, were reviewed. Cushing's syndrome was identified in four patients, normal suppression of cortisol values occurred in 84 patients and 12 patients exhibited false positive results. Differences in body weights, body mass indices or in-patient status did not distinguish between those subjects with normal suppression of plasma cortisol and those subjects who yielded false positive results. These data indicate that the simple overnight dexamethasone test substituted for the more cumbersome traditional 48-h dexamethasone test in 84 of 96 patients who did not have Cushing's syndrome. Thus the overnight test provides a useful screening procedure but a small percentage of patients, approximately 12.5%, will require additional procedures to exclude Cushing's syndrome.

Enzyme induction and inhibition

The rate and extent of drug metabolism significantly influences drug effect. Enzyme induction by increasing the metabolism of drugs may result in important drug interactions. Other implications of enzyme induction include alterations in the metabolism of endogenous substrates, vitamins and activity of extrahepatic enzyme systems. Similarly a wide range of drugs may produce clinically significant drug interactions following enzyme inhibition. Assessment of enzyme induction and inhibition in man involves diverse methods including the use of model drugs.

Clinically relevant anti-epileptic drug interactions

Anti-epileptic drugs frequently interact due to pharmacokinetic features (induction or inhibition of metabolism, production of active metabolites, low therapeutic indices) and the need for prolonged treatment with possible addition of other drugs to treat concomitant diseases. The most important pharmacokinetic interactions are those that inhibit phenytoin, carbamazepine and phenobarbitone metabolism and thus increase their toxicity. Drugs inhibiting metabolism include antibiotic macrolides, chloramphenicol, isoniazide, some sulphonamides, propoxyphene, cimetidine, valproic acid and sulthiame. Anti-epileptic drugs can induce hepatic microsomal enzymes and, therefore, may increase metabolism of corticosteroids, oral contraceptives, oral anticoagulants, cardiovascular agents, antibiotics, chemotherapeutic agents, psychotropic drugs and non-opiate analgesics, thereby reducing their efficacy. Advantageous pharmacodynamic interactions include synergism of ethosuximide plus valproic acid and of carbamazepine plus valproic acid. A pharmacodynamic mechanism may be responsible for the reduced sensitivity of chronically treated epileptics to some neuromuscular blockers.

The effect of anticonvulsants on the dexamethasone suppression test

Rates of non-suppression on the DST were compared in 19 psychiatric inpatients and anticonvulsants and 38 psychiatric inpatients not on anticonvulsants who were matched for age, sex, and diagnosis. Patients on anticonvulsants had a significantly higher rate of nonsuppression.

Specific determination of plasma dexamethasone by HPLC and RIA--application to standard dexamethasone suppression test in psychiatric patients

Three radioimmunoassays (RIA), with or without preparative HPLC, were applied to the monitoring of plasma dexamethasone (DXM) levels during standard dexamethasone suppression test (DST) in psychiatric patients. Due to the robotic ease of the fully automated HPLC process, precision of the chromatographic assay was equivalent to that of the direct assays, but prepurification improved both sensitivity and specificity. These improvements allowed the elucidation of the following features: (1) half (36) of the patients (68) displayed infranormal DXM levels (less than or equal to 0.40 ng/ml) whatever the cortisol response; (2) 22% (15) patients (68) with DXM levels in the low control range showed a strong inhibition of cortisol suppression. These observations raise some doubts on the validity of the DST test and introduce the following questions. (1) What is the dependence of cortisol suppression upon DXM absorption and catabolism? (2) Does plasma DXM measurement several hours after its physiological action still reflect its effect on the hypothalamo-hypophyseal axis? (3) What is the reliability of DXM direct assays when measuring low DXM levels in the presence of high cortisol?

Cutaneous and immunologic reactions to phenytoin

Phenytoin (diphenylhydantoin; Dilantin) is a highly effective and widely prescribed anticonvulsant and antiarrhythmic agent. Since 1938 it has been invaluable in the treatment of grand mal and psychomotor epilepsy. Hydantoin derivatives have been used medicinally for more than a half-century. In recent years dermatologists have broadened the indications for phenytoin use to include recessive dystrophic epidermolysis bullosa, linear scleroderma, and pachyonychia congenita. In spite of widespread use and popularity, it is interesting that the frequency of complications relating to drug therapy remains low, relatively speaking. Nevertheless, a broad spectrum of cutaneous and immunologic reactions to phenytoin have been reported. These range from tissue proliferative syndromes (side effects), drug hypersensitivity syndromes (allergic effects), and a possible linkage with lymphoma (idiosyncratic effects). Therapeutic and toxic reactions to this commonly prescribed drug are comprehensively reviewed, analyzed, and summarized in this monograph.

Dexamethasone bioavailability: implications for DST research

The bioavailability of dexamethasone (DEX) has recently been demonstrated to be a critical factor in determining Dexamethasone Suppression Test (DST) status in psychiatric patients. This brief review focuses on several aspects of DEX bioavailability as they relate to the use of the DST in neuroendocrine research. Several methodologies, including radioimmunoassay, high-performance liquid chromatography, and gas chromatography-mass spectrometry are available for quantification of DEX in biological fluids, although few detailed comparisons between methods have been reported. Surprisingly, little systematic research on the metabolism of DEX has been reported, but it appears that hepatic rather than renal mechanisms are the major source of DEX elimination. The marked variability in serum DEX levels following oral administration in psychiatric patients is also observed in normal controls and patients with Cushing's syndrome. A variety of drugs can modify serum DEX levels and thereby after the effectiveness of DEX in suppressing serum cortisol levels. Simultaneous measurement of serum DEX and cortisol levels appears to be necessary for the appropriate evaluation of DST results. This procedure may help explain many of the inconsistencies in recent DST research.

[Factors interfering with the dexamethasone suppression test]

The interpretation of the dexamethasone suppression test in endocrinology and psychiatry depends on several variables. False-positive results can be caused by stress, weight loss, alcohol withdrawal, treatment with diphenylhydantoin, phenobarbital, rifampicin, carbamazepine and lithium. Therapy with spironolactone, naloxone, alpha 1-mimetic agents and estrogen can be responsible for an increase in plasma-cortisol concentration. False-negative results are seen in patients with liver disease and can also result from therapy with benzodiazepines at high dosages, indomethacin and possibly methadone and ketoconazole.

Adverse effect of phenytoin on mineralocorticoid replacement with fludrocortisone in adrenal insufficiency

Two patients with longstanding adrenal insufficiency developed severe mineralocorticoid deficiency during concomitant phenytoin treatment. A 64-year-old man with primary adrenal insufficiency of 41 years duration was treated with phenytoin for an acute seizure disorder. He subsequently developed mineralocorticoid insufficiency despite taking his customary dosages of cortisone acetate and fludrocortisone. This responded to volume repletion and increased fludrocortisone requirement from 0.05 mg to 0.4 mg daily, which decreased to the former amount following discontinuation of phenytoin. A 42-year-old woman with primary adrenal insufficiency of 3 years duration and a lifelong seizure disorder treated with phenytoin incurred multiple, life-threatening episodes of mineralocorticoid insufficiency. Her fludrocortisone requirement was ultimately established as 2.0 mg daily with a normal hydrocortisone requirement and clearance rate. Fludrocortisone thus appears to be another synthetic steroid whose metabolism is sensitive to drugs that increase hepatic 6-beta-hydroxylation, such as phenytoin. Treatment with these inducing drugs may markedly alter mineralocorticoid requirements in patients with primary adrenal insufficiency.

Restoration of dexamethasone suppression by incomplete adenomectomy in Cushing's disease

Two cases of pituitary-dependent Cushing's disease are described. Selective transphenoidal adenomectomy was attempted in each, but proved incomplete. Although both patients had persistent Cushing's disease after the operation, both were shown to have been rendered fully dexamethasone suppressible. The pathophysiological basis of partial suppressibility is discussed, as are the criteria for the diagnosis of cure in Cushing's disease treated by selective adenomectomy.

Interactions between anticonvulsants and other commonly prescribed drugs

Many drug interactions can be demonstrated, but only a few are so clinically significant that they necessitate adjusting drug dosages. The same drug combination may produce changes of variable extent or direction in different individuals. The reasons for this variability include genetic control of the rate and inducibility of drug metabolism, and environmental factors such as contact with chemicals. Among antimicrobial agents, chloramphenicol may cause accumulation of phenytoin (PHT) and phenobarbital (PB), and isoniazid may cause PHT, carbamazepine (CBZ), and primidone (PRM) to accumulate. Erythromycin may cause accumulation of CBZ. Among anti-ulcer agents, antacids may reduce PHT concentration while cimetidine may cause accumulation of PHT, CBZ, and diazepam (DZP). Salicylates displace strongly binding drugs such as PHT, DZP, or valproate (VPA) from the binding sites in plasma proteins, which may lead to some decline of the total plasma level with an increase in the unbound drug percentage. Conversely, anticonvulsants may influence the dosage requirements of oral anticoagulants by inducing their metabolism. Failures of oral contraceptives have been attributed to anticonvulsants in some patients. Probably the most predictable interaction that necessitates dosage adjustment is accumulation of PB caused by VPA. Intentional inhibition of PRM metabolism by nicotinamide serves as an example of attempts to utilize an interaction for improved therapeutic effect.

Cortisol production during high dose dexamethasone therapy in neurological and neurosurgical patients

Simultaneous plasma dexamethasone and cortisol levels were followed at intervals over 8 hour periods on 40 occasions in 19 subjects who received regular high dosage dexamethasone therapy (rarely less than 12 mg a day) for various neurological and neurosurgical conditions. Lower dexamethasone doses (for example 2 mg daily for 2 days) normally suppress adrenal cortical production of cortisol to below 50 micrograms/l for at least 8 hours. However, in 12 of the 35 studies that did not take place at the first steroid dose or in subjects taking second daily bolus steroid dosage such suppression was not present 8 to 12 hours after dexamethasone intake, though it was shown that dexamethasone could suppress cortisol production in all these cases. Failure of maintained suppression despite the high steroid dose appeared to be related to rapid elimination of dexamethasone. These findings may help explain the relative rarity of adrenal failure in clinical neurological practice after high dosage steroid therapy is ceased.

Phenytoin impairs the bioavailability of dexamethasone in neurological and neurosurgical patients

Plasma concentration-time data after oral and intravenous administration of dexamethasone have been subjected to pharmacokinetic analysis in six neurological or neurosurgical patients taking the steroid with phenytoin, and in nine patients (one studied twice) taking dexamethasone without phenytoin. An additional patient was studied before and during phenytoin intake. Apparent volume of distribution was similar in the two groups, but the group treated with phenytoin had an almost statistically significantly shorter dexamethasone mean terminal half-life, an approximately trebled mean plasma clearance, and a mean oral bioavailability of the steroid of only 33%, compared with a mean 84% oral bioavailability in those not receiving phenytoin. To achieve a given plasma dexamethasone concentration, patients treated with the steroid and phenytoin may need oral dexamethasone doses several times those required by patients not receiving phenytoin.

Disposition of total and unbound prednisolone in renal transplant patients receiving anticonvulsants

Kidney transplant patients receiving phenytoin or phenobarbital may have decreased graft survival. These drugs have been shown to enhance the metabolism of glucocorticoids. We determined the disposition of total and unbound prednisolone in six stable kidney transplant patients receiving prednisone for immunosuppression and phenytoin or phenobarbital for a seizure disorder. Six similar patients not on anticonvulsants served as controls. A single intravenous dose of prednisolone was administered, and plasma samples were analyzed for prednisolone using a high-performance liquid chromatographic assay. Equilibrium dialysis was used to determine unbound prednisolone concentrations. Pharmacokinetic analysis showed that the half-life of prednisolone was shorter in the anticonvulsant group compared to the controls, based on both total (2.3 +/- 0.4 vs. 3.4 +/- 0.2 hr (SD), P less than 0.01) and unbound (1.7 +/- 0.3 vs. 2.4 +/- 0.2 hr, P less than 0.01) concentrations. Total drug clearance was 10.4 +/- 2.8 liters/hr (0.171 +/- 0.087 liters/hr X kg) in the anticonvulsant group versus 7.2 +/- 1.2 liters/hr (0.100 +/- 0.014 liters/hr X kg) in the controls (P less than 0.05). Unbound prednisolone clearance was 57.2 +/- 12.1 versus 46.4 +/- 8.7 liters/hr (P greater than 0.05) and for weight-corrected estimates 0.886 +/- 0.224 liters/hr X kg versus 0.644 +/- 0.115 liters/hr X kg (P less than 0.05) in the two groups, respectively. Thus, the disposition of prednisolone is altered by anticonvulsants in kidney transplant patients and may require dose alteration.

Bioavailability of oral dexamethasone during high dose steroid therapy in neurological patients

The pharmacokinetics and oral biovailability of dexamethasone were studied in 6 patients with neurological disease being treated with high dosages of the drug. A specific high performance liquid chromatographic assay was used to measure dexamethasone concentrations. Unlike the previously published mean figure of 0.78 for the oral bioavailability of the drug given in single doses to healthy volunteers, the mean bioavailability of dexamethasone in the patients studied was 0.53 +/- SD 0.40. It appeared more likely that this incomplete bioavailability was due to presystemic elimination than to poor absorption. The intravenous clearance of the drug was relatively high (0.4902 +/- SD 2291 1 kg-1, approximately 65% of expected hepatic plasma flow), the oral clearance higher (2.5804 +/- SD 3.2181 1 kg-1 h-1) while the absorption rate constant (4.8729 +/- 8.4998 h-1), suggested rapid absorption after oral administration. Prior phenytoin and possibly prior dexamethasone therapy is likely to have contributed to the higher clearance values of the drug in these patients than the values reported in healthy volunteers after single dose studies.

Pharmacokinetic interactions with antiepileptic drugs

A large number of pharmacokinetic interactions with antiepileptic drugs have been reported in recent years. Among the interactions affecting the disposition of anticonvulsants, the most important are probably those resulting in inhibition of the metabolism of phenytoin, phenobarbitone and carbamazepine. Drugs which have been shown to inhibit the metabolism of these anticonvulsants and to precipitate clinical signs of intoxication in epileptic patients include sulthiame, valproic acid, chloramphenicol, certain sulphonamides, phenylbutazone, isoniazid and propoxyphene. Interactions affecting the plasma protein binding of antiepileptic drugs are less likely to cause long-lasting alterations in response, but they are important because they change the relationship between serum drug concentrations and clinical effect. Anticonvulsant agents may induce important alterations in the pharmacokinetics of other drugs. Phenytoin and phenobarbitone may decrease the gastrointestinal absorption of frusemide and griseofulvin, respectively. Many of the drugs used in the treatment of the adult epilepsies, including phenytoin, phenobarbitone, primidone and carbamazepine, are potent inducers of the hepatic microsomal enzymes. This results in an increased rate of metabolism and decreased clinical efficacy of a number of drugs, including dicoumarol, steroid oral contraceptives, metyrapone, glucocorticoid agents, doxycycline, quinidine and vitamin D.

Drug interactions with phenytoin

Drug interactions with phenytoin are a frequent occurrence, although their clinical relevance has often been overemphasised. Probably the most important of such interactions are those resulting in inhibition of phenytoin metabolism: due to the saturable nature of phenytoin biotransformation even minor degrees of inhibition can produce disproportionate changes in both steady-state serum concentration and the magnitude of pharmacological effect. Phenytoin has marked enzyme-inducing properties and can stimulate the metabolism of many concurrently administered drugs, thereby reducing their therapeutic efficacy. Clinically important examples of such interactions include a reduction of the anticoagulant effect of dicoumarol, a decrease in the prophylactic efficacy of the contraceptive pill and failure of response to various corticosteroid agents when administered therapeutically or diagnostically. Unless complicated by additional mechanisms, plasma protein binding interactions with phenytoin are seldom of clinical significance. However, they may alter considerably the relationship between serum drug concentration and clinical response, a possibility which needs to be taken into account when interpreting serum phenytoin levels in clinical practice.

Effect of smoking on prednisone, prednisolone, and dexamethasone pharmacokinetics

The pharmacokinetics of oral prednisone and oral dexamethasone were examined in 18 healthy male adults. Eight subjects also received intravenous prednisolone and intravenous dexamethasone. Half of each group were cigarette smokers as confirmed by plasma thiocyanate concentrations. Plasma and urine concentrations of prednisone and prednisolone were assayed by high performance liquid chromatography, while plasma dexamethasone was measured by radioimmunoassay. There were no statistically significant differences between smokers and nonsmokers in the systemic availability of prednisolone (75 versus 84%), oral dose clearance of prednisone (29 verus 27 ml/min/kg), systemic prednisolone clearance (2.8 versus 2.9 ml/min/kg), or in the interconversion rates, volumes of distribution, or urinary recoveries of prednisone and prednisolone. Similarly, the pharmacokinetics of dexamethasone were unaffected by smoking. A limited correlation (r = 0.55) was found between the high oral dose clearances of prednisone and the lower values of dexamethasone (6.73 and 5.71 ml/min/kg in smokers and nonsmokers). A two- to threefold variability occurred in oral dose clearances of each steroid with partial intrasubject covariance. Unlike the anticonvulsants, which markedly induce corticosteroid metabolism, smoking has no effect on their pharmacokinetics and should not complicate therapy with these drugs.

Clinical implications of enzyme induction and enzyme inhibition

The pharmacological effect of a drug is partly dependent upon its concentration at its site of action, which in turn is partly dependent upon its rate of elimination. The rate of elimination of many lipophilic drugs is governed by the activity of the hepatic microsomal mixed-function oxidases. Consequently any alteration in the activity of these enzymes may result in a modification of drug action. A wide range of chemically unrelated substances may stimulate the activity of the mixed-function oxidases by enzyme induction. The drugs most frequently encountered as enzyme-inducing agents in man are barbiturates, rifampicin and phenytoin. Enhancement of drug metabolism by ethanol, tobacco smoking and diet may also involve enzyme induction. Enzyme induction is normally associated with a reduction in the drug efficacy but may also alter the toxicity of certain substances. Enzyme induction has been assessed in man by measuring changes in the pharmacokinetics of a marker drug, or changes in the disposition of endogenous compounds such as gamma-glutamyltranspeptidase, D-glucaric acid and 6beta-hydroxycortisol. The therapeutic problems associated with enzyme inhibition have received much less attention than those associated with enzyme induction. The effect on the rate of elimination of a particular drug will depend upon the fraction of the dose that is normally metabolised by the inhibited enzyme and on the affinity of the enzyme for the drug and the inhibitor. An alteration in the dosage schedule is usually only necessary for drugs with a small therapeutic ratio.

Influence of phenylbutazone, mofebutazone and aspirin on the pharmacokinetics of dexamethasone in the rat

Phenylbutazone suppresses the C-6 hydroxylation, absorption rate, bioavailability, and renal and plasma clearanceè rates of dexamethasone administered orally to normal and oedemateous rats. It increases the half life and the volume of distribution. Aspirin exerts an effect which is less pronounced and involves the enhancement of the C-6 hydroxylation. Aspirin suppresses the half life and renal clearance of dexamethasone and enhances its hepatic clearance. Mofebutazone does not exert any pronounced influence. Also, unlike phenylbutazone, it does not interfere with the gastrointestinal absorption of dexamethasone. More rapid onset of absorption, decrease of half life and increase of the contribution of renal clearance to total plasma clearance of dexamethasone, are characteristics of the oedematous condition in the rat. The contribution of renal clearance to the elimination of dexamethasone is much greater in the rat than in human subjects. The presence of a third unconjugated metabolite of dexamethaone in the urine of rat has been demonstrated.

Pharmacokinetics and bioavailability of prednisone and prednisolone in healthy volunteers and patients: a review

Limited information is available on the pharmacokinetics and bioavailability of prednisone and prednisolone in patients with different disease states. This is partly due to difficulty in measuring these drugs in biological fluids at the usual dosages prescribed to patients. This article attempts to comprehensively review these studies categorized under the following four sections: (1) bioavailability--healthy volunteers, patients with respiratory disease, patients with liver disease, patients with kidney disease, pediatric patients with various diseases, effect of antacids, effect of food, effect of other drugs (aminophylline, cholestyramine); (2) pharmacokinetics--healthy volunteers, patients with respiratory disease, patients with liver disease, patients with kidney disease, pediatric patients with various diseases, effect of other drugs, enzyme induction of steroids and the effect on the kinetics of steroids and other drugs; (3) protein binding; and (4) analytical methods. The literature is reviewed through August 1979.

Cushing's syndrome: a review of diagnostic tests

This review presents an analysis and interpretation of the published experimental data that form the basis for laboratory tests commonly used for screening, definitive diagnosis, and differential diagnosis in Cushing's syndrome. The single-dose overnight dexamethasone suppression test is excellent for screening outpatients since this test has a very low incidence of false-negative results (1.9% of 154 patients with Cushing's syndrome). The definitive diagnosis of Cushing's syndrome is best established by combining basal state measurements of the daily urine-free cortisol excretion and late evening plasma cortisol levels with the 2-mg low-dose dexamethasone suppression test. The etiology of Cushing's syndrome is best determined by combining measurements of basal state plasma adrenocorticotropin (ACTH) levels with the 8-mg high-dose dexamethasone suppression test. Under certain conditions, the basal state daily urine excretion of 17-hydroxycorticosteroids and 17-ketogenic steroids, the insulin tolerance test, and the metyrapone test may be useful in the definitive or differential diagnosis of Cushing's syndrome.