Effect of diphenylhydantoin on the metabolism of metyrapone and release of ACTH in man

A prospective study on the incidence and predictive factors of relative adrenal insufficiency in Korean critically-ill patients

This study was undertaken to evaluate the incidence and risk factors associated with relative adrenal insufficiency (RAI) in Korean critically-ill patients. All patients who were admitted to the Medical Intensive Care Unit (MICU) of Samsung Medical Center between January 1, 2006 and April 30, 2007 were prospectively evaluated using a short corticotropin stimulation test on the day of admission. RAI was defined as an increase in the serum cortisol level of <9 microg/dL from the baseline after administration of 250 microg of corticotropin. In all, 123 patients were recruited and overall the incidence of RAI was 44% (54/123). The presence of septic shock (P=0.001), the Simplified Acute Physiology Score (SAPS) II (P=0.003), the Sequential Organ Failure Assessment (SOFA) score (P=0.001), the mean heart rate (P=0.040), lactate levels (P=0.001), arterial pH (P=0.047), treatment with vasopressors at ICU admission (P=0.004), and the 28-day mortality (P=0.041) were significantly different between patients with and without RAI. The multivariate analysis showed that the SOFA score was an independent predictor of RAI in critically-ill patients (odd ratio=1.235, P=0.032). Our data suggest that RAI is frequently found in Korean critically-ill patients and that a high SOFA score is an independent predictor of RAI in these patients.

Reduced plasma nisoldipine concentrations in phenytoin-treated patients with epilepsy

PURPOSE

To assess whether phenytoin affects the pharmacokinetics of the dihydropyridine calcium antagonist nisoldipine.

METHODS

Twelve patients with epilepsy receiving chronic phenytoin therapy and 12 healthy control subjects matched for age and gender received a single oral dose of nisoldipine (40 and 20 mg, respectively). Blood samples were collected for up to 48 h for estimation of plasma nisoldipine levels by capillary gas chromatography.

RESULTS

Mean plasma nisoldipine concentrations were much lower in the patients. Geometric means for areas under the concentration-time curve (AUC0-tn) normalized to a 20-mg dose were 1.6 micrograms/L/h (95% confidence intervals, 0.6-3.8 micrograms/L/h) in the patients compared with 15.2 (10.7-21.6) micrograms/L/h in control subjects (p < 0.002).

CONCLUSIONS

These results suggest that phenytoin increases the first-pass metabolism of nisoldipine to a clinically important extent. In view of the magnitude and variability of interaction, use of nisoldipine in patients receiving chronic phenytoin therapy is contraindicated.

The effects of hepatic microsomal enzyme inducers on the pharmacokinetics of ouabain after portal and systemic administration to rats

The microsomal enzyme inducers 3-methylcholanthrene, phenobarbitone and pregnenolone-16alpha-carbonitrile (PCN) are known to affect other aspects of hepato-biliary disposition in addition to metabolism. This study was designed to determine if presystemic elimination of the non-metabolized xenobiotic ouabain could be altered by these inducers. Male Sprague-Dawley rats were pretreated with inducers or saline for four days. A day later, ouabain (0.5 mg kg-1) was administered into either the ileocolic vein (portal administration) or the femoral vein (systemic administration). Blood and bile samples were collected for up to 90 min after ouabain administration. Biliary excretion rate and cumulative biliary excretion of ouabain were increased by pretreatment with PCN (75 mg kg-1 day-1) relative to controls. Phenobarbitone pretreatment (75 mg kg-1 day-1) also increased these parameters, but to a lesser extent than PCN. In contrast, 3-methylcholanthrene pretreatment (20 mg kg-1 day-1) had no effect on biliary excretion. Plasma concentrations of ouabain were much lower after PCN pretreatment relative to controls, whereas neither phenobarbitone nor 3-methylcholanthrene had any effect. Similarly, clearance (both biliary and total) and volume of distribution were increased by PCN, but not by phenobarbitone or 3-methylcholanthrene pretreatment. Interestingly, the magnitude of biliary and plasma effects induced by PCN appeared to be comparable whether ouabain was administered portally or systemically. Pretreatment of rats with PCN, but not phenobarbitone or 3-methylcholanthrene was shown to increase total clearance of ouabain, mainly via an increase in biliary clearance. Furthermore, because the enhanced clearance occurs after systemic as well as after portal administration of ouabain, a significant change in hepatic presystemic elimination was not detected.

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.

Differential effects of valproic acid and enzyme-inducing anticonvulsants on nimodipine pharmacokinetics in epileptic patients

1. The single dose pharmacokinetics of orally administered nimodipine (60 mg) were investigated in normal subjects and in two groups of epileptic patients receiving chronic treatment with hepatic microsomal enzyme-inducing anticonvulsants (carbamazepine, phenobarbitone or phenytoin) and sodium valproate, respectively. 2. Compared with the values found in the control group, mean areas under the plasma nimodipine concentration curve were lowered by about seven-fold (P less than 0.01) in patients taking enzyme-inducing anticonvulsants and increased by about 50% (P less than 0.05) in patients taking sodium valproate. 3. Nimodipine half-lives were shorter in enzyme-induced patients than in controls (3.9 +/- 2.0 h vs 9.1 +/- 3.4 h, means +/- s.d., P less than 0.01), but this difference could be artifactual since in the patients drug concentrations declined rapidly below the limit of assay, thus preventing identification of a possible slower terminal phase. In valproate-treated patients, half-lives (8.2 +/- 1.8 h) were similar to those found in controls.

The occurrence of carbonyl reduction in continuous cell lines emphasizes the essentiality of this metabolic pathway

Using the ketone compound metyrapone (MPON) as a substrate for carbonyl reduction it has been verified for the first time that various permanent cell lines in culture express carbonyl reducing activity. This is even true for the dedifferentiated and fibroblastoid cell line V79, emphasizing the essentiality of this metabolic pathway. MPON reducing enzyme activities are located in the endoplasmic reticulum as well as in the cytoplasm of the cells. Compared to MPON-reductase in rat liver microsomes, no immunological homology to microsomal C2REV7 rat liver hepatoma cell MPON-reductase could be detected, indicating differences in antigenic determinants between the enzymes of the solid organ and respective cells in continuous culture.

Assessment of hypothalamic-pituitary-adrenal (HPA) axis dysfunction: comparison of ACTH stimulation, insulin-hypoglycemia and metyrapone

The response to ACTH stimulation, insulin-hypoglycemia and metyrapone in patients with suspected HPA axis dysfunction due to corticosteroid therapy (Group I, n = 10), or pituitary surgery (Group II, n = 7) and in a control population (Group III, n = 8) was studied. Group I patients had been maintained on a stable low dose of prednisone 5.0-7.5 mg/day for 1 month-16 yr (mean = 31 mos) prior to testing. Basal 08:00 h cortisol levels in this group were not different from control values. However, the mean responses to all three testing procedures were suppressed (Group I vs III, ACTH p less than 0.001, insulin p less than 0.01, metyrapone p less than 0.05). Group II patients had undergone surgery 1-26 months (mean = 10 mo) prior to testing and had been maintained subsequently on a stable dose of prednisone 5.0-7.5 mg/day. In this group basal mean 08:00 h cortisol and the cortisol response to ACTH and insulin-hypoglycemia were not significantly different from control values while the response to metyrapone was suppressed (Group II vs III p less than 0.02). Basal serum DHEA-S levels were suppressed in both Groups I and II when compared to Group III (p less than 0.001). Discordant responses to the three testing procedures were noted in 6 patients with suspected HPA dysfunction with abnormal test results in 1/6 using cortrosyn, 3/6 using insulin-hypoglycemia and 4/6 using metyrapone.(ABSTRACT TRUNCATED AT 250 WORDS)

Anticonvulsant therapy and cortisol elimination

The effect of anticonvulsant therapy on early morning concentration of cortisol in saliva and plasma was assessed in a group of epileptic patients receiving regular phenytoin medication and the results compared with those obtained from a group of normal subjects not receiving drug therapy. Values of cortisol in matched samples of plasma (331 +/- 23 nmol l-1, mean +/- s.e. mean, n = 6) and saliva (11.4 +/- 0.9 nmol l-1, mean +/- s.e. mean, n = 9) provided by epileptics did not differ significantly from those in the plasma (334 +/- 41 nmol l-1, mean +/- s.e. mean) and saliva (12.0 +/- 2.0 nmol-1, mean +/- s.e. mean) of healthy volunteers (n = 12). Six anticonvulsant-treated epileptics, together with six age and sex matched normal volunteers, each received intravenous dexamethasone (1 mg h-1) to determine the half-life of cortisol in plasma and saliva. In the anticonvulsant-treated group, the half-life of cortisol in plasma (73 +/- 5 min, mean +/- s.e. mean) and saliva (83 +/- 5 min, mean +/- s.e. mean) was reduced significantly (P less than 0.01 plasma, P less than 0.05 saliva) from that observed in healthy volunteers. In patients, the half life of cortisol and antipyrine showed a significant correlation (r2 = 0.75, P less than 0.05 plasma, r2 = 0.71, P less than 0.05 saliva). The antipyrine half-life in saliva was reduced significantly (P less than 0.02) and the antipyrine clearance rate, increased significantly (P less than 0.005) in the treated epileptic group, reflecting drug-induced microsomal enzyme production.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Pharmacokinetic interactions of antiepileptic drugs

The problem of antiepileptic drug interactions is significant in that many epileptic patients are treated with multiple drug therapy. Moreover, patients may also be receiving additional medication for other concurrent disorders. Most drug interactions are pharmacokinetic, involving changes in absorption, protein binding, metabolism, or excretion. As a result, plasma levels of the antiepileptic drug may decrease leading to exacerbation of seizures. Alternatively, plasma levels may rise resulting in toxic side effects. Similar changes may also occur with drugs given for other disorders. In this paper, possible mechanisms of drug interactions are discussed. This is followed by a description of clinically significant interactions involving phenytoin, carbamazepine, barbiturates, valproic acid, benzodiazepines, and succinimides. Potentially serious drug interactions may be minimized by using as few medications as possible and by regularly monitoring plasma levels of antiepileptic drugs.

First-pass elimination. Basic concepts and clinical consequences

First-pass elimination takes place when a drug is metabolised between its site of administration and the site of sampling for measurement of drug concentration. Clinically, first-pass metabolism is important when the fraction of the dose administered that escapes metabolism is small and variable. The liver is usually assumed to be the major site of first-pass metabolism of a drug administered orally, but other potential sites are the gastrointestinal tract, blood, vascular endothelium, lungs, and the arm from which venous samples are taken. Bioavailability, defined as the ratio of the areas under the blood concentration-time curves, after extra- and intravascular drug administration (corrected for dosage if necessary), is often used as a measure of the extent of first-pass metabolism. When several sites of first-pass metabolism are in series, the bioavailability is the product of the fractions of drug entering the tissue that escape loss at each site. The extent of first-pass metabolism in the liver and intestinal wall depends on a number of physiological factors. The major factors are enzyme activity, plasma protein and blood cell binding, and gastrointestinal motility. Models that describe the dependence of bioavailability on changes in these physiological variables have been developed for drugs subject to first-pass metabolism only in the liver. Two that have been applied widely are the 'well-stirred' and 'parallel tube' models. Discrimination between the 2 models may be performed under linear conditions in which all pharmacokinetic parameters are independent of concentration and time. The predictions of the models are similar when bioavailability is large but differ dramatically when bioavailability is small. The 'parallel tube' model always predicts a much greater change in bioavailability than the 'well-stirred' model for a given change in drug-metabolising enzyme activity, blood flow, or fraction of drug unbound. Many clinically important drugs undergo considerable first-pass metabolism after an oral dose. Drugs in this category include alprenolol, amitriptyline, dihydroergotamine, 5-fluorouracil, hydralazine, isoprenaline (isoproterenol), lignocaine (lidocaine), lorcainide, pethidine (meperidine), mercaptopurine, metoprolol, morphine, neostigmine, nifedipine, pentazocine and propranolol. One major therapeutic implication of extensive first-pass metabolism is that much larger oral doses than intravenous doses are required to achieve equivalent plasma concentrations. For some drugs, extensive first-pass metabolism precludes their use as oral agents (e. g. lignocaine, naloxone and glyceryl trinitrate).(ABSTRACT TRUNCATED AT 400 WORDS)

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.

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.

Reduction of oral bioavailability of lignocaine by induction of first pass metabolism in epileptic patients

1. The pharmacokinetics of lignocaine following single oral and intravenous doses have been investigated in six normal volunteers and in six patients receiving chronic antiepileptic drug therapy. 2. After intravenous administration, serum lignocaine levels declined biexponentially in all subjects. The serum clearance (mean +/- s.d.) was slightly higher in the patients (0.85 +/- 0.09 v 0.77 +/- 0.07 l/min) but the difference was not statistically significant. 3. Lignocaine bioavailability after oral administration was more than two-fold in the patients than in the normal subjects (0.15 +/- 0.06 v 0.37 +/- 0.09, P < 0.001). 4. It is suggested that the reduced bioavailability of lignocaine in the patients is a consequence of stimulation of hepatic first-pass metabolism by antiepileptic drugs.

Paracetamol disposition in normal subjects and in patients treated with antiepileptic drugs

1 The serum concentration profile of paracetamol has been determined after administration of single 1000 mg intravenous and oral doses in six normal subjects and six epileptic patients on chronic antiepileptic drug therapy. The urinary excretion of free and conjugated paracetamol has also been determined. 2 Following intravenous administration, serum paracetamol concentration declined with first-order kinetics. Both elimination rate and total body clearance were higher in the epileptic patients, although in neither case was the difference statistically significant. 3 The oral bioavailability (mean +/- s.e. mean) was significantly lower in the epileptic patients than in the normal subjects (0.77 +/- 0.03 and 0.89 +/- 0.02 respectively, P less than 0.01), whereas the urinary excretion total (free+conjugated) paracetamol was almost identical in the two groups. 4 It is suggested that the lower bioavailability of paracetamol in the epileptic patients results from enhancement of first-pass metabolism, secondary to enzyme induction.

Effect of diphenylhydantoin on patterns of insulin secretion in obese subjects

The effect of short-term treatment with diphenylhydantoin (DPH) on the insulin secretion patterns during OGTT and on the daily insulin profile was studied in obese patients. DPH treatment for 3 days with a dose of 300 mg/die (100 mg, 3 times daily) significantly decreased the insulin release after glucose ingestion, but did not alter the basal insulin level. No effect on the fasting glucose concentration as well as on the glucose profiles during OGTT was observed after short-term DPH treatment. A smaller decrease of plasma free fatty acid concentration during OGTT performed after DPH administration confirmed the inhibitory effect of the drug on insulin release. Short-term DPH treatment was also shown to decrease markedly the postpradial insulin release in obese patients. No difference was noted between plasma 11-OHCS and serum HGH concentrations during OGTT before and after DPH treatment. The possible therapeutic role of DPH in obesity is discussed.

Adrenal function in the carcinoid syndrome: effects of the serotonin antagonist cyproheptadine

Results of evaluations of adrenal function in 11 patients with carcinoid tumors are presented. Nine patients had tumors that made and secreted serotonin resulting in elevated 5-hydroxyindoleacetic acid (5-HIAA), elevated serum serotonin, and the carinoid syndrome; while two patients had tumors that did not make serotonin and that did not cause elevated 5-HIAA excretion or elevated serum serotonin. All of the patients had normal 24-hr 17-hydroxycorticosteroid excretion. In the group of patients with tumors actively secreting serotonin, the correlation between 17-hydroxycorticosteroid and 5-HIAA excretion (r = 0.44) was not significant. Six of these patients pretreated with cyproheptadine (CYPRO), a serotonin antagonist, experienced a 36% mean decrease in 17-hydroxycorticosteroid excretion, a finding that was not present when three of them were treated with triprolidine (TPRO), an antihistamine. Serum cortisol at 8 a.m. was normal in all patients except two whose values were mildly elevated, and these two patients showed evidence of suppression of ACTH secretion secondary to dexamethasone treatment. There was a significant positive correlation between serum-cortisol concentrations and 5-HIAA excretions (r = 0.73, p less than .05). Normal diurnal variation was present in six patients in whom it was determined. The serum-cortisol response to insulin-induced hypoglycemia in six patients who had carcinoid tumors actively secreting serotonin was not statistically different from that of 12 normal volunteers. Comparisons between these two groups were difficult because the carcinoid patients' fall in blood sugar was 50%, whereas that of the control group was to 38% of the fasting glucose concentration. Six patients with actively secreting carcinoid tumors responded to standard metyrapone testing with a mean increment of 22.8 +/- 2.5 mg/day in 17-hydroxycorticosteroids. This response was statistically different from the increment of 13.8 +/- 5.3 mg/day in 17-hydroxycorticosteroid excretion found in 34 age-matched hospitalized control patients. When the tests were repeated in four of the patients with carcinoid tumors after pretreatment with CYPRO, the increment in 17-hydroxycorticosteroid excretion was reduced well below the mean increment of the control group. Peak serum 11-deoxycortisol (Compound S) values during the test were also reduced. This decrease in the metyrapone response after CYPRO pretreatment was not due to changed peripheral cortisol metabolism, altered adrenal responsiveness to ACTH, interference with recovery of 17-hydroxysteroids by the Porter-Silber reaction, altered metyrapone metabolism, or reduced renal clearance of Compound S. These changes in adrenal response to metyrapone were not seen when the patients were pretreated with TPRO. Our data suggest that the alterations in adrenal function in our patients may be related to elevated serum serotonin. If CYPRO acts by antagonizing serotonin, these data may give support to the idea of serotoninergic control of cortisol secretion.

Interactions of antiepileptic drugs

Several interactions involving antiepileptic drugs are based on changes in the rate of their metabolism and elimination, with concomitant rise or fall of plasma levels. Thus, phenobarbital generally induces the production of the DPH metabolizing enzyme, but its presence inhibits the action of that enzyme. The net result depends upon the balance between these factors in individual patients. Either a decline, a rise, or no change of the DPH plasma level may occur after the onset of administration of phenobarbital. Drugs that may cause elevation of the DPH plasma level include disulfiram, sulthiame, bishydroxycoumarin, chloramphenicol, phenyramidol, benzodiazepines, sulfamethizole, and isoniazid. Isoniazid has been shown experimentally to be a strong inhibitor of DPH metabolism. The extent of DPH plasma level elevation by INH is related to the genetic make-up of individual patients. The highest and frequently toxic DPH plasma levels were seen in very slow INH inactivators. The incidence of clinically significant interactions is not high with most drug combinations; marked changes of antiepileptic drug levels occur only in apparently susceptible individuals. The effects of interactions are not necessarily detrimental; elevation of a low ineffective level may improve seizure control. A rise to a toxic level range requires reduction of the dose of primary drug or elimination of interfering drugs. Monitoring the blood levels of anti-epileptic drugs provides the best means to anticipate interactions and to regulate the doses when multiple medications have to be used.