Plasma inorganic fluoride concentrations during and after prolonged (greater than 24 h) isoflurane sedation: effect on renal function

We studied the effect of prolonged sedation (greater than 24 h) with isoflurane on plasma inorganic fluoride concentrations and renal function in 60 critically ill patients allocated randomly to receive either isoflurane or midazolam for sedation. In the isoflurane group, plasma fluoride increased from a mean concentration of 3.1 mumol/L to 20.0 mumol/L at the end of sedation, continued to increase to a peak of 25.3 mumol/L 16 h later, and then decreased exponentially (t1/2 = 111 h) to reach normal levels by the fifth day. In the midazolam group, the plasma fluoride increased from a mean concentration of 4.2 mumol/L to a peak of 6.8 mumol/L 12 h after starting the sedation and then decreased toward normal. Serum and urinary electrolytes, urine osmolality, and creatinine clearance during and after sedation were similar in the two groups. Isoflurane sedation was associated with an increase in plasma fluoride concentration without any clinical deterioration of renal function.

Effect of halothane anesthesia and trifluoroacetic acid on protein binding of benzodiazepines

The in vitro effect of the halothane metabolite, trifluoroacetic acid, on the protein binding of three different benzodiazepines (diazepam, lorazepam and midazolam) has been investigated. Furthermore, protein binding of these drugs was studied in serum from patients under the effect of halothane anesthesia (1-2.5%; 2.5 h). Trifluoroacetic acid, 4 mmol/l, displaced diazepam and midazolam from serum and produced a marked increase in the free percentage, but did not influence lorazepam binding. Moreover, 48 h after the end of halothane anesthesia, there were changes in protein binding of diazepam (3.9 +/- 0.3% at 48 h vs. 3.3 +/- 0.3% before halothane anesthesia; p less than 0.05). It can be concluded that halothane anesthesia (1-2.5%; 2.5 h) may temporarily potentiate the pharmacological effect of diazepam in the postoperative period following anesthetic procedures.

In vitro forecasting of drugs which may interfere with the biotransformation of midazolam

The biotransformation of midazolam is mediated by a cytochrome P-450 isozyme (P-450 IIIA) whose activity is highly variable. The kinetics of the 1'- and 4-hydroxylation of midazolam, the major routes of midazolam oxidation, by human liver microsomes have been examined to characterize further the cytochrome isozyme(s) catalysing these reactions, and to screen for drugs that might interfere with them. In hepatic microsomal preparation from two kidney donors (extensive and poor metabolisers of debrisoquine) KM values for 1'-hydroxylation were 4.2 and 6.1 microM (extensive and poor metabolisers, respectively), and for the 4-hydroxylation they were 14.7 and 18.1 microM, respectively. The corresponding Vmax values were 25.8 and 29.8 and 17.0 and 18.1 nmol.mg P-1.h-1. Both reactions appeared to be catalysed by the same or by coregulated isozymes. Midazolam hydroxylations in vitro are inhibited by many drugs, including nifedipine and other dihydropyridine-type calcium channel blockers, ergot alkaloids, cyclosporine, erythromycin and phenothiazine-type neuroleptics. A clinical case report illustrates the consequence of such a drug-drug interference with hepatic biotransformation; midazolam-induced sleep in a patient lasted for 6 days (t1/2 = 25 h).

Pharmacokinetic interactions with calcium channel antagonists (Part I)

Calcium channel antagonists are a diverse class of drugs widely used in combination with other therapeutic agents. The potential exists for many clinically significant pharmacokinetic interactions between these and other concurrently administered drugs. The mechanisms of calcium channel antagonist-induced changes in drug metabolism include altered hepatic blood flow and impaired hepatic enzyme metabolising activity. Increases in serum concentrations and/or reductions in clearance have been reported for several drugs used with a number of calcium channel antagonists. A number of reports and studies of calcium channel antagonist interactions have yielded contradictory results and the clinical significance of pharmacokinetic changes seen with these agents is ill-defined. The first part of this article deals with interactions between calcium antagonists and marker compounds, theophylline, midazolam, lithium, doxorubicin, oral hypoglycaemics and cardiac drugs.

Effect of physostigmine on the loss of consciousness induced by midazolam, etomidate and althesin

The effect of physostigmine, a cholinesterase inhibitor, on the loss of consciousness induced by three different intravenous induction anesthetics, namely midazolam, etomidate and althesin at ED50, was studied in three comparable groups of patients. Ten min before induction, the first and second groups received physostigmine 8 micrograms/kg and 16 micrograms/kg, respectively, and the third group received 2 ml of saline solution. Physostigmine 16 micrograms/kg resulted in a significant decrease in the percentage of unconscious patients with midazolam (from 50% to 10%), but it did not modify the incidence with etomidate or althesin. Physostigmine at doses of 8 micrograms/kg and 16 micrograms/kg could cause 6.7% and 10% nausea, respectively. Although the mechanism of the drug interaction of physostigmine and midazolam is unclear, physostigmine could be used clinically to reverse post-anesthetic somnolence induced by midazolam.

Midazolam withdrawal and discriminative motor control: effects of FG 7142 and Ro 15-1788

Rats chronically drank either water or midazolam solution (0.1 mg/ml) in daily, 3-h schedule-induced polydipsia sessions and were evaluated in daily motor control sessions after polydipsia when midazolam metabolite levels had fallen to zero (withdrawal). Under midazolam polydipsia, animals orally self-administered between 21 and 38 mg/kg daily. The effect of acute drug administration [midazolam (0.75-3 mg/kg, SC), FG 7142 (1-8 mg/kg, IP), Ro 15-1788 (10-20 mg/kg, IP)] on motor control performance was similar after either chronic water or midazolam polydipsia. Thus chronic, oral midazolam self-administration did not lead to tolerance to the motor impairment produced by SC midazolam, nor did the daily discontinuation lead to impaired motor performance, nor had these performances, which occurred after daily elevated midazolam metabolite levels had reached zero (withdrawal), become sensitized to the effects of either the benzodiazepine inverse agonist FG 7142 or the agonist Ro 15-1788.

Inhibitory effect of omeprazole on the metabolism of midazolam in vitro

To examine the effect of omeprazole on the hepatic drug metabolizing enzyme system microsomes from rat and human liver samples were incubated with midazolam (CAS 59467-70-8) in the absence and presence of various concentrations of omeprazole (CAS 73590-58-6), its sulfone metabolite and for comparison also with cimetidine. In the extracted incubation mixtures unchanged midazolam, a-OH-midazolam, 4-OH-midazolam and di-OH-midazolam were analyzed by HPLC. In both species omeprazole (and its sulfone) inhibited the formation of all three oxidized metabolites of midazolam and the corresponding IC50-values (range 0.2-1.3 mmol/l for rat microsomes and 0.2-1.5 mmol/l for human microsomes) were comparable to cimetidine (range 0.05 to 3.8 mmol/l). These results indicate that the oxidative metabolism of midazolam can be inhibited in vitro by omeprazole (and/or its sulfone metabolite) and this interaction should be considered if both drugs are administered concomitantly in man.

Is cyclosporin A an inhibitor of drug metabolism?

1. The potential for a drug interaction between cyclosporin A and midazolam was investigated since both compounds appear to be metabolized by the same cytochrome P-450 isoenzyme. 2. In vitro evaluation of the binding of cyclosporin A to rat microsomal cytochrome P-450 indicated a Ks-value of 0.4 microM. In further studies with rat liver microsomes IC50-values of 6, 8 and 70 microM cyclosporin A were determined for the inhibition of the metabolism of midazolam to its alpha-OH-,4-OH- and di-OH-metabolites, respectively. 3. Comparative studies with human liver microsomes indicated IC50-values of approximately 300 microM for the formation of alpha-OH-midazolam and of 65 microM for the formation of 4-OH-midazolam. 4. The pharmacokinetics of a single intravenous dose of midazolam (0.075 mg kg-1) was studied in nine patients receiving cyclosporin A to prevent rejection of their transplanted kidneys. The average steady state blood concentrations of cyclosporin A, measured by r.i.a. using a specific monoclonal antibody, varied during a dosing interval between 175 and 600 ng ml-1. 5. In these patients the hepatic elimination of midazolam was characterized by a mean t1/2 (+/- s.d.) of 2.3 +/- 1.2 h and a plasma clearance (CL) of 414 +/- 95 ml min-1. These values were not different from those of normal human subjects (t1/2 = 1.5 to 4 h, CL = 350 to 700 ml min-1). 6. From the results of the in vitro experiments it is concluded that cyclosporin A may potentially inhibit drug metabolism.(ABSTRACT TRUNCATED AT 250 WORDS)

The use of midazolam in premedication

Socio-psychological factors, such as increased anxiety in developed societies and cultures, and separation anxiety, particularly in children, justify the use of premedicants. In addition, the link between a central nervous "anxiety centre" and biochemical stress responses is blocked by an efficient anxiolytic. The elimination half-life of midazolam is longer in the elderly than in the young and in the obese than in the thin, which demands longer intervals between repeated doses in old and fat patients. The hypoxic ventilatory response is depressed in most patients and the ventilatory CO2 response in patients with chronic pulmonary disorders, which justifies increased monitoring of O2 saturations. It is important for the choice of dose and for estimating the duration of recovery time to know that midazolam is at least four times as potent as diazepam.

Postoperative sedation with midazolam in heart surgery patients: pharmacokinetic considerations

Midazolam remains one of the few drugs that can safely be used for the sedation of intubated patients in an intensive care unit. Midazolam pharmacokinetics in patients recovering from cardiac surgery were recently reported and found to be different from the drug's kinetics in young and healthy patients or volunteers. In particular, the elimination half-life was prolonged (10.6 h) and the metabolic clearance was reduced. For the clinician, pharmacokinetics parameters do not have a straightforward meaning. The purpose of the present paper is to review the pharmacokinetics of midazolam in this category of patients, and to examine what kind of practical information and dosing recommendation can be derived.

Pharmacokinetics of antibiotics in critically ill patients

Differences in pharmacokinetic data of aminoglycosides, ceftazidime and ceftriaxone between intensive care patients and volunteers or patients who are less severely ill, are described. Similar differences are observed for midazolam. In severely ill patients with normal renal function a wide interpatient variability of aminoglycoside half-life (t1/2) and increased distribution volume (Vd) are observed. This results in inadequate serum levels. A pharmacokinetic approach of drug dosing, based on serum concentrations in individual patients, is advised. For ceftazidime and ceftriaxone similar changes of t1/2 and Vd are observed. Since protein binding is frequently reduced in severely ill patients, the influence of altered binding of highly bound drugs on Vd and drug clearance is discussed. As both may be increased by reduced protein binding, the change of t1/2 to be expected is unpredictable. Dosing regimens should be based on pharmacokinetic data derived from patients whose severity of disease is comparable to that of the patients to be treated.

Isoflurane sedation for patients undergoing mechanical ventilation: metabolism to inorganic fluoride and renal effects

The metabolism and renal effects of isoflurane sedation were studied for 24 h in patients undergoing mechanical ventilation. Forty-six patients admitted to our intensive therapy unit were allocated randomly to receive either 0.1-0.6% isoflurane or midazolam 0.01-0.2 mg kg-1 h-1 for sedation. In 26 patients sedated with isoflurane, plasma inorganic fluoride increased from a mean concentration of 4.03 mumol litre-1 to 13.57 mumol litre-1 12 h after stopping sedation. Plasma inorganic fluoride concentrations in 20 patients sedated with midazolam were unchanged from baseline values (mean 5.32 mumol litre-1). Serum electrolyte, urea and creatinine concentrations, and urine output rates during and after sedation in patients who received isoflurane were similar to those who received midazolam. We conclude that, following isoflurane sedation for up to 24 h, metabolism to inorganic fluoride is insufficient to cause clinical renal dysfunction.

[Midazolam in anesthesiology]

Midazolam (MDZ) (8-chloro-6(2-fluoropheny 1)-1-methyl-4H-imidazol-[1,5a] [1,4]benzodiazepine) is an "annelated" benzodiazepine (BZDs) synthetized in 1976, characterized differently with the "classical" BZDs by a five-membered heterocycle fused on position 1,2 of the diazepine nucleus. This fused imidazol ring modifies the properties inherent in the "classical" BZDs in at the least three aspects: solubility, metabolisation and the stability in aqueous solution. MDZ, having similar properties with the "classical" benzodiazepines, has better local tolerance, faster onset of action, greater plasmatic clearance, shorter half-life elimination (1.7-2.4 hr) with no active metabolites. With a bioavailability of 92% (IV), 82-91% (IM) and 50-52% "per os", the CNS effects of MDZ are similar in all three ways. In anesthesiology we can administer MDZ as an anesthetic premedication ("per os", IM or IV), anesthetic induction and maintenance and IV sedation in locorregional anesthetic procedures or diagnostic and therapeutic explorations. MDZ has a great safety margin, moderate respiratory and cardiovascular effects and lacks of teratogenic or embryotoxic effects.

Oxidation of midazolam and triazolam by human liver cytochrome P450IIIA4

The metabolism of midazolam and triazolam to their 1'-hydroxy and 4-hydroxy metabolites was studied in microsomes of 15 human livers. The formation of both metabolites was inhibited by more than 90% by an antiserum directed against a pregnenolone 16 alpha-carbonitrile-inducible cytochrome P450 (P450PCN1) of rat liver. Moreover, midazolam hydroxylase activity was immunoprecipitated from solubilized human microsomes with polyclonal antibodies against rat P450PCN1 and the closely related human isozyme P450NF. A close correlation was observed between the amount of protein detected in immunoblots with these antibodies and the midazolam or triazolam hydrxylase activity. The formation of both metabolites of midazolam was inhibited by triacetyloleandomycin, a known inhibitor of cytochromes P450 of the IIIA family. Direct evidence that P450IIIA4 catalyzes the metabolism of midazolam was provided through the use of cDNA-directed expression. Monkey COS cells transfected with human P450PCN1 cDNA were able to catalyze both the 1'- and the 4-hydroxylation of midazolam. We conclude that the metabolism of midazolam and triazolam in human liver is predominantly mediated by cytochrome P450IIIA4. Two of 15 human livers expressed a second immunoreactive microsomal protein of higher apparent Mr and were more active in midazolam 1'-hydroxylation. Our data also provide evidence that the marked interindividual variation in the response to these widely used benzodiazepine drugs is due to variable hepatic metabolism.

Simultaneous determination of midazolam and its three hydroxy metabolites in human plasma by electron-capture gas chromatography without derivatization

Electron-capture gas chromatography was carried out to determine midazolam and its three hydroxy metabolites (1-hydroxymethylmidazolam, 4-hydroxymidazolam and 1-hydroxymethyl-4-hydroxymidazolam) in human plasma. The assay involves extraction from plasma, buffered to pH 9.3, into cyclohexane-dichloromethane (6:4) and analysis by gas chromatography. The use of an HP-17 cross-linked, capillary column makes derivatization unnecessary. The sensitivity of the method was 2-3 ng/ml for midazolam, 1-hydroxymethylmidazolam and 4-hydroxymidazolam, and 20 ng/ml for 1-hydroxymethyl-4-hydroxymidazolam. The extraction recovery of midazolam, 1-hydroxymethylmidazolam, 4-hydroxymidazolam and 1-hydroxymethyl-4-hydroxymidazolam was 99.3 +/- 2.4, 67.0 +/- 4.6, 92.7 +/- 4.7 and 28.7 +/- 6.3%, respectively. This gas chromatographic assay was used to assess the concentration-time profiles of midazolam and its metabolites in human plasma after rectal and intravenous administration of midazolam.

Extra-hepatic metabolism of midazolam

Six patients received 10 mg of midazolam intravenously during the anhepatic period of liver transplantation. Arterial blood was sampled during this time and for a similar period following revascularisation. The plasma was analysed using gas chromatography and electron capture detection (GC-ECD) for midazolam alpha-hydroxymidazolam and alpha-hydroxymidazolam glucuronide. Five of the six patients had small but significant concentrations of metabolites detected during the anhepatic period, demonstrating the presence of extra-hepatic sites of metabolism for this drug. The remaining patient had plasma concentrations of metabolites below the lower limit of detection (2 micrograms l-1). This may represent a pharmacogenetic abnormality or a temporary failure of midazolam metabolism secondary to the patients illness affecting the extra-hepatic sites of metabolism.

Pharmacokinetics of midazolam following intravenous and oral administration in patients with chronic liver disease and in healthy subjects

To study the effects of cirrhosis of the liver on the pharmacokinetics of midazolam single IV (7.5 mg as base) and p.o. (15.0 mg as base) doses of midazolam were administered to seven patients with cirrhosis of the liver and to seven healthy control subjects. One cirrhotic patient did not receive the oral dose. The distribution of midazolam in both study groups was alike as indicated by similar values of t1/2 alpha, V1 and Vss. Also the plasma protein binding of midazolam was unchanged in the patients with cirrhosis. The elimination of midazolam was significantly retarded in the patients as indicated by its lower total clearance (3.34 vs. 5.63 ml/min/kg), lower total elimination rate constant (0.400 vs. 0.721 h-1), and longer elimination half-life (7.36 vs. 3.80 h). The bioavailability of oral midazolam was significantly (P less than 0.05) higher in patients than controls (76% vs. 38%). The antipyrine-half-life was 32.4 h in the patients and 11.8 h in the controls. There were statistically significant (P less than 0.01) correlations between the clearances of the two drugs (r = 0.680) and between their half-lives (r = 0.755). The hypnotic effects of midazolam were similar in both groups. However, on a pharmacokinetic basis a reduced dosage of midazolam to patients with advanced cirrhosis of the liver is recommended.

Characterization of midazolam metabolism using human hepatic microsomal fractions and hepatocytes in suspension obtained by perfusing whole human livers

Isolated human hepatocytes provide a useful model for studying xenobiotic metabolism. However, in vitro studies using human hepatocytes are scarce due to the limited availability of this material. A new methodology is described for obtaining hepatocytes from a whole adult human liver. This procedure is based on (i) the rapid and intense in situ washing step of the organ with Eurocollins then glucose supplemented HEPES buffer (10 mM, pH 7.4) at 4 degrees in order to both minimize the warm ischemic period and remove erythrocytes, and (ii) a perfusion of collagenase solution (0.05% in 10 mM HEPES buffer at 37 degrees) throughout the portal vein according to a recirculated model. All perfused buffers are oxygenized. Hepatocyte viability averaged 85% as determined by Trypan Blue dye exclusion. The ability of these hepatocytes to catalyze certain metabolic transformations such as Phase I and Phase II reactions has been particularly investigated using the benzodiazepine drug, midazolam, as a substance probe. Freshly isolated human hepatocytes in suspension retained the ability to metabolize midazolam to its different hydroxylated derivatives--mainly the 1-hydroxy-midazolam--which was further conjugated with glucuronic acid. For a better understanding of the cytochrome P-450 mediated reactions, we studied the metabolism of midazolam in microsomal fractions prepared from twelve human livers. It was concluded that human microsomes (i) exhibited a Type I binding spectrum upon midazolam addition (Ks = 3.3 microM) and (ii) intensively metabolized the drug to its different derivatives. Furthermore, and since we demonstrated that midazolam was predominantly transformed by a single cytochrome P-450 enzyme, we could attribute the large inter-individual variations in midazolam metabolism to differences in human liver cytochrome P-450 content.

No evidence of a genetic polymorphism in the oxidative metabolism of midazolam

The benzodiazepine midazolam is rapidly eliminated by oxidative metabolism. In young healthy volunteers elimination half-life (t1/2) is about 2.4 hours. A recent study showed a prolonged t1/2 from 8 to 22 hours in 6.5% of surgical patients, and a genetic polymorphism of midazolam's metabolism has been suggested. Therefore, we measured in 168 surgical patients the elimination of midazolam and its major hydroxylated metabolite (alpha-OH-midazolam) in blood and urine. Co-medication, disease status, smoking habits and alcohol intake were recorded; normal liver and kidney functions were assessed by routine laboratory tests. Midazolam was administered intravenously (0.1 to 0.2 mg/kg) for the induction of anaesthesia. Blood was drawn 1.5, 3, 4.5 and 6 hours after application and urine was collected for 6 hours. Plasma protein binding of midazolam was determined by equilibrium dialysis. Midazolam and alpha-OH-midazolam were measured in plasma by specific gas-liquid chromatography and in urine by high performance liquid chromatography. Data for the dose-corrected area under plasma-level curve of midazolam (AUC-midazolam/dose: 1.23 +/- 961 x 10(5) h/ml; mean +/- SD) and for the metabolic plasma ratio (AUC of alpha-OH-midazolam/AUC-midazolam: 0.52 +/- 0.28) demonstrated a log-normal distribution. Likewise, the percentage of the unbound fraction of midazolam in plasma (5.0 +/- 2.4%), urinary excretion of alpha-OH-midazolam (55.9 +/- 22.7% of dose) and the values for t1/2 (2.9 +/- 1.1 hours) did indicate a unimodal distribution. Age, comedication and smoking habits did not affect the disposition of midazolam. However, patients with regular intake of alcohol had a higher (p less than 0.05) metabolic ratio. Only in 3 patients could a prolonged t1/2 of midazolam from 7.5 to 10.2 hours be detected, but plasma levels and urinary excretion of alpha-OH-midazolam in those individuals were found to be normal. Therefore it is very unlikely that the oxidative metabolism of midazolam exhibits a genetic polymorphism.

Pharmacokinetics and pharmacodynamics of midazolam in the enflurane-anesthetized dog

Midazolam (Mid) is widely used as an anesthetic adjunct. To test its anesthetic effect vs. concentration relationships, it is desirable to establish stable and predictable Mid concentrations in plasma (and brain). Therefore, the pharmacokinetics of Mid in the enflurane-anesthetized dog were determined, and the ability of Mid to reduce the enflurane concentration required for anesthesia was measured and correlated with the Mid concentration in plasma [MID]. Mongrel dogs (n = 9) were anesthetized with enflurane and the enflurane EC50 (MAC--the end-tidal concentration at which one-half of the dogs respond to the noxious stimulation of clamping of the tail, and one-half do not) was determined. Group 1 (n = 5) received Mid 2.5 mg/kg iv over 60 sec. Plasma for determination of [MID] was collected and the enflurane EC50 was determined repeatedly over the 7-8-hr period following injection. Based on the pharmacokinetic parameters determined for Group 1, dogs in Group 2 (n = 4) received Mid as a continuous infusion of 21 micrograms kg-1 min-1 for 5 hr accompanied by an initial loading dose (3 mg/kg infused over 20 min) designed to produce a stable [MID] of 1000 ng/ml in plasma. Enflurane MAC and [MID] were determined regularly during the infusion and for 6 hr after discontinuation of the infusion. There were no important differences in the pharmacokinetic parameters determined for Group 1 vs. Group 2: t1/2,z = 98 +/- 5 vs. 95 +/- 10 min (mean +/- SEM); V = 3.94 +/- 0.27 vs. 2.98 +/- 25 L/kg; Cl = 28.5 +/- 3.1 vs. 22.3 +/- 1.1 ml kg-1 min-1, respectively. When administered as a continuous intravenous infusion (Group 2), [MID] remained stable at 949 +/- 53 ng/ml for more than 5 hr. The enflurane EC50 was reduced by 55% and the reduction remained stable during the 5 hours of Mid infusion. After a single iv bolus dose or after discontinuation of the continuous infusion, the degree of enflurane EC50 reduction diminished toward the control (i.e., enflurane alone) value as [MID] declined. Mid-azolam's pharmacokinetics and plasma concentration vs. effect relationships have been determined to be consistent under two different experimental conditions.

Involvement of the macrolide antibiotic inducible cytochrome P-450 LM3c in the metabolism of midazolam by microsomal fractions prepared from rabbit liver

This report characterizes the cytochrome P-450 isozyme involved in midazolam metabolism. This study was undertaken into liver microsomal fractions prepared from untreated rabbits or animals treated with drugs known to specifically induce various cytochrome P-450 isozymes such as form LM2 by phenobarbital, LM4 and LM6 by 3-methylcholanthrene and beta-naphthoflavone, LM3a by ethyl alcohol and acetone, and LM3c by macrolide antibiotics (rifampicin, erythromycin and triacetyloleandomycin). Among this library of characterized microsomal preparations, only those obtained from macrolide antibiotic-treated rabbits exhibited a Type I binding spectrum upon addition of midazolam (Ks = 3.2-5.3 micrograms/ml; 10.6-17.5 microM) and significantly metabolized midazolam to its various hydroxylated metabolites (Km = 2.52 +/- 0.22 micrograms/ml; 8.32 +/- 0.73 microM and Vmax = 20 micrograms metabolites formed/min/mg proteins; 66 nmoles metabolites formed/min/mg proteins). The following observations further confirmed the specific involvement of the cytochrome P-450 LM3c isozyme: (i) only anti-cytochrome P-450 LM3c isozyme antibodies intensively inhibited midazolam metabolism, (ii) incubation of microsomes, prepared from TAO-treated rabbits, with midazolam in the presence of potassium ferricyanide which restored the functional cytochrome P-450 LM3c isozyme, increased midazolam metabolism to a similar extent, and (iii) in the presence of Cyclosporin A, a specific substrate of the rabbit cytochrome P-450 LM3c isozyme, midazolam metabolism was inhibited in a concentration-dependent manner. These data demonstrated that the rabbit cytochrome P-450 LM3c isozyme was predominantly involved in midazolam metabolism.