Effect of blood decrease on micafungin disposition in rats.

Micafungin (MCFG) is a novel echinocandin-class antifungal agent that extensively undergoes metabolic removal in the liver. In the present study, the influence of decreased blood volume on pharmacokinetic disposition of MCFG was examined using a rat model prepared by phlebotomy. In phlebotomized rats, hematocrit level and plasma albumin concentration were decreased by 50 and 15%, respectively. Regarding the pharmacokinetic parameters of MCFG, there were no significant differences in the total body clearance (CL(tot)) and elimination rate constant (k (e)) between control and phlebotomized rat groups. A slight increase was observed in the apparent volume of distribution at steady-state (Vd(ss)), but the degree of change was minimal. These findings demonstrate that the elimination capacity for MCFG is only slightly affected by severe anemia and moderate hypoalbuminemia, and provide experimental evidence for the preceding clinical studies suggesting that neither hematocrit level nor serum albumin concentration is a contributory factor for the metabolic clearance of MCFG.

Impact of plasma exchange on pharmacokinetic disposition of micafungin.

Change in the pharmacokinetic disposition of an antifungal agent micafungin (MCFG) by 8-hour plasma exchange (PE) with 3200 mL replacement was examined in a stem cell transplant recipient. On pharmacokinetic analysis of the time course of the serum concentrations of MCFG, it was determined that PE shortened the elimination half-life of MCFG from 16.5 hours to 6.3 hours. Total clearance (CL(tot)) was increased from 0.366 L/h to 0.932 L/h by PE. PE-dependent clearance (CL(pe)) accounted for approximately two-thirds of CL(tot), and PE was found to contribute to the removal of nearly 40% of the total body store of MCFG. It was confirmed that a significant amount of MCFG was excluded into apheresed plasma waste. In addition, adsorption of MCFG onto plasma-separating membrane was strongly suggested, because the CL(pe) exceeded the rate of plasma apheresis and MCFG concentrations in apheresed plasma were lower than those in circulating blood collected at the same time. The marked elimination of MCFG during PE can be explained by its low volume of distribution and high affinity for serum proteins. Judging from these findings as well as those of other reports, MCFG can be considered one of the drugs most susceptible to removal by PE. Our findings suggest that an increment in the regular dose of MCFG would be required at the next administration after PE.

Reduced elimination clearance of micafungin in rats with cholestatic hyperbilirubinemia.

We examined whether the pharmacokinetic disposition of micafungin (MCFG), an echinocandin class antifungal agent, is altered in hyperbilirubinemia using a rat model prepared by bile duct ligation (BDL). Serum bilirubin levels were increased depending upon the duration of BDL. The elimination rate constant and total body clearance (CL(tot)) of MCFG were reduced by 24% and 16%, respectively, after BDL for 1 h, but there was no significant change in the apparent volume of distribution at steady-state. The degree of reduction in the CL(tot) was much greater 7 days after BDL as compared with that 1 h after BDL (44% vs. 16%). However, the proportion of the biliary clearance in the CL(tot) was about 10%. This is similar to the extent of decrease in the CL(tot) by occlusion of the bile duct, demonstrating that decreased biliary excretion of MCFG makes only a minor contribution to its pharmacokinetic change. These findings suggest that the metabolic capacity of MCFG is markedly impaired in hepatic hypofunction secondary to hyperbilirubinemia, providing a fundamental explanation for the previous clinical report that there is a significant correlation between dose-adjusted plasma MCFG concentration and serum bilirubin levels.

Change in blood tacrolimus concentration by fluctuation of renal function in a bone marrow transplant patient.

The authors report a case showing a marked change in blood tacrolimus concentration due to modification of renal function in a bone marrow transplant recipient. Blood tacrolimus concentration was well controlled after transplantation, but an approximately threefold increase in the concentration was observed on day 10 even though the dosage was unchanged. Although there were no pronounced changes in hepatic enzyme activities in serum, marked elevations of renal function test values were noted; concentrations of serum creatinine (SCr) and blood urea nitrogen (BUN) were increased by more than 300% from the original levels. The tacrolimus concentration was gradually decreased by the dose reduction, but the dose-adjusted tacrolimus blood concentration (C/D) was increased contrary to the decreased tacrolimus concentration. The C/D of tacrolimus also began to decline from several days after the recovery of Scr and BUN levels and returned to the basal level. Our finding suggests that renal function has a significant effect on the pharmacokinetic disposition of tacrolimus, although this agent is almost completely eliminated by hepatic metabolism. Careful attention should be paid to alteration in tacrolimus blood concentration, especially when renal function fluctuates during post-transplant immunosuppressive therapy.

Withdrawal symptom after discontinuation of transdermal fentanyl at a daily dose of 0.6 mg.

Neurophysiologic disorders developed in three patients after discontinuation of transdermal fentanyl (TDF) at a daily dose of 0.6 mg (2.5 mg per a patch), although direct removal of a 2.5 mg patch is permitted by the manufacturer as the formulation has the lowest fentanyl content among all the commercially available patch formulations. These observations indicate that the discontinuation of TDF carries a risk for developing withdrawal symptoms even when using a 2.5 mg patch. To avoid such adverse events, we considered the necessity of gradual reduction in the daily fentanyl requirements. For this purpose, we covered part of the application surface of the patch with an insulating tape, and then increased the covered area in a stepwise manner. There were no apparent withdrawal signs during the procedure described above.

Pharmacokinetic behavior of micafungin in rats with carbon tetrachloride-induced acute hepatic failure.

We examined the pharmacokinetic behavior of micafungin, a novel antifungal agent, in rats receiving carbon tetrachloride (CCl4) at a single dose of 2.5 ml/kg. There was no significant change in the total clearance (CL(tot)) in CCl4-treated rats, while the steady-state volume of distribution (Vd(ss)) was significantly increased by CCl4 treatment. Alteration in the serum unbound fraction of micafungin after CCl4 treatment was unlikely in light of the serum albumin, bilirubin, creatinine, and urea nitrogen. The increased Vd(ss) was attributable to augmentation in the accessibility of micafungin to peripheral tissue without impairment of the intrinsic clearance, because slight enhancement of the tissue distribution of micafungin was confirmed following CCl4 treatment.

Decrease in oral bioavailability of ciclosporin by intravenous pulse of methylprednisolone succinate in rats.

We examined the effects of high-dose methylprednisolone on the bioavailability of orally administered ciclosporin in rats. To emulate the clinical protocol of methylprednisolone pulse therapy, methylprednisolone sodium succinate (MPS), a prodrug of methylprednisolone, was intravenously administered as repeated doses (66.3 mg kg(-1)) for 3 days. The area under the blood ciclosporin concentration versus time curve after oral administration was significantly reduced by 60% by pulse treatment with MPS. Based on our previous finding that the total body clearance of ciclosporin was reduced by about 20% by the same methylprednisolone pulse protocol, the extent of reduction in the oral bioavailability of ciclosporin was estimated to be approximately 50%, indicating a drug interaction between high-dose methylprednisolone and orally administered ciclosporin, which affected the absorption process. In rats treated with MPS, an in-situ efflux experiment using rhodamine-123 demonstrated that the reverse transport function of P-glycoprotein (P-gp) in the small intestine was significantly enhanced, although there was no significant increase in the intestinal microsomal activity of triazolam alpha- and 4-hydroxylation, metabolic probes for CYP3A. In addition, a significant decrease was observed in the amount of secreted bile acids serving as an enhancer of gastrointestinal absorption of ciclosporin in MPS treatment. To directly estimate the absorptive capacity, an in-situ absorption test was conducted using a closed-loop of small intestine in control and MPS-treated rats. Intestinal absorption of ciclosporin was significantly decreased, not only in the absence of bile flow but also by treatment with MPS, which well reflected the change in the in-vivo pharmacokinetic behaviour of ciclosporin after methylprednisolone pulsing. These results demonstrate that bioavailability of ciclosporin is markedly reduced by MPS pulse treatment, and the mechanism of this interaction was confirmed to involve enhancement of small-intestinal P-gp function and decrease in bile secretion.

Urinary 6beta-hydroxycortisol/17-hydroxycorticosteroids ratio as a measure of hepatic CYP3A4 capacity after enzyme induction.

BACKGROUND: The correlation between the urinary 6beta-hydroxycortisol/17-hydroxycorticosteroids (6beta-OHF/17-OHCS) ratio and the metabolic capacity of the most abundant form of hepatic cytochrome P450 (CYP3A4) after induction remains unclear. METHODS: Concentrations of 6beta-OHF and 17-OHCS in spot urine specimens obtained from 61 epileptic children receiving continuous carbamazepine therapy were measured by high-performance liquid chromatography. The relationship between the urinary 6beta-OHF/17-OHCS ratio and the serum carbamazepine concentration, corrected for dose and body weight, was examined. RESULTS: Serum carbamazepine was inversely associated with the urinary 6beta-OHF/17-OHCS ratio, and the hyperbolic relationship between the two parameters was statistically significant (P < 0.01). DISCUSSION: Carbamazepine is well known as a potent inducer and a substrate of hepatic CYP3A4. The present results suggest that measurement of the urinary 6betaOHF/17-OHCS ratio is helpful for assessing individuals' hepatic CYP3A4 capacity after enzyme induction.

Influence of intravenous methylprednisolone pulse treatment on the disposition of ciclosporin and hepatic CYP3A activity in rats.

We examined the effects of high-dose methylprednisolone (MP) on the disposition of ciclosporin (CsA) and hepatic microsomal CYP3A activity using rats. Methylprednisolone sodium succinate (MPS), a prodrug of MP, was intravenously administered as repeated doses (66.3 mg kg(-1)) for 3 days or as a single dose. In MP-treated rats, a significant increase was observed in the total body clearance (CL(tot)) and elimination rate constant (Ke) of intravenously administered CsA. The enzyme activities of triazolam hydroxylations and erythromycin N-demethylation in hepatic microsomes were also enhanced by about 50% by MP treatment, suggesting that the alteration in the CsA pharmacokinetics was due to significant induction of the hepatic CYP3A responsible for the metabolic conversion of CsA. In contrast, no significant changes in the values of CL(tot) and Ke were found following a single treatment with MP. On the other hand, MP inhibited the CYP3A-mediated triazolam hydroxylations in a concentration-dependent manner. The difference between the in-vivo and in-vitro inhibitory behaviours of MP was attributed to the rapid elimination of MP after biotransformation from MPS because the plasma MP concentration decreased with a half-life of 15 min immediately after reaching a level close to the inhibition constant for the triazolam 4-hydroxylation reaction (32.4 microM). Although there is a general consideration that MP cannot act as an enzyme inducer at maintenance doses, the present results strongly suggest that high-dose MP is likely to interact pharmacokinetically with CsA by inducing hepatic CYP3A. These results may provide basic explanations for the clinical experience that blood CsA levels are reduced during MP pulse therapy.

Depression of phenytoin metabolic capacity by 5-fluorouracil and doxifluridine in rats.

It has been found in clinical practice that the serum level of phenytoin, of which metabolism is mediated by hepatic CYP2C enzymes, was markedly elevated by co-administration of 5-fluorouracil (5-FU) and doxifluridine (5'-deoxy-5-fluorouridine; 5'-DFUR), a prodrug of 5-FU, but the detailed mechanisms are unclear. A study using rats was undertaken to examine the effects of 5-FU and 5'-DFUR on phenytoin metabolism in hepatic microsomes and phenytoin pharmacokinetics in-vivo. Neither 5-FU nor 5'-DFUR exhibited direct inhibitory effects on hepatic microsomal phenytoin p-hydroxylation, a major metabolic route catalysed by CYP2C in rats, as in humans. 5-FU and 5'-DFUR were injected intraperitoneally into male rats as single doses (1.68 mmol kg(-1)) and repeated doses (0.24 mmol kg(-1) for 7 days). Control rats received vehicle alone. A significant reduction in the activity of phenytoin p-hydroxylation was observed 4 days after the last administration irrespective of the agents and their treatment regimens, although the activity was unchanged on Day 1. Pharmacokinetic analysis of phenytoin revealed that the elimination rate constant and the total clearance was decreased by 70-75% in both the 5'-DFUR-treated and 5-FU-treated rats, indicating that the decrease in the metabolic capacity of phenytoin was responsible for the change in phenytoin disposition in-vivo. On the other hand, 5-FU significantly depressed the total P450 content, NADPH cytochrome c reductase activity and activities of progesterone hydroxylations. However, the depressive effects of 5'-DFUR were not very potent relative to those of 5-FU, which can be explained by the fact that 5-FU is derived from 5'-DFUR to only a small extent. According to a recent report, phenytoin p-hydroxylation and progesterone 2alpha-/21-hydroxylations share common CYP2C enzymes as their catalysts. Because there was a difference in the modulation profiles between phenytoin p-hydroxylation and progesterone 2alpha-/21-hydroxylations after exposure to 5'-DFUR, 5'-DFUR might modulate phenytoin metabolism without loss of catalytic ability for other substrates, unlike 5-FU. The present study suggested that the down-regulation of hepatic CYP2C enzymes occurs by 5-FU exposure even at a low level, and provided a fundamental explanation for the drug interaction encountered in clinical practice.

Fluctuation in therapeutic control associated with interchange of prednisolone tablet formulations: assessment of bioequivalence by dissolution test.

A 47-year-old woman received combination therapy with prednisolone (PSL), danazol, cepharanthin, ascorbic acid, and cimetidine for the treatment of idiopathic thrombocytopenic purpura. The platelet count was well controlled for over 1 year. Then the PSL tablet formulation was altered from Tablet A to Tablet B with the same treatment regimen, but the platelet counts fell drastically thereafter. However, the platelet counts recovered by changing the PSL tablet formulation back from Tablet B to Tablet A. In vitro dissolution testing was undertaken to assess bioequivalence between Tablet A and Tablet B. PSL in Tablet B was released more slowly compared with that in Tablet A regardless of the medium pH conditions, and the difference in the release rate between the two tablet formulations increased with increasing medium pH value. The difference exceeded the allowance limit (15%) for judgment of bioequivalence under conditions above pH 4, indicating that Tablet A and Tablet B might be nonbioequivalent. The intragastric pH of the patient was probably raised due to coadministration of cimetidine. Therefore the present results suggest that the disparity in the immunosuppressive effects between the two PSL tablet formulations was attributable to the difference in their dissolution behavior in the gastrointestinal tract. We consider that it is better to avoid interchanging PSL tablet formulations in clinical practice.

Moricizine, an antiarrhythmic agent, as a potent inhibitor of hepatic microsomal CYP1A.

We examined the inhibitory effect of moricizine (MOR) on hepatic cytochrome P-450 (CYP) in mice. Spectrophotometric analysis revealed that MOR had a relatively high affinity for CYP molecules. MOR most potently inhibited the CYP1A1-dependent ethoxyresorufin O-deethylation and the CYP1A2-dependent methoxyresorufin O-demethylation, among the metabolic reactions mediated by CYP1A, CYP2A, CYP2B, CYP2C, CYP2D, CYP2E, and CYP3A subfamilies expressed in untreated and CYP-inducer-treated hepatic microsomes. The inhibition constants (K(i)) for ethoxyresorufin and methoxyresorufin O-dealkylations were 0.43 and 0.98 micromol/l, respectively. These K(i) values were one to three orders of magnitude lower than those of cimetidine (CIM) and mexiletine (MEX) that have been accepted as the clinical inhibitors of CYP1A2 and were below the therapeutic serum concentration of MOR. Theophylline 3-demethylation and 8-hydroxylation in untreated hepatic microsomes, clinical probes for CYP1A2 activities, were subjected to marked and competitive inhibition by MOR with K(i) values similar to that of methoxyresorufin O-demethylation, and the inhibitory potency of MOR was much higher than those of CIM and MEX. In addition, the zoxazolamine paralysis time, an in vivo measure of the hepatic CYP1A2 capacity, was markedly prolonged by pretreatment of mice with MOR rather than CIM and MEX, while the prolonging effect of MOR on the pentobarbital sleeping time, an indicator of the metabolic function of phenobarbital-inducible CYP species, was not so pronounced as compared with the zoxazolamine paralysis time. These results indicate that MOR acts as a potent and preferential inhibitor of hepatic CYP1A enzymes in vitro and in vivo.

Probable metabolic interaction of doxifluridine with phenytoin.

OBJECTIVE: To report the marked elevation of the serum phenytoin concentration during treatment with antineoplastic agents. CASE SUMMARY: A 51-year-old Japanese woman, who was diagnosed with multiple brain metastatic tumors, was placed on oral phenytoin at a maintenance dose of 200 mg/d (3.8 mg/kg/d) to prevent seizures. The serum concentration of phenytoin was well controlled within the therapeutic range; no seizures occurred. Four months later, combination therapy with doxifluridine (5'-DFUR) 800 mg/d, cyclophosphamide 100 mg/d, and medroxyprogesterone acetate 800 mg/d was initiated because of further metastasis. Approximately 1 month after the start of concurrent treatment with the antineoplastic agents, the serum phenytoin concentration was elevated to fourfold of the original concentration. Staggering was observed at that time, but toxic symptoms gradually subsided with the decline in the serum phenytoin concentration after its withdrawal. DISCUSSION: A probable explanation for the marked elevation of serum phenytoin concentration is a reduction of the capacity of CYP2C-dependent phenytoin metabolism, and the antineoplastic agents could be involved in this event. The interaction of fluorouracil and phenytoin is known in clinical practice, and it is reported that the expression of hepatic CYP2C enzymes is depressed by exposure of rats to fluorouracil. 5'-DFUR, a prodrug of fluorouracil, was considered the likeliest candidate responsible for the interaction. This interaction was of clinical significance because of the great extent of changes in the serum phenytoin concentration. CONCLUSIONS: Clinicians should be aware of the elevation of serum phenytoin concentrations when phenytoin is given in combination with fluorouracil derivatives, including 5'-DFUR.

Pharmacokinetic analysis of theophylline to assess noncompliance in therapy.

OBJECTIVE: To report a case showing patient noncompliance, supported by outcomes of pharmacokinetic analysis of theophylline as a surrogate drug. CASE SUMMARY: A 45-year-old woman with severe hypertension was treated with a variety of oral antihypertensive drugs, but there was no improvement in her elevated blood pressure. Since we suspected that her intestinal drug absorption capacity was impaired, a theophylline absorption test was performed. When oral theophylline was given to the patient in tablet form, the apparent bioavailability was only 20%, which agreed with our hypothesis. However, the bioavailability of theophylline given in liquid form was almost 100%, and theophylline in tablet form was confirmed to be fully bioavailable when the test was performed under supervision by medical staff. DISCUSSION: The almost complete bioavailability of oral theophylline indicated that there was no impairment in intestinal absorption capacity. The low bioavailability of theophylline observed after tablet administration in the first trial was apparently a result of noncompliance, because the staff did not supervise administration to ensure that tablets were swallowed. Thus, the low response to antihypertensive therapy was attributed to patient noncompliance in taking the drugs, despite her insistence to the contrary. CONCLUSIONS: The use of theophylline was a novel approach to evaluating the absorbability of orally administered drugs in a patient suspected of poor compliance with therapy. A great difference in theophylline bioavailability between the supervised and unsupervised trials was strongly indicative of patient noncompliance. The possibility of impaired absorption was virtually ruled out.