E. E. Ohnhaus

Urinary excretion of 6β-hydroxycortisol and the time course measurement of enzyme induction in man

SummaryThe effect of enzyme induction by antipyrine, phenobarbitone and rifampicin on the time-course of urinary 6β-hydroxycortisol (6β-OHC) excretion was investigated in healthy volunteers. The drugs were given chronically for either seven or 14 days.Significant increases in 6β-OHC excretion were observed after 4 days administration of antipyrine (1.2 g), 13 days administration of phenobarbitone (100 mg), and only 2 days administration of rifampicin (0.6 or 1.2 g). During 14 days rifampicin administration (1.2 g) 6β-OHC excretion, for individual subjects, reached a maximum on Days 11–14 when excretion was significantly greater than on day 7. On stopping rifampicin, in a 7-day study, excretion decreased over the next six days, but still remained significantly elevated compared to the original control values.These studies show that measurement of urinary 6β-hydroxycortisol provides a simple non-invasive method with which to monitor the time-course of enzyme induction by drugs in man. However, the method cannot be used to predict clinically important drug interactions until the cytochrome P-450 enzyme responsible for cortisol 6β-hydroxylation has been fully characterized.

The effect of antipyrine and rifampin on the metabolism of diazepam

The elimination of diazepam and antipyrine and the urinary excretion of their metabolites were investigated in 21 healthy volunteers before and after 7 days of administration of antipyrine, 1200 mg, and rifampin, 600 or 1200 mg. After administration of antipyrine and rifampin in two doses, antipyrine total body clearance increased by 53% and 60% or 98%, respectively. The clearance to metabolite showed a preferential induction of the norantipyrine pathway with different proportions after antipyrine and rifampin; rifampin, 1200 mg, also enhanced the 4‐hydroxyantipyrine pathway further. After antipyrine, diazepam total body clearance was increased by 102%, affecting all metabolic pathways to a similar extent. After rifampin in both doses, diazepam total body clearance rose equally to 300% and desmethyl‐ and 3‐hydroxydiazepam metabolic clearance to 400%. Therefore rifampin preferentially affects norantipyrine or desmethyl‐ and 3‐hydroxydiazepam metabolic formation, suggesting induction of different (iso)zymes of cytochrome P‐450.

Selective inhibition of drug oxidation after simultaneous administration of two probe drugs, antipyrine and tolbutamide

SummaryThe effects of sulphaphenazole, cimetidine and primaquine on the disposition of antipyrine and tolbutamide in healthy volunteers have been investigated. The model substrates were administered simultaneously in order more clearly to define any selective effects of the potential inhibitors. Sulphaphenazole produced a significant increase in the half-life of tolbutamide (7.10 to 21.50 h) and a correponding decrease in its clearance (0.260 to 0.084 ml·min−1·kg−1). Clearance to hydroxytolbutamide (OHTOL) and carboxytolbutamide (COOHTOL) was also significantly decreased.In contrast, sulphaphenazole had no effect on the disposition of antipyrine. Administration of cimetidine did not significantly alter the disposition of either model drug. However, a 1.6-times higher dose of cimetidine did increase the half lives both of tolbutamide and antipyrine (6.21 to 9.04 h and 14.2 to 19.2 h, respectively) and decrease their clearance (0.226 to 0.148 and 0.50 to 0.31 ml·min−1 kg−1, respectively). Clearance to OHTOL and hydroxymethylantipyrine (HMA) was reduced.A single dose of primaquine had no demonstrable effect on tolbutamide disposition whereas the half-life of antipyrine was increased (12.1 to 15.0 h) and its clearance decreased (0.63 to 0.38 ml·min−1·kg−1). The partial clearance to HMA, 4-hydroxyantipyrine (OHA) and norantipyrine (NORA) was also significantly reduced.The two main inferences are first, that tolbutamide and antipyrine are metabolished by different forms of cytochrome P-450, and second that a battery of model substrates is needed to investigate the inhibitory effects of a drug in man.

Isolation of human hepatic microsomes and their inhibition by cimetidine and ranitidine

SummaryHuman hepatic microsomes were isolated from wedge biopsies of the liver from 13 patients undergoing abdominal surgery. Ultrasonic homogenisation was used to increase the yield of microsomal monooxygenase activity (7-ethoxycoumarin O-deethylase, NADPH-cytochrome c reductase), resulting in a 30% higher total enzyme activity per g liver than preparation by other techniques. In 4 individual microsomal preparations the influence of cimetidine and ranitidine on Michaelis-Menten kinetics of O-deethylation and of reductase activity were studied. Without the H2-receptor blocking drugs, enzyme kinetics of O-deethylation with a Km of 51.0±16.4 µM (n=3) were obtained using Lineweaver-Burke plots. Both, cimetidine and ranitidine inhibited the O-deethylation; cimetidine had a five-fold higher inhibitory affinity (Ki 1.01 and 3.94 mM) to the monooxygenase than ranitidine (Ki 4.96 and 17.70 mM) in the uninduced liver. However, in liver from a patient with induced enzyme activity (Km=478.0 µM), the Ki of ranitidine was similar to that of cimetidine (Ki ran 3.57 versus Ki cim 2.49 mM). The reductase activity was not inhibited by ranitidine and only marginally so by cimetidine.The results suggest that in human hepatic microsomes oxidative drug metabolism is inhibited by both H2-receptor antagonists. However, the inhibitory potency of the compounds seems to depend on the individual isozyme pattern of the hepatic microsomes. Thus, while cimetidine is an relatively nonspecific enzyme inhibitor, ranitidine might more selectively inhibit induced drug metabolizing enzymes.

IMMUNOMODULATORY PROPERTIES OF CIMETIDINE IN ARC PATIENTS

The immunomodulatory potency of cimetidine, a histamine H2 receptor antagonist, was investigated in 33 AIDS-related complex (ARC) patients performing detailed immunological and clinical evaluations. Cimetidine was administered orally in daily doses of 1200 mg for a period of 5 months with an interruption of therapy after the first 3 months for an interval of 3 weeks. Significant (P less than 0.05) elevations of immunoglobulins (IgG, IgA), complement C4, B-lymphocytes, and OKT4+ (helper/inducer) cells were found after cimetidine intake. The in vitro lymphocyte proliferative response to plant mitogens was significantly increased, and the in vivo cell-mediated hypersensitivity reaction assessed by intradermal application of seven recall antigens improved significantly. These effects were both reversible with the discontinuation of cimetidine and reproducible with repeated administration of the drug. Clinical data such as performance status, body weight, and fever were influenced favorably (P less than 0.05) by cimetidine. The frequency of diarrhea and the lymph node size were also diminished significantly. The data suggest that cimetidine may at least partially restore immunofunctions in AIDS-related complex.

Bioavailability and elimination of nitrendipine in liver disease.

SummaryTwenty one patients with liver disease (cirrhosis 11, chronic hepatitis 5 and acute hepatitis 5) and 6 healthy volunteers were given a single i.v. dose of nitrendipine 5 mg. Afterwords nitrendipine 20 mg once daily were administered orally for seven days.With the intravenous injection a significant increase in the AUC and elimination half-life of nitrendipine was found in patients with cirrhosis as compared to the normal volunteers. After chronic oral dosing, the area under the plasma concentration-time curve, AUC (0–24), was 94.5 ng ml−1 h and the plasma clearance CL was 1380.6 ml/min in the healthy controls; in patients with cirrhosis the AUC (0–24) h was significantly greater at 309.4 ng ml−1 h and CL had fallen to 686.6 ml/min. Considerable accumulation of nitrendipine was also found in the patients with chronic hepatitis. Nitrendipine could not be detected in urine from any of the subjects. Blood pressure and heart rate were not significantly influenced by the treatment in the various groups investigated. Antipyrine clearance in the patients with cirrhosis was correlated with the nitrendipine plasma clearance.Thus, accumulation of nitrendipine has been demonstrated in the patients with cirrhosis and chronic hepatitis.

Dose-dependence of the nifedipine-digoxin interaction?

The dose‐dependence of the nifedipine‐digoxin interaction was investigated in seven healthy subjects. After an adequate loading dose of digoxin for 2 weeks, 0.25 mg digoxin b.i.d. was given by mouth by itself. Afterwards, 0.25 mg digoxin was given twice a day for three 1‐week periods in combination with capsules of nifedipine, 5, 10, or 20 mg, respectively, given on a thrice‐daily basis. The study ended with a digoxin monotherapy phase lasting 7 days. All three doses of nifedipine significantly increased digoxin plasma concentrations and AUC compared with digoxin monotherapy. Thus, for example, the AUC was 10.16 ± 0.88 ng/ml · hr (X̄ ± SE) when digoxin was given alone and 12.33 ± 1.59 ng/ml · hr with concurrent nifedipine, 5 mg t.i.d. (P < 0.05). Nifedipine causes a slight but significant increase (15%) in digoxin plasma concentrations and AUC. This effect did not depend on the nifedipine dose given in the range studied.

Interaction of metoprolol, propranolol and atenolol with concurrent administration of cimetidine

SummaryPharmacokinetics of metoprolol, propranolol, and atenolol were investigated in six healthy volunteers following 7 days of oral monotherapy with these drugs, and after 7 days concurrent administration of each of these betareceptor antagonists with cimetidine. Application of cimetidine did not lead to any interaction with atenolol, whereas mean peak plasma levels of metoprolol were increased by 70%, and those of propranolol by 95% due to concurrent administration of cimetidine (P<0.05). The plasma level time curve (AUC) of the two above-mentioned beta blockers behaved similarly (P<0.05). Other kinetic parameters of these two drugs were not influenced to a statistically significant extent by cimetidine, despite the tendency for the elimination half-life of metoprolol and propranolol to be prolonged when cimetidine is added. Measurement of exercise-induced tachycardia on the sixth day of administration showed no differences between monotherapy with the beta blockers and combined treatment with each of them together with cimetidine. Apart from one volunteer who complained of anxiety, weakness, and sweating on the sixth day of cimetidine/metoprolol administration, no adverse effects could be observed during the combination therapy with cimetidine and the beta blockers, nor during monotherapy with beta blockers.ZusammenfassungDie Pharmakokinetik von Metoprolol, Propranolol und Atenolol wurde nach jeweils 7tägiger oraler Monotherapie mit diesen Substanzen und nach 7tägiger kombinierter Gabe jedes Betablockers zusammen mit Cimetidin bei 6 gesunden Probanden geprüft. Cimetidinapplikation führte zu keiner Änderung des kinetischen Verhaltens von Atenolol. Demgegenüber stiegen nach Cimetidingabe die maximalen Metoprolol-Plasmaspiegel durchschnittlich um 70% und die von Propranolol um 95% im Vergleich zur Monotherapie mit diesen beiden Betablockern an (P<0,05). Ähnlich verhielt sich die AUC beider Betablocker unter zusätzlicher Therapie von Cimetidin (P<0,05). Bis auf einen geringen, jedoch nicht signifikanten Anstieg der Eliminationshalbwertzeit von Metoprolol und Propranolol bei gleichzeitiger Cimetidingabe wurden andere kinetische Parameter dieser beiden Pharmaka durch Cimetidin nicht bemerkenswert verändert. Die am 6. Tag jeder Behandlungsphase gemessene Hemmung der belastungsabhängigen Tachykardie ergab keine signifikanten Unterschiede zwischen der Monotherapie mit den jeweiligen Betablockern und deren kombinierter Gabe mit Cimetidin. Außer einem Probanden, der am 6. Tag der Therapie mit Metoprolol und Cimetidin über Angstgefühl, Schwäche und Schweißausbruch klagte, wurden weder in der Behandlungsperiode von Cimetidin mit den 3 Betablockern noch in den Monotherapiephasen Nebenwirkungen beobachtet.

Interaction of bisoprolol with cimetidine and rifampicin

SummaryIn 6 healthy volunteers the pharmacokinetics of bisoprolol under steady-state conditions was investigated over three consecutive phases: over 7 days of 10 mg of bisoprolol once daily per os, 7 days of 10 mg of bisoprolol once daily plus 400 mg of cimetidine t.i.d. and 14 days of 10 mg of bisoprolol and 600 mg of rifampicin once daily with adequate intervals free of medication. After therapy with bisoprolol alone peak plasma levels (Cmaxss) of the beta-blocker were 55.5±6.4 ng/ml (x±SEM), area under the plasma level-time curve (AUCτ) was 597±70 ng/ml.h, total body clearance (CL) 15.8±1.8 l/h and elimination half-lives (t1/2β) 10.1±1.2 h. Cimetidine did not cause any significant changes in the pharmacokinetics of bisoprolol. Co-administration of rifampicin resulted in a decrease in Cmaxss (43.0±6.9 ng/ml), AUCτ (397±54 ng/ml·h) and t1/2β (6.2±0.4 h). Accordingly, total body clearance increased to 23.8±2.51/h (p<0.05). In conclusion bisoprolol showed a statistically significant but probably clinically not important interaction with the enzyme-inducing drug rifampicin, but not with the enzyme inhibitor cimetidine.

Effect of cisapride and metoclopramide on digoxin bioavailability

SummaryPharmacokinetics of digoxin were investigated in six healthy volunteers following one week of digoxin monotherapy 0.25 mg b.i.d., and during coadministration of metoclopramide 10 mg t.i.d. or cisapride 10 mg t.i.d.. Metoclopramide reduced the peak plasma concentration of digoxin from 1.5±0.2 ng/ml to 1.1±0.1 ng/ml (mean±SEM) (p=0.05), cisapride lowered the peak concentration to 1.3±0.1 ng/ml (p=0.14). Metoclopramide prolonged the time required to reach the peak concentration of digoxin from 2 hr to 2.7 hr (p=0.17), cisapride did not. Digoxin AUC0–12 (743±79 ng/ml.min) was reduced by 12% on coadministration of-cisapride (653±3 8 ng/ml.min, p=0.22) and by 19% on coadministration of metoclopramide (605±3 4 ng/ml.min, p=0.06).It is concluded that the gastrointestinal absorption of digoxin is reduced by both substances. Monitoring of the patient’s clinical status should be recommended when metoclopramide and cisapride are coadministered.