Morphine, morphine-6-glucuronide and morphine-3-glucuronide pharmacokinetics in newborn infants receiving diamorphine infusions

1The pharmacokinetics of morphine, morphine-6-glucuronide (M6G) and morphine-3-glucuronide (M3G) were studied in 19 ventilated newborn infants(24–41 weeks gestation) who were given a loading dose of 50 μg kg⁻¹ or 200 μg kg⁻¹ of diamorphine followed by an intravenous infusion of 15 μg kg⁻¹ h⁻¹ of diamorphine. Plasma concentrations of morphine, M3G and M6G were measured during the accrual to steady-state and at steady state of the diamorphine infusion. 2Following both the 50 μg kg⁻¹ or 200 μg kg⁻¹ loading doses the mean steady-state plasma concentration (±s.d.) of morphine, M3G and M6G were 86±52 ng ml⁻¹, 703±400 ng ml⁻¹ and 48±28 ng ml⁻¹ respectively and morphine clearance was found to be 4.6±3.2 ml min⁻¹ kg⁻¹. 3M3G formation clearance was estimated to be 2.5±1.8 ml min⁻¹ kg⁻¹, and the formation clearance of M6G was estimated to be 0.46±0.32 ml min⁻¹ kg⁻¹. 4M3G metabolite clearance was 0.46±0.60 ml min⁻¹ kg⁻¹, the elimination half-life was 11.1±11.3 h and the volume of distribution was 0.55±1.13 l kg⁻¹. M6G metabolite clearance was 0.71±0.36 ml min⁻¹ kg⁻¹, the elimination half-life was 18.2±13.6 h and the volume of distribution was 1.03±0.88 l kg⁻¹. 5No significant effect of the loading dose (50 μg kg⁻¹ or 200 μg kg⁻¹) on the plasma morphine or metabolite concentrations or their derived pharmacokinetic parameters was found. 6We were unable to identify correlations between gestational age of the infants and any of the determined pharmacokinetic parameters. 7M3G:morphine and M6G:morphine steady-state plasma concentration ratios were 11.0±10.8 and 0.8±0.8, respectively. 8The metabolism of morphine in neonates, in terms of the respective contributions of each glucuronide pathway, was similar to that in adults.     

The effects of co-administration of benzhexol on the peripheral pharmacokinetics of oral levodopa in young volunteers

1 The effects of benzhexol on the absorption and pharmacokinetics of an oral dose of levodopa have been studied in 10 young healthy volunteers. Subjects were given a suspension of levodopa (250 mg) 90 min after either benzhexol (5 mg) or placebo in a randomized cross over design with doses separated by at least 1 week; on each occasion carbidopa was given 1 h before and 5 h after the dose of levodopa. Soluble paracetamol and radiolabelled DTPA were given with the levodopa as markers of gastric emptying. 2 Most subjects showed two peaks in the levodopa plasma concentration–time curve on the placebo day, with the second minor peak occurring 1–2 h after the dose. After benzhexol administration all subjects showed two or more peak levodopa concentrations in plasma. Benzhexol administration caused a significant decrease in the maximum concentration (43%; P<0.05) of the initial peak and an increase (22%; P<0.1) in the maximum concentration of the second peak. This change in absorption profile caused by benzhexol significantly altered the ratios of the second peak compared with the initial peak for both the maximum concentrations (P<0.02) and for the AUC values (P<0.05). Benzhexol administration did not affect the total AUC of levodopa (7.30±1.09 vs 7.19±1.26 μg ml⁻¹ h; means±s.d.). 3 The plasma concentration–time curves for paracetamol showed similar profiles to those for levodopa and the ratios of the peak concentrations and AUC values for the second peak compared with the initial peak were increased significantly by benzhexol administration (P<0.05). The total AUC of paracetamol was not affected by benzhexol administration (39.4±8.2 vs 40.0±8.9 μg ml⁻¹ h; mean±s.d.) 4 Benzhexol altered the gastric emptying profile, shown by γ-scintigraphy, with a reduced extent of initial emptying prior to the establishment of the plateau which is characteristic of levodopa administration in the fasting state. In consequence the ratio of the second to the initial phase of emptying was significantly higher (P<0.01) following benzhexol treatment. 5 Benzhexol reduces the initial phase of gastric emptying after a dose of levodopa so that there is a decrease in the initial peak and a greater proportion of the dose is absorbed subsequently following the second phase of gastric emptying which occurs approximately 1 h later. Theoretically, this altered concentration-time profile could be an advantage for some patients with Parkinson''s disease.     

Dopamine natriuresis in salt-repleted, water-loaded humans: a dose-response study

Aims The purpose of the present study was to define the dose-response relationship between exogenous dopamine and systemic haemodynamics, renal haemodynamics, and renal excretory function at infusion rates in the range 0 to 12.5 μg kg⁻¹ min⁻¹ in normal volunteers. Methods While undergoing water diuresis, eight subjects were infused with 0, 1, 2, 3, 5, 7.5, 10 or 12.5 μg of dopamine kg⁻¹ min⁻¹ over 2 h in a randomized and double-blind fashion. On each study day, renal clearance studies were performed during a 1 h baseline period and subsequently during the second 1 h infusion period. Lithium clearance (CL_Li ) was used to estimate proximal tubular outflow. Results Cardiac output increased with the four highest doses. Mean arterial pressure followed a biphasic pattern with a decrease during the two lowest doses and a dose-dependent increase from the 7.5 μg kg⁻¹ min⁻¹ dose onwards. Effective renal plasma flow increased with all doses of dopamine, but peaked with the 3 μg kg⁻¹ min⁻¹ infusion rate [ from 617 (585–649) ml min⁻¹ with placebo to 915 (824–1006) ml min⁻¹ (means with 95% CI, P<0.001)]. None of the doses changed glomerular filtration rate (GFR). Sodium clearance (CL_Na ) and CL_Li were elevated with the four lowest doses but increased further from 7.5 μg kg⁻¹ min⁻¹ onwards. Compared with placebo, the percentage increase in CL_Na with increasing dose was 77 (5–159), 93 (13–172), 107 (24–190), 121 (60–181), 253 (65–441), 284 (74–494), and 212 (111–312) %, respectively. There were only small, inconsistent decreases in absolute proximal reabsorption rate (APR=GFR-CL_Li ). Fractional distal reabsorption of sodium (FDR_Na=(CL_Li-CL_Na )/CL_Li ) decreased with all doses, reaching its nadir with 7.5 μg kg⁻¹ min⁻¹ [from 95.9 (94.6–97.2) % with placebo to 91.5 (90.0–93.0) % (P<0.01)] whereafter a flat dose-response curve was observed. Conclusions In conclusion, the renal vasodilating effect of dopamine was maximal with 3 μg kg⁻¹ min⁻¹. The dose-dependent attenuation seen with higher doses is consistent with an increased α-adrenergic stimulation opposing the effect on dopaminergic receptors. The present CL_Li studies confirm that dopamine increases proximal tubular outflow. The results suggest that the natriuretic effect of depressor doses of dopamine was primarily caused by attenuation of the increase in distal sodium reabsorption normally seen after an increase in proximal tubular outflow. Pressor doses further increased sodium excretion, indicating the presence of pressure natriuresis at these high doses.     

The effect of omeprazole on the pharmacokinetics of metronidazole and hydroxymetronidazole in human plasma, saliva and gastric juice

Aims To evaluate the effect of omeprazole on the pharmacokinetics of metronidazole and hydroxymetronidazole in plasma, gastric juice and saliva following intravenous infusion or oral dosing of metronidazole. Methods Eight volunteers received single doses of metronidazole (400 mg) intravenously and orally, whilst taking placebo or omeprazole (40 mg, twice daily for 5 days) in a randomized 4-way crossover study. Metronidazole and hydroxymetronidazole concentrations in plasma, saliva and gastric juice samples were determined by h.p.l.c. Pharmacokinetic parameters for metronidazole and hydroxymetronidazole were calculated, and the significance of the mean differences in parameters between omeprazole and placebo co-administration was assessed using a two-tailed, paired t-test. Results There were no significant differences (P<0.05) in any of the plasma or saliva pharmacokinetic parameter values for metronidazole between volunteers receiving omeprazole or placebo when metronidazole was administered either as an intravenous infusion or orally. Following intravenous administration of metronidazole to the placebo group and omeprazole treated group respectively, the gastric transfer of metronidazole was significantly reduced from 15.5±10.4% to 2.6±1.0% of the dose (P=0.007; 95% CI of difference 4.8 to 21.0) with concomitant changes in the metronidazole AUC (from 77.5±18.0 μmol l⁻¹ h to 352.6±182.1 μmol l⁻¹ h; P=0.0003; 95% CI of difference 127.6 to 422.7), C_max (from 61.4±26.5 μmol l⁻¹ to 271.8±104.3 μmol l⁻¹; P=0.0001; 95% CI of difference 118.6 to 302.1). Similarly, the gastric juice AUC of hydroxymetronidazole was significantly reduced from 3.2±1.9 μmol l⁻¹ h to 1.5±0.8 μmol l⁻¹ h of the dose (P=0.0043; 95% CI of difference 0.4 to 3.0) with a concomitant change in C_max (from 5.0±2.5 μmol l⁻¹ to 3.0±1.2 μmol l⁻¹; P=0.0007; 95% CI of difference 0.7 to 3.4). Conclusions Omeprazole had little effect on the plasma and salivary pharmacokinetics of metronidazole (or its hydroxymetabolite) after intravenous or oral administration, but it did have a substantial effect on the pharmacokinetics of metronidazole and hydroxymetronidazole in gastric juice.     

The influence of the sparteine/debrisoquine genetic polymorphism on the disposition of dexfenfluramine

1 To determine whether dexfenfluramine is a substrate of cytochrome P450 2D6 (CYP2D6), its disposition has been studied in nine extensive (EM) and eight poor metabolizers (PM) of debrisoquine. 2 Following a 30 mg dose of dexfenfluramine hydrochloride, urine was collected in all subjects for 96 h post-dose and plasma samples were collected in 11 subjects (six EMs and five PMs). Dexfenfluramine and nordexfenfluramine were measured in urine by h.p.l.c. and in plasma by g.c. 3 Urinary recovery of dexfenfluramine was greater in PMs than EMs (4136±1509 μg vs 1986±792 μg; 95% CI of difference 926–3374; P<0.05) whereas that of nordexfenfluramine was similar in both phenotypes (PM: 1753±411 μg vs 1626±444 μg). 4 Dexfenfluramine AUC was higher in PMs (677±348 μg l⁻¹ h) than EMs (359±250 μg l⁻¹ h). The apparent oral clearance of dexfenfluramine was greater in EMs than PMs (93.6±42.4 l h⁻¹vs 45.6±19.5 l h⁻¹; 95% CI of difference 1.2–94.7; P<0.05). The renal clearance was similar in both phenotypes (EMs: 5.88±2.83 l h⁻¹; PMs 6.60±2.01 l h⁻¹), indicating that the higher urinary recovery of dexfenfluramine in PMs reflects higher plasma concentrations, rather than phenotype differences in the renal handling, of dexfenfluramine. 5 The apparent nonrenal clearance of dexfenfluramine was substantially lower (P<0.05; 95% CI of difference 3.0–94.1) in PMs (39.0±19.5 l h⁻¹) than EMs (87.6±41.2 l h⁻¹). 6 There was a significant inverse correlation (r_s=−0.776 95% CI −0.31–−0.94; n=11; P=0.005) between the debrisoquine metabolic ratio and the apparent nonrenal clearance of dexfenfluramine. 7 PMs had a higher incidence of adverse effects (nausea and vomiting) than EMs. 8 In conclusion, the metabolism of dexfenfluramine is impaired in PMs. Thus CYP2D6, the isoenzyme deficient in poor metabolizers of debrisoquine, must catalyse at least one pathway of dexfenfluramine biotransformation.     

Effect of simvastatin on cyclosporine unbound fraction and apparent blood clearance in heart transplant recipients

Aims To investigate the effects of lipid lowering therapy on the fraction unbound and dosage requirement of cyclosporine in heart transplant recipients. Methods Cyclosporine fraction unbound (fu) was measured ex vivo in plasma obtained from heart transplant recipients (n=12) before and after lipid lowering treatment, using equilibrium dialysis. Cyclosporine trough concentration data were also collected from cardiac transplant recipients (n=32) who received simvastatin for the treatment of hyperlipidaemia. Cyclosporine daily dosage and total concentration (monoclonal FPIA method) were recorded for periods up to 6 months before and after simvastatin administration. The total number of dose rate-concentration observations was 172 before and 135 after simvastatin administration respectively. Using a population pharmacokinetic approach (implemented in P-PHARM software) the ratio of dose rate to trough concentration at steady state (DR/C_sstrough ), an estimation of apparent clearance, was determined. The posterior Bayesian estimate of DR/C_sstrough was calculated for each patient before and after simvastatin administration. Results The mean fu increased by 29%, from 1.40±0.1% (mean±s.d.) to 1.82±0.22% after simvastatin administration (P<0.01). Mean trough concentrations of cyclosporine in whole blood were 349 μg l⁻¹ before and 242 μg l⁻¹ after simvastatin administration (P<0.0001). The mean cyclosporine daily dosage was 2.87 mg kg⁻¹ and 2.33 mg kg⁻¹ (NS), before and after simvastatin administration respectively. The average cyclosporine DR/C_sstrough was significantly increased from 24.5 l h⁻¹ before to 28.9 l h⁻¹ after simvastatin administration (P<0.05). Furthermore the median increase in cyclosporine DR/C_sstrough was 18 l h⁻¹ (−3.1 to 42.1 l h⁻¹, interquartile range). Conclusions Cyclosporine fraction unbound and clearance are increased following co-administration of lipid lowering agents, necessitating closer monitoring of cyclosporine total blood concentration when lipid lowering agents are administered concomitantly with cyclosporine.     

A pharmacokinetic and pharmacodynamic interaction study between nebivolol and the H2-receptor antagonists cimetidine and ranitidine

Aims The study was designed to investigate the effects of the H₂-receptor antagonists, cimetidine and ranitidine on the pharmacokinetics and pharmacodynamics of nebivolol in healthy volunteers. Methods Twelve healthy volunteers took part in a randomized placebo-controlled cross-over study. Each subject received on three separate occasions placebo, cimetidine (400 mg twice daily) or ranitidine (150 mg twice daily) for 24 h before and 48 h after a single oral dose of nebivolol (5 mg). Nebivolol and its individual (+) and (−) enantiomers were determined tby h.p.l.c. Results Ranitidine had no significant effect on nebivolol pharmacokinetics. Cimetidine, however, resulted in a 21–23% increase in C_max of unchanged nebivolol and of each enantiomer plus its hydroxylated metabolites. Cimetidine significantly (P<0.05) increased the AUC [mean±s.d. (95% C.I. of differences in mean)] for unchanged (±)-nebivolol [7.76±3.07 ng ml⁻¹h with placebo; 11.50±5.40 (1.75, 8.76) ng ml⁻¹h with cimetidine], (+)-nebivolol plus its hydroxylated metabolites [73.0±18.0 ng ml⁻¹h with placebo; 91.5±25.7 (1.0, 23.1) ng ml⁻¹h with cimetidine] and (−)-nebivolol plus its hydroxylated metabolites [101±32 ng ml⁻¹h with placebo; 123±38 (3.3, 27.0) ng ml⁻¹h with cimetidine]. Statistical analysis of the resting blood pressure and heart rate and exercise data did not suggest any consistent effects of ranitidine or cimetidine upon the pharmacodynamic effects of nebivolol. Conclusions There was no interaction between ranitidine and nebivolol. Although cimetidine inhibited nebivolol metabolism, it did not have a significant influence on the pharmacodynamics of the drug.     

Pharmacokinetic and pharmacodynamic interaction trial after repeated oral doses of imdapril and digoxin in healthy volunteers

Aims To investigate the potential pharmacokinetic and pharmacodynamic interaction between imidapril and digoxin. Methods AUC, C_max and t_max of imidapril, imidaprilat and digoxin were calculated and evaluated in a randomized, doubleblind three-period cross-over design in 12 healthy volunteers after 8 days treatment with the following combinations: digoxin 0.25 mg day⁻¹+placebo (D+P); imidapril 10 mg day⁻¹+placebo (I+P); imidapril 10 mg day⁻¹+digoxin 0.25 mg day⁻¹ (I+D). Results Mean AUC (0, 24 h) of digoxin was 10.4 (±4.9 s.d.) ng ml⁻¹ h (D+P) and 10.7 (±3.9 s.d.) ng ml⁻¹ h (I+D), respectively (90%-confidence intervals [ CI] for the ratio of (D+P) and (I+D): 0.91–1.27, point estimator [PE]: 1.06). Mean AUC (0, 24 h) of imidapril was 133 (±86 s.d.) ng ml⁻¹ h (I+P) and 108 (±52 s.d.) ng ml⁻¹ h (I+D), respectively (90%-CI: 0.76–0.94, PE 0.85). AUC (0, 24 h) of imidaprilat was 215 (±91 s.d.) ng ml⁻¹ h (I+P) and 194 (±54 s.d.) ng ml⁻¹ h (I+D), respectively (90%-CI: 0.80–1.08, PE 0.93). C_max was 19.9 (±8.7 s.d.) ng ml⁻¹ (I+P) and 15.9 (±5.3 s.d.) ng ml⁻¹ (I+D) (90%-CI: 0.67–1.00, PE 0.82). The results indicate a slight reduction of imidapril and imidaprilat plasma levels when coadministered with digoxin without any effect on digoxin plasma levels. Maximal ACE-inhibition was 79% (I+P) and 67% (I+D). Conclusions Grouped data analysis of imidaprilat plasma levels vs ACE-activity showed that for maximal inhibition of plasma ACE activity, imidaprilat plasma levels should exceed 10 ng ml⁻¹. Under digoxin and imidapril, more plasma concentrations of imidaprilat were seen under this level as after imidapril alone, this reduces the integral of the ACE-inhibition/time curves by about 20 to 30%.     

Comparison of nitroprusside and nitroglycerin in inhibition of angiotensin II and other vasoconstrictor-mediated contraction in human coronary bypass conduits

Aims To compare the effect of nitroprusside (SNP) and nitroglycerin (NTG) on angiotensin II (ANGII), endothelin-1 (ET-1), and α₁-adrenoceptor (phenylephrine, PE)-mediated contraction in internal mammary artery (IMA). Methods Human IMA segments (n=120) taken from 37 patients were studied. Concentration-relaxation curves for SNP and NTG were established in IMA precontracted with these vasoconstrictors. Concentration-contraction curves were also constructed in IMA rings incubated with SNP and NTG (0.1 and 1 μm ) for 10 min. Results Both SNP and NTG caused full relaxation with similar EC₅₀ s except NTG was four-fold more potent than SNP in PE-induced contraction (−7.92±0.06 vs−7.32±0.2 log m, mean±s.e. mean, P<0.01; 95% confidence interval for the difference of the means: 0.19, 1.01 log m ). Pretreatment with SNP (0.1 and 1 μm ) significantly depressed the contraction by ANGII from 56.6±7.7% (of 100 mm K⁺-contraction) to 18.3±8.6% and 3.9±2.1% (P=0.0001). In four rings treated with SNP, the contraction to ANGII was abolished whereas NTG did not depress ANGII-mediated contraction. Pretreatment with SNP (1 μm ), but not NTG, significantly depressed the magnitude of the PE-induced contraction from 4.7±1.2 to 1.7±0.4 g (P<0.05). Treatment with both SNP and NTG significantly increased the EC₅₀ (−5.09±0.17 log m, P=0.0007 for SNP and −5.40±0.06 log M, P=0.02 for NTG). Pretreatment with SNP did not significantly change either the magnitude or the EC₅₀ of the ET-1-induced contraction. Conclusions SNP may be advantageous compared with NTG in preventing coronary arterial graft contraction. However, once grafts have constricted to ANGII, α₁-adrenoceptor agonists, and ET-1, NTG may be only marginally advantageous.     

The effects of combined treatment with β1-selective receptor antagonists and lipid-lowering drugs on fat metabolism and measures of fatigue during moderate intensity exercise: a placebo-controlled study in healthy subjects

Aims We examined the effects of different combinations of β₁-selective adrenoceptor blockers and lipid-lowering drugs, on fat metabolism and fatigue during moderate intensity exercise in 14 healthy young volunteers. Methods The study was a randomized crossover design, each subject completing 5, 90 min walks at 50% of predetermined maximal oxygen uptake (VO₂ max), one following each 3 day treatment period with either: atenolol 100 mg and bezafibrate 400 mg, atenolol 100 mg and fluvastatin 40 mg, metoprolol CR 100 mg and bezafibrate 400 mg, metoprolol CR 100 mg and fluvastatin 40 mg, or placebo. Results Plasma free fatty acid (FFA) concentration during exercise was significantly reduced on all treatments, in comparison with placebo, P=0.0001. Following 90 min of exercise FFA levels were as follows: placebo 573 μmol l⁻¹ (105–1041), metoprolol CR+fluvastatin 277 μmol l⁻¹ (0–647), metoprolol CR+bezafibrate 182 μmol l⁻¹ (0–396), atenolol+fluvastatin 211 μmol l⁻¹ (0–511), and atenolol+bezafibrate 123 μmol l⁻¹ (0–352). Total fat oxidation during exercise was also reduced on all treatments in comparison with placebo: 38.1% (2–74), compared with 29.1% (0–61) on metoprolol CR+fluvastatin, P=0.02, 26.2% (2–51) on metoprolol CR+bezafibrate, P=0.002, 25.5% (3–48) on atenolol+fluvastatin, P=0.009, and 22.8% (0–47) on atenolol+bezafibrate treatment, P=0.0002. Plasma ammonia concentration was elevated on all treatments during exercise in comparison with placebo. After 90 min of exercise, plasma ammonia levels were as follows: placebo 37 μmol l⁻¹ (0–84), metoprolol CR+fluvastatin 56 μmol l⁻¹ (2–110), metoprolol CR+bezafibrate 79 μmol l⁻¹ (0–167), atenolol+fluvastatin 90 μmol l⁻¹ (10–170), and atenolol+bezafibrate 100 μmol l⁻¹ 26–174). In comparison with placebo, metoprolol CR+fluvastatin had the least adverse impact on measures of perceived exertion and the ‘feeling scale’ during exercise. Metoprolol CR+bezafibrate, atenolol+fluvastatin, and atenolol+bezafibrate treatments had greater adverse effects, particularly on perceived ‘cardiorespiratory effort’ and ‘feeling scale’ scores. Conclusions In healthy volunteers, combinations of β₁-selective blockers and lipid-lowering drugs were associated with significant reductions in fat metabolism, increased plasma ammonia levels, and raised the perception of effort during exercise, in comparison with placebo. Metoprolol CR+fluvastatin had the least effect, combinations metoprolol CR+bezafibrate and atenolol+fluvastatin had intermediate effects, and atenolol+bezafibrate had the most adverse effect.     

Effects of oral and inhaled corticosteroid on lymphocyte β2-adrenoceptor function in asthmatic patients

Aims We have previously demonstrated that a single dose of oral prednisolone but not single doses of inhaled fluticasone had facilitatory effects on lymphocyte β₂-adrenoceptor (AR) function. To address possible differences in steady-state time-course, the aim of this study was to determine if repeated dosing with inhaled fluticasone would have facilitatory effects on lymphocyte β₂-AR. Plasma cortisol was also evaluated as a measure of systemic bioactivity. Methods Ten asthmatic subjects, mean (s.e.mean) age 29 (3) years, FEV₁ 89 (5) % predicted, were randomised in a double-blind crossover study to receive inhaled placebo (PL), inhaled fluticasone 1000 μg day⁻¹ (F1000) and inhaled fluticasone 2000 μg day⁻¹, each for 4 days and also a single dose of oral prednisolone 50 mg (PRED). Prednisolone was given as open medication. The last dose of study drug was taken at 22.00 h and subjects attended the laboratory at 08.00 h the following day. Results β₂-AR density (B_max; fmol/10⁶ cells) was significantly increased after PRED compared with PL and inhaled fluticasone. B_max (geometric mean) after each treatment were: PL 1.51, F1000 1.20, F2000 1.20 and PRED 2.14 (a 1.4 fold difference PRED vs PL; 95% CI 1.05 to 1.95; P<0.001). There was significant (P<0.001) suppression of plasma cortisol (nmol l⁻¹ ) following F2000 and PRED compared with PL: 393.8, F1000 302.1, F2000 205.0 (95% CI F2000 vs PL 58.1 to 319.4) and PRED 87.0 (95% CI PRED vs PL 176.2 to 437.5). The estimated milligram equivalence ratio for adrenal suppression was calculated at 1:11 for fluticasone vs prednisolone. Conclusions Repeated dosing with high-dose inhaled fluticasone did not up-regulate lymphocyte β₂-AR as compared with a single dose of oral prednisolone, despite having significantly suppressed early morning plasma cortisol. This study confirms our previous finding of a dissociation in sensitivity between effects of inhaled corticosteroid on adrenal suppression and lymphocyte β₂-AR regulation, at least for doses up to 2 mg day⁻¹ of fluticasone.     

The involvement of CYP1A2 and CYP3A4 in the metabolism of clozapine

Aims Clozapine (CLZ), an atypical neuroleptic with a high risk of causing agranulocytosis, is metabolized in the liver to desmethylclozapine (DCLZ) and clozapine N-oxide (CLZ-NO). This study investigated the involvement of different CYP isoforms in the formation of these two metabolites. Methods Human liver microsomal incubations, chemical inhibitors, specific antibodies, and different cytochrome P450 expression systems were used. Results K_m and V_max values determined in human liver microsomes were lower for the demethylation (61±21 μm, 159±42 pmol min⁻¹ mg protein⁻¹ mean±s.d.; n=4), than for the N-oxidation of CLZ (308±1.5 μm, 456±167pmol min⁻¹ mg protein⁻¹; n=3). Formation of DCLZ was inhibited by fluvoxamine (53±28% at 10 μm ), triacetyloleandomycin (33±15% at 10 μm ), and ketoconazole (51±28% at 2 μm ) and by antibodies against CYP1A2 and CYP3A4. CLZ-NO formation was inhibited by triacetyloleandomycin (34±16% at 10 μm ) and ketoconazole (51±13% at 2 μm ), and by antibodies against CYP3A4. There was a significant correlation between CYP3A content and DCLZ formation in microsomes from 15 human livers (r=0.67; P=0.04). A high but not significant correlation coefficient was found for CYP3A content and CLZ-NO formation (r=0.59; P=0.09). Using expression systems it was shown that CYP1A2 and CYP3A4 formed DCLZ and CLZ-NO. K_m and V_max values were lower in the CYP1A2 expression system compared to CYP3A4 for both metabolic reactions. Conclusions It is concluded that CYP1A2 and CYP3A4 are involved in the demethylation of CLZ and CYP3A4 in the N-oxidation of CLZ. Close monitoring of CLZ plasma levels is recommended in patients treated at the same time with other drugs affecting these two enzymes.     

Inhibition of vasoconstriction by potassium channel opener aprikalim in human conduit arteries used as bypass grafts

Aims Potassium channel openers (KCOs) are of potential therapeutic value. Little is known about the effect of these drugs on human conduit arteries used as coronary bypass grafts. The purpose of this study was to determine the effect of the KCO aprikalim (RP52891) on human arteries used as coronary bypass grafts with emphasis on the possible difference in the inhibitory effect on depolarizing agent-mediated rather than receptor-mediated contraction. Methods Human internal mammary artery segments (IMA, n=88) taken from 28 patients were studied. Concentration-relaxation curves for aprikalim were established in IMA precontracted with three vasoconstrictors (K⁺, U46619, and phenylephrine). In IMA rings incubated with aprikalim (1 or 30 μm ) for 10 min concentration-contraction curves for the three vasoconstrictors were constructed. Results Aprikalim-induced relaxation was less in K⁺ (37.3±6.4%) than in U46619 (80.2±7.7%, P=0.002), or phenylephrine (67.5±7.0%, P=0.038) -precontracted IMA. The EC₅₀ for K⁺-(−5.40±0.12 log m ) was significantly higher than that for phenylephrine (−6.43±0.30 log m, P=0.007) but not significant compared with that for U46619 (−5.81±0.11, P >0.05). Pretreatment with aprikalim depressed the contraction by phenylephrine from 140.6±27.6% to 49.3±14.1% (P=0.002) and shifted the EC₅₀ 11.0-fold higher in rings treated with 1 μm aprikalim (P=0.007). Treatment of aprikalim did not significantly reduce the K⁺ and U46619-induced contraction (P >0.05) but shifted the concentration-contraction curves rightward (2.8-fold higher for K⁺, P<0.05 and 2.2-fold higher for U46619, P<0.05). Conclusions This study demonstrates that aprikalim has vasorelaxant effects in human conduit arteries used as coronary artery bypass grafts contracted by a variety of vasoconstrictors and this effect is vasoconstrictor-selective with greater potency for α₁-adrenoceptor agonists than for depolarizing agent K⁺. These findings provide information on the possible use of this KCO in various clinical settings.     

Fluvoxamine pharmacokinetics in healthy elderly subjects and elderly patients with chronic heart failure

AIMS

To investigate the effects of age and chronic heart failure (CHF) on the oral disposition kinetics of fluvoxamine.

METHODS

A single fluvoxamine dose (50 mg) was administered orally to 10 healthy young adults, 10 healthy elderly subjects and 10 elderly patients with CHF. Fluvoxamine concentration in plasma was measured for up to 96 h.

RESULTS

With the exception of apparent distribution volume, ageing modified all main pharmacokinetic parameters of fluvoxamine. Thus, peak concentration was about doubled {31 ± 19 vs. 15 ± 9 ng ml⁻¹; difference [95% confidence interval (CI)] 16 (3, 29), P < 0.05}, and area under the concentration–time curve was almost three times higher [885 ± 560 vs. 304 ± 84 ng h ml⁻¹; difference (95% CI) 581 (205, 957), P < 0.05]; half-life was prolonged by 63% [21.1 ± 6.2 vs. 12.9 ± 6.4 h; difference (95% CI) 8.2 (2.3, 14.1), P < 0.01], and oral clearance was halved (1.12 ± 0.77 vs. 2.25 ± 0.66 l h⁻¹ kg⁻¹; difference (95% CI) −1.13 (−1.80, −0.46), P < 0.001]. A significant inverse correlation was consistently observed between age and oral clearance (r=−0.67; P < 0.001). The coexistence of CHF had no significant effect on any pharmacokinetic parameters in elderly subjects.

CONCLUSIONS

Ageing results in considerable impairment of fluvoxamine disposition, whereas CHF causes no significant modifications. Therefore, adjustment of initial dose and subsequent dose titrations may be required in elderly subjects, whereas no further dose reduction is necessary in elderly patients with CHF.     

In vitro adrenaline and collagen-induced mobilization of platelet calcium and its inhibition by naftopidil, doxazosin and nifedipine

Aims The aim of the study was to obtain further information regarding the modes of action of doxazosin, naftopidil and nifedipine on platelet function.Methods We conducted an in vitro study of drug influences on adrenaline and collagen-induced mobilization of platelet calcium.‘fn2\Results In the presence of fibrinogen (300 μg ml⁻¹ ) both collagen (5 μg ml⁻¹ ) and adrenaline (16 μm ) stimulated the aggregation of washed platelets. Collagen induced a transient rise (+4.97±0.63 μm ) in platelet Ca²⁺ concentration, [Ca²⁺]_i, as measured using the photoprotein aequorin, which coincided with the onset of aggregation. Adrenaline induced a smaller rise (+3.6±0.96 μm ) which, however, occurred after the onset of aggregation. Naftopidil, an α₁-adrenoreceptor antagonist produced a concentration-dependent inhibition of collagen-induced Ca²⁺ mobilization, maximum inhibition (22.9±4%, P<0.05) occurring with 40 μm naftopidil. The inhibition of Ca²⁺ mobilization was not reflected by a concentration-dependent inhibition of platelet aggregation, although 40 μm naftopidil produced statistically significant inhibition (23.3±11.7%, P<0.05). The adrenaline-induced rise in [Ca²⁺]_i was inhibited dose dependently by naftopidil (e.g. 40 μm naftopidil, 100±0%, P<0.05), as was aggregation (40 μm naftopidil, 100±0%, P<0.05). Doxazosin, another α₁-adrenoreceptor blocker, inhibited Ca²⁺ mobilization induced by collagen to similar extents as for naftopidil (30 μm doxazosin, 17.4±2.5%, P<0.05), but did not inhibit platelet aggregation. It also inhibited the adrenaline-induced rise in [Ca²⁺]_i in a concentration-dependent manner (30 μm doxazosin, 37.6±13.7%, P<0.05), significant inhibitions of platelet aggregation also being produced (30 μm, 49.6±17.2%, P<0.05). As expected, the calcium channel blocker nifedipine produced concentration-dependent inhibitions of both collagen-induced Ca²⁺ mobilization (e.g. 28 μm nifedipine, 47.8±2.7%, P<0.05) and aggregation (28 μm, 55.1±9.2%, P<0.05).Conclusions These data indicate that the α₁-adrenoreceptor blockers, naftopidil and doxazosin, inhibit Ca²⁺ mobilization, this mechanism being possibly the means whereby these drugs inhibit platelet aggregation.     

Pharmacokinetics of trapidil,an antagonist of platelet derived growth factor,in healthy subjects and in patients with liver cirrhosis

1Pharmacokinetic parameters of trapidil (an antagonist of platelet derived growth factor) were evaluated in 12 healthy male subjects (study I) and in a group of 10 patients with liver cirrhosis (Child B) and five control subjects, respectively (study II). 2Investigations were carried out after a single dose trapidil (200 mg) and at steady state after application of 200 mg trapidil three times daily for 5 days (study 1) or 4 days (study II). 3Study I: The concentration-time curves of the terminal elimination phase of trapidil exhibited a slight convexity which might reflect nonlinear kinetics. The AUC of trapidil obtained after the first dose (20.5 [±7.0 s.d.] μg ml⁻¹ h) was markedly higher than the AUC determined at steady state (13.2 [±3.8 s.d.] μg ml⁻¹ h), the non-parametric 90% confidence intervals of the ratio day 5/day 1 was 0.58–0.73 (point estimator 0.64). 4Study II: AUC averaged (21.4 [±9.1 s.d.] μg ml⁻¹ h) in controls and (34.4 [±14.9 s.d.] μg ml⁻¹ h) in cirrhotic patients. The 90% confidence intervals for the difference group 1 vs group 2 was 0.95–2.97 (point estimator 1.48, P=0.066). At steady state, AUC averaged (13.7 [±5.7 s.d.] μg ml⁻¹ h) in controls and (20.8 [±6.8 s.d.] μg ml⁻¹ h) in cirrhotic patients (90% confidence intervals group 1 vs group 2: 0.88–2.20 [point estimator 1.45, P=0.05]). As seen in study I, the AUC of trapidil obtained after the first dose was markedly higher than the AUC determined at steady state , the non-parametric 90% confidence intervals of the ratio day 5/day 1 was 0.48–0.84 (point estimator 0.66) in control subjects and 0.54–0.72 (point estimator 0.64) in cirrhotic patients, respectively. 5An inverse correlation was seen between the results of the monoethylglycin-xilidid (MEGX)-test and the AUC of trapidil (single dose: r=−0.516, P=0.048; steady state: r=−0.548, P=0.042). 6Results of study I and study II indicate an autoinduction of trapidil metabolism after repeated oral doses. Although trapidil elimination is decreased in patients with liver cirrhosis (study II), the elimination half-life at steady state is relatively short (2.4 [±1.1 s.d.] h) and therefore should prevent cumulation of trapidil even in cirrhotic patients.     

Clinical efficacy and pharmacokinetics of artemisinin monotherapy and in combination with mefloquine in patients with falciparum malaria

1The aim of this study was to assess the pharmacokinetics, clinical efficacy and safety of artemisinin alone and in combination with mefloquine. 2Thirty-eight adults with symptomatic Plasmodium falciparum malaria were randomly assigned to receive either artemisinin (500 mg single dose followed by another 500 mg on day 1 and then 250 mg twice daily for 4 days) or artemisinin (500 mg single dose followed by 750 mg on day 1 and then 250 mg three times daily for one more day) in co-administration with mefloquine (250 mg three times daily for the first day). All drug administration was by the oral route. Patients were hospitalized at the Kibaha Designated District Hospital, Kibaha, Tanzania, for 6 days and a follow up for 3 weeks was performed. 3Treatment with the artemisinin/mefloquine combination resulted in a shorter parasite clearance time (PCT) of 24 (22, 27; 95% confidence interval) h vs 31 (27, 36) h and fever subsidence time (FST) of 14 (12, 16) h vs 20 (18, 23) h compared with artemisinin monotherapy. The 95% CI for the difference of the PCT and FST were 1.7, 12 and 3, 10, respectively. Parasites were detected in 7 out of 17 patients (41%) receiving artemisinin monotherapy at the 3rd and 4th week follow up visits. No parasites were detected after the combination therapy. 4The maximum plasma concentrations ( C_max) were similar after artemisinin monotherapy (615.4±387.0 ng ml⁻¹) and in combination with mefloquine (851.8±523.6 ng ml⁻¹). Elimination half-lives (t_1/2) were also identical at 2.2±0.6 h and 2.5±0.7 h, respectively. However, the AUC values were higher ( P<0.05) after combination therapy (3252±1873 ng  ml⁻¹ h) than after monotherapy (2234±1502 ng ml⁻¹ h). The oral clearance values were lower ( P<0.05) after combination therapy (195.4±86.9 l h⁻¹) than after monotherapy (314.3±189.4 l h⁻¹). PCT and FST normalized to initial parasitaemia correlated with AUC(0,  t) (r_s=0.56, P=0.02, r_s=0.58, P=0.01, respectively) and with C_max (r_s=0.62, P=0.01, r_s=0.68, P=0.005, respectively) in the artemisinin monotherapy only. 5One patient on the combination therapy developed a psychiatric condition and two patients on the monotherapy developed skin itch.     

Effect of high-fat, high-protein, and high-carbohydrate meals on the pharmacokinetics of a small dose of ethanol

Aims To investigate whether the relative amounts of fat, carbohydrate (CHO), or protein in a meal influence the pharmacokinetics of a small dose of ethanol. Methods Nine healthy men received ethanol (0.30 g kg⁻¹ body weight) on five occasions in a randomized cross-over fashion. On three occasions the dose of ethanol was consumed within 15 min of eating a standardized breakfast of similar volume and calorific value but containing different amounts of fat, CHO, and protein. On two other occasions the same dose of ethanol was ingested on an empty stomach (overnight fast) or administered by intravenous (i.v.) infusion over 30 min. Results The blood-ethanol profiles showed large inter and intraindividual variations, especially when ethanol was ingested after eating food. The peak blood-alcohol concentrations (BAC) were 16.6±4.0, 17.7±7.1, and 13.3±4.0 mg dl⁻¹ (mean±s.d.) after fat, CHO, and protein-rich meals and 30.8±4.3 and 54.3±6.4 mg dl⁻¹ after fasting and i.v. infusion, respectively. The corresponding areas under the concentration-time profiles (AUC) were 1767±549, 1619±760, 1270±406 mg dl⁻¹ min after fat, CHO, and protein-rich meals compared with 3210±527 and 4786±446 mg dl⁻¹ min after fasting and i.v. infusion, respectively. The time required to eliminate ethanol from the blood was shortened by 1–2 h in the fed-state. Conclusions Drinking ethanol after eating a meal, regardless of the nutritional composition, decreases the systemic availability of ethanol. Because gastric emptying is slow and more prolonged with food in the stomach, the delivery of ethanol to the duodenum and the liver will be highly variable as will the hepatic clearance of ethanol. Provided that portal venous BAC remains fairly low and ethanol metabolizing enzymes are not fully saturated then part of the dose of ethanol can be cleared by hepatic first-pass metabolism (FPM), as one consequence of Michaelis-Menten elimination kinetics.     

Haemodynamic effects of adrenomedullin in human resistance and capacitance vessels

Aims The haemodynamic effects of adrenomedullin and calcitonin gene-related peptide (CGRP) were studied in resistance and capacitance vessels of healthy volunteers. Methods Adrenomedullin and CGRP were infused into the brachial artery of eight healthy subjects on two separate occasions at doses between 0.3–30 pmol min⁻¹. Forearm blood flow was measured using venous occlusion plethysmography. Venodilatation to adrenomedullin and CGRP was assessed in a further eight subjects by infusing the peptides at doses between 0.3–10 pmol min⁻¹ into a dorsal hand vein preconstricted with noradrenaline. Venodilator responses were measured as percentage reduction in noradrenaline preconstriction. Results Adrenomedullin and CGRP at a dose of 30 pmol min⁻¹, produced an increase in forearm blood flow of 288±42% and 252±30% respectively (mean±s.e. mean, P<0.001). At doses between 3 and 10 pmol min⁻¹ adrenomedullin was significantly more potent than CGRP. The vasodilatation to both peptides was of similar duration with a biological half-life of approximately 18 min. Adrenomedullin reversed constriction in dorsal hand veins by 84±2% (P<0.001) at a dose of 10 pmol min⁻¹. CGRP produced a similar effect reversing constriction by 72±12% at the same dose (P<0.01). In veins, adrenomedullin was also more potent than CGRP at doses between 0.3 and 3 pmol min⁻¹. Conclusions The lowest dose of adrenomedullin producing significant arteriolar dilatation was calculated to produce plasma levels similar to those found in heart failure. These findings suggest that in pathophysiological conditions such as heart failure circulating levels of adrenomedullin may be within a range capable of influencing vascular resistance directly.     

Disposition and effects of flurbiprofen enantiomers in human serum and blister fluid

Aims To investigate the pharmacokinetics of the enantiomers of flurbiprofen and inhibition of prostanoid production in blister fluid and serum. Methods Eleven healthy volunteers received 75 mg R-, 75 mg S-flurbiprofen or no medication in a randomized 3-way cross-over study. Flurbiprofen concentrations were determined by h.p.l.c. TXB₂ and PGE₂ were determined by enzyme immunoassay and chemiluminescence immunoassay respectively. Results S-flurbiprofen produced almost complete (>99%vs baseline) inhibition of thromboxane B₂ (TXB₂ ) in serum in all volunteers and significant inhibition of prostaglandin E₂ (PGE₂ ) generation in blister fluid, but there was a considerable inter-individual variation in the response ranging from −78 to +190% change from control PGE₂ AUC. After administration of R-flurbiprofen, there was a mean maximum TXB₂ inhibition of 65.2±15.0% in serum but no significant changes of PGE₂ levels in blister fluid were observed. The pharmacokinetic parameters in serum and blister fluid were not significantly different between enantiomers. R- to S-inversion did not occur to a clinically relevant extent. For R-flurbiprofen, the complex rate constant of transfer into blister fluid was greater at the u.v.-exposed site (0.110±0.050) than at the control site (0.079±0.026, P<0.05) which corresponded to a higher AUC and C_max of R-flurbiprofen in u.v.-exposed blister as compared with control. For inhibition of TXB₂ generation after administration of S-flurbiprofen, a sigmoidal log-linear concentration–response relationship was established in all subjects (EC₅₀: 0.123±0.092 μg ml⁻¹ ). In contrast, inhibition of PGE₂ production in blister showed no clear concentration-response relationship when correlated with concentrations of S-flurbiprofen in either serum or blister fluid. After administration of R-flurbiprofen, no concentration-effect relationship could be established. Conclusions It is concluded that the blister model may have value for studying the pharmacokinetics and pharmacodynamics of antiinflammatory drugs in humans. Interestingly, inter-individual variation in the pharmacokinetics of flurbiprofen enantiomers could not account for the variability in response observed in the blister model.