Feedback modeling of non-esterified fatty acids in rats after nicotinic acid infusions

A feedback model was developed to describe the tolerance and oscillatory rebound seen in non-esterified fatty acid (NEFA) plasma concentrations following intravenous infusions of nicotinic acid (NiAc) to male Sprague-Dawley rats. NiAc was administered as an intravenous infusion over 30 min (0, 1, 5 or 20 μmol kg⁻¹ of body weight) or over 300 min (0, 5, 10 or 51 μmol kg⁻¹ of body weight), to healthy rats (n = 63), and serial arterial blood samples were taken for measurement of NiAc and NEFA plasma concentrations. Data were analyzed using nonlinear mixed effects modeling (NONMEM). The disposition of NiAc was described by a two-compartment model with endogenous turnover rate and two parallel capacity-limited elimination processes. The plasma concentration of NiAc was driving NEFA (R) turnover via an inhibitory drug-mechanism function acting on the formation of NEFA. The NEFA turnover was described by a feedback model with a moderator distributed over a series of transit compartments, where the first compartment (M ₁) inhibited the formation of R and the last compartment (M _N ) stimulated the loss of R. All processes regulating plasma NEFA concentrations were assumed to be captured by the moderator function. The potency, IC ₅₀, of NiAc was 45 nmol L⁻¹, the fractional turnover rate k _out was 0.41 L mmol⁻¹ min⁻¹ and the turnover rate of moderator k _tol was 0.027 min⁻¹. A lower physiological limit of NEFA was modeled as a NiAc-independent release (k _cap ) of NEFA into plasma and was estimated to 0.032 mmol L⁻¹ min⁻¹. This model can be used to provide information about factors that determine the time-course of NEFA response following different modes, rates and routes of administration of NiAc. The proposed model may also serve as a preclinical tool for analyzing and simulating drug-induced changes in plasma NEFA concentrations after treatment with NiAc or NiAc analogues.     

Concentration-effect relationship of the positive chronotropic and hypokalaemic effects of fenoterol in healthy women of childbearing age

Objective: To analyse fenoterol-induced tachycardia and hypokalaemia, the most important and most frequent adverse effects of tocolytic therapy with β₂-adrenoceptor agonists in females of childbearing age. Methods: The study was performed as a double blind, randomised, placebo controlled, cross over trial. Seven healthy women aged 22–38 y, received intravenous infusions of fenoterol at 3 different rates within the therapeutic range for tocolysis (0.5,1.0, and 2.0 μg⋅min⁻¹) and placebo. The time courses of the plasma concentrations of fenoterol and potassium, and the heart rate were analysed with mixed effects pharmacokinetic-pharmacodynamic (PKPD) modeling using NONMEM. Results: The plasma concentration-time course followed a linear two compartment model. Fenoterol-induced tachycardia was described by a linear concentration-effect model with baseline. The estimated baseline and slope parameters were 78 beats⋅min⁻¹ and 0.032 beats⋅min⁻¹⋅μg⁻¹⋅l, respectively. Fenoterol-induced hypokalaemia could be described by a physiological indirect response model including feedback; the Estimated basal plasma potassium concentration was 3.93 mmol⋅l⁻¹ and the slope factor for the fenoterol-induced relative increase in the efflux of potassium from the extracellular space was 6.22^*10⁻⁴ ng⋅l⁻¹. Conclusion: The estimated population parameters permitted calculation of the expected time course of tachycardia and hypokalaemia in women after the initiation of tocolysis with fenoterol over the clinically relevant concentration range, and prediction of its variability. Based on simulation, our model predicted that a continous infusion of 2.0 μg⋅min⁻¹ (highest rate examined) would increase heart rate to 113 beats⋅min⁻¹ at steady state and lower the plasma potassium concentration to 2.77 mmol⋅l⁻¹ 1.5 h after beginning the infusion. Thereafter, the plasma potassium concentration would slowly return to normal. Received: 7 December 1995/Accepted in revised form: 29 April 1996     

Inhibition of CYP2C9 by selective serotonin reuptake inhibitors: in vitro studies with tolbutamide and (S)-warfarin using human liver microsomes

Objective: To investigate the in vitro potential of selective serotonin reuptake inhibitors (SSRIs) to inhibit two CYP2C9-catalysed reactions, tolbutamide 4-methylhydroxylation and (S)-warfarin 7-hydroxylation. Methods: The formation of 4-hydroxytolbutamide from tolbutamide and that of 7-hydroxywarfarin from (S)-warfarin as a function of different concentrations of SSRIs and some of their metabolites was studied in microsomes from three human livers. Results: Both tolbutamide 4-methylhydroxylation and (S)-warfarin 7-hydroxylation followed one enzyme Michaelis-Menten kinetics. Kinetic analysis of 4-hydroxytolbutamide formation yielded a mean apparent Michaelis-Menten constant (K_m) of 133 μM and a mean apparent maximal velocity (V_max) of 248 pmol · min⁻¹ · mg⁻¹; formation of 7-hydroxywarfarin yielded a mean K_m of 3.7 μM and a mean V_max of 10.5 pmol · min⁻¹ ·  mg⁻¹. Amongst the SSRIs and some of their metabolites tested, only fluvoxamine markedly inhibited both reactions. The average computed inhibition constant (K_i) values and ranges of fluvoxamine when tolbutamide and (S)-warfarin were used as substrate, were 13.3 (6.4–17.3) μM and 13.0 (8.4–18.7) μM, respectively. The average K_i value of fluoxetine for (S)-warfarin 7-hydroxylation was 87.0 (57.0–125) μM. Conclusion: Amongst the SSRIs tested, fluvoxamine was shown to be the most potent inhibitor of both tolbutamide 4-methylhydroxylation and (S)-warfarin 7-hydroxylation. Fluoxetine, norfluoxetine, paroxetine, sertraline, desmethylsertraline, citalopram, desmethylcitalopram had little or no effect on CYP2C9 activity in vitro. This is consistent with in vivo data indicating that amongst the SSRIs, fluvoxamine has the greatest potential for inhibiting CYP2C9-mediated drug metabolism. Received: 20 July 1998 / Accepted in revised form: 6 October 1998     

A two-compartment effect site model describes the bispectral index after different rates of propofol infusion

Different estimates of the rate constant for the effect site distribution (k_e0) of propofol, depending on the rate and duration of administration, have been reported. This analysis aimed at finding a more general pharmacodynamic model that could be used when the rate of administration is changed during the treatment. In a cross-over study, 21 healthy volunteers were randomised to receive a 1 min infusion of 2 mg/kg of propofol at one occasion, and a 1 min infusion of 2 mg/kg of propofol immediately followed by a 29 min infusion of 12 mg kg⁻¹ h⁻¹ of propofol at another occasion. Arterial plasma concentrations of propofol were collected up to 4 h after dosing, and BIS was collected before start of infusion and until the subjects were fully awake. The population pharmacokinetic-pharmacodynamic analysis was performed using NONMEM VI. A four-compartment PK model with time-dependent elimination and distribution described the arterial propofol concentrations, and was used as input to the pharmacodynamic model. A standard effect compartment model could not accurately describe the delay in the effects of propofol for both regimens, whereas a two-compartment effect site model significantly improved the predictions. The two-compartment effect site model included a central and a peripheral effect site compartment, possibly representing a distribution within the brain, where the decrease in BIS was linked to the central effect site compartment concentrations through a sigmoidal E_max model.     

The oxidative metabolism of metoprolol in human liver microsomes: inhibition by the selective serotonin reuptake inhibitors

Objective: Biotransformation of metoprolol to α-hydroxymetoprolol (HM) and O-demethylmetoprolol (ODM) is mediated by CYP2D6. The selective serotonin reuptake inhibitors (SSRIs) are known to inhibit CYP2D6. The aim was to study in vitro the potential inhibitory effect of SSRIs on metoprolol biotransformation. Methods: Using microsomes from two human livers, biotransformation of metoprolol to α-hydroxymetoprolol (HM) and O-demethylmetoprolol (ODM) as a function of the concentrations of the SSRIs and of some of their metabolites was studied. Results: The kinetics of the formation of both metabolites are best described by a biphasic enzyme model. The estimated values of V_max and k_M for the high affinity site are for the α-hydroxylation in human liver HL-1 32 pmol mg⁻¹ min⁻¹ and 75 μmol · l⁻¹ respectively, and in human liver HL-9 39 pmol mg⁻¹ · min⁻¹ and 70 μmol · l⁻¹ respectively; for the O-demethylation in HL-1 131 pmol mg⁻¹ min⁻¹ and 95 μmol · l⁻¹ respectively, and in HL-9 145 pmol mg⁻¹ min⁻¹ and 94 μmol · l⁻¹ respectively. Quinidine is for both pathways a potent inhibitor of the high-affinity site, with K_i values ranging from 0.03 to 0.18 μmol · l⁻¹. Fluoxetine, norfluoxetine and paroxetine are likewise potent inhibitors, with K_i values ranging from 0.30 to 2.1 μmol · l⁻¹ fluvoxamine, sertraline, desmethylsertraline, citalopram and desmethylcitalopram are less potent inhibitors, with K_i values above 10 μmol · l⁻¹. Conclusion: The rank order of the SSRIs for inhibition of metoprolol metabolism is comparable to that reported in the literature for other CYP2D6 substrates, with fluoxetine, norfluoxetine and paroxetine being the most potent. These findings need further investigation to determine their clinical relevance. Received: 24 October 1997 / Accepted: 17 January 1998     

Pharmacokinetic–Pharmacodynamic Modeling of the Hydroxy Lerisetron Metabolite L6-OH in Rats: An Integrated Parent–Metabolite Model

Purpose The twofold aim of this study was to characterize in vivo in rats the pharmacokinetics (PK) and pharmacodynamics (PD) of L6-OH, a metabolite of lerisetron with in vitro pharmacological activity, and evaluate the extent to which L6-OH contributes to the overall effect. Methods The PK of L6-OH was determined directly postmetabolite i.v. dose (PK-1), and also simultaneously for L (lerisetron concentration) and for generated L6-OH after lerisetron dose (200 μg kg⁻¹, i.v.), using Nonlinear Mixed Effects Modeling with an integrated parent–metabolite PK model (PK-2). Surrogate effect was measured by inhibition of serotonin-induced bradycardia. Protein binding was assayed via ultrafiltration and all quantification was performed via liquid chromatography-electrospray ionization-mass spectrometry. Results L6-OH showed elevated plasma and renal clearances, and volume of distribution (PK-1). The in vivo potency (PD) of L6-OH was high (EC₅₀ = 0.098 ng mL⁻¹ and EC_50unbound = 0.040 ng mL⁻¹). Total clearance for L (PK-2) in the presence of generated L6-OH (CL_L = CL_→L6-OH + CL_n) was 0.0139 L min⁻¹. Most of this clearance was L6-OH formation (F_c = 99.6%), but only an 8.6% fraction of L6-OH was released into the bloodstream. The remainder undergoes biliar and fecal elimination. The parameters estimated from PK-2 were used to predict concentrations of L6-OH (Cp_L6) generated after a lerisetron therapeutic dose (10 μg kg⁻¹) in the rat. These concentrations are needed for the PD model and are below the quantification limit. Cp_L6max was less than the EC₅₀ of L6-OH. Conclusions We conclude that after lerisetron administration, L6-OH is extensively formed in the rat but it is quickly eliminated; therefore, besides being equipotent with the parent drug, the L6-OH metabolite does not influence the effect of lerisetron.     

Pharmacokinetics and pharmacodynamics of ranitidine in renal impairment

Objective: The pharmacodynamics and pharmacokinetics of ranitidine were examined in subjects with varying degrees of renal function to determine the effect of this condition on acid-antisecretory activity. Methods: Subjects with creatinine clearances (C_Cr) ranging from 0 to 213 ml · min⁻¹ received single 50-mg and 25-mg i.v. doses of ranitidine. This was followed by determination of serum and urine ranitidine concentrations, and continuous gastric pH monitoring for 24 h. Results: Serum ranitidine concentrations were described by a two-compartment model linked to a sigmoidal E_max model describing gastric pH. Ranitidine renal clearance, ranging from 0 to 1003 ml · min⁻¹, correlated with C_PAH (r ² = 0.707), while non-renal clearance was unaltered. Steady-state volume of distribution decreased by half in severe renal impairment. No changes in the effective concentration at half-maximal response (EC₅₀), maximal response (E_max), or basal response (E₀) were observed. Thus, renal elimination of ranitidine declined in parallel with renal function, while sensitivity to the pharmacologic effect (gastric pH elevation) was unaltered. Ranitidine was well tolerated in these renally impaired subjects. Conclusion: These data indicate that the current recommendation for renal impairment dose reduction (by two-thirds when C_Cr<50 ml · min⁻¹) might result in under-treating moderately impaired patients, and suggests a less conservative dose reduction (by half when C_Cr<10 ml · min⁻¹) to avoid therapeutic failure while remaining within the wide margin of safety for this drug. Received: 10 September 1996 / Accepted in revised form: 7 December 1996     

Lack of correlation between fluvoxamine clearance and CYP1A2 activity as measured by systemic caffeine clearance

Objective: Evidence exists to suggest that fluvoxamine is metabolized by CYP1A2. The present study was undertaken in order to further elucidate the role of CYP1A2 in fluvoxamine disposition. Methods: Twelve healthy non-smoking male volunteers participated in this cross-over study. Six subjects received first fluvoxamine 50 mg as a single oral dose and, some weeks later, caffeine 200 mg as a single oral dose. The other six subjects received the drugs in reverse order. Serum concentrations of fluvoxamine, caffeine and paraxanthine were measured and standard pharmacokinetic parameters were calculated. Results: There were no significant correlations between caffeine clearance and fluvoxamine oral clearance (r _s= −0.30; P = 0.43) or between the paraxanthine/caffeine ratio in serum 6 h after caffeine intake and fluvoxamine oral clearance (r _s= −0.18; P = 0.58). Conclusion: CYP1A2 does not appear to be of major importance in the metabolism of fluvoxamine. Received: 10 July 1998 / Accepted in revised form: 4 October 1998     

Extrahepatic glucuronidation of propofol in man: possible contribution of gut wall and kidney

Objective: Results from clinical pharmacokinetic studies of propofol indicate that this i.v. anaesthetic agent may undergo significant extrahepatic glucuronidation. We have investigated whether glucuronidation of propofol takes place in the kidney and/or the gut wall. First, propofol concentrations were measured in arterial (radial artery) and portal venous blood of 12 cirrhotic patients with trans internal jugular porto-systemic shunting (TIPSS). Results: In 7 of the 12 patients arterial propofol concentrations were higher than portal venous concentrations. In the remaining patients, propofol concentrations were higher in the portal vein than the radial artery. Since an additional study in 5 patients anaesthetized with propofol while undergoing cholecystectomy showed propofol and an acid-labile conjugate of it in bile, it is difficult to interpret the results in patients with TIPSS due to the possibility of enterohepatic cycling. Next, in vitro studies with human liver (n = 5), kidney (n = 5) and small intestinal (n = 5) microsomes showed that all three tissues were capable of forming propofol glucuronide. V_max for propofol glucuronidation was approximately 3 to 3.5 times higher in kidney (5.56 nmol ⋅ min⁻¹⋅ mg⁻¹ protein) than liver (1.80 nmol ⋅ min⁻¹⋅ mg⁻¹ protein) and small intestine (1.61 nmol ⋅ min⁻¹⋅ mg⁻¹ protein). Conclusion: Based on these in vitro results, it is concluded that extrahepatic glucuronidation in the small intestine and especially in the kidney may contribute to the overall glucuronidation of propofol in man. Received: 28 July 1995/Accepted in revised form: 30 October 1995     

Pharmacokinetics and pharmacodynamics of clevidipine in healthy volunteers after intravenous infusion

Objective: To determine the pharmacokinetics and pharmacodynamics of clevidipine, a new ultrashort-acting calcium antagonist, in healthy male volunteers following a constant rate infusion. Methods: Eight healthy male volunteers received 1030 nmol · min⁻¹ of clevidipine together with a tracer dose of ³[H]-clevidipine for 1 h as an i.v. infusion. Frequent venous blood samples and effect recordings were obtained during ongoing infusion and up to 32 h following termination of the infusion. The excretion of radioactivity in urine and faeces was followed for 7 days. Results: A two-compartment model gave the best fit to the individual clevidipine blood levels, resulting in a mean blood clearance of 0.14 (0.03) l · min⁻¹ · kg⁻¹ and a mean volume of distribution at steady state of 0.6 (0.1) l · kg⁻¹. The initial half-life was 1.6 (0.3) min, and the terminal half-life was 15 (5) min. The maximum concentration of the metabolite H 152/81 was reached 2.2 (1.3) min following termination of the infusion. The mean terminal half-life of the inactive primary metabolite was 9.5 (0.8) h and the mean recovery of the radioactive dose reached 83 (3)%. Following termination of the 1 h infusion, the effect on blood pressure (BP) and heart rate was back to pre-dose values within 15 min. Conclusion: Clevidipine is a high clearance drug, which is rapidly metabolized to the corresponding inactive acid. The t_max value of the primary metabolite, and a virtually identical value of the initial half-life and the half-life for elimination from the central compartment, indicate that the initial rapid decline of the post-infusion blood levels is mainly due to elimination rather than distribution. The duration of action of clevidipine is short. Received: 23 September 1998 / Accepted in revised form: 20 November 1998     

Subcellular/tissue distribution and responses to oil exposure of the cytochrome P450-dependent monooxygenase system and glutathione S-transferase in freshwater prawns (Macrobrachium malcolmsonii, M. lamarrei lamarrei)

Subcellular fractions (mitochondrial, cytosolic and microsomal) prepared from the tissues (hepatopancreas, muscle and gill) of freshwater prawns Macrobrachium malcolmsonii and Macrobrachium lamarrei lamarrei were scrutinized to investigate the presence of mixed function oxygenase (MFO) and conjugating enzymes (glutathione-S-transferase, GST). Cytochrome P450 (CYP) and other components (cytochrome b₅; NADPH-cytochrome c (CYP) reductase and NADH-cytochrome c-reductase activities) of the MFO system were predominantly present in the hepatic microsomal fraction of M. malcolmsonii and M. lamarrei lamarrei. The results are in agreement with the notion that monooxygenase system is mainly membrane bound in the endoplasmic reticulum, and that the hepatopancreas is the major metabolic tissue for production of biotransformation enzymes in crustaceans. Further, the prawns were exposed to two sublethal (0.9 ppt (parts per thousand) and 2.3 ppt) concentrations of oil effluent. At the end of 30th day, hydrocarbons and detoxifying enzymes were analysed in the hepatopancreas. The accumulations of hydrocarbon in the tissues gradually increased when exposed to sublethal concentrations of oil effluent and were associated with significantly enhanced levels of cytochrome P450 (180.6±6.34 pmol mg⁻¹ protein (P<0.05 versus control, 136.5±7.1 pmol mg⁻¹ protein) for 2.3 ppt and 305.6±8.5 pmol mg⁻¹ protein (P<0.001 versus control, 132.3±6.8 pmol mg⁻¹ protein] for 0.9 ppt of oil exposed M. malcolmsonii; 150±6.5 pmol mg⁻¹ protein (P<0.01 versus control, 84.6±5.2 pmol mg⁻¹ protein) for 2.3 ppt and 175±5.5 pmol mg⁻¹ protein (P<0.01 versus control, 87.6±5.4 pmol mg⁻¹ protein) for 0.9 ppt of oil exposed M. lamarrei lamarrei), NADPH cytochrome c-reductase activity (14.7±0.6 nmol min⁻¹ mg⁻¹ protein (P<0.05 versus control, 6.8±0.55 nmol min⁻¹ mg⁻¹ protein) for 2.3 ppt and 12.1±0.45 nmol min⁻¹ mg⁻¹ protein (P<0.01 versus control, 6.9±0.42 nmol min⁻¹ mg⁻¹ protein) for 0.9 ppt of oil exposed M. malcolmsonii; 12.5±0.31 nmol min⁻¹ mg⁻¹ protein (P<0.001 versus control, 4.6±0.45 nmol min⁻¹ mg⁻¹ protein) for 2.3 ppt and 9.6±0.32 nmol min⁻¹ mg⁻¹ protein (P<0.01 versus control, 4.9±0.41 nmol min⁻¹ mg⁻¹ protein) for 0.9 ppt of oil exposed M. lamarrei lamarrei) and cytochrome b₅ (124.8±3.73 pmol mg⁻¹ protein (P<0.01 versus control, 76.8±4.2 pmol mg⁻¹ protein) for 2.3 ppt and 115.3±3.86 pmol mg⁻¹ protein (P<0.01 versus control, 76.4±4.25 pmol mg⁻¹ protein) for 0.9 ppt of oil exposed M. malcolmsonii and 110±3.11 pmol mg⁻¹ protein (P<0.01 versus control, 63.7±3.24 pmol mg⁻¹ protein) for 2.3 ppt and 95.3±2.63 pmol mg⁻¹ protein (P<0.01 versus control, 61.4±2.82 pmol mg⁻¹ protein) for 0.9 ppt of oil exposed M. lamarrei lamarrei). The enhanced levels of biotransformation enzymes in oil-exposed prawns demonstrate a well-established detoxifying mechanism in crustaceans, and the response offers the possibility of use as a biomarker for the early detection of oil pollution.Special Issue on Biomarkers of Marine Pollution and Bioremediation     

An understanding of flow- and diffusion-limited vs. Carrier-mediated hepatic transport: A simulation study

Concentration-dependent changes in the hepatic extraction ratio E and tissue accumulation of drugs were examined in a simulation study, wherein plasma protein binding, flow, and mode of entry were altered. A tubular flow model that described carrier-mediated (influx:K _ml =20 μM,V _max1=1000 nmol min⁻¹;effux,K _m2=200 μM,V _max2=250 nmol min⁻¹), flow-limited (influx clearanceCL _in=efflux clearanceCL _ef=50 ml min⁻¹), or diffusion-limited (CL _in=CL _ef=0.1 ml min⁻¹) hepatocytic entry was employed; drug removal was solelyvia biliary excretion (K _m3=100 μM,V _max3=1500 nmol min⁻¹). Other parameter space and the combination of carriermediated transport and passive diffusion were also explored. Increased plasma protein binding reduced the hepatic extraction of the substrate, and in some instances, constituted the ratecontrolling factor, especially at lower input concentrations for which tighter binding existed. Increased flow rate also brought about a reduction inE, affectingE almost inversely when values ofE were low (e.g., for the diffusion-limited case or at higher input concentration). Tissue accumulation patterns and the apparent tissue distribution equilibrium ratio, i.e., tissue to plasma unbound concentration ratioK _p, differed among the systems. The behavior ofK _p may be used as an identifier for the mode of drug transport: A declining (concave-down)K _p curve or a parabolicK _p that approached unity with input concentration (C _In) is associated with carrier-mediated entry; a risingK _p curve that approaches unity withC _In suggests flow limitation; and a waning concaveupK _p curve of very low magnitude represents diffusion limitation. Since the unbound tissue concentration (C _t) differs from the logarithmic average of the unbound input and output concentrations in plasma ( ) for carrier-mediated and diffusion-limited systems, excretion parameters may be obtained only upon fitting of the overall excretion ratevs.C _t in the Michaelis-Menten equation; whereas when data are fitted with , the rate-limiting step, influx, or deviations of influx, efflux, and excretion, will be obtained. WhenC _t equals , as in flow-limited systems, accurate excretion parameters will be provided with the fitting of data against eitherC _t or . This work was supported by the Medical Research Council of Canada (MA-9765, MA-9104) and the National Institutes of Health (GM-38250). Dr. W. P. Geng was supported by a postdoctoral fellowship in Basic Pharmacology from the Merck Sharp and Dohme Research Laboratories, West Point, PA. Karen Poon was supported by an Ontario Graduate Scholarship.     

Bayesian pharmacokinetic estimation of vinorelbine in non-small-cell lung cancer patients

Objective: To develop a population pharmacokinetics of vinorelbine in a population of non-small-cell lung cancer (NSCLC) patients using a Bayesian estimation in order to calculate for any further patient, individual pharmacokinetic parameters from few blood samples. Methods: Vinorelbine was given by a 15-min infusion (30 mg · m⁻²) to eight patients with NSCLC. Its serum concentration was determined by HPLC and its pharmacokinetics was described by a three-compartment open model with elimination from the central compartment. Volume of the central compartment (V₁) and rate constants (k₁₀, k₁₂, k₂₁, k₁₃, k₃₁) were selected as population pharmacokinetic parameters and computed by non-linear regression (two-step approach) from 14 to 18 concentration measurements per course. Subsequently, these parameters were used by the Bayesian estimator to calculate individual pharmacokinetics from only 2 or 3 measured concentrations. Results: The population mean values (CV%) of V₁, k₁₀, k₁₂, k₂₁, k₁₃, k₃₁, CL, t _1/2 were respectively 21 l (55%), 3.2 h⁻¹ (29%), 7.7 h⁻¹ (74%), 1.3 h⁻¹ (67%), 4.7 h⁻¹ (53%), 0.04 h⁻¹ (20%), 57 l · h⁻¹ (31%) and 43 h (36%). The comparison of results obtained from the Bayesian estimator and from the three-compartment model showed that CL and t _1/2 were well predicted (relative deviation: ±12 to 22%) by the Bayesian method using only two blood samples. Conclusion: We demonstrated that Bayesian estimation allows, at minimal cost and minimal disturbance for the patient, the determination of several vinorelbine pharmacokinetic parameters and therefore dose adaptation from as few as two drug concentrations, measured at 6 h and 24 h after infusion. Received: 4 July 1997 / Accepted in revised form: 15 November 1997     

Effects of dopamine on veins in humans: comparison with noradrenaline and influence of age

Objective: To compare the venoconstricting effect of dopamine with that of noradrenaline and to investigate the influence of age on the responsiveness to dopamine in human subjects. Methods: In eight young and eight elderly male subjects, increasing doses of dopamine or noradrenaline were infused into a dorsal hand vein and its diameter was measured using a linear variable differential transformer. Results: There was no significant difference between the maximum venoconstriction (E_max) for dopamine and that for noradrenaline. The infusion rate to induce 50% of E_max (ED₅₀) for dopamine in the young and elderly subjects was 363 ng · min⁻¹ and 352 ng · min⁻¹, and the ED₅₀ for noradrenaline was 40.7 ng · min⁻¹ and 43.8 ng · min⁻¹, respectively. Neither in the E_max nor in the ED₅₀ for these drugs were there significant differences between the young and elderly subjects. Conclusion: The venoconstricting effect of dopamine is 5–20 times less than that of noradrenaline, and aging does not influence the responsiveness to dopamine and noradrenaline in human subjects. Received: 29 August 1997 / Accepted in revised form: 5 February 1998     

Pharmacokinetics of oxypolygelatine in healthy volunteers

Objective/methods: The pharmacokinetics of the plasma substitute oxypolygelatine (OPG) were studied in 12 healthy volunteers after single-dose administration of 27 ml · kg⁻¹ body weight, with a maximum of 2000 ml. OPG was determined in plasma and urine over 48 h after the infusion. Peak plasma OPG concentrations at the end of the infusion were determined to 4.600 (623) μg · ml⁻¹, the area under the plasma concentration/time curve (AUC_0∞) was calculated to 70.135 (15.861) μg · h · ml⁻¹. Results: The model-independently calculated volume of distribution came to 23.1 (4.8) l with a clearance total is (Cl_tot) of 24.6 (6.8) ml · min⁻¹. The initial half-life according to a three-compartment model came to 0.3 (0.2) h, followed by a distribution half-life of 3.1 (2.6) h and a terminal elimination half-life of 13.4 (2.2) h. Cumulative urinary excretion of OPG was 64% after 48 h. Conclusion: This low recovery rate may be explained by the distribution of OPG into the extravascular space and subsequent degradation in tissue sites. Received: 9 June 1998 / Accepted in revised form: 23 November 1998     

Dilatory effects of phosphodiesterase inhibitors on human hand veins in vivo

Objective: To compare the venodilator potencies of the phosphodiesterase (PDE) III inhibitors amrinone and enoximone with the unspecific PDE inhibitors theophylline and pentoxifylline in human hand veins in vivo. Methods: Eighteen healthy nonsmokers (16 men and two women) were studied using the dorsal hand vein technique. After preconstriction with the selective α₁-adrenergic-receptor agonist phenylephrine dose–response curves were constructed for amrinone (1–270 μg · min⁻¹), enoximone (1–270 μg · min⁻¹), theophylline (5–1500 μg · min⁻¹) and pentoxifylline (2–877 μg · min⁻¹) in a random order on separate occasions. Due to limitation in the maximum dose infused in order to avoid systemic effects, full dose–response curves could not be constructed for pentoxifylline. In this case, the individual dose of pentoxifylline and theophylline producing 50% venodilation were compared. Results: All PDE inhibitors induced dose-dependent venodilation. The value of maximum venodilation was the same for amrinone, enoximone and theophylline. The infusion rate needed to induce 50% of maximum venodilation (ED₅₀) was not significantly different for amrinone (geometric mean, 8.8 μg · min⁻¹) and enoximone (14.2 μg · min⁻¹), whereas the ED₅₀ of theophylline (84.0 μg · min⁻¹) was significantly higher than either amrinone or enoximone. The dose necessary to dilate the vein to 50% the maximum dilation (as determined during sodium chloride infusion) was significantly higher for pentoxifylline than for theophylline (409 vs 71 μg · min⁻¹). Conclusions: These findings demonstrate that enoximone and amrinone have similar venodilatory potency which is six times higher than that of theophylline. The least potent vasodilator in this study was pentoxifylline. Received: 16 September 1997 / Accepted in revised form: 4 December 1997     

Comparison of Skin Esterase Activities from Different Species

Purpose Many topically applied drugs contain esters that are hydrolyzed in the skin. Minipigs have emerged as potential models of human dermatology and, in some aspects, may be superior to commonly used rat skin. The aims of this study were to evaluate the suitability of minipig and rat skin as in vitro models of human epidermal esterase activity. Methods Naphthyl acetate and para-nitrophenyl acetate were tested as prototypical substrates of carboxylesterases from skin, plasma, and liver. Reaction products were monitored by high-performance liquid chromatography/ultraviolet analysis. Results Hydrolysis efficiency in skin was higher than plasma, but lower than liver. The esterase efficiency of rat skin microsomes (580–1100 min⁻¹ mg⁻¹) was two to three orders of magnitude higher than human (1.3–4.2 min⁻¹ mg⁻¹) and minipig microsomes (1.2–4.2 min⁻¹ mg⁻¹). Rat skin cytosol (80–100 min⁻¹ mg⁻¹) was 2- to 10-fold more efficient than human (2.4–67 min⁻¹ mg⁻¹) or minipig cytosol (18–61 min⁻¹ mg⁻¹). Most importantly, human skin fractions displayed kinetics of hydrolysis very similar to minipig skin. Conclusions These studies show minipig skin as an appropriate, potentially valuable model for human epidermal ester metabolism and support the use of minipig skin in preclinical development of topically applied compounds.     

Pharmacokinetic-pharmacodynamic (PK-PD) modeling of cardiovascular effects of metoprolol in spontaneously hypertensive rats: a microdialysis study

The present work addressed possible alterations in the pharmacokinetics and the in vivo pharmacodynamic of metoprolol (MET) in spontaneously hypertensive (SH) rats and Wistar Kyoto (WKY) animals by means of the microdialysis technique. The correlation between MET unbound plasma concentrations and its pharmacological effects, such as heart rate and blood pressure change, was also examined in SH and WKY rats by the application of a PK-PD model. MET dialysate concentrations and its chronotropic and blood pressure effect were determined during 3 h after the administration of 3 and 10 mg.kg⁻¹ of the drug. A PK-PD model with a separate effect compartment was used to analyse the data. A good correlation between plasma MET concentrations and its hypotensive and chronotropic effect was found in all experimental groups. Although a greater maximal effect (E_max) for the antihypertensive effect of MET was observed in SH rats (WKY: E_max: −17±1 mmHg; SH: E_max: −28±4 mmHg; P<0.05 versus WKY rats), no differences were found in the concentration yielding half-maximal response (IC₅₀) comparing SH (IC₅₀: 583±146 ng.ml⁻¹) and WKY animals (IC₅₀: 639±187 ng.ml⁻¹). The bradycardic effect of MET was greater in SH rats (E_max: −29±1%, P<0.05 versus WKY rats) than in WK animals (E_max: −22±2%), but no differences were observed in the IC₅₀ comparing both experimental groups (WKY: IC₅₀: 187±53 ng.ml⁻¹; SH: IC₅₀: 216±62 ng.ml⁻¹). Pharmacokinetic analysis shows that the volume of distribution of MET was greater in SH rats (Vd: 3.4±0.5 l, P<0.05 versus WKY rats) with regard to Wistar Kyoto (WKY) animals (Vd: 1.9±0.2 l). The results suggest that the pharmacokinetic behaviour of metoprolol are modified in SH rats, resulting in an increased volume of distribution. A greater maximal efficacy to the hypotensive effect of metoprolol was observed in SH rats, suggesting participation of β-adrenoceptors in the maintenance of the hypertension. Also, a greater chronotropic response to metoprolol was found in the hypertensive group compared with WKY animals, suggesting that, at least in part, the greater cardiac effect of metoprolol explained the enhanced hypotensive response of the beta blocker in the SH animals.     

Pharmacokinetics and systemic availability of the antihypertensive agent indoramin and its metabolite 6-hydroxyindoramin in healthy subjects

Summary We have studied the pharmacokinetics and absolute systemic availability of indoramin (50 mg) given orally in solution or as a tablet with reference to intravenously administered drug (0.15 mg/kg) in 9 healthy volunteers. After intravenous administration the median apparent volume of distribution was 6.3l·kg⁻¹, plasma clearance was 20.0 ml·min⁻¹·kg⁻¹, and terminal half-time was 4.1 h. When given by tablet indoramin was absorbed with moderate rapidity, with a median t_max of 1.5 h. The median systemic availability was 24%. After oral administration in solution the drug was more rapidly absorbed, with a median t_max of 1.0 h (p<0.01). The median systemic availability was 43% (15–85%). Plasma concentrations of an active metabolite, 6-hydroxyindoramin, after single oral doses in either dosage form, were of a similar order to those of unchanged drug and fell with similar rapidity. After intravenous administration, however, concentrations of the metabolite were negligible.     

Lack of enantiomeric influence on the brain cytoprotective effect of ibuprofen and flurbiprofen

R(−) enantiomers of the 2-arylpropionic acid derivatives ibuprofen and flurbiprofen weakly inhibit cyclooxygenase (COX) activity. However, a possible cytoprotective effect has been proposed. The aim of the study is to investigate the possible mechanism of this effect. An in vitro hypoxia–reoxygenation model in rat brain slices was used (n = 6 rats per group). After reoxygenation, we measured LDH efflux (neuronal death), brain prostaglandin E₂ (PGE₂) concentration, interleukins (IL)-1β and 10, oxidative and nitrosative stress (lipid peroxides, glutathione, 3-nitrotyrosine, and nitrites/nitrates). Anti-COX activity was measured in human whole blood. Racemic, R(−), and S(+) enantiomers of ibuprofen and flurbiprofen were tested. All compounds had a cytoprotective effect with IC₅₀ values in the range of 10⁻⁵ M. R(−) enantiomers did not significantly inhibit brain PGE₂. The concentration of IL-1β was reduced by 53.1% by the racemic form, 30.6% by the S(+) and 43.2% by the R(−) enantiomer of ibuprofen. The IL-10 concentration increased significantly only with S(+)-flurbiprofen (33.1%) and R(−)-flurbiprofen (26.1%). Lipid peroxidation was significantly reduced by all three forms of flurbiprofen. Nitrite + nitrate concentrations were reduced by racemic, S(+), and R(−)-flurbiprofen. Peroxynitrite formation (3-nitrotyrosine) was significantly reduced by racemic and S(+)-ibuprofen. COX inhibition is not the main mechanism of cytoprotection for these compounds. Their influence on inflammatory mediators and oxidative and nitrosative stress could account for the potential cytoprotective effect of R(−) enantiomers.