Quinidine pharmacokinetics in man: Choice of a disposition model and absolute bioavailability studies

Parameters describing disposition and absolute oral bioavailability of quinidine were determined in ten normal male volunteers using a specific assay. Various models were compared for their ability to describe the experimental data. An intravenous quinidine gluconate and an oral quinidine sulfate solution were administered (3.74 mg/kg quinidine base). In three subjects the intravenous and oral studies were repeated. One-, two-, and three-compartment models with zeroand first-order input were fitted to the plasma concentrations. The selection of the best model was made by the Akaike information criterion and by eye. After intravenous administration, plasma concentrationtime curves could be adequately described by a twocompartment model. Mean disposition constants (±SD) were obtained from individualized fits (V₁: 0.398 ±0.336 liter/kg, Vd_area: 2.53±0.72 liter/kg, : 0.316±0.294 min^–1, : 0.00204 ± 0.00262 min¹, k₂: 0.0305 ±0.010 min^–1). A clearance of 4.9 ±1.5 ml/min/kg was observed. After oral administration, threecompartment models were needed to describe the observed data in some cases. Absorption was in most cases best described by a zeroorder rather than by a firstorder process. The time to peak concentration varied from 23 to 121 min, the lag time was always less than 3 min, and the mean elimination rate constant was 0.00171 min^–1. The mean oral bioavailability of quinidine was 0.70 ±0.17.This study was supported by funds from Food and Drug Administration Contract No. 223-74-3145. T. W. G. acknowledges support from the Swiss National Science Foundation, N. H. G. H. received a NIH fellowship for training in Clinical Pharmacology (GM 00001).     

Opioid receptor agonists activate pertussis toxin-sensitive G proteins and inhibit adenylyl cyclase in canine cardiac sarcolemma

Although both opioid receptors and endogenous opioids are abundant in cardiac tissues, the signal transduction pathways of opioids in cardiac sarcolemmal membranes have yet to be identified. In highly purified canine cardiac sarcolemmal membranes, binding of the opioid receptor antagonist [³H]diprenorphine and effects of , and agonists on low K_m GTPase and adenylyl cyclase were measured. Equilibrium binding of [³H]diprenorphine revealed a maximal binding capacity of 7.2 pmol/mg protein and a K_d of 1.3 nmol/1. In the presence of GTP, (D-Pen^2,5, p-Cl-Phe⁴)enkephalin and (D-Arg⁶)dynorphin A 1-13 fragment both inhibited adenylyl cyclase by 20–25% (from 206 ± 30to164 ± 28 pmol·min^{– 1}·mgprotein^–1, EC₅₀6 mol/Landfrom254 ± 109to204 ± 90 pmol·min^{– 1}·mg protein^–, EC₅₀ 8 pmol/L, respectively; P<0.001). Both substances stimulated low Km GTPase by 20%and13%,respectively(from12.7 ± 3.0 to 15.2 ± 3.7 pmol·min^–1.mgprotein^–1,EC₅₀ 12 mol/L, P<0.01, and from 9.1 ± 2.8 to 10.4 ± 3.2 pmol-min^{– 1}·mg protein^–1, EC₅₀ 6 mol/L, P<0.05, respectively). These effects were blocked by the opioid receptor antagonist naltrexone and by pretreatment of sarcolemmal membranes with pertussis toxin. The opioid receptor agonists (D-AIa², Me Phe⁴, Gly-[ol]⁵)enkephalin and morphiceptin had no effect on either cardiac adenylyl cyclase or low Km GTPase activities. These data suggest that in cardiac sarcolemma, opioid receptors are coupled to pertussis toxin sensitive G proteins and mediate inhibition of adenylyl cyclase activity.     

Absolute Bioavailability and Absorption Profile of Cyanamide in Man

A pharmacokinetic study of cyanamide, an inhibitor of aldehyde dehydrogenase (EC1.2.1.3) used as an adjuvant in the aversive therapy of chronic alcoholism, has been carried out in healthy male volunteers following intravenous and oral administration. Cyanamide plasma levels were determined by a sensitive HPLC assay, specific for cyanamide. After intravenous administration cyanamide displayed a disposition profile according to a two-compartmental open model. Elimination half-life and total plasma clearance values ranged from 42.2 to 61.3 min and from 0.0123 to 0.0190 L · kg ^–1 · min^–1, respectively. After oral administration of 0.3, 1.0, and 1.5 mg/kg ± SEM values of C_max, t_max (median) and AUC were 0.18 ± 0.03, 0.91 ± 0.11, and 1.65 ± 0.27 g · ml ^–1 ; 13.5, 13.5, and 12 min; and 8.59 ± 1.32, 45.39 ± 1.62, and 77.86 ± 17.49 g · ml ^–1 · min, respectively. Absorption was not complete and the oral bioavailability, 45.55 ± 9.22, 70.12 ± 4.73, and 80.78 ± 8.19% for the 0.3, 1.0, and 1.5 mg/kg doses, respectively, increased with the dose administered. The models that consider a first-order absorption process alone (whether with a fixed or variable bioavailability value as a function of dose) or with loss of drug due to presystemic metabolism (with zero-order or Michaelis–Menten kinetics) were simultaneously fitted to plasma level data obtained following 1 mg/kg iv and 0.3, 1.0, and 1.5 mg/kg oral administrations. The model that best fit the data was that with a first-order absorption process plus a loss by presystemic metabolism with Michaelis–Menten kinetics, suggesting the presence of a saturable first-pass effect.     

The pharmacokinetics of carbamazepine

Summary The time-courses of plasma carbamazepine concentrations were followed in six apparently healthy adult subjects who, at different times, took single oral drug doses of 200, 400, 500, 600, 700, 800 and 900 mg. There were some suggestions of impaired bioavailability of the drug when given in tablet form. The following values were obtained for various pharmacokinetic parameters:k _abs =0.176±0.209 h^–1;k=0.0203±0.0055 h^–1; T_1/2=37.5±13.1 h; V_D=0.825±0.1041 · kg^–1; Clearance=0.0163±0.0061 l · kg^–1. The elimination rate constant showed a statistically significant increase with increasing drug dose. This may help explain the clinical observation that the rate of rise of steady state plasma carbamazepine concentrations tends to decrease with dose increase in patients taking carbamazepine alone.     

In Vitro Characterization of the Erythrocyte Distribution of Methazolamide: A Model of Erythrocyte Transport and Binding Kinetics

The rate and extent of binding of methazolamide to human erythrocytes was studied in vitro. All experiments were carried out at physiological temperature (37C) and pH (7.4). Methazolamide (MTZ) buffer concentrations were analyzed by HPLC. Distributional equilibrium between buffer and washed red blood cells was achieved after 1 hr. Results of equilibrium studies were consistent with two classes of binding sites for MTZ within the erythrocyte: a low affinity, high capacity site (CA-I) and a high affinity, low capacity site (CA-II). A two-binding site model was fitted to experimental data generating estimates for binding parameters Ka₁ (0.0017 ± 0.00022 M ^–1 ) nM₁ (636 ± 5.23 M), Ka₂ (0.46 ± 0.0083 M ^–1 ), and nM₂ (80.9 ± 0.389 M). Based upon these findings, kinetic studies were performed in order to characterize the rate of drug distribution. The rate of erythrocyte uptake of MTZ was mathematically modeled using a series of differential equations describing drug diffusion across the red blood cell membrane and subsequent complexation with intracellular binding sites. The model assumed that penetration of MTZ into the red blood cells was passive but drug binding to the carbonic anhydrase isozymes was not instantaneous. Using a novel curve fitting technique, parameter estimates of RBC membrane permeability (0.0102 ± 0.000618 cm/min), and binding rate constants k_–1 (0.254 ± 0.0213 min ^–1 ), k₁ (0.0022 ± 0.00020 ml/g-min), k_–2 (1.59 ± 0.0358 min ^–1 ), and k₂ (3.1 ± 0.035 ml/g-min) were obtained. The model characterized the observed biphasic decline of MTZ buffer concentrations over time and may help explain the prolonged residence of MTZ in vivo.     

Endothelial cells contain beta adrenoceptors

Summary The direct identification of beta adrenoceptors in endothelial cell cultures has not been possible until the advent of a new beta-adrenergic radioligand, [¹²⁵I]iodocyanopindolol ([¹²⁵I]ICYP). Using [¹²⁵I]ICYP, we report thes successful identification of a beta adrenoceptor in cultured bovine aortic endothelial cells. At 37°C, specific binding is saturable, stable and reversible. There is a single class of binding sites (21,500±2,900 sites/cell) with an equilibrium dissociation constant (K _d) of 109±26 pM. The rate constant of association, k ₁, is 1.22×10⁹ M^–1 min^–1 and of dissociation, k _–1, is 0.01 min^–1. Binding studies on monolayers of endothelial cells grown in microtiter plates yield similar data (K _d=53±9 pM, B _max=20,000±1,900 sites/cell). Stereoselectivity of binding for the (–)-isomer is demonstrable for both agonists and antagonists. A series of adrenergic agonists competes with [¹²⁵I]ICYP for binding with an order of potency suggesting beta₂ subselectivity; isoproterenol (0.73 M) > epinephrine (15 M) > norepinephrine (71 M). Furthermore, the beta₂ inhibitor butoxamine is more potent than the beta₁ inhibitor practolol (7.7 M vs 22 M respectively). The GTP analogue, Gpp(NH)p, reduces isoproterenol affinity to 1.9 M and increases the Hill coefficient from 0.62–0.90.     

Biliary elimination of indomethacin in man

Summary The biliary elimination of indomethacin 100 mg p.o. has been investigated in patients with a T-tube drain after cholecystectomy, who had normal or abnormal liver function (Group In=5, and Group IIn=4, respectively). In plasma, the concentration maximum (Group I 9.5±1.4 µmol·1^–1; Group II 19.4±3.8 µmol·1^–1) and the total clearance (Group I 1.56±0.28 ml·min^–1·kg^–1, Group II 0.5±0.06 ml·min^–1·kg^–1) were significant different in the two groups. The bile:plasma ratio (bpr) in Group I was 1.3±0.33 and 10.1±3.2, respectively, for indomethacin (In) and conjugated In. The conjugated fraction of In was 87.5±1.9% of the total concentration. In Group II the bpr was lower (0.65±0.1). In is eliminated in bile by diffusion and conjugated In by active secretion. In Group I 0.99±0.1 mg In and 6.1±0.7 mg conjugated In and in Group II 2.1±0.3 mg In (p<0.05) were eliminated in bile. No influence of In on the biliary lipids was observed or on the bile acid-independent fraction of bile flow. It is concluded that the total plasma clearance of In was dependent of liver function. Conjugated In undergoes enterohepatic circulation.     

The pharmacokinetics ofα-human atrial natriuretic polypeptide in healthy subjects

Summary We have analysed the pharmacokinetics of-human atrial natriuretic polypeptide (-hANP) in healthy subjects, using a two-compartment open model following bolus intravenous injection. The plasma half-times for the fast and slow components were 1.7±0.07 min and 13.3±1.69 min respectively. V₁ (the volume of the central compartment), V_z (volume of distribution) and V_ss (volume of distribution at steady-state) were 5370±855 ml (89.5±14.3 ml·kg^–1), 32000±4620 ml (533±77.0 ml·kg^–1), and 11900±1530 ml (198±25.5 ml·kg^–1) respectively. The mean plasma clearance was 1520±121 ml·min^–1 (25.4±2.0 ml·min^–1·kg^–1.     

The pharmacokinetics of amiloride-hydrochlorothiazide combination in the young and elderly

Summary The pharmacokinetics of amiloride and hydrochlorothiazide were studied in 12 healthy young volunteers following a single dose of a fixed combination of amiloride and hydrochlorothiazide and in 11 elderly hypertensive patients at steady-state. Following modelling of the single dose data, simulated steady-state plasma concentrations for the 2 drugs were generated to examine the effect of age and/or hypertension on pharmacokinetics.The apparent systemic plasma clearance for both amiloride and hydrochlorothiazide was significantly reduced in the elderly when compared to the young (from 753 to 325 ml·min^–1, amiloride; and from 418 to 157 ml·min^–1, hydrochlorothiazide). The plasma concentrations at steady state for both drugs were greatly increased in the elderly patients (Amiloride: from 7 to 25 ng·ml^–1, C_ss,max; from 2 to 8 ng·ml^–1, C_ss,min; and from 4 to 14 ng·ml^–1, C_av; Hydrochlorothiazide: from 184 to 651 ng·ml^–1, C_ss,max; from 31 to 121 ng·ml^–1, C_ss,min; and from 89 to 273 ng·ml^–1, C_av).The decreased clearance of the diuretics in the elderly was believed due to deterioration of renal function, and there was a significant correlation between the plasma clearance of hydrochlorothiazide and creatinine clearance in both age groups (r=0.62, young;r=0.72, elderly).As a result of the pharmacokinetic findings caution may be indicated in the clinical dosage of the diuretics particularly when in fixed dose combination.     

Stability of Lidocaine in Aqueous Solution: Effect of Temperature,pH, Buffer,and Metal Ions on Amide Hydrolysis

The degradation of lidocaine in aqueous solution obeys the expression k _obs = (k _H+[H ⁺] + k _o ) [H⁺]/([H ⁺ ] + K _a + k_o K _a([H ⁺ ] + K _a) where k _H+ is the rate constant for hydronium ion catalysis, and k _o and k_o are the rate constants for the spontaneous (or water-catalyzed) reactions of protonated and free-base lidocaine. At 80°C, the rate constants for these processes are 1.31 × 10^–7 M ^–l sec^–1, 1.37 × 10^–9 sec^–1, and 7.02 × 10^–9sec^–1; the corresponding activation energies are 30.5, 33.8, and 26.3 kcal mol^–1, respectively. It was found that the room temperature pH of maximum stability is 3–6 and that lidocaine is more reactive in the presence of metal ions such as Fe²⁺ and Cu²⁺. The dissociation constant, K _a, for lidocaine at 25–80°C was also measured at 0.1 M ionic strength and a plot of pK _a versus 1/T gave a slope of (1.88 ± 0.05) × 10³ K^–1 and intercept 1.56 ± 0.16.     

Analysis of Antiplatelet Effect of Ticlopidine In Humans: Modeling Based on Irreversible Inhibition of Platelet Precursors in Bone Marrow

The relationship between plasma concentration of ticlopidine and its inhibitory effect on platelet aggregation in human was analyzed using a pharmacokinetic/pharmacodynamic (PK/PD) model. The data of plasma concentration and inhibitory effect on platelet aggregation were taken from the literature. A two-compartment open model was fitted to plasma ticlopidine concentrations. Assuming that ticlopidine acts on platelet precursors in the bone marrow, the apparent reaction rate constant of ticlopidine and platelet precursors (K), apparent transformation rate constant of platelet precursors (k_r) and apparent elimination rate constant of platelets (k _e ) were estimated. The estimated values ± S.D. were 1.01 ± 1.08 ml g ^–1 hr^–1 for K, 0.265 ± 0.259 hr^–1 for k_r and 0.0747 ± 0.0112 hr ^–1 for k _e . The antiaggregation effects of ticlopidine on platelets after administration of 100, 200, and 300 mg (bid for 8 days) were simulated using the PD parameters of K, k_r, and k_e. While the antiaggregation effect reached steady state within 3–4 days without dose dependency of the interval, the maximum effect increased with dose. Furthermore, changing the elimination rate constant of ticlopidine from the central compartment in the model significantly changed the duration of inhibitory effect of ticlopidine on platelet aggregation. Therefore, the reported long duration of antiplatelet effect after discontinuation of ticlopidine, which is believed to be irreversible binding to the platelet, might have been partially caused by the delayed plasma elimination after a long therapy of ticlopidine. On the other hand, the mean life-span of platelets in the blood estimated by 1/k_e after administration of ticlopidine was 14 hr, far below the life-span of platelets in the blood. For a more detailed analysis of the antiplatelet effect of ticlopidine, the possible contribution of reversible binding of the drug to glycoprotein IIb/IIIa should be considered in future PK/PD models.     

Pharmacokinetics and pharmacodynamics of midazolam in the enflurane-anesthetized dog

Midazolam (Mid) is widely used as an anesthetic adjunct. To test its anesthetic effect vs. concentration relationships, it is desirable to establish stable and predictable Mid concentrations in plasma (and brain). Therefore, the pharmacokinetics of Mid in the enflurane-anesthetized dog were determined, and the ability of Mid to reduce the enflurane concentration required for anesthesia was measured and correlated with the Mid concentration in plasma [MID]. Mongrel dogs (n=9) were anesthetized with enflurane and the enflurane EC₅₀ (MAC—the end-tidal concentration at which one-half the dogs respond to the noxious stimulation of clamping of the tail, and one-half do not) was determined. Group 1 (n=5) received Mid 2.5mgJkg iv over 60 sec. Plasma for determination of [MID] was collected and the enflurane EC₅₀ was determined repeatedly over the 7–8-hr period following injection. Based on the pharmacokinetic parameters determined for Group 1, dogs in Group 2 (n=4) received Mid as a continuous infusion of 21 kg^–1 min^–1 for 5hr accompanied by an initial loading dose (3 mg/kg infused over 20 min) designed to produce a stable [MID] of 1000 ng/ml in plasma. Enflurane MAC and [MID] were determined regularly during the infusion and for 6hr after discontinuation of the infusion. There were no important differences in the pharmacokinetic parameters determined for Group 1 vs. Group 2: t_1/2,z=98±5 vs. 95±10min (mean ±SEM); V=3.94±0.27 vs. 2.98±25 L/kg; Cl=28.5±3.1 vs. 22.3±1.1 ml kg^–1 min^–1,respectively. When administered as a continuous intravenous infusion (Group 2), [MID] remained stable at 949 ± 53 ng/ml for more than 5hr. The enflurane EC₅₀ was reduced by 55% and the reduction remained stable during the 5 hours of Mid infusion. After a single iv bolus dose or after discontinuation of the continuous infusion, the degree of enflurane EC₅₀ reduction diminished toward the control (i.e., enflurane alone) value as [MID] declined. Midazolam's pharmacokinetics and plasma concentration vs. effect relationships have been determined to be consistent under two different experimental conditions.     

Physiological Indirect Effect Modeling of the Antilipolytic Effects of Adenosine A1-Receptor Agonists

The relationship between blood concentrations of the adenosine A ₁ -receptor agonist N⁶ -(p-sulfophenyI)adenosine (SPA) and its effect on both plasma nonesterified fatty acid (NEFA) and glycerol release was described on the basis of an integrated pharmacokinetic–pharmacodynamic model. An indirect response model rather than a hypothetical link model was used to account for the delayed response. For that purpose an empirical solution to the differential equation describing the physiological indirect response model is presented. The model-estimated rate constant for the output of the glycerol response was compared to the elimination rate constant after exogenous administration of glycerol. In a crossover designed study, chronically cannulated male Wistar rats were subjected to either SPA administration (120 g/kg for 15 min) for measurement of the effects on glycerol, or glycerol administration for determination of glycerol pharmacokinetics. Glycerol pharmacokinetics was determined in the presence of a stable level of SPA (171±6ng/ml) to suppress endogenous glycerol levels completely. The indirect response model adequately described the relationship between SPA concentrations and plasma glycerol levels. The PD parameter estimates for EC ₅₀ , E_max , and Hill factor were 23±2 ng/ml, 74±3% (change from baseline), and 3.3±0.5, respectively. These values were not different from those obtained when analyzing the data on basis of the differential equation directly. Furthermore the EC₅₀ values for the reduction in glycerol or NEFA levels were identical (23±2 and 21±3 ng/ml, respectively) indicating that both PD end points reflect the same physiological process. The concentration–time profile after administration of glycerol could be described best on the basis of a biexponential function. The value for k_out in the PK/PD model (0.19±0.03 min ^–1 ) corresponded very well to the terminal elimination rate constant determined after iv administration of glycerol (0.25±0.03 min ^–1 ). In conclusion, the antilipolytic effects of adenosine A ₁ -receptor agonists can be described by the indirect suppression model. The rate constant describing the delay between concentration and glycerol effect was shown to be a true reflection of the removal of glycerol.     

Influence of isoprenaline and salbutamol on the threshold of electrically inuced ventricular fibrillation in anaesthetized guinea pigs

Summary Fibrillation threshold and heart rate were measured after increasing doses of (–)-isoprenaline and (±)-salbutamol. The initial values of 212±10 A (x±SEM) and 211±10 A were decreased to 74±10 A by low doses of (–)-isoprenaline (10^–11 to 10^–10 moles/kg i.v.), and to 89±11 A by (±)-salbutamol (10^–9 to 10^–8 moles/kg i.v.). Higher doses, paradoxically, increased fibrillation threshold to initial values, (–)-isoprenaline: 2×10^–9 moles/kg; (±)-salbutamol: 2×10^–6 moles/kg. A linear increase in heart rate per 10-fold increase of either drug was observed, (–)-isoprenaline: 25 beats·min^–1; (±)-salbutamol: 14 beats·min^–1. The apparent ₂-selective property of salbutamol is documented by its low potency in changing fibrillation threshold and heart rate.     

Transport characteristics of L-carnosine and the anticancer derivative 4-toluenesulfonylureido-carnosine in a human epithelial cell line

Purpose. The aim of the present study was to evaluate whether the transepithelial transport of the anticancer compound 4-toluenesulfonylureido-carnosine (Ts-carnosine) and the dipeptide moiety L-carnosine was due to a hPepT1 carrier-mediated flux. Methods. Transport experiments were conducted using Caco-2 cell monolayers and either reversed-phase HPLC-UV or liquid scintillation counting methods for quantification. pK_a, LogD, and LogP were determined using the Sirius GlpK_a meter. Results. L-carnosine was transported across the apical membrane with a K_m,app of 2.48 ± 1.16 mM and a V_max of 2.08 ± 0.34 nmol · cm^–2 · min^–1 and across the basolateral membrane with a K_m,app of 7.21 ± 3.17 mM and a V_max of 0.54 ± 0.10 nmol · cm^–2 · min^–1, and transepithelially with a P_app of 4.46 · 10^–2 ± 6.4 · 10^–6 cm · min^–10. Ts-carnosine had an affinity (K_i) for hPepT1 of 2.33 ± 0.54 mM; however, the transepithelial transport was low as compared to that of L-carnosine. Conclusions. L-carnosine was transported across both the apical and basolateral membrane of Caco-2 cell monolayers in a carrier-mediated manner however, the transepithelial transport followed apparent simple non-saturable kinetics. Ts-carnosine had an affinity for hPepT1 but a relatively low transepithelial transport. This indicates that the transepithelial transport of L-carnosine and Ts-carnosine is not hPepT1 carrier-mediated and that L-carnosine is not a suitable dipeptide moiety for hPepT1-mediated absorption of sulfonamide-type anticancer compounds.     

Pharmacokinetics of cimetidine after subchronic administration

Summary The influence of cimetidine on its own pharmacokinetics after subchronic administration was assessed in 8 healthy volunteers, aged 26–29 years. On control Day 1, each subject received cimetidine 300 mg i.v., and serum and urine samples were obtained. Each subject was initiated on cimetidine 600 mg b.i.d. orally for 2 weeks. There were 3 further study days repeated after 1 and 2 weeks of cimetidine dosing and 1 week after stopping cimetidine. There was no significant difference in the mean total body clearance of cimetidine among the 4 study days. Mean elimination t_1/2 and V were similarly unchanged. However mean renal clearance (CL_R) and f_e were significantly increased following 2 weeks of drug dosing (CL_R 5.41 ml·min^–1 kg^–1; f_e 0.61) compared to control (CL_R 4.00 ml·min^–1·kg^–1; f_e 0.48). Although the non renal clearance was reduced from control values of 4.29 to 3.51 ml·min^–1·kg^–1 following 2 weeks of dosing the difference was not significant. Dosage adjustment of cimetidine appears unnecessary after short-term dosing in the presence of normal renal function.     

Pharmacokinetic interaction of chloroquine and methylene blue combination against malaria

Objective The combination of chloroquine and methylene blue is potentially effective for the treatment of chloroquine-resistant malaria caused by Plasmodium falciparum. The aim of this study was to investigate whether methylene blue influences the pharmacokinetics of chloroquine.Methods In a randomized, placebo-controlled, parallel group design, a 3-day course of therapeutic oral doses of chloroquine (total 2.5 g in male, 1.875 g in female participants) with oral co-administration of placebo or 130 mg methylene blue twice daily for 3 days was administered to 24 healthy individuals. Chloroquine, desethylchloroquine, and methylene blue concentrations were determined by means of HPLC/UV or LC/MS/MS assays in whole blood, plasma, and urine for 28 days after the last dose.Results During methylene blue exposure, the area under the chloroquine whole blood concentration–time curve normalized to body weight (AUC_{0-24 h}/BW) yielded a trend of reduction (249±98.2 h g l^–1 kg^–1 versus 315±65.0 h g l^–1 kg^–1, P=0.06). The AUC_{0-24 h}/BW of desethylchloroquine was reduced by 35% (104±40.3 h g l^–1 kg^–1 versus 159±66.6 h g l^–1 kg^–1, P=0.03), whereas the metabolic ratio between chloroquine and desethylchloroquine remained unchanged (2.25±0.49 versus 1.95±0.42, P=0.17). The renal clearance of chloroquine and the ratio between chloroquine in whole blood and plasma remained unchanged (P>0.1).Conclusion Oral co-administration of methylene blue appears to result in a small reduction of chloroquine exposure which is not expected to be clinically relevant and thus represents no concern for further development as an anti-malarial combination.     

Binding characteristics of (+)-, (±)- and (-)-[125Iodo] cyanopindolol to guinea-pig left ventricle membranes

Summary [¹²⁵Iodo]cyanopindolol [(±)-ICYP], a potent and selective ligand for -adrenoceptors, exhibited complex biphasic dissociation kinetics. Consequently, in receptor binding studies, the pure (+)- and (-)-enantiomers of ICYP were synthesised and their equilibrium and kinetic binding characteristics were investigated on a membrane preparation of guinea pig left ventricle containing almost only ₁-adrenoceptors.All three ligands, i. e. (+)-, (-)- and (±)-ICYP, bind to -adrenoceptors as assessed by competition experiments with different -blocking agents; irrespective of the ligand used, the same dissociation constant was found for the competitor. In a first series of saturation binding experiments performed in a low concentration range of free ligand (0–250 pM), ICYP showed the following dissociation constants: K _D=93, 9 and 23 pM, and number of binding sites: B _max=40,128 and 124 fmoles/mg protein for (+)-, (-)-, and (±)-ICYP, respectively. Asexpected, (±)-ICYP showed the same B _max as (-)-ICYP, whereas its K _D was approximately two times higher than that of (-)-ICYP. Surprisingly, the B _max of (+)-ICYP represented only 30% of the B _max of (-)-ICYP. All three ligands bound apparently to a single class of binding sites.In dissociation experiments, the enantiomers of ICYP showed biphasic dissociation curves as observed for the racemic ligand. (+)-, (-)- and (±)-ICYP showed a rapidly dissociating (k _-1=0.488, 0.047 and 0.049 min^–1) and a slowly dissociating component (k _-2=0.0205, 0.0033 and 0.0025 min^–1). The ratio slow dissociating/fast dissociating component represented respectively for (+)-, (-)- and (±)-ICYP 40/60, 90/10 and 90/10. For all three ligands, the association rate constants were of the same order of magnitude (ca. 10⁹ M^–1 min^–1), typical for a diffusion controlled reaction.In contrast to equilibrium binding studies, the existence of multiple receptor affinity sites was evident from the biphasic dissociation behaviour observed especially with the nonracemic ligands (+)-ICYP and (-)-ICYP.Simulation of theoretical saturation curves performed with the ratios of high versus low affinity sites and the K _D-values suggested by kinetic analysis, indicated that the delineation into two affinity states might be visible in saturation experiments, under certain conditions.Therefore, equilibrium binding studies were repeated with an increased number of ligand concentrations covering a large concentration range of 0–800 pM. Simultaneous analysis of saturation curves from the same experiment using three different ligands, provided more accurate estimates of the ratio of high and low affinity sites, as well as the affinity constants of the ligand for each receptor affinity state, in good agreement with the results from kinetic analysis.The contribution of the (+)enantiomer in the binding of the racemic ligand under low receptor concentrations could be neglected since dissociation characteristics of (±)- and (-)-ICYP were identical. A model that explains the biphasic dissociation of (±)-ICYP by differential binding of both enantiomers could be rejected. Kinetic and equilibrium binding characteristics of the three radioligands were not influenced by the guanylnucleotide Gpp(NH)p (10^–4 M).The antagonist ICYP binds to -adrenoceptors in a high and low affinity state which are probably interconvertible.Abbreviations CYP cyanopinodolol - ICYP [¹²⁵Iodo]cyanopindolol - HYP (±)-hydroxybenzylpindolol - IHYP (±)-[¹²⁵Iodo]hydroxybenzylpindolol - ³H-DHA (-)-[³H]dihydroalprenolol - ³H-HBI (±)-[³H]-hydroxybenzylisoproterenol - ISA intrinsic sympathomimetic activity; Gpp(NH)p, guanyl-5yl-imidodiphosphate - FM 24 1-(2-exobicyclo[2,2,1]hept-2yl-phenoxy)-3-[(1-methylethyl)amino]-2-propranolol Part of this work has been presented at the Spring Meeting of the German Pharmacological Society, Mainz, March 10–13, 1981     

Early stage autoinduction of carbamazepine metabolism in humans

Six healthy young adult male volunteers were given two 600 mg (2540 moles) oral doses of carbamazepine (CBZ) 5 days apart. Serial concentrations of CBZ and its 10,11-epoxy (CBZ-epoxide) and 10,11-dihydro-10,11-trans-dihydroxy (CBZ-diol) metabolites in plasma, and daily excretions of these substances and the 2-hydroxy (2-OH-CBZ), 3-hydroxy (3-OH-CBZ) and 9-hydroxymethyl-10-carbamoylacridan (acridan) metabolites in urine were followed for 5 days after each dose.Pharmacokinetic analysis showed that autoinduction of CBZ metabolism was present within 6–10 days of the initial drug dose. The mean oral clearance of CBZ increased from 1.48 to 1.74 l·h^– (difference 0.26 l·h^–, 95% confidence interval 0.11 to 0.41 l·h^–) and the mean percentage urinary recovery of the amount of CBZ eliminated increased from 41.8% to 44.6% (difference 2.8%, 95% confidence interval 0.5 to 5%) between the two studies 5 days apart.The data for daily clearance to metabolite and the time-courses of the plasma CBZ-epoxide to CBZ and CBZ-diol to CBZ concentration ratios suggested that autoinduction had begun by the second day after CBZ intake, and involved not only the epoxide-diol pathway but, to a lesser extent, the oxidations to phenolic derivatives.