Disposition kinetics in dogs of diethyldithiocarbamate,a metabolite of disulfiram

Following the intravenous infusion of sodium diethyldithiocarbamate to dogs, the disposition kinetics of diethyldithiocarbamate (DDC), a metabolite of disulfiram, were assessed. Approximately 27% of the administered dose was S-methylated, this process exhibiting a mean first-order rate constant of 0. 0569 min^–1 (t_1/2=12.2 min), while the remainder was eliminated by other routes having a rate constant of 0.148 min^–1 (t_1/2=4.68 min). The methyl diethyldithiocarbamate (MeDDC) formed from DDC showed an elimination rate constant of 0.0141 min^–1 (t_1/2=49.2 min). These observations are discussed in the light of previous investigations where the presence of MeDDC has rarely been sought or reported. A few comparisons with prior studies, in which DDC or disulfiram was administered, are made by retrospective kinetic evaluation of published data. The results are discussed in relation to the duration of action of disulfiram in man.Glossary A plasma concentration intercept at the cessation of infusion (mass/volume) - A _T simplifying constant (mass/volume/time) - AUC _M area under the plasma concentration-time curve for MeDDC (mass × time/volume) - b time variable; equalst during infusion, equalsT after the cessation of infusion - B plasma concentration intercept at the cessation of infusion (mass/volume) - B _T simplifying constant (mass/volume/time) - C _D plasma concentration of DDC at any timet (mass/volume) - C _M plasma concentration of MeDDC, expressed as DDC, at any timet (mass/volume) - C _T plasma concentration of total DDC, expressed as DDC, at any timet;C _T=C_D+C_M (mass/volume) - C _t plasma concentration of total DDC, expressed as DDC, at any timet (mass/volume) - Cl _D total body clearance of DDC (volume/time) - Cl _M total body clearance of MeDDC (volume/time) - DDC diethyldithiocarbamate - f fraction of DDC that is methylated;f=K _DM/K _D - K _A apparent first-order rate constant (reciprocal time) - K _B apparent first-order rate constant (reciprocal time) - K _D apparent first-order rate constant for the elimination of DDC by all routes (reciprocal time) - K _M apparent first-order rate constant for the elimination of MeDDC by all routes (reciprocal time) - K _DE apparent first-order rate constant for the elimination of DDC by all routes except methylation (reciprocal time) - K _DM apparent first-order rate constant for theS-methylation of DDC (reciprocal time) - MeDDC methyl diethyldithiocarbamate - NaDDC sodium diethyldithiocarbamate (trihydrate) - Q zero-order infusion rate constant (mass/time) - Q ₁ zero-order infusion rate constant for the faster of two consecutive infusions (mass/time) - Q ₂ zero-order infusion rate constant for the slower of two consecutive infusions (mass/time) - t elapsed time since dosing (e.g., infusion) commenced - t elapsed time since the cessation of infusion - T duration of infusion (time) - T ₁ duration of the faster of two consecutive infusions (time) - T ₂ total duration of infusion when two consecutive infusions are administered (time) - V _D apparent volume of distribution of DDC - V _M apparent volume of distribution of MeDDC This work was supported by the Atkinson Charitable Foundation (Toronto, Ontario, Canada) and the Non-Medical Use of Drugs Directorate, Health and Welfare Canada (Grant No. 1212-5-206).     

Bioavailability of two hydrochlorothiazide preparations

Summary Eight healthy volunteers received hydrochlorothiazide 75 mg as Dichlotride and Esidrex. Maximal plasma levels were significantly (p<0.05) higher after Dichlotride than Esidrex, 512±189 and 376±70 ng/ml, respectively. However, the bioavailability of the two brands of hydrochlorothiazide did not differ significantly as judged by comparison of the AUC_09h and AUC₀, and the urinary recovery of hydrochlorothiazide during 48 hrs.Supported by the Swedish Medical Research Council (Grant No. B 75-04X-227-12 C) and Hässle-Ciba-Geigy AB, Gothenburg, Sweden     

Can the Enhanced Renal Clearance of Antibiotics in Cystic Fibrosis Patients Be Explained by P-Glycoprotein Transport?

Purpose. To investigate in vitro if P-glycoprotein (P-gp) transport can differentiate between antibiotic drugs exhibiting increased active renal clearance (CL_r) in cystic fibrosis (CF) patients (i.e., dicloxacillin, trimethoprim) and drugs that do not exhibit this phenomenon (i.e., cefsulodin, sulfamethoxazole). Methods. Transport studies were carried out in MDCK (wild type) and MDR1-MDCK (P-gp overexpressing) cells that were grown to confluence on Transwell inserts. [¹⁴C]-mannitol transport and transepithelial electrical resistance (TEER) were measured to validate the integrity of the cells. Drug concentrations were analyzed using HPLC. Results. Dicloxacillin and trimethoprim are substrates of P-gp (BA/AB ratios in MDR1-MDCK cells are 32 and 50, respectively). P-gp inhibitors (i.e., GG918, cyclosporine, ketoconazole, vinblastine) decreased the BA transport of dicloxacillin and trimethoprim and increased the AB transport of trimethoprim while non-P-gp inhibitors (e.g., PAH) had no effect. In contrast, cefsulodin and sulfamethoxazole are not substrates of P-gp (BA/AB values in MDCK and MDR1-MDCK cells are 1). Conclusions. Our in vitro studies suggest that P-glycoprotein may play a role in increasing renal clearance of drug substrates in CF patients. Dicloxacillin and trimethoprim, which are both substrates of P-gp, show increased active renal clearance in CF patients while cefsulodin and sulfamethoxazole, which are not P-gp substrates, do not show increased active renal clearance in CF patients.     

Role of human liver microsomal CYP2C9 in the biotransformation of lornoxicam

Objective: The nature of the enzyme(s) catalysing the biotransformation of lornoxicam to one of its major metabolites, 5-hydroxy-lornoxicam, has been investigated in human liver microsomes. The reaction kinetics were characterised, the affinity of lornoxicam for three major human drug metabolising cytochrome P-450 isozymes (CYP2C9, CYP2D6 and CYP3A4) was determined, and inhibition of the reaction by known substrates (diclofenac, ibuprofen, mefenamic acid, phenytoin, tolbutamide and warfarin) and the prototype inhibitor (sulphaphenazole) of CYP2C9 was investigated. Results: Lornoxicam 5-hydroxylation displayed single enzyme Michaelis-Menten kinetics, with a K_M of 3.6 mol·l^-1 and a V_max of 2.6 nmol·h^-1·mg^-1 microsomal protein. The apparent affinity of lornoxicam was high for CYP2C9, but negligible for CYP3A4 and CYP2D6. Inhibition of lornoxicam 5-hydroxylation by CYP2C9 substrates and sulphaphenazole was comparable in all livers preparations, values predicted from their K_M or K_i for CYP2C9 determined in separate studies assuming competitive inhibition. Sulphaphenazole competitively and completely inhibited lornoxicam 5-hydroxylation (K_i=0.31 mol·l^-1) as well as lornoxicam clearance (K_i=0.33 mol·l^-1), partial metabolic clearance (f_m)=0.95). Conclusion: 5-Hydroxylation appears to be the only cytochrome P-450 catalysed metabolic reaction of lornoxicam by human liver microsomes and this major in vivo biotransformation pathway is catalysed virtually exclusively by CYP2C9.Supported in part by Hafslund Nycomed Pharma AG (Linz, Austria) and by a grant of the Forschungsförderungsfond der Gewerblichen Wirtschaft Österreichs     

维生素K_1微乳剂在大鼠体内的药物动力学

目的建立大鼠血浆中维生素K_1的高效液相色谱测定法,并应用于经灌胃给予维生素K_1微乳剂后的维生素K_1药动学研究。方法大鼠经灌胃给予维生素K,微乳剂3.77 mg·kg^(-1),于给药后不同时间采集血样,采用无水乙醇-乙醚(体积比为1:3)提取的方法处理血浆样品。采用反相高效液相色谱法测定维生素K_1的血药浓度,色谱柱为Thermo C_(18)柱(150 mm×4.6 mm,5μm),流动相为无水乙醇-水(体积比为90:10),流速为1.1 mL·min(-1),于给药后不同时间采集血样,采用无水乙醇-乙醚(体积比为1:3)提取的方法处理血浆样品。采用反相高效液相色谱法测定维生素K_1的血药浓度,色谱柱为Thermo C_(18)柱(150 mm×4.6 mm,5μm),流动相为无水乙醇-水(体积比为90:10),流速为1.1 mL·min^(-1),检测波长为270 nm,柱温为35℃。结果血液中的内源性物质不干扰测定。维生素K_1的线性为48～2 400μg·L(-1),检测波长为270 nm,柱温为35℃。结果血液中的内源性物质不干扰测定。维生素K_1的线性为48～2 400μg·L^(-1),r=0.995 3,定量下限为48μg·L(-1),r=0.995 3,定量下限为48μg·L^(-1),精密度、准确度、回收率均符合生物样品测定要求。主要药物动力学参数:t_(max)=1.0 h,t_(1/2)=7.61 h,ρ_(max)=0.77 mg·L(-1),精密度、准确度、回收率均符合生物样品测定要求。主要药物动力学参数:t_(max)=1.0 h,t_(1/2)=7.61 h,ρ_(max)=0.77 mg·L^(-1),AUC_(0-t)=2.87 mg·h·L(-1),AUC_(0-t)=2.87 mg·h·L^(-1),AUC_(0-∞)=3.21 mg.h·L(-1),AUC_(0-∞)=3.21 mg.h·L^(-1)。结论该方法适用于维生素K,微乳剂在大鼠体内的药物动力学研究。     

Pharmacokinetic interactions of timolol with vasodilating drugs,food and phenobarbitone in healthy human volunteers

Summary Pharmacokinetic interactions of oral timolol maleate 10 mg, with food (3566 kJ), single oral doses of prazosin 1 mg and dihydralazine 25 mg, and with a 1 week pretreatment with phenobarbitone 100 mg daily were examined in a randomized crossover study in 12 healthy volunteers. After fasting, the peak level (C_max=29.1±3.2 ng/ml; mean±SEM) was reached at 1.3±0.1 h (T_max). The total area under the serum concentration-time curve (AUC^0–) was 154.4±33.8 ng×h/ml, total clearance (Cl_tot) 751.5±90.6 ml/min, renal clearance (Cl_ren) 97.2±10.1 ml/min, elimination half-life (t_1/2) 2.9±0.3 h and 24-h recovery in urine (X _u ^0–24 ) 11.1±1.4% of the dose. Food and prazosin did not significantly affect the fate of timolol maleate. Dihydralazine enhanced C_max (38.2±4.6 ng/ml) only when compared to phenobarbitone treatment, and did not affect any other parameters. Phenobarbitone pretreatment somewhat lowered C_max (25.5±3.9 ng/ml), AUC^0– (117.5±22.1;p<0.05 vs food) and X _u ^0–24 (8.7±1.2%), evidently by increasing Cl_tot (957.5±116.9 ml/min;p<0.05 vs food), but it did not affect Cl_ren. It is concluded that the pharmacokinetics of timolol maleate can be altered to a limited extent in opposite directions by dihydralazine and phenobarbitone.     

Relationship between the ocular and systemic disposition of flurbiprofen: The effect of altered protein dynamics at steady state

The differences in flurbiprofen disposition in the aqueous humor and the plasma were examined after systemic doses. Steady state plasma concentrations of flurbiprofen (20–60 g/mL) were achieved via intravenous infusion to albino rabbits. Flurbiprofen demonstrated linear systemic kinetics throughout the dosing range, with constant body clearance and unbound fraction in plasma. At steady state, aqueous humor drug concentrations depended on the corresponding plasma drug concentration. Two clearance terms—CL_so, the systemic clearance to ocular tissues, and CL_os, the ocular clearance to systemic circulation—were used. After systemic doses, the drug concentration in the aqueous humor was related to that in the plasma as well as to the ratio of these two clearances. Flurbiprofen was extensively bound to plasma proteins and showed limited ocular distribution; its CL_so to CL_so tratio was very small. Thus, the concentration of flurbiprofen in the aqueous humor after systemic doses was lower than that obtained after ophthalmic doses. A plasmapheresis technique was utilized to lower the plasma protein concentrations to 60% of normal levels. As a consequence, flurbiprofen demonstrated reduced aqueous humor protein concentrations, increased unbound fractions in the plasma and the aqueous humor, elevated aqueous humor drug concentrations, and elevated total body clearance. The unbound body clearance stayed unchanged. Our study indicated that a drug should present a significant CL_so/CL_os ratio in order to achieve therapeutic concentrations in the eye via systemic doses. The drug-protein binding kinetics can be different between the plasma and the aqueous humor circulations. Because the ocular compariment is very small compared to the overall systemic distribution of flurbiprofen, it has little effect on the steady state systemic concentrations.     

The Site of Absorption in the Small Intestine Determines Diltiazem Bioavailability in the Rabbit

Purpose. Since the ability of the small intestine to biotransform a drug may decrease in distal segments of the intestine, this study aimed to assess whether the site of administration in the small intestine could affect the systemic bioavailability of diltiazem and its two active metabolites, N-desmethyldiltiazem (MA) and desacetyldiltiazem (Ml). Methods. Five mg/kg of diltiazem were administered into the lumen of the proximal (0–30 cm, n = 9) or the distal (150–180 cm) small intestine (n = 7) of anesthetized New Zealand rabbits. Blood samples were drawn from the femoral artery for 6 hours, and diltiazem, MA and M1 were assayed by HPLC. Results. The area under the curve (AUC₀ )of diltiazem administered into the distal small intestine was larger than that estimated when diltiazem was given in the proximal segment (14.20 ± 2.82 vs 8.14 ± 0.88 µg.min/ml, p < 0.05), due to a lower diltiazem oral clearance (440 ± 78 vs 660 ± 55 ml/min/kg, p < 0.05). The AUC_{0 360} of MA was not affected by the site of diltiazem administration, but the AUC_{0 360} of M1 was increased when diltiazem was administered in the distal segment of the small intestine. When administered into the distal segment of the intestine, the molar sum of diltiazem and its active metabolites was 48% greater than when delivered into the 0–30 cm segment of the small intestine; as a consequence, absorption of diltiazem in distal segments of the small intestine may enhance its pharmacological response. Conclusions. The site of absorption into the intestine modulates the bioavailability of diltiazem and its two active metabolites.     

Interindividual differences in the pharmacokinetics of digitoxin and digoxin during long-term treatment

Summary Patients suffering from congestive heart failure received maintenance doses of digitoxin (N=10) or digoxin (N=8). The plasma glycoside concentration was determined, and after a single dose of³H-digitoxin or³H-digoxin, the decline and excretion of radioactivity were measured over a period of 7 (digitoxin) and 3 days (digoxin). Plasma radioactivity declined with a ^x T_1/2 between 77 and 234 h (mean 138 h) in the case of digitoxin and with a ^x T_1/2 between 9.2 and 38.6 h (mean 23.5 h) for digoxin. A close correlation between ^x T_1/2 and excreted radioactivity and ^x T_1/2 and total plasma level was found for digitoxin. In 4 patients TLC of urine showed that interindividual variations in digitoxin elimination could possibly be attributed to variation in metabolism, resulting in the production of different metabolites. Predicted digitoxin plasma levels agreed well with measured values. The maintenance dose could be calculated from the total body clearance (V_Cl) and a presumed plasma glycoside level. The recommended technique facilitates dosage calculations in patients treated with digitoxin.Abbreviations AUC ₀ ^x area under curve - average steady state plasma concentration - F fraction of the dose which is absorbed - D dose - D_M maintenance dose - V_Cl plasma (body) clearance - dosage interval - kinetic parameter of the corresponding - ³H glycoside     

Does the carrier of chromaffin granules transport the protonated or the uncharged species of catecholamines?

Summary By osmotic lysis in the presence of urea ghosts (60–100 nmol catecholamine/mg prot.) were prepared from chromaffin granules (4–6 mol catecholamine/mg prot.) of the bovine adrenal medulla. In the presence of 1–300 mol/l³H-catecholamine and ATP-Mg²⁺, ghosts show a net uptake of catecholamine. The net uptake is sensitive to reserpine or agents (uncouplers and ammonium) which diminish the electrochemical potential difference for protons at the granule membrane (p). The same uptake was found by³H-counting or by fluorimetric measurements. At various pH-values (pH 6.2–82.) theK _m andV _max of the ATP-stimulated rate of uptake of³H-catecholamine into ghosts was determined (at 30°C) to identify the species of catecholamine (protonated, uncharged, or anionic) which is the substrate for the granule carrier. The pH difference (pH=pH_out-pH_in) and the electrical potential difference () were determined to calculate p under conditions of³H-catecholamine uptake. When the pH_out was increased (pH 6.2, 7.4, 8.2), the apparentK _m of uptake decreased (50, 5, 1–2 mol/l), showing a linear relation between pH and logarithm ofK _m. TheK _m was calculated for the uncharged catecholamine (with pK₁=8.8 and pK₂=10.0); it was nearly pH-independent and amounted to about 0.2 mol/l. TheV _max declined only in the extreme pH-range. Between pH 6.6 and 7.8V _max and p showed a slight increase from 16 to 20 nmoles/(mg prot.·min) and from 110 to 140 mV, resp. In the same pH-range the pH_in inside ghosts increased from pH 5.2 to 5.7, whereas was constant (30 mV). At constant pH_out (=7.3) ammonium (0–30 mmol/l) caused an increase of pH_in from 5.5 to 6.6. The increase of pH_in was accompanied by an increase ofK _m from 5 to 20 mol/l³H-catecholamine and by a decrease of bothV _max and p from 20 to 5 nmoles/(mg prot.·min) and from 123 to 85mV, respectively. From the dependence of theK _m of uptake on pH_out is concluded that the uncharged species of catecholamine is transported, whereas the dependence ofK _m on pH_in suggests that the translocation of the catecholamine-carrier complex across the granule membrane is not the rate-limiting step of catecholamine uptake.A preliminary account was presented to the Deutsche Pharmakologische Gesellschaft (Kobold and Burger 1983)     

Model-independent assessment of accumulation kinetics based on moments of drug disposition curves

Summary The bounds of the accumulation profile can be predicted on the basis of the mean disposition residence time (MDRT) of a drug. The time to reach 90% of the plateau level (t _0.9) is less than 3.7 MDRT. This prediction can be improved if, in addition, the variance of disposition residence time (VDRT, CV _D ² =VDRT/MDRT²), or the terminal exponential coefficient (), is known. For CV _D ² 1 or MDRT1, the time to reach steady state (t_0.9) approaches 2.3 MDRT (limiting case of monoexponential drug disposition curve). Conditions are stated under which can be regarded as the principal determinant of the accumulation rate.     

Pharmacodynamics and pharmacokinetics of xipamide in patients with normal and impaired kidney function

Summary The effect of a single oral dose of 40 mg xipamide on urinary excretion of Na⁺, K⁺, Cl^–, Ca²⁺ and Mg²⁺ in healthy subjects and in patients with varying degrees of renal impairment was compared with various conventional diuretics. Xipamide caused marked excretion of Na⁺ and Cl^–, whereas the diuretic produced only moderate kaliuresis; urinary excretion of Ca²⁺ was increased in proportion to Na⁺, like the loop diuretics. Xipamide affected electrolyte excretion even in patients with a creatinine clearance below 30 ml/min, as do the loop diuretics, too. Therefore, the pharmacodynamic characteristics of xipamide are more like those of a loop diuretic than of a thiazide. Xipamide was good bioavailable, its t_1/2 was 7 h and urinary recovery of the undegraded drug was 40% of the given dose. In renal insufficiency, t_1/2 increased from 7 to only 9 h, yielding a moderate increase in the AUC. Urinary recovery of the drug was reduced in proportion to the reduction in the creatinine clearance of the patient. Therefore, significant extrarenal elimination of the diuretic must be postulated, which suffices to prevent significant drug accumulation in renal failure.     

L-type Ca<Superscript>2+</Superscript> channels activation and contraction elicited by myricetin on vascular smooth muscles

The effects of myricetin (3,3,4,5,5,7-hesahydroxyflavone), a natural flavonoid found in edible plants, were studied on vascular smooth muscle L-type Ca²⁺ channels by comparing its mechanical, radioligand binding, and electrophysiological properties to those of the Ca²⁺ channel agonist (S)-(-)-Bay K 8644.In rat aorta rings, both myricetin and (S)-(-)-Bay K 8644 induced contractile responses, which were dependent upon prior exposure to K⁺. At 15 mM K⁺ (K15) the pEC₅₀ values for myricetin and (S)-(-)-Bay K 8644 were 4.43±0.03 and 7.92±0.13, respectively. Furthermore, the maximum tension response to myricetin was not significantly different from that elicited by either (S)-(-)-Bay K 8644 or K60. The Ca²⁺ channel blockers nifedipine, verapamil and diltiazem antagonised and fully reverted myricetin-, (S)-(-)-Bay K 8644- as well as K60-induced contractions. Both myricetin and (S)-(-)-Bay K 8644 potentiated rat aorta ring responses to K⁺, shifting the K⁺ concentration-response curve to the left. (S)-(-)-Bay K 8644, but not myricetin, inhibited in a concentration-dependent manner (+)-[³H]PN200–110 binding in porcine aortic membranes. Electrophysiological recordings from single rat tail artery myocytes, under amphotericin B-perforated as well as conventional methods, showed that both myricetin and (S)-(-)-Bay K 8644 increased L-type Ba²⁺ current (I_Ba(L)) and shifted the maximum of the current-voltage relationship by 10 mV in the hyperpolarising direction, without, however, modifying the threshold potential. Furthermore, (S)-(-)-Bay K 8644 accelerated both activation and inactivation kinetics of I_Ba(L) while myricetin slowed down the activation kinetics. Finally, both (S)-(-)-Bay K 8644 and myricetin slowed down deactivation kinetics of I_Ba(L).These results suggest that myricetin induces vasoconstriction by activating L-type Ca²⁺ channel with similar efficacy but a site of action different to that of (S)-(-)-Bay K 8644.Abbreviations I_Ba(L) L-type Ba²⁺ current - PSS Physiological salt solution - V_h Holding potential     

Pharmacokinetics and pharmacodynamics of etizolam are influenced by polymorphic CYP2C19 activity

Objective To study the effect of erythromycin on metabolism of quetiapine in Chinese suffering from schizophrenia.Methods Nineteen patients received multiple doses of quetiapine (200 mg, twice daily) with or without co-administered erythromycin (500 mg, three times daily). Blood samples were collected at specified time intervals for determination of plasma concentrations of quetiapine and some of its metabolites.Results With erythromycin co-administration: for quetiapine, maximal plasma concentration (C _max), area under concentration–time curve of 0– h (AUC_0–) and terminal-phase elimination half-life time (t _1/2) increased 68, 129 and 92%, respectively, and clearance (CL) and terminal elimination rate constant (K _e) decreased 52% and 55%, respectively; for quetiapine sulfoxide (QTP-SF), C _max, AUC_0– and AUC ratio decreased 64, 23, and 70%, respectively, and t _1/2 increased 211%; for 7-hydroxy-quetiapine (QTP-H), K _e and AUC ratio decreased 61% and 45%, respectively, and t _1/2 increased 203%; for 7-hydroxy-N-desalkyl-quetiapine (QTP-ND), C _max, AUC_0– and AUC ratio decreased 36, 40 and 71%, respectively.Conclusion Erythromycin has a noticeable effect on the metabolism of quetiapine. When quetiapine is co-administered with CYP3A inhibitors such as erythromycin, the dosing regimen should be modified according to quetiapine TDM.     

Compartment- and model-independent linear plateau principle of drugs during a constant-rate absorption or intravenous infusion

A simple general equation is derived to show the linear plateau principle under various conditions during or after a constant or changing rate of absorption or intravenous infusion. The time required to cause a certain fraction (f_t) of the total shift or change between the two steady-state plasma concentrations is equal to the time required for the cumulative (from time zero) plasma area, AUC_0t, to reach the same fraction of AUC₀ assumed to be obtained after an instantaneousintravenous dosing. The role of the terminal biological half-life and the importance of the earlydistribution phase and its exponential half-life or lives in the plateau principle are discussed.Clinical implications and applications to multiple dosage regimens are also discussed.     

Vitamin K1, vitamin K1 epoxide and warfarin interrelationships in the dog

The cyclic interconversion of vitamin K₁ and vitamin K₁ epoxide, in the presence and absence of administration of warfarin, was studied in two dogs, using tritiated vitamin K₁ and vitamin K₁ epoxide. Warfarin, at therapeutic doses, completely blocked the conversion of vitamin K₁ epoxide to vitamin K₁. Alternative routes of metabolism of vitamin K₁ epoxide appear to account for about one-third of its clearance from plasma. Warfarin did not influence the clearance of vitamin K₁ from plasma. Nonetheless, it reduced the volume of distribution of vitamin K₁ (but not of vitamin K₁ epoxide), possibly by displacing vitamin K₁ from specific binding sites in the liver. In the absence of warfarin, approximately 10 per cent of an administered dose of vitamin K₁ appeared as vitamin K₁ epoxide in plasma, whereas 45 per cent of an administered dose of vitamin K₁ epoxide appeared in plasma as vitamin K₁. Warfarin increased the former percentage to 65 per cent by inhibiting reconversion of the epoxide, as shown by the absence of vitamin K₁ in plasma when vitamin K₁ epoxide was administered in the presence of warfarin.     

Effect of a diffusional barrier to a metabolite across hepatocytes on its kinetics in “enzyme-distributed” models: A computer-aided simulation study

The effect of a diffusional barrier to a metabolite between the blood and hepatocytes on elimination kinetics of formed and preformed metabolites was predicted under various enzymic distributions in the liver by computer- aided simulation. Sequential metabolism by which the primary metabolite (MI) is generated from the parent drug (D) and further metabolized to the terminal metabolite (MII) by enzymes A and B, respectively, was chosen for the simulation. Moreover, four models of enzymic distribution patterns were defined with regard to the hepatic blood flow path. The extraction ratios for the preformed and formed metabolites (designated as E_m and E_pm, respectively) were simulated by varying both the average intrinsic clearance of enzyme B ( ) and the permeability of hepatocytes for MI ( ), while keeping the average intrinsic clearance of enzyme A ( ) equal to hepatic blood flow (Q). When a rapid equilibrium of MI between the blood and hepatocytes held, i.e., was large relative to Q, E_m was equal to or higher than E_pm for all models, as previously shown by Pang and Stillwell. By contrast, it was found that when a diffusional barrier for MI existed, i.e., was small relative to Q, E_m was equal to or lower than E_pm. Furthermore, it was observed that the smaller became, the larger the difference between E_m and E_pm became. We further simulated the effect of the intrinsic clearance ( ) for a metabolic pathway, which competes for that by enzyme A. on the E _pm value. In the model assuming even distribution of all the enzymes along the flow path, irrespective of the value, a similar effect of on E_pm was observed when the value was relatively small ( ). By contrast, in the case of uneven enzymic distributions of enzymes A and B, the effect of the value on the relationship between P_m and E_pm occurred to some extent. From these simulations, it was concluded that lower membrane permeability ( ) both diminishes the entry of preformed metabolite into the hepatocytes and accelerates the removal of intracellularly formed metabolite (through sequential metabolism) by diminishing its efflux, yielding lower E_m than E_pm. When becomes small ( ), these mechanisms for lower E_m than E_pm predominate over other mechanisms such as the presence of a competing metabolic route and uneven distribution of enzymes.     

Coenzyme Q10 attenuates cyanide-activation of the ATP-sensitive K+ channel current in single cardiac myocytes of the guinea-pig

Summary The effect of coenzyme Q₁₀ (CoQ₁₀) on the cyanide (CN^–)-induced ATP-sensitive K⁺ channel current (K_ATP) was examined in single atrial myocytes, using the patch clamp technique. Superfusion of the cells with a CN^–/low glucose bathing solution induced an outward current in the whole-cell clamp condition. Glibenclamide (1 M) abolished this current, indicating that the current was carried through the K_ATP channel. After steady-state activation by CN^–, pinacidil (a K_ATP channel opener, 300 M) failed to further increase the current. In cell-attached patches, CN^–, when applied to the bath, induced bursting openings of an 80 pS channel (the K_ATP channel). In cells preincubated for 30 min in a solution containing CoQ₁₀ (100 g/ml), CN^–-activation of the K_ATP channel was markedly attenuated both at the whole cell and at the single channel level. At the steady-state effect of CN^– in CoQ₁₀-treated cells, pinacidil (300 M) activated the current to the maximum level achieved by CN^– in the control cells. These results suggest that CoQ₁₀ reduces in the CN^–-induced KATP current not by affecting the channel itself but by preventing depletion of intracellular ATP caused by CN^–. Send offprint requests to Y. Kurachi at Mayo Foundation     

Biliary Excretion of 17β-Estradiol 17β-d-Glucuronide Is Predominantly Mediated by cMOAT/MRP2

Purpose. The mechanism for the biliary excretion of 17-estradiol170-d-glucuronide (E₂17G), a cholestatic metabolite of estradiol, isstill controversial. The purpose of the present study is to examine thetransport of E₂17G across the bile canalicular membrane. Methods. We examined the uptake of [³H]E₂17G by isolatedcanalicular membrane vesicles (CMVs) prepared from Sprague-Dawley (SD)rats and Eisai Hyperbilirubinemic rats (EHBR) whose canalicularmultispecific organic anion transporter/multidrug resistance associatedprotein 2 (cMOAT/MRP2) function is hereditarily defective. Also,in vivo biliary excretion of intravenously administered [³H]E₂17Gwas examined. Results. In CMVs prepared from SD rats, but not from EHBR, amarked ATP-dependent uptake of [³H]E₂17G was observed.Moreover, E₂17G competitively inhibited the ATP-dependent uptake of[³H]2,4-dinitrophenyl-S-glutathione (DNP-SG). In addition, nosignificant inhibitory effect of verapamil (100 M) and PSC-833 (5 M) onthe uptake of [³H]E₂17G was observed. In vivo, the biliary excretionof intravenously administered [³H]E₂17G was severely impaired inEHBR while the biliary excretion of [³H]E₂17G in SD rats wasreduced by administering a cholestatic dose (10 mol/kg) unlabeledE₂17G, but not by PSC-833 (3 mg/kg). Conclusions. The transport of E₂17G across the bile canalicularmembrane is predominantly mediated by cMOAT/MRP2.     

Pharmacokinetics of hydrochlorothiazide in man

Summary Hydrochlorothiazide (hct) was administered orally in four different doses (12.5, 25, 50 and 75 mg), to eight healthy volunteers. Plasma and urine concentrations of hct were determined by GLC. Maximal plasma levels were found at 1.5–5 h, and averaged 70, 142, 260 and 376 ng × ml^–1 respectively. The peak plasma levels and AUC_09h of hct were highly correlated (p<0.001) with the dose. The decline in the plasma curve was biphasic in those experiments in which the plasma levels of hct could be determined for at least 24 h. The half life of the slower phase lay between 5.6 and 14.8 h. The urinary recovery of hct, which represented the gastrointestinal absorption, averaged 65 to 72 per cent of the dose. The mean renal plasma clearance did not vary with the dose and averaged 319 to 345 ml × min^–1. The diuresis during the 10 h after hct 12.5 mg exceeded that after placebo by a mean of 800 ml. The diureses was not increased further after higher doses of hct. The maximal natriuretic effect (+ 100 mmol), too, was found after the 12.5 mg dose. The excretion of potassium, however, rose with increasing doses; the maximal increment, after 75 mg hct, averaged 25 mmol. The excretion of calcium was significantly increased after 50 mg hct (+ 0.6 mmol). The maximal effect on magnesium excretion occurred after 25 mg hct (+ 0.5 mmol). In healthy volunteers there was no correlation between peak plasma level of hct or AUC_09h and the renal excretion of water and electrolytes.Financially supported by the Swedish Medical Research Council (Grant Nr. R 76-04x-227-12c) and Hässle-Ciba-Geigy AB, Gothenburg, Sweden