The unit impulse response procedure for the pharmacokinetic evaluation of drug entry into the central nervous system

The unit impulse response theory has been adapted to characterize the transport profile of drugs into the central nervous system (CNS). From the obtained input function, the cumulative plasma volume (V) cleared by transport into the CNS in time can be calculated. Simulation studies demonstrated that transport governed by passive diffusion resulted in a linear relationship between V and time, while the slope of the line, the blood-brain barrier (BBB) clearance, proved to be an adequate and model independent parameter to characterize drug transport into the CNS. The error in the result of the numerical procedure could be limited to less than 10% of the theoretically predicted value. Superposition of 5 or 10% random noise on simulated data did not result in significant differences between the calculated and theoretically predicted clearance values. Simulations of carrier-mediated transport resulted in nonlinear transport curves; the degree of nonlinearity, and thus the detectability, was dependent on the initial degree of saturation of the system, the rate of desaturation, as caused by drug elimination processes and the noise level on the data. In vivo experiments in the rat were performed, using atenolol, acetaminophen, and antipyrine as model drugs. Linear transport relationships were obtained for all drugs, indicating that transport was dependent on passive diffusion or a low affinity carrier system. BBB-clearance values were 7 +/- 1 microliters/min for atenolol, 63 +/- 7 microliters/min for acetaminophen and 316 +/- 25 microliters/min for antipyrine. These experiments validate the applicability of the presented technique in in vivo studies.     

pH-Metric Solubility. 2: Correlation Between the Acid-Base Titration and the Saturation Shake-Flask Solubility-pH Methods

Purpose. The objective of this study was to compare the results of anormal saturation shake-flask method to a new potentiometricacid-base titration method for determining the intrinsic solubility and thesolubility-pH profiles of ionizable molecules, and to report thesolubility constants determined by the latter technique. Methods. The solubility-pH profiles of twelve generic drugs (atenolol,diclofenac.Na, famotidine, flurbiprofen, furosemide,hydrochlorothiazide, ibuprofen, ketoprofen, labetolol.HCl, naproxen, phenytoin, andpropranolol.HCl), with solubilities spanning over six orders ofmagnitude, were determined both by the new pH-metric method and by atraditional approach (24 hr shaking of saturated solutions, followed byfiltration, then HPLC assaying with UV detection). Results. The 212 separate saturation shake-flask solubilitymeasurements and those derived from 65 potentiometric titrations agreed well.The analysis produced the correlation equation:log(1/S)_titration = ±0.063(± 0.032)+ 1.025(±0.011) log(1/S)_shake-flask,s = 0.20, r² = 0.978.The potentiometrically-derived intrinsic solubilities of the drugs were:atenolol 13.5 mg/mL, diclofenac.Na 0.82 g/mL, famotidine 1.1mg/mL, flurbiprofen 10.6 g/mL, furosemide 5.9 g/mL,hydrochlorothiazide 0.70 mg/mL, ibuprofen 49 g/mL, ketoprofen 118 g/mL,labetolol.HCl 128 g/mL, naproxen 14 g/mL, phenytoin 19 g/mL, andpropranolol.HCl 70 g/mL. Conclusions. The new potentiometric method was shown to be reliablefor determining the solubility-pH profiles of uncharged ionizable drugsubstances. Its speed compared to conventional equilibriummeasurements, its sound theoretical basis, its ability to generate the fullsolubility-pH profile from a single titration, and its dynamic range (currentlyestimated to be seven orders of magnitude) make the new pH-metricmethod an attractive addition to traditional approaches used bypreformulation and development scientists. It may be useful even todiscovery scientists in critical decision situations (such as calibratingcomputational prediction methods).     

Stereoselective analysis of the disposition of tosufloxacin enantiomers in man

Summary The pharmacokinetics of tosufloxacin enantiomers after oral administration of racemic tosufloxacin were examined in healthy volunteers. Only small differences were observed in time to peak concentration (2.6±0.3 [mean ± SEM] h for (+)-tosufloxacin vs 2.4±0.2 h for (–)-tosufloxacin), elimination half-life (3.61±0.24 h vs 3.49±0.23 h), and area under the curve (2.78±0.19 h·g/ml vs 2.87±0.19 h·g/ml); however, peak concentration (0.40±0.03 g/ml vs 0.44±0.03 g/ml), renal clearance (226±10 ml/min vs 202±10 ml/min), and urinary recovery (35.4±2.2% vs 32.4±1.9%) differed significantly between enantiomers.     

Evaluation of Renal Tubular Secretion and Reabsorption of Levofloxacin in Rats

Purpose. Levofloxacin, a quinolone antibacterial drug, is a zwitterion at physiological pH. We examined the effect of cationic and anionic drugs on renal excretion of levofloxacin by means of in vivo clearance to characterize the mechanisms of renal excretion of this drug. Methods. In vivo clearance was studied in male Wistar albino rats. A bolus dose of 2.85 mg/kg of levofloxacin was administered, followed by a constant infusion of 7.08 g/min. Cimetidine, tetraethylammonium, or p-aminohippurate was administered as a bolus and incorporated into the infusion solution. After reaching steady state, urine and blood concentrations were measured, and pharmacokinetic parameters were calculated. Results. Renal clearance was 2.56 ± 0.42 ml/min in control, which accounted for 34% of the total body clearance. Renal clearance was significantly decreased to 0.83 ± 0.25 ml/min by cimetidine (p<.05), corresponding to 32% of the control value. The cationic drug, tetraethylammonium also reduced the renal clearance of levofloxacin, but the effect of the anionic drug, p-aminohippurate, was slight. The clearance ratio of levofloxacin, which was calculated by renal clearance divided by the plasma unbound fraction and the glomerular filtration rate, was 1.60 ± 0.38 in the control and it was decreased to 0.68 ± 0.17 and 1.11 ± 0.22 by cimetidine and tetraethylammonium, respectively. Conclusions. The present results suggest that the renal excretion of levofloxacin in rats involves tubular secretion and reabsorption, in addition to glomerular filtration, and that tubular secretion is inhibited by cimetidine.     

Noradrenaline transport by rat heart sympathetic nerves: A re-examination of the role of sodium ions

Summary The effect of sodium ion on ³H-(–)-noradrenaline (0.0875 to 0.5 M) transport by rat heart atrial hemi-appendages incubated in vitro has been studied, and the following observations made: a) When sodium was omitted (choline and lithium substitution) there was no evidence for active noradrenaline transport, and only a component that did not show saturation kinetics up to 1 M noradrenaline, remained. b) Omission of sodium or addition of 4×10^–5 M desipramine inhibited noradrenaline transport to exactly the same extent, and their effects were not additive. Alprenolol did not reduce this sodium-independent transport, but tropolone lowered it somewhat. c) No evidence for corticosterone-sensitive noradrenaline transport (uptake-2) was found in this preparation at the low amine concentrations used. d) In control medium, the kinetic parameters of transport were: K _m: 0.59 ± 0.063 M and V _max: 2.44 ± 0.43 (pmoles/mg protein/min). With 26 mM sodium and the rest substituted by choline, K _m:2.26 ± 0.70 M (P0.001) and V _max: 2.74 ± 0.43 (pmoles/mg protein/min) (not significant). Also with 26 mM sodium, but with sucrose substitution, K _m: 0.76 ± 0.13 M (N.S.) and V _max: 1.06 ± 0.13 (pmol/mg/min) (P<0.05). Such results indicate that sodium only modifies the affinity of the transport system for noradrenaline, without changing V _max, and that changes in the latter are only a consequence of a reduction of the ionic strength. e) When noradrenaline transport was studied at different concentrations of external sodium, at constant ionic strength and with precautions to minimize the noradrenaline-releasing effect of low sodium, it was found that the data could be best represented by two hyperbolas placed in series. This suggests that the noradrenaline carrier has two sites for sodium, that do not interact with each other. When the same experiments were repeated in the absence of chloride, it was found that the noradrenaline transport system had lost virtually all its affinity for sodium. f) The effect of prolonged tissue incubation in the absence of sodium was found to produce a relatively small inactivation of noradrenaline transport. Such phenomenon was enhanced by raising the calcium concentration to 2 mM.     

Single-dose toxicokinetics of aluminum in the rat

The toxicokinetics of aluminum (Al) in male Wistar rats was studied after single intragastric (IG) doses of 1000 and 12000 g Al/kg and intravenous (IV) doses of 10, 100, 1000, and 12000 g Al/kg. Serial blood samples, daily samples of urine and feces as well as brain, liver, kidney, spleen, quadriceps muscle, and femur samples were collected. Al was measured by atomic absorption spectrometry. Al blood profiles after IV doses were adequately described by a two-compartment open model. Al toxicokinetics was dose dependent and appeared to plateau at 12000 g/kg. At IV doses between 10 and 1000 g/kg the terminal half-life of elimination from whole blood (t_1/2) increased from 29.9±7.8 to 209.3±32.6 min, and the total body clearance (CL) decreased from 2.45±0.64 to 0.28±0.03 ml min^–1 kg^–1. Following an IV bolus of 10 and 100 g/kg the administered Al was recovered completely from urine (94.4%±9.9% and 98.5%±3.2%). Twenty-nine days after the IV dose of 1000 g/kg daily renal excretion decreased to baseline values while only 55.1%±8.0% of the dose was excreted. Nineteen days after the single IV dose of 1000 g/kg Al accumulated in liver (28.1±7.7 versus 1.7±0.5 g/g of control rats) and spleen (72.5±21.1 versus <0.4 g/g). After the single 1000 g/kg IG dose no absorption of Al was detectable. The IG dose of 12000 g/kg resulted in a maximum blood Al level of 47.9±12.4 g/l after 50 min. The blood concentration time curve fitted a one-compartment open model with a half-life of absorption of 28.2±3.6 min and a t_1/2 of 81.2±20.2 min. Cumulative renal Al excretion was 0.18%±0.10% of the dose and oral bioavailability was 0.02%. Seventeen days after the 12000 g/kg IG dose the Al content in femur samples was increased (2.7±1.3 versus 0.6±0.4 g/g). In no case was fecal elimination of incorporated Al observed.     

Renal Secretion of the Antiviral Nucleoside Analog AM188 Is Inhibited by Probenecid,p-Aminohippuric Acid,and Cimetidine in the Isolated Perfused Rat Kidney

Purpose. To investigate the effects of potential inhibitors of membrane transport on the tubular secretion of AM188, an antiviral guanosine analog, in the isolated perfused rat kidney (IPK). Methods. AM188 was administered to the IPK perfusate as a bolus/infusion regimen. In inhibitor groups, probenecid, p-aminohippuric acid (PAH), cimetidine, or nitrobenzylthioinosine was added to the perfusing medium. Results. In control IPKs, the ratio of renal clearance of AM188 (CL_R) to GFR was 7.7 ± 0.51 (mean ± SD). The CL_R/GFR ratio for AM188 was 6.20 ± 0.41*, 2.85 ± 0.20*, 1.45 ± 0.07*, and 0.80 ± 0.01* when the concentration of probenecid in perfusate was 10, 50, 100, and 1000 M, respectively (*p < 0.05 compared to control group); the ratio was 7.71 ± 0.38, 6.02 ± 0.42*, 1.71 ± 0.15*, and 0.91 ± 0.07* for the PAH group and 6.42 ± 1.70*, 5.33 ± 1.53*, 3.16 ± 0.81*, and 1.21 ± 0.20* for the cimetidine group when the concentrations were 10, 100, 1000 and 10,000 M, respectively; and the ratio was 5.33 ± 0.21* when the concentration of nitrobenzylthioinosine was 5 M. Conclusions. These results suggest that renal tubular secretion of AM188 involves organic anion and cation transport systems.     

Effect of rifampicin treatment on the kinetics of mexiletine

Summary To study the effects of enzyme induction on its pharmacokinetics, a single oral dose of the new antiarrhythmic agent mexiletine hydrochloride 400 mg was administered to 8 healthy volunteers before and after treatment with rifampicin 300 mg b.i.d. for ten days. The absorption and distribution of mexiletine were not changed after rifampicin, but its elimination half-life fell from 8.5±0.8 h (mean±SE) to 5.0±0.4 h (p<0.01), and its nonrenal clearance increased from 435±68 ml/min to 711±101 ml/min (p<0.01). The mean renal clearance of mexiletine did not change, but it showed an exponential correlation with urinary pH. The amount of unchanged mexiletine excreted in urine over two days decreased from 32±7 to 18±3 mg (p<0.01). The half-life of antipyrine fell from 11.8±0.4 to 5.5±0.3 h and its clearance increased from 40±3 ml to 74±3 ml/min (p<0.01). There was a significant (p<0.05) positive linear correlation between both the half-lives and the clearances of antipyrine and mexiletine. The clearances were positively correlated with serum -glutamyl transpeptidase. The results suggest that the dosage of mexiletine should be adjusted when enzyme inducing drugs are started or stopped during therapy with it.     

Extensive Biliary Excretion of the Model Opioid Peptide [D-PEN2,5] Enkephalin in Rats

Purpose. This study was designed to test the hypothesis that the enzymatically stable opioid peptide, [D-pen^2,5] enkephalin (DPDPE), is excreted extensively into bile. Methods. Following an i.v. bolus dose of DPDPE (10 mg/kg) to rats, concentrations of DPDPE in serum, bile, liver homogenate and urine were measured by a novel capillary zone electrophoresis method. Data were analyzed to recover the fundamental pharmacokinetic parameters (volumes of distribution; distribution and elimination rate constants governing DPDPE systemic and biliary disposition). Parallel in vitro experiments were performed to evaluate the partitioning of DPDPE between erythrocytes and plasma, as well as to assess the degree of binding of DPDPE to serum proteins. Results. The majority of the administered dose (~80%) was recovered from bile as intact peptide. DPDPE disposition was best described by a two-compartment model with Michaelis-Menten elimination (K_m: 37.5 ± 11 g/ml; V_max: 1143 ± 368 g/min/kg) from the central compartment into bile, suggestive of an active hepatic transport system. DPDPE was associated with a distributional space of 486 ± 62 ml/kg. In vitro incubation of DPDPE with whole blood showed that ~65% of the peptide was associated with erythrocytes. The difference between concentrations of DPDPE in erythrocytes and plasma was statistically significant (29.2 ± 4.9 vs. 18.1 ± 3.1 g/ml, p < 0.05), but not between whole blood and plasma (21.3 ± 2.8 vs. 18.1 ± 3.1 g/ml, p > 0.05). Concentration-independent binding of DPDPE to serum proteins was evidenced between 10 and 100 (g/ml, with an unbound fraction of 0.517 ± 0.182. Conclusions. DPDPE undergoes extensive biliary excretion after i.v administration in rats. The apparent nonlinearity in the biliary excretion of DPDPE revealed by the pharmacokinetic modeling strongly suggests the existence of an active transport system(s) in hepatocytes which may mediate the rapid disappearance of DPDPE from the systemic circulation.     

Changes in the excretion of endogenous glycine conjugate as a possible artifact in toxicological experiments

Changes in the urinary excretion of hippuric acid (HIA) and phenaceturic acid (PUA) as well as their metabolic precursors, i.e. benzoic (BA) and phenylacetic acid (PAA), in rats housed in glass metabolic cages for 4 days were monitored using gas-liquid chromatography. The amount of HIA excreted was 128±63 mol/kg for female and 79±43 mol/kg for male rats in the first 24 h and decreased to 11±7 mol/kg (p< 0.01) for female and 3.2±2.4 mol/kg (p< 0.001) for male rats on the 2nd day. These values remained nearly at the same level until the end of the experiment. The amount of PUA decreased from 48±12 mol/kg on the 1st day to 22±9 mol/kg (p< 0.05) on the 2nd day by male rats, whereas by the females the decrease from 30±9 mol/kg to 21±8 mol/kg was not significant. The decrease in the excretion of glycine conjugates was compensated by a parallel increase in the level of unconjugated BA and PAA.     

Chronopharmacokinetics of beta-receptor blocking drugs of different lipophilicity (propranolol,metoprolol, sotalol,atenolol) in plasma and tissues after single and multiple dosing in the rat

Summary Comparative pharmacokinetic studies with the -receptor blocking drugs propranolol, metoprolol, sotalol and atenolol, differing greatly in lipophilicity, and their main route of elimination were performed in light-dark-synchronized rats after equimolar single (6 moles/kg) or multiple (6x6 moles/kg) drug application. Drug concentrations were determined in plasma and various target organs of the drugs, e.g. heart, muscle, lung and brain, after drug application in the light period (L) and dark period (D), respectively. After single drug administration pharmacokinetic parameters of all drugs depended on the L and D conditions. Elimination half-lives in plasma and organs were shorter during D than during L. No L-D-differences were found in initial drug concentrations of the hydrophilic drugs sotalol and atenolol. In contrast, C₀-values of the lipophilic propranolol in highly perfused organs (muscle, lung, brain) and of metoprolol in muscle tissue were significantly higher in D than in L. No obvious temporal dependency was found in other pharmacokinetic parameters (AUC, plasma clearance,V _d) with the exception inV _d of propranolol. Due to the different physico-chemical properties of the compounds inter-drug-differences in pharmacokinetic parameters including drug accumulation into lung and brain tissue were observed. Multiple drug dosing abolished the circadian-phase-dependency in the elimination half-lives of the drugs due to an increase in D. Only for the highly lipophilic propranolol half-lives in highly perfused organs were still shorter in D than in L. It is concluded that L-D-differences in drug half-lives after single dose application are mainly due to circadian variations in drug elimination with a higher hepatic (propranolol, metoprolol) or renal (sotalol, atenolol) elimination in the activity period of rats during D. Additional studies with propranolol on heart rate of conscious rats revealed that a maximum in -receptor blockade was achieved at 10 moles/kg in L but not in D. Thus, it is assumed that abolition of circadian-phase-dependency in half-lives after 6x6 moles/kg of the drugs may be due to the longer lasting and more pronounced -receptor blockade after multiple drug dosing over a period of several hours in D. Thereby, liver-flow-dependent elimination of propranolol and metoprolol and renal elimination of sotalol and atenolol is reduced to base-line levels found in L.Parts of this work were presented at the 22nd Spring Meeting (Lemmer 1981) and at the Joint Meeting (Lemmer et al. 1983a) of the German Pharmacological Society     

Analysis of Nonlinear Hepatic Clearance of a Cyclopentapeptide,BQ-123, with the Multiple Indicator Dilution Method Using the Dispersion Model

Purpose. To bridge in vitro, in situ and in vivo kinetic analyses of the hepatic clearance of a cyclopentapeptide, BQ-123, by using dispersion models that assume nonlinear pharmacokinetics. Methods. Rat livers were perfused by the multiple indicator dilution method with doses of BQ-123 ranging from 1-1000 g. The outflow dilution curves were fitted to a two-compartment dispersion model that was solved numerically by the finite difference method. Further, in vivo plasma concentrations of BQ-123 after bolus injection were analyzed with a hybrid physiological model that incorporates the hepatic dispersion model. Results. The calculated Michaelis-Menten constants (K_m = 12.0 M, V_max = 321 pmol/min/10⁶ cells, P_dif = 1.2 l/min/10⁶ cells) were comparable to those obtained previously from the in vitro isolated hepatocyte experiment (K_m = 9.5 M, V_max = 517 pmol/min/10⁶ cells, P_dif =1.1 l/min/10⁶ cells). The plasma concentrations of BQ-123 at doses of 1-25 mg/kg were explained well by the hybrid physiological model. Conclusions. These results suggest that carrier-mediated transport on the sinusoidal membrane was responsible for the in vivo hepatic elimination of BQ-123.     

Salbutamol disposition and dynamics in conscious rabbits: Influence of the route of administration and of the dose

This study assessed the influence of dose and route of administration on salbutamol kinetics and hypokaliemic effect. Salbutamol plasma kinetics were studied in a first group of 6 rabbits who received 60, 800, and 60 g/kg by the intravenous (iv), oral (po), and intratracheal (it) routes, respectively, at 1-week intervals. A second group of 6 rabbits received 120, 2400, and 120 g/kg of salbutamol by the same three routes. Multiple blood samples were withdrawn to assay salbutamol and potassium. Following iv salbutamol (60 g/kg), total plasma clearance was 82±5 ml/min per kg, apparent volume of distribution was 5.0±0.5 l/kg, and terminal half- life was 41±2 min. Similar values were estimated when 120 g/kg of salbutamol was administered iv or was given po or it. The bioavailability of po and it salbutamol was approximately 1 and 20%, respectively. For the first group, the maximal decrease in plasma potassium elicited by salbutamol was 0.80±0.19, 0.48±0.22, and 0.78±0.46 mmol/l, and for the second group, maximal decrement was 1.31±0.37, 0.70±0.24, and 0.84±0.17 mmol/l for the iv, po, and it routes, respectively. Compared to salbutamol peak plasma concentrations, maximal decrease in plasma potassium appeared between 60 and 108 min later for the iv route, 90 and 25 min later for po and it routes, and for this reason, the hypokaliemic effect was not associated to salbutamol plasma concentrations. The hypokaliemic effect was dependent upon the route, e.g., po>it>iv. It is concluded that (i) salbutamol plasma kinetics are first-order independently of the route of administration, and (ii) salbutamol hypokaliemic effect is modulated by the dose and the route of administration.List of abbreviations AUC Area under salbutamol plasma concentration-time curve - cl_INT Salbutamol intrinsic clearance - cl_T Salbutamol total plasma clearance - c_MAX Salbutamol maximal plasma concentration - F Fraction of the dose of salbutamol reaching the systemic circulation - iv Intravenous route of administration - it Intratracheal route of administration - po Oral route of administration - V_area Salbutamol apparent volume of distribution - T ₂ ¹ Salbutamol half-life of the terminal phase - t_MAX Time to observe the maximal decrease in plasma potassium - e_MAX Predicted maximal effect of salbutamol - EC₅₀ Concentration of salbutamol eliciting 50% of e_MAX Supported by the Medical Research Council of Canada (MT-10874). Sylvie Perreault is recipient of a Bourse Formation de troisième cycle des Fonds de la Recherche en Santé du Québec.     

Responses of human bone marrow progenitor cells to fluoro-ara-AMP,homoharringtonine, and elliptinium

Summary The cytotoxicity of the investigational anticancer drugs fluoro-ara-AMP, homoharringtonine, and elliptinium on normal human granulocyte-macrophage colony-forming units in culture (GM-CFU) was investigated using a bilayer soft agar system. For each drug, the dose-dependent survival curve on a semilogarithmic plot formed a straight line. The Do were: 0.51 g/ml (fluoro-ara-AMP), 0.004 g/ml (homoharringtonine) and 0.026 g/ml (elliptinium). The in vitro toxicity of drugs on bone marrow progenitor cells did not correlate with the relative myelosuppressive potency observed in vivo.     

Characterization of P-glycoprotein Mediated Transport of K02, a Novel Vinylsulfone Peptidomimetic Cysteine Protease Inhibitor,Across MDR1-MDCK and Caco-2 Cell Monolayers

Purpose. Here we characterized the transport properties of morpholine-urea-phenylalanine- homophenylalanine-vinylsulfone-phenyl (K02), a newly developed peptidomimetic cysteine protease inhibitor, across monolayers of P-gp-expressed MDR1 transfected MDCK cells (MDR1-MDCK) and Caco-2 cells. Methods. MDR1-MDCK, MDCK and Caco-2 cells, grown to confluence on Transwell insert membranes, were used to investigate transcellular transport of [¹⁴C]-K02. Results. The basolateral to apical (B-A) flux of 10 M [^I4C]-K02 across MDR1-MDCK cells was markedly greater than its apical to basolateral (A-B) flux (ratio = 39). This specific B-A transport was temperature dependent and saturable, with an apparent Michaelis-Menten constant and maximum velocity of 69.1 ± 19.5 M and 148.9 ± 16.3 pmol/min/cm², respectively. This B-A flux was significantly inhibited by cyclosporine (IC₅₀ = 17.1 ± 0.7 M), vinblastine (IC₅₀ = 75.9 ± 13.0 M) and verapamil (IC₅₀ = 236 ± 63 M). In Caco-2 cell monolayers, the B-A flux was reduced about 50% compared to that in MDR1-MDCK and the A-B flux was increased about 8-fold. The apparent Michaelis-Menten constant and maximum velocity values for the B-A transport were 71.8 ± 45.9 M and 35.3 ± 9.0 pmol/min/ cm². This B-A flux was also significantly inhibited by P-gp substrates/ inhibitors. Western blots showed that the P-gp expression in MDR1-MDCK cells was about 10-fold that in Caco-2 cells. Conclusions. K02 is transported by P-gp in both MDR1-MDCK and Caco-2 cells, and the in vitro interactions between K02 and various P-gp substrates may provide strategies to overcome the bioavailability barrier by intestinal P-gp. __     

Hepatic Disposition of Fexofenadine: Influence of the Transport Inhibitors Erythromycin and Dibromosulphothalein

Purpose. To examine the disposition of fexofenadine in the isolated perfused rat liver and the influence of erythromycin and dibromosulphthalein (DBSP) on the hepatic uptake and biliary excretion of fexofenadine. Methods. Livers from four groups of rats were perfused in a recirculatory manner with fexofenadine HCl added as a bolus (125, 250, 500, or 1000 g) to perfusate. Livers from another three groups of rats were perfused with 250 g of fexofenadine HCl. With one group as control, erythromycin (4.0 g/ml) or DBSP (136 g/ml) was added to the perfusate of the other groups. In all experiments, perfusate and bile were collected for 60 min; in addition, livers from the second experiment were retained for assay. Fexofenadine was determined in perfusate, bile, and homogenized liver by HPLC. Results. The area under the curve (AUC) of fexofenadine was linearly related to concentration. It was unchanged from control (12,800 ± 200 ng·h/ml) by erythromycin (14,400 ± 2000 ng·h/ml), but was increased 95% by DBSP (25,000 ± 2600 ng·h/ml, P <0.001). The ratios of the concentrations of fexofenadine in liver/perfusate were decreased significantly by DBSP; those for bile/liver were increased by erythromycin. Conclusions. Erythromycin reduced the canalicular transport of fexofenadine into bile, whereas DBSP reduced uptake across the sinusoidal membrane.     

Effect of Hydralazine on the Elimination of Antipyrine in the Rat

The concomitant administration of hydralazine with metoprolol or propranolol substantially increases the oral bioavailability of these beta-blockers, presumably via reduction of the first-pass effect. It has been suggested that this effect may be secondary to a decrease in the intrinsic clearance of propranolol, possibly by inhibition of oxidative metabolism. To examine the possibility that hydralazine alters oxidative metabolism in vivo, the effect of hydralazine on the pharmacokinetics of antipyrine was examined in the rat. The oral administration of hydralazine hydrochloride, 7.5 mg/kg, 15 min prior to antipyrine administration reduced antipyrine clearance from 9.66 ± 1.18 to 8.19 ± 0.76 ml/min/kg (P < 0.05). Hydralazine was observed to cause substantial hypothermia. The study was repeated in temperature-regulated animals and no alteration in antipyrine clearance was found. Two doses of hydralazine in temperature-regulated rats also failed to alter antipyrine clearance. Thus, it appears that the effect of hydralazine on antipyrine clearance is secondary to the hypothermic effect of hydralazine and not due to a direct inhibition of cytochrome P-450-mediated enzyme activity.     

Testosterone reinforcement: intravenous and intracerebroventricular self-administration in male rats and hamsters

Rationale Anabolic steroids are drugs of abuse. However, the potential for addiction remains unclear. Testosterone induces conditioned place preference in rats and oral self-administration in hamsters.Objectives To determine if male rats and hamsters consume testosterone by intravenous (IV) or intracerebroventricular (ICV) self-administration.Methods With each nose-poke in the active hole during daily 4-h tests in an operant conditioning chamber, gonad-intact adult rats and hamsters received 50 g testosterone in an aqueous solution of -cyclodextrin via jugular cannula. The inactive nose-poke hole served as a control. Additional hamsters received vehicle infusions.Results Rats (n=7) expressed a significant preference for the active nose-poke hole (10.0±2.8 responses/4 h) over the inactive hole (4.7±1.2 responses/4 h). Similarly, during 16 days of testosterone self-administration IV, hamsters (n=9) averaged 11.7±2.9 responses/4 h and 6.3±1.1 responses/4 h in the active and inactive nose-poke holes, respectively. By contrast, vehicle controls (n=8) failed to develop a preference for the active nose-poke hole (6.5±0.5 and 6.4±0.3 responses/4 h). Hamsters (n=8) also self-administered 1 g testosterone ICV (active hole:39.8±6.0 nose-pokes/4 h; inactive hole: 22.6±7.1 nose-pokes/4 h). When testosterone was replaced with vehicle, nose-poking in the active hole declined from 31.1±7.6 to 11.9±3.2 responses/4 h within 6 days. Likewise, reversing active and inactive holes increased nose-poking in the previously inactive hole from 9.1±1.9 to 25.6±5.4 responses/4 h. However, reducing the testosterone dose from 1 g to 0.2 g per 1 l injection did not change nose-poking.Conclusions Compared with other drugs of abuse, testosterone reinforcement is modest. Nonetheless, these data support the hypothesis that testosterone is reinforcing.     

Pharmacokinetics of Plasmid DNA in the Rat

Purpose. The pharmacokinetics of plasmid DNA after IV bolus administration in the rat by following supercoiled (SC), open circular (OC), and linear (L) pDNA forms of the plasmid.Methods. SC, OC, and L pDNA were injected at 2500, 500, 333, and 250 g doses. The concentrations in the bloodstream of OC and L pDNA were monitored.Results. SC pDNA was detectable in the bloodstream only after a 2500 g dose, and had a clearance of 390(±50) ml/min and V_d of 81(±8) ml. The pharmacokinetics of OC pDNA exhibited non-linear characteristics with clearance ranging from 8.3(±0.8) to 1.3(±0.2) ml/min and a V_d of 39(±19) ml. L pDNA was cleared at 7.6(±2.3) ml/min and had a V_d of 37(±17) ml. AUC analysis revealed that 60(±10) % of the SC was converted to the OC form, and nearly complete conversion of the OC pDNA to L pDNA. Clearance of SC pDNA was decreased after liposome complexation to 87(±30) ml/min. However the clearance of OC and L pDNA was increased relative to naked pDNA at an equivalent dose to 37(±9) ml/min and 95(±37) ml/min respectively.Conclusions. SC pDNA is rapidly metabolized and cleared from the circulation. OC pDNA displays non-linear pharmacokinetics. Linear pDNA exhibits first order kinetics. Liposome complexation protects the SC topoform, but the complexes are more rapidly cleared than the naked pDNA.     

Dose dependent pharmacokinetics of nitroglycerin after multiple intravenous infusions in healthy volunteers

Evaluation of the pharmacokinetics of nitroglycerin has been hindered in the past by the lack of specific and sensitive analytical procedures, and the unavailability of parenteral nitroglycerin and infusion sets which did not adsorb nitroglycerin. The purpose for this present study was to determine the pharmacokinetic parameters of nitroglycerin and the dinitrate metabolites after multiple intravenous infusions of nitroglycerin in healthy volunteers. Six volunteers received variable infusion rates of nitroglycerin. Generally, at 0, 40, 80, and 120 min, the infusion rates were adjusted to 10, 20, 40, and 10 g/min, respectively. Plasma samples were drawn and analyzed for nitroglycerin and its 1,2-and 1,3-dinitraie metabolites using capillary GC. Steady-state nitroglycerin plasma concentrations attained at 10, 20, 40, and 10g/min were 0.44±0.31, 1.32±0.71, 4.23±1.50 and 1.04±0.43 ng/ml, respectively. As the infusion rate was increased, the steady-state concentrations increased disproportionately. When the dose was decreased from 40 to 10g/min, the steady-state nitroglycerin concentrations were always higher than those at the initial low infusion rate. Thus, in the majority of subjects, a hysteretic type of response was present. The hysteresis observed in the dose versus steady-state concentration curve may be explained by either end-product inhibition or saturable binding of nitroglycerin to blood vessels. The clearance values (5.5 to 71 l/min) were very high and far exceed the maximum possible hepatic clearance suggesting that nitroglycerin is metabolized by organs other than liver. Clearance was not directly related to plasma concentrations but was found to decrease to a constant value (approximately 11±6 l/min< as nitroglycerin concentrations initially increased.Supported in part by NIH Grant HL 32243 and by a grant from Key Pharmaceuticals, Inc., Miami, Florida. During the course of the work, PKN received support as an AFPE James F. Hoge Memorial Fellow.