The Pharmacokinetics and Electroencephalogram Response of Remifentanil Alone and in Combination with Esmolol in the Rat

Purpose. The goal of this study was to determine if the co-administration of esmolol (ES), a short acting cardioselective -blocker, significantly alters the pharmacokinetics and/or pharmacodynamics of remifentanil (REMI), an ultra short-acting opioid, in the rat. Methods. Sprague-Dawley rats (N = 8, Wt. = 325 ± 15g) were surgically implanted with stainless steel cerebrocortical EEG electrodes three days before the study. Each rat was dosed with REMI (15 g/ kg/min), and REMI & ES (15 g/kg/min and 600 g/kg/min) for 21 minutes in a random crossover design. Six serial blood samples were collected over 25 minutes into test-tubes containing 0.5ml acetonitrile. Blood samples were extracted with methylene chloride and analyzed by a validated GC-MS assay. EEG was captured and subjected to power spectral analysis (0.1–50 Hz) for spectral edge (97%). Results. No significant differences (p < 0.05) were found in clearance (REMI = 287 + 73 ml/min/leg vs. REMI & ES = 289 ± 148 ml/ min kg) or Vd (REMI = 286 ± 49 ml/kg vs REMI & ES = 248 + 40 ml/kg). A linked sigmoid E_max PK-PD model was used and the pharmacodynamic parameters were not statistically different. Mean E_max and EC₅₀ after REMI were 18.0 ± 6.0 Hz and 32 ± 12 ng/ml; and after REMI + ES were 19 + 4.8 Hz and 26 + 8.6 ng/ml. Conclusions. At the doses tested, there is no pharmacokinetic or pharmacodynamic interaction between remifentanil and esmolol in the rat.     

Pharmacokinetics and pharmacodynamics of a new reformulated microemulsion and the long-chain triglyceride emulsion of propofol in beagle dogs

Background and purpose:

Microemulsion propofol was developed to eliminate lipid solvent-related adverse events of long-chain triglyceride emulsion (LCT) propofol. We compared dose proportionality, pharmacokinetic and pharmacodynamic characteristics of both formulations.

Experimental approach:

The study was a randomized, two-period and crossover design with 7-day wash-out period. Microemulsion and LCT propofol were administered by zero-order infusion (0.75, 1.00 and 1.25 mg·kg⁻¹·min⁻¹) for 20 min in 30 beagle dogs (male/female = 5/5 for each rate). Arterial samples were collected at preset intervals. The electroencephalographic approximate entropy (ApEn) was used as a measure of propofol effect. Dose proportionality, pharmacokinetic and pharmacodynamic bioequivalence were evaluated by non-compartmental analyses. Population analysis was performed using nonlinear mixed effects modelling.

Key results:

Both formulations showed dose proportionality at the applied dose range. The ratios of geometric means of AUC_last and AUC_inf between both formulations were acceptable for bioequivalence, whereas that of C_max was not. The pharmacodynamic bioequivalence was indicated by the arithmetic means of AAC (areas above the ApEn time curves) and E₀ (baseline ApEn)–E_max (maximally decreased ApEn) between both formulations. The pharmacokinetics of both formulations were best described by three compartment models. Body weight was a significant covariate for V₁ of both formulations and sex for k₂₁ of microemulsion propofol. The blood-brain equilibration rate constants (k_e0, min⁻¹) were 0.476 and 0.696 for microemulsion and LCT propofol respectively.

Conclusions and implications:

Microemulsion propofol was pharmacodynamically bioequivalent to LCT propofol although pharmacokinetic bioequivalence was incomplete, and demonstrated linear pharmacokinetics at the applied dose ranges.     

Sex Specificity in Methadone Analgesia in the Rat: A Population Pharmacokinetic and Pharmacodynamic Approach

Purpose. To quantify the extent to which a sex-specific dichotomy in the temporal evolution of the analgesic effect, after intravenous (i.v.) methadone injection in the rat, relates to the pharmacokinetics (PK) and pharmacodynamics (PD) that mediate the dose-to-effect pathway. Methods. Tail-flick analgesia was measured after i.v. methadone injection (0.35 mg/kg) in female (n = 16) and male (n = 16) Sprague-Dawley rats. The PK were evaluated in separate female (n = 56) and male (n = 56) rats after they had received the same dose of methadone i.v. (0.35 mg/kg). A bicompartmental model described the kinetics and a sigmoid E_max model-related drug effect vs. simulated concentrations (pharmacodynamics) at the times of effect measurement. All model parameters as well as interanimal and assay variabilities were estimated with a mixed-effects population method using the program NONMEM. Results. The area under the effect-time curve (AUCE_0-120) was (mean ± interanimal SD) 1859 ± 346 min in the females, which was significantly lower than the 4871 ± 393 min in the males (P < 0.0001). On the contrary, the profiles of concentration vs. time were higher in females and, therefore, corresponded inversely to the effect vs. time-relative magnitudes. The central volume of distribution, V1, was 1.94 ± 0.37 l/kg for female rats and 3.01 ± 0.33 l/kg for male rats. Also, the central clearance was 0.077 ± 0.006 l/min/kg and 0.102 ± 0.005 l/min/kg, respectively, for female and male rats. Both parameters differed significantly between sexes (P < 0.0001). The pharmacodynamic maximum observed effect parameter (E_max) was 37% ± 29% in female rats and 85% ± 16% in male rats, and these values were significantly different (P < 0.0001). The parameter for the concentration eliciting half of E_max (EC₅₀) was 24.1 ± 7.5 g/l in female rats and 20.3 ± 2.9 g/l in male rats, and the Hill-related exponent, , was 6.3 ± 3.9 in female rats and 5.5 ± 4.1 in male rats. These parameters did not differ significantly (at the P < 0.05 level). Conclusions. A sex-specific dichotomy in the methadone antinociceptive effect, in the rat, was not proportionally related to plasma concentrations. Each sex corresponded to a distinct subpopulation of the PK parameters and one of the pharmacodynamic parameters (E_max). When the course of a drug involves PK or PD subpopulations, PK/PD modeling can afford the safest prediction of the effect-time evolution for a particular dose.     

Altered Disposition and Effect of Lerisetron in Rats with Elevated Alpha1-acid Glycoprotein Levels

Purpose. To examine the effect of changes in plasma 1-acid glycoprotein (AAG) levels on the pharmacokinetics (PK) and pharmacodynamics (PD) of lerisetron, a novel serotonin 5-HT₃ receptor antagonist, in the rat. Methods. After subcutaneous administration of turpentine oil, AAG was significantly elevated compared with controls. The PK of unchanged lerisetron (UL; high-performance liquid chromatography with radioactivity monitoring) and total lerisetron (TL; unchanged + changed, scintillation counting) was characterized post intravenous (i.v.) ¹⁴C lerisetron (50 g/kg) in control and turpentine oil pretreated rats. The PK (0 – 180 min) was described by a two-compartmental model. Protein binding of lerisetron in vitro was measured using an ultrafiltration technique. The effect of lerisetron (5 g/kg, i.v.) over 180 min was measured in anesthetized rats (control and pretreated) with the Bezold-Jarisch reflex (inhibition of bradycardia after 16 g/kg serotonin i.v.) as the endpoint. PD parameters were estimated by sigmoid E_max models. Results. The unbound fraction was significantly diminished in pretreated rats (mean ± SEM) (6.60 ± 1.23% vs. control 14.4 ± 1.40%, P < 0.05). Volume of distribution (V) and clearance for UL and TL were significantly decreased when compared to the controls (P < 0.0001 for UL and P < 0.05 for TL). Plasma clearance based on unbound concentration for UL did not differ between groups but the unbound V and steady-state unbound V remained decreased (P < 0.05 and P < 0.0001). Pretreated rats showed a significantly diminished drug effect: the area under the E-t curve over 180 min was (mean ± SEM) 5189 ± 657.7 in control animals vs. 3486 ± 464.4 in the pretreated group (P < 0.05). The EC₅₀ (concentration at half maximum effect) for UL and TL were increased in pretreated rats and were not compensated when the unbound concentration was used. Conclusions. An increase in AAG causes alterations in the PK and PD of lerisetron, and because this is not compensated with the unbound concentration, we suggest that mechanisms not linked to protein binding may be involved.     

Pharmacokinetics of quinidine and three of its metabolites in man

Disposition parameters of quinidine and three of its metabolites, 3-hydroxy quinidine, quinidine N-oxide, and quinidine 10,11-dihydrodiol, were determined in five normal healthy volunteers after prolonged intravenous infusion and multiple oral doses. The plasma concentrations of individual metabolites after 7 hr of constant quinidine infusion at a plasma quinidine level of 2.9±(SD) 0.3 mg/L were: 3-hydroxy quinidine, 0.32±0.06 mg/L; quinidine N-oxide, 0.28±0.03 mg/L; and quinidine 10,11-dihydrodiol, 0.13±0.04 mg/L. Plasma trough levels after 12 oral doses of quinidine sulfate every 4 hr averaged: quinidine, 2.89±0.50 mg/L; 3-hydroxy quinidine, 0.83±0.36 mg/L; quinidine N-oxide, 0.40±0.13 mg/L; and quinidine 10,11-dihydrodiol, 0.38±0.08 mg/L. Relatively higher plasma concentrations of 3-hydroxy quinidine metabolite after oral dosing probably reflect first-pass formation of this quinidine metabolite. A two-compartment model for quinidine and a one-compartment model for each of the metabolites described the plasma concentration-time curves after both i.v. infusion and multiple oral doses. Mean (±SD) disposition parameters for quinidine from individual fits, after i.v. infusion were as follows: V ₁ ,0.37±0.09 L/kg; ₁,0.094±0.009 min ^–1; ₂, 0.0015±0.0002 min^–1; EX2, 0.013±0.002 min^–1;clearance (Cl_Q),3.86±0.83 ml/min/kg. Both plasma and urinary data were used to determine metabolic disposition parameters. Mean (±SD) values for the metabolites after i.v. quinidine infusion were as follows: 3-hydroxy quinidine: formation rate constant k_mf,0.0012±0.0005 min ^–1,volume of distribution, V_m,0.99±0.47 L/kg; and elimination rate constant, k_mu 0.0030±0.0002 min ^–1.Quinidine N-oxide: k_mf,0.00012±0.00003 min ^–1; V_m,0.068±0.020 L/kg; and k_mu,0.0063±0.0008 min ^–1.Quinidine 10,11-dihydrodiol: k_mf,0.0003±0.0001 min ^–1; V_m,0.43±0.29 L/kg; and k_mu,0.0059±0.0010 min ^–1.Oral absorption of quinidine was described by a zero order process with a bioavailability of 0.78. Concentration dependent renal elimination of 3-hydroxy quinidine was observed in two out of five subjects studied.This work was supported by funds from the grants GM 26691 and GM 28072 from the National Institute of General Medical Sciences, NIH. A. Rakhit was the recipient of a Training Grant Traineeship from NIH. T. W. Guentert is grateful for support from the Swiss National Science Foundation.Professor Sidney Riegelman. deceased April 4, 1981.     

Improved lung delivery from a passive dry powder inhaler using an Engineered PulmoSphere powder

Purpose. To assess the pulmonary deposition and pharmacokinetics of an engineered PulmoSphere® powder relative to standard micronized drug when delivered from passive dry powder inhalers (DPIs). Methods. Budesonide PulmoSphere (PS_bud) powder was manufactured using an emulsion-based spray-drying process. Eight healthy subjects completed 3 treatments in crossover fashion: 370 g budesonide PulmoSphere inhaled from Eclipse® DPI at target PIF of 25 L·min^-1 (PS_bud25), and 50 L·min^-1 (PS_bud50), and 800 g of pelletized budesonide from Pulmicort® Turbuhaler® at 60 L·min^-1(TH_bud60). PS_bud powder was radiolabeled with ^99mTc and lung deposition determined scintigraphically. Plasma budesonide concentrations were measured for 12 h after inhalation. Results. Pulmonary deposition (mean ± sd) of PS_bud was 57 ± 7% and 58 ± 8% of the nominal dose at 25 and 50 L·min^-1, respectively. Mean peak plasma budesonide levels were 4.7 (PS_bud25), 4.0 (PS_bud50), and 2.2 ng·ml^-1 (TH_bud60). Median t_max was 5 min after both PS_bud inhalations compared to 20 min for Turbuhaler (P < 0.05). Mean AUCs were comparable after all inhalations, 5.1 (PS_bud25), 5.9 (PS_bud50), and 6.0 (TH_bud60) ng·h·ml^-1. The engineered PS_bud powder delivered at both flow rates from the Eclipse® DPI was twice as efficiently deposited as pelletized budesonide delivered at 60 L·min^-1 from the Turbuhaler. Intersubject variability was also dramatically decreased for PS_bud relative to TH_bud. Conclusion. Delivery of an engineered PulmoSphere formulation is more efficient and reproducible than delivery of micronized drug from passive DPIs.     

Rat Jejunal Permeability and Metabolism of μ-Selective Tetrapeptides in Gastrointestinal Fluids from Humans and Rats

Purpose. To study intestinal transport and metabolism of three new -selective tetrapeptide enkephalin analogues, LEF537, LEF553 and TAPP These peptides are stabilized against enzymatic hydrolysis by having a D-aminoacid in position 2 and a blocked COOH-terminal. Methods. We used a single-pass perfusion technique to study the transport of the peptides in rat jejunum. To reduce luminal and/or brush-border metabolism during the perfusion we used protease inhibitors (Pefabloc^® SC, bestatin and thiorphan). The rate of metabolism was studied by incubations in rat jejunal homogenate, rat jejunal fluid and human gastric and jejunal fluid with and without these inhibitors. Results. The jejunal permeabilities (P_eff) of the peptides were 0.43–0.78 10^–4 cm/s without inhibitors and 0.09–0.45 10^–4 cm/s in presence of the inhibitors. All three peptides were rather rapidly degraded by enzymes in rat jejunal homogenate with half-lives of between 11.9 ± 0.5 and 31.7 ± 1.5 min. The addition of inhibitors to the homogenate prolonged the half-lives substantially for LEF553 (167 ± 35 min) and TAPP (147 ± 2 min), but only slightly for LEF537 (16.4 ± 0.5 min). LEF553 and TAPP were both hydrolyzed in rat and human jejunal fluid, while LEF537 was metabolized less in these fluids. When LEF553 and TAPP were incubated with intestinal fluid in the presence of inhibitors, metabolism was almost completely inhibited. There was no metabolism for any of the peptides in human gastric juice. Conclusions. The replacement of the terminal free carboxylic group with an amide group did not increase the stability of the peptides in jejunal tissue enough to allow successful oral drug delivery.     

Pharmacokinetics of intravenous chloramphenicol sodium succinate in adult patients with normal renal and hepatic function

The pharmacokinetics of chloramphenicol (CAP) and total chloramphenicol succinate (CAPS) were studied in eight hospitalized adult patients with normal renal and hepatic function receiving intravenous chloramphenicol sodium succinate therapy. The steady-state peak concentrations of CAP (8.4–26.0 g/ml) occurred at an average of 18.0 min (range 5.4–40.2) after cessation of the chloramphenicol sodium succinate infusion. Unhydrolyzed CAPS prodrug, representing 26.0±7.0% of the dose, was recovered unchanged in the urine indicating that the bioavailability of CAP from a dose of intravenous chloramphenicol succinate is not complete. A pharmacokinetic model was developed for simultaneous fitting of CAP and CAPS plasma concentration data. Pharmacokinetic parameters determined by simultaneous fitting were: V, 0.81±0.18 liters/kg; t_1/2, 3.20 ±1.02 hr; CL_B, 3.21±1.27 ml/min/kg for chloramphenicol; and V, 0.38±0.13 liters/kg; t_1/2, 0.57±0.12hr; CL_B, 7.72±1.87 ml/min/kg for total chloramphenicol succinate.Supported in part by Faculty Research Council Grant VF648 from the University of North Carolina.     

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.     

Protein Deposition from Dry Powder Inhalers: Fine Particle Multiplets as Performance Modifiers

Purpose. To evaluate the use of carrier-based dry powder aerosols for inhalation delivery of proteins and examine the effect of fine particle excipients as potential formulation performance modifiers. Methods. Bovine serum albumin (BSA) was co-processed with malto-dextrin by spray-drying to produce model protein particles. Aerosol formulations were prepared by tumble mixing protein powders with -lactose monohydrate (63–90 m) or modified lactoses containing between 2.5 and 10% w/w fine particle lactose (FPL) or micronised polyethylene glycol 6000. Powder blends were characterised in terms of particle size distribution, morphology and powder flow. Formulation performance in Diskhaler^® and Rotahaler^® devices was investigated using a twin stage impinger operating at 60 1 min^–1. Results. Inhalation performance of binary ordered mixes prepared using BSA-maltodextrin and lactose (63–90 m) was improved by addition of FPL and micronised PEG 6000. For the addition of 5% w/w FPL the protein fine particle fraction (0.5–6.4 m) using the Diskhaler^® was increased from 31.7 ± 2.4% to 47.4 ± 2.2%. Inclusion of FPL and micronised PEG 6000 changed the bulk properties of inhalation powders and reduced powder flow but did not affect device emptying. Unexpectedly, improvements in performance were found to be independent of the order of addition of FPL to the ternary powder formulations. SEM studies revealed that this was probably the result of a redistribution of protein particles between the coarse carrier lactose component and added FPL during mixing. Conclusions. Fine particle excipients can be used to improve the performance of carrier-based protein dry powder aerosols. Mechanistically, enhancement of performance is proposed to result from a redistribution of protein particles from coarse carrier particles to the fine particle component in the ternary mix.     

A comparison between haemodynamic effects of vasopressin analogues

Some analogues of arginine vasopressin (AVP) reportedly possess hypotensive properties, and two such peptides are Cys¹-Tyr²-Phe³-Val⁴-Asn⁵-Cys⁶-Pro⁷-d-Arg⁸-Gly⁹-NH₂ (VD-AVP) and d(CH₂)₅-Cys¹-d-Tyr(Et)²-Arg³-Val⁴-Asn⁵-Cys⁶-Lys⁷-Lys⁸-ethylenediamine⁹ (TA-LVP). In the present investigation we examined the effects of TA-LVP (0.3, 1.0 and 3.0 g/kg/min), VD-AVP (0.3, 1.0 and 3.0 g/kg/min) and AVP (1.0, 3.0, 10 ng/kg/min) on haemodynamics, blood volume (BV) and plasma troponin levels in anaesthetised rats. Infusion of TA-LVP significantly (P<0.05) reduced blood pressure (–45±3%; n=8; mean ± SEM), mean circulatory filling pressure (P_mcf; –41±3%), and cardiac output (CO; –59±4%). The reduction in CO at a lower dose of TA-LVP was due to reduced venous tone, while at higher doses the reduction was predominantly the result of reduced BV (–35±4%). The large decrease in BV during the infusion of TA-LVP, substantially increased resistance to venous return (50±11%), which was the main contributor in reducing CO. Administration of AVP significantly increased blood pressure (41±4%) and arterial resistance (98±16%) without any impact on P_mcf and BV, while significantly reducing CO (–26±5%). Infusion of VD-AVP did not produce hypotension, but produced a modest but significant reduction in CO (–18±5%) and insignificant but moderate increases in peripheral resistance (30±12%) and resistance to venous return (28±8%). Plasma troponin levels were not affected by any of the peptides. The hypotensive action of TA-LVP was due to a reduction in CO as a result of a reduced pre-load, while the pressor effect of AVP increased after-load sufficiently to impede flow, reducing CO. VD-AVP was devoid of any hypotensive effects, suggesting that V₂-vasopressin receptors are most likely to play a limited role in the control of cardiac and vascular function in these animals.     

Pharmacokinetics and absolute bioavailability of carteolol,a new β-adrenergic receptor blocking agent

Summary The pharmacokinetics and absolute bioavailability of a new nonselective -adrenoreceptor blocking agent, carteolol, were investigated after administration of single intravenous and oral doses to eight normal volunteers. Plasma and urine drug concentrations were measured by an HPLC method. The pharmacokinetic parameters after intravenous dosing were obtained by a two-compartment analysis: elimination or -phase t_1/2 4.7±0.3 h; Vc, 0.74±0.101/kg; Vd, 4.05±0.48 l/kg; Cl, 10.13±0.94 ml/min/kg; Cl_R, 6.56±0.58 ml/min/kg; and Cl_NR, 3.57±0.40 ml/min/kg. The absolute bioavailability obtained from plasma data was 83.7±8.0%, which was consistent with that derived from analysis of urine of 82.7±4.2%. The amounts excreted unchanged in urine up to 48 h after the intravenous and oral doses were 65.0±1.5% and 53.8±3.2% of the administered doses, respectively. The t_1/2 for removal of the drug derived from plasma and urine findings after intravenous and oral dosing were similar, which indicates that the main route of elimination of carteolol is via the kidneys. As the Cl_R of carteolol exceeded the Cl of creatinine there may be renal tubular secretion of the drug.     

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.     

Renal response to amino acid infusion in rats: effect of dopamine receptor antagonists and benserazide

Previously, we have found that feeding is a dominant factor controlling urinary dopamine excretion (U_DA) in conscious rats (Mühlbauer and Osswald 1992). Since the renal response to feeding is also characterized by an increase in glomerular filtration rate (GFR), we wanted to investigate in a first step whether the feeding-induced elevations of GFR and U_DA could be causally related phenomena. Therefore, we studied the influence of dopamine synthesis and dopamine receptor blockade on the renal response to amino acid infusion (AA) in thiopental anesthetized rats. AA infusion (n = 7) increased GFR by 33±7% (P<0.001) and U_DA by 87±19% (P<0.001). In the presence of benserazide (BZD, n = 5), an inhibitor of dopamine synthesis, infused i.v. at a dose of 30 g/min/kg, U_DA was suppressed to values below detection limit and the AA-induced GFR increase was abolished. Continuous intravenous infusion of the DA₁ receptor antagonist SCH 23390 (SCH, n = 7) in a dose of 4.0 g/kg/min did not prevent the AA-induced increase in GFR (33±3%, P<0.001) and U_DA (97±12%, P< 0.001). In contrast, S-sulpiride (SUL), a specific DA₂ receptor antagonist, infused continuously i.v. in a dose of 5 g/kg/min, completely abolished the AA-induced GFR increase, while U_DA was increased 1.6-fold (P<0.01). Like BZD, both dopamine receptor antagonists did not affect renal sodium excretion substantially.Our results suggest, that endogenous dopamine could act as a mediator in the renal response to amino acid infusion in the rat, most likely by activation of DA₂ receptors. Correspondence to:B. Mühlbauer at the above address     

Use of Stable Isotopes for Evaluation of Drug Delivery Systems: Comparison of Ibuprofen Release in Vivo and in Vitro from Two Biphasic Release Formulations Utilizing Different Rate-Controlling Polymers

Certain delivery systems are intended to release the active ingredient in different phases to obtain the desired therapeutic effect. For these formulations, such as a bilayer tablet, it is desirable to distinguish and measure the release of drug from the different phases simultaneously. Mass spectrometric methods were developed to measure three ibuprofen isotopomers in serum and two in dissolution fluid. The analytical methods were linear (r 0.992) over the concentration range of interest and recovery was greater than 99.2% for all isotopomers. Coadministration of [²H₀]ibuprofen, [²H₄]ibuprofen, and [²H₇]ibuprofen to male beagles demonstrated that the isotopomers were bioequivalent and verified the absence of any kinetic isotope effect due to deuterium incorporation (p = 0.286). These methods were then used to evaluate a bilayer tablet formulation composed of an immediate release layer of 100 mg [²H₄]ibuprofen and a sustained release layer with a drug load of 300 mg [²H₀]ibuprofen. Two different rate-controlling polymer matrices that provided similar in vitro dissolution profiles were compared in the sustained release phase, while the immediate release formulation remained the same. In male beagles, the HPMC matrix delivered a significantly greater amount of ibuprofen (p < 0.05). The AUC was threefold greater for HPMC (1067 ± 437 nmole * h/ml) versus EUDRAGIT^® (320 ± 51), and C_max was nearly four times greater (145 ± 62.1 nmole/ml for HPMC versus 37.9 ± 14.4 for EUDRAGIT^®). Although T_max for HPMC (3.4 ± 1.9 h) lagged behind EUDRAGIT^® (2.0 ± 0.82 h), the difference was not significant (p > 0.05). The immediate release layer was absorbed to the same extent as an oral solution (containing [²H₇]ibuprofen) that was administered concomitantly with the bilayer tablet. Using the stable isotope markers also demonstrated that the release rates of the two layers were independent of each other, both in vivo and in vitro. Stable isotope techniques are a useful tool in the development of biphasic release formulations since they can be used to determine proper drug load of each phase as well as the appropriate rate of release.     

Pharmacokinetic and thrombolytic properties of unglycosylated recombinant tissue-type plasminogen activator (BM 06.021) produced in Escherichia coli

Summary Recombinant tissue-type plasmogen activator (rt-PA) was produced in Escherichia coli cells in order to obtain an unglycosylated rt-PA (BM 06.021) with increased thrombolytic potency due to altered pharmacokinetic properties. The pharmacokinetics were studied in rabbits upon intravenous infusion of 200 kU/kg over 30 min. The thrombolytic dose-response effects were evaluated in a rabbit model with ¹²⁵I-labeled venous thrombi upon intravenous infusion over 4 h. The thrombolytic effects after intravenous bolus injection of 200 kU/kg BM 06.021 were investigated in a canine model of coronary artery thrombosis. All studies were performed comparing BM 06.021 with glycosylated rt-PA (alteplase).BM 06.021 demonstrated a longer (p < 0.05) half-life (5.6±2.6 vs. 2.1±0.3 min) and a lower (p(0.05) clearance rate (7.5±0.8 vs. 22.2±3.1 ml ·min^–1·kg^–1) than alteplase in rabbits upon intravenous infusion. The doseresponse curve of BM 06.021 for thrombolysis in a rabbit model of jugular vein thrombosis was located to the left of that for alteplase with a 2.1-fold lower effective dose of 50% thrombolysis (ED₅₀) of BM 06.021 (207 vs. 436 kU/kg). Intravenous bolus injection of 200 kU/kg BM 06.021 induced the same reperfusion rate (4/6) as intravenous infusion of 800 kU/kg alteplase over 90 min in a canine model of coronary artery thrombosis. The residual thrombus wet weight did not significantly differ between BM 06.021 and alteplase (5.7 ± 1.8 vs. 6.3 ± 1.1 mg).The results indicate that unglycosylated rt-PA (BM 06.021) has a higher in vivo thrombolytic potency than glycosylated rt-PA (alteplase). The increased potency seems to be due to the altered pharmacokinetic properties of BM 06.021, because the in vitro specific fibrinolytic activities of BM 06.021 and alteplase are virtually identical (783 vs. 800 kU/mg). Send offprint requests to U. Martin at the above address     

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.     

Accumulation Kinetics of Propranolol in the Rat: Comparison of Michaelis– Menten-Mediated Clearance and Clearance Changes Consistent with the “Altered Enzyme Hypothesis”

(+)-Propranolol was infused at two rates into the pyloric vein (a portal vein tributary) of 15 male Sprague Dawley rats until apparent steady-state conditions were established (i.e., 8 hr at each rate). One group (n = 7) received the high dose (40 µg/min/kg) first, and in the other group (n = 8) the low dose (20 µg/kg/min) was used to initiate treatment. Free and total serum concentrations of propranolol were measured. When the low dose was given first, the apparent steady-state concentrations achieved during low- and high-rate infusion steps were 166 ± 37 and 774 ± 235 ng/mL, respectively. These data are consistent with a simple Michaelis–Menten kinetic model and the key parameters of such a model (V _max and K _m) were estimated. However, a crucial test of such a model (and one which should give insight regarding the relevance of an altered enzyme hypothesis) is to reverse the order of infusion steps since, in a system controlled by Michaelis–Menten kinetics, the same steady-state concentrations should be achieved regardless of the order in which infusion steps are given. When the sequence of infusion rates was reversed, steady-state concentrations were 492 ± 142 and 298 ± 79 ng/mL for the high and low infusion rates, respectively. Clearly, a history of high-dose exposure reduces the intrinsic clearance of total drug (CL_SS) during a subsequent low-dose exposure (i.e., the apparent steady-state levels during the low-dose pyloric vein infusions were significantly different; P < 0.001). When these data were corrected for plasma protein binding, the same trends emerged. For example, the intrinsic clearance of free drug (CL_Uss) during low dose treatment, when this treatment was given first, was 27.4 ± 7.3 L/min/kg. However, when the low dose was given as the last step, CL_Uss was only 14.4 ± 5.6 L/min/kg. This highly statistically significant decrease in CL_Uss (P < 0.002) is inconsistent with any simple Michaelis–Menten model, but it is consistent with an altered enzyme hypothesis.     

Buccal Transmucosal Delivery of Calcitonin in Rabbits Using Thin-Film Composites

Purpose. Salmon Calcitonin (sCT) is used to treat hypercalcemia resulting from Paget's disease and osteoporosis. sCT is available either in a sterile injectable form or nasal spray. Alternative and more cost-effective dosage forms for the delivery of calcitonin are needed. We sought to deliver sCT transmucosally using a previously reported mucoadhesive bilayer thin-film composite (TFC) via the buccal route. Methods. Forty micrograms of salmon calcitonin (200-IU) was loaded on preformed TFCs. In vitro release of sCT from TFCs was monitored in phosphate-buffered saline (10 mM, pH 7.4) at 37°C. Female New Zealand White rabbits (n = 6) were dosed with 40 g of sCT either by injection via the ear vein or by applying sCT-loaded TFCs directly on the buccal pouch. Blood was collected at various times, and the plasma sCT and calcium concentrations were quantified. WinNonlin® was used to determine the relevant pharmacokinetic parameters. Results. In vitro, over 80% of sCT was released from the TFCs within 240 min. Super Case-II transport was indicated as the primary release mechanism. Rabbits injected intravenously had C _max, Cls, Vss, and AUC_0-inf values of 75.1 ± 6.5 ng/mL, 20.7 ± 3.3 mL/min, 637 ± 141 mL, and 1925 ± 237 ng*min/mL, respectively. Rabbits dosed via the buccal route had C _max, Cls, and AUC_{0-400 min} values of 4.6 ± 1.6 ng/mL, 22.0 ± 5.9 mL/min, and 842.9 ± 209.7 ng*min/mL, respectively. The relative bioavailability for rabbits treated with the TFCs was 43.8 ± 10.9% with a CV of 24.9%. The reductions in plasma calcium levels after administration of sCT by both the intravenous and buccal route were comparable. Conclusions. The TFCs effectively delivered therapeutically efficacious amounts of sCT across the buccal mucosa in rabbits.     

Esterase-Sensitive Cyclic Prodrugs of Peptides: Evaluation of an Acyloxyalkoxy Promoiety in a Model Hexapeptide

Purpose. To evaluate a cyclic acyloxyalkoxycarbamate prodrug of a model hexapeptide (H-Trp-Ala-Gly-Gly-Asp-Ala-OH) as a novel approach to enhance the membrane permeation of the peptide and stabilize it to metabolism. Methods. Conversion to the linear hexapeptide was studied at 37°C in aqueous buffered solutions and in various biological milieus having measurable esterase activities. Transport and metabolism characteristics were assessed using the Caco-2 cell culture model. Results. In buffered solutions the cyclic prodrug degraded chemically to the linear hexapeptide in stoichiometric amounts. Maximum stability was observed between pH 3–4. In 90% human plasma (t _1/2 = 100 ± 4 min) and in homogenates of the rat intestinal mucosa (t _- = 136 ± 4 min) and rat liver (t _- = 65 ± 3 min), the cyclic prodrug disappeared faster than in buffered solution, pH 7.4 (t _- = 206 ± 11 min). Pretreatment of these media with paraoxon significantly decreased the degradation rate of the prodrug. When applied to the apical side of Caco-2 cell monolayers, the cyclic prodrug (t _- = 282 ± 25 min) was significantly more stable than the hexapeptide (t _- = 14 min) and at least 76-fold more able to permeate (P _app = 1.30 ± 0.15 × 10^–7 cm/ s) than the parent peptide (P _app 0.17 × 10^–8 cm/s). Conclusions. Preparation of a cyclic peptide using an acyloxyalkoxy promoiety reduced the lability of the peptide to peptidase metabolism and substantially increased its permeation through biological membranes. In various biological media the parent peptide was released from the prodrug by an apparent esterase-catalyzed reaction, sensitive to paraoxon inhibition.