Mamillary Model Research Articles

Mechanism-Based Pharmacokinetic Modeling of Absorption and Disposition of a Deferoxamine-Based Nanochelator in Rats.

Deferoxamine (DFO) is an effective FDA-approved iron chelator. However, its use is considerably limited by off-target toxicities and an extremely cumbersome dose regimen with daily infusions. The recent development of a deferoxamine-based nanochelator (DFO-NP) with selective renal excretion has shown promise in ameliorating animal models of iron overload with a substantially improved safety profile. To further the preclinical development of this promising nanochelator and to inform on the feasibility of clinical development, it is necessary to fully characterize the dose and administration-route-dependent pharmacokinetics and to develop predictive pharmacokinetic (PK) models describing absorption and disposition. Herein, we have evaluated the absorption, distribution, and elimination of DFO-NPs after intravenous and subcutaneous (SC) injection at therapeutically relevant doses in Sprague Dawley rats. We also characterized compartment-based model structures and identified model-based parameters to quantitatively describe the PK of DFO-NPs. Our modeling efforts confirmed that disposition could be described using a three-compartment mamillary model with elimination and saturable reabsorption both occurring from the third compartment. We also determined that absorption was nonlinear and best described by parallel saturable and first-order processes. Finally, we characterized a novel pathway for saturable SC absorption of an ultrasmall organic nanoparticle directly into the systemic circulation, which offers a novel strategy for improving drug exposure for nanotherapeutics.

Pharmacokinetic Modeling of Ceftiofur Sodium Using Non-linear Mixed-Effects in Healthy Beagle Dogs.

Ceftiofur (CEF) sodium is a third-generation broad-spectrum cephalosporin commonly used in an extra-label manner in dogs for the treatment of respiratory and urinary system infections. To contribute to the literature supporting CEF use in companion animals, we have developed a compartmental, non-linear mixed-effects (NLME) model of CEF pharmacokinetics in dogs (PK). We then used the mathematical model to predict (via Monte Carlo simulation) the duration of time for which plasma concentrations of CEF and its pharmacologically active metabolites remained above minimum inhibitory concentrations (respiratory tract Escherichia coli spp.). Twelve healthy beagle dogs were administered either 2.2 mg/kg ceftiofur-sodium (CEF-Na) intravenously (I.V) or 2.2 mg/kg CEF-Na subcutaneously (S.C). Plasma samples were collected over a period of 72 h post-administration. To produce a measurement of total CEF, both CEF and CEF metabolites were derivatized into desfuroylceftiofur acetamide (DCA) before analysis by UPLC-MS/MS. No adverse effects were reported after I.V or S.C dosing. The NLME PK models were parameterized using the stochastic approximation expectation maximization algorithm as implemented in Monolix 2018R2. A two-compartment mamillary model with first-order elimination and first-order S.C absorption best described the available kinetic data. Final parameter estimates indicate that CEF has a low systemic clearance (0.25 L/h/kg) associated with a low global extraction ratio E = 0.02) and a moderate volume of distribution (2.97 L/kg) in dogs. The absolute bioavailability after S.C administration was high (93.7%). Gender was determined to be a significant covariate in explaining the variability of S.C absorption. Our simulations predicted that a dose of 2.2 mg/kg CEF-Na S.C would produce median plasma concentrations of CEF of at least 0.5 μg/mL (MIC50) for ~30 h.

Population Pharmacokinetics of Azithromycin in Whole Blood, Peripheral Blood Mononuclear Cells, and Polymorphonuclear Cells in Healthy Adults

Azithromycin's extensive distribution to proinflammatory cells, including peripheral blood mononuclear cells (PBMCs) and polymorphonuclear cells (PMNs), may be important to its antimicrobial and anti-inflammatory properties. The need to simultaneously predict azithromycin concentrations in whole blood (“blood”), PBMCs, and PMNs motivated this investigation. A single-dose study in 20 healthy adults was conducted, and nonlinear mixed effects modeling was used to simultaneously describe azithromycin concentrations in blood, PBMCs, and PMNs (simultaneous PK model). Data were well described by a four-compartment mamillary model. Apparent central clearance and volume of distribution estimates were 67.3 l/hour and 336 l (interindividual variability of 114 and 122%, respectively). Bootstrapping and visual predictive checks showed adequate model performance. Azithromycin concentrations in blood, PBMCs, and PMNs from external studies of healthy adults and cystic fibrosis patients were within the 5th and 95th percentiles of model simulations. This novel empirical model can be used to predict azithromycin concentrations in blood, PBMCs, and PMNs with different dosing regimens.

Population Pharmacokinetic Analysis for Hydrocodone Following the Administration of Hydrocodone Bitartrate Extended-Release Capsules

Hydrocodone is a semi-synthetic narcotic analgesic and antitussive. Although hydrocodone products have been on the market for over 50 years, relatively little is known about its pharmacokinetics. Additionally, there are no published reports of population pharmacokinetic analyses for hydrocodone. Furthermore, current labeling of hydrocodone-containing products provides little guidance in terms of the impact of patient descriptors on the pharmacokinetics of hydrocodone. The objectives of this analysis were to develop a population pharmacokinetic model that characterizes the pharmacokinetics of hydrocodone following single and multiple oral doses of hydrocodone extended-release capsules (hydrocodone bitartrate ER capsules) in healthy subjects and patients, to examine the impact of patient descriptors on pharmacokinetic parameters and to assess the dose-proportionality of hydrocodone pharmacokinetic. A total of 4,714 plasma hydrocodone concentrations from 220 subjects were available for this analysis. The data were extracted from seven studies (five phase 1 and two phase 2 studies). A two-compartment open mamillary model with linear elimination and a complex absorption model was used to fit the data, using NONMEM(®) version 7.1.2 software. The absorption model involved two sequential first-order absorption processes with the delay in the first process accomplished by means of multiple transit compartments. Covariate analysis was performed using standard forward selection followed by backward elimination processes. Model evaluation was performed using a prediction-corrected visual predictive check (pcVPC). The population estimates of apparent oral central volume of distribution and apparent oral linear clearance were 714 L and 64.4 L/h, respectively. The first absorption process was rapid. Creatinine clearance and body surface area (BSA) were statistically significant predictors of the apparent oral clearance and apparent oral volume of distribution. The pcVPC indicated that the model provided a robust and unbiased fit to the data. A linear model for hydrocodone elimination provided an adequate fit to the observed data over the entire dose range, which supports that hydrocodone bitartrate ER capsules exhibit dose-proportional pharmacokinetics. The formulation of hydrocodone bitartrate ER capsules results in absorption profiles that are variable across and within subjects. Despite the variability in absorption profiles, a relatively simple model provided an adequate fit to the data. Creatinine clearance and BSA were statistically significant predictors of the apparent oral clearance and apparent oral volume of distribution. Absorption characteristics of hydrocodone bitartrate ER capsules should still allow effective plasma concentrations of hydrocodone to be reached quickly and for effective concentrations to be maintained for a long period.

Comparative canine Pharmacokinetics–Pharmacodynamics of Fospropofol Disodium Injection, Propofol Eemulsion, and Cyclodextrin-Enabled Propofol Solution Following Bolus Parenteral Administration

The pharmacokinetics and pharmacodynamics of fospropofol (FP) disodium injection, propofol emulsion (PE), and cyclodextrin-enabled propofol (CDP) solution following bolus parenteral administration in dogs was evaluated. Three healthy male beagle dogs were treated in a three-way cross-over study (14 day washout period) with 6 mg/kg propofol equivalents. Blood samples were collected predose and at 16 points postdose through 1440 min and analyzed for propofol and FP, when appropriate. From 5 min predose to 30 min postdose, brain electrical activity [electroencephalography (EEG)] was recorded and analyzed by power spectrum analysis techniques. Each formulation appeared to be well tolerated with transient discomfort observed in the PE and CDP animals and minor excitability in the FP animals prior to loss of consciousness. Blood propofol followed three-compartment pharmacokinetic behavior and derived parameters were not statistically different except for elimination half-life from the CDP formulation and onset, and duration of anesthesia from the FP formulation. The effect site concentrations at 50% the maximum EEG effect for the FP and CDP formulations were approximately one-half that of the PE formulation. Onset and duration of anesthesia are correlated with modeled effect site propofol concentrations. The implications of formulation on pain on injection and propofol activity are discussed.

Thiopentone Elimination in Newborn Infants

Background: Thiopentone elimination has been described using Michaelis-Menten pharmacokinetics in adults after prolonged infusion or overdose, but there are few reports of elimination in neonates. Methods: Time-concentration profiles for neonates (n=37) given single-dose thiopentone were examined using both first-order (constant clearance) and mixed-order (Michaelis-Menten) elimination processes using nonlinear mixed effects models. These profiles included a 33-week post-menstrual age (PMA) neonate given an overdose. A two-compartment mamillary model was used to fit data. Parameter estimates were standardized to a 70 kg person using allometric models. Results: There were 197 observations available for analysis from neonates with a mean post-menstrual age of 35 (SD 4.5) weeks and a mean weight of 2.5 (SD 0.9) kg. They were given a mean thiopentone dose of 3 (SD 0.4) mg/kg as a rapid bolus. Clearance at 26 weeks PMA was 0.015 l/min/70 kg and increased to 0.119 l/min/70 kg by 42 weeks PMA. The maximum rate of elimination (V(max) ) at 26 weeks PMA was 0.22 mg/min/70 kg and increased to 4.13 mg/min/70 kg by 42 weeks PMA. These parameter estimates are approximately 40% adult values at term gestation. The Michaelis constant (K(m) ) was 28.3 [between subject variability (BSV) 46.4%, 95% confidence interval (CI) 4.49-99.2] mg/l; intercompartment clearance was 0.44 (BSV 97.5%, 95% CI 0.27-0.63) l/min/70 kg; central volume of distribution was 46.4 (BSV 29.2%, 95% CI 41.7-59.8) l/70 kg; peripheral volume of distribution was 95.7 (BSV 70.3%, 95% CI 61.3-128) l/70 kg. Conclusions: Both first-order and mixed-order processes satisfactorily described elimination. First-order elimination adequately described the time-concentration profile in the premature neonate given an overdose. Clearance is immature in the pre-term neonate although there is rapid maturation around 40 weeks PMA, irrespective of post-natal age. (Less)

Pharmacokinetics and pharmacodynamics – is there anything new?

Pharmacokinetics and pharmacodynamics – is there anything new?

Thiopentone elimination in newborn infants: exploring Michaelis-Menten kinetics

Thiopentone elimination has been described using Michaelis-Menten pharmacokinetics in adults after prolonged infusion or overdose, but there are few reports of elimination in neonates. Time-concentration profiles for neonates (n=37) given single-dose thiopentone were examined using both first-order (constant clearance) and mixed-order (Michaelis-Menten) elimination processes using nonlinear mixed effects models. These profiles included a 33-week post-menstrual age (PMA) neonate given an overdose. A two-compartment mamillary model was used to fit data. Parameter estimates were standardized to a 70 kg person using allometric models. There were 197 observations available for analysis from neonates with a mean post-menstrual age of 35 (SD 4.5) weeks and a mean weight of 2.5 (SD 0.9) kg. They were given a mean thiopentone dose of 3 (SD 0.4) mg/kg as a rapid bolus. Clearance at 26 weeks PMA was 0.015 l/min/70 kg and increased to 0.119 l/min/70 kg by 42 weeks PMA. The maximum rate of elimination (V(max)) at 26 weeks PMA was 0.22 mg/min/70 kg and increased to 4.13 mg/min/70 kg by 42 weeks PMA. These parameter estimates are approximately 40% adult values at term gestation. The Michaelis constant (K(m)) was 28.3 [between subject variability (BSV) 46.4%, 95% confidence interval (CI) 4.49-99.2] mg/l; intercompartment clearance was 0.44 (BSV 97.5%, 95% CI 0.27-0.63) l/min/70 kg; central volume of distribution was 46.4 (BSV 29.2%, 95% CI 41.7-59.8) l/70 kg; peripheral volume of distribution was 95.7 (BSV 70.3%, 95% CI 61.3-128) l/70 kg. Both first-order and mixed-order processes satisfactorily described elimination. First-order elimination adequately described the time-concentration profile in the premature neonate given an overdose. Clearance is immature in the pre-term neonate although there is rapid maturation around 40 weeks PMA, irrespective of post-natal age.

A Physiologically Based, Recirculatory Model of the Kinetics and Dynamics of Propofol in Man

The disposition of propofol in man is commonly described using a three-compartment mamillary model. However, these models do not incorporate blood flows as parameters. This complicates the representation of the changes in blood flows that may occur in surgical patients. In contrast, complex physiologically based models are derived from data sets (e.g., tissue:blood partition coefficients) that may not be readily collected in man. Alternatively, the authors report a recirculatory model of propofol disposition in a "standard" man that incorporates detailed descriptions of the lungs and brain, but with a lumped description of the remainder of the body. The model was parameterized from data in the literature using a "meta-modeling" approach. The first-pass passage of propofol through the venous vasculature and the lungs was a function of the injected drug mixing with cardiac output and passing through a three-"tank in series" model for the lungs. The brain was represented as a two-compartment model defined by cerebral blood flow and a permeability term. The Bispectral Index was a linear function of the mean brain concentration. The remainder of the body was represented by compartment systems for the liver, fast distribution and slow distribution. The model was a good fit of the data and was able to predict other data not used in the development of the model. The model may ultimately find a role in improving the fidelity of patient simulators currently used to train anesthetists and for clinical practice simulation to optimize dosing and management strategies.

In vitro and in vivo evaluation of a sulfobutyl ether β‐cyclodextrin enabled etomidate formulation

In this study, we report the formulation and in vivo evaluation of etomidate in an aqueous solution using sulfobutyl ether‐7 β‐cyclodextrin (SBE‐CD, Captisol®) as a solubilizing agent. The phase‐solubility behavior of etomidate as a function of SBE‐CD concentration was evaluated, and accelerated solution stability studies of 2mg/mL etomidate in a 5% w/v SBE‐CD solution were conducted. The intravenous administration of the SBE‐CD etomidate formulation in dogs was compared with Amidate®, the commercial etomidate drug product formulated with propylene glycol as a cosolvent. The etomidate plasma concentration‐time data were fit to a three‐compartment mamillary model and the derived standard pharmacokinetic parameters were not statistically different between the two formulations (n = 4, p > 0.050). Concurrent pharmacodynamic analysis provided statistically equivalent maximum effects and median inhibitory concentrations for the two formulations. In vivo hemolysis after intravenous administration of Amidate® was 10‐fold higher than the SBE‐CD formulation. Whereas Amidate® cannot be given subcutaneously because of the cosolvent in the formulation, a 12mg/mL aqueous solution of etomidate in 20% (w/v) SBE‐CD was well tolerated by this route. The results suggest that the SBE‐CD formulation is a viable clinical drug product with a reduced side‐effect profile.

Pharmacokinetic Profile of the Induction Dose of Propofol in Patients with Severe Burns

The pharmacokinetics of propofol were studied in nine severely burnt patients undergoing general anaesthesia with enflurane/fentanyl for escarectomy. All patients received an intravenous bolus dose of propofol 2 mg/kg, and blood concentrations of propofol were measured over the subsequent 12 hours using high performance liquid chromatography with fluorescent detection. Propofol blood concentrations, showing a very rapid initial decline and a long terminal elimination phase, were best fitted by a triexponential function describing a 3-compartment open mamillary model with rapid distribution from the central compartment, extensive redistribution and elimination from the central compartment. When compared with nonburnt normal controls, the patients with burns showed lower propofol blood concentrations, lower baseline albumin blood levels (p ≤ 0.001), smaller areas under the concentration-time curve (p ≤ 0.001), larger volumes of distribution (p ≤ 0.001), and higher total body clearance (p ≤ 0.001). Pathophysiological systemic responses to the burn injury and particularly the presence of oedema and changes in albumin concentrations were probably responsible for the significant alterations in the disposition kinetics of propofol in the patients with burns.

Pharmacokinetics of eltanolone following bolus injection and constant rate infusion.

Disposition of intravenous anaesthetic eltanolone was studied when administered as a bolus injection (B) of 0.75 mg/kg and constant rate intravenous infusion at 2 mg/kg/hr (12) and 3.5 mg/ kg/hr (13.5) for 2 hr in healthy male volunteers. Venous blood samples were collected for 12 hr and 20 hr following bolus injection and intravenous infusion, respectively. Serum eltanolone concentrations were determined by a specific gas chromatographic mass spectrometric assay. Using a nonlinear regression analysis, the individual data sets were best fitted by a three-compartment mamillary model with central elimination. Derived pharmacokinetic parameters expressed as median and 95% confidence intervals indicated an initial fast distribution with a half-life of 1.80 (0.23-5.47) min (B), 1.44 (0.97-2.06) min (12) and 1.44 (0.95-2.39) min (13.5), an intermediate phase with a half-life of 35.4 (28.7-45.2) min (B), 39.6 (31.0-47.9) min (12) and 35.4 (33.3-44.9) min (13.5) and a moderately short terminal phase with a half-life of 3.8 (2.7-5.9) hr (B), 5.0 (4.2-6.1) hr (12) and 4.6.(4.0-4.8) hr (13.5). The serum clearance after bolus injection was 1.37 (1.23-1.67) L/hr/kg and after infusion was 1.36 (1.25-1.52) L/hr/kg (12) and 1.17 (1.11-1.31) L/hr/kg (13.5). The pharmacokinetics of eltanolone appear to be linear over the dosage range studied. Pharmacokinetic parameters obtained after bolus injection were very much similar to the parameters obtained after infusion with the exception of t1/2 beta which was longer after the infusion (significant) and the volume of central compartment which was lower after infusion (not significant). Context sensitive times were estimated for a 30%, 50% and 80% drop in the concentration of eltanolone after different infusion times. A 30% drop in concentration is estimated to take about 2 to 3 min. A 50% drop in concentration is estimated to take about 8 min when duration of infusion is 3 hr and reaches a value of about 10 min by a duration of infusion of 10 hr. A 80% drop in concentration is estimated to take about 55 min following an infusion of 1 hr and it reaches a value of 70-80 min following an infusion of 10 hr.

Mild Hypothermia Alters Propofol Pharmacokinetics and Increases the Duration of Action of Atracurium

Mild intraoperative hypothermia is common.We therefore studied the effects of mild hypothermia on propofol pharmacokinetics, hepatic blood flow, and atracurium duration of action in healthy volunteers. Six young volunteers were studied on two randomly assigned days, at either 34 degrees C or 37 degrees C. Anesthesia was induced with thiopental, 3 mg/kg, and maintained with 70% N2 O and 0.6% isoflurane. Core hypothermia was induced by conductive and convective cooling. On the other study day, normothermia was maintained by a Bair Hugger Registered Trademark (Augustine Medical, Inc., Eden Prairie, MN) forced-air warmer. Propofol, 1 mg/kg lean body mass (LBM), then was given, followed by a 4-h infusion at 5 mg centered dot kg-1 centered dot h-1. After 2 h, atracurium 0.5 mg/kg was administered as an intravenous bolus. Indocyanine green was administered for estimation of hepatic blood flow. Arterial blood was assayed for propofol and indocyanine green concentration. Pharmacokinetic analysis was performed using NONMEM. Results are reported as means +/- SEM. Propofol blood concentrations averaged approximate equals 28% more at 34 degrees C than at 37 degrees C (P < 0.05). Hepatic blood flow decreased 23% +/- 11% in normothermic volunteers during the propofol infusion, and 33% +/- 11% in hypothermic volunteers (P = not significant). A three-compartment mamillary model fitted the data best. Inclusion of hepatic blood flow change from the prepropofol baseline as a covariate for total body clearance significantly improved the fit. The intercompartmental clearances were decreased in the presence of hypothermia. Core hypothermia prolonged the time to recovery of the first twitch in the train-of-four to 10% of its control value (T1 = 10%) after atracurium administration by approximate equals 60% (P < 0.05), from 44 +/- 4 min to 68 +/- 7 min. In contrast, T1 = 25%-75% remained unchanged. We conclude that 3 degrees C of core hypothermia increased propofol blood concentrations and prolonged atracurium duration of action. Hepatic blood flow was decreased during propofol administration, and this change was a significant predictor of propofol clearance, indicating that the effect of propofol on hepatic blood flow impairs the clearance of propofol itself. (Anesth Analg 1995;80:1007-14)

Pharmacokinetic parameters relevant to recovery from opioids.

Several recent studies have suggested that the terminal half-lives of many drugs do not predict the rate of washout of drug after the relatively short durations of infusions used in anesthesia. Many anesthetic drugs fit a three-compartment mamillary model, with three volumes of distribution (central [V1] and peripheral [V2 and V3]) and three clearances (elimination or metabolic [Cl1] and distribution [Cl2 and Cl3]). It has been suggested that a large V3:Cl3 ratio contributes to rapid recovery after infusion. We investigated the role of each of these primary pharmacokinetic parameters to determine values of each that would contribute to rapid recovery after various dosing schemes. Three sets of computer simulations were performed based on a three-compartment mamillary model for fentanyl, alfentanil, and sufentanil. Set I predicted the change in plasma concentration of each drug after a bolus if each pharmacokinetic parameter were independently increased by 5%. Set II predicted the time for an 80%, 50%, or 20% decrease in plasma concentration after infusions of varying duration (the 80%, 50%, and 20% decrement time, respectively) if each pharmacokinetic parameter were independently increased by 5%. Set III calculated the percent change in each pharmacokinetic parameter alone that would give a 30% shorter decrement time after infusions of varying duration. Set I demonstrated that after a bolus dose to obtain identical initial plasma concentrations, the drug with a larger V1 had a higher plasma concentration than did the parent drug at all subsequent times. The drug with a larger Cl1 had a lower plasma concentration than did the parent drug at all times. A larger V2, V3, Cl2, or Cl3 led to a lower plasma concentration at times soon after the bolus and subsequently to a higher plasma concentration than did the parent drug. Set II demonstrated that after an infusion, increasing V1 led to a longer decrement time and increasing Cl1 led to a shorter decrement time for infusions of all durations. Increasing V2, V3, Cl2, or Cl3 led to a shorter decrement time when the infusion had been short and when a small decrease in plasma concentration was desired. Increasing each of these four parameters led to a longer decrement time when the infusion had been long and when a larger decrease in plasma concentration was desired. Set III demonstrated that a smaller V1 or a larger Cl1 always led to a shorter decrement time. For infusions of short duration and for a small decrease in plasma concentration, a larger V2, V3, Cl2, or Cl3 led to the desired decrease in decrement time. For infusions of longer duration and for larger decreases in plasma concentration, a smaller V2, V3, Cl2, or Cl3 was able to decrease the decrement time by 30%. This study proposes qualitative guidelines for pharmacokinetic properties desirable in anesthetic drugs. If a rapid decrease in plasma concentration is desired after an infusion, it is always beneficial to have a small V1 and a large Cl1. For infusions of short duration, after which only a small decrease in plasma concentration is required, it is beneficial to have a larger V2, V3, Cl2, and Cl3. For infusions of longer duration, after which a large decrease in plasma concentration is desired, it is beneficial to have a smaller V2, V3, Cl2, and Cl3. These proposals may be beneficial for planning clinical trials of new drugs.

The Pharmacokinetics of the New Short-acting Opioid Remifentanil (GI87084B) in Healthy Adult Male Volunteers

Remifentanil (GI87084B) is a new short-acting opioid with a unique ester structure. Metabolism of remifentanil by ester hydrolysis results in very rapid elimination. The aim of this study was to characterize in detail the pharmacokinetic profile of remifentanil in healthy male volunteers. Ten healthy adult male volunteers received a zero-order infusion of remifentanil at doses ranging from 1 to 8 micrograms.kg-1.min-1 for 20 min. Frequent arterial blood samples were drawn and analyzed by gas chromatographic mass spectroscopy to determine the remifentanil blood concentrations. The raw pharmacokinetic data were analyzed using three different parametric compartmental modeling methods (traditional two-stage, naive pooled data, and NONMEM). The raw pharmacokinetic data also were analyzed using numeric deconvolution and a nonparametric moment technique. A computer simulation using hte pharmacokinetic parameters of the NONMEM compartmental model was performed to provide a more intuitively meaningful and clinically relevant description of the pharmacokinetics. The simulation estimated the time necessary to achieve a 50% decrease in remifentanil concentration after a variable-length infusion. For each parametric method, a three-compartment mamillary model that accurately describes remifentanil's concentration decay curve was constructed. The NONMEM analysis population pharmacokinetic parameters included a central clearance of 2.8 l/min, a volume of distribution at steady state of 32.8 l, and a terminal half-life of 48 min. The mean results of the nonparametric moment analysis included a clearance of 2.9 l/min, a volume of distribution at steady state of 31.8 l, and a mean residence time of 10.9 min. The computer simulation revealed the strikingly unique pharmacokinetic profile of remifentanil compared to that of the currently available fentanyl family of opioids. Remifentanil is a new, short-acting opioid with promising clinical potential in anesthesiology.

Disposition of Propofol Administered as Constant Rate Intravenous Infusions in Humans

The disposition of the intravenous anesthetic propofol was studied when administered as a constant rate infusion at 3, 6, and 9 mg.kg-1.hr-1 for at least 2 hr to three groups of six patients each undergoing surgery under regional anesthesia. Arterial blood samples were collected at selected times during and up to 8 hr after infusion. Whole blood propofol concentrations were determined by high-performance liquid chromatography with fluorescence detection. Using a non-linear least-squares regression analysis, the individual data sets were best fitted by a three-compartment open mamillary model with central elimination in 17 patients. In one patient a biexponential equation was more appropriate. Derived pharmacokinetic parameters expressed as mean values +/- SD indicated an initial fast distribution (t1/2 pi; 2.8 +/- 1.2 min), with an intermediate phase (t1/2 alpha; 31.4 +/- 14.7 min), and a long terminal phase (t1/2 beta; 355 +/- 227 min), a large volume of distribution at steady state (Vss, 287 +/- 213 L), and a high blood clearance (Clb, 1.7 +/- 0.3 L/min). The function of drug in the central compartment in the terminal phase was low (Fc, 0.02). The elimination rate constant (K10, 0.1190 +/- 0.0351 min-1) was large compared with the other transfer rate constants and was responsible for the large amount of drug eliminated during distribution. The fraction of drug eliminated during the terminal phase amounted to 0.28. The slow return of drug from remote tissues (K31, 0.0033 +/- 0.0013 min-1) was rate limiting in the ultimate elimination.(ABSTRACT TRUNCATED AT 250 WORDS)

Parameter identification in N-compartment mamillary models

We consider a general mamillary model with a central compartment (compartment 1) and n−1 peripheral compartments, each bidirectionally connected to the first. Elimination is allowed from any compartment and effectively occurs from the system. With input introduced into an arbitrary compartment and measurement performed in an arbitrary compartment, explicit equations are given to derive the parameters of the model from the input-output procedure. The calculations include essentially the determination of the roots of a polynomial plus some elementary algebra. If input and measurement are performed in the same compartment, then a set of 2 n elementary combinations of the model parameters can be uniquely determined. However, the model parameters themselves can only be localized, each within an interval. These intervals are explicitly calculated and their width discussed.

Clinical pharmacokinetics of ciclosporin A in bone marrow transplantation patients

Ciclosporin A (CsA) plasma concentrations were monitored by radioimmunoassay after administration IV and PO to 15 patients before allogeneic bone marrow transplantation. The clearance (10.4 +/- 5.41 1/h), half-life (26.4 +/- 14.6 h), and bioavailability (16% +/- 10%) calculated directly from experimental data showed wide interindividual variations. The kinetics after IV infusion, monitored over 2-5 days, appeared to be triphasic. A three-compartment open mamillary model was thus used to simulate the distribution and elimination kinetics of CsA. The distributive delay between administration PO and absorption in the plasma was approximated using a series of six additional compartments connected with a single transfer rate constant. Then, for each patient, the experimental data corresponding to both IV and PO administration were fitted using a single set of rate constants. Clearance and bioavailability calculated from model parameters correlated well with direct estimations obtained from the areas under the concentration time curves. Drug monitoring and linear modeling will be used in an attempt to achieve stable predetermined plasma concentrations of CsA in bone marrow transplantation patients receiving the drug either IV or PO.

Amphotericin B pharmacokinetics in humans.

The pharmacokinetics of amphotericin B were studied in two patients at the conclusion of long-term therapy for disseminated histoplasmosis. The distribution kinetics of this drug were adequately described by a three-compartment mamillary model with a total distribution volume averaging 4 liters/kg. The elimination phase half-life of amphotericin B was approximately 15 days, reflecting slow release of amphotericin B from a peripheral compartment. In accordance with previous reports, renal excretion accounted for only 3% of total amphotericin B elimination. The pharmacokinetic model for one of the patients also was used to compare the simulated amphotericin B serum levels that would be expected if initial therapy followed two recommended regimens.

The effects of pesticides on wildlife: Exposure and residues

The errors in analysing a wildlife specimen for organochlorine insecticides are relatively small, despite the sensitive techniques that are needed. Single determinations should be correct within a factor of two, unless there is a bias. Variation between animals is large, has a skew distribution, and there is a risk of biassed selection for samples. Other complications arise from use of wet or dry weights, and from analysis of tissues instead of whole organisms. The relationships between exposure and amounts in animals are consistent with a mamillary model, although plateau levels may not be maintained if constant exposure to insecticide lasts long enough. The transition from sublethal to lethal concentrations can occur very rapidly, because fat reserves may be mobilised, when the concentration of insecticide in tissues changes abruptly. It is unlikely that predators accumulate the insecticide contained in their prey. All the evidence suggests that aquatic predators acquire their insecticide directly from the water, not from their food. It is important in terrestrial food chains to compare like with like. If terrestrial predators have higher concentrations than their prey it probably indicates either a difference in metabolic and excretory rates, or selection by the predator of the more contaminated individuals from their prey species. Pesticide residues in wildlife are important as indications of the potential hazard to wildlife from pesticides. They are also used to survey the degree of environmental contamination, but quantitative statements are fraught with difficulties.