Simulation for the Analysis of Distorted Pharmacodynamic Data

A simulation study was conducted to compare the performance of alternative approaches for analyzing the distorted pharmacodynamic data. The pharmacodynamic data were assumed to be obtained from the natriurertic peptide-type drug, where the diuretic effect arises from the hyperbolic (E_max) dose–response model and is biased by the dose-dependent hypotensive effect. The nonlinear mixed effect model (NONMEM) method enabled assessment of the effects of hemodynamics on the diuretic effects and also quantification of intrinsic diuretic activities, but the standard two-stage (STS) and naive pooled data (NPD) methods did not give accurate estimates. Both the STS and the NONMEM methods performed well for unbiased data arising from a one-compartment model with saturable (Michaelis–Menten) elimination, whereas the NPD method resulted in inaccurate estimates. The findings suggest that nonlinearity and/or bias problems result in poor estimation by NPD and STS analyses and that the NONMEM method is useful for analyzing such nonlinear and distorted pharmacodynamic data.     

Hydralazine Pharmacokinetics and Interaction with Food: An Evaluation of the Dog as an Animal Model

The intravenous and oral dose-dependent pharmacokinetics of hydralazine and the effect of concurrent administration of food with hydralazine in dogs were evaluated for comparison with published human data. Four dogs were given intravenous and oral doses of hydralazine at 0.25, 1.0, 2.5, and 4.0 mg/kg. In addition, the oral 2.5 mg/kg dose was given with a meal. Blood samples were collected at appropriate intervals and analyzed for hydralazine. Pharmacokinetic analysis showed that AUC_oral/ dose (5552 to 13218 mg-min/ml) and F (0.36 to 0.77) increased significantly with dose, indicating saturation of first-pass metabolism, as is seen in humans. Total-body clearance (70 ml/min/kg) and steady-state volume of distribution (9 L/kg) were similar to human values. The bioavailability of hydralazine in the dog was decreased by 63% when the dose was given with a meal, which is comparable to some human data. It was concluded that the dog may be a useful model in which to study mechanisms of the hydralazine-food interaction.     

Application of Mean Residence-Time Concepts to Pharmacokinetic Systems with Noninstantaneous Input and Nonlinear Elimination

Equations describing the mean residence time (MRT) of drugs in the body are derived for drugs that are administered by first-and zero-order rates into systems with Michaelis–Menten elimination. With computer simulations, the validity of these equations, the differences between them, and the conventional approach using the AUMC/AUC or the summation of mean times are demonstrated by examining calculations of the percentage of the administered dose eliminated at the MRT and AUMC/AUC. The effects of the absorption rate on the AUC and on the approximate and true MRT values in a nonlinear pharmacokinetic system are also illustrated with computer simulations. It was previously found that the true MRT_iv = V _ss · AUC_iv/dose for an iv bolus. The total MRT (sum of input and disposition) of a drug after noninstantaneous administration was found to be a function of the MRT_iv, two values of AUC (iv and non-iv), and exactly how the drug is administered expressed as the mean absorption time (MAT). In addition, a theoretical basis is proposed for calculation of the bioavailability of drugs in both linear and nonlinear pharmacokinetic systems.     

Pharmacokinetics and pharmacodynamics of recombinant tissue-type plasminogen activator following intravenous administration in rabbits: a comparison of three dosing regimens

Recombinant tissue-type plasminogen activator (rt-PA) is indicated for the treatment of acute myocardial infarction as a dose of up to 100 mg. Several clinical trials have suggested that higher patency rates can be achieved with a rapid drug administration. A study was conducted in rabbits to determine whether pharmacokinetics provides an explanation for the higher patency rates. Alteplase plasma concentration versus time profiles were compared following three dosing regimes: an accelerated 90 min, a standard 3 h, and a double-bolus regimen. The accelerated and double-bolus regimens resulted in higher initial rt-PA plasma concentrations compared to the standard regimen. No difference in the rt-PA clearance was noted between the standard and accelerated regimens. The rt-PA plasma clearance was slower following the double-bolus administration compared to either infusion regimen, suggesting a saturation of rt-PA clearance in rabbits. The estimated V_max/K_m ratio, the intrinsic metabolic clearance, was 14–19 h⁻¹ using a Michaelis–Menten model. The infusion regimens resulted in a ∼15% maximum depletion of α₂-antiplasmin levels compared to 29% for the double-bolus regimen. In summary, the higher patency following rapid rt-PA administration may be due, at least in part, to the higher rt-PA plasma concentrations. © 1998 John Wiley & Sons, Ltd.     

Influence of Dose Range on Degree of Nonlinearity Detected in Dose-Proportionality Studies for Drugs with Saturable Elimination: Single-Dose and Steady-State Studies

Deviation from proportionality occurs when the ratio of area under the curve (AUC) values is not equal to the ratio of administered doses. The degree of nonlinearity (f _NL) can be quantitated as the ratio of AUCs divided by the ratio of doses. We explore positive deviation from proportionality (f _NL > 1) using the classical Michaelis–Menten model of nonlinear elimination after a single dose (n = 1) or at steady state (ss). The degree of nonlinearity is related to the ratio of the highest dose to the lowest dose (Rd = D _H/D _L): f _NL ⁿ⁼¹ = (2 + Rd · )/(2 + ), f _NL ^ss= (Rd · –1) /(Rd · –Rd), where is the ratio of the initial concentration after the lowest dose to the K _m ( = D _L/K _m · V) and is the ratio of the V _max to the average rate of input for the highest dose ( = V _max/F · D _H). From these relationships, we find that (1) for single-dose studies, K _m is the important Michaelis–Menten parameter, while V _max is important at steady state; (2) the degree of nonlinearity cannot exceed the ratio of doses in single-dose studies, and when doses in extreme excess of K _m· V are chosen, the degree of nonlinearity is equal to the dose range; and (3) at steady state, the degree of nonlinearity can exceed the ratio of doses and approaches infinity as the average input rate approaches V _max. Literature data (phenytoin and ethanol) support these findings. We conclude that the degree of nonlinearity is not a useful measure of nonlinearity in and of itself and propose percentage saturation as being more informative.     

Nonlinear pharmacokinetics of tissue-type plasminogen activator in three animal species: a comparison of mathematical models

A recent study presented plasma concentrations of tissue-type plasminogen activator in three different animal species and at several different dose levels. A three-compartment mammillary model with capacity-limited elimination (of Michaelis–Menten form) was postulated to describe the data. In the present study, several alternative model structures are examined with the view of determining whether better fits can be obtained, whether linear models are significantly worse than nonlinear models, and whether all three compartments are really necessary. © 1998 John Wiley & Sons, Ltd.     

Intestinal absorption pathway of γ-aminobutyric acid in rat small intestine

Intestinal absorption of γ-aminobutyric acid (GABA), as a model compound for γ-aminoacids, has not been extensively studied from the kinetic viewpoint. Since data from our laboratory suggested that some competition arises between intestinal absorption of β-alanine and GABA and since our intent was to maintain the aqueous stagnant diffusion layer in order to approach absorption tests to in vivo physiological conditions, a rat jejunum in situ study was undertaken in order to gain an insight into the mechanism of GABA absorption. In the present paper, results from assays using isotonic perfusion solutions with starting GABA concentrations ranging from 1 to 50 mM are reported. They show that the intestinal absorption of the γ-aminoacid can be apparently described as a specialized transport mechanism which obeys Michaelis-Menten and first-order kinetics. Parameter values found were V_m = 13.99 ± 2.37 mM h^?1, K_m = 3.87 ± 0.63 mM, and k_a(passive) = 0.362 ± 0.120 h^?1. Through the perfusion of 5 mM β-alanine solutions containing variable concentrations of GABA (from 5 to 50 mM), a partially competitive inhibition of β-alanine absorption was apparently characterized.     

Investigating a potential mechanism of Cd resistance in Chironomus riparius larvae using kinetic analysis of calcium and cadmium uptake

The uptake kinetics of waterborne Ca and Cd, both independently and in combination, were examined in C. riparius larvae, which are extremely Cd tolerant. Larvae exposed to Ca (100-2500mumolL(-1)), exhibited classic Michaelis-Menten saturation kinetics for Ca influx, measured using (45)Ca as a radio-tracer. The maximum rate of Ca influx (J(max)(Ca)) was 0.39mumolg(-1)h(-1), and the Ca concentration where the carrier reached half saturation (K(M)(Ca)) was 289mumolL(-1). Cd influx was measured using (109)Cd as a radio-tracer in larvae exposed to Cd (0-1400mumolL(-1)) while the Ca concentration was set to the K(M)(Ca). This revealed a J(max)(Cd) (2.26mumolg(-1)h(-1)) which was nearly 6-fold higher that of Ca. This unusually high capacity for Cd uptake is in accordance with the huge tissue Cd burdens that chironomid larvae are able to accumulate during high level exposures. The apparent K(M)(Cd) (1133mumolCdL(-1)), when recalculated to account for the background Ca level, was still high (567mumolCdL(-1)), suggesting that this organism has a low affinity for Cd relative to most aquatic animals, indeed lower or comparable to its affinity for Ca. In consequence, even well above environmentally relevant Cd exposures, C. riparius does not accumulate Cd at the expense of Ca, thereby avoiding internal hypocalcaemia, in contrast to most other organisms which are much more sensitive to Cd. However, Ca influx was significantly reduced when 1200mumolCdL(-1) was added to Ca exposures (96-2410mumolL(-1)). Michaelis-Menten analysis revealed a similar J(max)(Ca) in Cd-exposed and control larvae (i.e. exposed only to Ca), but that the apparent K(M)(Ca) was many-fold higher in larvae which were simultaneously exposed to Ca and Cd. Conversely, increasing Ca concentrations (96-2410mumolL(-1)) progressively inhibited Cd uptake from a Cd exposure concentration (1200mumolL(-1)), providing additional support for a common transport system. These results suggest that the interaction of Cd and Ca in C. riparius is one of simple competitive interaction, and that the unusual Cd transport kinetics (low affinity, high capacity) relative to fairly standard Ca transport kinetics help explain the unusual tolerance that this organism has to acute Cd exposure.     

The dynamics of dopamine metabolism in various regions of rat brain

Dopamine metabolism was studied in various regions of rat brain by following the decline of 3,4-dihydroxyphenylacetic acid (DOPAC) and 3-methoxy-4-hydroxyphenylacetic acid (HVA) from brain after treatment with pargyline, or from the accumulation of the acids after treatment with probenecid. The decline of DOPAC and HVA after pargyline treatment appeared exponential in all regions of brain studied with half-lives of about 13 min for HVA and 6.5 min for DOPAC. DOPAC was the major metabolite of dopamine, with various brain regions producing between 2–5 times more DOPAC than HVA. HVA accumulated after treatment with probenecid but the accumulation in 1 h did not account for all of the HVA apparently eliminated from brain. DOPAC accumulated in some regions of brain (medulla, hypothalamus and midbrain) and not in others (cerebellum, cortex, striatum and hippocampus) after probenecid treatment. We conclude that dopamine metabolism is not uniform in brain and that the accumulation of DOPAC and HVA in brain after probenecid treatment only accounts for a minor fraction of the dopamine formed in brain.     

Pharmacokinetic Model of Ascorbic Acid in Healthy Male Volunteers During Depletion and Repletion

Purpose. To develop a new pharmacokinetic model for ascorbic acid (vitamin C) since no previously published model describes ascorbic acid absorption and disposition over a broad physiologic range of doses and plasma concentrations. Methods. A new model was developed through exploratory simulations. The model was fitted to pharmacokinetic data obtained from seven healthy volunteers who underwent ascorbic acid depletion then gradual repletion. Concentrations of ascorbic acid were measured in plasma and urine. Final pharmacokinetic model parameter estimates were obtained using nonlinear regression analysis. Results. The new model included saturable absorption, distribution and renal tubular reabsorption parameters. The model described ascorbic acid concentrations in plasma, cells, and urine during depletion and gradual repletion phases with a residual error less than 15%. Conclusions. The model was useful for obtaining a new understanding of the likely causes for the complex concentration-time profile observed during gradual repletion. At doses of 200 to 2500 mg per day, the plateau in pre-dose concentrations is largely due to apparent saturation of tissue uptake and less a function of oral bioavailability and renal excretion than previously thought.     

Successful Prediction of Human Pharmacokinetics by Improving Calculation Processes of Physiologically Based Pharmacokinetic Approach

The physiologically based pharmacokinetics (PBPK) model is a major mechanistic approach for predicting human pharmacokinetics (PK) using drug-specific and physiological parameters but has been difficult to use for human PK prediction with acceptable accuracy. Here, we report a newly developed PBPK approach that incorporates the mechanism of albumin-mediated membrane penetration in the liver and interspecies correlation for unbound tissue fractions. To verify the utility of our PBPK approach, we used 12 drugs that are mainly eliminated by hepatic metabolism to compare the prediction accuracy with a conventional PBPK approach and to observe human PK parameters. We found the predictive accuracy for total clearance (CL_tot), distribution volume at the steady state (V_ss), elimination half-life (t_1/2), and plasma concentration at the last measurable time point (C_last) of our PBPK approach to show better absolute average fold error and percentage within 2-fold error (1.6-1.8 and 67%-92%, respectively) compared with values obtained from the conventional PBPK approach (2.1-2.4 and 42%-67%, respectively). As our approach can use parameters obtained in early drug screening, it could help accelerate successful nomination of drug candidates by optimizing the pharmacokinetics of new chemical entities by directly using predicted human PK profiles.     

Quantitative Structure–Activity Study on Human Pharmacokinetic Parameters of Benzodiazepines Using the Graph Theoretical Approach

The graph theoretical indices for a series of 13 benzodiazepines were calculated using a graph-path topological method. The total molecule, the ring fragments, and combinations of ring fragments were subjected to a quantitative structure–activity analysis using eight pharmacokinetic parameters. The metabolic clearance and the blood-to-plasma concentration ratios were most highly correlated with the graph theoretical indices, with R values of 0.975 and 0.938, respectively. These correlations were found when the diazepine + benzo fragment and phenyl fragment were used to calculate the graph-path indices. Terminal disposition half-life was correlated with the benzo + diazepine fragment, with R = 0.969. Truncating the graph-path codes by eliminating cycles in the total molecule markedly improved the correlation coefficients. When compared to the graph-path indices for the total molecule, the correlation coefficients for the terminal disposition half-life and metabolic clearance data rose from 0.721 to 0.935 and from 0.770 to 0.968, respectively, using the graph-path indices of the truncated molecule. Intrinsic clearance of unbound drug also was poorly correlated with the total molecule (r < 0.7) but rose significantly using the graph-path indices of the truncated moleucle (r = 0.971 and 0.975 for the well-stirred and parallel-tube models, respectively.)     

Time-dependent variations in the organ extraction ratios of acetaminophen in rat

The pharmacokinetics of a single 40mg/kg dose of acetaminophen was investigated at 09h00 and 21h00 in Sprague-Dawley rats synchronized to a 12-h light-dark cycle. Acetaminophen was administered by the intraarterial, intravenous, intraperitoneal, and oral routes in order to determine the contribution of the gastrointestinal tract, liver, and lung to the oral extraction ratio of the drug. A mean oral extraction ratio of 0.46 was obtained at 21h00 as compared to 0.39 at 09h00. The mean extraction ratios of the gastrointestine, liver, and lung were 0.05, 0.41, and 0 at 09h00 and 0.18, 0.24, and 0.13 at 21h00, respectively. These results indicate that the extrahepatic metabolism of acetaminophen is important at 21h00, but is barely detectable at 09h00, whereas the hepatic extraction ratio is higher at 09h00 than at 21h00. Thus, there are temporal variations in the disposition of acetaminophen in the rat.This work was supported by grant MA-6469 from the Medical Research Council of Canada.     

A preliminary study of dexamethasone treatment on pituitary–adrenal responsivity in major depression

Major depression is characterized by overactivity of the hypothalamic–pituitary–adrenal (HPA) axis. Dexamethasone (DEX), the glucocorticoid agonist, has been shown to be effective in the treatment of depression. We chose to examine the impact of a short course of DEX treatment on depressive symptomatology, and on the pituitary‐adrenal response to CRH administration. In this preliminary study, five subjects with major depression were treated for 4 days with 3 mg DEX; a CRH test was performed before and after treatment. Four subjects showed a reduction in ACTH (p=0·01) and cortisol output (p<0·01) following DEX treatment. All subjects showed a drop in depression scores after treatment; the Hamilton Depression score fell by 11·4±1·7 (mean±SEM) from baseline (p=0·01) and the Beck Depression score by 9·2±2·5 (mean±SEM) from baseline (p=0·01); this represented a reduction by almost 50 per cent from baseline levels on both depression indices. We suggest that the impact of DEX treatment on depressive symptoms may reflect a restraining influence on an overactive HPA, with a normalization of pituitary–adrenal response to CRH drive. Larger studies are required to investigate this further and to ascertain whether the mood and neuroendocrine changes induced by dexamethasone are sustained. Copyright © 1999 John Wiley & Sons, Ltd.     

A Kinetic Model for Spray-Freezing of Pharmaceuticals

Spray freeze-drying (SFD), which includes spray-freezing into droplets and dynamic vacuum drying, presents a promising alternative approach to manufacture dried pharmaceuticals more efficiently than conventional vial freeze-drying. Without reliable predictive models for the SFD conditions of interest, any respective process development still relies on empirical approaches. In this work, we propose an improved modeling framework to describe the fast freezing (<1 s) that sub-millimeter droplets undergo in the present SFD process. The modeled freezing rate accounts for both the kinetics of ice growth and droplet heat transfer mechanisms. Computational fluid dynamics (CFD) simulations and experiments on bulk spray-freezing are combined to refine and validate the proposed reduced-order model. While this study is limited to water-sucrose solutions, the present modeling approach can be extended to other pharmaceutical excipients. For the cooling rates of interest, model results indicate that droplets with initial sucrose concentration higher than 20% w/w will transit to a glassy state before completion of crystallization and, consequently, devitrification is expected during post spray-freezing manipulation of the bulk material. In practice, such compact model does not only allow quantification of process parameters that cannot be measured in real time but also enable the choice of optimal spraying conditions for production of free-flowing, high-quality frozen droplets that meet the target product profile.     

Hyperhydration Effect on Pharmacokinetic Parameters and Detection Sensitivity of Recombinant Human Erythropoietin in Urine and Serum Doping Control Analysis of Males

Excessive fluid intake, that is, hyperhydration, may be adopted by athletes as a masking method during antidoping sample collection to influence the excretion patterns of doping agents and, therefore, manipulate their detection. The aim of this exploratory study was to assess the hyperhydration effect on the detection sensitivity of recombinant human erythropoietin (rHuEPO) by sodium N-lauroyl sarcosinate (“sarkosyl”) polyacrylamide gel electrophoresis analysis. The influence of hyperhydration on the serum and urinary pharmacokinetic (PK) profiles of rHuEPO was also investigated. Seven healthy physically active nonsmoking Caucasian males participated in a 31-day clinical study comprising a baseline (days 0, 1-3, and 8-10) and a drug phase (days 15-17, 22-24, and 29-31). Epoetin beta was administered subcutaneously at a single dose of 3000 IU on days 15, 22, and 29. Hyperhydration was applied in the morning on 3 consecutive days (days 1-3, 8-10, 22-24, and 29-31), that is, 0, 24, and 48 h after first fluid ingestion. Water and a commercial sports drink were used as hyperhydration agents (20 mL/kg body weight). Serum and urinary concentration-time profiles were best described by a one-compartment PK model with zero-order absorption. Delayed absorption was observed after hyperhydration and, therefore, lag time was introduced in the PK model. Results showed no significant difference (p > 0.05) on serum or urinary erythropoietin concentrations under hyperhydration conditions. A trend for decreasing volume of distribution and increasing clearance after hyperhydration was observed, mainly after sports drink consumption. However, no significant differences (p > 0.05) due to hyperhydration for any of the serum PK parameters calculated by noncompartmental PK analysis were observed. Renal excretion of endogenous erythropoietin and rHuEPO, as reflected on the urinary cumulative amount, was increased approximately twice after hyperhydration and this supports the nonsignificant difference on the urinary concentrations. Analysis of serum and urine samples was able to detect rHuEPO up to 72 h after drug administration. The detection window of rHuEPO remained unaffected after water or sports drink ingestion. Hyperhydration had no effect on the detection sensitivity of EPO either in serum or urine samples.     

A Novel Pharmacokinetic Bridging Strategy to Support a Change in the Route of Administration for Biologics

Determination of appropriate pharmacokinetic end point to bridge different dosing regimens is often a challenge when developing a new route of administration. Trough concentrations (C_trough) are often considered the most relevant PK end point to predict efficacy (ACR20/DAS28) in the treatment of rheumatoid arthritis for biologics. However, no systematic research has been conducted to evaluate this approach. We developed a novel strategy to predict the most relevant PK variables that may be used to support a change in the route of administration for biological products. Our analysis indicated that matching only C_trough when switching from intravenous dosing to subcutaneous dosing with decreasing dosing interval may result in a lower treatment response. If only average concentration (C_ave) is considered as the relevant variable, our analysis showed that treatment response may be worsened when switching from subcutaneous dosing to intravenous dosing with increasing dosing interval. The study results demonstrated that matching a single pharmacokinetic end point (C_trough or C_ave) may not be sufficient to ensure efficacy when switching between intravenous dosing and subcutaneous dosing. A practical novel pharmacokinetic bridging approach is provided to support a change in the route of administration for biological products.     

NMR structural characterization of cecropin A(1–8) – magainin 2(1–12) and cecropin A(1–8) – melittin(1–12) hybrid peptides

Abstract: In order to elucidate the structure–antibiotic activity relationships of the peptides, the three-dimensional structures of two hybrid peptides, CA(1–8) – MA(1–12) and CA(1–8) – ME(1–12) in trifluoroethanol-containing aqueous solution were investigated by NMR spectroscopy. Both CA(1–8) – MA(1–12) and CA(1–8) – ME(1–12) have strong antibacterial activity but only CA(1–8) – ME(1–12) has hemolytic activity against human erythrocytes. CA(1–8) – MA(1–12) has a hydrophobic 3₁₀-helix of only two turns combined with one short helix in the N-terminus with a flexible hinge section in between. CA(1–8) – MA(1–12) has a severely bent structure in the middle of the peptide. These structural features as well as the low hydrophobicity of CA(1–8) – MA(1–12) seem to be crucial for the selective lysis against the membrane of prokaryotic cells. CA(1–8) – ME(1–12) has an α-helical structure of about three turns in the melittin domain and a flexible structure with one turn in the cecropin domain connected with a flexible hinge section in between, and these might be the structural features required for membrane distruption against prokaryotic and eukaryotic cells. The central hinge region (Gly9-Ile10-Gly11) in an amphipathic antibacterial peptide is considered to play an important role in providing the conformational flexibility required for ion channel formation of the C-terminal hydrophobic α-helix on cell membrane.