Validation of a limited sampling model to determine etoposide area under the curve.

STUDY OBJECTIVE: To validate the utility of a previously reported 3-point limited sampling model (LSM) for determining etoposide area under the curve to infinity (AUC(infinity)). DESIGN: Secondary analysis of data from two clinical trials of etoposide. SETTING: University medical center clinical research center. PATIENTS: Thirty-four patients with different malignancies. INTERVENTIONS: Etoposide was administered as a 2-hour infusion to 34 patients. Serial plasma samples were drawn over 24 hours after the infusion and analyzed for etoposide by high-performance liquid chromatography. MEASUREMENTS AND MAIN RESULTS: The 3-point LSM AUC was compared with a 14-point actual AUC calculated by the linear trapezoidal rule. Actual and predicted AUC(infinity) by the LSM were highly correlated (r=0.97, p<0.0001). The LSM predictions had a mean absolute error of 10.9% (95% CI -14.1, -5.3) and a mean error of -9.7% (95% CI 6.9, 14.9). Nine patients with poor AUC(infinity) estimations by the LSM (error > 12%) tended to have abnormally low or high peak concentrations. CONCLUSION: Our findings suggest the development of more robust LSM using other techniques, such as pharmacostatistical models, that can accommodate a greater degree of pharmacokinetic variability.

Interaction of cyclosporin derivatives with the ATPase activity of human P-glycoprotein.

1. P-glycoprotein, a 170-180 kDa membrane glycoprotein that mediates multidrug resistance, hydrolyses ATP to efflux a broad spectrum of hydrophobic agents. In this study, we analysed the effects of three MDR reversing agents, verapamil, cyclosporin A and [3'-keto-Bmt1]-[Val2]-cyclosporin (PSC 833), on the adenosine triphosphatase (ATPase) activity of human P-glycoprotein. 2. P-glycoprotein was immunoprecipitated with a monoclonal antibody (MRK-16) and the P-glycoprotein-MRK-16-Protein A-Sepharose complexes obtained were subjected to a coupled enzyme ATPase assay. 3. While verapamil activated the ATPase, the cyclosporin derivatives inhibited both the substrate-stimulated and the basal P-glycoprotein ATPase. No significant difference was observed between PSC 833 and cyclosporin A on the inhibition of basal P-glycoprotein ATPase activity. PSC 833 was more potent than cyclosporin A for the substrate-stimulated activity. 4. Kinetic analysis indicated a competitive inhibition of verapamil-stimulated ATPase by PSC 833. 5. The binding of 8-azido-[alpha-32P]-ATP to P-glycoprotein was not altered by the cyclosporin derivatives, verapamil, vinblastine and doxorubicin, suggesting that the modulation by these agents of P-glycoprotein ATPase cannot be attributed to an effect on ATP binding to P-glycoprotein. 6. The interaction of the cyclosporin derivatives with ATPase of P-glycoprotein might present an alternative and/or additional mechanism of action for the modulation of P-glycoprotein function.

Description of blood pressure changes in patients beginning cyclosporin A therapy.

Cyclosporin A (CyA) is the primary immunosuppressive agent for the prophylaxis of rejection episodes in renal, cardiac, liver, and other transplants. Recently, its use in autoimmune diseases has been investigated as well. Although several studies have produced promising results, nephrotoxicity and hypertension can result from CyA treatment, and their development must be understood in order to facilitate patient management. This article describes the diastolic blood pressure (DBP) responses in two populations of patients during three months of CyA therapy. Study A involved psoriasis patients and Study B involved postoperative renal transplant patients. The relationship between blood pressure and systemic CyA exposure and other covariates was evaluated using linear mixed effects modeling. Temporal patterns of blood pressure changes with varying duration of CyA exposure were investigated. In Study A, the psoriasis patients showed transient exposure-related increases in DBP on CyA. These elevations, while statistically significant, were clinically insignificant. In Study B, the renal transplant patients showed no CyA-related rises in DBP. In neither study was there evidence for a difference in effect on DBP between Sandimmune and Neoral, the two formulations of CyA presently approved for marketing by the Food and Drug Administration, after differences in CyA exposure were taken into account.

Perspectives in pharmacokinetics. Physiologically based pharmacokinetic modeling as a tool for drug development.

Since the pioneering work of Haggard and Teorell in the first half of the 20th century, and of Bischoff and Dedrick in the late 1960s, physiologically based pharmacokinetic (PBPK) modeling has gone through cycles of general acceptance, and of healthy skepticism. Recently, however, the trend in the pharmaceuticals industry has been away from PBPK models. This is understandable when one considers the time and effort necessary to develop, test, and implement a typical PBPK model, and the fact that in the present-day environment for drug development, efficacy and safety must be demonstrated and drugs brought to market more rapidly. Although there are many modeling tools available to the pharmacokineticist today, many of which are preferable to PBPK modeling in most circumstances, there are several situations in which PBPK modeling provides distinct benefits that outweigh the drawbacks of increased time and effort for implementation. In this Commentary, we draw on our experience with this modeling technique in an industry setting to provide guidelines on when PBPK modeling techniques could be applied in an industrial setting to satisfy the needs of regulatory customers. We hope these guidelines will assist researchers in deciding when to apply PBPK modeling techniques. It is our contention that PBPK modeling should be viewed as one of many modeling tools for drug development.

Pharmacokinetics of SDZ 64-412, a novel antiasthmatic agent, following intravenous, oral, and inhalation dosing in the rat.

The pharmacokinetics of SDZ 64-412, an antiasthmatic agent, were investigated following intravenous, oral, and inhalation dosing in rats. 14C-SDZ 64-412 was administered intravenously (2.75 mg kg-1) and orally (5.5 mg kg-1, 110 mg kg-1), whereas non-radiolabeled drug (5.04 mg kg-1) was administered using nose-only inhalation chambers. Radioactivity and parent drug concentrations in blood, lung, and excreta were determined at designated times post-dose. SDZ 64-412 was rapidly and extensively (approximately 80%) absorbed following both oral doses, although absorption appeared to be prolonged with increasing dose. The absorbed drug was shown to undergo extensive and saturable first-pass metabolism. The bioavailability of the parent drug, calculated by dose-normalized AUC and deconvolution methods, was only 10-15% from the low dose, but increased to approximately 40% following the high dose. Following inhalation dosing, SDZ 64-412 concentrations in blood and lungs increased rapidly, and did not decline immediately after termination of dosing. The inhalation dose yielded a bioavailability of approximately 40%, and AUC of the drug in lungs was approximately 25 times greater than in blood. In general, SDZ 64-412 was extensively distributed and rapidly eliminated from the systemic circulation. Biliary excretion was the predominant route of radioactivity excretion. The present findings suggest that inhalation administration provides a viable means of delivery of SDZ 64-412.

The hydration of proteins in nearly anhydrous organic solvent suspensions.

Water sorption isotherms at 27 degrees C have been measured for lysozyme and chymotrypsin in suspensions of toluene, di(n-butyl) ether, n-propanol, and a solution of 1M n-propanol in benzene. Sorption isotherms for the different suspensions are compared by converting solvent water content to the thermodynamic activity of water in each solvent. The sorption behavior is also compared to that for the two proteins hydrated from the vapor phase. At low water activities, all sorption isotherms are similar when compared on the basis of water activity. However, at higher activities, water sorption by the proteins in the organic suspensions is suppressed relative to the sorption of water vapor. The greatest suppression is observed for n-propanol, which suggests that the suppression may be due to a competition for water-binding sites on the protein by the organic solvent. Sorption isotherms at low water activities have also been predicted using a thermodynamic model in which it is assumed that water binds selectively to the ionizable residues on the surface of the protein. A comparison of predicted and measured sorption isotherms shows that the model can provide reasonable estimates of water sorption in nonpolar or moderately polar organic solvent suspensions at low levels of hydration.

Computer simulations of cell-target encounter including biased cell motion toward targets: single and multiple cell-target simulations in two dimensions.

In order for immune cells to carry out many of their functions, including clearance of infectious agents from tissue, they must first encounter their targets in the tissue. This encounter process is often the rate-limiting step in the overall function. Most immune cells exhibit chemotactic ability, and previous continuum models for encounter rates and dynamics have shown that chemotaxis can be a great advantage to cells by greatly increasing encounter rates relative to those for randomly moving cells. This paper describes computer simulations of discrete cell-target encounter events in two dimensions, for the two cases considered by the continuum models: where only a single cell and a single target are present, and where many cells and targets are present. The results of these simulations verify our previous model predictions that a small amount of chemotactic bias dramatically decreases the encounter time, while further increases in the amount of bias have a much smaller effect. Chemotactic ability is shown to be an important determinant of the kinetics of target clearance, and its effects depend on the initial cell-target ratio and the initial distributions of cells and targets. To the best of our knowledge, this work provides the first computer simulations of particle-target encounter in which there is biased motion of particles toward their targets, and is therefore of general interest beyond specific application to immune cell function.

Mathematical analysis of cell-target encounter rates in three dimensions. Effect of chemotaxis.

Efficient and rapid immune response upon challenge by an infectious agent is vital to host defense. The encounter of leukocytes (white blood cells of the immune system) with their targets is the first step in this response. Analysis of the kinetics of this process is essential not only to understanding dynamic behavior of the immune response, but also to elucidating the consequences of many leukocyte functional abnormalities. The motion of leukocytes in the presence of targets typically involves a directed, or chemotactic component. These immune cells orient the direction of their motion in the presence of gradients in chemical attractants generated by pathogens. Fisher and Lauffenburger (1987. Biophys. J. 51:705-716) developed a model for macrophage/bacterium encounter in two dimensions which includes chemotaxis, and applied it to the particular system of alveolar macrophages (phagocytic leukocytes on the lung surface). Their model showed that macrophage/target encounter is likely the rate-limiting step in clearance of bacteria from the lung surface (Fisher, E. S., D. A. Lauffenburger, and R. P. Daniele. 1988. Am. Rev. Resp. Dis. 137:1129-1134). We have extended this model to analyze the effects of cell motility properties and geometric parameters on cell-target encounter in three dimensions. The differential equation governing encounter time in three dimensions is essentially the same as that in two dimensions, except for changed probability values. Our results show that more highly directed motion is necessary in three dimensions to achieve substantially decreased encounter times than in two dimensions, because of the increased search dimensionality. These general results were applied to the particular system of neutrophils operating in three dimensions in response to a bacterial challenge in connective tissue. Our results provide a plausible rationalization for both the chemotactic and chemokinetic behavior observed in neutrophils. That is, these cells exhibit in vitro a greater chemotactic bias and a more dramatic variation of speed with attractant concentration than alveolar macrophages, and our results indicate that these behaviors can have a greater influence in three-dimensional connective tissue infection situations than in two-dimensional lung surface infection cases. In addition, we show that encounter apparently is not generally the rate-limiting step in this neutrophil response. These findings have important implications for correlating in vitro measured defects in cell motility and chemotaxis properties with in vivo functions of host defense against infection.