Robert J. Bauer

A Survey of Population Analysis Methods and Software for Complex Pharmacokinetic and Pharmacodynamic Models with Examples

An overview is provided of the present population analysis methods and an assessment of which software packages are most appropriate for various PK/PD modeling problems. Four PK/PD example problems were solved using the programs NONMEM VI beta version, PDx-MCPEM, S-ADAPT, MONOLIX, and WinBUGS, informally assessed for reasonable accuracy and stability in analyzing these problems. Also, for each program we describe their general interface, ease of use, and abilities. We conclude with discussing which algorithms and software are most suitable for which types of PK/PD problems. NONMEM FO method is accurate and fast with 2-compartment models, if intra-individual and interindividual variances are small. The NONMEM FOCE method is slower than FO, but gives accurate population values regardless of size of intra- and interindividual errors. However, if data are very sparse, the NONMEM FOCE method can lead to inaccurate values, while the Laplace method can provide more accurate results. The exact EM methods (performed using S-ADAPT, PDx-MCPEM, and MONOLIX) have greater stability in analyzing complex PK/PD models, and can provide accurate results with sparse or rich data. MCPEM methods perform more slowly than NONMEM FOCE for simple models, but perform more quickly and stably than NONMEM FOCE for complex models. WinBUGS provides accurate assessments of the population parameters, standard errors and 95% confidence intervals for all examples. Like the MCPEM methods, WinBUGSs efficiency increases relative to NONMEM when solving the complex PK/PD models.

Population pharmacokinetics and pharmacodynamics of the anti-CD11a antibody hu1124 in human subjects with psoriasis

The pharmacokinetics of hu1124, a human anti-CD11a antibody, were investigated in human subjects with psoriasis. CD11a is a subunit of LFA-1, a cell surface molecule involved in T cell mediated immune responses. Subjects received a single dose of 0.03, 0.1, 0.3, 0.6, 1, 2, 3, or 10 mg/kg of hu1124 intravenously over 1–3 hr. Blood samples were collected at selected times from 60 min to 72 days after administration. Plasma samples were assayed for hu1124 by ELISA, and pharmacokinetic analyses were performed on the drug plasma concentrations. As the dose of hu1124 was increased, the clearance decreased from 322 ml/day per kg at 0.1 mg/kg to 6.6 ml/day per kg at 10 mg/kg of hu1124. The plasma hu1124 concentration–time profile suggested that the clearance of hu1124 was saturable above 10 μg/ml. In addition, treatment with hu1124 caused a rapid reduction in the level of CD11a expression on CD3-positive lymphocytes (T cells) to about 25% of pretreatment levels. Regardless of the hu1124 dose administered, cell surface CD11a remained at this reduced level as long as hu1124 was detectable (>0.025 μg/ml) in the plasma. When hu1124 levels fell below 3 μg/ml, the drug was rapidly cleared from the circulation and expression of CD11a returned to normal within 7–10 days thereafter. In vitro, half-maximal binding of hu1124 to lymphocytes was achieved at about 0.1 μg/ml and saturation required more than 10 μg/ml. One of the receptor-mediated pharmacokinetic/pharmacodynamic models which was developed describes the dynamic interaction of hu1124 binding to CD11a, resulting in the removal of hu1124 from the circulation and reduction of cell surface CD11a. The model accounts for the continually changing number of CD11a molecules available for removing hu1124 from the circulation based on prior exposure of cells expressing CD11a to hu1124. In addition, the model also accounts for saturation of CD11a molecules by hu1124 at drug concentrations of approximately 10 μg/ml, thereby reducing the clearance rate of hu1124 with increasing dose.

Pharmacokinetic-pharmacodynamic-efficacy analysis of efalizumab in patients with moderate to severe psoriasis

PurposeEfalizumab is a humanized anti-CD11a monoclonal antibody that demonstrated efficacy in the treatment of patients with psoriasis. The objective of this study was to perform a pharmacokinetic (PK)–pharmacodynamic (PD)–efficacy (E) modeling analysis with intersubject variability assessment to increase our understanding of the interaction of efalizumab with CD11a on T cells and consequent reduction in severity of disease in psoriasis patients.MethodsA total of 6,329 samples from 240 patients in five Phase I and II clinical studies were used in the analysis. For the analysis, plasma efalizumab concentration was used as the PK measurement, the percent of predose CD11a was used as the PD measurement, and the psoriasis area and severity index was used as the measure of efficacy. A receptor-mediated PK/PD model was developed that describes the dynamic interaction of efalizumab binding with CD11a. In the efficacy model, the rate of psoriasis skin production is directly proportional to the amount of free surface CD11a on T cells, which is offset by the rate of skin healing. An additional CD11a-independent component to psoriasis skin production accounted for incomplete response to efalizumab therapy. A Monte Carlo parametric expectation maximization method implemented in the ADAPT II program was used to obtain the estimate of population parameters and inter- and intrasubject variability.Results and ConclusionsThe final model described the PK/PD/E data in psoriasis patients reasonably well. In addition, simulations using the final model suggested that efalizumab administered less frequently could possibly be more convenient with similar efficacy.

An Overview of the Pharmacokinetics and Pharmacodynamics of Efalizumab: A Monoclonal Antibody Approved for Use in Psoriasis

Efalizumab is a recombinant humanized monoclonal IgG1 antibody shown to be efficacious for the treatment of moderate to severe chronic plaque psoriasis. Efalizumab, a targeted inhibitor of T cell interactions, binds to the CD11a subunit of lymphocyte function—associated antigen 1 (LFA‐1), thereby preventing LFA‐1 binding to intercellular adhesion molecule 1 (ICAM‐1). The authors review the pharmacokinetic and pharmacodynamic data from the efalizumab clinical development program and discuss how these data led to selection of the optimal weekly subcutaneous (SC) dose of efalizumab (1.0 mg/kg) in adults. Efalizumab SC dosages of 1.0 mg/kg/wk or greater exerted maximal pharmacodynamic effects for CD11a expression and available CD11a binding sites on T lymphocytes. Dosages greater than 1.0 mg/kg/wk SC did not provide additional benefits; moreover, higher doses did not alter the safety profile. During long‐term administration of efalizumab, serum levels were generally stable and pharmacodynamic markers remained maximally affected.

ING-1, a Monoclonal Antibody Targeting Ep-CAM in Patients with Advanced Adenocarcinomas

Purpose: To determine the feasibility of administration, safety, toxicity, immunogenicity, pharmacokinetics, maximum tolerated dose, and biodistribution of ING-1, a high-affinity, Human-Engineered monoclonal antibody (heMAb) to the Mr 40,000 epithelial cell adhesion molecule Ep-CAM, in patients with advanced adenocarcinomas. Experimental Design: ING-1 was initially administered to patients as a 1-hour intravenous infusion every 3 weeks. Toxicity and pharmacokinetic data led to the evaluation of a weekly schedule. The distribution of iodine-131 (131I)-labeled ING-1 was studied. Results: Twenty-five patients received 82 courses of ING-1. Minimal toxicity was initially observed at the 0.03-, 0.10-, and 0.30-mg/kg dose levels. A patient dosed at 1.0 mg/kg developed acute pancreatitis with severe abdominal pain, nausea, and vomiting. A patient dosed at 0.3 mg/kg had an asymptomatic amylase and lipase elevation to 502 units/L and 1,627 units/L, respectively. Both patients made uncomplicated recoveries. No other dose-limiting toxicities were observed. Regardless of dose, the volume of distribution (mean ± SEM) was 46.6 ± 1.6 mL/kg. ING-1 clearance decreased with increasing dose. To minimize toxicity and increase dose intensity, we then administered ING-1 weekly. No significant toxicity was observed in 7 patients dosed at 0.1 mg/kg. Studies of 131I-labeled ING-1 biodistribution showed radiolocalization to colorectal and prostate cancers. A patient with colorectal cancer had an 80% decrement in the levels of carcinoembryonic antigen. Conclusion: The recommended dose for ING-1 is 0.10 mg/kg by intravenous infusion weekly. The absence of severe toxicity at this dose, low immunogenicity, and preliminary evidence of ING-1 tumor localization and antitumor efficacy support the further clinical development of this antibody to treat Ep-CAM–positive malignant diseases.

Kinetic Approach to Pathway Attenuation Using XOMA 052, a Regulatory Therapeutic Antibody That Modulates Interleukin-1β Activity

Many therapeutic antibodies act as antagonists to competitively block cellular signaling pathways. We describe here an approach for the therapeutic use of monoclonal antibodies based on context-dependent attenuation to reduce pathologically high activity while allowing homeostatic signaling in biologically important pathways. Such attenuation is achieved by modulating the kinetics of a ligand binding to its various receptors and regulatory proteins rather than by complete blockade of signaling pathways. The anti-interleukin-1β (IL-1β) antibody XOMA 052 is a potent inhibitor of IL-1β activity that reduces the affinity of IL-1β for its signaling receptor and co-receptor but not for its decoy and soluble inhibitory receptors. This mechanism shifts the effective dose response of the cytokine so that the potency of IL-1β bound by XOMA 052 is 20–100-fold lower than that of IL-1β in the absence of antibody in a variety of in vitro cell-based assays. We propose that by decreasing potency of IL-1β while allowing binding to its clearance and inhibitory receptors, XOMA 052 treatment will attenuate IL-1β activity in concert with endogenous regulatory mechanisms. Furthermore, the ability to bind the decoy receptor may reduce the potential for accumulation of antibody·target complexes. Regulatory antibodies like XOMA 052, which selectively modulate signaling pathways, may represent a new mechanistic class of therapeutic antibodies.

Monte Carlo Parametric Expectation Maximization (MC-PEM) Method for Analyzing Population Pharmacokinetic/Pharmacodynamic Data

We have developed a practical parametric implementation of the expectation-maximization (EM) methodology which accurately evaluates point estimates of population parameters from pharmacokinetic (PK)/Pharmacodynamic (PD) data without linearizing the expectation step. This algorithm is called the Monte Carlo Parametric Expectation Maximization (MC-PEM) method. In our implementation of this method, the PK/PD parameters are modeled to be multivariate normally or log-normally distributed among subjects, and observed data are modeled to have measurement error that is normally or log-normally distributed about the predicted value for each subject, similar to the manner in which NONMEM models population data. In addition, population parameters may be modeled to patient characteristics (covariates), and intra-subject error coefficients may also be determined. The MC-PEM method was first tested on simulated sparse data (one datum per subject) generated from a simple one-compartment model and was found to accurately estimate the population parameter estimates in all cases. In contrast, NONMEM first order conditional estimation

A Phase I Safety And Pharmacokinetic Study Of A Recombinant Amino Terminal Fragment Of Bactericidal/ Permeability-increasing Protein In Healthy Male Volunteers

A phase I pharmacokinetic and safety clinical trial of rBPI23, a recombinant amino terminal fragment of bactericidal/permeability-increasing protein, was conducted in healthy male volunteers. rBPI23 was administered as a 5 or 30 min infusion at doses of .1 to 1 mg/kg. The pharmacokinetics of rBPI23 in human subjects were described by a bi-exponential disposition function with evidence of concentration-dependent kinetics. The α half-life increased significantly with increasing dose, from 4–5 min at .1 mg/kg to 7–8 min at 1 mg/kg. The β half-life varied between 18 and 29 min regardless of dose and the clearance varied from 5 to 10 mL/min/kg. Very little, if any, of the administered rBPI23 was excreted intact in the urine. Electrocardiograms, ionized calcium concentration, prothrombin and partial prothrombin times, hematologic parameters, and blood chemistries remained normal. Furthermore, no antibody response to rBPI23 was observed in any of the subjects.

In Vitro and In Vivo Pharmacology and Pharmacokinetics of a Human Engineered™ Monoclonal Antibody to Epithelial Cell Adhesion Molecule

ING-1(heMAb), a Human Engineered monoclonal antibody to epithelial cell adhesion molecule (Ep-CAM), was evaluated for its in vitro and in vivo activity. The dissociation constant of ING-1(heMAb) for binding to Ep-CAM on HT-29 human colon tumor cells was 2 to 5 nM, similar to chimeric ING-1. In antibody-dependent cellular cytotoxicity and complement-dependent cytotoxicity assays, ING-1(heMAb) caused a concentration-dependent lysis of BT-20 breast, MCF-7 breast, HT-29 colon, and CACO-2 colon tumor cells, with maximum cytolysis at approximately 1 microg/ml. After an intravenous injection in rats, plasma ING-1(heMAb) levels declined with an alpha half-life of 8 to 11 hours, and a beta half-life of 20 days, typical of an IgG in a species without the target for ING-1. In nude mice with human HT-29 colon tumors, plasma ING-1(heMAb) levels declined more rapidly than in non-tumor-bearing mice, suggesting an enhanced clearance via the tumor-associated human Ep-CAM. In nude mice, intravenous treatments with ING-1(heMAb) twice a week for 3 weeks significantly suppressed the growth of human HT-29 colon and PC-3 prostate tumors in a dose-dependent manner, with 1.0 mg/kg providing the greatest benefit. These results indicate that Human Engineered ING-1(heMAb) is a high-affinity antibody with potent in vitro activity that targets and suppresses the growth of human tumors in vivo.

ALTERATION OF THE PHARMACOKINETICS OF SMALL PROTEINS BY IODINATION

The pharmacokinetics of several proteins were investigated using two different assays. A 23 kDa recombinant protein fragment of bactericidal/permeability-increasing protein (rBPI23) was radiolabeled with 125I using Iodo-beads and administered rats. Plasma samples were collected and assayed for 125I-rBPI23 by radioactivity. In a separate experiment, rBPI23 was administered to rats and plasma samples were assayed for rBPI23 by ELISA. The clearance determined from plasma concentrations of 125I-rBPI23 measured by radioactivity was about 2.5-fold lower than that of rBPI23 determined by ELISA. In addition, the steady state volumes of distribution and mean residence times of 125I-rBPI23 measured by radioactivity were four-fold and 10-fold greater, respectively, compared to those measured by the ELISA method. By studying several proteins with a range of molecular weights, we found that the pharmacokinetics of proteins below about 60 kDa were different when assayed by radioactivity or ELISA, but those of proteins with molecular weights of at least 80 kDA revealed only minor differences. To determine which assay method yielded the correct plasma pharmacokinetic profile, rBPI23 was metabolically labeled with 35S-methionine and administered to rats, and plasma samples were assayed by radioactivity. The concentration-time profile assessed by this method was very close to that determined by ELISA. Exposing rBPI23 to chloramine-T (the oxidant used in the iodination process) and measuring its plasma concentration by ELISA revealed pharmacokinetics similar to those of the iodinated protein measured by radioactivity. In contrast, radiolabeling rBPI23 using iodinated Bolton-Hunter reagent (which avoids exposing the protein to oxidant), and measuring 125I-rBPI23 by radioactivity, yielded pharmacokinetics that were similar, although not identical, to the pharmacokinetics of rBPI23 measured by ELISA. Thus, our data suggest that directly iodinating low-molecular-weight proteins by oxidation procedures alters their clearance from the blood, preventing reliable determination of pharmacokinetic parameters.