Leonid Kagan

Systems for region selective drug delivery in the gastrointestinal tract: biopharmaceutical considerations.

Background: The design of a proper region-specific oral drug delivery system has to take into consideration the differences in anatomy, physiology and absorption characteristics that exist between segments within the gastrointestinal (GI) tract, as well as transit kinetics of the dosage form and the site of drug release within the GI tract. Objective: The aim of this review is to examine the various biopharmaceutical aspects of region-specific drug delivery in the GI tract. Methods: This review is mainly focused on pharmacokinetic and pharmacodynamic aspects of region-selective drug delivery with special emphasis on drug absorption pathways (para- and transcellular, absorption and efflux transporters, lymphatic uptake etc.). It includes a discussion of gastroretentive systems, colonic delivery, and lipid-based formulations. The review also addresses targeted therapy of local diseases within the GI tract. Conclusions: The advances in pharmaceutical technology allow for the development of a variety of region-specific drug delivery systems for oral administration to optimize local and systemic therapy.

Interspecies Scaling of Receptor-Mediated Pharmacokinetics and Pharmacodynamics of Type I Interferons

ABSTRACTPurposeTo develop an integrated mechanism-based modeling approach for the interspecies scaling of pharmacokinetic (PK) and pharmacodynamic (PD) properties of type I interferons (IFNs) that exhibit target-mediated drug disposition (TMDD).MethodsPK and PD profiles of human IFN-β1a, IFN-β1b, and IFN-α2a in humans, monkeys, rats, and mice from nine studies were extracted from the literature by digitization. Concentration-time profiles from different species were fitted simultaneously using various allometric relationships to scale model-specific parameters.ResultsPK/PD profiles of IFN-β1a in humans and monkeys were successfully characterized by utilizing the same rate constant parameters and scaling the volume of the central compartment to body weight using an allometric exponent of 1. Concentration and effect profiles of other IFNs were also well described by changing only the affinity of the drug to its receptor. PK profiles in rodents were simulated using an allometric exponent of −0.25 for the first-order elimination rate constant, and no receptor-binding was included given the lack of cross-reactivity.ConclusionsAn integrated TMDD PK/PD model was successfully combined with classic allometric scaling techniques and showed good predictive performance. Several parameters obtained from one IFN can be effectively shared to predict the kinetic behavior of other IFN subtypes.

Role of P-glycoprotein in region-specific gastrointestinal absorption of talinolol in rats

P-Glycoprotein (PGP) is nonuniformly distributed along the gastrointestinal (GI) tract; however, the data regarding regional differences in PGP function in the intestine are controversial. The aim of this work was to investigate the role of PGP efflux in region-specific absorption of talinolol from the GI tract in rats. Plasma talinolol concentrations were measured after several modes of administration, including high (40 mg/kg) and low (4 mg/kg) dose levels, to different segments of the GI tract (stomach versus colon), and codosing with PGP inhibitors (verapamil or cyclosporine). The bioavailability (F) of talinolol after high-dose administration to the stomach was significantly greater than that achieved by the low dose (approximately 18 versus 2%). Coadministration of low-dose talinolol with cyclosporine increased F by approximately 5-fold (p < 0.01). For the high dose, codosing with PGP inhibitors did not increase the extent of absorption. Talinolol demonstrated poor colonic absorption that was significantly increased by coadministration with cyclosporine (F = 0.76 versus 8.1%). Oral verapamil significantly increased systemic clearance and the steady state volume of distribution of intravenous talinolol. A semiphysiological model was developed that successfully captured the pharmacokinetic profiles of talinolol after various modes of administration. PGP-mediated efflux appears to be a major factor responsible for GI region-specific absorption of talinolol in rats, and gastroretentive dosage forms may provide an advantage in the delivery of talinolol and PGP substrate drugs.

Mechanisms of subcutaneous absorption of rituximab in rats.

Absorption of monoclonal antibodies (mAbs) after s.c. injection results from the interplay among several kinetic processes. The aims of this study were to investigate the absorption mechanisms of rituximab in rats by using slow s.c. infusion and coadministration with nonspecific IgG or hyaluronidase, and to evaluate the predictive performance of the pharmacokinetic model previously developed to describe the nonlinear absorption behavior of mAbs. Rituximab serum concentrations were measured after s.c. coadministration with nonspecific IgG and hyaluronidase to rats. Several dose levels and different injection sites were evaluated. For the back site, 6.5- and 2.6-fold decreases in the area under the concentration-time curve were obtained after coadministration with IgG for 1 and 10 mg/kg doses compared with administration of rituximab alone. For the abdomen, only a minor reduction in concentrations was observed. Hyaluronidase increased the rate of s.c. absorption and the bioavailability (1.9- and 1.6-fold for the back and the abdomen injection of 10 mg/kg). Our previously established pharmacokinetic model provided excellent predictions of the effect of nonspecific IgG on rituximab absorption. In conclusion, the magnitude of the effect of absorption modifiers is dependent on the site of injection and the dose level of rituximab. Pharmacokinetic profiles further support the hypothesis that neonatal Fc receptor–mediated transport is a major determinant of s.c. absorption of mAbs.

Pharmacokinetic Modeling of the Subcutaneous Absorption of Therapeutic Proteins

Subcutaneous injection is an important route of administration for therapeutic proteins that provides several advantages over other modes of parenteral delivery. Despite extensive clinical use, the exact mechanism underlying subcutaneous absorption of proteins is not completely understood, and the accuracy of prediction of absorption of biotherapeutics in humans remains unsatisfactory. This review summarizes a variety of models that have been developed for describing the pharmacokinetics of therapeutic proteins administered by subcutaneous injection, including single- and dual-pathway absorption models. Modeling of the lymphatic uptake and redistribution, absorption of monoclonal antibodies and insulin, and population analysis of protein absorption are discussed. The review also addresses interspecies modeling and prediction of absorption in humans, highlights important factors affecting the absorption processes, and suggests approaches for future development of mechanism-based absorption models.

Interspecies Modeling and Prediction of Human Exenatide Pharmacokinetics

ABSTRACTPurposeTo develop a model-based approach for interspecies scaling of the preclinical pharmacokinetics of exenatide and to predict concentration-time profiles in humans.MethodsA target-mediated drug disposition (TMDD) model was simultaneously fit to concentration-time profiles of exenatide over a wide range of intravenous (IV) and subcutaneous (SC) doses obtained from mice, rats, and monkeys. Allometric relationships were incorporated into the model to scale parameters based on species body weight. Human pharmacokinetic profiles following IV and SC administration were simulated using the final model structure and parameter estimates and compared to clinical data.ResultsThe final model provided a good simultaneous fit to all animal data and reasonable parameter estimates. Exenatide receptor binding affinity and baseline receptor concentrations were species-dependent. Absorption parameters from rat provided the best prediction of exenatide SC absorption in humans, but good predictions could also be obtained using allometric scaling of preclinical absorption parameters.ConclusionsA TMDD model combined with allometric scaling was successfully used to simultaneously describe preclinical data for exenatide from three animal species following both IV and SC administration. The majority of model parameters could be shared among the animal species and further used for projecting exenatide behavior in humans.

Effect of mode of administration on guaifenesin pharmacokinetics and expectorant action in the rat model

AIM Guaifenesin is a very commonly used and prescribed oral expectorant drug. However, its mechanism of action is not completely elucidated and the available information is limited. The purpose was to evaluate whether guaifenesin action on respiratory tract secretion is mediated through a reflex stimulation of the gastric mucosa or by the systemic exposure due to the absorption of the drug to the blood circulation. METHODS Guaifenesin was administered to rats by various routes: intravenous bolus, oral gavage, and gastric, jejunal or cecal infusions (through surgically implanted catheters). Phenol red respiratory tract secretion (after intraperitoneal or intravenous injection) was used as a marker for degree of expectorant action. Administration of saline by gavage was used as control. RESULTS Respiratory secretion following oral bolus was approximately 2-fold higher (p<0.05) than that of control. Following IV administration the increase of respiratory secretion did not occur despite the fact that systemic exposure to guaifenesin was 1.5-fold higher than following oral administration. The abdominal surgery was found to eliminate the effect of guaifenesin although it did not change systemic absorption. Guaifenesin was equally absorbed from all parts of the gastrointestinal tract. CONCLUSIONS It was demonstrated that expectorant action of guaifenesin is mediated by stimulation of the gastrointestinal tract and not by the systemic exposure to the drug.

Physiologically Based Pharmacokinetic Model of Amphotericin B Disposition in Rats Following Administration of Deoxycholate Formulation (Fungizone®): Pooled Analysis of Published Data

The time course of tissue distribution of amphotericin B (AmB) has not been sufficiently characterized despite its therapeutic importance and an apparent disconnect between plasma pharmacokinetics and clinical outcomes. The goals of this work were to develop and evaluate a physiologically based pharmacokinetic (PBPK) model to characterize the disposition properties of AmB administered as deoxycholate formulation in healthy rats and to examine the utility of the PBPK model for interspecies scaling of AmB pharmacokinetics. AmB plasma and tissue concentration–time data, following single and multiple intravenous administration of Fungizone® to rats, from several publications were combined for construction of the model. Physiological parameters were fixed to literature values. Various structural models for single organs were evaluated, and the whole-body PBPK model included liver, spleen, kidney, lung, heart, gastrointestinal tract, plasma, and remainder compartments. The final model resulted in a good simultaneous description of both single and multiple dose data sets. Incorporation of three subcompartments for spleen and kidney tissues was required for capturing a prolonged half-life in these organs. The predictive performance of the final PBPK model was assessed by evaluating its utility in predicting pharmacokinetics of AmB in mice and humans. Clearance and permeability–surface area terms were scaled with body weight. The model demonstrated good predictions of plasma AmB concentration–time profiles for both species. This modeling framework represents an important basis that may be further utilized for characterization of formulation- and disease-related factors in AmB pharmacokinetics and pharmacodynamics.

Type I Interferon Receptor is a Primary Regulator of Target-Mediated Drug Disposition of Interferon-β in Mice

The purpose of this study is to evaluate the primary mechanism through which interferon (IFN)-β exhibits target-mediated drug disposition (TMDD) and whether the theoretical assumptions of TMDD models are consistent with experimental pharmacokinetic (PK) data. Recombinant murine IFN-β was administered as an intravenous injection at two dose levels (0.5 and 1 million IU/kg) to male wild-type (WT) and type-I IFN-α/β receptor subunit (IFNAR-1) knockout (KO) mice (A129S7/SvEvBrd strain). Sampling was conducted at various times (n = 3/time point), and plasma was analyzed for IFN-β concentrations using a validated enzyme-linked immunosorbent assay. The pharmacodynamic (PD) biomarker was IP-10 mRNA that was isolated from the distal femur bone and quantified using reverse transcription-polymerase chain reaction. An integrated model that includes rapid-binding TMDD and an indirect mechanism of drug action was used to characterize the PK/PD profiles. For an experimental control, PK profiles of recombinant murine erythropoietin (muEPO), another drug that exhibits TMDD, were determined after a single intravenous dose (0.5 μg/kg) in WT and KO animals. The concentration-time profiles for IFN-β differed substantially at initial times for the WT and KO mice at the same dose levels. These differences are characteristic of ligands exhibiting receptor-mediated disposition and were well described by a rapid-binding TMDD model. No differences in muEPO PK were observed in the control study. In summary, the intact IFNAR receptor is a primary regulator of in vivo IFN-β exposure. An integrated PK/PD model was successfully used to assess the receptor-mediated disposition and dynamics of IFN-β.

Interspecies Pharmacokinetic Modeling of Subcutaneous Absorption of Rituximab in Mice and Rats

PurposeTo investigate the effect of dose level and anatomical site of injection on the pharmacokinetics of rituximab in mice, and to evaluate the utility of a pharmacokinetic model for describing interspecies differences in subcutaneous absorption between mice and rats.MethodsRituximab serum concentrations were measured following intravenous and subcutaneous administration at the back and abdomen of mice. Several approaches were compared for scaling model parameters from estimated values in rats.ResultsThe bioavailability of rituximab following subcutaneous injection was inversely related to the dose level and was dependent on the site of injection in mice. The overall rate of absorption was faster in mice as compared to rats. Subcutaneous absorption profiles were well described using the proposed structural model, in which the total receptor concentration, the affinity of rituximab to the receptor, and the degradation rate constant were assumed to be species independent.ConclusionsSubcutaneous absorption processes show similar trends in rats and mice, although the magnitude differs between species. A mathematical model that combines the absorption of free and bound antibody with presystemic degradation successfully captured rituximab pharmacokinetics in both species, and approaches for sharing and scaling parameters between species were identified.