Erik Sjögren

In vivo methods for drug absorption - comparative physiologies, model selection, correlations with in vitro methods (IVIVC), and applications for formulation/API/excipient characterization including food effects.

This review summarizes the current knowledge on anatomy and physiology of the human gastrointestinal tract in comparison with that of common laboratory animals (dog, pig, rat and mouse) with emphasis on in vivo methods for testing and prediction of oral dosage form performance. A wide range of factors and methods are considered in addition, such as imaging methods, perfusion models, models for predicting segmental/regional absorption, in vitro in vivo correlations as well as models to investigate the effects of excipients and the role of food on drug absorption. One goal of the authors was to clearly identify the gaps in todays knowledge in order to stimulate further work on refining the existing in vivo models and demonstrate their usefulness in drug formulation and product performance testing.

In silico predictions of gastrointestinal drug absorption in pharmaceutical product development : Application of the mechanistic absorption model GI-Sim

Oral drug delivery is the predominant administration route for a major part of the pharmaceutical products used worldwide. Further understanding and improvement of gastrointestinal drug absorption predictions is currently a highly prioritized area of research within the pharmaceutical industry. The fraction absorbed (fabs) of an oral dose after administration of a solid dosage form is a key parameter in the estimation of the in vivo performance of an orally administrated drug formulation. This study discloses an evaluation of the predictive performance of the mechanistic physiologically based absorption model GI-Sim. GI-Sim deploys a compartmental gastrointestinal absorption and transit model as well as algorithms describing permeability, dissolution rate, salt effects, partitioning into micelles, particle and micelle drifting in the aqueous boundary layer, particle growth and amorphous or crystalline precipitation. Twelve APIs with reported or expected absorption limitations in humans, due to permeability, dissolution and/or solubility, were investigated. Predictions of the intestinal absorption for different doses and formulations were performed based on physicochemical and biopharmaceutical properties, such as solubility in buffer and simulated intestinal fluid, molecular weight, pK(a), diffusivity and molecule density, measured or estimated human effective permeability and particle size distribution. The performance of GI-Sim was evaluated by comparing predicted plasma concentration-time profiles along with oral pharmacokinetic parameters originating from clinical studies in healthy individuals. The capability of GI-Sim to correctly predict impact of dose and particle size as well as the in vivo performance of nanoformulations was also investigated. The overall predictive performance of GI-Sim was good as >95% of the predicted pharmacokinetic parameters (C(max) and AUC) were within a 2-fold deviation from the clinical observations and the predicted plasma AUC was within one standard deviation of the observed mean plasma AUC in 74% of the simulations. GI-Sim was also able to correctly capture the trends in dose- and particle size dependent absorption for the study drugs with solubility and dissolution limited absorption, respectively. In addition, GI-Sim was also shown to be able to predict the increase in absorption and plasma exposure achieved with nanoformulations. Based on the results, the performance of GI-Sim was shown to be suitable for early risk assessment as well as to guide decision making in pharmaceutical formulation development.

The Multiple Depletion Curves Method Provides Accurate Estimates of Intrinsic Clearance (CLint), Maximum Velocity of the Metabolic Reaction (Vmax), and Michaelis Constant (Km): Accuracy and Robustness Evaluated through Experimental Data and Monte Carlo Simulations

The use of multiple depletion curves for the estimation of maximum velocity of the metabolic reaction (Vmax), the Michaelis constant (Km), and intrinsic clearance (CLint) was thoroughly evaluated by means of experimental data and through a series of Monte Carlo simulations. The enzyme kinetics of seven compounds were determined using the multiple depletion curves method (MDCM), the traditional initial formation rate of metabolite method (IFRMM), and the “in vitro t½” method, and the parameter estimates that were derived from the three methods were compared. The impact of a change in enzyme activity during the incubation period on the parameter estimates and the possibility to correct for this were also investigated. The MDCM was in good overall agreement with the IFRMM. Correction for a change in enzyme activity was possible and resulted in a better concordance in CLint estimates. The robustness of the method in coping with different rates of substrate turnover and variable starting concentrations were also demonstrated through Monte Carlo simulations. Furthermore, the limitations imposed by assumptions inherent in the in vitro t½ method were demonstrated both experimentally and by simulations. This study demonstrates that the MDCM is a robust and efficient method for estimating enzyme kinetic variables with high accuracy and precision. The method may potentially be used in a wide range of applications, from pure enzyme kinetics to in vitro-based predictions of the pharmacokinetics of compounds with multiple and/or unknown metabolic pathways.

Human in Vivo Regional Intestinal Permeability : Quantitation Using Site-Specific Drug Absorption Data

Application of information on regional intestinal permeability has been identified as a key aspect of successful pharmaceutical product development. This study presents the results and evaluation of an approach for the indirect estimation of site-specific in vivo intestinal effective permeability (Peff) in humans. Plasma concentration-time profiles from 15 clinical studies that administered drug solutions to specific intestinal regions were collected and analyzed. The intestinal absorption rate for each drug was acquired by deconvolution, using historical intravenous data as reference, and used with the intestinal surface area and the dose remaining in the lumen to estimate the Peff. Forty-three new Peff values were estimated (15 from the proximal small intestine, 11 from the distal small intestine, and 17 from the large intestine) for 14 active pharmaceutical ingredients representing a wide range of biopharmaceutical properties. A good correlation (r(2) = 0.96, slope = 1.24, intercept = 0.030) was established between these indirect jejunal Peff estimates and jejunal Peff measurements determined directly using the single-pass perfusion double balloon technique. On average, Peff estimates from the distal small intestine and large intestine were 90% and 40%, respectively, of those from the proximal small intestine. These results support the use of the evaluated deconvolution method for indirectly estimating regional intestinal Peff in humans. This study presents the first comprehensive data set of estimated human regional intestinal permeability values for a range of drugs. These biopharmaceutical data can be used to improve the accuracy of gastrointestinal absorption predictions used in drug development decision-making.

Direct In Vivo Human Intestinal Permeability (Peff ) Determined with Different Clinical Perfusion and Intubation Methods.

Regional in vivo human intestinal effective permeability (Peff ) is calculated by measuring the disappearance rate of substances during intestinal perfusion. Peff is the most relevant parameter in the prediction of rate and extent of drug absorption from all parts of the intestine. Today, human intestinal perfusions are not performed on a routine basis in drug development. Therefore, it would be beneficial to increase the accuracy of the in vitro and in silico tools used to evaluate the intestinal Peff of novel drugs. This review compiles historical Peff data from 273 individual measurements of 80 substances from 61 studies performed in all parts of the human intestinal tract. These substances include: drugs, monosaccharaides, amino acids, dipeptides, vitamins, steroids, bile acids, ions, fatty acids, and water. The review also discusses the determination and prediction of Peff using in vitro and in silico methods such as quantitative structure-activity relationship, Caco-2, Ussing chamber, animal intestinal perfusion, and physiologically based pharmacokinetic (PBPK) modeling. Finally, we briefly outline how to acquire accurate human intestinal Peff data by deconvolution of plasma concentration-time profiles following regional intestinal bolus dosing.

The pharmacokinetics and hepatic disposition of repaglinide in pigs: mechanistic modeling of metabolism and transport.

The predictive power of using in vitro systems in combination with physiologically based pharmacokinetic (PBPK) modeling to elucidate the relative importance of metabolism and carrier-mediated transport for the pharmacokinetics was evaluated using repaglinide as a model compound and pig as the test system. Repaglinide was chosen as model drug as previous studies in humans have shown that repaglinide is subject to both carrier-mediated influx to the liver cells and extensive hepatic metabolism. A multiple sampling site model in pig was chosen since it provides detailed in vivo information about the liver disposition. The underlying assumption was that both metabolism and carrier-mediated transport are also important for the hepatic disposition of repaglinide in pigs. Microsomes and primary hepatocytes were used for in vitro evaluation of enzyme kinetics and cellular disposition, respectively. In vitro data were generated both with and without metabolism inhibitors (ketoconazole, bezafibrate and trimethoprim) and transport inhibitors (diclofenac and quinine) providing input into a semi-PBPK model. In vivo data were also generated with and without the same enzyme and transporter inhibitors, alone and in combination. The pigs were given repaglinide as intravenous infusions with and without inhibitors in a sequential manner, i.e., a control phase and a test phase. Parameters describing the passive and carrier-mediated flux as well as metabolism were estimated in the control phase. The result from test phase was used to gain further knowledge of the findings from the control phase. The in vivo pig model enabled simultaneous sampling from plasma (pre- and postliver and peripheral) as well as from bile and urine. A semi-PBPK model consisting of 11 compartments (6 tissues + 5 sampling sites) was constructed for the mechanistic elucidation of the liver disposition, in vitro based in vivo predictions, sensitivity analyses and estimations of individual pharmacokinetic parameters. Both in vitro and in vivo results showed that carrier-mediated influx was important for the liver disposition. The in vivo findings were supported by the result from the test phase where hepatic clearance (4.3 mL min⁻¹ kg⁻¹) was decreased by 29% (metabolism inhibition), 43% (transport inhibition) and 57% (metabolism + transport inhibition). These effects were in good agreement with predicted levels. This study suggests that both metabolism and carrier-mediated uptake are of significant importance for the liver disposition of repaglinide in pigs.

In Silico Modeling of Gastrointestinal Drug Absorption: Predictive Performance of Three Physiologically Based Absorption Models.

Gastrointestinal (GI) drug absorption is a complex process determined by formulation, physicochemical and biopharmaceutical factors, and GI physiology. Physiologically based in silico absorption models have emerged as a widely used and promising supplement to traditional in vitro assays and preclinical in vivo studies. However, there remains a lack of comparative studies between different models. The aim of this study was to explore the strengths and limitations of the in silico absorption models Simcyp 13.1, GastroPlus 8.0, and GI-Sim 4.1, with respect to their performance in predicting human intestinal drug absorption. This was achieved by adopting an a priori modeling approach and using well-defined input data for 12 drugs associated with incomplete GI absorption and related challenges in predicting the extent of absorption. This approach better mimics the real situation during formulation development where predictive in silico models would be beneficial. Plasma concentration-time profiles for 44 oral drug administrations were calculated by convolution of model-predicted absorption-time profiles and reported pharmacokinetic parameters. Model performance was evaluated by comparing the predicted plasma concentration-time profiles, Cmax, tmax, and exposure (AUC) with observations from clinical studies. The overall prediction accuracies for AUC, given as the absolute average fold error (AAFE) values, were 2.2, 1.6, and 1.3 for Simcyp, GastroPlus, and GI-Sim, respectively. The corresponding AAFE values for Cmax were 2.2, 1.6, and 1.3, respectively, and those for tmax were 1.7, 1.5, and 1.4, respectively. Simcyp was associated with underprediction of AUC and Cmax; the accuracy decreased with decreasing predicted fabs. A tendency for underprediction was also observed for GastroPlus, but there was no correlation with predicted fabs. There were no obvious trends for over- or underprediction for GI-Sim. The models performed similarly in capturing dependencies on dose and particle size. In conclusion, it was shown that all three software packages are useful to guide formulation development. However, as a consequence of the high fraction of inaccurate predictions (prediction error >2-fold) and the clear trend toward decreased accuracy with decreased predicted fabs observed with Simcyp, the results indicate that GI-Sim and GastroPlus perform better than Simcyp in predicting the intestinal absorption of the incompletely absorbed drugs when a higher degree of accuracy is needed. In addition, this study suggests that modeling and simulation research groups should perform systematic model evaluations using their own input data to maximize confidence in model performance and output.

Regional Intestinal Permeability of Three Model Drugs in Human.

Currently there are only a limited number of determinations of human Peff in the distal small intestine and none in the large intestine. This has hindered the validation of preclinical models with regard to absorption in the distal parts of the intestinal tract, which can be substantial for BCS class II-IV drugs, and drugs formulated into modified-release (MR) dosage forms. To meet this demand, three model drugs (atenolol, metoprolol, and ketoprofen) were dosed in solution intravenously, and into the jejunum, ileum, and colon of 14 healthy volunteers. The Peff of each model drug was then calculated using a validated deconvolution method. The median Peff of atenolol in the jejunum, ileum, and colon was 0.45, 0.15, and 0.013 × 10(-4) cm/s, respectively. The corresponding values for metoprolol were 1.72, 0.72, and 1.30 × 10(-4) cm/s, and for ketoprofen 8.85, 6.53, and 3.37 × 10(-4) cm/s, respectively. This is the first study where the human Peff of model drugs has been determined in all parts of the human intestinal tract in the same subjects. The jejunal values were similar to directly determined values using intestinal single-pass perfusion, indicating that the deconvolution method is a valid approach for determining regional Peff. The values from this study will be highly useful in the validation of preclinical regional absorption models and in silico tools.

Treatment of intermediate stage hepatocellular carcinoma: a review of intrahepatic doxorubicin drug-delivery systems

The biopharmaceutical properties of doxorubicin delivered via two drug-delivery systems (DDSs) for the palliative treatment of unresectable hepatocellular carcinoma were reviewed with relation to the associated liver and tumor (patho)physiology. These two DDSs, doxorubicin emulsified with Lipiodol(®) and doxorubicin loaded into DC Bead(®) are different regarding tumor delivery, release rate, local bioavailability, if and how they can be given repeatedly, biodegradability, length of embolization and safety profile. There have been few direct head-to-head comparisons of these DDSs, and in-depth investigations into their in vitro and in vivo performance is warranted.

Concomitant intake of alcohol may increase the absorption of poorly soluble drugs.

Ethanol can increase the solubility of poorly soluble and hence present a higher drug concentration in the gastrointestinal tract. This may produce a faster and more effective absorption resulting in variable and/or high drug plasma concentrations, both of which can lead to adverse drug reactions. In this work we therefore studied the solubility and absorption effects of nine diverse compounds when ethanol was present. The apparent solubility was measured using the μDiss Profiler Plus (pION, MA) in four media representing gastric conditions with and without ethanol. The solubility results were combined with in-house data on solubility in intestinal fluids (with and without ethanol) and pharmacokinetic parameters extracted from the literature and used as input in compartmental absorption simulations using the software GI-Sim. Apparent solubility increased more than 7-fold for non-ionized compounds in simulated gastric fluid containing 20% ethanol. Compounds with weak base functions (cinnarizine, dipyridamole and terfenadine) were completely ionized at the studied gastric pH and their solubility was therefore unaffected by ethanol. Compounds with low solubility in intestinal media and a pronounced solubility increase due to ethanol in the upper gastric compartments showed an increased absorption in the simulations. The rate of absorption of the acidic compounds indomethacin and indoprofen was slightly increased but the extent of absorption was unaffected as the complete doses were readily absorbed even without ethanol. This was likely due to a high apparent solubility in the intestinal compartment where the weak acids are ionized. The absorption of the studied non-ionizable compounds increased when ethanol was present in the gastric and intestinal media. These results indicate that concomitant intake of alcohol may significantly increase the solubility and hence, the plasma concentration for non-ionizable, lipophilic compounds with the potential of adverse drug reactions to occur.