Effect of erythrocytes on the hepatic distribution kinetics of antipyrine

Hepatic transporter-mediated pharmacokinetic drug-drug interactions: Recent studies and regulatory recommendations

Transporter-mediated drug-drug interactions are one of the major mechanisms in pharmacokinetic-based drug interactions and correspondingly affecting drugs' safety and efficacy. Regulatory bodies underlined the importance of the evaluation of transporter-mediated interactions as a part of the drug development process. The liver is responsible for the elimination of a wide range of endogenous and exogenous compounds via metabolism and biliary excretion. Therefore, hepatic uptake transporters, expressed on the sinusoidal membranes of hepatocytes, and efflux transporters mediating the transport from hepatocytes to the bile are determinant factors for pharmacokinetics of drugs, and hence, drug-drug interactions. In parallel with the growing research interest in this area, regulatory guidances have been updated with detailed assay models and criteria. According to well-established preclinical results, observed or expected hepatic transporter-mediated drug-drug interactions can be taken into account for clinical studies. In this paper, various methods including in vitro, in situ, in vivo, in silico approaches, and combinational concepts and several clinical studies on the assessment of transporter-mediated drug-drug interactions were reviewed. Informative and effective evaluation by preclinical tools together with the integration of pharmacokinetic modeling and simulation can reduce unexpected clinical outcomes and enhance the success rate in drug development.

Prediction of human pharmacokinetics—evaluation of methods for prediction of hepatic metabolic clearance

Methods for prediction of hepatic clearance (CLH) in man have been evaluated. A physiologically-based in-vitro to in-vivo (PB-IVIV) method with human unbound fraction in blood (fu,bl) and hepatocyte intrinsic clearance (CLint)-data has a good rationale and appears to give the best predictions (maximum ∼2-fold errors; < 25% errors for half of CL-predictions; appropriate ranking). Inclusion of an empirical scaling factor is, however, needed, and reasons include the use of cryopreserved hepatocytes with low activity, and inappropriate CLint- and fu,bl-estimation methods. Thus, an improvement of this methodology is possible and required. Neglect of fu,bl or incorporation of incubation binding does not seem appropriate. When microsome CLint-data are used with this approach, the CLH is underpredicted by 5- to 9-fold on average, and a 106-fold underprediction (attrition potential) has been observed. The poor performance could probably be related to permeation, binding and low metabolic activity. Inclusion of scaling factors and neglect of fu,bl for basic and neutral compounds improve microsome predictions. The performance is, however, still not satisfactory. Allometry incorrectly assumes that the determinants for CLH relate to body weight and overpredicts human liver blood flow rate. Consequently, allometric methods have poor predictability. Simple allometry has an average overprediction potential, > 2-fold errors for ∼1/3 of predictions, and 140-fold underprediction to 5800-fold overprediction (potential safety risk) range. In-silico methodologies are available, but these need further development. Acceptable prediction errors for compounds with low and high CLH should be ∼50 and ∼10%, respectively. In conclusion, it is recommended that PB-IVIV with human hepatocyte CLint and fu,bl is applied and improved, limits for acceptable errors are decreased, and that animal CLH-studies and allometry are avoided.

Prediction of human pharmacokinetics – evaluation of methods for prediction of volume of distribution

The aim was to evaluate and review methods for prediction of the steady-state volume of distribution (VD,ss) of xenobiotics in man. For allometry, ˜30–40% of predictions are classified as incorrect, humans and animals belong to different VD,ss categories for ˜30% of the compounds, maximum prediction errors are large (>10-fold), the b-exponent ranges between −0.2 and 2.2 (averaging ˜0.8–0.9), and >2-fold prediction errors are found for 35% of the substances. The performance is consistent with species differences of binding in and outside the vasculature. The largest errors could potentially lead to very poor prediction of exposure profile and failure in clinical studies. A re-evaluation of allometric scaling of unbound tissue volume of distribution demonstrates that this method is less accurate (27% of predictions >2-fold errors) than a previous evaluation demonstrated. By adding molecular descriptor information, predictions based on animal VD,ss data can be improved. Improved predictions (˜1/10 of allometric errors) can also be obtained by using the relationship between unbound fraction in plasma (fu,pl) and VD,ss for each substance (method suggested by the author). A physiologically-based 4-compartment model (plasma, red blood cells, interstitial fluid and cell volume) together with measured tissue-plasma partitioning coefficients in rats, fu,pl, interstitial-plasma concentration ratio of albumin, organ weight and blood flow data has been successfully applied. Prediction errors for one basic and one neutral drug are only 3–5%. The data obtained with this comparably laboratory-intensive method are limited to these two compounds. A similar approach where predicted tissue partitioning is used, and a computational model, give prediction errors similar to that of allometry. Advantages with these are the suitability for screening and avoidance of animal experiments. The evaluated methods do not account for potential active transport and slow dissociation rates.

Influence of erythrocytes on the hepatic distribution kinetics of urea and thiourea

The role of erythrocyte on the hepatic distribution kinetics of urea and thiourea was investigated in the in situ isolated perfused rat liver. Perfusion experiments were conducted using Krebs-bicarbonate buffer delivered via the portal vein in a single pass mode at a total flow rate of 15 ml/min. With urea, superimposable unimodal effluent curves were obtained in the presence and absence of erythrocytes, indicating that its distribution kinetics is not affected by erythrocytes. With thiourea, effluent curves were unimodal in the absence of erythrocytes but bimodal in the presence of erythrocytes. The maximum frequency output at the first peak increased from 0.017+/-0.002 to 0.042+/-0.006 s(-1) with an increase in the bolus hematocrit from 0.40 to 0.75, indicating that some thiourea fraction is retained by the erythrocytes on transit through the liver. Although the fractional output associated with the first peak was very similar (11.9% versus 11.5%), whether the perfusate contained unlabelled thiourea or not, this fraction was reduced from 17 to 5% with a decrease in the incubation time before injection from 30 min to 40s. However, there was no evidence for a capacity limitation; a 30-min period of pre-incubation of either radiolabelled thiourea alone or combined with a high concentration of unlabelled thiourea had minimal effect on effluent profiles.

Estimation of hepatic distributional volumes using non-labeled reference markers

Hepatic distributional volumes were investigated in the in situ perfused rat liver. Perfusion experiments were conducted using Krebs bicarbonate buffer delivered via the portal vein in single-pass mode at a total flow rate of 15 mL/min. A bolus dose of normal erythrocytes (RBC, vascular marker) and Evans blue (EB, extracellular marker) respectively was administered in the presence and absence of protein. At the end of the experiment, liver total water content was determined by desiccation and freeze-drying methods. Similar moment analysis results and superimposable effluent curves were obtained in the presence (RBC, mean transit time [MTT]: 7.31 +/- 0.45 s and volume of distribution [V]: 0.17 +/- 0.01 mL/g; EB, MTT: 10.9 +/- 0.62 s and V: 0.25 +/- 0.02 mL/g) and in the absence (RBC, MTT: 7.55 +/- 0.84 s and V: 0.18 +/- 0.02 mL/g; EB, MTT: 9.24 +/- 0.77 s and V: 0.20 +/- 0.02 mL/g) of protein, which indicates that the hepatic distribution of RBC and EB within the liver is not influenced by protein. Furthermore, the almost identical results obtained with the desiccation and freeze-drying methods clearly suggest that the freeze-drying method can be used as an alternative to desiccation for the estimation of liver water content.