Gian Camenisch

Review of theoretical passive drug absorption models: Historical background, recent developments and limitations

In the drug discovery process the optimization of a promising lead to an orally bioavailable drug remains a difficult task. Recent progress in the understanding of the role of physicochemical properties in membrane permeability relevant to important processes such as drug absorption and blood-brain barrier crossing, brings rational drug delivery more within reach. In the last thirty years a number of theoretical transport and absorption models have been developed to describe mathematically how a drug is being passively transported from its site of administration to its site of action and how a compound passes a membrane. The goal of such models is to rationalize the physical significance of the observed non-linear structure-permeability relationships. The models are based on various views on the composition of the biological membranes and on the underlying diffusion and distribution mechanisms. Often simplifications reducing the mathematical complexity are made. We review here a selection of the most important models and discuss modern views on the role of lipophilicity and various pathways through membranes.

Pharmacokinetics of the Oral Direct Renin Inhibitor Aliskiren in Combination With Digoxin, Atorvastatin, and Ketoconazole in Healthy Subjects: The Role of P-Glycoprotein in the Disposition of Aliskiren

This study investigated the potential pharmacokinetic interaction between the direct renin inhibitor aliskiren and modulators of P‐glycoprotein and cytochrome P450 3A4 (CYP3A4). Aliskiren stimulated in vitro P‐glycoprotein ATPase activity in recombinant baculovirus‐infected Sf9 cells with high affinity (Km 2.1 μmol/L) and was transported by organic anion‐transporting peptide OATP2B1‐expressing HEK293 cells with moderate affinity (Km 72 μmol/L). Three open‐label, multiple‐dose studies in healthy subjects investigated the pharmacokinetic interactions between aliskiren 300 mg and digoxin 0.25 mg (n = 22), atorvastatin 80 mg (n = 21), or ketoconazole 200 mg bid (n = 21). Coadministration with aliskiren resulted in changes of <30% in AUCτ and Cmax,ss of digoxin, atorvastatin, o‐hydroxy‐atorvastatin, and ρ‐hydroxy‐atorvastatin, indicating no clinically significant interaction with P‐glycoprotein or CYP3A4 substrates. Aliskiren AUCτwas significantly increased by coadministration with atorvastatin (by 47%, P < .001) or ketoconazole (by 76%, P < .001) through mechanisms most likely involving transporters such as P‐glycoprotein and organic anion‐transporting peptide and possibly through metabolic pathways such as CYP3A4 in the gut wall. These results indicate that aliskiren is a substrate for but not an inhibitor of P‐glycoprotein. On the basis of the small changes in exposure to digoxin and atorvastatin and the <2‐fold increase in exposure to aliskiren during coadministration with atorvastatin and ketoconazole, the authors conclude that the potential for clinically relevant drug interactions between aliskiren and these substrates and/or inhibitors of P‐glycoprotein/CPY3A4/OATP is low.

Influence of breast cancer resistance protein (Abcg2) and p-glycoprotein (Abcb1a) on the transport of imatinib mesylate (Gleevec®) across the mouse blood–brain barrier

Imatinib, a protein tyrosine kinase inhibitor, may prevent the growth of glioblastoma cells. Unfortunately, its brain distribution is restricted by p‐glycoprotein (p‐gp or multidrug resistance protein Mdr1a), and probably by breast cancer resistance protein (Bcrp1), two efflux pumps expressed at the blood–brain barrier (BBB). We have used in situ brain perfusion to investigate the mechanisms of imatinib transport across the mouse BBB. The brain uptake of imatinib in wild‐type mice was limited by saturable efflux processes. The inhibition of p‐gp, by valspodar and zosuquidar, increased imatinib uptake (2.5‐fold), as did the deficiency of p‐gp in Mdr1a/1b(−/−) mice (5.5‐fold). Perfusing imatinib with the p‐gp/Bcrp1 inhibitor, elacridar, enhanced the brain uptake of imatinib in wild‐type (4.1‐fold) and Mdr1a/1b(−/−) mice (1.2‐fold). However, the brain uptake of imatinib was similar in wild‐type and Bcrp1(−/−) mice when it was perfused at a non‐saturating concentration. The brain uptake of CGP74588, an active metabolite of imatinib, was low. It was increased by perfusion with elacridar (twofold), but not with valspodar and zosuquidar. CGP74588 uptake was 1.5 times greater in Bcrp1(−/−) mice than in wild‐type mice. These data suggest that imatinib transport at the mouse BBB is limited by p‐gp and probably by Bcrp1, and that CGP74588 transport is restricted by Bcrp1.

Interaction of the antiviral drug telaprevir with renal and hepatic drug transporters

Telaprevir is a new, direct-acting antiviral drug that has been approved for the treatment of chronic hepatitis C viral infection. First data on drug-drug interactions with co-medications such as cyclosporine, tacrolimus and atorvastatin have been reported recently. Drug transporting proteins have been shown to play an important role in clinically observed drug-drug interactions. The aim of this study was therefore to systematically investigate the potential of telaprevir to inhibit drug transporting proteins. The effect of telaprevir on substrate uptake mediated by drug transporters located in human kidney and liver was investigated on a functional level in HEK293 cell lines that over-express single transporter. Telaprevir was shown to exhibit significant inhibition of the human renal drug transporters OCT2 and MATE1 with IC(50) values of 6.4 μM and 23.0 μM, respectively, whereas no inhibitory effect on OAT1 and OAT3 mediated transport by telaprevir was demonstrated. Liver drug transporters were inhibited with an IC(50) of 2.2 μM for OATP1B1, 6.8 μM for OATP1B3 and 20.7 μM for OCT1. Our data show that telaprevir exhibited significant potential to inhibit human drug transporters. In view of the inhibitory potential of telaprevir, clinical co-administration of telaprevir together with drugs that are substrates of renal or hepatic transporters should be carefully monitored.

Prediction of organic anion-transporting polypeptide 1B1- and 1B3-mediated hepatic uptake of statins based on transporter protein expression and activity data.

Organic anion-transporting polypeptides (OATP) 1B1 and OATP1B3 are drug transporters mediating the active hepatic uptake of their substrates. Because they exhibit overlapping substrate specificities, the contribution of each isoform to the net hepatic uptake needs to be considered when predicting drug-drug interactions. The relative contribution of OATP1B1- and OATP1B3-mediated uptake of statins into hepatocytes was estimated based on either relative transporter protein expression data or relative activity data. Therefore, kinetics of eight statins and OATP1B1- and OATP1B3-specific reference substrates was determined in OATP1B1- and OATP1B3-expressing human embryonic kidney 293 cells and in human cryopreserved hepatocytes. Absolute OATP1B1 and OATP1B3 protein abundance was determined by liquid chromatography-tandem mass spectrometry in all expression systems. Transporter activity data generated in recombinant cell lines were extrapolated to hepatocyte values using relative transporter expression factors (REF) or relative activity factors (RAF). Our results showed a pronounced OATP1B1 and comparatively low OATP1B3 protein expression in the investigated hepatocyte lot. Based on REF scaling, we demonstrated that the active hepatic uptake clearances of reference substrates, atorvastatin, pravastatin, rosuvastatin, and simvastatin were well predicted within twofold error, demonstrating that OATP1B1 and OATP1B3 were major contributors. For other statins, the net hepatic uptake clearance was underpredicted, suggesting the involvement of other hepatic uptake transporters. Summarized, we showed that REF- and RAF-based predictions were highly similar, indicating a direct transporter expression-activity relationship. Moreover, we demonstrated that the REF-scaling method provided a powerful tool to quantitatively assess the transporter-specific contributions to the net uptake clearance of statins in hepatocytes

Novel In Vitro-In Vivo Extrapolation (IVIVE) Method to Predict Hepatic Organ Clearance in Rat

ABSTRACTPurposeDrug elimination in the liver consists of uptake, metabolism, biliary excretion, and sinusoidal efflux from the hepatocytes to the blood. We aimed to establish an accurate prediction method for liver clearance in rats, considering these four elimination processes. In vitro assays were combined to achieve improved predictions.MethodsIn vitro clearances for uptake, metabolism, biliary excretion and sinusoidal efflux were determined for 13 selected compounds with various physicochemical and pharmacokinetic properties. Suspended hepatocytes, liver microsomes and sandwich-cultured hepatocytes were evaluated as in vitro models. Based on the individual processes, in vivo hepatic clearance was calculated. Subsequently, the predicted clearances were compared with the corresponding in vivo values from literature.ResultsUsing this in vitro-in vivo extrapolation method good linear correlation was observed between predicted and reported clearances. Linear regression analysis revealed much improved prediction for the novel method (r2 = 0.928) as compared to parameter analysis using hepatocyte uptake only (r2 = 0.600), microsomal metabolism only (r2 = 0.687) or overall hepatobiliary excretion in sandwich-cultured hepatocytes (r2 = 0.321).ConclusionsIn this new attempt to predict hepatic elimination under consideration of multiple clearance processes, in vivo hepatic clearances of 13 compounds in rats were well predicted using an IVIVE analysis method based on in vitro assays.

Predicting human hepatic clearance from in vitro drug metabolism and transport data: a scientific and pharmaceutical perspective for assessing drug-drug interactions.

Membrane transporters and metabolism are major determinants of the hepatobiliary elimination of drugs. This work investigates several key questions for drug development. Such questions include which drugs demonstrate transporter‐based clearance in the clinic, and which in vitro methods are most suitable for drug classification, i.e. transporter‐ vs metabolism‐dependent compound class categories. Additional questions posed are: what is the expected quantitative change in exposure in the presence of a transporter‐ and/or metabolism‐inhibiting drug, and which criteria should trigger follow‐up clinical drug–drug interaction studies.

Shapes of membrane permeability–lipophilicity curves: Extension of theoretical models with an aqueous pore pathway

The objective of this study was to rationalize the shape of membrane permeability-lipophilicity curves, when considering, in addition to the usual transcellular route, a parallel diffusion pathway through aqueous pores as present in biological membranes. The theoretical influence of different pH in donor and acceptor compartment and the molecular weight on the permeability curves was studied. We combined and extended two previously proposed absorption models, namely one describing diffusion through a simple membrane (two stagnant aqueous and two organic layers in series, no pores) as the sum of the two distribution steps at both membrane interfaces, and a second theoretical model considering the sum of different diffusional resistances through stagnant layers and membrane, respectively. Under certain conditions the equivalence of the two-step distribution model and the diffusional resistance model can be demonstrated. Incorporation of an aqueous diffusion pathway leads to an extended two-step distribution model. This theoretical membrane permeation model will permit a more physicochemical-based interpretation of permeation data and shows that combined log D values and molecular weight are important determinants for membrane transport processes through, e.g. Caco-2 monolayers and the mucosal GI membranes. We have demonstrated that the well-known sigmoidal permeability-lipophilicity relationship should be considered as a molecular weight-dependent set of sigmoidal relationships.

Comparison of passive drug transport through Caco-2 cells and artificial membranes

Abstract Impregnated artificial membranes have been used to study the mechanism of passive diffusion by trans- and para-epithelial transport routes of a structurally diverse set of compounds through Caco-2 cells. The selected compounds of which Caco-2 permeability data have been reported in the literature differed in their physicochemical properties, such as 1-octanol/water distribution coefficient, charge and molecular weight. Experimentally observed Caco-2 permeability-lipophilicity relationships are rationalized as a function of permeation pathway and the above-mentioned physicochemical properties. Impregnated artificial membranes appear to mimic the passive diffusion through in vitro monolayer cell cultures.

In vitro blood distribution and plasma protein binding of the iron chelator deferasirox (ICL670) and its iron complex Fe-[ICL670]2, for rat, marmoset, rabbit, mouse, dog and human

Deferasirox (Exjade, ICL670) is an orally active iron chelator. Two molecules of deferasirox can form a complex with ferric iron (Fe-[ICL670]2) that can be excreted, reducing body iron overload. The blood binding parameters across species and the interaction with human serum albumin were analyzed for deferasirox and its iron complex. Both molecules were very highly bound to plasma proteins in all the tested species with unbound fractions in plasma in the range of 0.4 to 1.8% and 0.2 to 1.2% for deferasirox and Fe-[ICL670]2, respectively; binding of the iron complex was either similar or higher in all the species. The high plasma protein binding was in line with a distribution mainly into the plasma fraction of blood; the fraction in plasma was around 100% for Fe-[ICL670]2 in all the species and 65 to 95% for deferasirox depending on the species. Investigations with isolated proteins pointed to serum albumin as the principal binding protein for deferasirox and its iron complex in human plasma. Competition binding experiments indicated that deferasirox at high concentrations displaced markers from the two main drug binding sites of human albumin, whereas Fe-[ICL670]2 displaced only warfarin. In the context of the pharmacokinetic properties of deferasirox and Fe-[ICL670]2, the data indicate the importance of plasma protein binding for their disposition and support a comparison of the pharmacokinetics of deferasirox and its iron complex across species. The low likelihood of clinically relevant drug displacement by deferasirox in plasma is discussed.