Johan Nicolaï

Sandwich-cultured hepatocytes: utility for in vitro exploration of hepatobiliary drug disposition and drug-induced hepatotoxicity

Introduction: The sandwich-cultured hepatocyte (SCH) model has become an invaluable in vitro tool for studying hepatic drug transport, metabolism, biliary excretion and toxicity. The relevant expression of many hepatocyte-specific functions together with the in vivo-like morphology favor SCHs over other preclinical models for evaluating hepatobiliary drug disposition and drug-induced hepatotoxicity. Areas covered: In this review, the authors highlight recommended procedures required for reproducibly culturing hepatocytes in sandwich configuration. It also provides an overview of the SCH model characteristics as a function of culture time. Lastly, the article presents a summary of the most prominent applications of the SCH model, including hepatic drug clearance prediction, drug–drug interaction potential and drug-induced hepatotoxicity. Expert opinion: When human (cryopreserved) hepatocytes are used to establish sandwich cultures, the model appears particularly valuable to quantitatively investigate clinically relevant mechanisms related to in vivo hepatobiliary drug disposition and hepatotoxicity. Nonetheless, the SCH model would largely benefit from better insight into the fundamental cell signaling mechanisms that are critical for long-term in vitro maintenance of the hepatocytic phenotype. Studies systematically exploring improved cell culture conditions (e.g., co-cultures or extracellular matrix modifications), as well as in vitro work identifying key transcription factors involved in hepatocyte differentiation are currently emerging.

Verapamil hepatic clearance in four preclinical rat models: towards activity-based scaling

The current study was designed to cross‐validate rat liver microsomes (RLM), suspended rat hepatocytes (SRH) and the isolated perfused rat liver (IPRL) model against in vivo pharmacokinetic data, using verapamil as a model drug. Michaelis‐Menten constants (Km), for the metabolic disappearance kinetics of verapamil in RLM and SRH (freshly isolated and cryopreserved), were determined and corrected for non‐specific binding. The ‘unbound’ Km determined with RLM (2.8 µ m) was divided by the ‘unbound’ Km determined with fresh and cryopreserved SRH (3.9 µ m and 2.1 µ m, respectively) to calculate the ratio of intracellular to extracellular unbound concentration (Kpu,u). Kpu,u was significantly different between freshly isolated (0.71) and cryopreserved (1.31) SRH, but intracellular capacity for verapamil metabolism was maintained after cryopreservation (200 vs. 191 µl/min/million cells). Direct comparison of intrinsic clearance values (Clint) in RLM versus SRH, yielded an activity‐based scaling factor (SF) of 0.28–0.30 mg microsomal protein/million cells (MPPMC). Merging the IPRL‐derived Clint with the MPPMC and SRH data, resulted in scaling factors for MPPGL (80 and 43 mg microsomal protein/g liver) and HPGL (269 and 153 million cells/g liver), respectively. Likewise, the hepatic blood flow (61 ml/min/kg b.wt) was calculated using IPRL Clint and the in vivo Cl. The scaling factors determined here are consistent with previously reported CYP450‐content based scaling factors. Overall, the results show that integrated interpretation of data obtained with multiple preclinical tools (i.e. RLM, SRH, IPRL) can contribute to more reliable estimates for scaling factors and ultimately to improved in vivo clearance predictions based on in vitro experimentation. Copyright

Transport-Metabolism Interplay of Atazanavir in Rat Hepatocytes

The aim of this study was to explore the mechanisms governing the intra- to extracellular unbound concentration ratio (Kpu,u) for the HIV protease inhibitor atazanavir (ATV) in rat hepatocytes. We had previously proposed a new method to determine Kpu,u by using the unbound Km values from metabolism studies with suspended rat hepatocytes and rat liver microsomes. Following that method, we determined that the value of ATV Kpu,u was 0.32, indicating that ATV hepatocellular clearance is uptake rate–limited. This hypothesis was supported by the linear correlation between Kpu,u and active uptake clearance (P = 0.04; R2=0.82) in the presence of increasing concentrations of the uptake transport inhibitor losartan. Moreover, in contrast to an expected increase of Kpu,u upon inhibition of ATV metabolism, a decrease of Kpu,u was observed, suggesting an increased impact of sinusoidal efflux. In summary, involvement of active uptake transport does not guarantee high intracellular accumulation; however, it has a key role in regulating intracellular drug concentrations and drug metabolism. These findings will help improve future in vitro–to–in vivo extrapolations and likewise physiologically based pharmacokinetic models.

In vitro disposition profiling of heterocyclic compounds

Compound libraries that are screened for biological activity commonly contain heterocycles. Besides potency, drug-like properties need to be evaluated to ensure in vivo efficacy of test compounds. In this context, we determined hepatic and intestinal disposition profiles for 17 heterocyclic compounds. All studied compounds showed rapid uptake in suspended rat hepatocytes, whereas metabolism was poor and the rate-limiting step in hepatic elimination. In vitro assays demonstrated a relatively low solubility and high intestinal permeability. Based on these in vitro data, heterocycles were categorized in the biopharmaceutics classification system (BCS) and the biopharmaceutics drug disposition classification system (BDDCS) to predict disposition characteristics before clinical data are available. Our findings emphasized the importance to use hepatocytes in addition to microsomes to study metabolism, since the latter lack non-microsomal enzymes and cellular context. Moreover, intracellular exposure should be considered to gain insight in the relevant fraction of the compound available at the enzymatic site. Finally, the study reveals discrepancies associated with the classification of heterocycles in BCS versus BDDCS. These probably originate from the binary character of both systems.

Role of the OATP Transporter Family and a Benzbromarone-SensitiveEfflux Transporter in the Hepatocellular Disposition of Vincristine

PurposeVincristine is known to interfere with OATP-mediated uptake of other compounds, hinting that vincristine itself could be a substrate of OATP transporters. The present study therefore aimed to investigate the role of OATP transporters in the hepatocellular disposition of vincristine.MethodsVincristine uptake was studied in suspended rat and human hepatocytes as well as OATP-transfected Chinese hamster ovary (CHO) cells in the absence and presence of OATP transporter inhibitors. Membrane vesicles containing MDR1 or MRP1/2/3 were used to directly assess the role of these efflux transporters in vincristine disposition.ResultsUptake in suspended rat hepatocytes was temperature-dependent and could be inhibited by a range of OATP inhibitors. Furthermore, the MRP-inhibitor benzbromarone, but none of the tested MDR1 inhibitors, reduced vincristine efflux in rat and human suspended hepatocytes. OATP1B1-, OATP1B3- and OATP2B1- transfected CHO cells showed significantly increased vincristine uptake as compared to wild-type cells. Moreover, uptake in OATP-transfected CHO cells was reduced by OATP inhibitors. However, uptake studies in suspended human hepatocytes showed that only 10% of the total vincristine uptake process could be attributed to OATP-mediated transport. Studies with transporter-expressing membrane vesicles confirmed vincristine as an MDR1 substrate, while MRP1/2/3-mediated transport of vincristine could not be observed with this model system.ConclusionsOur findings show the involvement of OATP transporters in the disposition of vincristine in rat and human hepatocytes. However, in both species, hepatic uptake is overshadowed by a benzbromarone-sensitive efflux mechanism, possibly MRP3.

DOSE-FINDING STUDY AND PHARMACODYNAMIC ASSESSMENT OF PROPOFOL FOR (SEMI-)ELECTIVE INTUBATION IN NEONATES

Introduction The optimal propofol dose and its safety for procedural sedation in neonates is controversial. The aim of this study was to perform a prospective propofol dose-finding study with pharmacodynamic (PD) assessment during (semi-)elective intubation in neonates. Methods Patients were stratified in 4 groups by postmenstrual and postnatal age. The first patient in each group received 1 mg/kg intravenous propofol bolus. Dosing for the next patient was determined using the up-and-down method. Propofol ED50 doses [effective dose for successful intubation (as well as extubation in INSURE -intubation, surfactant, extubation-cases) in 50% of patients] were calculated, with simultaneous assessment of clinical scores, continuous vital sign monitoring and propofol blood concentrations (3 and 12h after dosing). Results Thirty-five neonates (weight 540–3290 g) were included. Using initial and total propofol dose ranges of 0.5–2 and 0.5–4.5 mg/kg respectively, median propofol ED50 range for preterm neonates <10 days was 0.480–1.287 mg/kg. Clinical recovery was not attained at the end of the scoring period (21 minutes). A median decrease in mean arterial blood pressure (MABP) between −29.41% and −39.09% from baseline, and a short-lasting decrease in peripheral (SaO2) and regional cerebral (rScO2) oxygen saturation was documented. Variability in MABP, SaO2 and rScO2 was not explained by weight, age or propofol concentrations. Conclusions Low propofol doses sufficiently sedate neonates for intubation, but clinical recovery takes time and is accompanied by permissive hypotension. Propofol ED50 doses are provided and need integration in a prospective validation approach. Finally, feasibility of continuous monitoring for neonatal pharmacodynamic research was demonstrated.