Background: Transmembrane domain interactions in gap junction channels are poorly understood. Results: Tryptophan substitution experiments involving all four TM domains of Cx32 revealed tight packing. Conclusion: After modeling, tight packing was found to occur in the midregion. Pore-facing residues were highly sensitive to substitution, whereas lipid-facing residues were variably tolerant. Significance: Connexin-based channels are more densely packed than their innexin-based counterparts.

Aim: Determination of paritaprevir and ritonavir in rat liver tissue samples. Results: We successfully validated a UPLC-MS/MS method to measure paritaprevir and ritonavir in rat liver using deuterated internal standards (d8-paritapervir and d6-ritonavir). The method is linear from 20 to 20,000 and 5 to 10,000 pg on column for paritaprevir and ritonavir, respectively, and is normalized per milligram tissue. Interday and intraday variability ranged from 0.591 to 5.33% and accuracy ranged from -6.68 to 10.1% for quality control samples. The method was then applied to the measurement of paritaprevir and ritonavir in rat liver tissue samples from a pilot study. Conclusion: The validated method is suitable for measurement of paritaprevir and ritonavir within rat liver tissue samples for PK studies.

The liver is crucial to pharmacology, yet substantial knowledge gaps exist in the understanding of its basic pharmacologic processes. An improved understanding for humans requires reliable and reproducible liver sampling methods. We compared liver concentrations of paritaprevir and ritonavir in rats by using samples collected by fine-needle aspiration (FNA), core needle biopsy (CNB), and surgical resection. Thirteen Sprague-Dawley rats were evaluated, nine of which received paritaprevir/ritonavir at 30/20 mg/kg of body weight by oral gavage daily for 4 or 5 days. Drug concentrations were measured using liquid chromatography-tandem mass spectrometry on samples collected via FNA (21G needle) with 1, 3, or 5 passes (FNA 1 , FNA 3 , and FNA 5 ); via CNB (16G needle); and via surgical resection. Drug concentrations in plasma were also assessed. Analyses included noncompartmental pharmacokinetic analysis and use of Bland-Altman techniques. All liver tissue samples had higher paritaprevir and ritonavir concentrations than those in plasma. Resected samples, considered the benchmark measure, resulted in estimations of the highest values for the pharmacokinetic parameters of exposure (maximum concentration of drug in serum [C max ] and area under the concentration-time curve from 0 to 24 h [AUC 0 -24 ]) for paritaprevir and ritonavir. Bland-Altman analyses showed that the best agreement occurred between tissue resection and CNB, with 15% bias, followed by FNA 3 and FNA 5 , with 18% bias, and FNA 1 and FNA 3 , with a 22% bias for paritaprevir. Paritaprevir and ritonavir are highly concentrated in rat liver. Further research is needed to validate FNA sampling for humans, with the possible derivation and application of correction factors for drug concentration measurements.KEYWORDS hepatocyte isolation, liver biopsy, liver drug concentration, liver fineneedle aspiration T he liver is the primary site of drug metabolism, and hepatocytes are principally responsible for hepatic metabolic capacity as a result of the abundance of phase I and II drug-metabolizing enzymes and transporters (1-3). Due to the liver's fundamental role in drug metabolism, understanding the relationship between intrahepatic and plasma drug concentrations is potentially important for predicting drug-drug interactions, hepatotoxicity, and therapeutic efficacy. To date, most methods used to determine intrahepatic drug concentrations have relied on in vitro techniques (e.g., microsomal experiments and sandwich cultures) that work well as isolated systems. Inaccuracies develop, however, when these models are used for other purposes, as they