ABSTRACT:To predict volume of distribution at steady-state (V ss ), empirical (e.g., allometry) and mechanistic (using physicochemical property data and plasma protein binding) methods have been used. None of these approaches has been able to predict V ss accurately for the total compliment of a wide range of drugs. Therefore, alternative approaches would be of value. This study evaluates the utility of in vitro nonspecific tissue-binding measurements in predicting V ss for a wide range of drugs in rats. Literature as well as proprietary compounds were studied. It was found that in vitro tissue-binding measurements combined with calculated effects of the pH partition hypothesis often predict V ss more accurately than other available mechanistic methods and that this approach can compliment existing methods. The V ss values for some compounds were not accurately predicted using either nonspecific tissue-binding experiments or other available mechanistic methods. The V ss for these drugs may not be describable by nonspecific tissue binding alone; there may be significant specific components to the mechanism of distribution for these drugs, such as pH-dependent uptake into lysosomes (primarily strongly basic drugs), active transport, and/or enterohepatic recirculation. A lack of prediction for certain drugs warrants further investigation into these mechanisms and their application to more accurate prediction of V ss by mechanistic means.The apparent volume of distribution at steady state (V ss ) is one of the primary descriptive terms derived from pharmacokinetic (PK) analysis of the concentration-time profile after intravenous administration of drugs.
Human genetic evidence has identified the voltage-gated sodium channel Na V 1.7 as an attractive target for the treatment of pain. We initially identified naphthalene sulfonamide 3 as a potent and selective inhibitor of Na V 1.7. Optimization to reduce biliary clearance by balancing hydrophilicity and hydrophobicity (Log D) while maintaining Na V 1.7 potency led to the identification of quinazoline 16 (AM-2099). Compound 16 demonstrated a favorable pharmacokinetic profile in rat and dog and demonstrated dose-dependent reduction of histamine-induced scratching bouts in a mouse behavioral model following oral dosing. KEYWORDS: Sodium channel, Na V 1.7, Na V 1.5, pain, histamine scratching model H uman genetics has implicated the voltage-gated sodium channel Na V 1.7, which is expressed in nociceptive sensory neurons in dorsal root ganglia (DRG), 1 as a compelling target for pain.2−4 The primary challenge associated with the development of Na V 1.7 inhibitors has historically been achieving selectivity over the other eight Na V isoforms. These isoforms are differentially expressed throughout the body, but inhibition of Na V 1.5, which is expressed in cardiac tissue, is of particular concern as it has been shown to prolong the cardiac QRS wave in humans. 5,6 Previous efforts, including our own, have met with limited success. 4 Here we report the characterization, structure−activity relationship (SAR) and optimization of a series of sulfonamide-derived Na V 1.7 inhibitors. These efforts delivered an isoform-selective compound that was effective in a histamine-induced scratching model that is representative of Na V 1.7 target engagement. Recently Pfizer and Icagen described a series of heteroarylsulfonamide Na V 1.7 inhibitors with high levels of selectivity over Na V 1.5. 7−9 These results were reproduced by our group and are exemplified by compound 1 (Figure 1A). The lack of Na V 1.5 activity was noteworthy, and we envisioned this as a good starting point for our own lead optimization efforts, which would initially be aimed at addressing some of the liabilities and shortcomings associated with this class of compounds. Namely, this series suffered from low passive permeability and high clearance in rodents. We believed constraining the linker within a bicyclic core such as indole 2 or naphthalene 3 would afford a similar conformation and potentially help address the pharmacokinetic liabilities of this class of compounds. An overlay of global minima conformations of compounds 1, 2, and 3 supported this hypothesis ( Figure 1B), 10 and we were pleased to find that 2 and 3 were potent Na V 1.7 inhibitors and showed greater than 200-fold selectivity over Na V 1.5 ( Figure 1C). 11 In general, analogues in the naphthalene series demonstrated superior Na V 1.7 inhibition compared to the corresponding indole analogues, thus the naphthalene scaffold was chosen for further optimization. Additional profiling showed 3 also suffered from high clearance; however, we believed that 3 represented a promising starting poin...
ABSTRACT:In vitro intrinsic metabolic clearance (CL int ) is used routinely for compound selection in drug discovery; however, in vitro CL int often underpredicts in vivo clearance (CL). Forty-one proprietary compounds and 16 marketed drugs were selected to determine whether permeability and efflux status could influence the predictability of CL from in vitro CL int obtained from liver microsomal and hepatocyte incubations. For many of the proprietary compounds examined, rat CL was significantly underpredicted using the well stirred model incorporating both fraction of unbound drug in blood and fraction of unbound drug in the microsomal or hepatocyte incubation. Further analysis revealed that the accuracy of the prediction was differentiated by permeability and P-glycoprotein-(Pgp) and mouse breast cancer resistance protein (mBcrp)-mediated efflux. For proprietary compounds with passive permeability greater than 5 ؋ 10 ؊6 cm/s and efflux ratios less than 5 in both P-gp-and mBcrp-expressing cells, CL int provided reasonable prediction. The average -fold error (AFE) was 1.8 for rat liver microsomes (RLMs) and 2.3 for rat hepatocytes. In contrast, CL was dramatically underpredicted for compounds with passive permeability less than 5 ؋ 10 ؊6 cm/s; AFEs of 54.4 and 29.2 were observed for RLM and rat hepatocytes, respectively. In vivo CL was also underpredicted for compounds that were good efflux substrates (permeability >5 ؋ 10 ؊6 cm/s). The AFEs were 7.4 and 8.1for RLM and rat hepatocytes, respectively. A similar relationship between permeability, efflux status, and human CL prediction reported in the literature was observed for 16 marketed drugs. These data show that permeability and efflux status are determinants for the predictability of CL from in vitro metabolic CL int .In vitro metabolic clearance (CL int ) derived from liver microsomal incubations is used routinely to aid compound selection in drug discovery. Hepatocytes, which possess intact cellular membranes, complete enzymatic systems, and cofactors, have also become a common option for determination of CL int . Successful prediction of in vivo clearance (CL) from CL int in liver microsomes or hepatocytes has been reported for some compounds. Obach (1999) reported that CL was predicted within a 2-fold error for 16 of 29 compounds from CL int in human liver microsomes using the well stirred model incorporating both fraction of unbound drug in blood (fu b ) and fraction of unbound drug in the microsomal or hepatocyte incubation (fu inc ).Likewise, CL int from human hepatocytes provided prediction within a 2-fold error for 11 of 37 (Brown et al., 2007) or 23 of 56 compounds (Riley et al., 2005). However, a tendency toward underprediction was observed for many drugs (Obach, 1999;Ito and Houston, 2004;Riley et al., 2005;Brown et al., 2007;Stringer et al., 2008). Marked underprediction of CL from in vitro metabolic CL int in humans and/or animals has been reported for compounds such as montelukast, troglitazone, and quinotolast (Naritomi et al., 2001(Naritomi e...
This study was designed to characterize breast cancer resistance protein (Bcrp) knockout Abcg2(Ϫ/Ϫ) rats and assess the effect of ATP-binding cassette subfamily G member 2 (Abcg2) deletion on the excretion and pharmacokinetic properties of probe substrates. Deletion of the target gene in the Abcg2(Ϫ/Ϫ) rats was confirmed, whereas gene expression was unaffected for most of the other transporters and metabolizing enzymes. Biliary excretion of nitrofurantoin, sulfasalazine, and compound A [2-(5-methoxy-2-((2-methyl-1,3-benzothiazol-6-yl)amino)-4-pyridinyl)-1,5,6,7-tetrahydro-4H-pyrrolo[3,2-c]pyridin-4-one] accounted for 1.5, 48, and 48% of the dose in the Abcg2(ϩ/ϩ) rats, respectively, whereas it was decreased by 70 to 90% in the Abcg2(Ϫ/Ϫ) rats. Urinary excretion of nitrofurantoin, a significant elimination pathway, was unaffected in the Abcg2(Ϫ/Ϫ) rats, whereas renal clearance of sulfasalazine, a minor elimination pathway, was reduced by Ͼ90%. Urinary excretion of compound A was minimal. Systemic clearance in the Abcg2(Ϫ/Ϫ) rats decreased 22, 43 (p Ͻ 0.05), and 57%, respectively, for nitrofurantoin, sulfasalazine, and compound A administered at 1 mg/kg and 27% for compound A administered at 5 mg/kg. Oral absorption of nitrofurantoin, a compound with high aqueous solubility and good permeability, was not limited by Bcrp. In contrast, the absence of Bcrp led to a 33-and 11-fold increase in oral exposure of sulfasalazine and compound A, respectively. These data show that Bcrp plays a crucial role in biliary excretion of these probe substrates and has differential effects on systemic clearance and oral absorption in rats depending on clearance mechanisms and compound properties. The Abcg2(Ϫ/Ϫ) rat is a useful model for understanding the role of Bcrp in elimination and oral absorption.
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