Differential role of P-glycoprotein and breast cancer resistance protein in drug distribution into brain, CSF and peripheral nerve tissues in rats.

1. This study was designed to evaluate how the absence of P-glycoprotein (Pgp, Mdr1a), breast cancer-resistance protein (Bcrp, Abcg2) or both affects drug distribution into sciatic nerves, brain and cerebrospinal fluid (CSF) in rats. 2. Pgp substrate (loperamide), BCRP substrates (dantrolene and proprietary compound X) and dual substrates (imatinib and proprietary compound Y) were well distributed into sciatic nerves with comparable nerve to plasma concentration ratios between wild-type and knockout (KO) rats. 3. Brain exposure increased substantially in Mdr1a(-/-) rats for loperamide and in Mdr1a(-/-)/Abcg2(-/-) rats for imatinib and compound Y, but minimally to modestly in Abcg2(-/-) rats for dantrolene and compound X. The deletion of Mdr1a or Abcg2 alone had little effect on brain distribution of compound Y. 4. While CSF to unbound brain concentration ratio remained ≥3 in the KO animals for dantrolene, compounds X and Y, it was reduced to 1 in the Mdr1a(-/-)/Abcg2(-/-) rats for imatinib. 5. The data indicate that Pgp and Bcrp do not play significant roles in drug distribution into peripheral nerve tissues in rats, while working in concert to regulate brain penetration. Our results further support that CSF concentration may not be a good surrogate for unbound brain concentration of efflux substrates.

Identification of a potent, state-dependent inhibitor of Nav1.7 with oral efficacy in the formalin model of persistent pain.

Clinical human genetic studies have recently identified the tetrodotoxin (TTX) sensitive neuronal voltage gated sodium channel Nav1.7 (SCN9A) as a critical mediator of pain sensitization. Herein, we report structure-activity relationships for a novel series of 2,4-diaminotriazines that inhibit hNav1.7. Optimization efforts culminated in compound 52, which demonstrated pharmacokinetic properties appropriate for in vivo testing in rats. The binding site of compound 52 on Nav1.7 was determined to be distinct from that of local anesthetics. Compound 52 inhibited tetrodotoxin-sensitive sodium channels recorded from rat sensory neurons and exhibited modest selectivity against the hERG potassium channel and against cloned and native tetrodotoxin-resistant sodium channels. Upon oral administration to rats, compound 52 produced dose- and exposure-dependent efficacy in the formalin model of pain.

In vitro and in vivo pharmacokinetic characterizations of AMG 900, an orally bioavailable small molecule inhibitor of aurora kinases.

AMG 900 is a small molecule being developed as an orally administered, highly potent, and selective pan-aurora kinase inhibitor. The aim of the investigations was to characterize in vitro and in vivo pharmacokinetic (PK) properties of AMG 900 in preclinical species. AMG 900 was rapidly metabolized in liver microsomes and highly bound to plasma proteins in the species tested. It was a weak Pgp substrate with good passive permeability. AMG 900 exhibited a low-to-moderate clearance and a small volume of distribution. Its terminal elimination half-life ranged from 0.6 to 2.4 h. AMG 900 was well-absorbed in fasted animals with an oral bioavailability of 31% to 107%. Food intake had an effect on rate (rats) or extent (dogs) of AMG 900 oral absorption. The clearance and volume of distribution at steady state in humans were predicted to be 27.3 mL/h/kg and 93.9 mL/kg, respectively. AMG 900 exhibited acceptable PK properties in preclinical species and was predicted to have low clearance in humans. AMG 900 is currently in Phase I clinical testing as a treatment for solid tumours. Preliminary human PK results appear to be consistent with the predictions.

Relationship between passive permeability, efflux, and predictability of clearance from in vitro metabolic intrinsic clearance.

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- (P-gp) and mouse breast cancer resistance protein (mBcrp)-mediated efflux. For proprietary compounds with passive permeability greater than 5 x 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 x 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 x 10(-6) cm/s). The AFEs were 7.4 and 8.1 for 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).