Mebane H, Turnbach ME, Randich A. Open in a separate window Figure 1 Structure of GSK269984A GSK269984A has been shown to possess analgesic efficacy in models of inflammation [16]. In the rat complete Freund’s adjuvant (CFA) model of inflammatory pain, orally administered GSK269984A produced a dose-dependent reversal of hypersensitivity (E3 mg kg?1, orally) in a rat model of chronic inflammatory joint pain [18] and full reversal of hypersensitivity (equivalent to rofecoxib) was achieved at 10 mg kg?1. In drug metabolism and pharmacokinetic (DMPK) studies the metabolic stability of GSK269984A was profiled using microsomes derived from mouse, rat, dog, monkey and human liver. This revealed a low intrinsic clearance (CLi) across all species ( 0.7 ml min?1 g?1 liver) [16]. Further studies, undertaken with hepatocytes and S9 fraction to include phase 2 metabolic pathways, similarly provided evidence of low CLi and low metabolic turnover across all tested species. Interestingly, the CLi data for GSK269984A were found not to predict the PK profile observed across three pre-clinical species. These data showed GSK269984A blood clearance (CLb) to be high in the monkey and moderate in the rat and dog, which was reflected in the respective species terminal half-lives [16]. Oral bioavailability (relatively high in the rat (94%), moderate in the dog (39%) and low in the monkey (7%) [16]) was therefore considered to be limited by first-pass hepatic extraction in each of the species. Whilst the solubility and permeability data suggested that the compound should diffuse well across cell membranes, and there was no evidence that GSK269984A is a P-glycoprotein (P-gp) substrate, the steady-state volume of distribution (2.1 l ITM2B kg?1) and similar in the dog and monkey (0.6 l kg?1) even though the plasma protein binding was comparable across all three species (99.9% (rat and human). 99.8% (dog) and 99.7% (monkey). This would suggest that, in the rat at least, drug transporters (other than P-gp) may be involved in the distribution of GSK269984A. Further profiling of GSK269984A revealed a low potential for inhibition of CYP1A2, 2C9, Napabucasin 2C19, 2D6 and 3A4 (I1 m). Preliminary studies to investigate GSK269984A biotransformation (rat and human liver S9 fraction, as well as rat, monkey and human hepatocytes), revealed the formation of an acyl glucuronide in all species. For the purposes of GSK269984A clinical dose predictions, three alternative scenarios were considered Napabucasin (Table 1). Collectively, they raised considerable uncertainty with respect to the likely human PK profile. Firstly, based on simple allometric scaling of PK parameters obtained from rat, dog and monkey, the human PK predictions indicated a high CLb (90% of liver blood flow), low 0.3 l kg?1), low oral bioavailability (10%), and a short terminal half-life (0.2 h). Using these predictions, it was estimated that a daily dose of 11 g GSK269984A would be required to maintain efficacious plasma concentrations, and would necessitate an unacceptably frequent dosing regimen (450 mg h?1) to accommodate the short terminal half-life. Table 1 Predicted PK parameters and dose for GSK269984A; comparison with known PK parameters for marketed NSAIDs log W [51]. Predicted terminal half-life (20% liver blood flow), with a larger 2.1 l kg?1), a longer terminal half-life of 7.8 h and an oral bioavailability of 80%. Using these PK estimates, a single once daily dose of 325 mg GSK269984A would be sufficient to maintain efficacious concentrations. A third scenario was based on reference to the known human PK parameters for commonly available nonsteroidal anti-inflammatory drugs (NSAIDs) containing the carboxylic acid moiety (akin to GSK269984A), exemplified by the propionic acid (ibuprofen, naproxen) Napabucasin and acetic acid class (diclofenac, indomethacin) ([19] and see Table 1). Such compounds have relatively Napabucasin low clearance and volumes of distribution, but good oral bioavailability and half-lives that would support an acceptable dosing regimen. If GSK269984A is assumed to have a comparably low human CLb (20% liver blood flow), low 0.2 l kg?1), and a moderate 2 h) and oral bioavailability (50%), then a total daily dose of 550 mg of GSK269984A can be predicted to maintain efficacious concentrations. Furthermore, a half-life of Napabucasin 2 h would support a dosage regimen of 200 mg three times a day. In view of the absence of an correlation of CLb across the pre-clinical species, the use of simple allometric scaling to facilitate human PK predictions for GSK269984A was recognized to be of questionable value, and posed a significant risk.