Estimating human ADME properties, pharmacokinetic parameters and likely clinical dose in drug discovery.

Introduction: Prediction of human absorption, distribution, metabolism, and excretion (ADME) properties, therapeutic dose and exposure has become an integral part of compound optimization in discovery. Incorporation of drug metabolism and pharmacokinetics into discovery projects has largely tempered historical drug failure due to sub-optimal ADME. In the current era, inadequate safety and efficacy are leading culprits for attrition; both of which are dependent upon drug exposure. Therefore, prediction of human pharmacokinetics (PK) and dose are core components of de-risking strategies in discovery. Areas covered: The authors provide an overview of human dose prediction methods and present a toolbox of PK parameter prediction models with a proposed framework for a consensus approach valid throughout the discovery value chain. Mechanistic considerations and indicators for their application are discussed which may impact the dose prediction approach. Examples are provided to illustrate how implementation of the proposed strategy throughout discovery can assist project progression. Expert opinion: Anticipation of human ADME, therapeutic dose and exposure must be deliberated throughout drug discovery from virtual/initial synthesis where key properties are considered and similar molecules ranked, into development where advanced compounds can be subject to prediction with greater mechanistic understanding and data-driven model selection.

Application of an in vitro OAT assay in drug design and optimization of renal clearance.

1. Optimization of renal clearance is a complex balance between passive and active processes mediated by renal transporters. This work aimed to characterize the interaction of a series of compounds with rat and human organic anion transporters (OATs) and develop quantitative structure-activity relationships (QSARs) to optimize renal clearance. 2. In vitro inhibition assays were established for human OAT1 and rat Oat3 and rat in vivo renal clearance was obtained. Statistically significant quantitative relationships were explored between the compounds' physical properties, their affinity for OAT1 and oat3 and the inter-relationship with unbound renal clearance (URC) in rat. 3. Many of the compounds were actively secreted and in vitro analysis demonstrated that these were ligands for rat and human OAT transporters (IC50 values ranging from <1 to >100 µM). Application of resultant QSAR models reduced renal clearance in the rat from 24 to <0.1 ml/min/kg. Data analysis indicated that the properties associated with increasing affinity at OATs are the same as those associated with reducing URC but orthogonal in nature. 4. This study has demonstrated that OAT inhibition data and QSAR models can be successfully used to optimize rat renal clearance in vivo and provide confidence of translation to humans.

Use of hepatocytes to assess the contribution of hepatic uptake to clearance in vivo.

The wealth of information that has emerged in recent years detailing the substrate specificity of hepatic transporters necessitates an investigation into their potential role in drug elimination. Therefore, an assay in which the loss of parent compound from the incubation medium into hepatocytes ("media loss" assay) was developed to assess the impact of hepatic uptake on unbound drug intrinsic clearance in vivo (CL(int ub in vivo)). Studies using conventional hepatocyte incubations for a subset of 36 AstraZeneca new chemical entities (NCEs) resulted in a poor projection of CL(int ub in vivo) (r2 = 0.25, p = 0.002, average fold error = 57). This significant underestimation of CL(int ub in vivo) suggested that metabolism was not the dominant clearance mechanism for the majority of compounds examined. However, CL(int ub in vivo) was described well for this dataset using an initial compound "disappearance" CL(int) obtained from media loss assays (r2 = 0.72, p = 6.3 x 10(-11), average fold error = 3). Subsequent studies, using this method for the same 36 NCEs, suggested that the active uptake into human hepatocytes was generally slower (3-fold on average) than that observed with rat hepatocytes. The accurate prediction of human CL(int ub in vivo) (within 4-fold) for the marketed drug transporter substrates montelukast, bosentan, atorvastatin, and pravastatin confirmed further the utility of this assay. This work has described a simple method, amenable for use within a drug discovery setting, for predicting the in vivo clearance of drugs with significant hepatic uptake.