Effect of P-glycoprotein-mediated efflux on cerebrospinal fluid concentrations in rhesus monkeys

Calculation of an Apical Efflux Ratio from P-Glycoprotein (P-gp) In Vitro Transport Experiments Shows an Improved Correlation with In Vivo Cerebrospinal Fluid Measurements in Rats: Impact on P-gp Screening and Compound Optimization

P-glycoprotein (P-gp) is a major blood-brain barrier (BBB) efflux transporter. In vitro approaches, including bidirectional efflux ratio (ER), are used to measure P-gp-mediated transport, but findings can be inconsistent across models. We propose a novel, more physiologically relevant, in vitro model: unidirectional apical efflux ratio (AP-ER)-a ratio of permeability rates at the apical side of the BBB with and without P-gp inhibitor. To test our approach, ER and AP-ER were calculated for 3227 structurally diverse compounds in porcine kidney epithelial cells (LLC-PK1) overexpressing human or mouse P-gp and classified based on their passive transcellular P-gp permeability or charged properties. In vivo rat infusion studies were performed for selected compounds with high ER but low AP-ER. One-third of the 3227 compounds had bidirectional ER that was much higher than AP-ER; very few had AP-ER higher than ER. Compounds with a large difference between AP-ER and ER were typically basic compounds with low-to-medium passive permeability and high lipophilicity and/or amphiphilicity, leading to strong membrane binding. Outcomes in the human model were similar to those in mice, suggesting AP-ER/ER ratios may be conserved for at least two species. AP-ER predicted measured cerebrospinal fluid (CSF) concentration better than ER for the five compounds tested in our in vivo rat infusion studies. We report superior estimations of the CSF concentrations of the compounds when based on less resource-intensive AP-ER versus classic ER. Better understanding of the properties leading to high P-gp-mediated efflux in vivo could support more efficient brain-penetrant compound screening and optimization. SIGNIFICANCE STATEMENT: To address inconsistencies associated with the historical, bidirectional efflux ratio (ER) calculation of P-glycoprotein-mediated transport, we propose to use the novel, more physiologically relevant, unidirectional apical efflux ratio (AP-ER) model. In vitro experiments suggested that compounds with strong membrane binding showed the largest difference between AP-ER and ER, and in vivo infusion studies showed that AP-ER predicted cerebrospinal fluid concentrations of compounds better than ER; outcomes in the human model were similar to those in mice.

A Practical Perspective on the Evaluation of Small Molecule CNS Penetration in Drug Discovery

The separation of the brain from blood by the blood-brain barrier and the bloodcerebrospinal fluid (CSF) barrier poses unique challenges for the discovery and development of drugs targeting the central nervous system (CNS). This review will describe the role of transporters in CNS penetration and examine the relationship between unbound brain (Cu-brain) and unbound plasma (Cu-plasma) or CSF (CCSF) concentration. Published data demonstrate that the relationship between Cu-brain and Cu-plasma or CCSF can be affected by transporter status and passive permeability of a drug and CCSF may not be a reliable surrogate for CNS penetration. Indeed, CCSF usually over-estimates Cu-brain for efflux substrates and it provides no additional value over Cu-plasma as the surrogate of Cu-brain for highly permeable non-efflux substrates. A strategy described here for the evaluation of CNS penetration is to use in vitro permeability, P-glycoprotein (Pgp) and breast cancer resistance protein efflux assays and Cu-brain/Cu-plasma in preclinical species. Cu-plasma should be used as the surrogate of Cu-brain for highly permeable non-efflux substrates with no evidence of impaired distribution into the brain. When drug penetration into the brain is impaired, we recommend using (total brain concentration * unbound fraction in the brain) as Cu-brain in preclinical species or Cu-plasma/in vitro Pgp efflux ratio if Pgp is the major limiting mechanism for brain penetration.

Prediction of Human Brain Penetration of P-glycoprotein and Breast Cancer Resistance Protein Substrates Using In Vitro Transporter Studies and Animal Models

Four P-glycoprotein (P-gp) and breast cancer resistance protein (BCRP) substrates with human cerebrospinal fluid (CSF) concentrations and preclinical neuropharmacokinetics were used to assess in vitro-in vivo extrapolation of brain penetration in preclinical species and the ability to predict human brain penetration. Unbound brain (Cb,u), unbound plasma (Cp,u), and CSF compound concentrations (CCSF) were measured in rats and nonhuman primates (NHPs), and the unbound partition coefficients (Cb,u/Cp,u and CCSF/Cp,u) were used to assess brain penetration. The results indicated that for P-gp and BCRP dual substrates, brain penetration was severally impaired in all species. In comparison, for P-gp substrates that are weak or non-BCRP substrates, improved brain penetration was observed in NHPs and humans than in rats. Overall, NHP appears to be more predictive of human brain penetration for P-gp substrates with weak or no interaction with BCRP than rat. Although CCSF does not quantitatively correspond to Cb,u for efflux transporter substrates, it is mostly within 3-fold higher of Cb,u in rat and NHP, suggesting that CCSF can be used as a surrogate for Cb,u. Taken together, a holistic approach including both in vitro transporter and in vivo neuropharmacokinetics data enables a better estimation of human brain penetration of P-gp/BCRP substrates.

Restricting CNS penetration of drugs to minimise adverse events: role of drug transporters

Some drug discovery approaches can benefit from restricting the access of compounds to the central nervous system (CNS) to minimise the risk of side-effects. Designing compounds that act as substrates for efflux transporters in the blood–brain barrier can achieve CNS restriction without significantly impairing absorption in the intestine. In vitro assays can be deployed to optimise a balance between passive permeability and active efflux via the ABC family transporters P-glycoprotein (P-gp, ABCB1) and Breast Cancer Resistance Protein (BCRP, ABCG2) whilst in vivo estimates of distribution of unbound concentrations of drug are needed to understand pharmacologically relevant exposure in peripheral and central compartments. This strategy can deliver significant CNS restriction whilst retaining good oral bioavailability, cell penetration and pharmacological activity. The possible risks of targeting P-gp and BCRP in orally delivered drugs are discussed.

Structure-based design of β-site APP cleaving enzyme 1 (BACE1) inhibitors for the treatment of Alzheimer's disease

The amyloid hypothesis asserts that excess production or reduced clearance of the amyloid-β (Aβ) peptides in the brain initiates a sequence of events that ultimately lead to Alzheimer's disease and dementia. The Aβ hypothesis has identified BACE1 as a therapeutic target to treat Alzheimer's and led to medicinal chemistry efforts to design its inhibitors both in the pharmaceutical industry and in academia. This review summarizes two distinct categories of inhibitors designed based on conformational states of "closed" and "open" forms of the enzyme. In each category the inhibitors are classified based on the core catalytic interaction group or the aspartyl binding motif (ABM). This review covers the description of inhibitors in each ABM class with X-ray crystal structures of key compounds, their binding modes, related structure-activity data highlighting potency advances, and additional properties such as selectivity profile, P-gp efflux, pharmacokinetic, and pharmacodynamic data.

Use of in vivo animal models to assess pharmacokinetic drug-drug interactions

Animal models are used commonly in various stages of drug discovery and development to aid in the prospective assessment of drug-drug interaction (DDI) potential and the understanding of the underlying mechanism for DDI of a drug candidate. In vivo assessments in an appropriate animal model can be very valuable, when used in combination with in vitro systems, to help verify in vivo relevance of the in vitro animal-based results, and thus substantiate the extrapolation of in vitro human data to clinical outcomes. From a pharmacokinetic standpoint, a key consideration for rational selection of an animal model is based on broad similarities to humans in important physiological and biochemical parameters governing drug absorption, distribution, metabolism or excretion (ADME) processes in question for both the perpetrator and victim drugs. Equally critical are specific in vitro and/or in vivo experiments to demonstrate those similarities, usually both qualitative and quantitative, in the ADME properties/processes under investigation. In this review, theoretical basis and specific examples are presented to illustrate the utility of the animal models in assessing the potential and understanding the mechanisms of DDIs.