Rupert P. Austin

A unified model for predicting human hepatic, metabolic clearance from in vitro intrinsic clearance data in hepatocytes and microsomes

The aim of this study was to evaluate a unified method for predicting human in vivo intrinsic clearance (CLint, in vivo) and hepatic clearance (CLh) from in vitro data in hepatocytes and microsomes by applying the unbound fraction in blood (fub) and in vitro incubations (fuinc). Human CLint, in vivo was projected using in vitro data together with biological scaling factors and compared with the unbound intrinsic clearance (CLint, ub, in vivo) estimated from clinical data using liver models with and without the various fu terms. For incubations conducted with fetal calf serum (n = 14), the observed CLint, in vivo was modeled well assuming fuinc and fub were equivalent. CLint, ub, in vivo was predicted best using both fub and fuinc for other hepatocyte data (n = 56; r2 = 0.78, p = 3.3 × 10–19, average fold error = 5.2). A similar model for CLint, ub, in vivo was established for microsomal data (n = 37; r2 = 0.77, p = 1.2 × 10–12, average fold error = 6.1). Using the model for CLint, ub, in vivo (including a further empirical scaling factor), the CLh in humans was also calculated according to the well stirred liver model for the most extensive dataset. CLint, in vivo and CLh were both predicted well using in vitro human data from several laboratories for acidic, basic, and neutral drugs. The direct use of this model using only in vitro human data to predict the metabolic component of CLh is attractive, as it does not require extra information from preclinical studies in animals.

A Method for Measuring the Lipophilicity of Compounds in Mixtures of 10

Lipophilicity is an important parameter for any potential drug candidate. Accurate and efficient lipophilicity measurements facilitate the development of high-quality predictive in silico models that support the design of future drugs. Lipophilicity estimates derived from the traditional 1-octanol/water shake flask techniques have been the most widely employed and are therefore the best understood. This technique can be considered to give a good measure of a compound’s lipophilicity, albeit slower and more labor intensive to run compared with some other methodologies. Herein is described and validated an efficient 1-octanol/water shake flask technique that has sufficient capacity to be run as a primary screen within the drug discovery process. This is achieved by the simultaneous measurement of the distribution coefficients of mixtures of up to 10 compounds using high-performance liquid chromatography and tandem mass spectrometry. Concerns regarding ion pair partitioning that could result in erroneous results due to interactions between compounds within a mixture are discussed.

Cathepsin C Inhibitors: Property Optimization and Identification of a Clinical Candidate

A lead generation and optimization program delivered the highly selective and potent CatC inhibitor 10 as an in vivo tool compound and potential development candidate. Structural studies were undertaken to generate SAR understanding.

A Highly Automated Assay for Determining the Aqueous Equilibrium Solubility of Drug Discovery Compounds

Aqueous solubility is an important physicochemical parameter for any potential drug candidate, and high-throughput kinetic assays are frequently used in drug discovery to give an estimate of a compounds aqueous solubility. However, the aqueous solubility data from an equilibrium (thermodynamic) shake-flask technique is considered more relevant, but is slower and more labor intensive to generate. A highly automated aqueous equilibrium solubility shake-flask technique is described and validated on a set of 15 marketed drugs, whose aqueous solubilities cover four orders of magnitude. The assay uses a Tecan Freedom Evo 200 liquid handling robot (Tecan Group Ltd., Männerdorf, Switzerland) with integrated appliances for the transportation, decapping and recapping, and centrifugation of sample tubes. These bespoke automation solutions help overcome the labor intensive steps associated with performing conventional, gold standard, aqueous equilibrium solubility shake-flask measurements, enabling the assay to be used as a primary-wave drug discovery screen.

A kinetic method for the determination of plasma protein binding of compounds unstable in plasma: Specific application to enalapril

Traditional methods for the determination of plasma protein binding (PPB), such as equilibrium dialysis and ultrafiltration, normally operate on a timescale ranging from tens of minutes to several hours and are not suitable for measuring compounds that have significant chemical degradation on this timescale. One such compound is enalapril. Although stable in human plasma enalapril is subject to rapid esterase-catalyzed hydrolysis in rat plasma. A method has been developed which allows the extent of rat PPB of enalapril to be determined from initial rates kinetics of the adsorption of the unstable compound to dextran coated charcoal (DCC). The method has been applied to stable compounds, and the results are consistent with those from traditional equilibrium dialysis experiments. The experimental method is simple to run, requires no specialized equipment, and can potentially be applied to other compounds that show instability in plasma where traditional experimental techniques are unsuitable.

Discovery and evaluation of a novel monocyclic series of CXCR2 antagonists.

Antagonism of the chemokine receptor CXCR2 has been proposed as a strategy for the treatment of inflammatory diseases such as arthritis, chronic obstructive pulmonary disease and asthma. Earlier series of bicyclic CXCR2 antagonists discovered at AstraZeneca were shown to have low solubility and poor oral bioavailability. In this Letter we describe the design, synthesis and characterisation of a new series of monocyclic CXCR2 antagonists with improved solubility and good pharmacokinetic profiles.

Unusual steric effects in sulfonyl transfer reactions

The hydrolysis of N-acyl-β-sultams generally occurs with ring opening and S–N fission in contrast to the C–N fission observed in analogous acyclic N-acyl sulfonamides. Similar to other β-sultams, the N-acyl derivatives are at least 106 more reactive than N-acylsulfonamides. However, the α-substituted 4-isopropylidene β-sultam is relatively unreactive and undergoes alkaline hydrolysis with C–N fission leaving the strained 4-membered β-sultam ring intact. This reduction in reactivity is shown to be due to steric strain introduced in the transition state for attack at the sulfonyl centre. (Z)-4-Ethylidene-β-sultam shows similar behaviour with preferential C–N fission whereas the (E)-4-ethylidene isomer and 4-isopropyl-β-sultam revert to hydrolytic ring opening with S–N fission.

Survival of the Fittest: Time-To-Event Modeling of Crystallization of Amorphous Poorly Soluble Drugs

The objective of this study was to gain a quantitative understanding of the link between physicochemical properties and long-term and time-censored amorphous stability of poorly water-soluble drugs using parametric time-to-event modeling. Previously published data on amorphous stability and physicochemical properties of 25 structurally diverse neutral, poorly soluble compounds were used. To describe the general shape of the survival curve (probability of event at time >t), Constant, Gompertz, and Weibull hazard functions and their linear combinations were tested. For a selected Weibull hazard base model, the effect of each physicochemical covariate was investigated, with combined influence of enthalpy of fusion (Hf) and molecular weight (Mr) showing the highest statistical significance. The covariate model was used to simulate survival curves and calculate the median survival time for different values of Hf and Mr. It was found that a decrease in Hf or an increase in Mr contribute to longer survival times. The derived model equation was validated against external data sets consisting of 11 compounds. It showed better predictive ability than a previously published multiple linear regression model incorporating Hf and Mr. The proposed Weibull covariate model may assist in faster and more cost-effective decision making in the pre-formulation phase of drug development, where compound properties and appropriate drug formulation strategies are investigated.

In Vitro Models for Plasma Binding and Tissue Storage

Measurement of the extent of plasma protein binding (PPB) and tissue disposition together with an understanding of the molecular properties that control binding is of fundamental importance in the drug discovery process. The extent of plasma protein binding of a candidate drug molecule has an influence on a number of critical areas such as the determination of safety margins in toxicology studies, the efficacy of a drug candidate, and its metabolism and pharmacokinetics. The measurement of tissue storage and a molecular understanding of tissue affinity has, historically, not been studied to the same extent as plasma protein binding. However, the now widespread use of physiologically-based pharmokinetics (PBPK) modeling has necessitated a greater understanding of tissue disposition and the properties controlling it.

Hydrolysis of a sulfonamide by a novel elimination mechanism generated by carbanion formation in the leaving group

The alkaline hydrolysis of N-alpha-methoxycarbonyl benzyl-beta-sultam occurs 10(3) times faster than the corresponding carboxylate and with rapid D-exchange at the alpha-carbon: the pH rate profile indicates pre-equilibirum CH ionisation and together with formation of benzoyl formate as a product this suggests a novel mechanism for hydrolysis.