In Vitro ADME Pharmacology
The assessment of the Absorption, Distribution, Metabolism, and Excretion (in vitro ADME Pharmacology) properties of compounds is a critical component supporting the development of compounds within discovery and development.
Properties of ADME Pharmacology
Absorption
Permeability Assays
Drug permeability is an important component in the oral absorption of drugs. The permeability of a drug across a membrane is dependent on the passive permeability as well as the susceptibility of the drug to efflux or uptake by drug transporter proteins. At Sygnature, several assays to study permeability are available:
Distribution
Protein Binding
Several in vitro binding assays are available including Plasma Protein Binding (PPB), Brain Tissue Binding and Blood Plasma Partitioning (BPP). There is flexibility to adapt protocols based on specific customer requirements.
Plasma Protein Binding
Drugs may bind to a wide variety of plasma proteins, including α-glycoprotein and albumin, and the degree of binding can impact the pharmacokinetic and pharmacodynamic parameters of a drug. Only the free drug in plasma is available for passive diffusion to extravascular or tissue sites, where it is available to elicit a pharmacological effect on the target.
The leading approach for assessing plasma protein binding is an assay utilising the Rapid Equilibrium Dialysis (RED) device. In this system, the impact of non-specific binding is minimised compared to other methods such as ultrafiltration and HT-dialysis, which are relatively slow to reach equilibrium. Sygnature’s Plasma Protein Binding assay uses RED to measure the percentage binding of a test compound to plasma proteins in human and preclinical species.
Brain Tissue Binding
The composition of the brain is very different from plasma. Plasma has twice as much protein as the brain and the brain has 20-fold more lipids than plasma. It is the brain unbound concentrations that dictate receptor occupancy and hence target engagement, and, for a compound with no active transport, this should equal the unbound plasma concentration at steady-state.
Sygnature’s Brain Tissue Binding assay uses RED to measure the percentage binding of a test compound to brain tissue. Brain tissue binding is species-independent, and as such, brain tissue binding of rats can be used to obtain binding of other species and strains in drug discovery.
Blood Plasma Partitioning
Knowledge of Blood Plasma Partitioning (BPP) of compounds enables a rational choice of appropriate biological fluid, either whole blood, plasma, or serum, for bioanalysis of in vivo PK samples, or to correct scaling of in vitro clearance data to in vivo.
Sygnature’s Blood Plasma Partitioning assay offers a specific and robust assay to measure these parameters in a variety of species using fresh blood from human and preclinical species.
Metabolism
Metabolic Stability
Metabolism of small molecules occurs primarily through the cytochrome P450 (CYP) family of enzymes located in the hepatic endoplasmic reticulum but can also occur through non-CYP enzymes, including esterases and Phase II glucuronosyl- and sulfotransferases for instance.
- Plasma stability
- Blood stability
- Metabolic stability in liver microsomes
- Metabolic stability in hepatocytes
Intestinal/Lung Tissue Metabolic Stability
In addition to the liver, metabolism can also occur in other tissues, including the GI tract, lung, skin and nasal mucosa, affecting absorption.
- First-pass metabolism that includes both intestinal and hepatic metabolism following oral dosing of a compound can impact oral bioavailability. If inhalation is the proposed route, lung metabolism can play a significant role. Sygnature’s intestinal metabolic stability or lung metabolic stability assay uses subcellular fractions such as microsomes from humans and all preclinical species to assess intestinal metabolism.
Drug Interaction Assays
CYP450 Inhibition
Potential drug-drug interactions between metabolising enzymes and investigational drugs are usually assessed during the optimisation process.
The contribution of a specific metabolizing enzyme to an investigational drug’s clearance is considered significant if the enzyme is responsible for > 25% of the drug’s elimination. It is recommended that phenotyping studies and CYP induction tests are conducted during candidate selection, or earlier where a particular liability is suspected, to investigate detrimental interactions.
CYP Induction
Cytochrome P450 (CYP) induction by a drug can accelerate the metabolism of a co-administered victim drug significantly, causing serious drug-drug interactions.
To date induction of CYP1A2, CYP2B6, CYP2C8, CYP2C9, CYP2C19, and CYP3A4 have been described. Whilst CYP1A2 induction involves the Aryl hydrocarbon receptor (AhR) and induction of CYP2B6 is mediated by the constitutive androstane receptor (CAR), the CYP2C isoforms and CYP3A4 are co-induced via the pregnane X receptor (PXR). Therefore, the current FDA guidelines suggest investigating the induction of CYP1A2, CYP2B6 and CYP3A4.
Traditionally, CYP induction has been measured via the assessment of enzyme activity in human hepatocytes. To address inter-individual variability, hepatocyte preparations from at least three donors had to be used.
HepaRG is an immortalised human hepatic cell line highly rated by scientists for its hepatocyte-like phenotype, e.g. its expression of drug-metabolising enzymes drug transporters and transcription factors.
Therefore, HepaRG provides an excellent, highly reproducible and cost-efficient system to study CYP induction during drug discovery or the early phases of drug development.
Determination of mRNA levels by quantitative revers-transcription polymerase chain reaction (qRT-PCR) has proven superior over enzyme activity because the latter could lead to false negative results when CYP induction was masked by simultaneous inhibition.
Excretion
With access to some of the most sensitive UPLC-MS instruments in the industry, such as 3 Sciex 5500 Qtrap and 3 Q Exactive Focus, analysis of bile, faeces or urine samples as well as tissue can be performed to quantify compound and metabolite levels.