Inhibition of cytochrome P450 enzymes is one of the major reasons for drug-drug interactions (DDI).
Therefore, detailed CYP inhibition profiles are now required for the registration of novel molecular entities. Using either liver microsomes or recombinant CYP preparations like Bactosomes, in combination with specific probe substrates we can assess inhibition of individual CYPs by your discovery compounds. As a reliable and cost-effective initial screening we recommend to assess inhibition potencies against the 5 most relevant CYP isoforms, i.e. CYP1A2, CYP2C9, CYP2C19, CYP2D6 and CYP3A4 using Cypex Bactosomes. Please see Weaver R. et al., 2003, DMD 31:7, p. 955-966 for reference. Please talk to us now about availability of other CYP isoform inhibition assays.
By introducing a 30 minute pre-incubation in the absence or presence of NADPH, we can distinguish between direct, time-dependent and metabolism-dependent inhibition. Our inhibition assays are generally performed using seven concentrations covering four orders of magnitude. Please feel free to inquire about more detailed investigations e.g. Ki/KI determination.
Direct inhibition, sometimes referred to as reversible inhibition, is the most basic form of enzyme inhibition. It is assessed by measurement of an enzyme activity in the presence of increasing concentration of inhibitor without a pre-incubation step. This is the classic in vitro inhibition assay which, if positive, yields an IC50-value as a result.
Time-dependent Inhibition (TDI)
TDI of CYPs refers to a change in enzyme inhibition during an in vitro incubation or dosing period in vivo. Riley, Grime and Weaver, Expert Opin Drug Metab Toxicol. 2007 Feb;3(1):51-66. TDI can be the cause of serious drug-drug interactions (DDI). TDI is usually measured by comparing potency (IC50) in direct inhibition to potency after a 30 minute pre-incubation in the absence of NADPH. In the case of a TDI the resulting IC50-value will be significantly lower than the IC50-value observed in the direct inhibition experiment.
Metabolism-dependent Inhibition (MDI)
MDI can be observed when the product of a metabolic reaction is a more potent inhibitor than the parent compound. MDI is measured by introducing a 30 minute pre-incubation in the presence of NADPH. In the case of a MDI the resulting IC50-value will be significantly lower than the IC50-value observed in the direct inhibition experiment.
Once MDI is detected, the apparent inactivation constant (KI) and the maximal inactivation rate constant (kinact) are determined to estimate the potential DDI risk.
Please talk to us now about the experimental design that best suits your needs.