Читать книгу Physiologically Based Pharmacokinetic (PBPK) Modeling and Simulations - Sheila Annie Peters - Страница 55

To evaluate a compound’s potential to be a victim of DDI, it is necessary to identify the specific enzymes involved in its metabolism. A common experimental approach to reaction phenotyping (Harper and Brassil, 2008; Zhang et al., 2009) is the use of cDNA‐expressed recombinant enzyme systems, in which the test compound is incubated with a panel of individually expressed human recombinant enzymes. A typical panel of CYP enzymes includes CYPs 1A2, 2A6, 2B6, 2C8, 2C9, 2C19, 2D6, 2E1, 3A4, and 3A5. Using the known CYP abundances^{60, 61}, the percentage contribution of individual CYPs to the overall oxidative metabolism of a drug candidate (f_m,CYP ) can be estimated:

(2.2)

CL_int,i is the intrinsic clearance of the ith CYP isoform in individually expressed recombinant enzyme. The application of this method to estimate the relative contributions of the individual UGT enzymes to the overall glucuronidation rate is not so feasible today as the relative abundances of the various UGT enzyme activities in human liver are not well‐established. Altenatively, reaction phenotyping experiments are also done by incubating the test compound in hepatocytes, microsomes, or some other in vitro preparation, using normal tissues as the enzyme source with inclusion of selective chemical or immunoinhibitors of specific enzymatic pathways. By performing a series of incubations with various inhibitors, and comparing the relative rates of metabolism, one can identify which inhibitor reduces the overall metabolism to the greatest extent and thereby uncover the metabolic pathway that contributes the most to the clearance of a compound. The use of hepatocytes guarantees the full range of phase I and phase II enzymes but is limited by the availability of specific inhibitors to some enzymes (example, UGT enzymes). The percentage contribution of an enzyme isoform to the metabolism of a victim drug provides a measure of the extent of its dependence on that isoform (f_m,CYP ). In addition, a good assessment of victim potential requires knowledge of the fractions eliminated in bile and urine in order to get the fraction of compound metabolized (f_m ).