TABLE 3

Kinetic parameters for 8-hydroxyefavirenz formation from efavirenz enantiomers

Wild-type CYP2B6 and all variants were coexpressed with wild-type POR.1 and cytochrome b5. Results (Vmax and Embedded Image and n) are the parameter estimate and S.E. of the estimate, determined by nonlinear regression analysis of the Hill equation, over the substrate concentration range 0.25–40 μM S-efavirenz and 0.11–45 μM R-efavirenz.

CYP2B6 VariantS-8-Hydroxyefavirenz Formation from S-EfavirenzR-8-Hydroxyefavirenz Formation from R-Efavirenz
VmaxEmbedded ImagenClmaxVmaxEmbedded ImagenClmax
pmol/min per picomolesμMml/min per nanomolespmol/min per picomolesμMml/min per nanomoles
CYP2B6.14.2 ± 0.27.7 ± 0.62.1 ± 0.30.270.57 ± 0.0916.1 ± 4.41.5 ± 0.30.019
CYP2B6.4a4.5 ± 0.15.9 ± 0.32.5 ± 0.20.390.78 ± 0.067.0 ± 1.11.2 ± 0.10.071
CYP2B6.53.3 ± 0.17.3 ± 0.71.4 ± 0.10.250.44 ± 0.026.5 ± 0.81.8 ± 0.30.034
CYP2B6.63.4 ± 0.111.8 ± 0.51.8 ± 0.10.150.41 ± 0.0514.5 ± 3.81.2 ± 0.20.018
CYP2B6.71.5 ± 0.15.0 ± 0.31.5 ± 0.10.160.14 ± 0.016.3 ± 1.31.6 ± 0.40.011
CYP2B6.91.7 ± 0.18.1 ± 1.01.2 ± 0.10.130.13 ± 0.0313.5 ± 5.61.3 ± 0.30.006
CYP2B6.16b0.19 ± 0.010.012 ± 0.002
CYP2B6.174.4 ± 0.29.3 ± 0.81.7 ± 0.20.240.38 ± 0.079.2 ± 4.301.2 ± 0.30.055
CYP2B6.18b0.20 ± 0.010.009 ± 0.001
CYP2B6.19c3.4 ± 0.113.4 ± 0.91.5 ± 0.10.130.65 ± 0.3331 ± 281.2 ± 0.40.013
CYP2B6.261.5 ± 0.15.0 ± 0.31.8 ± 0.10.150.21 ± 0.014.2 ± 0.71.9 ± 0.40.025
  • a CYP2B6.4 (only) showed substrate inhibition with R-efavirenz. Results in the table for CYP2B6.4and R-efavirenz were from the Hill equation and the substrate concentration range 0.11–18 μM. Data were also analyzed with the model for substrate inhibition (LiCata model, x = 3) over the substrate concentration range 0.11–45 μM, yielding Vmax = 0.90 ± 0.18, K = 9.0 ± 3.5, n = 1.1 ± 0.1, Clmax = 0.74, Vi = 0.09 ± 0.19, and Ki = 38 ± 11 μM.

  • b CYP2B6.16 and CYP2B6.18 rates were measured at a fixed substrate concentration of 40 μM S- and R-efavirenz.

  • c For R-efavirenz hydroxylation by CYP2B6.19, an alternative Michaelis-Menten model of linear regression analysis at low substrate concentrations generated a ratio of Vmax/Km = 0.019.