TABLE 3

Kinetic parameters for the metabolism of N-desmethyl-TAM to α-hydroxy N-desmethyl-TAM, endoxifen, and N-didesmethyl-TAM in HLMs Since endoxifen was formed very slowly in HG06 (undetectable at substrate concentrations <25 μM), the kinetic parameters estimated from a single-site binding equation were overestimated and are not listed in the table; only mean values from HG23 and HG112 are presented. Data for the formation of N-didesmethyl-TAM were characterized by biphasic kinetics in HG23 and HG112 (low and high affinity) and by monophasic kinetics in HG06. The low affinity Km values were in millimolar ranges, and the kinetic data that represent the high affinity system (Vmax1, Km1, and Vmax1/Km1) are presented here.


HLMs

α-Hydroxy N-Desmethyl-TAMa

Endoxifena

N-Didesmethyl-TAM
Vmax
Km
Vmax/Km
Vmax
Km
Vmax/Km
Vmax
Km
Vmax/Km
HG06 36.0 5.8 6.2 0.06 203 95.8 2.1
HG23 19.7 5.9 3.4 32.2 5.9 5.4 26.6 8.8 3.0
HG112 94.3 4.7 20.2 6.0 4.5 1.4 62.2 4.3 14.6
   Mean 50.0 5.5 9.9 19.1 5.2 3.4 97.3 36.3 6.6
   ±S.D.
39.2
0.7
9.0


2.5
93.2
51.6
6.9
  • Vmax, pmol/min/mg of protein; Km, μM; and Vmax/Km, μl/min/mg of protein.

  • a Formation rates (apparent) vs. substrate concentrations were best fit to a one-site binding equation using a nonlinear regression analysis (see Data Analysis)