Abstract
Paclitaxel is a substrate of the mdr1 P-glycoprotein (Pgp). The objective of the present study was to determine the kinetics of the Pgp-mediated efflux and its contribution to the overall efflux of paclitaxel at the clinically achievable concentration range of 1 to 1500 nM. Human breast carcinoma BC19 cells that were derived from MCF7 cells by mdr1 transfection and show a >10-fold higher level of the Pgp protein were used to measure the uptake and efflux of [3H]paclitaxel. A computational model of intracellular paclitaxel pharmacokinetics was developed to analyze for the Pgp efflux parameters. The results show a saturable Pgp-mediated efflux in BC19 cells; the dissociation constant was 14 nM, and the maximal efflux rate was 2.8 × 10−4 pmol/h/cell. The contribution of Pgp-mediated efflux to the total efflux decreased with increasing extracellular drug concentrations; the Pgp efflux accounted for 86 and 34% of total efflux at 1 and 1500 nM, respectively. The validity of the model was confirmed by the close agreement between the model-predicted data and the experimentally obtained data (∼6% deviation) describing the effect of cell density and intracellular-to-extracellular concentration gradient on the kinetics of drug accumulation and efflux. In conclusion, our results indicate that the Pgp-mediated efflux represents a major efflux mechanism of paclitaxel at the low end of the clinically observed drug concentration range, but accounts for only a minor part of the efflux at higher concentrations in BC19 cells.
Footnotes
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This work was partially supported by Research Grants R37CA49816 and R01CA63363 from the National Cancer Institute, National Institutes of Health, Department of Health and Human Services.
- Abbreviations:
- Pgp
- P-glycoprotein
- Jmax and KM,Pgp
- maximum rate and dissociation constant of Pgp-mediated efflux, respectively
- Ctotal,c and Ctotal,m
- total drug concentrations (i.e., free plus bound) in cells and medium, respectively
- Cfree,c and Cfree,m
- free drug concentrations in cells and medium, respectively
- Vc
- volume of cells
- CLf,d and CLPgp
- clearance of free drug by passive diffusion and Pgp efflux, respectively
- Bmax,c and Bmax,m
- maximum drug binding capacity in cells and medium, respectively
- Kd,c and Kd,m
- dissociation constants for drug binding to saturable binding sites in cells and medium, respectively
- NSB
- proportionality constant for nonsaturable binding in cells
- Vone cell
- mean volume of a single cell
- ICN
- initial cell number at time 0
- kcell number
- rate constant for changes in cell number
- kBmax,c
- rate constant for increase inBmax,c
- Bmax,c(t) and Bmax,c,initial
- Bmax,c at time t and 0, respectively
- Received February 26, 2001.
- Accepted May 1, 2001.
- The American Society for Pharmacology and Experimental Therapeutics
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