Kinetic parameters for inhibition of daunorubicin (DNR) and calcein-AM (C-AM) efflux from four progressively resistant (P6, P12, P36, P72) Ehrlich ascites tumor cell lines
The parameters shown are Kapp (in μM), the apparent inhibition constant, i.e., the concentration of blocker that gives half-maximal inhibition of DNR or C-AM efflux; Ki (in μM), the intrinsic affinity of the blocker for the pump; Si/So, the ratio of intra- to extracellular drug concentration at steady state.
Cell Line | ||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Drug | P6 Si/So = 0.88 | P12 Si/So = 0.66 | P36 Si/So = 0.46 | P72 Si/So = 0.34 | Mean Ki | |||||||||||
| Kapp | Ki | Kapp | Ki | Kapp | Ki | Kapp | Ki | |||||||||
| DNR | ||||||||||||||||
| Verapamil | 9.4 | (9.4)a | 5.3 | (5.3) | 6.4 | (6.4) | 6.4 | (6.4) | 6.9 ± 0.9 | |||||||
| Cyclosporin A | 0.36 | 0.31 | 0.811 | 0.54 | 1.30 | 0.60 | 1.60 | 0.54 | 0.50 ± 0.06 | |||||||
| Vinblastine | 1.1 | 0.97 | 3.6 | 2.38 | 4.7 | 2.2 | 4.5 | 1.5 | 1.76 ± 0.32 | |||||||
| XR9576 | 0.03 | 0.02 | 0.06 | 0.04 | 0.113 | 0.05 | 0.113 | 0.04 | 0.04 ± 0.01 | |||||||
| C-AM | ||||||||||||||||
| Verapamil | 8.0b | 6.4 | 4.4 | 4.2 | 5.8 ± 0.9 | |||||||||||
| Cyclosporin A | 1.7 | 1.6 | 1.6 | 1.8 | 1.68 ± 0.05 | |||||||||||
| Vinblastine | 6.5 | 5.7 | 4.7 | 5.4 | 5.6 ± 0.4 | |||||||||||
| XR9576 | 0.34 |
| 0.19 |
| 0.19 |
| 0.14 |
| 0.22 ± 0.04 | |||||||
↵ a The apparent and intrinsic constants for inhibition of DNR efflux by verapamil are identical.
↵ b For C-AM, the ratio Si/So is zero, because any C-AM that enters the cellular compartment is immediately hydrolyzed by intracellular esterases to free calcein; therefore, Si is kept at zero concentration while So remains constant.