Abstract
In cell culture systems with aqueous buffers, concentration-response curves to lipophilic inhibitors are difficult to establish because plateau effects (Imax) are often not reached because of limited drug solubility. Consequently, the inhibitory potency of a compound will not be definable using IC50 values (concentration exerting 50% of Imax). Since alternative potency measures f2 values, the concentrations required to double baseline signals have been proposed. Using both methods, we reevaluated the concentration-response curves of calcein assays with 78 compounds in three different cell culture systems and found a close correlation between both methods (rs = 0.93–0.99, p ≤ 0.0028). These findings suggest that f2 values are a valuable alternative to define rank orders of highly lipophilic inhibitors as a basis for the prediction of pharmacological interaction properties in clinical settings. Although it was only tested for inhibition of P-glycoprotein, it seems likely that this method may be transferred to other assays with other proteins.
Footnotes
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Article, publication date, and citation information can be found at http://dmd.aspetjournals.org.
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doi:10.1124/dmd.105.007377.
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ABBREVIATIONS: ABC, ATP-binding cassette; f1.5, f2, f3, and f4, concentration needed to increase baseline fluorescence by a factor of 1.5, 2, 3, and 4, respectively; rS, Spearman rank correlation coefficient; DMSO, dimethyl sulfoxide; LY335979, zosuquidar; GG918, elacridar; IC50, concentration leading to 50% of Imax; Imax, maximum effect; Imin, minimum effect; pBCEC, porcine brain capillary endothelial cell; P-gp, P-glycoprotein; SDZ-PSC833, valspodar.
- Received September 14, 2005.
- Accepted November 2, 2005.
- The American Society for Pharmacology and Experimental Therapeutics
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