Table 2. RF Values of the 12 Selected Compounds in the Resistant HT29/DOX and A549/DOX Cell Lines.

	HT29/DOX		A549/DOX
	RFa		RFa
compounds	1 μM	3 μM	1 μM	3 μM
1	14.9	18.7	21.5	30.2
2	38.4	44.5	54.0	64.3
4	21.7	41.1	39.5	65.0
5	44.4	85.7	70.4	155.0
10	40.2	47.5	65.1	85.4
11	10.8	29.7	17.6	49.8
13	14.1	4.15	16.5	11.6
14	46.2	63.2	67.4	103.0
19	60.5	87.0	92.2	131.6
20	13.1	16.2	19.2	31.5
22	37.6	61.9	61.9	82.4
23	27.2	26.6	37.0	41.3

^aInhibition of the P-gp transport activity on two resistant cell lines, expressed as RF that is the ratio between the IC₅₀ of doxorubicin alone and in the presence of modulators (RF = IC₅₀ of doxorubicin – modulator/IC₅₀ of doxorubicin + modulator).