Cytoxicity dose range is too narrow

[feng zeng]

Is there some people could give me some suggestions about my experiment
design

I test two compounds' cytoxicity using HT-29 and Zr 75-1 cell line.
I set the treatment doses as: 200um,100um,50um,25um,12.5um,6.25um
cell seeding at 5x10*3 cells/well, incubated 24h for treatment
after adding compound, incubated another 24h
then use MTS as detection reagents


I got result as: the three high doses (200um, 100um,50um) killed all the
cells
while the three low doses (25um,25um,12.5um) could not kill any cell.

So I calculator the IC50 between 50um and 25um.
But why the dose range is so small??

Or this is normal, I just asked a stupid question??
ANY COMMENTS and SUGGESTION  are highly appreciated.

Thanks to all

Feng