How do I assess the ISI(50) concentration in an MTT-Assay
mathematically? ISI (50%) is the concentration where 50% of
cell-proliferation is inhibited by a certain drug.

IC50 values can be derived using linear interpolation, or from
standard dose response curves in which the mean normalized
fluorescence can be plotted against the drug concentration.

Draw a graph of the fluorescence response (y) to the dose of the drug (x).
Asymptotic or sigmoidal dose response curves can be fitted using a
four-or two-parameter curve fitting analysis. The derived IC50 drug
concentration corresponds to a response halfway between the upper and
lower asymptotes of the four-parameter logistic model.
The curve can be fitted using origin software. 

For eg.
for a asymptotic curve
y=b+{(a-b)/(1+(x/c)d) }
is a equation where the value c is ic50 and 
a,b, d are the parameters of the asymptotes.
So basically we need to know the type of curve that is resulting from
the graph & then look in for the best possible curve fitting; then
follows the calculation of IC50.

Have fun,

sorry for the typo

the equation is  

y=b+{(a-b)/(1+(x/c)^d) }

I would like to add that the MTT assay isn't the best available to
measure cell proliferation because results vary with the metabolic
activity of the cells.  So you could have a small amount of cells,
very active, that give a reading equal to a larger number of less
active cells.  There are assays that use a DNA intercalating dye to
quantitate cell number which I usually recommend to people who are
using MTT conversion.  One good product(not to be a commercial) is
called CyQuant, and it is sold by the Molecular Probes division of
Invitrogen.