How can the IC50 of a compound be determined in a cytotoxicity assay by the Spearman-Karber method if the value of n (usually number of animals) is uncertain with cells?
It is not the correct method of analysis. The cytotoxicity IC50 is the concentration that kills half the cells. You can make a plot of compound concentration versus % cell killing, then use a nonlinear least squares program to calculate the IC50 using the Hill equation.
% inhibition = 100[I]n/(IC50n + [I]n)
where [I] is the inhibitor concentration, n is the Hill (cooperativity) coefficient, the number 100 represents the maximal % inhibition.