You can calculate that parameter by non linear regression or find some help at https://www.ncbi.nlm.nih.gov/pubmed/?term=Experimental+models+in+Chagas+disease%3A+a+review+of+the+methodologies+applied+for+screening+compounds+against+Trypanosoma+cruzi or at Article In vitro and in vivo experimental models for drug screening ...
The half maximum inhibitory concentration (IC50) measures the power of a substance in a specific biological or biochemical function. IC50 can be calculated by this formula: Y = a * X + b. I have given the link below which can be helpful for you.