*This page demonstrates how IC _{50} values are determined from competition
experiments using different types of plots. It is important to go through this page
even if you are not doing competition studies.*

The IC_{50} concentration for the unlabeled ligand in a competition experiment
is the concentration required to inhibit of the radiolabeled ligand to the specific site
by 50%

Although the IC_{50} value can be determined many times by eye, the most
accurate method for determining IC_{50} values is to use non-linear regression
analysis using the equation for a sigmoid plot.

Where Top is the top of the curve

Bottom is the bottom of the curve

Y is the amount bound (either as cpm or as % of control)|

X is the concentration of unlabeled ligand

In the graph shown above the bottom of the curve was zero. 50% of bound was halfway
between 1000 cpm and 0 cpm. Estimate 50% bound and the IC_{50} for the following
curves?

What point on the Y-axis represents 50% binding of the radioligand to
the specific biding site? What is the IC_{50}? |

What point on the Y-axis represents 50% binding of the radioligand to
the specific biding site? What is the IC_{50}? |

50% bound: 600 1000 - (1000-200)/2 = 600 Log of IC _{50} = -9.2IC _{50}= 6.3 X 10^{-10} MIC _{50} = 0.63 nMNote on a negative log scale you
need to read the axis from right to left rather than from left to right. That is the
Log IC |