Table 3.

Methods for measuring radioligand dissociation rates: advantages and limitations

Method	Removal of free radioligand after pre-incubation with receptor	Excess unlabelled ligand in wash- out medium	Advantages and limitations
1	Yes	Yes	Correct koff*
2	No	Yes	Correct koff*
3	Yes	No	Rebinding possible
4	Only dilution	No	Rebinding possible and approximate koff#

^*Only if both radioligand and unlabelled ligand interact competitively, that is, when they bind to overlapping sites at the receptor.

^#Due to the establishment of a new mass-action equilibrium with diluted free radioligand in the wash-out medium.