Payload selection is another critical factor that defines the success of an ADC. An ideal ADC payload is a highly potent small molecule with lack of specificity. Currently, there are two broad categories of payloads
for conjugation to a mAb. The first category is referred to as radionuclide antibody conjugates (RACs), where a radionuclide emitting radiation penetrates into the targeted cells of the solid tumor to induce a sufficient
lethal response with no or minimal damage to the surrounding healthy cells. The second and a major category for antibody payloads include high potency synthetic or natural product small molecules. ADC payloads approved or under development are for the most part cytotoxic agents with picomolar or sub-picomolar potencies.

References:

1.Bakhtiar R. Biotechnol Lett. 2016 Oct;38(10):1655-64.