Hello Daehoon Kim

The conditioned media will contain biologically active components secreted from previously cultured cells (cancer cells) that have been released into the media by the cancer cells. If you use FBS containing media to culture the cancer cells, then the effects of the active components secreted by the cancer cells will either get masked or inhibited because FBS is a rich source of nutrients containing several components like growth factors, hormones, cytokines and other biologically active molecules which may interact with the components secreted by the cancer cells in the conditioned media.

So, when you collect the cancer cell conditioned media to study the effects of substances secreted by cancer cells on other cells, you will not get the proper interaction of the secreted substances on other cells because of FBS interference. Therefore, you must use FBS-free media to culture the cancer cells and collect the conditioned media after 24 or 48 hours.

Best.