C8 Ceramide (d18:1.8:0) is a cell-permeable analog of naturally occurring ceramides. Unlike C2 ceramide, it is metabolized within cells to generate natural ceramides, often causing a dramatic increase in cellular ceramide levels. [1] For this reason, treatment of cells with C8 ceramide (d18:1.8:0) may more closely mimic the effects of ceramide elevation than does treatment with shorter chain ceramide analogs. C8 Ceramide (d18:1.8:0) likely mediates many diverse biological activities, as do natural ceramides. It promotes the differentiation of human keratinocytes [2] and induces apoptosis of both human leukemia cells and the U937 cell line.[1] [3]

Reference:
[1]. Karasavvas, N., Erukulla, R.K., Bittman, R., et al. Stereospecific induction of apoptosis in U937 cells by N-octanoyl-sphingosine stereoisomers and N-octyl-sphingosine. The ceramide amide group is not required for apoptosis. Eur. J. Biochem. 236(2), 729-737 (1996).
[2]. Wakita, H., Tokura, Y., Yagi, H., et al. Keratinocyte differentiation is induced by cell-permeant ceramides and its proliferation is promoted by sphingosine. Archives of Dermatological Research 286, 350-354 (1994).
[3]. Geley, S., Hartmann, B.L., and Kofler, R. Ceramides induce a form of apoptosis in human acute lymphoblastic leukemia cells that is inhibited by Bcl-2, but not by CrmA. FEBS Letters 400, 15-18 (1997).