Effect of cell density on cytotoxicity of ether lipid analogues in variants of B16 murine melanoma.

Ether lipids are defined here as analogues of naturally occurring lysophosphatidylcholines with cytotoxic activity against neoplastic cells. The activity of 1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine (ET18OMe) and 3-hexadecylmercapto-2-methoxymethyl-propyl-1-phosphocholine (Ilmofosine) (BM 41.440) was tested in variants of B16 murine melanoma, grown in adhesion cultures (B16F1 with low metastatic potential; B16F10 and B16BL6 with high metastatic potential). Cytotoxicity was evaluated by counting the cells that survived after 24 h of drug exposure. Cholesterol, sphingomyelin, total phospholipid and phosphatidylcholine levels were determined. After 24 h of drug exposure, cultures of the B16BL6 variant contained a larger number of cells, especially when high drug concentrations (100-250 microM) were used, than cultures of the B16F1 and B16F10 variants. The sensitivity to ET18OMe of the three variants was evaluated at different cell densities (at each density the dose was equalized per number of cells/well; 0.1 mumol/10(6) cells/well). In B16F1 and B16F10 cultures the dose-response curve was not affected by the number of cells/well, while in B16BL6 no more than 20% of the cells were killed at all cell densities measured. A linear relationship was noted between cell density and cholesterol/phospholipid and sphingomyelin/phosphatidylcholine ratios in the resistant variant B16BL6, confirming that lipid composition modulates the cytotoxic activity of ether lipids.