Methionine dependence of tumours: A biochemical strategy for optimizing paclitaxel chemosensitivity in vitro

Methionine dependence is a unique feature of cancer cells characterized by growth and cell cycle arrest (typically in S and G2/M) under conditions of methionine depletion. Following replenishment of media with methionine, the cell cycle blockade is reversible and during this recovery period, cells may become more susceptible to the action of cell cycle specific drugs. The response of a panel of methionine dependent (HTC, Phi-1, PC3 and 3T3) cells to vinblastine and paclitaxel was compared to methionine independent Hs-27 cells under conditions of methionine depletion (M^-H⁺; methionine depleted media supplemented with homocysteine) and starvation (M^-H^-; media without methionine or homocysteine). All cell lines were significantly more resistant to both agents under M^-H⁺ and M ^-H^- conditions compared to controls under normal culture conditions [M⁺H^-]; however, the magnitude of resistance was reduced in the methionine independent Hs-27 cells. During recovery from methionine depletion and starvation, the response of the methionine dependent cells to vinblastine and paclitaxel was significantly enhanced compared to controls. Although the activity of vinblastine on the Hs-27 cell line was comparable to controls, these methionine independent cells became significantly more resistant to paclitaxel during recovery studies (IC50 = 2.13 ± 0.5 μM) compared to control cultures (IC50 = 0.13 ± 0.15 μM). Whilst the mechanism responsible for this remains uncertain, the increased activity of paclitaxel against methionine dependent cells in conjunction with the decreased activity against Hs-27 cells suggests that methionine depletion strategies may enhance the therapeutic index of paclitaxel.