Raloxifene attenuation of 5-FU/methotrexate cytotoxicity in human breast cancer cells: the importance of sequence in combination chemotherapy.

BACKGROUND: The selective estrogen receptor modulator raloxifene (RAL), used to treat and prevent osteoporosis, is under investigation for its use in the treatment and prevention of breast cancer. RAL in combination with the antimetabolites methotrexate (MTX) and 5-fluorouracil (5-FU) has not been extensively studied. Because RAL and the antimetabolites target different phases of the cell cycle and exhibit different mechanisms of action and clinical toxicity, the effects of sequence of administration on the growth inhibition of MCF-7 human breast cancer cells were investigated. MATERIALS AND METHODS: MCF-7 human breast cancer cells were exposed to vehicle alone, 10 microM MTX, 1 microM 5-FU, 10 microM RAL, 10 microM RAL 24 hours prior to 1 microM 5-FU followed 2 hours later by 10 microM MTX, and 1 microM 5-FU 2 hours prior to 10 microM MTX followed 24 hours later by 10 microM RAL. The cells were evaluated for viability and proliferation. The retinoblastoma (Rb) protein, a cell cycle regulator which when phosphorylated allows the progression of cells from G1- to S-phase, was used as a marker to determine the effects of early RAL and late RAL on cellular progression at the molecular level. RESULTS: Early RAL administration exhibited a cell viability of 66.83 +/- 6.17% of the control. However, late RAL administration exhibited cell viability 39.40 +/- 17.03% of the control. Late RAL was a more cytotoxic combination than RAL alone or early RAL. These findings from manual cell counting were also supported by cell flow cytometric analysis and Western blot data. CONCLUSION: Late RAL in combination with 5-FU and MTX, due to greater cytotoxicity, is a more desirable combination to treat breast cancer than RAL alone.

Sequence- and time-dependent antagonism between raloxifene and methotrexate in human breast cancer cells.

The selective estrogen receptor modulator (SERM) and an agent for the prevention of osteoporosis in postmenopausal women, raloxifene (Ral), decreased high-dose methotrexate (MTX) cytotoxicity in MCF-7 breast cancer cells. When Ral is given at least 24 hours prior to MTX, the resultant interaction is antagonistic. However, when breast cancer cells are exposed to Ral 24 hours after MTX, the interaction between Ral and MTX is not antagonistic. The proliferation of cells exposed to 10 microM Ral and 10 microM MTX alone or in combination with Ral 24 hours prior to MTX was in had the following order: MTX > Ral 24 hours prior to MTX > Ral. MTX administration 24 hours prior to Ral had the following inhibitory effects on the growth of cells: MTX 24 hours prior to Ral > or = MTX > Ral 24 hours prior to MTX > Ral > control (no drug exposure). To determine if the antagonistic interaction between Ral and MTX was a function of sequence and time, cells were exposed to Ral 24 hours and 36 hours prior to MTX exposure. The percentages of control rates were 43.48 +/- 3.90% and 54.43 +/- 2.93%, respectively, from a 24 hours and 36 hours exposure of Ral prior to MTX. The growth rates after 24 h and 36 h exposures to MTX alone were 30.30 +/- 0.61% and 33.11 +/- 2.27% of control rates, respectively. These studies suggest that: (a) the interactions between Ral and MTX are sequence-dependent; (b) Ral antagonizes the effect of MTX when Ral administration precedes MTX; and (c) Ral antagonism to MTX is a function of time.