Light-Induced Toxic Effects of Tamoxifen: A Chemotherapeutic and Chemopreventive Agent

Abstract

Tamoxifen is a powerful drug used to treat breast cancer patients, and more than 500,000 women in the U. S. are being treated with this drug. In our study, tamoxifen is found to be photomutagenic in Salmonella typhimurium TA102 at concentrations as low as 0.08 muM and reaches maximum photomutagenicity at 0.4 muM under a light dose equivalent to 20 min sunlight. These concentrations are comparable to the plasma tamoxifen concentration of 0.4 to 3 muM for patients undergoing tamoxifen therapy. The toxicity seems to be the result of DNA damage and/or lipid peroxidation caused by light irradiation of tamoxifen. The DNA damage caused by irradiation of PhiX174 DNA in the presence of tamoxifen appears to be formation of DNA-tamoxifen covalent adducts, not single strand/double strand cleavages, and there is no oxygen involvement. This is confirmed by EPR experiments that carbon-centerd radicals are formed by light irradiation of tamoxifen and there is no singlet oxygen formation. Although superoxide radical is formed, it is not involved in DNA damage.

Figure 1

Tamoxifen, trans-2-[4-(1,2-diphenyl-1-butenyl)phenoxy]-N,N-dimethylethylamine.

Figure 2

Salmonella typhimurium TA102 survival in the presence of tamoxifen with or without light irradiation. Light dose were 1.2 J/cm² UVA and 2.1 J/cm² visible (5 min) and 4.6 J/cm² UVA and 8.5 J/cm² of visible light (20 min). Bacteria colonies were grown in Petri dishes with nutrient agar. All data points were the average reading from three Petri dishes.

Figure 3

Photomutagenicity of tamoxifen with Salmonella typhimurium TA102 with 1.2 J/cm² UVA and 2.1 J/cm² visible (5 min) and 4.6 J/cm² UVA and 8.5 J/cm² of visible light (20 min) irradiation or in the presence of S9 mix. The points are the average reading of three parallel experiments.

Figure 4

Damage of ΦX-174 supercoiled plasmid DNA (27 μM in base pairs) caused by light-irradiation of tamoxifen (TAM) in solution. Left: concentration dependence experiment; Right: irradiation time dependence experiment. Light dose was 4.6 J/cm² of UVA and 8.5 J/cm² of visible light.

Figure 5

Effect of scavengers on light induced DNA damage by tamoxifen (200 μM). All experiments were tested with 27 μM ΦX174 DNA and irradiated for 20 min by 300 W Xe lamp except the negative control. Lane N: negative controls in the dark; lane P: positive control with tamoxifen and irradiation; lanes 3, 4, 5, 6, 7, and 8 are in the presence of nitrogen (N₂), 50 mM histidine (His), 200 units SOD, 50 mM NaN₃, 50 mM KI, 100% D₂O, and 1.0 M mercaptoethanol (SH).

Figure 6

Superoxide radical generation by tamoxifen (TAM) by UV (330 nm or 280 nm) irradiation with spin trap BMPO. Samples contained 25 mM BMPO in 50% ethanol.

Figure 7

Time dependence of superoxide generation by tamoxifen (TAM) under UV (330 nm) irradiation. Samples contained 10 mM Tamoxifen and 20 mM BMPO in 50% ethanol at room temperature.

Figure 8

Time-dependent formation of carbon-centered radicals induced by UV (330 nm) irradiation of tamoxifen (TAM, 5 mM) in solution. The radicals were trapped with PBN.