A molecular insight into the phototoxic reactions observed with vemurafenib, a first-line drug against metastatic melanoma

The electronic properties of vemurafenib (VB) provide a rational basis for understanding its strong UVA-induced phototoxicity. Thus, solvation of hydrophobic VB by hydrogen bonding solvents controls its photophysical, photochemical and photosensitizing properties. Addition of phosphate buffered saline (PBS) to methanol (MeOH) induces a bathochromic shift of the VB absorbance spectrum and a fluorescence emission ( λ max = 450 nm, quantum yield ( Φ ) = 0.011). Phosphorescence ( λ max = 461 nm) is observed at 77 K in MeOH. 308 nm laser flash spectroscopy demonstrates that the lifetimes ( τ ) and quantum yields of the VB triplet state in deaerated MeOH ( τ MeOH = 0.41 μs, λ max ∼ 380 nm), MeOH-PBS and HSA solutions markedly depend on the microenvironment. A long-lived radical (half-life >200 μs) is also formed. The state is quenched by O 2 and electron donors (Cys and 2′-deoxyguanosine) at a rate constant >1 × 10 9 M −1 s −1 . UVA-irradiation of VB in air-saturated MeOH or MeOH-PBS solutions produces a UVA-absorbing photoproduct ( Φ ∼ 5 × 10 −4 ). VB photosensitizes Trp destruction by type I (radical formation) and type II (singlet oxygen ( 1 O 2 ) formation) photodynamic reactions ( Φ = 0.005). Singlet oxygen production is further demonstrated by the VB-photosensitized His oxidation ( Φ MeOH = 0.006). Photochemical and photosensitizing properties of vemurafenib in relation to its phototoxicity.