The influence of light sources on sunitinib measurements with photoisomerization.

Sunitinib is an orally administered tyrosine kinase inhibitor. Therapeutic drug monitoring is an important component of the follow-up of patients because of high interpatient variability in the pharmacokinetics of sunitinib and large variabilities in its efficacy and toxicity. The aim of the present study was to examine the light stability of sunitinib and confirm the effects of light exposure on sunitinib measurements by LC-MS/MS. Sunitinib and its active metabolite, SU12662 convert Z isomers to E isomers with exposure to light. The Z-E photoisomerization ratio reached a plateau at 35% for both E isomers in methanol within 15 min of normal light exposure (700 lux). However, the Z isomer of the sunitinib and SU12662 peak area ratios in plasma decreased by 10% within 15 min. These results suggest that sunitinib samples need to be handled without light exposure in all sample preparation steps. Alternatively, it should be measured after the sample has reached photoisomerical equilibrium. These results suggest that the sunitinib therapeutic range changes depending on light conditions during sample handling in sunitinib and SU12662 measurements.