Combretastatin A4 Disodium Phosphate Forms Aggregates In Solution Leading To Exciton Transfer

Abstract

Combretastatin A4 disodium phosphate (CA4P) has been effective in the treatment of solid tumors. The side effects of CA4P could be reduced using targeted delivery where CA4P loading/release can be studied using CA4P fluorescence. However, the fluorescence properties of CA4P are poorly characterized. This work reports the effects of drug concentration on the excitation and emission properties of CA4P. The excitation spectrum shows a broad peak with a maximum at 328nm. The spectrum becomes narrow and the emission maximum shifts to 356nm when the CA4P concentration is increased. The emission spectrum also shows a red shift from 398 to 406nm, in the same drug concentration range (0.1-5.0mM). This spectral shift is typical for exciton transfer probably due to the formation of J-aggregates, where an excited monomer in the aggregate transfers its electronic excitation energy through Coulombic interactions to a ground-state monomer in the same aggregate. When excited at 356nm, the emission intensity is proportional to [CA4P] up to ∼1.75mM; thereafter, the intensity decreases. When excited at 328nm, the biphasic change persists but occurs at 0.175mM. We propose that the emission upon excitation at 328nm and 356nm comes from CA4P monomers and aggregates, respectively. An increase in drug concentration leads to aggregation, decreasing the number of monomers in solution and therefore the fluorescence intensity due to 328nm excitation drops. The decrease in fluorescence intensity, due to 356nm excitation, observed at [CA4P]>1.75mM is probably due to some subtle changes in the optical properties of the aggregates. These results have been applied to develop an assay capable of following the leakage of the encapsulated CA4P from the liposomes in real time. To the best of our knowledge, this is the first report on optically active aggregates formed by CA4P.