Molecular binding of self-assembling peptide EAK16-II with anticancer agent EPT and its implication in cancer cell inhibition

The current drug delivery techniques involve encapsulation, targeting and controlled release of the drug with various molecules or nanoparticles, but rarely has the drug molecular state or form been investigated. It is necessary to deliver a drug with a prescribed molecular state in order to maximize drug therapeutic effects. Here we present two facile methods to characterize molecular states of the anticancer drug ellipticine (EPT) encapsulated in the self-assembling peptide EAK, and relate the different molecular states of EPT to their respective cancer inhibition efficacies. The first method is UV-based, where drug loading capacity of a particular molecular state was determined. The experimental data corroborated a molecular binding model, where peptide–drug interaction was assumed to be electrostatic in nature. The developed model could elucidate a unique pH effect on protonated EPT loading capacity. The second method is based on fluorescence characteristics of EPT, which could differentiate the two molecular states: protonated and crystalline of EPT in situ. The inner filter effect was, however, found with this method, presenting an ineluctable obstacle in quantitative analysis of fluorescence data. A correction method for the inner filter effect was thus developed. With this approach, concentrations of EPT at different molecular states in its peptide complex solutions were determined. In vitro cytotoxicity assay was applied to evaluate the efficacy of the two molecular states of EPT, showing that protonated EPT was more efficient at killing cancer cells than crystalline EPT. The molecular binding model and two characterization methods for EAK–EPT complexation could be extended to other carrier-drug systems.

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