Structural diversity in the host-guest complexes of the antifolate pemetrexed with native cyclodextrins: gas phase, solution and solid state studies

The complexation of the antifolate pemetrexed (PTX) with native cyclodextrins was studied. This process, along with the findings gathered for the structurally related folic acid was treated as a model for exploiting host-guest interactions of this class of guest molecules in the gas phase, in solution and in the solid state. Mass spectrometry was employed for the investigation of the architecture and relative gas-phase stabilities of these supramolecular complexes. The mode of complexation was further tracked by 1D and 2D NMR proving the formation of the exclusion-type complex with alpha-CD and pseudorotaxane inclusion-type complexes with beta-, and gamma-CDs. UV-vis titrations at pH 7.4 gave association constants for the obtained complexes. The stability of the complexes increases in the series: alpha-CD/PTX < gamma-CD/PTX << beta-CD/PTX. The association of PTX with a monomer cyclodextrin equivalent methyl alpha-D-glucopyranoside - was investigated for a deeper understanding of the type of host-guest interactions. Solid state studies of PTX/CDs were performed using FTIR-ATR and Raman spectroscopy techniques.