Inclusion complexes of 2-methoxyestradiol with dimethylated and permethylated β-cyclodextrins: models for cyclodextrin–steroid interaction

Summary The interaction between the potent anticancer agent 2-methoxyestradiol (2ME) and a series of cyclodextrins (CDs) was investigated in the solid state using thermal analysis and X-ray diffraction, while the possibility of enhancing its poor aqueous solubility with CDs was probed by means of equilibrium solubility and dissolution rate measurements. Single crystal X-ray diffraction studies of the inclusion complexes between 2ME and the derivatised cyclodextrins heptakis(2,6-di-O-methyl)-β-CD (DIMEB) and heptakis(2,3,6-tri-O-methyl)-β-CD (TRIMEB) revealed for the first time the nature of the encapsulation of a bioactive steroid by representative CD host molecules. Inclusion complexation invariably involves insertion of the D-ring of 2ME from the secondary side of each CD molecule, with the 17-OH group generally hydrogen bonding to a host glycosidic oxygen atom within the CD cavity, while the A-ring and part of the B-ring of 2ME protrude from the secondary side. In the case of the TRIMEB·2ME complex, there is evidence that complexation proceeds with mutual conformational adaptation of host and guest molecules. The aqueous solubility of 2ME was significantly enhanced by CDs, with DIMEB, TRIMEB, randomly methylated β-CD and hydroxypropyl-β-CD being the most effective hosts. The 2:1 host–guest β-CD inclusion complex, prepared by two methods, yielded very rapid dissolution in water at 37 °C relative to untreated 2ME, attaining complete dissolution within 15 minutes (co-precipitated complex) and 45 minutes (complex from kneading).


Relevant PXRD traces, DSC traces and FTIR spectra confirming complex
formation between the hosts RAMEB and HPBCD and the guest 2ME (p. s3) 3. Thermoanalytical characterization of the DIMEB2ME and TRIMEB2ME inclusion complexes (p. s4) 4. Geometrical data for the host DIMEB in the inclusion complex DIMEB2ME (Table S1) (p. s6) 5. Molecular overlay of 2ME molecules (uncomplexed and complexed with DIMEB) (p. s7) 6. Additional hydrogen bond data for DIMEB2ME (Table S2) (p.s s8) 7. Simulated X-ray photographs for the TRIMEB2ME complex (p. s9) 8. Overlay of host molecules A and B in the TRIMEB2ME complex (p. s9) 9. Overlay of host molecules C and D in the TRIMEB2ME complex (p. s10) 10. Geometrical parameters for the host molecules A-D in the inclusion complex TRIMEB2ME (Table S3) (p. s10) 11. Overlay of the 2ME molecules in TRIMEB2ME complex unit C and the DIMEB2ME complex (p. s11) 12. Dissolution profiles for 2ME and two series of binary products of 2ME and CDs (p. s12) s2 1. PXRD traces and single crystal XRD data confirming complex formation between -CD and 2ME. Figure S1: Experimental PXRD patterns of 2ME, -CD and various preparations containing the two components.
Interpretation: The traces labelled -CD-2ME (1:1) neat grinding and -CD-2ME (2:1) mixture do not indicate complex formation as they contain peaks from both of the starting materials. Instead, a distinctly different, common PXRD trace results from the products of kneading and co-precipitation, indicating probable complex formation.
Interpretation: Close similarities between the PXRD trace for complex KOFJEU and those for the putative complex between -CD and 2ME suggested that these phases are isostructural. Subsequent investigation of the single crystals obtained by co-precipitation of -CD and 2ME were found to belong to the space group C222 1 , with a = 19.376(2), b = 24.053(2), c = 32.412(2) Å at −160 °C.

Relevant PXRD traces, DSC traces and FTIR spectra confirming complex Formation between the hosts RAMEB and HPBCD and the guest 2ME
Descriptions and interpretations of the figures below appear in the manuscript. Figure S3: Experimental PXRD patterns of 2ME, RAMEB and various preparations containing the two components.  Figure S5: FTIR spectra of RAMEB, 2ME and the spectrum of the putative inclusion complex 'RAM2ME'.

DIMEB)
6. Additional hydrogen bond data for TRIMEB2ME (Table S2) Hydrogen bonds based on numbering in Figure 2 Atom labels O···O Distance ( NOTE: In Figure 2e, water oxygen atoms labelled 8, 9, 10 are the symmetry-related counterparts of 11, 12 and 13 respectively, the former set being located at (iv) 1+x, y, −1+z and the latter belonging to the asymmetric unit (symmetry operator x, y, z).
*e.s.d.s not calculated by PLATON due to low site-occupancy of atoms involved.
11. Overlay of the 2ME molecules in TRIMEB2ME complex unit C and the DIMEB2ME complex Figure S16: Overlay of the 2ME molecules in the DIMEB2ME complex (green) and in unit C of the TRIMEB2ME complex. (R.M.S.D = 0.083 Å with a maximum atomic deviation of 0.161 Å).
s12 12. Dissolution profiles for 2ME and two series of binary products of 2ME and CDs Figure S17: Comparative dissolution profiles for 2ME in physical mixtures (pm) with the native cyclodextrins -and -CD as well as RAMEB and HPBCD. Figure S18: Dissolution profile of the inclusion complex (-CD) 2 2ME and those of a series of 2ME/CD samples resulting from kneading (kn) as well as 1:1 inclusion complexes of HPBCD and RAMEB.