The Studies On The Inclusion Complex Of16,17α-Epoxyprogesterone With Cyclodextrin

The solubility of steroids with a remarkable hydrophobic nature is very low, which is considered as the rate-limiting step of steroid biotransformation. To overcome this undesirable problem, cyclodextrins(CDs), which are inert to microorganisms, have been widely used to form inclusion complexes with the steroids through non-covalent interactions to enhance their water solubility for the hydrophobic nature of the interior cavity and hydrophilic exterior torus. In this paper, the inclusion complexes of16,17a-epoxyprogesterone (EP) with CDs were prepared and characterized. The interaction mechanism, the driving forces between the host and the guest, and the influence factors were also investigated. The experiment results are significant in the study of CD-promoted biotransformation of steroids as well as in the application of CDs in drug delivery.The EP-β-CD inclusion complex and the EP-RM-β-CD complex were prepared by coprecipitation method and stirring method respectively and subsequently characterized by means of TG-DTA, DSC, XRPD, SEM and FTIR techniques. The results clearly revealed that A-ring of EP molecules was predominantly included in the cavity of CDs. It has been found that water solubility of EP was increased by inclusion with CDs and the dissolution studies showed that such inclusion complex offered a significant improvement in the dissolution rate compared to EP alone and the physical mixture.The existence of cyclodextrin causes hypochromic effect and a little blue shift in the UV spectra of EP. EP formed more stable complexes with the derivatives than β-CD which might be attributed to the extension of the hydrophobic cavity and provision of greater inclusion ability. Phase solubility diagrams indicated that the values of the apparent stability constant K1:1and the enhancement factor followed by the order of:RM-β-CD> HP-β-CD>β-CD. The water soluble inclusion complexes with1:1stoichiometry were formed between EP and the derivatives whereas β-CD/EP gave both1:1soluble and2:1insoluble complexes.In the thermodynamic study, the negative value of the Gibbs energy (AG<0) suggests that the inclusion process is a spontaneous one. Although the solubility of EP increased upon increasing the temperature, the stability constant values decreased. This was not unexpected since cyclodextrin complexes usually dissociate upon increasing the temperature of the solution. Complex formation for EP and RM-β-CD is driven by both favorable enthalpy (ΔH<0) and entropy (ΔS>0) changes. However, the thermodynamic functions for β-CD-EP inclusion complex showed that complexation is driven by enthalpy (AH<0) but retarded by entropy (AS<0) changes. The association ability of RM-β-CD decreased as long as the hydrophobicity of the medium increased due to increasing organic solvent content or the more hydrophobic organic solvent added. All these observations revealed a clear contribution of the hydrophobic effect to the driving force of complexation. The results of hydrophobic effect indicated that49.3%of the tendency for EP-β-CD inclusion complex formation is driven by hydrophobic character of EP, while other factors including specific interactions (dipole-dipole interaction, hydrogen bonding) contribute about-8.08KJ/mol (attractive). Whereas97.4%of the tendency for EP-RM-β-CD inclusion complex formation is driven by hydrophobic character of EP, other factors including specific interactions contribute about+1.09KJ/mol (repulsive).The association constants increased between HP-β-CD and the steroids as the increasing hydrophobic character of the guests and followed the order of:RSA>EP>RS> Testosterone(TS)>Androstenedione(AD)>Hydrocortisone(HC). It was found that the greatest influence on the association constants is the structural features of ring A of these compounds but the substituents of ring D also alter the complex stability to an appreciable degree.Compared with EP alone and the physical mixture, EP-RM-β-CD inclusion complex offered a marked improvement in the initial bioconversion rates and the last degree of conversion. Taking into consideration that the solubility of the substrate EP is very low, which may limit the maximal usefulness of the steroid microbial transformation, the strategy could be successfully employed in the design of novel formulation of EP in microbial transformation applications.