Drugs-in-cyclodextrins-in liposomes: a novel concept in drug delivery

Abstract

Inclusion complexes of tritiated dehydroepiandrosterone (DHEA), retinol (R) and retinoic acid (RA) were formed with14C-labelled 2-hydroxypropyl-β-cyclodextrin (HPβCD) or unlabelled β-cyclodextrin (βCD) polymers 2009 and 2010 (Mol. Wt 4000–4500 and 8700, respectively) at various molar ratios. Formation of inclusion complexes was confirmed by the complete or partial solubilization of the drugs used and by the simultaneous elution of drug and HPβCD radioactivities following molecular sieve chromatography of the complex solutions. Inclusion complex solutions (also containing ‘void’ cyclodextrins) were subsequently entrapped into dehydration-rehydration vesicles (DRV liposomes). Ratios of entrapment values (% of amounts used) for drugs and cyclodextrin (HPβCD) approximating unity were taken to denote entrapment that did not discriminate between complexes and void cyclodextrin. Near unity ratios and highest entrapment values (e.g., up to32.3 ± 11.9% (DHEA) and31.9 ± 11.8% (HPβCD) of the materials used; distearoyl phosphatidylcholine (DSPC) DRV) were achieved with liposomes made of phospholipids with a high gel liquid crystalline transition temperature (Tc) or, when equimolar (to the phospholipid) cholesterol was also present, with all phospholipids, regardless of theirTc. When DSPC liposomes (without or with equimolar cholesterol) containing drug (DHEA, R or RA) complex solutions with cyclodextrins were exposed to rat blood plasma at 37°C for up to 60 min, cyclodextrin (HPβCD) retention was nearly complete (0.7–11.9% released at 60 min). However, release of drugs was considerable with values being significantly greater for DHEA (60.2–62.0%) than for R or RA (26.6 and 26.8%, respectively). Experiments with DRV containing both car☐yfluorescein (CF) (as a marker of vesicle stability) and inclusion complex solutions revealed that entrapped cyclodextrins do not destabilize liposomes. Instead, data suggest that during or after the entrapment of complex solution into liposomes, some of the included drug is displaced from the cyclodextrin cavity by phospholipid and/or cholesterol (to a degree probably dependent on the stability constant of the complex) to end up in the lipid bilayer in a state which, on incubation, ensures rapid release into the media. Results suggest that entrapment of water-insoluble (or certain soluble drugs) in the form of cyclodextrin-inclusion complexes into the aqueous phase of liposomes may circumvent some of the problems associated with their entrapment as such.