The acidic microclimate in poly(lactide-co-glycolide) microspheres stabilizes camptothecins

Purpose. The camptothecin (CPT) analogue, 10-hydroxycamptothecin (10-HCPT) has been shown previously to remain in its acid-stable land active) lactone form when encapsulated in poly(lactide-co-glycolide) (PLGA) microspheres ( 1). The purpose of this study was to determine the principal mechanism(s) o f 10-HCPT stabilization. Methods. CPTs were encapsulated in PLGA 50:50 microspheres by standard solv ent evaporation techniques. Microspheres were eroded in pH 7.4 buffer at 37 degrees C. The ratio of encapsulated lactone to carboxylate was determined by HPLC as a function of time, initial form of drug encapsulated, fraction of co-encapsulated Mg(OH)(2), CPT lipophilicity, and drug loading. Two tec hniques were developed to assess the microclimate pH, including: i) measure ment of H+ content of the dissolved microspheres in an 80:20 acetonitrile/H 2O mixture and iii confocal microscopy of an encapsulated pH-sensitive dye, fluorescein. Results. The encapsulated carboxylate converted rapidly to the lactone afte r exposure to the release media, indicating the lactone is favored at equil ibrium in the microspheres. Upon co-encapsulation of Mg(OH)(2), the trend w as reversed, i.e., the lactone rapidly converted to the carboxylate form. M easurement of -log(hydronium ion activity) (pa(H)*) of dissolved microspher es with pH-electrode and pH mapping with fluorescein revealed the presence of an acidic microclimate. From the measurements of H+ and water contents o f particles hydrated for 3 days, a microclimate pH was estimated to be in t he neighborhood of 1.8. The co-encapsulation of Mg(OH)(2) could both increa se the pa(H)* reading and neutralize pH in various regions of the microsphe re interior. Varying the drug lipophilicity and loading revealed that the p recipitation of the lactone could also stabilize CPT. Conclusions, PLGA microspheres prepared by the standard solvent evaporation techniques develop an acidic microclimate that stabilizes the lactone form of CPTs. This microclimate may be neutralized by co-encapsulating a base s uch as Mg(OH)(2), as suggested by previous work with poly(ortho eaters) (2) .