The acidic microclimate in poly(lactide-co-glycolide) microspheres stabilizes camptothecins.
OBJECTIVE
The camptothecin (CPT) analogue, 10-hydroxycamptothecin (10-HCPT) has been shown previously to remain in its acid-stable (and 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) of 10-HCPT stabilization.


METHODS
CPTs were encapsulated in PLGA 50:50 microspheres by standard solvent 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 techniques were developed to assess the microclimate pH, including: i) measurement of H+ content of the dissolved microspheres in an 80:20 acetonitrile/H2O mixture and ii) confocal microscopy of an encapsulated pH-sensitive dye, fluorescein.


RESULTS
The encapsulated carboxylate converted rapidly to the lactone after exposure to the release media, indicating the lactone is favored at equilibrium in the microspheres. Upon co-encapsulation of Mg(OH)2, the trend was reversed, i.e., the lactone rapidly converted to the carboxylate form. Measurement of -log(hydronium ion activity) (paH*) of dissolved microspheres with pH-electrode and pH mapping with fluorescein revealed the presence of an acidic microclimate. From the measurements of H+ and water contents of particles hydrated for 3 days, a microclimate pH was estimated to be in the neighborhood of 1.8. The co-encapsulation of Mg(OH)2 could both increase the paH* reading and neutralize pH in various regions of the microsphere interior. Varying the drug lipophilicity and loading revealed that the precipitation 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 such as Mg(OH)2, as suggested by previous work with poly(ortho esters) (2).
Shenderova
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A
Division of Pharmaceutics and Pharmaceutical Chemistry, College of Pharmacy, Ohio State University, Columbus 43210, USA.
Burke
T G
TG
Schwendeman
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SP
D000970:Antineoplastic Agents / Q000737:chemistry*; D002166:Camptothecin / Q000031:analogs & derivatives* / Q000737:chemistry*; D004337:Drug Carriers; D004347:Drug Interactions; D004355:Drug Stability; D004566:Electrodes; D005456:Fluorescent Dyes; D006863:Hydrogen-Ion Concentration; D007783:Lactones / Q000737:chemistry; D008276:Magnesium Hydroxide / Q000737:chemistry; D008834:Microclimate; D008863:Microspheres; D011098:Polyglactin 910 / Q000737:chemistry; D014867:Water / Q000737:chemistry
D000970:Antineoplastic Agents; D004337:Drug Carriers; D005456:Fluorescent Dyes; D007783:Lactones; D014867:Water; D011098:Polyglactin 910; C028098:10-hydroxycamptothecin; D008276:Magnesium Hydroxide; D002166:Camptothecin