NMR spectroscopic evaluation of the internal environment of PLGA microspheres

The internal environment of poly(lactide-co-glycolide) (PLGA) microspheres was characterized using 31P and 13C solid-state and solution NMR spectroscopy. Physical and chemical states of encapsulated phosphate- and histidine-containing porogen excipients were evaluated using polymers with blocked (i.e., esterified) or unblocked (free acid) end groups. Spectroscopic and gravimetric results demonstrated that the encapsulated porogen deliquesced upon hydration at 84% relative humidity to form a solution environment inside the microspheres. Dibasic phosphate porogen encapsulated in unblocked PLGA was partially titrated to the monobasic form, while in the same formulation 13C NMR showed partial protonation of the histidine imidazole. Similarly, encapsulated monobasic phosphate was partially converted to phosphoric acid. Coencapsulation of monobasic and dibasic phosphate porogens resulted in a single peak on hydration, indicating chemical exchange between discrete excipient microphases. Exogenous buffer addition differentiated external from internal, nontitratable, excipient populations. Microspheres containing dibasic phosphate porogen were hydrated with fetal calf serum, incubated at 37 degrees C, and characterized by 31P NMR through the polymer erosion phase. Within 48 h the 31P chemical shift moved over 2 ppm upfield and the line width narrowed to <60 Hz; there was little additional change through day 14. This indicated complete conversion to the monobasic phosphate form throughout the polydisperse sample and that pH remained below 4 but above the phosphoric acid p K a during matrix erosion.