The preparation of injectable sustained release microspheres of TRH was investigated. The drug was encapsulated in copoly(dl-lactic/glycolic acid) (PLGA) using an in-water drying method through a w/o/w emulsion. The predominant factors influencing the entrapment ratio of TRH in the microspheres and the release rate of TRH from the microspheres were the loading amount and the dissociation state of the drug. Microspheres of TRH encapsulated as a free base at an adequate concentration provided a small initial burst followed by a zero-order prolonged release regardless of the high water solubility of the drug. The addition of strong acids to the inner aqueous phase interfered with the encapsulation efficacy and caused a large initial burst. The ionic interaction between the basic functional group of TRH and the carboxylic acid end group of PLGA during the preparation is necessary to produce a rigid matrix of the microspheres. This method is applicable to the microencapsulation of other water-soluble basic peptides in PLGA.
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