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The determination of in vitro release kinetics of peptides from poly(D,Llactide-co-glyolide) (PLGA) microspheres generally requires optimization of the test conditions for a given formulation. This is particularly important when in vitro/in vivo correlation should be determined. Here, the somatostatin analogue vapreotide pamoate, an octapeptide, was microencapsulated into PLGA 50:50 by spray-drying.The solubility of this peptide and its in vitro release kinetics from the microspheres was studied in various test media. The solubility of vapreotide pamoate was approx.20-40 μg/mL in 67 mM phosphate buffer saline (PBS) of pH 7.4, but increased to approx. 500-1000 μg/mL at a pH of 3.5. At low pH, the solubility increased with the buffer concentration (1-66 mM). Very importantly, proteins (aqueous BSA solution or human serum) appeared to solubilize the peptide pamoate, resulting in solubilities ranging from 900 to 6100 μg/mL. The release rate was also greatly affected by the medium composition. Typically, in PBS of pH 7.4, only 33±1% of the peptide were released within 4 days, whereas 53±2% and 61±0.9% were released in 1% BSAsolution and serum, respectively. The type of medium was found critical for the estimation of the in vivo release. The in vivo release kinetics of vapreotide pamoate from PLGA microspheres following administration to rats were qualitatively in good agreement with those obtained in vitro using serum as release medium. Finally, sterilization by γ-irradiation had only a minor effect on the in vivo pharmacokinetics.

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The purpose of this study was to design poly(lactide-co-glycolide) (PLGA) microspheres for the continuous delivery of the somatostatin analogue vapreotide over 2-4 weeks. The microspheres were produced by spray-drying and the desired characteristics, i.e. high encapsulation efficiency and controlled release over 2-4 weeks, achieved through optimizing the type of polymer, processing solvent and coencapsulated additive. The in vitro release was tested in fetal bovine serum preserved with 0.02% of thiomersal. Furthermore, formulations were injected intramuscularly into rats to obtain pharmacokinetic profiles. Encapsulation efficiency was between 34 and 91%, depending on the particular formulation. The initial peptide release (within 6 h) was lowest, i.e. <20%, when acetic acid was used as processing solvent and highest, i.e. 57%, with dichloromethane. The various coencapsulated additives generally lowered the encapsulation efficiency by 15-30%.The best formulation in terms of low burst and effective drug serum levels (> 1 ng/ml) over 21 to 28 days in rats was the one made with end-group uncapped PLGA 50:50, the solvent acetic acid and the additive polyethyleneglycol. In conclusion, the optimization of formulation parameters allowed us to produce vapreotide-loaded PLGA microspheres of suitable characteristics for therapeutic use.

The development of a long-acting delivery system for D-Phe-Cys-Tyr-D-Trp-Lys-Val-Cys-Trp-NH2 (RC-160), an octapeptide analog of somatostatin, required the establishment of a method for determining the concentration of this analog in serum during treatment. A sensitive and specific radioimmunoassay (RIA) for RC-160 was developed and used for following the rate of liberation of this peptide from microcapsules of poly(DL-lactide-coglycolide). Antibodies were generated in a rabbit against RC-160 coinugated to bovine serum albumin with glutaraldehyde. At an antiserum dilution of 1:100,000, the antibodies bound approximately 25% of added radiolabeled RC-160. Somatostatin octapeptide analogs that had a disulfide bridge showed crossreactivity with the antiserum, but analogs without the disulfide bridge and other peptides tested did not crossreact. The minimum detectable dose of RC-160 was 10 pg. Intra-and interassay coefficients of variation ranged from 9.1% to 12.8% and from 14% to 30%, respectively. The RIA was suitable for direct determination of RC-160 in serum. Eleven prototype batches of microcapsules were tested in rats, and the rate of release of the analog from the microcapsules was followed. An improved batch of microcapsules made from RC-160 pamoate maintained high serum levels of RC-160 for more than 30 days after intramuscular injection. The RIA should be of value for monitoring levels of this analog in serum during long-term therapy.Superactive analogs of somatostatin such as D-Phe-CTs-Tyr-D-Trp-Lys-Val-Cys-Trp-NH2 (RC-160) and D-Phe-Cs-TyTrLys-Val-C's-Thr-NH2 (RC-121) show a high potency and a long duration of action in tests for inhibition of growth hormone release and much lower activity for suppression of insulin, glucagon, and gastric acid secretion (1, 2). This indicates a marked selectivity in their biological actions, which could make them useful for the treatment of endocrinerelated diseases such as type I diabetes mellitus and diabetic retinopathy.There is also experimental evidence that somatostatin analogs might be used to treat pancreatic cancer (3) by inhibiting the release of gastrointestinal hormones such as secretin, gastrin, and cholecystokinin or by stimulating the dephosphorylation of membrane receptors for the epidermal growth factor (4). In addition, somatostatin analogs of this class inhibit the growth of experimental prostate and mammary tumors by suppressing the levels of prolactin, growth hormone, and insulin-like growth factor I (IGF-I, somatomedin C) (5) and could be used as adjuncts to therapy with analogs of luteinizing hormone-releasing hormone (LH-RH).Thus, therapeutic application of these analogs in the treatment of various hormone-sensitive tumors is likely (6).The use of RC-160 and related analogs for the therapy of hormone-dependent tumors and endocrine disorders would be greatly enhanced by delayed delivery systems capable of maintaining controlled levels of the peptide over an extended period of time. Consequently, we started the development of such a long-acti...