Many drugs, especially proteins and peptides, when administered by conventional method (e.g. oral administration in the form of tablets or capsules and intravenous or intramuscular injections), do not cause the desired therapeutic activity. Therefore, the therapeutic applicability of these pharmaceutically active proteins/peptides is limited because of lack of suitable delivery systems. At present a number of delivery systems for peptides/proteins is under investigation, among which liposomes, polymeric nanoparticles and microspheres, as well as implants from biodegradable polymers and hydrogels.

ABA triblock copolymers (block A: poly(N-(2-hydroxypropyl)methacrylamide mono/dilactate), block B: PEG) have thermosensitive behavior. These polymers are soluble in water below the Lower Critical Solution Temperature (LCST) of the thermosensitive A block and gellify above this temperature. The LCST of this polymer can be modulated by HPMA-mono/di lactate ratio.

The aim of this study was to design an injectable formulation with these polymers from which opioid peptides are released for a period of around 10 days.

Thermosensitive polymers (with different length of PEG) were synthesized using radical polymerization and characterized by DSC, rheology and Static Light Scatering measurements. Hydrogels with different water contents and loaded with two opoid peptides (biphalin, β-casomorphin) were prepared by bringing aqueous solutions of the ABA block copolymers above the LCST of the thermosensitive blocks. The released peptides were quantified using HPLC.

The obtained results show that hydrogels based on thermosensitive poly(HPMAm-mono/di lactate)-PEG4000-poly(HPMAm-mono/di lactate) triblock copolymer showed a sustained release of the peptides for 5 till 10 days. These systems are therefore suitable candidates for further in vivo evaluations.

Acknowledgements

The investigations were supported by a research grant cooperation Polish-Netherlands scholarship.

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