IN-VITRO STUDY FOR THE ASSESSMENT OF POLY(L-ASPARTIC ACID) AS A DRUG CARRIER FOR COLON-SPECIFIC DRUG-DELIVERY

Selective delivery of drugs to the colon for the treatment of large bo wel disorders can be achieved using prodrugs which are cleaved by enzy mes produced by microorganisms residing in the large intestine. Since it is known that enzymes of gut microflora are able to cleave certain peptide and eater bonds, the ability of poly(L-aspartic acid) (weight- average MW: 30000) to act as a drug carrier for the model drug dexamet hasone (DX) was investigated. The ester prodrug (10% w/w drug loading) was synthesized in high yield using dicyclohexylcarbodiimide as dehyd rating agent in dimethylformamide. Prodrug incubations were carried ou t in the homogenized luminal contents of various GI-tract segments of male Sprague-Dawley rats at 37 degrees C and pH 6.5 (stomach at pH 4.5 ) and prodrug concentrations which provide enzyme saturated conditions . DX concentrations were measured by HPLC. In addition, incubations we re done with the homogenized mucosa and muscle tissues at pH 4.5 (pH o f the lysosomal compartment) and at pH 6.5. The half-life t(1/2) of ch emical hydrolysis at these pH-values (37 degrees C) was 114 days and 2 .8 days, respectively. Enzymatic activity of the mucosa and the tissue samples was low ([10 mu g DX/g mucosa or tissue per h); in contrast, markedly higher activities were found in the luminal contents. The cec um and colon contents showed significantly (P < 0.01) higher enzymatic activities (60 and 80 mu g DX/g content per h, respectively) than the contents of the stomach or the small intestine (5-20 pg DX/g content per h). With the intestinal contents of colitic rats, lower enzymatic activities were found than with the contents of the conventional rats. Negligible enzymatic activities in the cecum and colon contents of ge rm-free animals indicate that bacterial enzymes are responsible for th e cleavage of the prodrug. Incubations with human feces samples from c olitic patients and non-colitic patients result in prodrug hydrolysis of 26-40 mu g DX/g feces per h with no significant difference between the two groups. According to the present results, poly(l-aspartic acid ) seems to be a suitable drug carrier for colon-specific drug delivery .