AIM:To investigate Time- and pH-dependent colon-specificdrug delivery systems(CDDS)for orally administered diclofenacsodium(DS)and 5-aminosalicylic acid(5-ASA),respectively.METHODS:DS tablets and 5-ASA pellets were coated byethylcellulose(EC)and methacrylic acid copolymers(Eudragit~(?) L100 and S100),respectively.The in vitro releasebehavior of the DS coated tablets and 5-ASA coated pelletswere examined,and then in vivo absorption kinetics of DScoated tablets in dogs were further studied.RESULTS:Release profile of time-dependent DS coatedtablets was not influenced by pH of the dissolution medium,but the lag time of DS release was primarily controlled bythe thickness of the coating layer.The thicker the coatinglayer,the longer the lag time of DS release is.On thecontrary,in view of the pH-dependent 5-ASA coated pellets,5-ASA release was significantly governed by pH.Moreover,the 5-ASA release features from the coated pellets dependedupon both the combination ratio of the Eudragit~(?) L100 andS100 pH-sensitive copolymers in the coating formulation andthe thickness of the coating layer.The absorption kineticstudies of the DS coated tablets in dogs demonstrated thatin vivo lag time of absorption was in a good agreement within vitro lag time of release.CONCLUSION:Two types of CDDS,prepared herein bymeans of the regular coating technique,are able to achievesite-specific drug delivery targeting at colon following oraladministration,and provide a promising strategy to controldrug release targeting the desired lower gastrointestinal region. AIM: To investigate Time- and pH-dependent colon-specific drug delivery systems (CDDS) for orally administered diclofenacsodium (DS) and 5-aminosalicylic acid (5-ASA), respectively. METHODS: DS tablets and 5-ASA pellets were coated by ethylcellulose (EC) and methacrylic acid copolymers (Eudragit® L100 and S100), respectively. The in vitro release behavior of the DS coated tablets and 5-ASA coated pelletswere examined, and then in vivo absorption kinetics of DScoated tablets in dogs were further studied.RESULTS: Release profile of time-dependent DS coated tablets was not influenced by pH of the dissolution medium, but the lag time of DS release was required by the thickness of the coating layer. the thicker the coating layer, the longer the lag time of DS release is. Of thecontrary, in view of the pH-dependent 5-ASA coated pellets, 5-ASA release was significantly governed by pH. Moreover, the 5-ASA release features from the coated pellets dependedupon both combination combination of the Eudragit ~ (?) L10 0 and S100 pH-sensitive copolymers in the coating formulation and the thickness of the coating layer. The absorption kinetic studies of the DS coated tablets in dogs demonstrates that in vivo lag time of absorption was in a good agreement within vitro lag time of release. CONCLUSION: Two types of CDDS, prepared herein bymeans of the regular coating technique, are able to achievesite-specific drug delivery targeting at colon following oraladministration, and provide a promising strategy to control drug release targeting the desired lower gastrointestinal region.