European Journal of Pharmaceutical Sciences Vol.34 Nr. Supplement 1, S40 ; Betulin is a widespread naturally occurring triterpenene, found in large quantities in birch bark. Despite the knowledge on betulin-derived compounds as anti-HIV agents, their wider antiviral spectrum remains poorly studied. In this study, a set of 68 betulin derivatives was screened against Semliki Forest virus (SFV), a member of Alphavirus genus and a commonly used model for replication of positive-stranded RNA viruses. The basic structure of betulin was moderately active against SFV (IC50 45 µM), but antiviral potency was enhanced particularly by certain cyclic ether or ester substituents, as demonstrated by 28-O-tetrahydropyranylbetulin and 3,28-betulinyl di(2-sulfobenzoate) (IC50 values 17 and 11 µM, respectively). Even though some derivatives raised also toxic responses in Huh-7 cells, therapeutic indices higher than 50 were obtained for many of the tested compounds. Due to their non-nucleoside structure and relatively low toxicity, betulin-derived compounds make a competitive contribution to the few inhibitors of these pathogens. ; Betulin is a widespread naturally occurring triterpenene, found in large quantities in birch bark. Despite the knowledge on betulin-derived compounds as anti-HIV agents, their wider antiviral spectrum remains poorly studied. In this study, a set of 68 betulin derivatives was screened against Semliki Forest virus (SFV), a member of Alphavirus genus and a commonly used model for replication of positive-stranded RNA viruses. The basic structure of betulin was moderately active against SFV (IC50 45 µM), but antiviral potency was enhanced particularly by certain cyclic ether or ester substituents, as demonstrated by 28-O-tetrahydropyranylbetulin and 3,28-betulinyl di(2-sulfobenzoate) (IC50 values 17 and 11 µM, respectively). Even though some derivatives raised also toxic responses in Huh-7 cells, therapeutic indices higher than 50 were obtained for many of the tested compounds. Due to their non-nucleoside structure and relatively low toxicity, betulin-derived compounds make a competitive contribution to the few inhibitors of these pathogens. ; 19th Helsinki Drug Research 2008. Helsinki, Finland, 9 - 11 June 2008