11 2021 203 83 The present invention addresses the problem of providing: a compound and a pharmaceutical composition that exhibit superior efficacy against tumors especially tumors that have acquired gemcitabine resistance. The present invention provides thionucleoside derivatives represented by general formula (1) or salts thereof. (In the formula: R represents an optionally protected hydroxyl group an optionally substituted C alkoxy group or the like R represents an optionally substituted C alkoxy group an optionally substituted C cycloalkoxy group or the like and R represents a hydrogen atom or the like). Further provided are pharmaceutical compositions containing these thionucleoside derivatives or salts thereof.
