The present invention relates to fluoropiperidine compounds of formula (I), their stereoisomers, isotopic forms or pharmaceutically acceptable salts thereof as 5-HT6 receptor antagonists. In particular the present invention discloses the methods of preparation, pharmaceutical composition, combinations and use of fluoropiperidine compounds, their stereoisomers, isotopic forms or pharmaceutically acceptable salts thereof. In formula (I), R1 represents phenyl or pyridyl; wherein the phenyl or pyridyl is optionally substituted with one or more groups selected from halogen, (C1-6)-alkyl or halo(C1-6 )-alkyl; R2 represents hydrogen or (C1-6)-alkyl; R3 represents hydrogen or (C1-6)-alkyl; or R2 and R3 can combine together to form (C3-6)- cycloalkyl; R4 represents hydrogen, (C1-6)-alkyl or halo(C1-6)-alkyl; R5 represents hydrogen, (C1-6)-alkyl, halo(C1-6)-alkyl or -(CH2)0-3- (C3-6)-cycloalkyl. The compounds can be used in the treatment of cognitive disorder mediated by the 5-Hydroxytryptamine 6 receptor, wherein said cognitive disorder is selected from the group consisting of dementia in Alzheimer's disease, dementia in Parkinson's disease, dementia in Huntington's disease, dementia associated with Down syndrome, dementia associated with Tourette's syndrome, dementia associated with post menopause, frontotemporal dementia, Lewy body dementia, Vascular dementia, dementia in HIV, dementia in Creutzfeldt-Jakob disease, substance-induced persisting dementia, dementia in Pick's disease, dementia in schizophrenia, dementia in general medical conditions and senile dementia.