The present invention relates to pure 5-HT6 receptor antagonists selected from 1-[(2-Bromophenyl)sulfonyl]-5-methoxy-3-[(4-methyl-1-piperazinyl)methyl]-1H-indole； 1-[(4-Fluorophenyl)sulfonyl]-5-methoxy-3-[(4-methyl-1-piperazinyl)methyl]-1H-indole； or 1-[(4-Isopropylphenyl)sulfonyl]-5-methoxy-3-[(4-methyl-1-piperazinyl)methyl]-1H-indole, or their pharmaceutically acceptable salt(s) thereof, in combination with or as adjunct to a N-Methyl-D-aspartate (NMDA) receptor antagonist and their use in the treatment of cognitive disorders. The invention further relates to the pharmaceutical composition containing the said combination, and the use of the combination in an improved combination therapy for the treatment of cognitive disorders, such as Alzheimer’s disease, schizophrenia, Parkinson’s disease, Lewy body dementia, vascular dementia or frontotemporal dementia.
