Potential cognitive enhancing and disease modification effects of SSRIs for Alzheimer’s disease

OBJECTIVE Selective serotonin reuptake inhibitors (SSRIs) have increased cognitive performance in some clinical studies of Alzheimer's disease (AD), but it is has been difficult to dissociate whether this is due to direct effects on cognition (neurochemical or disease-modifying) or a secondary effect of mood stabilization. We performed a systematic review for preclinical and human clinical trial evidence to support the use of SSRIs specifically for the management of cognitive decline in AD. DATA SOURCES (1) PUBMED without language restrictions from 1950s until 2004 and updated August 2006, terms: "serotonin uptake inhibitors"[MeSH] AND ("Alzheimer disease"[MeSH] OR "Cognition Disorders"[MeSH]) NOT "Parkinson disease"[MeSH] AND (Clinical Trial[ptyp] OR Letter[ptyp] OR Meta-Analysis[ptyp] OR Randomized Controlled Trial[ptyp]) AND "alzheimer disease" [MESH] OR "Alzheimer*" combined with AND to "ssri*" OR "serotonin reuptake inhibitors" [MESH] NOT Review[ptyp]. (2) Cochrane Database of Systematic Reviews, keywords "SSRI" and "Alzheimer's". STUDY SELECTION The PubMed search yielded 57 hits. Of these, 23 were included in this review for their specificity to SSRI use in AD or indications on efficacy beyond depressive symptoms. The other 34 citations were excluded because: (1) depression or other mood or behavioral disturbance severity was the reported outcome measure, (2) effects of SSRIs on cognition were confounded by concomitant use of other drugs, (3) subjects described were young adults, and/or (4) subjects had traumatic brain injury. The Cochrane Database of Systematic Reviews, 3rd Quarter 2006, yielded six citations related to SSRIs. DATA EXTRACTION Data extracted from clinical trials included name of SSRI tested, cognitive outcome measures, and adverse events reported, which could include cognitive worsening. DATA SYNTHESIS Preclinical evidence for use of SSRIs to enhance cognition in AD includes an effect at the hippocampus through carbonic anhydrase activation or stimulation of hippocampal neurogenesis. The chemical structure of paroxetine, and not intrinsic SSRI activity, may also affect APP ectodomain expression to reduce amyloid plaque formation. Clinical trials in AD generally have not assessed cognitive outcomes independently from mood or behavior stabilization. Currently, clinical studies in AD only indirectly support the use of SSRIs for disease modification by confirming a serotonergic deficit during the course of illness. CONCLUSIONS Lack of supportive evidence for SSRIs as cognition enhancers or disease modifiers in AD is the result of omissions in clinical trial design, as opposed to reporting of negative outcomes. The preclinical evidence warrants the study of SSRIs in AD using mood, behavior, cognition, neurochemistry, and possibly neuroimaging as outcome variables.