Neuropathological and biochemical assessments of an Alzheimer's disease patient treated with the γ-secretase inhibitor semagacestat.

Amyloid deposition has been implicated as the key determinant of Alzheimer's disease (AD) pathogenesis. Interventions to antagonize amyloid accumulation and mitigate dementia are now under active investigation. We conducted a combined clinical, biochemical and neuropathological assessment of a participant in a clinical trial of the γ-secretase inhibitor, semagacestat. This patient received a daily oral dose of 140 mg of semagacestat for approximately 76 weeks. Levels of brain amyloid-β (Aβ) peptides were quantified using enzyme-linked immunosorbent assays (ELISA). Western blot/scanning densitometry was performed to reveal BACE1, presenilin1, amyloid precursor protein (APP) and its proteolysis-produced C-terminal peptides APP-CT99 and APP-CT83 as well as several γ-secretase substrates. To serve as a frame of reference, the ELISA and Western analyses were performed in parallel on samples from neuropathologically confirmed non-demented control (NDC) and AD subjects who did not receive semagacestat. Neuropathology findings confirmed a diagnosis of AD with frequent amyloid deposits and neurofibrillary tangles in most areas of the cortex and subcortical nuclei as well as cerebellar amyloid plaques. Mean levels of Tris-soluble Aβ40 and glass-distilled formic acid (GDFA)/guanidine hydrochloride (GHCl)-extractable Aβ40 in the frontal lobe and GDFA/GHCl-soluble Aβ40 in the temporal lobe were increased 4.2, 9.5 and 7.7-fold, respectively, in the semagacestat-treated subject compared to those observed in the non-treated AD group. In addition, GDFA/GHCl-extracted Aβ42 was increased 2-fold in the temporal lobe relative to non-treated AD cases. No major changes in APP, β- and γ-secretase and CT99/CT83 were observed between the semagacestat-treated subject compared to either NDC or AD cases. Furthermore, the levels of γ-secretase substrates in the semagacestat-treated subject and the reference groups were also similar. Interestingly, there were significant alterations in the levels of several γ-secretase substrates between the NDC and non-treated AD subjects. This is the first reported case study of an individual enrolled in the semagacestat clinical trial. The subject of this study remained alive for ~7 months after treatment termination, therefore it is difficult to conclude whether the outcomes observed represent a consequence of semagacestat therapy. Additional evaluations of trial participants, including several who expired during the course of treatment, may provide vital clarification regarding the impacts and aftermath of γ-secretase inhibition.