Background: Symptomatic treatments of Alzheimer’s Disease (AD) with cholinesterase inhibitors
and/or memantine are relatively ineffective and there is a need for new treatments targeting the underlying
pathology of AD. In most of the failed disease-modifying trials, patients have been allowed to continue taking
symptomatic treatments at stable doses, under the assumption that they do not impair efficacy. In recently
completed Phase 3 trials testing the tau aggregation inhibitor leuco-methylthioninium bis (hydromethanesulfonate)
(LMTM), we found significant differences in treatment response according to whether patients were
taking LMTM either as monotherapy or as an add-on to symptomatic treatments.
Methods: We have examined the effect of either LMTM alone or chronic rivastigmine prior to LMTM treatment
of tau transgenic mice expressing the short tau fragment that constitutes the tangle filaments of AD. We
have measured acetylcholine levels, synaptosomal glutamate release, synaptic proteins, mitochondrial complex
IV activity, tau pathology and Choline Acetyltransferase (ChAT) immunoreactivity.
Results: LMTM given alone increased hippocampal Acetylcholine (ACh) levels, glutamate release from synaptosomal
preparations, synaptophysin levels in multiple brain regions and mitochondrial complex IV activity,
reduced tau pathology, partially restored ChAT immunoreactivity in the basal forebrain and reversed deficits
in spatial learning. Chronic pretreatment with rivastigmine was found to reduce or eliminate almost all these
effects, apart from a reduction in tau aggregation pathology. LMTM effects on hippocampal ACh and synaptophysin
levels were also reduced in wild-type mice.
Conclusion: The interference with the pharmacological activity of LMTM by a cholinesterase inhibitor can be
reproduced in a tau transgenic mouse model and, to a lesser extent, in wild-type mice. Long-term pretreatment
with a symptomatic drug alters a broad range of brain responses to LMTM across different transmitter systems
and cellular compartments at multiple levels of brain function. There is, therefore, no single locus for the negative
interaction. Rather, the chronic neuronal activation induced by reducing cholinesterase function produces
compensatory homeostatic downregulation in multiple neuronal systems. This reduces a broad range of treatment
responses to LMTM associated with a reduction in tau aggregation pathology. Since the interference is
dictated by homeostatic responses to prior symptomatic treatment, it is likely that there would be similar interference
with other drugs tested as add-on to the existing symptomatic treatment, regardless of the intended
therapeutic target or mode of action. The present findings outline key results that now provide a working
model to explain interference by symptomatic treatment.