The prevention of
tau protein aggregations is a therapeutic goal for the treatment of
Alzheimer's disease (AD), and
hydromethylthionine (HMT) (also known as leucomethylthioninium-
mesylate [LMTM]), is a potent inhibitor of tau aggregation in vitro and in vivo. In two Phase 3 clinical trials in AD, HMT had greater pharmacological activity on clinical endpoints in patients not receiving approved symptomatic treatments for AD (
acetylcholinesterase (AChE) inhibitors and/or
memantine) despite different mechanisms of action. To investigate this drug interaction in an animal model, we used tau-transgenic L1 and wild-type NMRI mice treated with
rivastigmine or
memantine prior to adding HMT, and measured changes in hippocampal
acetylcholine (ACh) by microdialysis. HMT given alone doubled hippocampal ACh levels in both mouse lines and increased stimulated ACh release induced by exploration of the open field or by infusion of
scopolamine.
Rivastigmine increased ACh release in both mouse lines, whereas
memantine was more active in tau-transgenic L1 mice. Importantly, our study revealed a negative interaction between HMT and symptomatic AD drugs: the HMT effect was completely eliminated in mice that had been pre-treated with either
rivastigmine or
memantine.
Rivastigmine was found to inhibit AChE, whereas HMT and
memantine had no effects on AChE or on
choline acetyltransferase (ChAT). The interactions observed in this study demonstrate that HMT enhances
cholinergic activity in mouse brain by a mechanism of action unrelated to AChE inhibition. Our findings establish that the drug interaction that was first observed clinically has a neuropharmacological basis and is not restricted to animals with tau aggregation pathology. Given the importance of the
cholinergic system for memory function, the potential for commonly used AD drugs to interfere with the treatment effects of disease-modifying drugs needs to be taken into account in the design of clinical trials.