The MT-associated protein tau forms filamentous inclusions within neurons in several CNS disorders, including Alzheimer’s disease (AD)1
and certain frontotemporal dementias [1
]. Collectively, these diseases are referred to as tauopathies, and alterations in normal tau structure and/or function likely play a causative role in the neuropathology of these various conditions. Indeed, tau gene (MAPT
) mutations cause frontotemporal dementia with Parkinsonism linked to chromosome 17 (FTDP-17) [4
], and dementia ratings in AD are correlated with the extent of tau brain deposits [6
Tau is highly expressed in neurons, where it plays a critical role in MT stabilization [8
] and axonal transport. The tau hyperphosphorylation that occurs in all tauopathies [10
] results in reduced tau binding to MTs and a decreased ability to promote MT assembly [12
]. Moreover, tau phosphorylation has been reported to enhance its propensity to fibrillize [17
] and this could cause destabilization of MTs due to the depletion of free tau.
The concept that neuropathology can result from tau loss-of-function is supported by studies that showed MT abnormalities and axonal transport deficits in motor axons of transgenic (Tg) mice that over-express human tau [19
]. Importantly, impaired tau function was compensated for in these mice by treatment with the MT-stabilizing agent paclitaxel, as drug absorption by motor neurons at peripheral neuromuscular junctions resulted in increased MT density and marked improvement in motor function [20
]. These data suggest that MT-stabilizing drugs that are utilized in oncology may be potential therapeutics for AD and other tauopathies. However, paclitaxel and most related taxanes are thought to have poor blood-brain barrier (BBB) permeability and are thus likely unsuitable for the treatment of human tauopathies, where pathology is predominantly in the brain. Accordingly, compensation for tau loss-of-function in tauopathies will require brain-penetrant MT-stabilizing agents.
The relative inability of paclitaxel and analogues to enter the brain is believed to be due in part to these compounds acting as substrates for the P-glycoprotein (Pgp) transporter that resides in endothelial cells that form the BBB [21
]. Taxane derivatives have been prepared that retain anti-mitotic activity in Pgp-expressing cells [23
], but this activity appeared to result from the compounds inhibiting Pgp function [26
]. Similarly, other taxanes such as TXD258 [28
] and RPR-109881A [29
] have been reported to accumulate in the brain, although the former is a modest Pgp substrate that may saturate the transporter at high blood drug levels [28
]. Because Pgp plays an important role in shielding the brain from undesirable xenobiotics, inhibitors of Pgp or substrates that impede Pgp function could be detrimental as therapeutics for chronic disorders like AD and related tauopathies. Accordingly, we [30
] and others [31
] have reported on the synthesis of novel taxane analogs that are neither Pgp substrates or inhibitors. Here we report on the further characterization of such molecules, as well as examples from the epothilone class of MT-stabilizing agents. The compounds were evaluated in cell-based assays to assess whether they interact with Pgp and to gauge their membrane permeability. Moreover, because there is little published information on the brain penetration of existing MT-stabilizing agents in vivo
, we compared the brain and plasma levels of these compounds in mice after systemic administration. Finally, we determined whether these molecules can affect CNS MTs in mice, utilizing a marker of MT stabilization (i.e., tubulin acetylation). We find that members of the epothilone series of MT-stabilizing compounds have much greater BBB penetration than the tested taxanes. Moreover, certain of the epothilones cause a significant increase in MT stabilization within the CNS. Thus, we have identified candidate MT-stabilizing compounds that are suitable for testing in Tg mouse models of tauopathy, and certain of these may hold promise as possible therapeutic agents for AD and related tauopathies.