Tamoxifen Directly Interacts with the Dopamine Transporter.

The selective estrogen receptor modulator tamoxifen increases extracellular dopamine in vivo and acts as a neuroprotectant in models of dopamine neurotoxicity. We investigated the effect of tamoxifen on dopamine transporter (DAT)-mediated dopamine uptake, dopamine efflux, and [3H]WIN 35,428 [(-)-2-ß-carbomethoxy-3-ß-(4-fluorophenyl)tropane] binding in rat striatal tissue. Tamoxifen dose-dependently blocked dopamine uptake (54% reduction at 10 µM) and amphetamine-stimulated efflux (59% reduction at 10 µM) in synaptosomes. It also produced a small but significant reduction in [3H]WIN 35,428 binding in striatal membranes, indicating a weak interaction with the substrate binding site in the DAT. Biotinylation and cysteine accessibility studies indicated that tamoxifen stabilizes the outward-facing conformation of the DAT in a cocaine-like manner and does not affect surface expression of the DAT. Additional studies with mutant DAT constructs D476A and I159A suggested a direct interaction between tamoxifen and a secondary substrate binding site of the transporter. Locomotor studies revealed that tamoxifen attenuates amphetamine-stimulated hyperactivity in rats but has no depressant or stimulant activity in the absence of amphetamine. These results suggest a complex mechanism of action for tamoxifen as a regulator of the DAT. Due to its effectiveness against amphetamine actions and its central nervous system permeant activity, the tamoxifen structure represents an excellent starting point for a structure-based drug-design program to develop a pharmacological therapeutic for psychostimulant abuse.

Tamoxifen and its active metabolites inhibit dopamine transporter function independently of the estrogen receptors.

As one of the primary mechanisms by which dopamine signaling is regulated, the dopamine transporter (DAT) is an attractive pharmacological target for the treatment of diseases based in dopaminergic dysfunction. In this work we demonstrate for the first time that the commonly prescribed breast cancer therapeutic tamoxifen and its major metabolites, 4-hydroxytamoxifen and endoxifen, inhibit DAT function. Tamoxifen inhibits [3 H]dopamine uptake into human DAT (hDAT)-N2A cells via an uncompetitive or mixed mechanism. Endoxifen, an active metabolite of tamoxifen, asymmetrically inhibits DAT function in hDAT-N2A cells, showing a preference for the inhibition of amphetamine-stimulated dopamine efflux as compared to dopamine uptake. Importantly, we demonstrate that the effects of tamoxifen and its metabolites on the DAT occur independently of its activity as selective estrogen receptor modulators. This work suggests that tamoxifen is inhibiting DAT function through a previously unidentified mechanism.