Biomedical Engineering Reference
In-Depth Information
The binding sites for antidepressants at their main targets: the NET and SERT are poorly
described. Remarkably, it was recently reported that TCAs, such as clomipramine, imipramine, and
desipramine, have activity at the bacterial homologue LeuT
Aa
; hence, it was observed that the com-
pounds were capable of noncompetitively inhibiting substrate binding to the transporter. The effect
was only seen with high concentrations of the compounds and, thus, their afi nity for LeuT
Aa
is
substantially lower than that observed for the NET/SERT. Nonetheless, it has been possibly to crys-
tallize LeuT
Aa
in complex with these compounds. The structures showed that clomipramine, desip-
ramine, and imipramine bind in an extracellular-facing vestibule about 11 Å above the occluded
substrate-binding site, apparently stabilizing the extracellular gate in a closed conformation. The
TCAs are cradled by the carboxy-terminal half of transmembrane helix 1 (TM1), the aminoterminal
regions of TM6 and TM10, the approximate midpoint of TM3, and the sharp turn of ECL4.
The structures of TCA-bound LeuT
Aa
obviously raise the key question whether they describe a
binding mode for inhibitors that can be generalized to their mammalian counterparts. Mutations
suggested that desipramine might interact in a similar fashion with the SERT; however, previous
mutagenesis has supported that SSRIs, such as citalopram, l uoxetine, and sertraline as well as the
TCA clomipramine, bind deeper in the transporter structure in a site more close to the substrate-
binding site, i.e., mutation of Tyr95 in TM1 and/or Ile172 in TM3 of SERT substantially decreased
the afi nity for these compounds. Most signii cantly, the combined mutation of Tyr95 (Y95F) and
Ile172 (I172M) decreased transporter afi nity ~10,000-fold for escitalopram. The recent evidence
for a buried binding site for cocaine and related inhibitors in DAT (see earlier) also strongly argues
against that the TCA-binding mode seen in LeuT
Aa
can be generalized to other transporters.
14.3.4 O
THER
B
IOGENIC
A
MINE
T
RANSPORTER
I
NHIBITORS
The examples of additional biogenic amine inhibitors include the GBR (from Royal Gist-Brocades)
analogues that are highly selective for DAT and mazindol (Figure 14.4) that inhibits NET with
one and two orders of magnitude higher potency than DAT and SERT, respectively. Finally, the
amphetamine derivative methylphenidate (Figure 14.4) is a potent blocker of primarily DAT
and NET, and often used for treatment of narcolepsy and attention dei cit hyperactivity disorder
(ADHD). Not much is known about the molecular basis for the interaction of these compounds with
the transporters.
14.4 INHIBITORS OF GLYCINE AND GABA TRANSPORTERS:
SPECIFICITY, USE, AND MOLECULAR MECHANISM OF ACTION
Other SLC6 family transporters than the biogenic amine transporters are targets for drugs or for
drug discovery. GAT-1 is, for example, the target for the antiepileptic drug tiagabine; however, the
molecular basis for its interaction with GAT-1 is not known. Recently, the
N
-dithienyl-butenyl deriv-
ative of
N
-methyl-exo-THPO (4-methylamino-4,5,6,7-tetrahydrobenzo[d]isoxazol-3-ol) (EF-1502)
has been shown to inhibit not only GAT-1 but also the betaine carrier (BGT-1) and to act as a very
efi cient anticonvulsant whose action is synergistic with that of tiagabine (see also Section 15.4).
Thus, BGT-1 is likely to be an important antiepileptic drug target. The explanation for the observa-
tions might be related to a differential distribution of BGT-1 and GAT-1. While GAT-1 is localized
to synaptic sites, BGT-1 is localized to astrocytes and possibly extrasynaptic loci in the neurons;
hence the efi cacy of EF-1502 owing to its interaction with BGT-1 could be explained by modulation
of extracellular GABA concentrations at extrasynaptic sites (for further details about GABA recep-
tors and transporters see Chapter 15).
The high-afi nity glycine transporters (GlyT1 and GlyT2) might also represent interesting drug
targets. Physiologically, GlyT1 appears to play a role in astroglial control of glycine availability at
NMDA receptors whereas GlyT2 is likely to play a fundamental role in glycinergic inhibition as
rel ected in a lethal neuromotor dei ciency in GlyT2 knockout mice. The putative role of GlyT1 in
