Druggable site near the upper vestibule determines the high affinity and
P2X3 selectivity of Sivopixant, a clinical candidate for refractory
chronic cough
Abstract
Background and Purpose: The ionotropic purinergic trimeric receptor P2X3
is a new drug target other than the opioid receptor for the treatment of
refractory chronic cough (RCC). However, the only marketed P2X3
antagonist, Gefapixant/AF-219, has a side effect of taste disorders due
to simultaneous action on the human P2X2/3 (hP2X2/3) heterotrimer.
Therefore, selective molecules with high affinity for the hP2X3
homotrimer and low affinity for the hP2X2/3 heterotrimer have potential
in iteration 2.0 RCC drug development, such as Sivopixant/S-600918, a
clinical phase II RCC candidate with lower taste disturbance than
Gefapixant. S-600918 and its analogue
(3-(4-((3-chloro-4-isopropoxyphenyl)amino)-3-(4-methylbenzyl)-2,6-dioxo-3,6-dihydro-1,3,5-triazin-1(2H)-yl)propanoic
acid (DDTPA) exhibit both high affinity and high selectivity for hP2X3
homotrimers compared to hP2X2/3 heterotrimer. The mechanism of its
druggable site and this high selectivity is not clear. Experimental
Approach: Here, we reveal a novel allosteric mechanism that
distinguishes this drug candidate from other P2X3 inhibitors through
chimera construction, site covalent occupation, metadynamics,
mutagenesis, and electrophysiology. Key Results: We suggest that the
tri-symmetric site adjacent to the upper vestibule determines the high
affinity and selectivity of S-600918/DDTPA for hP2X3. Only four amino
acids of the hP2X2 upper body domain swapped with hP2X3, allow the
hP2X2/3 heterotrimer to gain comparable affinity for S-600918/DDTPA as
the hP2X3 homotrimer. Conclusion and Implications: Thus, we have
revealed the molecular basis for the cough suppressive effects and
reduced side effects of new RCC clinical candidates from the perspective
of receptor-ligand recognition, which may provide information critical
for the development of new drugs targeting P2X3 for indications such as
RCC, idiopathic pulmonary fibrosis (IPF), and primary hypertension.