Repurposing of the PDE5 Inhibitor Sildenafil for the Treatment of Persistent Pulmonary Hypertension in Neonates.

Nitric oxide (NO), an important endogenous signaling molecule released from vascular endothelial cells and nerves, activates the enzyme soluble guanylate cyclase to catalyze the production of cyclic guanosine monophosphate (cGMP) from guanosine triphosphate. cGMP, in turn, activates protein kinase G to phosphorylate a range of effector proteins in smooth muscle cells that reduce intracellular Ca2+ levels to inhibit both contractility and proliferation. The enzyme phosphodiesterase type 5 (PDE5) curtails the actions of cGMP by hydrolyzing it into inactive 5'-GMP. Small molecule PDE5 inhibitors (PDE5is), such as sildenafil, prolong the availability of cGMP and therefore, enhance NO-mediated signaling. PDE5is are the first-line treatment for erectile dysfunction but are also now approved for the treatment of pulmonary arterial hypertension (PAH) in adults. Persistent pulmonary hypertension in neonates (PPHN) is currently treated with inhaled NO, but this is an expensive option and around 1/3 of newborns are unresponsive, resulting in the need for alternative approaches. Here the development, chemistry and pharmacology of PDE5is, the use of sildenafil for erectile dysfunction and PAH, are summarized and then current evidence for the utility of further repurposing of sildenafil, as a treatment for PPHN, is critically reviewed.

Deletion of murine slc29a4 modifies vascular responses to adenosine and 5-hydroxytryptamine in a sexually dimorphic manner.

Equilibrative nucleoside transporter 4 (ENT4), encoded by SLC29A4, mediates the flux of both 5-hydroxytryptamine (5-HT) and adenosine across cell membranes. We hypothesized that loss of ENT4 function in mice would modify the effects of these established regulators of vascular function. Male and female wild-type (WT) and slc29a4-null (ENT4-KO) mice were compared with respect to their hemodynamics and mesenteric vascular function. Male ENT4-KO mice had a complete loss of myogenic tone in their mesenteric resistance arteries. This was accompanied by a decrease in blood flow in the superior mesenteric artery in the male ENT4-KO mice, and a reduced responsiveness to 5-HT. In contrast, endothelium-dependent relaxations of mesenteric arteries from female ENT4-KO mice were more sensitive to Ca2+ -activated K+ (KCa ) channel blockade than WT mice. Female ENT4-KO mice also demonstrated an enhanced vasodilatory response to adenosine in vivo that was not seen in males. Ketanserin (5-HT2A inhibitor) and GR55562 (5-HT1B/1D inhibitor) decreased 5-HT-induced tone, but only ketanserin inhibited the relaxant effect of 5-HT in mesenteric arteries. 5-HT-evoked increases in tone were elevated in arteries from ENT4-KO mice upon block of endothelial relaxant pathways, with arteries from female ENT4-KO mice showing the greatest increase. Adenosine A2b receptor expression was decreased, while other adenosine transporter subtypes, as well as adenosine deaminase and adenosine kinase were increased in mesenteric arteries from male, but not female, ENT4-KO mice. These findings indicate that deletion of slc29a4 leads to sex-specific changes in vascular function with significant consequences for regulation of blood flow and pressure by adenosine and 5-HT.