PDE5 Inhibitors as Potential Tools in the Treatment of Cystic Fibrosis

Abstract

Despite great advances in the understanding of the genetics and pathophysiology of cystic fibrosis (CF), there is still no cure for the disease. Using phosphodiesterase type 5 (PDE5) inhibitors, we and others have provided evidence of rescued F508del-CFTR trafficking and corrected deficient chloride transport activity. Studies using PDE5 inhibitors in mice homozygous for the clinically relevant F508del mutation have been conducted with the aim of restoring F508del-CFTR protein function. We demonstrated, by measuring transepithelial nasal potential difference in F508del mice following intraperitoneal injection of sildenafil, vardenafil, or taladafil at clinical doses are able to restore the decreased CFTR-dependent chloride transport across the nasal mucosa. Moreover, vardenafil, but not sildenafil, stimulates chloride transport through the normal CFTR protein. We developed a specific nebulizer setup for mice, with which we demonstrated, through a single inhalation of PDE5 inhibitors, local activation of CFTR protein in CF. Significant potential advantages of inhalation drug therapy over oral or intravenous routes include rapid onset of pharmacological action, reduced systemic secondary effects, and reduced effective drug doses compared to the drug delivered orally; this underlines the relevance and impact of our work for translational science. More recently, we analyzed the bronchoalveolar lavage of CF and wild-type mice for cell infiltrates and expression of pro-inflammatory cytokines and chemokines; we found that the CFTR activating effect of vardenafil, selected as a representative long-lasting PDE5 inhibitor, breaks the vicious circle of lung inflammation which plays a major role in morbi-mortality in CF. Our data highlight the potential use of PDE5 inhibitors in CF. Therapeutic approaches using clinically approved PDE5 inhibitors to address F508del-CFTR defects could speed up the development of new therapies for CF.

Keywords: CFTR; PDE5 inhibitors; cystic fibrosis; sildenafil; taladafil; vardenafil.

Figure 1

Substrate specificity of the different families of PDE. Although a high degree of homology has been observed within the catalytic domain of PDEs, slight structural differences in these domains determine the specificity of substrate of PDEs (Xu et al., 2004).

Figure 2

Structures of the three clinically approved phosphodiesterase type 5 inhibitors. Sildenafil, vardenafil, and tadalafil have been approved for treatment of erectile dysfunction. Sildenafil and taladafil have also been approved as a treatment for pulmonary arterial hypertension. Sildenafil citrate is designated chemically as 1-[[3-(6,7-dihydro-1-methyl-7-oxo-3-propyl-1Hpyrazolo[4,3-d]pyrimidin-5-yl)-4-ethoxyphenyl]sulfonyl]-4-methylpiperazine citrate. Vardenafil HCl is designated chemically as piperazine, 1-[[3-(1,4-dihydro-5-methyl-4-oxo-7-propylimidazo[5,1-f][1,2,4]triazin-2-yl)-4-ethoxyphenyl]sulfonyl]-4-ethyl-, monohydrochloride. Taladafil is designated chemically as pyrazino[1′,2′:1,6]pyrido[3,4-b]indole-1,4-dione,6-(1,3-benzodioxol-5-yl)-2,3,6,7,12,12a-hexahydro-2-methyl-, (6R,12aR)-.

Figure 3

Representative tracings of nasal potential difference (PD) measurements in wild-type (WT) and F508del-CF (CF) mice 24 h after placebo (saline) or vardenafil (single i.p. dose of 0.14 mg/kg body weight). Tracings show sequential response of the nasal surface to perfusion successively with basal solution, basal solution with 10⁻⁴ M amiloride (amil), chloride-free solution plus amiloride (0 Cl), and chloride-free solution with amiloride plus 10–5 M forskolin (forskolin). Arrows indicate change of solutions. As illustrated, basal values and amiloride response are not influenced by vardenafil treatment. However, chloride secretion (difference between values obtained at the end of the test and the end of the amiloride phase) is restored in CF animals and the effect lasts at least 24 h after vardenafil treatment (Lubamba et al., 2008).

Figure 4

Effect of parenteral (i.p.) administration of sildenafil (A; 0.7 mg/kg body weight) and of vardenafil (B; 0.14 mg/kg body weight) on CFTR-dependent chloride secretion assessed by means of the nasal potential difference (PD) in wild-type (WT) and F508del-CF mice. Vardenafil stimulates chloride secretion of the wild-type CFTR. The correcting effect of vardenafil lasts at least 24 h. Data are expressed as mean ± SEM of 14–15 placebo treated animals and six animals treated with PDE5 inhibitors (Lubamba et al., 2008).

Figure 5

Anti-inflammatory effect of in vivo treatment, by i.p. injection, of a single therapeutic dose of vardenafil (vard) to wild-type (A) and F508del-CF (B) mice on the inflammatory response induced by lipopolyssaccharide from P. aeruginosa (LPS). Biosynthesis of CCL-2 is significantly reduced in the bronchoalveolar lavage of vardenafil-treated CF and non-CF animals (Lubamba et al., 2012b).