Repurposing erectile dysfunction drugs tadalafil and vardenafil to increase bone mass

Abstract

We report that two widely-used drugs for erectile dysfunction, tadalafil and vardenafil, trigger bone gain in mice through a combination of anabolic and antiresorptive actions on the skeleton. Both drugs were found to enhance osteoblastic bone formation in vivo using a unique gene footprint and to inhibit osteoclast formation. The target enzyme, phosphodiesterase 5A (PDE5A), was found to be expressed in mouse and human bone as well as in specific brain regions, namely the locus coeruleus, raphe pallidus, and paraventricular nucleus of the hypothalamus. Localization of PDE5A in sympathetic neurons was confirmed by coimmunolabeling with dopamine β-hydroxylase, as well as by retrograde bone-brain tracing using a sympathetic nerve-specific pseudorabies virus, PRV152. Both drugs elicited an antianabolic sympathetic imprint in osteoblasts, but with net bone gain. Unlike in humans, in whom vardenafil is more potent than tadalafil, the relative potencies were reversed with respect to their osteoprotective actions in mice. Structural modeling revealed a higher binding energy of tadalafil to mouse PDE5A compared with vardenafil, due to steric clashes of vardenafil with a single methionine residue at position 806 in mouse PDE5A. Collectively, our findings suggest that a balance between peripheral and central actions of PDE5A inhibitors on bone formation together with their antiresorptive actions specify the osteoprotective action of PDE5A blockade.

The NO–cGMP–PKG pathway in skeletal remodeling

“…Among 11 known PDE families, PDE5A, 6, and 9A are known to specifically bind to and degrade cGMP. Taqman assays using mRNA from mouse skeletal tissue evaluating the expression of 11 PDEs showed high levels of Pde5a expression, which was comparable to the expression of receptor activator of nuclear factor κB (RANK) ( Tnfrsf11a ) and RANKL ( Tnfsf11 ) 15 . Immunohistochemistry confirmed the colocalization of RUNX2 and PDE5A in osteoblast precursors; however, PDE5A was not stained in osteoclast precursors.…”

“…In vivo studies showed protective skeletal effects of PDE5A inhibition, which are summarized in Table 1. In wild‐type mice, tadalafil and vardenafil significantly increased areal and volumetric BMD, including better microarchitecture, compared with vehicle‐treated mice 15 . Tadalafil and vardenafil increased serum NO, cGMP, and PKG expression, as well as BMD, in ovariectomized rats 59 .…”

“…Immunohistochemistry confirmed the colocalization of RUNX2 and PDE5A in osteoblast precursors; however, PDE5A was not stained in osteoclast precursors. Microarray analysis of human osteoblasts and osteoclasts also showed the presence of PDE5A only in osteoblasts 15 . Furthermore, the PDE5 inhibitors sildenafil and vardenafil increased cGMP concentration significantly in calvarial cells from neonatal mice 55 …”

“…sGC converts guanosine triphosphate to the secondary messenger cGMP and inorganic pyrophosphate and increases the NO–cGMP–PKG signal. The sGCs ( Gucy1a2 and Gucy1a3 ) were are expressed in mouse skeletal tissue 15 . Moreover, a sGC agonist, cinaciguat, increased intracellular cGMP concentration in murine primary osteoblast 51 …”

“…All three NOS isoforms were found in whole bone tissue or bone‐derived cells, such as osteoblasts or osteoclasts, by PCR and immunohistochemistry. eNOS expression was reported in osteoblasts, stromal cells, and osteoclasts 11–15 . eNOS is calcium‐ and calmodulin‐dependent and constitutively synthesizes picomolar concentrations of NO 16,17 .…”