Proof-of-Concept and Mechanistic Studies to Repurpose Erectile Dysfunction Drugs for Elderly Females

fda批准的四种治疗勃起功能障碍的药物，即他达拉非、伐地那非、西地那非和阿瓦那非的安全性和有效性记录，是基于它们能有效抑制细胞酶磷酸二酯酶5A （PDE5A）的能力。PDE5A水解环鸟苷单磷酸（cGMP），使PDE5A抑制剂刺激一氧化氮- cGMP -蛋白激酶G （PKG）通路。1991年，我们首次记录了一氧化氮调节破骨细胞，即吸收骨的细胞（PMID: 1849281）。多项研究已经证实，调节这一通路对骨重塑的两个组成部分——破骨细胞的骨吸收和成骨细胞的骨形成都有强有力的影响。由于观察到老年男性、糖尿病男性和接受前列腺癌雄激素剥夺治疗的男性的勃起功能障碍和骨质疏松症同时发生，我们试图研究他达拉非和伐地那非对骨骼的作用。总的假设是PDE5A抑制剂可以用于男性勃起功能障碍和骨质疏松症的联合治疗，甚至可能仅用于女性骨质疏松症。我们在小鼠研究中发现，他达拉非和伐地那非通过刺激骨形成和抑制骨吸收来增加骨量，尤其是在雌性小鼠中（Kim et al， PNAS, in press）。尽管净骨增加，但药物的合成代谢作用被一种独特的交感传递信号所拮抗，这种信号来自蓝斑、中脑白斑和下丘脑室旁核的pde5a阳性中枢神经元。注意到大多数骨质疏松症药物要么是抗吸收的，要么是抗合成代谢的，任何双作用药物都会

The record of safety and efficacy of the four FDA–approved drugs for erectile dysfunction, namely tadalafil, vardenafil, sildenafil and avanafil, is predicated on their ability to potently inhibit the cellular enzyme, phosphodiesterase 5A (PDE5A). PDE5A hydrolyzes cyclic guanosine monophosphate (cGMP), so that PDE5A inhibitors stimulate the nitric oxide–cGMP–protein kinase G (PKG) pathway. In 1991, we documented for the first time that nitric oxide regulates the osteoclast, the cell that resorbs bone (PMID: 1849281). Multiple studies have since established robust effects of modulating this pathway on both components of bone remodeling – bone resorption by osteoclasts and bone formation by osteoblasts. Prompted by observations that erectile dysfunction and osteoporosis track together in older men, in men with diabetes, and in men receiving androgen–deprivation therapy for prostate cancer, we sought to investigate the action of tadalafil and vardenafil on bone. The overarching hypothesis was that PDE5A inhibitors could be repurposed for the co–therapy of erectile dysfunction and osteoporosis in men and, even perhaps, solely for osteoporosis in women. We found in mouse studies that tadalafil and vardenafil increased bone mass, importantly in female mice, by stimulating bone formation and inhibiting bone resorption (Kim et al, PNAS, In press). Despite net bone gain, the anabolic action of the drugs was antagonized by a unique sympathetic relay signature originating from central PDE5A–positive neurons in the locus coeruleus, raphe pallidus and hypothalamic paraventricular nucleus. Noting that most osteoporosis drugs are either anti–resorptive or anabolic, any dual–acting agent will have unique value particularly with oral use. Therefore, towards the potential for repurposing PDE5A inhibitors for osteoporosis, our current goal is to understand precisely how the drugs work on bone and to evaluate preclinical efficacy in models of bone loss. In Specific Aim 1, using global and cell–selective knock out mice, we will determine whether the drugs inhibit PDE5A to activate the NO–cGMP–PKG2 pathway in bone, and if so, which cell is the primary target. In Specific Aim 2, we will comprehensively map the distribution of PDE5A in brain at the single transcript level by RNAscope, and interrogate PDE5A–positive nodes through AAV–mediated Pde5a overexpression or knock down. In Specific Aim 3, we will study the ability of tadalafil, vardenafil, sildenafil and/or avanafil to trigger bone gain in 1–year–old aging mice; to prevent hypogonadal bone loss in rats and mice; and to restore bone that is already lost 28 weeks following ovariectomy in rats. Together, our mechanistic and efficacy studies should provide a firm foundation for future clinical trials towards repurposing PDE5A inhibitors for the prevention and treatment of osteoporosis.