**5. PDE-5 inhibitors reverse cardiac dysfunction in Duchenne muscular dystrophy**

Duchenne muscular dystrophy (DMD) is a degenerative, muscle-wasting disease caused by mutations in the dystrophin gene. The total loss of dystrophin mainly affects skeletal muscle and results in impaired respiratory function, primarily in older boys (Finsterer & Stöllberger, 2003; Adamo, 2010). Due to remarkable improvement of noninvasive respiratory support in the recent past, the lifespan of patients with DMD has increased. Unfortunately, this was also associated with an increase in the incidence of complications and eventual mortality from cardiomyopathy (McNally, 2008). Cardiomyopathy is a delayed symptom of the disease that usually develops by the second decade of life, with more than 90% of patients presenting clinical symptoms by 18 y of age (Finsterer & Stöllberger, 2003). Loss of cardiac dystrophin eventually leads to dilated cardiomyopathy, which manifests as congestive heart failure in at least 20% of patients (Finsterer & Stöllberger, 2003). Current treatment options for heart failure associated with DMD include angiotensin converting enzyme inhibitors and β-blockers. Despite the moderate benefits provided by these medications in patients with systolic heart failure, similar advantages have not been observed in dystrophic patients with features of systolic and diastolic dysfunction (Bushby, 2003). These findings highlight the need for treatments that slow the development of cardiomyopathy in DMD and improve cardiac function in older patients with established cardiomyopathy.

It has been shown that stimulation of cGMP synthesis by overexpression of cardiac-specific neuronal (n)NOS reduces impulse-conduction defects in dystrophin-deficient (mdx) mice (Wehling-Henricks et al., 2005; Wehling et al., 2001). Similarly, increased particulate guanylyl cyclase activity in young mdx mice has also been shown to decrease susceptibility to cardiac damage during sympathetic stress (Khairallah et al., 2008). These findings clearly implicate reduced NO-cGMP signaling as a key contributor to myocardial pathogenesis in patients with DMD. Therefore, it is plausible that restoration of NO signaling, particularly by preservation of cGMP, may provide therapeutic benefit to dystrophic hearts. In a recent study, Adamo et al. tested whether chronic inhibition of PDE-5 with sildenafil would reverse cardiac dysfunction in the mdx mouse model of DMD (Adamo et al., 2010)

Chronic sildenafil treatment prevented LV functional deficits in aging mdx mice. Furthermore, late sildenafil treatment, i.e. after developing cardiomyopathy, reversed the established symptoms.

Conventional echocardiography and tissue Doppler analysis were used to monitor the development of LV dysfunction in aging mdx mice. Both the myocardial performance index (MPI) and ratios of early diastolic velocity (Ea) to peak velocity with atrial contraction (Aa) were calculated. MPI is a sensitive measure of left ventricular systolic and diastolic performance, whereas the Ea/Aa largely reflects diastolic function. The majority of patients with DMD exhibit diastolic dysfunction and impaired myocardial performance, which can be identified by increased MPI (Bahler et al., 2005). This dysfunction usually precedes the

Phosphodiesterase-5 Inhibitors Improve Left Ventricular Function in Failing Hearts 109

cardiomyopathy seen in the mdx mice used in this study.Overall, the findings of this study suggest that PDE-5 inhibitors may be an effective treatment for DMD-associated

Fig. 8. Sildenafil reverses cardiac dysfunction in the mdx model of Duchenne Muscular

Following years of basic research examining the cardioprotective effects of PDE-5 inhibitors against ischemia, a recent study by Guazzi et al. demonstrated that sildenafil improves LV diastolic function, cardiac geometry, and clinical status in patients with stable systolic heart

**6. Clinical use of PDE-5 inhibitors in patients with heart failure** 

Dystrophy

cardiomyopathy at early and late stages of the disease.

onset of systolic heart failure and dilated cardiomyopathy (Markham et al., 2006). Mdx mice show these same echocardiographic abnormalities (Jearawiriyapaisarn et al., 2010; Townsend et al., 2007).

Three different sildenafil treatment regimens were used: 1- long-term chronic sildenafil treatment starting at 1 month of age, 2- long-term treatment starting at 12 months with echocardiographic measurements taken 3 months later to assess whether established dysfunction could be reversed, and 3- a similar treatment starting at 12 months, but with multiple measurements to determine the time course of the reversal. The results showed impaired LV performance in mdx mice (increased MPI) by 11 to 13 months of age compared with treated and untreated WT controls. As mice approached 15 months of age, mdx mice continued to demonstrate impaired LV function whereas WT control mice began to show a slight age-related decline in cardiac performance. Although sildenafil did not have an effect on cardiac performance in WT mice, mdx mice that received chronic sildenafil treatment starting at 1 month of age retained a relatively normal MPI with age, indicating that sildenafil attenuated the cardiomyopathy in mdx mice. Furthermore, late sildenafil treatment following well-established cardiomyopathy at 12 months of age completely reversed LV dysfunction by age 15 months, as evidenced by normal MPI at that time point. Taken together, these results demonstrate that chronic treatment with sildenafil mitigates the progression of LV dysfunction and late treatment also reverses established LV dysfunction in mdx mice (Figure 8).

In order to better understand the underlying cause for the improvement in the MPI by sildenafil, which could be a result of effects on systolic or diastolic function, the authors measured the Ea/Aa using tissue Doppler imaging to more directly evaluate diastolic function in mdx mice. This parameter largely reflects the diastolic (chamber relaxation and filling) capacity of the LV. As shown in Figure 8B, diastolic dysfunction (indicated by Ea/Aa <1) was observed in mdx mice as early as 8 months of age. Moreover, chronic sildenafil treatment reduced the progression of diastolic dysfunction in mdx mice through 15 months of age. Even when sildenafil treatment was initiated after LV dysfunction was established at 12 months of age, it markedly reversed the diastolic dysfunction within 3 months. Based on this result, the authors suggest that diastolic dysfunction is a major component of the impaired MPI observed in 11- to 13-month-old mdx mice.

Cardiac remodeling after injury can result in hypertrophy, increased fibrosis and systolic dysfunction of the heart. However, cardiomyopathy in mdx mice is characterized by slow, progressive cell death, followed by compensatory hypertrophy of the surviving cardiomyocytes. In order to study the impact of sildenafil on cardiac dimensions and remodeling, the authors used M-mode echocardiography to determine LV dimensions in conscious mdx mice. By 12 months of age, the LV wall thickness of mdx mice was increased and the LV mass index (LVMI) was larger compared with sildenafil-treated mdx mice. Taken together, the anti-hypertrophic effect of sildenafil, coupled with the prevention of diastolic dysfunction, suggest that sildenafil may also have protective effects on some aspects of cardiac remodeling. However, the authors did not find any difference in the FS of 12-month-old, conscious mdx mice compared with WT controls or sildenafil-treated mdx mice, nor did they observe any effect on heart rate. This indicates a lack of major systolic dysfunction in these animals up to 12 month of age. Although systolic dysfunction may develop later in life, it appears that diastolic dysfunction plays a more prominent role in the

onset of systolic heart failure and dilated cardiomyopathy (Markham et al., 2006). Mdx mice show these same echocardiographic abnormalities (Jearawiriyapaisarn et al., 2010;

Three different sildenafil treatment regimens were used: 1- long-term chronic sildenafil treatment starting at 1 month of age, 2- long-term treatment starting at 12 months with echocardiographic measurements taken 3 months later to assess whether established dysfunction could be reversed, and 3- a similar treatment starting at 12 months, but with multiple measurements to determine the time course of the reversal. The results showed impaired LV performance in mdx mice (increased MPI) by 11 to 13 months of age compared with treated and untreated WT controls. As mice approached 15 months of age, mdx mice continued to demonstrate impaired LV function whereas WT control mice began to show a slight age-related decline in cardiac performance. Although sildenafil did not have an effect on cardiac performance in WT mice, mdx mice that received chronic sildenafil treatment starting at 1 month of age retained a relatively normal MPI with age, indicating that sildenafil attenuated the cardiomyopathy in mdx mice. Furthermore, late sildenafil treatment following well-established cardiomyopathy at 12 months of age completely reversed LV dysfunction by age 15 months, as evidenced by normal MPI at that time point. Taken together, these results demonstrate that chronic treatment with sildenafil mitigates the progression of LV dysfunction and late treatment also reverses established LV

In order to better understand the underlying cause for the improvement in the MPI by sildenafil, which could be a result of effects on systolic or diastolic function, the authors measured the Ea/Aa using tissue Doppler imaging to more directly evaluate diastolic function in mdx mice. This parameter largely reflects the diastolic (chamber relaxation and filling) capacity of the LV. As shown in Figure 8B, diastolic dysfunction (indicated by Ea/Aa <1) was observed in mdx mice as early as 8 months of age. Moreover, chronic sildenafil treatment reduced the progression of diastolic dysfunction in mdx mice through 15 months of age. Even when sildenafil treatment was initiated after LV dysfunction was established at 12 months of age, it markedly reversed the diastolic dysfunction within 3 months. Based on this result, the authors suggest that diastolic dysfunction is a major component of the

Cardiac remodeling after injury can result in hypertrophy, increased fibrosis and systolic dysfunction of the heart. However, cardiomyopathy in mdx mice is characterized by slow, progressive cell death, followed by compensatory hypertrophy of the surviving cardiomyocytes. In order to study the impact of sildenafil on cardiac dimensions and remodeling, the authors used M-mode echocardiography to determine LV dimensions in conscious mdx mice. By 12 months of age, the LV wall thickness of mdx mice was increased and the LV mass index (LVMI) was larger compared with sildenafil-treated mdx mice. Taken together, the anti-hypertrophic effect of sildenafil, coupled with the prevention of diastolic dysfunction, suggest that sildenafil may also have protective effects on some aspects of cardiac remodeling. However, the authors did not find any difference in the FS of 12-month-old, conscious mdx mice compared with WT controls or sildenafil-treated mdx mice, nor did they observe any effect on heart rate. This indicates a lack of major systolic dysfunction in these animals up to 12 month of age. Although systolic dysfunction may develop later in life, it appears that diastolic dysfunction plays a more prominent role in the

Townsend et al., 2007).

dysfunction in mdx mice (Figure 8).

impaired MPI observed in 11- to 13-month-old mdx mice.

cardiomyopathy seen in the mdx mice used in this study.Overall, the findings of this study suggest that PDE-5 inhibitors may be an effective treatment for DMD-associated cardiomyopathy at early and late stages of the disease.

Fig. 8. Sildenafil reverses cardiac dysfunction in the mdx model of Duchenne Muscular Dystrophy
