Hypothesis

Abstract

The hypothesis that paroxetine decreases morbidity and mortality in patients with heart failure (HF) is plausible but unproven. Basic research demonstrates that inhibition of G protein-coupled receptor kinase 2 (GRK2) both in vitro and in vivo in the myocardium may be beneficial. G protein-coupled receptor kinase 2 antagonism is purported to exert cardioprotective effects immediately following myocardial injury by blunting toxic overstimulation on a recently injured heart. In addition, chronic overexpression of GRK2 inhibits catecholamine induction of vital positive chronotropic and ionotropic effects required to preserve cardiac output leading to worsening of congestive HF. In cardiac-specific GRK2 conditional knockout mice, there is significant improvement in left ventricular wall thickness, left ventricular end-diastolic diameter (LVEDD), and ejection fraction (EF) compared to controls. Paroxetine is a selective serotonin reuptake inhibitor which was recently shown to have the ability to directly inhibit GRK2 both in vitro and in vivo. At physiologic temperatures, paroxetine inhibits GRK2-dependent phosphorylation of an activated G-protein-coupled receptor with a half maximal inhibitory concentration of 35 micromoles, a substantially greater affinity than for other G protein-coupled receptor kinases. In a randomized trial in mice with systolic HF and depressed EF postmyocardial infarction, those treated with paroxetine had a 30% increase in EF, improved contractility, and LVEDD and wall thickness compared to those treated with medical therapy alone. While further basic research may continue to elucidate plausible mechanisms of benefit and observational studies will contribute important relevant information, large scale randomized trials designed a priori to do so are necessary to test the hypothesis.

Introduction

Heart failure (HF) is an increasingly common clinical and public health problem in the United States and worldwide. In the United States alone, it is the most common cause of hospitalization for patients aged 65 years and older. Heart failure also confers a significantly higher case fatality rate than any other manifestation of cardiovascular disease. ¹ Advances in treatment during the last 25 years have reduced mortality from 50% to 10% per year. ² Among the targets of recent therapeutic advances are beta adrenergic receptors (βARs) which are typical G protein-coupled receptors (GPCRs) with a principal role in regulating cardiac rate and contractility in response to catecholamine release. There are 3 different βAR subtypes, all of them expressed in the human heart, however, by far the predominant subtype is the β₁AR representing 75% to 80% of total βAR density, with the remaining ∼15% to 17% being β₂AR and ∼2% to 3% β₃AR (under normal conditions). ³ Activation of β₁AR leads to an increase in cyclic adenosine monophosphate (cAMP) exerting positive chronotropic and ionotropic effects. Further, immediately following cardiac insult, catecholamine response plays an integral role in preserving cardiac output via the GPCR–cAMP pathway. ³ However, chronic exposure to catecholamines has been shown to have deleterious cardiac effects mainly because it induces upregulation of GPCR kinase 2 (GRK2) in the heart. This kinase leads to significant cardiac βAR dysfunction and diminished cardiac inotropic and adrenergic reserves, acting in concert with βarrestin 1. The actions of GRK2/ βarrestin 1 blunt the catecholaminergic response potentially preventing toxic overstimulation on a recently injured heart. ⁴ In addition, chronic elevation of GRK2 levels leads to a reduction in β₁AR density (receptor downregulation) resulting in a shift of the β₁AR: β₂AR ratio from 75:25 (the ratio under normal physiologic circumstances) to ∼50:50. ^3,5,6,7 This prohibits the catecholaminergic response of β₁AR from inducing the necessary positive chronotropic and ionotropic effects required to preserve cardiac output, while the β₂AR cannot function properly, either, due to severe functional desensitization. The body attempts to compensate by increasing sympathetic tone and catecholamine release. ^5,7 This, in turn, further potentiates the GRK2-dependentdesensitization of the β₁AR pathway in the heart leading to worsening HF. ^3,5,7

In Vivo and In Vitro Studies of GRK2 Antagonism in HF

The inhibition of GRK2 has been studied extensively over the last 30 years in vitro and more recently in vivo using mouse models. Total global genetic deletion of GRK2 (also known as βAR kinase 1) results in death in utero due to heart malformations. ⁷ Mice with cardiac-specific, partial GRK2 deficiency in the absence and presence of myocardial infarction (MI)-induced HF exhibited enhanced inotropy in response to catecholamines but also accelerated development of catecholamine toxicity upon chronic isoproterenol treatment. ^8,9 These findings suggested that cardiomyocyte GRK2 is not essential for cardiac development since the embryonic lethality was due to extracardiac effects of GRK2 deletion. In addition, in the adult heart, cardiac GRK2 is a major factor regulating inotropic and lusitropic tachyphylaxis to catecholamines, thereby contributing to its cardioprotective effects in acute cardiomyopathy. ¹⁰ Once HF has set in however, as in the case of an MI, cardiac GRK2 contributes significantly to the βAR dysfunction and further deterioration of cardiac function and adrenergic reserve. The mice expressing a cardiac GRK2 inhibitor (bARKct) had a 25% lower rate of cardiac dilatation (P < .0001) and double the cardiac function determined by echocardiography performed at 0, 7, and 18 weeks (P < .0001). ⁵ The bARKctmice fared better than the wild-type on beta-blocker therapy alone. ⁵ In addition, the bARKctarm on metoprolol fared the best with an average survival of 14 weeks compared with 9 weeks for the wild-type beta-blocker group (P < .0001). ⁵ Furthermore, the wild-type beta-blocker arm had no functional benefits on ejection fraction, wall thickness, and end-diastolic diameter compared with the nonbeta-blocker group. The authors proposed that the antiarrhythmic effects of metoprolol in HF may have accounted for the observed morbidity and mortality benefits.

In addition, conditional cardiac-specific GRK2 knockout mice induced by tamoxifen 11 days postinduced MI, had better prognosis compared with wild type. Interestingly, in the 10 days following MI, mortality between the 2 groups were similar. ¹¹ After induction of the knockout gene at day 11, however, the GRK2 knockout group fared significantly better. ¹¹ In addition, measurements of left ventricular hemodynamics displayed preserved responsiveness to βAR stimulation. Furthermore, in the knockout mice, real-time polymerase chain reaction analysis of isolated cardiac myocytes 5 weeks post-MI had less induction of atrial natriuretic peptide and brain natriuretic peptide, both markers of HF. These findings add support to the role of GRK2 in progression to HF. ¹¹ Lastly, heart-to-body-weight was significantly increased in the wild-type group when compared with the knockout group secondary to cardiac hypertrophy. ¹¹

Paroxetine as a GRK2 Antagonist

Paroxetine is a selective serotonin reuptake inhibitor that has been found to have the unique ability to directly inhibit GRK-2 both in vitro and in vivo. At physiologic temperatures, paroxetine binds to the GRK2 receptor at its adenosine triphosphate binding site inhibiting GRK2-dependent phosphorylation of an activated GPCR with an half maximal inhibitory concentration of 35 micromoles thereby preventing its activation. ¹ In addition, paroxetine has a 13- to 16-fold lower affinity for other GRK receptors. In addition, mice treated with paroxetine initiated 2 weeks post-MI at doses leading to similar serum concentration of the drug as used to treat depression in humans demonstrated a significantly better measured outcome compared with mice receiving standard post-MI medical therapy. Left ventricular ejection fraction increased by 30% (P < .0001) compared with the control and fluoxetine groups. ¹² Contractility was increased by 2.5 times, and left ventricular end diastolic volume was nearly restored to baseline (P < .0004). ¹² Left ventricular wall thickness remained stable without hypertrophy when compared with the fluoxetine and control groups. In summary, there was an increase in cardiac output and ejection fraction without compromising cardiac demand. ¹²

Research Challenges in Hypothesis Testing in Humans

Testing the hypothesis in humans that paroxetine decreases morbidity and mortality in patients with Congestive Heart Failure (CHF) will be particularly challenging. In mouse models, paroxetine binds with adequate affinity to the GRK2 receptor, which in turn leads to improved functional and mortality benefits. It is uncertain whether paroxetine will have a similar binding affinity and downstream effect in humans. Although large scale randomized trials are necessary to test the hypothesis, ¹³ further support for the hypothesis would derive from retrospective analyses of completed trials of HF. Despite the inherent confounding by indication, such observational analyses would contribute important and relevant information to the hypothesis that patients with HF who are prescribed paroxetine experience morbidity and mortality benefits compared to those prescribed other Selective Serotonin Re-uptake Inhibitors (SSRI’s) as well as those given standard medical therapy alone. As residual confounding by indication can never be excluded, such observational comparisons would be hypothesis formulating, not testing. ^14,15

Conclusion

On the basis of the current totality of evidence, we hypothesize that patients with recent MI and signs of HF or left ventricular dysfunction treated with paroxetine, a GRK2 antagonist, will have decreased morbidity and mortality when compared to patients who have not received a GRK2 antagonist. As HF is the leading cause of hospitalization for patients aged 65 and older this, hypothesis, if proven, would have important clinical and public health implications. While further basic research may continue to elucidate plausible mechanisms of benefit, large scale randomized trials designed a priori to do so are necessary to test the hypothesis. ^11,12,13 For a disease as common and serious as HF, any benefit on morbidity or mortality would have enormous clinical and public health significance in the United States and worldwide.