Increasing synaptic noradrenaline, serotonin and histamine enhances in vivo binding of phosphodiesterase-4 inhibitor (R)-[11C]rolipram in rat brain, lung and heart

Advances in Cyclic Nucleotide Phosphodiesterase-Targeted PET Imaging and Drug Discovery

Cyclic nucleotide phosphodiesterases (PDEs) control the intracellular concentrations of cAMP and cGMP in virtually all mammalian cells. Accordingly, the PDE family regulates a myriad of physiological functions, including cell proliferation, differentiation and apoptosis, gene expression, central nervous system function, and muscle contraction. Along this line, dysfunction of PDEs has been implicated in neurodegenerative disorders, coronary artery diseases, chronic obstructive pulmonary disease, and cancer development. To date, 11 PDE families have been identified; however, their distinct roles in the various pathologies are largely unexplored and subject to contemporary research efforts. Indeed, there is growing interest for the development of isoform-selective PDE inhibitors as potential therapeutic agents. Similarly, the evolving knowledge on the various PDE isoforms has channeled the identification of new PET probes, allowing isoform-selective imaging. This review highlights recent advances in PDE-targeted PET tracer development, thereby focusing on efforts to assess disease-related PDE pathophysiology and to support isoform-selective drug discovery.

A comprehensive review on the potential therapeutic benefits of phosphodiesterase inhibitors on cardiovascular diseases

Phosphodiesterases are a group of enzymes that hydrolyze cyclic nucleotides, which assume a key role in directing intracellular levels of the second messengers' cAMP and cGMP, and consequently cell function. The disclosure of 11 isoenzyme families and our expanded knowledge of their functions at the cell and molecular level stimulate the improvement of isoenzyme selective inhibitors for the treatment of various diseases, particularly cardiovascular diseases. Hence, future and new mechanistic investigations and carefully designed clinical trials could help reap additional benefits of natural/synthetic PDE inhibitors for cardiovascular disease in patients. This review has concentrated on the potential therapeutic benefits of phosphodiesterase inhibitors on cardiovascular diseases.

Analysis of (R)- and (S)-[(11)C]rolipram kinetics in canine myocardium for the evaluation of phosphodiesterase-4 with PET

PURPOSE

(R)-[(11)C]rolipram and (S)-[(11)C]rolipram have been proposed to investigate phosphodiesterase-4 and, indirectly, cAMP-mediated signaling with PET. This study assessed binding of these tracers to phosphodiesterase-4 in canine myocardium.

PROCEDURES

Seven dogs underwent (R)-[(11)C]rolipram and (S)-[(11)C]rolipram dynamic PET imaging at baseline and with co-injection of saturating doses of (R)-rolipram. Dual-input compartment models were applied to estimate the volumes of distribution (V(T)).

RESULTS

The model comprising one compartment for unmetabolized tracer and one compartment for labeled metabolites provided excellent fits to data acquired with (S)-[(11)C]rolipram at baseline and with both enantiomers during co-injection scans. Use of two compartments for unmetabolized (R)-[(11)C]rolipram at baseline was warranted according to Akaike and Schwarz criteria. V(T) estimates obtained with these models were robust (CV ≤ 8.2%) and reproducible (CV ≤ 15%).

CONCLUSION

An important fraction (~65%) of the V (T) of (R)-[(11)C]rolipram at baseline reflects specific binding. Thus, the latter may be a useful index of phosphodiesterase-4 levels in canine myocardium.

Reduced CGP12177 binding to cardiac β-adrenoceptors in hyperglycemic high-fat-diet-fed, streptozotocin-induced diabetic rats

INTRODUCTION

Abnormal sympathetic nervous system and β-adrenoceptor (β-AR) signaling is associated with diabetes. [(3)H]CGP12177 is a nonselective β-AR antagonist that can be labeled with carbon-11 for positron emission tomography. The aim of this study was to examine the suitability of this tracer for evaluation of altered β-AR expression in diabetic rat hearts.

METHODS

Ex vivo biodistribution with [(3)H]CGP12177 was carried out in normal Sprague-Dawley rats for evaluation of specific binding and response to continuous β-AR stimulation by isoproterenol. In a separate group, high-fat-diet feeding imparted insulin resistance and a single intraperitoneal injection of streptozotocin (STZ) or vehicle evoked hyperglycemia (blood glucose >11 mM). [(3)H]CGP12177 biodistribution was assessed at 2 and 8 weeks post-STZ to measure β-AR binding in heart, 30 min following tracer injection. Western blotting of β-AR subtypes was completed in parallel.

RESULTS

Infusion of isoproterenol over 14 days did not affect cardiac binding of [(3)H]CGP12177. Approximately half of rats treated with STZ exhibited sustained hyperglycemia and progressive hypoinsulinemia. Myocardial [(3)H]CGP12177 specific binding was unchanged at 2 weeks post-STZ but significantly reduced by 30%-40% at 8 weeks in hyperglycemic but not euglycemic STZ-treated rats compared with vehicle-treated controls. Western blots supported a significant decrease in β(1)-AR in hyperglycemic rats.

CONCLUSIONS

Reduced cardiac [(3)H]CGP12177 specific binding in the presence of sustained hyperglycemia corresponds to a decrease in relative β(1)-AR expression. These data indirectly support the use of [(11)C]CGP12177 for assessment of cardiac dysfunction in diabetes.

Venlafaxine-induced acute eosinophilic pneumonia

INTRODUCTION

Acute Eosinophilic Pneumonia (AEP) is a severe syndrome which can be potentially induced by many reasons, including drugs. It is characterized by pulmonary infiltrates, peripheral blood eosinophilia and respiratory failure. AEP has rarely been associated with antidepressant treatment.

CASE REPORT

We report a case of an 80-year-old woman who presented with fever, lung infiltrates, peripheral blood eosinophilia and acute respiratory failure. All evidence charge venlafaxine as the only possible causal factor. The syndrome rapidly resolved after discontinuation of the drug and upon reception of corticosteroids in low doses. The patient had a past medical history of AEP induced by sertraline and a recent medical history of Acute Lung Injury on the context of acute pancreatitis during treatment with venlafaxine.

DISCUSSION

Pathophysiological mechanisms implicated in the development of AEP in our patient seems to be associated with eotaxin and serotonin eosinophilic-specific chemoattracting action.

CONCLUSION

This is a case report with clinical adverse reaction of AEP in two antidepressant agents (venlafaxine and sertraline) with a similar neurochemical mechanism of action via the serotoninergic system.

Alterations of pre- and postsynaptic noradrenergic signaling in a rat model of adriamycin-induced cardiotoxicity

BACKGROUND

Altered sympathetic nervous system signaling is known to play a role in the cardiotoxicity of the anthracycline chemotherapeutic agents, but the interaction of pre- and postsynaptic function is not well understood.

METHODS AND RESULTS

Our aim was to study the noradrenergic signaling in an established rat model of adriamycin cardiotoxicity (15 mg/kg administered i.p. over 2 weeks) using radiotracers having potential applicability for imaging with positron emission tomography (PET). Ex vivo biodistribution was performed 1 and 3 weeks post-adriamycin treatment with the noradrenaline analogue [(11)C]meta-hydroxyephedrine ([(11)C]HED), beta-adrenergic receptor antagonist [(3)H]CGP12177, and phosphodiesterase-4 inhibitor (R)-[(11)C]rolipram. Cardiac function (echocardiographic parameters) and heart/body weight ratio were not affected. Myocardial retention of [(11)C]HED, [(3)H]CGP12177, and (R)-[(11)C]rolipram were unchanged 1 week post-adriamycin. Compared to controls, 3 weeks post-treatment [(3)H]CGP12177 uptake decreased (left ventricle free wall and septum; P < 0.05), while [(11)C]HED and (R)-[(11)C]rolipram uptake were unaffected. Following acute increase in myocardial noradrenaline levels with desipramine treatment, (R)-[(11)C]rolipram retention increased in the left atrium, right ventricle, left ventricle free wall and septum (P < 0.05) in vehicle-, but not adriamycin-treated animals.

CONCLUSION

Our results suggest that adriamycin-induced toxicity exhibits no change in presynaptic noradrenaline uptake, but decreased beta-adrenergic receptors in cardiac tissues, supporting a role for PET imaging of noradrenaline signaling in the study of anthracycline cardiotoxicity.

Effects of cAMP-dependent protein kinase activator and inhibitor on in vivo rolipram binding to phosphodiesterase 4 in conscious rats

Rolipram is a selective inhibitor of phosphodiesterase-4 (PDE4), and positron emission tomography (PET) using [(11)C]rolipram can monitor the in vivo activity of this enzyme that is part of the cAMP second messenger cascade. cAMP-dependent protein kinase (PKA) phosphorylates PDE4 and increases both enzyme activity and affinity for rolipram. In the present PET study, we examined effects of PKA modulators in conscious rats on the binding of [(11)C](R)-rolipram in comparison to the much less active enantiomer [(11)C](S)-rolipram. Unilateral injection of a PKA activator (dibutyryl-cAMP) and a PKA inhibitor (Rp-adenosine-3',5'-cyclic monophosphorothioate) into the striatum significantly increased and decreased, respectively, the binding of [(11)C](R)-rolipram. These effects were not caused by changes in blood flow or delivery of radioligand to brain, since these agents had no effect on the binding of [(11)C](S)-rolipram binding. These results support the value of measuring in vivo [(11)C](R)-rolipram binding in brain to assess responses to physiological or pharmacological challenges to the cAMP second messenger system.

Reduced in vivo phosphodiesterase-4 response to acute noradrenaline challenge in diet-induced obese rats

Altered sympathetic nervous activity has been linked to the development and persistence of obesity, partly relating to overfeeding. Binding of the selective, positron-emitting phosphodiesterase-4 (PDE4) inhibitor (R)-[11C]rolipram provides a direct index of the cAMP-hydrolyzing enzyme PDE4. This study examines progressive alterations in PDE4 in a high-fat-fed obese animal model. (R)-[11C]Rolipram was injected into diet-induced obese (DIO) and diet-resistant (DR) rats; the animals were killed after 45 min, tissues were extracted, and radioactivity was quantified. Responsiveness of PDE4 to acute noradrenaline (NA) stimulation was determined by 3 h pretreatment with the NA reuptake inhibitor desipramine. There was minimal variance in caloric intake, weight gain, fasting glucose, insulin, and energy expenditure (indirect calorimetry) measures. Basal (R)-[11C]rolipram binding was comparable between DIO and DR rats at 2 or 8 weeks of feeding. The normal increase of PDE4 levels in response to elevated NA by desipramine pretreatment was ablated in PDE4-rich tissues, including brain, heart, and skeletal muscle, of DIO animals after 8 weeks of high-fat diet. Lean DR rats maintained PDE4 responsiveness indicative of a normal NA signal transduction.

Dual inhibitors of phosphodiesterase-4 and serotonin reuptake

A new class of multitarget compounds was synthesized by linking a novel selective serotonin reuptake inhibitor (SSRI) to a PDE4 inhibitor. The new dual PDE4 inhibitor/SSRI showed antidepressant-like activity in the forced swim test in mice The SSRIs 2-{5-[3-(5-fluoro-2-methoxy-phenyl)-ethyl]-tetrahydro-furan-2-yl}-ethylamine (14) and 2-{5-[3-(5-fluoro-2-methoxy-phenyl)-propyl]-tetrahydro-furan-2-yl}-ethylamine (15) were both individually linked to the PDE4 inhibitor 4-(3,4-dimethoxy-phenyl)-4a,5,8,8a-tetrahydro-2H-phthalazin-1-one (19), via a five-carbon chain. The dual PDE4 inhibitor/SSRI 2-{5-[3-(5-fluoro-2-methoxy-phenyl)-ethyl]-tetrahydro-furan-2-yl}-ethylamine)-pentyl]-4,5,8,8a-tetrahydro-2H-phthalazin-1-one (21) showed potent and selective serotonin reuptake inhibition (IC(50) value of 127 nM). The dual PDE4 inhibitor/SSRI 21 also inhibited PDE4D3 with a K(i) value of 2.0 nM. The dual PDE4 inhibitor/SSRI was significantly more effective than the individual SSRI alone or fluoxetine in the forced swim test at standard doses. On a molar basis, the antidepressant-like effect of the dual PDE4 inhibitor/SSRI 21 showed a 129-fold increase in in vivo efficacy compared to fluoxetine.

Cortico-striatal cyclic AMP-phosphodiesterase-4 signalling and stereotypy in the deer mouse: attenuation after chronic fluoxetine treatment

Motor stereotypies, described as repetitive, topographically invariant and seemingly purposeless behaviours, are common to several developmental and neuropsychiatric disorders. While drug induced stereotypy has been extensively studied, the neurobiology of spontaneous stereotypy is poorly understood. Deer mice present with naturalistic stereotypic behaviours that are selectively suppressed by fluoxetine. We studied basal cyclic adenosine monophosphate (cAMP) levels and phosphodiesterase (PDE) type 4 activity in prefrontal cortex and striatum of high, low and non-stereotypic deer mice, as well as response in high stereotypic mice to chronic fluoxetine treatment (20 mg/kg/dayx21 days intraperitoneally). Cortical cAMP levels were associated with stereotypic behaviour, being significantly elevated in low and high stereotypic mice compared to non-stereotypic animals, with a similar trend in the striatum. In both brain regions, there was a significant inverse correlation between PDE4 activity and stereotypic behaviour. In the prefrontal cortex, PDE4 activity was significantly reduced in both low and high stereotypic mice compared to their non-stereotypic controls, while in the striatum, only high stereotypic mice showed a significant reduction in PDE4 activity. Fluoxetine significantly attenuated stereotypies in high stereotypic animals, together with a reduction in cortical cAMP levels and PDE4 activity, without noteworthy effects in the striatum. Spontaneous stereotypy in deer mice is thus characterized by raised cAMP and reduced PDE4 enzyme activity, particularly in the prefrontal cortex, and is modified by chronic treatment with fluoxetine.

Phosphodiesterase 2 and 5 inhibition attenuates the object memory deficit induced by acute tryptophan depletion

The underlying mechanism of short-term memory improvement after inhibition of specific phosphodiesterases (PDEs) is still poorly understood. The present study aimed to reveal the ability of PDE5 and PDE2 inhibitors, that increase cyclic guanosine monophosphate (cGMP) and both cyclic adenosine monophosphate (cAMP) and cGMP, respectively, to reverse an object recognition deficit induced by acute tryptophan depletion. Acute tryptophan depletion is a pharmacological challenge tool to lower central serotonin (5-hydroxytryptamine; 5-HT) levels by depleting the availability of its dietary precursor tryptophan. Short-term object memory was tested in male Wistar rats by exposing them to the object recognition task. First, the effects of acute tryptophan depletion upon object recognition 2 h after administration of the nutritional mixture were established. Subsequently, acute tryptophan depletion was combined with the PDE5 inhibitor vardenafil (1, 3 and 10 mg/kg) or with the PDE2 inhibitor BAY 60-7550 (0.3, 1 and 3 mg/kg), 30 min prior to testing. Acute tryptophan depletion significantly lowered plasma tryptophan levels and impaired object recognition performance. Vardenafil (3 and 10 mg/kg) and BAY 60-7550 (3 mg/kg) were able to attenuate the acute tryptophan depletion induced object recognition impairment. Thus, both PDE5 and PDE2 inhibition improved short-term object recognition performance after an acute tryptophan depletion induced deficit. The underlying mechanisms, however, remain poorly understood and further studies are needed to determine whether the present findings can be explained by a direct effect of enhanced cAMP and cGMP levels upon 5-HT activity, or even other neurotransmitter systems, and possibly an interaction with synthesis of nitric oxide or effects upon cerebral blood flow function.

The PDE4 inhibitor rolipram reverses object memory impairment induced by acute tryptophan depletion in the rat

RATIONALE

The selective type IV phosphodiesterase inhibitor, rolipram, has been shown to improve long-term memory and can reverse the cholinergic deficit caused by scopolamine. However, the underlying mechanisms of action of rolipram remain obscure.

OBJECTIVES

The present study investigates the effect of rolipram in a serotonergic-deficit model of acute tryptophan depletion (ATD). In addition, the levels of plasma tryptophan (TRP) were compared to object recognition performance.

MATERIALS AND METHODS

The experiments were conducted using male Wistar rats. The time-dependent effect of ATD treatment (a gelatin-based protein mixture) on plasma TRP levels (0, 1, 3, and 6 h after injection) and object recognition task (ORT) performance (0.5, 1, 3, and 6 h after ATD treatment) was examined. The effect of rolipram (0, 0.01, 0.03, and 0.1 mg/kg, i.p.) was tested in the condition in which ATD induced a clear memory deficit.

RESULTS

ATD significantly lowered the plasma TRP ratio (TRP/Sigmalarge neutral amino acid) with a maximum of 48%, approximately 1 h after administration. Furthermore, ATD impairs ORT performance when administered 3 h before testing. Rolipram (0.1 mg/kg) reversed the memory deficit induced by ATD in a dose-dependent manner.

CONCLUSIONS

On the basis of previous studies and the ability to reverse a serotonergic deficit, we suggest that rolipram may act through elevation of cyclic adenosine monophosphate levels and subsequent increase in neurotransmitter release.

In vivo selective binding of (R)-[11C]rolipram to phosphodiesterase-4 provides the basis for studying intracellular cAMP signaling in the myocardium and other peripheral tissues

INTRODUCTION

Phosphodiesterase-4 (PDE4) enzymes specifically break down the second messenger cAMP, thereby terminating the intracellular signaling cascade that plays an essential role in neurohormonal modulation of many physiological systems. PDE4 activity and expression are regulated by cAMP levels, suggesting that measurement of PDE4 provides an index of intracellular cAMP signaling.

METHODS

Male Sprague-Dawley rats were administered (R)- or the less active enantiomer (S)-[11C]rolipram and sacrificed 30 min later with tracer retention measured in various tissues. Co-injections with saturating doses of unlabeled (R)-rolipram, (S)-rolipram and Ro 20-1724, as well as subtype-selective PDE inhibitors vinpocetine, Bay 60-7550, cilostazol and zaprinast were used to establish binding selectivity for PDE4 over PDE1, PDE2, PDE3 and PDE5 subtypes, respectively. Autoradiography was performed to substantiate results of biodistribution studies in the myocardium.

RESULTS

In vivo (R)-[11C]rolipram retention was dose-dependently reduced by co-injections of (R)-rolipram and (S)-rolipram (ED50 values of 0.03 mg/kg and 0.2 mg/kg, respectively). Vinpocetine, Bay 60-7550, cilostazol and zaprinast had no effect on (R)-[11C]rolipram binding, while (R)-rolipram and Ro 20-1724 reduced the tracer uptake to nonspecific levels in PDE4-rich tissues.

CONCLUSIONS

In addition to the brain, (R)-[11C]rolipram binds selectively to PDE4 across all cardiac regions, skeletal muscle, lungs and pancreas, but not in the adipose tissues. In vivo findings were confirmed by in vitro autoradiography studies, suggesting that (R)-[11C]rolipram can be applied to evaluate alterations in central and peripheral PDE4 levels and cAMP-mediated signaling.