Stereoselective inhibition of serotonin re-uptake and phosphodiesterase by dual inhibitors as potential agents for depression

doi:10.1016/j.bmc.2008.10.065

Bioorganic & Medicinal Chemistry

Volume 17, Issue 1, 1 January 2009, Pages 337-343

https://doi.org/10.1016/j.bmc.2008.10.065 Get rights and content

Abstract

Multi-target compounds where more than one functional activity is incorporated into the same molecule may have advantages in treating disease states. Selective serotonin re-uptake inhibitors (SSRIs)^a (i.e., (R)- and (S)-norfluoxetine) were chemically linked to a PDE4 inhibitor via a five carbon bridge. The new dual PDE4 inhibitor/SSRIs (i.e., (R)-8 and (S)-8) showed moderately potent but highly selective serotonin re-uptake inhibition (IC₅₀ values of 173 and 42 nM, respectively) in vitro. The dual PDE4 inhibitor/SSRIs (R)-8 and (S)-8 also inhibited PDE4D2 (i.e., K_i values of 106 and 253 nM, respectively). Due to the synergistic functional activity, PDE4 inhibitor/SSRIs may be effective in treating diseases such as depression.

Graphical abstract

Introduction

The human serotonin transporter (hSERT) is a plasma membrane protein responsible for the re-uptake of 5-hydroxytryptamine (5-HT). Transport of 5-HT is sensitive to nanomolar concentrations of SERT re-uptake inhibitors including selective serotonin re-uptake inhibitors (SSRIs).¹ A central hypothesis of antidepressant action is that SSRI re-uptake inhibitor functional activity is due to an elevation of excitatory neurotransmitters such as 5-HT at post-synaptic sites of the brain. Fluoxetine is an SSRI that enhances 5-HT neurotransmission in vitro and in vivo by decreasing 5-HT clearance via uptake inhibition.² When animals are treated repeatedly with SSRIs such as fluoxetine, the SERT is down-regulated.³ The extent of SSRI re-uptake inhibition after repeated treatment is greater than that seen after acute drug treatment due to the increase in 5-HT concentrations.3, 4 Fluoxetine is selective in that it does not potently bind to the human dopamine (hDAT) or human norepinephrine (hNET) transporters and selective re-uptake inhibition of the hSERT is associated with antidepressant efficacy.⁵

Cyclic nucleotide phosphodiesterases (PDEs) comprise a diverse group of enzymes that are important regulators of signal transduction. PDEs are classified into 11 families based on sequence homology, substrates and regulation by modulators. Enzymes in the PDE4 family are particularly important in neuropsychopharmacology.⁶ PDE4 hydrolyzes cyclic AMP formed by stimulation of beta adrenergic receptor-linked adenylyl cyclase in brain cortical slices.⁷ Rolipram or other selective inhibitors of PDE4 has antidepressant activity in both preclinical⁸ and clinical tests⁹ and produces memory-enhancing effects in animal models.10, 11 When animals are repeatedly treated with SSRIs, PDE4 is up-regulated.¹² This may be a consequence of 5-HT receptor-mediated cAMP signaling. Increased cAMP augments the expression of a number of PDE4 variants in neurons.⁶ Thus, one adaptation of PDE4 that occurs in response to repeated treatment with SSRIs increases 5-HT receptor-mediated cAMP signaling. With repeated treatment, this effect will be blunted as tolerance develops because PDE4 is up-regulated and cAMP hydrolysis is increased.

A dual PDE4 inhibitor/SSRI offers advantages beyond simple additive effects of administration of the individual agents including providing greater symptomatic efficacy and better utility. The ‘message-address’ concept of a dual agent could afford proximal inhibition of PDE4 thus keeping ample cAMP concentrations present near the activated transporter for greater functional selectivity. Dual PDE4 inhibitor/SSRIs can affect local control in a stimulus-selective manner because of the compartmentalization of the PDE enzymes. The greater PDE4 inhibitor potency of the dual agent will allow lower doses to be used and decrease side effects. Because CNS disorders are recognized as poly-etiological in nature, drugs that modulate multiple targets will contribute to the multi-factorial processes in disease treatment. Reports of the use of multi-functional or multi-modal drugs for CNS targets have increased. For example, single molecular entities for treatment of cognition impairment, motor dysfunction, depression and neurodegeneration that combine one or more of the following properties: (a) cholinesterase inhibition, (b) inhibition or activation of acetylcholine receptors, (c) anti-inflammatory activity, (d) monoamine oxidase inhibition, nitric oxide production, (e) neuroprotection, (f) anti-apoptotic activity and (g) activation of mitochondrial-dependent cell-survival genes and proteins has been reported.¹² New antidepressants with dual serotonin transporter and 5-HT1A receptor affinity have been synthesized.¹³ A dual inhibitor of acetylcholinesterase and the serotonin transporter for use in Alzheimer’s Disease was reported.¹⁴

When animals are treated repeatedly with SSRIs such as fluoxetine, PDE4 is up-regulated¹⁵ but dual PDE4 inhibitors/SSRIs offer the advantage of blocking the effect of up-regulation of PDE4. While PDE4 expression will still increase, its hydrolytic activity will be blocked and thus, the overall increase in serotonin receptor-mediated cAMP signaling will be preserved with repeated SSRI treatment. A chemical biology strategy for synthesis of dual function compounds that selectively inhibit PDE4 and 5-HT re-uptake was devised. Compounds with SSRI potency and PDE4 inhibition potency were chemically synthesized and combined by a five carbon linker. Dual inhibitor diastereomers (8) were found to be potent re-uptake inhibitors of the SERT and inhibitors of PDE4D2. The successful synthesis of dual SSRI/PDE4 inhibitors based on fluoxetine and phthalazinones and their biological evaluation as inhibitors of biogenic amine transporters and PDE4D2 was done and is reported herein.

Section snippets

Chemistry

The chemical synthesis of dual SSRI/PDE4 inhibitors consisted of coupling norfluoxetine through a five carbon linker with a known PDE4 inhibitor (i.e., a phthalazinone) to afford the target compound. The enantioselective synthesis of the norfluoxetine portion of the dual inhibitor was obtained by combining the requisite (R)- or (S)-1-chloro-3-(hydroxy)-3-phenyl-propane with the appropriate Mitsunobu reagent (i.e., made by combining triphenylphosphine, DIAD and trifluoromethyl-p-cresol¹⁶)

Conclusions

In summary, the stereoselective synthesis of dual PDE4 inhibitor/SSRIs based on norfluoxetine linked to a phthalazinone PDE4 inhibitor was accomplished and the target compounds were examined in vitro. The results show that N-substitution of the phthalazinone results in a significant increase in PDE4D2 inhibitory potency. However, attachment of norfluoxetine via a five carbon linker to the phthalazinone PDE4 inhibitor decreased PDE4D2 inhibitory potency compared with the compound without the

General

Chemicals used in this study were of the highest purity available. Commercially available reagents including (R)- and (S)-α,α,α-trifluoromethyl-p-cresol were purchased from Aldrich Chemical Company (Milwaukee, WI) or VWR (San Diego, CA) and were used as received. 2′-Fluo-AHC-cAMP was from Axxora LLC (San Diego, CA). All moisture sensitive reactions were carried out in flame-dried glassware under an argon atmosphere. Tetrahydrofuran (THF) and toluene were freshly distilled from calcium hydride

Acknowledgments

The analytical help of Luke Guo and John Buza is gratefully acknowledged. The financial support of the HBRI is gratefully acknowledged.

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Stereoselective inhibition of serotonin re-uptake and phosphodiesterase by dual inhibitors as potential agents for depression

Abstract

Graphical abstract

Introduction

Section snippets

Chemistry

Conclusions

General

Acknowledgments

Eur. J. Pharmacol.

Clin. Ther.

Prog. Nucleic Acid Res. Mol. Biol.

Neuropharmacology

Trends Pharmacol. Sci.

Bioorg. Med. Chem.