# Heterocyclic Nitrogen Compounds as Potential PDE4B Inhibitors in Activated Macrophages
## 1 Introduction  
Cyclic-nucleotide phosphodiesterase (PDEs) are found in all organisms and classified into three classes I, II, and III. These enzymes catalyze the hydrolysis of cyclic adenosine \(3^{\prime}5^{\prime}\)-monophosphate (cAMP) and cyclic guanosine \(3^{\prime}5^{\prime}\)-monophosphate (cGMP) to adenosine \(5^{\prime}\)-monophosphate (AMP) and guanosine \(5^{\prime}\)-monophosphate (GMP), respectively, to regulate the cellular levels of such important secondary messengers responding to various stimuli in different physiological conditions. In humans, twenty-one PDE genes encode about 100 isoforms of PDEs classified into eleven families (PDE1 to PDE11) with a specific tissue distribution and cellular localization [1]. Certain diseases, including cardiovascular disease, neurodegeneration, and inflammatory diseases, are associated with malfunctions in specific types of PDEs. By considering their specific biochemical properties, transcriptional regulation, and tissue expression, the inhibition of PDEs has attracted the interest of many researchers. Many selective inhibitors of PDEs are identified as anti-depressants, anti-thrombotics, and anti-asthmatics agents [2], and as a treatment for male erectile dysfunction [3]. The cAMP PDEs are sensitive to the competitive inhibitors rolipram and piclamilast (Figure 1), and they have shown functional properties related to inflammatory diseases [4]. Indeed, PDE4 inhibitors are a large group of molecules with anti-inflammatory activity used for the treatment of chronic obstructive pulmonary disease, rheumatoid arthritis, type 2 diabetes, septic shock, and atopic dermatitis [1]. PDE4s include subfamily genes PDE4A-D expressed in immune and inflammatory cells, including T cells, B cells, neutrophils, dendritic cells, monocytes, and macrophages. The isoform PDE4B is upregulated early after stimulation and was found to be induced in monocytes activated by lipopolysaccharide (LPS) [5]. Furthermore, PDE4B was the only isoform expressed in LPS-stimulated mice macrophages [6]. This induction is characterized by an increase in inflammatory mediators such as reactive oxygen species (ROS), and interaction with inflammatory signal proteins like NF-E2-related factor 2 (Nrf2) or Tumor Necrosis Factor \(\alpha\) (TNF-\(\alpha\)) [7]. Moreover, it was demonstrated that in PDE4B null mice, LPS-stimulated leukocytes are characterized by low levels of TNF-\(\alpha\)[8]. Finally, the gene promoter analysis showed that the PDE4B promoter contains sequence motifs to NF-kB, a master transcription factor of macrophages, which are involved in LPS activation and the inflammatory response [9]. Recently, it was demonstrated that macrophages, the main actors of innate immunity [10,11], regulate their response by metabolic rewiring [12] through the production of mediators of inflammation, and by regulating gene expression [13, 14, 15] for an immunometabolic response. This metabolic rewiring in LPS-stimulated macrophages is characterized by the activation of the citrate pathway [16], which supports the synthesis of ROS, nitric oxide (NO), and prostaglandandins, and contributes to histone acetylation [17]. In light of the involvement of PDE4B in the inflammatory response, together with its activation in LPS-stimulated macrophages and its role as a regulator of cAMP protein kinase A (PKA), the central regulator of many cellular and metabolic processes, the research of new more selective inhibitors of PDE4B takes on importance. The compounds that we have chosen for this study are the indoles (**1** and **2**), the purines (**3** and **4**), and the carboxyles (**5** and **6**). As it is well known, these classes of compounds were widely studied for their anticancer, antidiabetic, antiviral, antioxidant, anti-inflammatory properties, etc. [18, 19, 20, 21, 22]. For this reason, new PDE4B potential inhibitors (**1-6**, Figure 2) have been synthesized as reported in Schemes 1-3 and biologically tested.