Synthesis and exploration of 2-morpholino-4-phenylthiazol-5-yl acrylamide derivatives for their effects against carbonic anhydrase I, II, IX and XII isoforms as a non-sulfonamide class of inhibitors
Graphical abstract
Novel series of 2-morpholino-4-phenylthiazol-5-yl acrylamide derivatives (8a–s) have been synthesized and explored as a nonsulfonamide class of carbonic anhydrase (CA, EC 4.2.1.1) inhibitors. The CA inhibitory results revealed that most of the compounds showed good activity against hCA II, IX, and XII whereas none of them were active against hCA I (Ki >100 μM). It is observed that the physiologically most important cytosolic hCA II was inhibited by these molecules in the range of Ki 9.3–77.7 μM. It is also found the both the transmembrane isoforms hCA IX and XII were also inhibited by these molecules with Ki ranging between 54.7–96.7 μM and 4.6–8.8 μM, respectively. The binding modes of the active compounds within the catalytic pockets of hCA II, IX and XII were evaluated by docking studies. This new non-sulfonamide class of selective inhibitors of hCA II, IX and XII over the hCA I isoform may be used for further understanding the physiological roles of some of these isoforms in various pathologies.
Introduction
Carbonic anhydrases (CAs, EC 4.2.1.1) are the essential enzymes that effectively catalyze the reversible hydration of CO2 to bicarbonate and proton, an important reaction for many physiological processes.1 This Zn(II) containing metalloenzyme (α-CA) has sixteen different isoforms (I–XV, VA and VB) in mammals (Fig. 1), which may promote pathological situations under their malfunctioning and/or altered expression.2 Therefore, it is important to develop an effective approach that can selectively inhibit isoforms involved in different diseases. Although, there are several commercially available CAIs, such as acetazolamide, ethoxzolamide, dorzolamide, well known for their anticonvulsant, anti-glaucoma, and anti-infective activities,3 none of them shows selectivity for a specific isoenzyme, therefore causing side effects. Nowadays, the major concern of the researchers working in this field is to develop new concept for designing new families of compounds that are capable to block specifically the process catalyzed by one or two isoforms of these enzymes.
In this perspective, privileged scaffolds such as thiazole, morpholine, phenyl acrylamide etc., are frequently used for the synthesis of various heterocyclic compounds with wide ranges of pharmacological activities. As an important class of heterocyclic compounds, the thiazoles are associated with almost all the biological and pharmacological activities such as antimicrobial (Sulfathiazole) (Fig. 2A), antiretroviral (Ritonavir) (Fig. 2B), antifungal (Abafungin) (Fig. 2C), and antineoplastic (Tiazofurin) (Fig. 2D) with recent applications for the treatment of allergies, hypertention, inflammation, schizophrenia, bacterial, HIV infections, hypnotics, and more recently for the treatment of pain, as fibrinogen receptor antagonists with antithrombotic activity and as new inhibitors of bacterial DNA gyrase B.4
The morpholine linked thiazole moiety was found to be relevant in the structure of inhibitors of the DNA binding domain of the androgen receptor (Fig. 2E),5 PI3K inhibitors (Fig. 2F, G)6 and as antifungal agents (Fig. 2H).7 On the hand acrylamide derivatives were explored by various researchers for numerous biological activities such as anticancer (Fig. 2I),8 antiparasite (Fig. 2J),9 antioxidant (Fig. 2K),10 and also as a lipid lowering agent (Fig. 2L).11
Coming to CA inhibition, sulfonamides were the most investigated class of CA inhibitors (CAIs) that bind to the metal ion from the enzyme active site by displacing the metal-bound hydroxide ion/water which is situated at the bottom of a (15 Å) deep active site pocket and coordinated by three amino acid residues, His94, His96 and His119 in a tetrahedral geometry (Fig. 3A).12, 13 On the other hand, the carboxylates were found to be the most complicated family of inhibitors, showing a variety of inhibition mechanism such as monodentate coordination of carboxylate to the metal ion, or anchoring to the zinc coordinated water, or binding at the entrance or even outside the active site cavity (Fig. 3).14, 15, 16, 17
Recently, one of us presented17 a comparison study of carbon versus sulfur based zinc binding groups where the carbon based compounds showed an interesting inhibition profile against all the mammalian isoforms of carbonic anhydrase CA I–XV, although with a lower efficacy compared to the sulfonamides (Fig. 4A). It has also been explained that the inhibition depends on: the pKa of the ZBG, its geometry (tetrahedral, i.e. sulfur-based, versus trigonal, i.e. carbon-based ZBGs), orientation of the organic scaffold induced by the nature of the ZBG where the carboxamide has shown very good inhibition potency against CA isoforms.18 Di Fiore et al. showed that the hydroxamate have inhibited all 12 CA isoforms with inhibition constants in the range of 0.94–179 μM and are less effective with compared to the sulfonamide but exhibiting a comparable activity with reference to that of the N-substituted sulfonamides (Fig. 4B).19 Also Carradori et al. has described the amide derivatives of Probenecid as selective inhibitors of carbonic anhydrase IX and XII (Fig. 4C).20 Based on these literatures findings on carbonic anhydrase and the diverse activity associated with thiazole our endeavor to discover novel CA inhibitors and hence this particular scaffold non-sulfonamide based 2-morpholinothiazolophenylacrylamide has been synthesized and evaluated for CA inhibitions.
Section snippets
Chemistry
The current work was designed to selectively inhibit hCA I, II, IX, XII isoforms, with derivatives which do not incorporate a sulfonamide moiety. The designed molecule 2-morpholino-4-phenylthiazol-5-yl cinnamamide (8a–s) was synthesized according to the general synthetic plan depicted in Scheme 1. Acyl chloride (1a–d) was refluxed with KSCN in dry CH3CN, cooled to room temperature and followed by addition of a solution of morpholine to give N-(morpholine-4-carbonothioyl)benzamide (2a–d).
Conclusion
In conclusion, we have synthesized 2-morpholinothiazole phenylacrylamide derivatives (8a–s) and screened against the various CA isoforms. Among them compound 8k, 8m, 8n, 8o and 8r are shown notable inhibitory activity against three isoforms hCA II, IX and XII with Ki in the range of 4.6–96.7 μM. Since these compounds are not belonging to neither classical nor non-classical CAIs functionality, thus it is anticipated that further optimization and exploration of such kind of novel scaffolds will
General
All the reagents implemented here in the present work were purchased from commercial suppliers and used without further purification. All the solvents were purified and dried using standard methods prior to use. All reactions were performed under atmospheric pressure and reactions involving air- or moisture-sensitive compounds were performed under a nitrogen atmosphere using dried glassware and syringe techniques to transfer solutions. Melting points (mp) were measured on Stuart digital
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
BS, CSD and PS are thankful to Department of Pharmaceuticals, Ministry of Chemicals and Fertilizers, Govt. of India, New Delhi, for awarding the NIPER Ph.D fellowship.
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