Recent Developments and Biological Activities of N-Substituted Carbazole Derivatives: A Review

Carbazoles represent an important class of heterocycles. These have been reported to exhibit diverse biological activities such as antimicrobial, antitumor, antiepileptic, antihistaminic, antioxidative, anti-inflammatory, antidiarrhoeal, analgesic, neuroprotective and pancreatic lipase inhibition properties. A series of carbazole derivatives such as N-substituted carbazoles, benzocarbazoles, furocarbazoles, pyrrolocarbazoles, indolocarbazoles, imidazocarbazoles, etc. have been synthesized. The N-substituted derivatives have gained the attention of researchers due to their therapeutic potential against neurological disorders and cell proliferation. Herein an attempt is made to review the medicinal importance of recently synthesized N-substituted carbazoles.


Introduction
Heterocycles are inextricably woven into the life processes [1]. The importance of heterocycles in drug discovery is one of the major areas in medicinal chemistry [2]. There are a vast number of pharmacologically active heterocyclic compounds, many of which are being used clinically, such as vincristine, morphine, chloroquine, meperidine, sulphadiazine, etc. Sulphur-and nitrogen-containing OPEN ACCESS heterocyclic compounds have maintained the interest of researchers through decades of historical development of organic synthesis [3,4].

Antimicrobial Activity
The Spread of drug resistant bacteria has badly affected the efficiency of many known antibacterial agents [34], while the emergence of fungal infections in the immuno-compromised population has also significantly increased over past few decades [35,36]. Carbazoles are considered to be one of the important classes of antimicrobial agents [37,38].

Anti-Cancer Activity
Cancer has emerged as one of the most alarming disease in the last few decades throughout the world. It is a multifactorial disease contributing towards uncontrolled growth and invasion of abnormal cells leading to the formation of tumors [49]. Pim-kinases control various proteins involved in significant biological processes such as cell cycle progression and apoptosis. Over expression of pim-kinases have been observed in human leukaemia and lymphoma, prostate, pancreatic and colon cancer contributed to tumorigenesis. For these reasons, pim-kinases are considered as important targets for the development of new anticancer drugs.
Signal transducers and activators of transcription (STATs) are latent cytoplasmic transcription factors that upon activation, results into the transduction of signals from the cell membrane to the nucleus. The seven STATs namely STAT1, STAT2, STAT3, STAT4, STAT5A, STAT5B and STAT6 have so far been identified in mammals. Among them, STAT3 is the most intimately linked to tumorigenesis. Saturnino et al. [57] have synthesized a series of N-alkylcarbazole derivatives and evaluated their activity on STAT3. Compounds 57-59 were revealed to inhibit the STAT3 activation by 50%, 90% and 95%, respectively, at a concentration of 50 µM (Figure 18).

Neuroprotective Activity
Neuroprotection refers to the strategies and relative mechanisms able to defend the central nervous system (CNS) against neuronal injury due to both acute (e.g., stroke or trauma) and chronic neurodegenerative disorders [58,59]. Increased oxidative stress has been recognized as a common culprit of many neurological disorders including Alzheimer's disease, Parkinson's disease and stroke [60,61].
The β-Amyloid (Aβ) deposition is one of the hallmarks of Alzheimer's disease. Therefore inhibition of Aβ peptide accumulation may be a preventive strategy for Alzheimer's disease [62].
Zhu et al. [63] in 2013 have synthesized a series of N-substituted carbazoles and studied their neuroprotective effects on neuronal cells HT22 against cell injury induced by glutamate or homocysteic acid. The compound 2-phenyl-9-(p-tolyl)-9H-carbazole (60) displayed considerable neuroprotective ability at the concentration as low as 3 µM which might result from its antioxidative activity with GSH-independent mechanism. The compounds substituted with bulky groups such as methoxy-phenyl (compound 61), t-butyl-phenyl (62), trifluoro-phenyl (63), and N,N-dimethyl-phenyl (64) at the N-position of the carbazole also possessed significant neuroprotective activity at a concentration of 30 µM (Figure 19). It was observed that the presence of a substituent at the N-position of carbazole is essential for neuroprotective activity. A series of N-alkyl-carbazole derivatives synthesized by Saturnino et al. [64], have been investigated for their ability to promote an increase of soluble amyloid-β (Aβ) peptides. Among the tested compounds, 65 was found to be responsible for a 30%-35% increase in the concentration of soluble Aβ peptides whereas compound 66 showed 65%-70% increase in concentration of soluble Aβ peptides at 10 µM ( Figure 20). In both compounds, the distance between oxygen on the chain and the N-atom of carbazole scaffold play a decisive role in the interaction with Aβ peptides. Among many genes reported to impact adult neurogenesis is the gene encoding NPAS3, a central nervous system-specific transcription factor that is associated with learning disability and mental illness [65,66]. NPAS3 deficit mice displayed the behavioral abnormalities [67] and a profound loss of adult hippocampal neurogenesis [68]. An aminopropyl-carbazole 67, designated as P7C3, exerts its pro-neurogenic activity by protecting newborn neurons from apoptosis and was capable of enhancing hippocampal neurogenesis in NPAS3 defecit mice. Pieper et al. [69] carried out a SAR study with thirty seven analogues of P7C3. The compound P7C3A20 (compound 68) was found to possess greater potency and efficacy than P7C3, whereas the (R)-enantiomer of P7C3-OMe (69) retained an activity equivalent to that of the parent compound ( Figure 21). MacMillan et al. [70] have developed a series of P7C3 derivatives and, among them, compounds 70-74 were found to be more active than the parent compound ( Figure 22).  An aminopropyl-carbazole (P7C3) has been reported to block 1-methyl-4-phenyl-1,-2,-3, -6-tetrahydropyridine (MPTP)-mediated cell death of dopaminergic neurons in Parkinson's disease. Cortes et al. [71] have screened P7C3 analogues (P7C3-S7, P7C3-S8, P7C3-S25, P7C3-S40, P7C3-S41, P7C3-S54, P7C3-S165 and P7C3-S184) for neuroprotective efficacy. The compounds P7C3-S7 (75), P7C3-S25 (76) and (S)-enantiomer of P7C3-S41 (77) were found to be active in MPTP protection assay. The compound P7C3-S184 (78) has been reported as a β-secretase inhibitor and prevented the formation of Aβ-peptide from amyloid precursor protein thereby proposed as a therapeutic approach for Alzheimer's disease ( Figure 23). Yoon et al. [72] have developed and screened 25 aminopropyl-carbazole derivatives that can enhance neurogenesis of cultured neural stem cells. Among these analogues, compounds 79-81 demonstrated excellent proneurogenic and neuroprotective activity with no apparent toxicity ( Figure 24

Anti-Epileptic and Antinociceptive Activities
Epilepsy is the most frequent neurologic infection characterized by excessive temporary neuronal discharge [73]. The overall prevalence of the disease is 1.0% of the population and up to 50 million people worldwide. Previous studies showed that a significant percent of individuals (20%-30%) using anti-epileptic drugs are resistant to the currently used therapeutic agent [74]. Therefore researchers are trying to find more active and less toxic compounds to control the seizures and produce a more comfortable life for the patients [75,76].
The N-substituted carbazoles 82-87 have been introduced by Rjamanickam et al. [77] and were screened for their antilepileptic and antinociceptive activities ( Figure 25). Compound 86 showed highly significant anti-epileptic potential at a concentration of 20 mg/kg. This may be due to the substituent, 2-(2,-3-dimethyl-phenyl)-amino-benzoic acid. The compounds 82-87 have been reported to possess analgesic potential. The details of the antinociceptive activity of these compounds are given in Table 1

Conclusions
Carbazole is a unique template associated with several biological activities. Due to the diverse and versatile biological properties of carbazole derivatives, particularly N-substituted carbazoles, they are of great interest to the research community. In particular, their antimicrobial, anticancer and antinociceptive activities makes these compounds attractive candidates, not only for microbe borne diseases, but also for the several other conditions like Alzheimer's disease and epilepsy. This article has presented a comprehensive review of potent N-substituted carbazoles reported for their biological activities. More research must be carried out to explore their therapeutic potential for many other diseases like AIDS, hepatitis and diabetes.