Therapeutic potential of heterocyclic pyrimidine scaffolds

Heterocyclic compounds offer a high degree of structural diversity and have proven to be broadly and economically useful as therapeutic agents. Comprehensive research on diverse therapeutic potentials of heterocycles compounds has confirmed their immense significance in the pathophysiology of diseases. Heterocyclic pyrimidine nucleus, which is an essential base component of the genetic material of deoxyribonucleic acid, demonstrated various biological activities. The present review article aims to review the work reported on therapeutic potentials of pyrimidine scaffolds which are valuable for medical applications during new generation.


Introduction
Pyrimidine is the six membered heterocyclic organic colorless compound containing two nitrogen atoms at 1st and 3rd positions (Fig. 1). The name of the pyrimidine was first applied by Pinner from the combination of two words pyridine and amidine). Pyrimidines (1,3-diazines) and their fused analogues form a large group of heterocyclic compounds. Pyrimidine which is an integral part of DNA and RNA imparts diverse pharmacological properties. The pyrimidine have been isolated from the nucleic acid hydrolyses and much weaker base than pyridine and soluble in water [1]. Pyrimidine and its derivatives have been described with a wide range of biological potential i.e. anticancer [2], antiviral [3], antimicrobial [4], antiinflammatory [5], analgesic [6], antioxidant [7] and antimalarial [8] etc.

Antimicrobial activity
The growing health problems demands for a search and synthesis of a new class of antimicrobial molecules which are effective against pathogenic microorganisms. Despite advances in antibacterial and antifungal therapies, many problems remain to be solved for most antimicrobial drugs available. The extensive use of antibiotics has led to the appearance of multidrug resistant microbial pathogens which necessitated the search for new chemical entities for treatment of microbial infections [9].
Anupama et al. synthesized a series of 2,4,6-trisubstituted pyrimidines by reacting chalcone with guanidine hydrochloride. All the synthesized derivatives were confirmed by physicochemical properties and spectral data (IR, NMR and elemental analyses) and screened their in vitro antimicrobial activity against bacterial and fungal strains by cup plate method using Mueller-Hinton agar medium. Among the derivatives tested, compounds, a1, a2 and a3 exhibited promising activity against microbial strains (B. pumilis, B. subtilis, E. coli, P. vulgaris. A. niger and P. crysogenium) and showed activity comparable with standard drugs. Structure activity relationship (SAR) studies indicated that compounds, a1, a2 and a3 having dimethylamino, dichlorophenyl and fluorine substituent on the phenyl ring at 4th position respectively exhibited better antimicrobial activity (Table 1 methyl]thio}-6-methylpyrimidines from pyrimidine-benzimidazole combination. All the synthesized derivatives were fully characterized by 1 H-NMR, 13 C-NMR and HRMS study and screened its in vitro antimicrobial activity against Gram-positive bacteria (Staphylococcus aureus, Bacillus subtilis), Gram-negative bacteria (Escherichia coli, Stenotrophomonas maltophilia) and fungi (Candida albicans). The minimum inhibitory concentration (MIC) of the target compounds was determined by broth microdilution method and compared to two commercial antibiotics (levofloxacin and fluconazole). Among the entire synthesized derivatives, compounds, a4 and a5 were found to be the most active antimicrobial agents ( Table 2, Fig. 2). Structure activity relationship showed that aromatic amines at pyrimidine ring are beneficial for the antimicrobial activity. Besides, the aniline containing para-substituted groups (especially Cl and Br) is more beneficial for the activity [10].
El-Gaby et al. developed a new class of pyrrolo [2,3d]pyrimidines containing sulfonamide moieties and screened its in vitro antifungal activity against four species of fungi viz: Aspergillus ochraceus (Wilhelm), Penicillium chrysogenum (Thom), Aspergillus fleavus (Link) and Candida albicans (Robin) Berkho by disc diffusion technique. Most of the synthesized molecules in this series were found to possess antifungal activity (Table 3, Fig. 2) towards all the microorganisms' used especially, compound a6 exhibited a remarkable antifungal activity which is comparable to the standard fungicide drug mycostatin [11].
Hilmy et al. developed a new series of pyrrolo[2,3-d] pyrimidine derivatives. The synthesized compounds were confirmed by IR, NMR, Mass and elemental analysis study and evaluated its antimicrobial activity against bacterial (Staphylococcus aureus, Escherichia coli) and fungal (Candida albicans) organisms was carried out by serial dilution method. All synthesized derivatives showed that good antimicrobial activity, especially, compounds, a7, a8, a9 were exhibited the better antimicrobial activity and compared with the standard drug (ampicillin and fluconazole) ( Table 4, Fig. 2) [12].
Holla et al. developed a new class of pyrazolo [3,4-d] pyrimidine derivatives. The synthesized derivatives were analyzed for N content and their structures were confirmed by IR, NMR and Mass spectral data and screened their antibacterial activity against Escherichia coli, Staphylococcus aureus, Pseudomonas aeruginosa and Bacillus subtilis by disk diffusion method and antifungal activity against Aspergillus flavus, Aspergillus fumigates, Candida albicans, Penicillium marneffei and Trichophyton mentagrophytes by serial plate dilution method. All synthesized pyrazolo [3,4-d]pyrimidine derivatives in this series showed that good antimicrobial and fungal activity against bacterial and fungal strains, especially compounds, a10 displayed very good antibacterial activity (Table 5, Fig. 2) and a11 exhibited antifungal activity (Table 6    and a17 showed excellent antibacterial and antifungal activity than the standard drugs ciprofloxacin and clotrimazole respectively (Tables 8, 9, Fig. 3) [15].

Anticancer activity
Cancer is a multifaceted disease that represents one of the leading causes of mortality in developed countries. Worldwide, one in eight deaths are due to cancer and it is the second most common cause of death in the US, exceeded only by heart disease. Chemotherapy is the mainstay for cancer treatment, the use of available chemotherapeutics is often limited due to undesirable side effects. It is important to identify new molecules and new targets for the treatment of cancer [17].
Shao et al. synthesized a new derivatives of 2,4,5-trisubstituted pyrimidine CDK inhibitors as potential antitumour agents. The synthesized 2,4,5-trisubstituted pyrimidine derivatives were evaluated for their antitumour activity against a panel of cancer cell lines including colorectal, breast, lung, ovarian, cervical and pancreatic cancer cells. Among the synthesized derivatives, compound b1, possessing appreciable selectivity for CDK9 over other CDKs, is capable of activating caspase 3, reducing the level of Mcl-1 anti-apoptotic protein and inducing cancer cell apoptosis (Table 15, Fig. 5) [21].
Cocco et al. synthesized a new class of 6-thioxopyrimidine derivatives and its molecular structures were confirmed by IR, NMR and elemental analyses study. The synthesized derivatives were evaluated their in vitro anticancer potential against multiple panels of 60 human cancer cell lines by Sulforhodamine B assay. All synthesized 6-thioxopyrimidine derivatives exhibited good anticancer potential, especially, compound b2 showed the best cytotoxicity (Table 16, Fig. 5) [2].    A new library of sulfonamide derivatives was synthesized and investigated for its in vitro and in vivo antitumor potential by El-Sayed et al. Preliminary biological study revealed that compounds, b3, b4 and b5 showed the highest affinity to DNA and highest percentage increase in lifespan of mice inoculated with Ehrlich ascites cells over 5-flurouracil was taken as standard drug ( properties and spectral data (IR, NMR, Mass and elemental analyses) and screened for their anticancer activity against human cancer cell lines i.e. PC-3 prostate and A-549 lung. Some of the tested compounds exhibited high growth inhibitory potential against PC-3 cell, among them, compounds, b6 and b7 showed relatively potent antitumor potential (Table 18 [3,4-d]pyrimidine derivatives by microwave (MW) irradiation method and evaluated its in vitro antitumor potential against human leukaemia (HL-60) cancer cell line by MTT assay. The preliminary results demonstrated that some of compounds exhibited potent antitumor inhibitory potential than doxorubicin (standard drug), especially compounds, b13 and b14 exhibited higher antitumor activity due to presence of CF group in its molecule structure (Table 20, Fig. 6) [26].
Lv et al. synthesized a new series of 2-phenylpyrimidine coumarin derivatives and evaluated its in vitro antiproliferative activity against CNE2, KB and Cal27 cancer cell lines. The results showed that most of the derivatives had a favorable effect on resisting tumor cell proliferation, among them, compound b32 exhibited the best antiproliferative activity and comparable to the standard drug (Table 28, Fig. 7) [36].

Antiviral activity
Antiviral nucleoside compounds inhibit viral genome replication by acting as mimetics of the natural nucleosides. Nucleoside analogues (NAs) can either act as chain     [3,4-d]pyrimidine-5,7-dione 1-oxide nucleoside was synthesized by Xu et al. and screened for its in vitro anti-vesicular stomatitis virus (VSV) activity in Wish cell. All the synthesized derivatives showed obvious anti-VSV potential whereas, compound c1 with ribofuranoside enhanced the anti-VSV potential by approximately 10-18 times compared to didanosine and acyclovir (standard drugs), respectively (Table 29, Fig. 8) [37].
A series of pyrazole and fused pyrazolo pyrimidines was synthesized by Rashad et al. and studied for their antiviral activity against hepatitis-A virus (HAV) and herpes simplex virus type-1 (HSV-1). The substituted pyrazole and fused pyrazolopyrimidine derivatives, c8 and c9 revealed higher anti-HSV-1 activity at concentration of 10 µg/10 5 cells and antiviral results are compared with amantadine and acyclovir (Fig. 8) [41].

Antimalarial activity
Malaria is the most serious and widespread parasitic disease because of its prevalence, virulence and drug resistance, having an overwhelming impact on public health in developing regions of the world. Plasmodium falciparum is the main cause of severe clinical malaria and death. Endemic mapping indicates that P. falciparum and P. vivax account for 95% of the malarial infections [43]. According to a WHO report, malaria accounted for 207 million cases and an estimated 627,000 deaths worldwide in 2013 [8].
Kumar et al. synthesized a new series of 4-aminoquinoline-pyrimidine hybrids and evaluated its antimalarial potential. Several compounds showed promising in vitro antimalarial activity against both CQ sensitive and CQ-resistant strains with high selectivity index. The in vitro evaluation of these hybrids against D6 and W2 strains of P. falciparum depicted the antimalarial activity in the nanomolar range. Also, these hybrids exhibited high selectivity indices and low toxicity against the tested cell lines. Compounds (d1, d2 and d3) (Fig. 9) exhibited very potent antimalarial activity with IC 50 = 0.033, 0.019 and 0.028 µM respectively which were comparable to the standard drug chloroquine (IC 50 = 0.035 µM) against CQ-sensitive strain [8].
Maurya et al. developed a new series of novel N-substituted 4-aminoquinoline-pyrimidine hybrids via simple and economic route and evaluated its antimalarial activity. Most compounds showed potent antimalarial activity against both CQ-sensitive and CQ-resistant strains with high selectivity index. All the compounds were found to be non-toxic to the mammalian cell lines. The most active compound d4 was analyzed for heme binding activity using UV spectrophotometer. Compound d4 was found to interact with heme and a complex formation between compound d4 and heme in a 1:1 stoichiometry ratio was determined using job plots. The interaction of these hybrids was also investigated by the molecular   docking studies in the binding site of wild type Pf-DHFR-TS and quadruple mutant Pf-DHFR-TS (Table 33, Fig. 9) [44]. Agarwal et al. developed a new series of 2,4,6-trisubstituted-pyrimidines and evaluated its in vitro antimalarial activity against Plasmodium falciparum. All the synthesized compounds showed good antimalarial activity against Plasmodium falciparum whereas, compound d5 exhibited higher antimalarial activity than pyrimethamine used as standard drug (Table 34, Fig. 9) [43].
Pretorius et al. synthesized a new library of quinolinepyrimidine hybrids and evaluated its in vitro antimalarial activity against the D10 and Dd2 strains of Plasmodium falciparum. The compounds were all active against both strains. However, hybrid (d6, Fig. 9) featuring piperazine linker stood as the most active of all. It was found as potent as CQ and PM against the D10 strain and possessed a moderately superior potency over CQ against the Dd2 strain (IC 50 : 0.157 vs 0.417 µM) and also displayed activity comparable to that of the equimolar fixed combination of CQ and PM against both strains [45].
A series of N-aryl and heteroaryl sulfonamide derivatives of meridianins were prepared by Yadav et al. and screened for its antimalarial activity against D6 and W2 strains of Plasmodium falciparum. Especially, compounds, d11 and d12 displayed promising antiplasmodial activity and comparable to the standard drugs (Table 36, Fig. 9) [47].

Anti-inflammatory activity
Non-steroidal anti-inflammatory drugs (NSAIDs) are among the most widely used therapeutics, primarily for the treatment of pain, rheumatic arthritis and various types of inflammatory conditions. However, their use is mainly restricted by their well known and serious adverse gastrointestinal side effects such as gastroduodenal erosions, ulcerations and nephrotoxicity [6].
Gondkar et al. prepared a new class of substituted 1,2,3,4-tetrahydropyrimidine and screened its in vitro anti-inflammatory activity by inhibition of protein denaturation method using diclofenac (standard drug). The results revealed that almost all the tested compounds showed potent anti-inflammatory potential. All synthesized derivatives were tested their in vitro anti-inflammatory activity using inhibition of albumin denaturation technique compared to standard diclofenac. Derivatives,  e11, e12, e13, e14 and e15 (Fig. 10) showed significant in vitro anti-inflammatory activity with % inhibition of albumin denaturation 98, 97, 90, 94, and 96% respectively [50].
Mohamed et al. synthesized a new library of thio containing pyrrolo [2,3-d]pyrimidine derivatives and carried out its in vitro anti-inflammatory potential using the carrageenan-induced rat paw oedema assay. The potency and duration of action was compared to ibuprofen was taken as standard drug. From tested compounds, compounds e21, e22 and e23 showed best anti-inflammatory activity (Table 42, Fig. 11) [52].
Sondhi et al. synthesized new derivatives of pyrimidine and screened their anti inflammatory activity carried out using carrageenin-induced paw oedema assay. All compounds exhibited good activity whereas, compound e24 was found to be most active one comparable to the standard drug ibuprofen (Table 43, Fig. 11) [53].

Antioxidant activity
Oxidative stress seems to play a significant role in various human diseases, including cancers. Antioxidant compounds are the agents that neutralize free radicals, which scavenge reactive oxygen species, may have potent value in preventing the onset and propagation of oxidative diseases such as neurovascular, cardiovascular diseases. Pyrimidine and its derivatives have recently attracted the attention of medicinal chemists in exploring their potential as antioxidant agents [1].
Bhalgat et al. developed a new class of novel pyrimidines and its triazole fused derivatives and investigated             Fig. 12) [56].

Antileishmanial activity
Leishmaniasis, a vector-borne parasitic disease, is a major cause of concern in developing countries. The disease is caused by more than 20 species of protozoan Leishmania and transmitted by the bite of female phlebotomine sand flies. Leishmaniasis has traditionally been classified into three major clinical forms: visceral leishmaniasis (VL), cutaneous leishmaniasis (CL) and mucocutaneous   [57].
A new series of substituted aryl pyrimidine derivatives was synthesized by Suryawanshi et al. and evaluated for its in vitro antileishmanial potential against intracellular amastigotes of Leishmania donovani using reporter gene luciferase assay. All synthesized compounds showed promising IC 50 values ranging from 0.5 to 12.9 µM. Selectivity indices (S.I.) of all these compounds are far better than sodium stibogluconate (SSG) and miltefosine used as standard drugs. On the basis of good selectivity indices compounds were further screened their in vivo antileishmanial activity against L. donovani/hamster model. Compounds, g1, g2 and g3 showed good inhibition (Table 48, Fig. 13) of parasitic multiplication that is 88.4, 78.1 and 78.2%, respectively at a daily dose of 50 mg/kg × 5 days, when administered intraperitoneally [57].
A new class of pyrido[2,3-d]pyrimidine derivatives was designed and synthesized by Ibrahim and Ismail. The pyrido[2,3-d]pyrimidine derivatives were evaluated for their in vitro anti-proliferative activity against A431a, SNU638b, HCT116 and inhibition of CDK2-Cyclin A, CDK4/Cyclin D and EGFR enzyme. In this class, the antiproliferative and CDK2-Cyclin A inhibitory activity of compounds, h17 and h18 (Fig. 15) was significantly more active than roscovotine (as standard drug) with IC 50 values of 0.3 and 0.09 µM respectively [64].