Research paperStructure based medicinal chemistry-driven strategy to design substituted dihydropyrimidines as potential antileishmanial agents
Graphical abstract
Introduction
Leishmaniasis is an infectious disease caused by Leishmania parasites belonging to genus Leishmania in the family Trypanosomatidae. It is transmitted into host blood by the bite of female sand fly [1]. These parasites can attack on mammals and human beings. When sandfly bites an infected organism parasite pass from the blood of infected organism into the gut of the sandfly. These parasites grow in the gut of sandfly for 8–20 days. When an infected sandfly bites a healthy organism it delivers these parasites (metacyclicpromastigote) in the blood of that organism. The infected stage, metacyclicpromastigotes inoculated in the rupture skin where it is phagocytosed by macrophages. The promastigotes multiplies and turn into amastigotes and infect many tissues which gradually express the disease. There are more than 20 Leishmanial protozoan species, that are responsible for a number of clinical forms of leishmaniasis such as cutaneous, diffuse cutaneous, disseminated cutaneous, mucocutaneous, visceral and post-kala-azar dermal leishmaniasis (PKDL). Leishmaniasis has many symptoms, such as destructive mucosal inflammation, skin lesions, ulcer and visceral infection (effects internal organs such as liver and spleen), fever and sometime anemia. This disease is prevalent in 88 countries, including Asia, Africa and Latin America. Visceral leishmaniasis (VL) is the most devastating and fatal form of leishmaniasis. It is caused by Leishmania donovani and Leishmania infantum. VL patients are prone to bacterial co-infection including tuberculosis and pneumonia etc. Post-kala-azar dermal leishmaniasis (PKDL), a form of dermal leishmaniasis caused by L. donovani, is a sequel of VL and develops months to years in VL cure patients [2].
In recent years, a plethora of investigational compounds has been investigated for antileishmanial activity. The major approved antileishmanial drugs are: pentavalent antimony drugs (meglumine antimoniate and sodium stibogluconate), pentamidine, miltefosine and amphotericin B. All these available treatments are not satisfactorily significant and have many draw backs such as renal dysfunction, nausea, anorexia, fever etc. There are also some reports of cardiac deaths. Besides investigational drugs, natural products are valuable sources in finding lead compounds. Flavonoids such as quercetin and luteolin emerged as potent antileishmanial agents against L. donovani. Similarly, natural products like lichochalcone A, iridoids, naphtoquinones, quinolone alkaloids, saponins, lignans and coumarins have shown promising antileishmanial activities [3], [4], [5]. Immunotherapy is considered as best alternative for the treatment of VL. Despite intense attempts to develop a prophylactic vaccine, there is no safe and efficacious vaccine against leishmaniasis due to inadequate knowledge of early immune response and poor understanding of parasite pathogenesis. However, Leish-111f + MPL-SE vaccine has been proved promising to control VL [6], [7]. Apart from chemotherapy/vaccination, leishmaniasis can be controlled by taking some safety measures like reservoir eradication, use of insect repellent, protective clothing and use of fine-mesh netting to prevent exposure to female sand fly [8].
Chalcones (1,3-diaryl-2-propen-1-ones), belonging to the flavonoid family, have been reported to possess many pharmacological activities. Zhai et al. reported oxygenated chalcones as potent inhibitors of L. major with IC50 in the range of 4.0–10.5 mM [3]. Foroumadi et al. investigated chromene-based chalcones namely, 1-(6-methoxy-2H-chromen-3-yl)3-phenylpropen-1-ones and 3-(6-methoxy-2H-chromen-3-yl)-1-phenyl-propen-1-ones for their antileishmanial activity against promastigotes form of Leishmania major [9]. These chalcones exhibited excellent activity at non-cytotoxic concentrations. Narender et al. reported promising antileishmanial activity of naturally occurring chromenochalcones [10]. 3,4-Dihydropyrimidine (DHPM) is the most attractive derivative of pyrimidines for a medicinal chemist. This structural motif may also be described as a derivative of cyclic urea. These non-planar heterocyclic compounds have received considerable attention of the pharmaceutical industry because of their interesting multifaceted pharmacological profiles. In the exploration of new and more potent antileishmanial compounds to diversify the current inhibitors, it is essential to design novel and potent inhibitors. Singh et al. identified potent dihydropyrimidine (DHPM) based derivatives targeting Pteridine reductase (PTR1) [11], [12]. Recently, Kaur et al. reported monastrol, a dihydropyrimidine based KSP inhibitor, as a potent antileishmanial agent [13].
In our group, a major part of our research is focused on computer-aided drug design with subsequent synthesis and testing of new chemical entities as putative drugs for the treatment of various diseases [14], [15], [16]. Our group recently identified a series of N-(1-methyl-1H-indol-3-yl)methyleneamines and eight new 3,3-diaryl-4-(1-methyl-1Hindol-3-yl)azetidin-2-ones against Leishmania major [17]. In another study, we identified a variety of 2-aryl- and 5-nitro-2-arylbenzimidazoles as new antileishmanial agents with IC50 values ranging from 0.62 to 0.92 μg/ml [18]. In continuation of our endeavor and considering the pharmacological importance of DHPM scaffold, it was planned to design and synthesize a variety of DHPM-based potent antileishmanial compounds to diversify the current inhibitors.
Section snippets
The design strategy
Molecular docking has contributed a lot in the identification of novel small drug-like scaffolds exhibiting high binding affinity and selectivity for the target. Hence, we extended our study to investigate in silico binding orientation of the synthesized DHPMs. Pteridine reductase (PTR1) is an important enzyme responsible for Pteridine salvage in leishmania and other trypanosomatid protozoans. PTR1 contributes to antifolate resistance and is responsible for the failure of conventional therapies
Conclusion
In summary, we have carried out medicinal chemistry-driven structure based modifications of 3,4-dihydropyrimidine core. It is clear from the SAR exploration around DHPM 8 analogues that certain aromatic substituents at 4-position on ring B of the chalcone moiety are important for antileishmanial activity. Attempt to decrease the number of rotatable bonds resulted in increased potency presumably due to hydrogen bond donor pattern of 4-OH group. In terms of potency, SAR exploration in Series 2
General
All the reagents and solvents were purchased from standard commercial vendors and were used without any further purification. Sonication was performed in Elma E 30H (Germany) ultrasonic cleaner with a frequency of 37 KHz and a nominal power of 250 W. 1H and 13C NMR spectra were recorded in deutrated solvents on a Bruker spectrometer at 300 and 75 MHz respectively using tetramethylsilane (TMS) as internal reference. Chemical shifts are given in δ scale (ppm). Melting points were determined in
Acknowledgments
The Higher Education Commission (HEC), Pakistan is thankfully acknowledged for providing financial support to Umer Rashid for its startup grant under IPFP program (HEC No: PM-IPFP/HRD/HEC/2011/346). The authors are also thankful to Chemistry Department, Quaid-i-Azam University and National Institute of Health (NIH), Islamabad, Pakistan for providing facilities.
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