Synthesis, in vitro antibacterial and antiprotozoal evaluation of 2-phenyl-4-amino quinazolines

Abstract

ESKAPE pathogens, including Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter species, are prominent contributors to global nosocomial infections. These pathogens possess the ability to evade the biocidal action of antibiotics, representing a paradigm shift in resistance, transmission, and pathogenesis. Multidrug resistance is common, with currently very few to no effective drugs. There is, therefore, an urgent need to identify new compounds active against ESKAPE pathogens. Human African trypanosomiasis (HAT) is caused by Trypanosoma brucei (T.b.) parasites. Greater than 90% of HAT cases are primarily attributed to T.b. gambiense, which is prevalent in western and Central Africa, and causes the chronic form of the disease. T.b. rhodesiense, primarily infects animals and occasionally humans, causing HAT in southern and eastern Africa. HAT disease progresses through an early stage involving parasites residing in the lymphatic system and bloodstream (haemolymphatic stage), followed by a late stage during which trypanosomes cross the blood-brain barrier into the central nervous system and cause neurological damage (meningoencephalitic stage). T.b. brucei causes severe and often fatal disease referred to as nagana in domestic animals. Drugs available for both nagana and HAT are limited in number, efficacy, and spectrum of activity. They have poor drug-like properties such as low oral bioavailability and high toxicities, hence the need to identify new compounds with antitrypanosomal activity. Leishmaniasis, caused by Leishmania parasites and transmitted by infected sandflies, manifests in various clinical forms depending on the infecting Leishmania species and the host's immune response. Cutaneous leishmaniasis affects the skin, mucosal leishmaniasis affects the mucous membranes, and visceral leishmaniasis affects internal organs such as the lungs and liver. Visceral leishmaniasis is considered an opportunistic infection in immunocompromised individuals, notably those with the human immunodeficiency virus (HIV) infection. Current leishmanial treatments are not ideal due to intravenous administration, toxicity concerns, and high cost. Tuberculosis (TB), caused by the bacterium Mycobacterium tuberculosis (M. tuberculosis), is one of the oldest diseases affecting humans and remains a significant global health threat. TB is classified into pulmonary TB and extrapulmonary TB based on clinical manifestations. Pulmonary TB affects the lungs, while extrapulmonary TB affects other organs. The emergence of drug-resistant M. tuberculosis strains and the co-infection of TB with HIV have exacerbated the management of TB, and TB treatment failure is currently on the rise. This makes it necessary to search for new molecules for treating TB. Quinazoline is a heterocyclic ring system composed of fused benzene and pyrimidine rings. Its discovery dates back to 1903 when the first quinazoline-based compound, 2-methyl-1,3-aryl-4-quinazoline, was isolated from the Chinese plant aseru. This compound possesses sedative and somniferous properties. Subsequent research focused on creating chemical derivatives of quinazoline and exploring their biological activities. By 1980, there were approximately 50 different derivatives, many with various pharmacological effects, including sedative, tranquilising, analgesic, anticonvulsant, antitussive, myorelaxant, antirheumatic, hypotensive, antiallergic, bronchodilating, antidiabetic, cholagogue, diuretic, antimalarial, and spermicidal activities. Several drug molecules containing the quinazoline ring have been developed for various therapeutic purpose, including bunazosin, trimetrexate, vandetanib, prazosin, alfuzosin, gefitinib, and erlotinib. These compounds are used in cardiovascular medicine, antimicrobial therapy, and cancer treatment. Overall, their diverse pharmacological activities have made this scaffold valuable for drug discovery. In this project, a series of 2,4-disubstituted quinazolines were synthesised successfully. These compounds were subjected to in vitro screening to evaluate their potential as antitubercular, antiprotozoal, antibacterial and antifungal agents, as well as their cytotoxicity. Among the tested compounds, 3a-3c, 3e, 3f, and 5a-5c demonstrated promising antitubercular activity, with minimum inhibitory concentration (MIC), which represents the lowest concentration at which a substance inhibits bacterial/fungal growth, values below 9 μM. This indicates they are potent against the TB-causing bacterium. However, none of the compounds showed activity against ESKAPE pathogens. The MIC values against these bacteria were higher than 32 μg/mL and a Dmax < 50. Dmax (maximum non-inhibitory concentration) values below 50 indicate the concentration at which the compound does not have any inhibitory effect. In terms of antifungal activity, Compounds 2b, 3b, 3d, and 3f exhibited activity against Cryptococcus neoformans var. grubii, with MIC values equal to or below 32 μg/mL. Compound 2b also displayed potent activity against Candida albicans, with an MIC value below 32 μg/mL. Compounds 5a-5c displayed the best antitrypanosomal activity (IC50 < 7 μM). Overall, the synthesised 2,4-disubstituted quinazolines demonstrated promising antitubercular, antifungal, and antiprotozoal activities in vitro. The compounds demonstrating promising antitubercular, antifungal, and antitrypanosomal activities in this study hold significant potential for future drug development. Their effectiveness against Mtb, Cryptococcus neoformans var. grubii, and Candida albicans suggests they could contribute to the development of novel treatments against these human pathogens. Additionally, the potent antitrypanosomal activity of Compounds 5a-5c highlights their potential in addressing trypanosomiasis. These findings pave the way for further research and exploration of these compounds as potential candidates for new drugs or therapeutic interventions against a range of infectious diseases. In summary the synthesis of the 2,4-disubstituted quinazolines includes the following key steps: As a starting material, 2-aminobenzamide was used to react with different aldehydes in a cyclisation reaction mediated by iodine to form Compound 1 in 61% yield. Compound 1 underwent chlorination using thionyl chloride to form Compound 2 in 55% yield. Compound 3 underwent nucleophilic substitution reaction (nucleophilic substitution reactions belong to a category of chemical reactions where an electron-rich nucleophile reacts with a positively charged electrophile, leading to the displacement of a leaving group) in the presence of appropriate amines under refluxing conditions to furnish Compounds 3-5 with yields ranging from 5-54%. Reactions were monitored using thin-layer chromatography (TLC).