Synthesis and antimycobacterial activity of ciprofloxacin-triazole hybrids
Tuberculosis (TB) is a public health crisis – infecting a quarter of the world’s population, also, inflicting 10.4 million morbidities and 1.7 mortalities in 2016. TB is effortlessly acquired through the inhalation of droplets containing the bacilli Mycobacterium tuberculosis (Mtb) from an infected individual. The occurrence of multi-, extensively-, and totally drug-resistant strains of Mtb, co-infection with human immunodeficiency virus (HIV), and the complex, prolonged and substantial treatment regimen with its associated side-effects result in poor patient compliance and creates an irrefutable demand for the development of novel antimycobacterial agents to address these issues. Within this body of work, the skeleton of a second-line antimycobacterial agent, i.e. ciprofloxacin (CPX), was utilised to devise a new chemical entity with potent antimycobacterial activity. However, poor solubility and the rise of resistance against the fluoroquinolones class of drugs was recognised as a major drawback associated with CPX that requires improvement. Two series of CPX derivatives were synthesised, with each series a different strategy in mind for targeting Mtb. In Series 1 (analogues), CPX was derivatised in position N-15 and/or further modifications were made by replacing the hydroxyl-group in position C-11 of the carboxylic acid, with either an ester or an amide. In Series 2 (hybrids), hybrid drugs – consisting of CPX and 1,2,3-traizole (chosen for its reported antimycobacterial activity) – were prepared when utilising Huisgen’s copper alkyne-azide cycloaddition ‘‘click’’ reaction. In both series, cholesterol was employed as targeting moiety since it gets taken up by Mtb who uses it as energy (carbon) source after metabolism. Anchoring CPX to cholesterol will result in the transport of the active drug into the bacterium through the mce4-transporter. Characterisation of all synthesised compounds was achieved by means of nuclear magnetic resonance (NMR), mass spectrometry (MS), infrared spectrometry (IR), and the purity of compounds determined with the help of high performance liquid chromatography (HPLC). Both series of compounds were assessed in vitro for their antimycobacterial activity against the human virulent H37Rv strain of Mtb in the commonly employed Middlebrook 7H9 Broth base media, using either a GFP or MABA reported assay. Overall, analogues with the carboxylic acid retained in position C-3 were mostly found to be active (MIC90 < 10 μM), the amides inactive (MIC90 >125 μM) and the esters had mixed activities (MIC90 1 – 125 μM). The analogues containing the highly lipophilic cholesteryl moiety were mostly inactive. The activity of these fluoroquinolones was revealed in fact to be structure specific. This was equally seen with the hybrids. Additionally, the structure-activity relationship (SAR) revealed the least electron donating or withdrawing the substituent on the phenyl ring the more anti-Mtb active the hybrid. Cytotoxicitities of synthesised compounds were assessed using normal human fetal lung fibroblast (WI38), human embryonic kidney cells (HEK-293), and Chinese hamster ovary (CHO) mammalian cell lines. The active analogues 9, 10, 11 and 13, and the hybrids 26, 28, 29, 30 and 32 were nontoxic towards these cell lines, with selectivity indices (SI) greater than 10 – showing high specificity towards Mtb. On account of the efficacy and safety criteria, analogues 10 and 11 and hybrid 28 with MIC90 below 10 μM, and SI-values greater 27 were the most promising hit candidates lending themselves for further development.
- Health Sciences