Synthesis and in vitro anti-tubercular evaluation of 6-nitroquinolone-3-carboxamides

Abstract

Tuberculosis (TB) is a disease that has been a major scourge of humankind since time immemorial. It is an airborne disease that mainly affects the lungs and it is caused by Mycobacterium tuberculosis (Mtb)─ a pathogenic bacterium in the family mycobacteriaceace. The disease is predominant in developing countries (African countries included), currently affecting one third of the world's population. HIV/AIDS pandemic has promoted TB to thrive, with 215000 TB related deaths among HIV positive individuals reported globally in 2018. Moreover, the emergence of drug resistant strains of Mtb also promotes the high prevalence of TB. Globally, an estimated 10 million people fell ill with TB in 2018 and among these, 484000 incident cases and 214000 deaths were caused by MDR-Mtb. The highest TB burden is in men who accounted for 57% of all TB cases in 2018. Women accounted for 32%, children below the age of 15 years accounted for 11% and people living with HIV accounted for 8.6%. MDR-Mtb strains are caused by the lack of compliance to treatment by TB patients. TB treatment still largely depends on chemotherapeutic agents discovered 40-60 years ago. The current chemotherapy for drug susceptible Mtb consists of the four first-line drugs: isoniazid, rifampicin, ethambutol and pyrazinamide, which have to be taken daily for a minimum of 6 months. The regimen is effective if taken as prescribed and has shown high efficacy in achieving cure rates around 90-95% both in treatment under the oversight of TB control programmes and trial conditions. However, because of the long treatment period, numerous side effects, and severe adverse events like hepatotoxicity, patients find it hard to take their medication daily for over 6 months as prescribed. Fortunately, in 2019, WHO consolidated a new guidance in the second line MDR-Mtb drug treatment. Among these drugs, two new drugs (delamanid and bedaquiline) were added; making them the first TB drugs with novel mechanism of action after 40 years. Despite these current advances in TB chemotherapy, there are now undeniable evidence that resistance to the second-line regimen is now emerging. Thus, this further resistance leads to extensively drug resistance Mtb (XDR-Mtb). The emergence of MDR/XDR-Mtb indicates the importance to search for new TB drugs with a novel mechanism of action not observed with current anti-tubercular agents deployed currently. Nitro-containing compounds have been highlighted as unique chemotype exhibiting potent inhibitory activity against Mtb through a novel mode of action. Most notably, this class of compounds have been validated to demonstrate anti-tubercular activity by acting as suicide inhibitors of DprE1. However, they also suffer from poor drug-like properties such as short half-life and poor aqueous solubility. Thus, to circumvent the shortcomings of nitro-containing ant-TB leads, we herein report the synthesis and evaluation of new nitro-containing compounds around the quinolone scaffold. It is noteworthy to mention that more than 70 % of newly approved anti-bacterial are constructed around the quinolone scaffold. All target compounds (3a, 5-25) were screened in-vitro for inhibitory activity against the green-fluorescent protein reporter strain of Mtb using middlebrook 7H9 media supplemented with casitone, glucose and tyloxpol. The anti-tubercular activity was reported as the minimum inhibitory concentration required to inhibit 90% (MIC90) of bacteria population. The MIC90 was evaluated at day 14 following incubation of Mtb with target compounds. No precipitation was observed during screening and compounds were soluble in screening media. 13 out of the 22 compounds evaluated were active against Mtb, exhibiting activity in the range of 0.244-31.865 µM. 12 out of the 13 active compounds exhibit MIC₉₀ values < 10 µg/ml, which gives a high hit rate of > 50% for this compound set. In conclusion, the synthesised compounds evaluated against the gfp reporter strain of Mtb showed great activity. Preliminary assessments of these compounds indicated potential good physico-chemical properties. They are less lipophilic with cLogP values ranging from 0.72-3.67 and have molecular weight < 500 Da. These results are encouraging and suggest that this new class of compounds is worth further exploring for potential anti-mycobacterial drugs.