Towards a New Class of Anti-Tuberculosis Drugs: Design and Synthesis of AnPRT TS Analogues

Tuberculosis (TB) is an infectious disease caused by the bacterium Mycobacteria tuberculosis (Mtb). In 2021 over 10 million people were diagnosed with TB. It is the number two cause of death behind COVID-19, being attributed to 1.6 million deaths in 2021. Although TB is curable the current multidrug treatment regimens are lengthy, which leads to low adherence and unfavourable treatment outcomes. This has given rise to antibiotic resistance, with 170,000 cases confirmed to be resistant to at least one of the first-line drugs in 2021. The COVID-19 pandemic has also had a significant impact on the global TB burden. This highlights the need for a new class of anti-tuberculosis drugs to tackle this current health crisis.

Tryptophan (Trp) is an amino acid that is important for the survival and virulence of Mtb. The Mtb Trp biosynthetic pathway is absent in mammals, therefore the inhibition of this pathway represents a great opportunity for the development of novel tuberculosis drugs.

Our group is experienced in transition state analysis (TSA) techniques, which is a sophisticated and rational technique to design potent enzyme inhibitors. This is exemplified by Mundesine®, a drug used for the treatment of peripheral T-cell lymphoma in Japan. In pursuit of a new class of anti-tuberculosis drug candidates, a series of potential TS analogues of anthranilate phosphoribosyl transferase (AnPRT), an enzyme involved in Trp biosynthesis pathway, were designed and synthesised.

For the synthesis of the first generation of compounds we used 5-O-tert-butyldimethylsilyl-1,N-dehydro-1,4-dideoxy-1,4-imino-2,3-O-isopropylidene-N-oxide-D-ribitol as a common intermediate. Seven targets were successfully isolated and the synthetic route leading to these targets has been optimised. These iminoribitol derivatives were then tested against the intended target AnPRT, using an enzyme-coupled assay. Unfortunately, the enzymatic assay revealed that the compounds tested displayed minimal inhibitory activity. Co-crystallisation with the enzyme and subsequent x-ray crystallography confirmed that this series of compounds did not bind in the active site of the enzyme. In order to facilitate entry into the active site, a more flexible second and third generation of acyclic phosphate and acyclic phosphonate derivatives were designed.

The synthesis of the second generation of targets proved to be challenging and required several alterations to the initial proposed synthetic route. This resulted in the preparation of a single compound. The third generation required less optimisation. Although there were difficulties with the final purification, five compounds were successfully isolated. Evaluating the inhibitory activity of these compounds will assist in drawing structure activity relationships between our compounds and the enzyme, which can be used for the refinement of future generations of inhibitors.