Bicyclic Acylguanidine Bioisosteres for Tuberculosis Indications

Prior to the FDA-approval of bedaquiline at the end of 2012, there was not a novel tuberculosis chemotherapeutic introduced into the market for 40 years. As a result, drug-resistant tuberculosis was problematic to treat and heavily relied on the development of novel drugs with novel drug targets. Bedaquiline was the first drug approved by the FDA that disrupts mycobacteria energy metabolism. Despite its efficacy in treating multi-drug resistant and extensively drug-resistant tuberculosis, prolonged use of bedaquiline is potentially fatal due to cardiotoxic side effects. As a result, the target product profile for a novel chemotherapeutic to treat drug-resistant tuberculosis requires a relatively non-toxic drug for prolonged use.

It was previously known that 5,6-disubstituted amiloride acylguanidines are potently tuberculocidal towards replicating and persisting mycobacteria (lead acylguanidine 4 MIC = 4 μM). However, due to in vivo hydrolysis of the acylguanidine, novel bioisosteres of this pharmacophore were required. Through the design, diversity-oriented synthesis from a common methyl ester intermediate and evaluation of amide bond and non-amide bond acylguanidine bioisosteres it was determined that an intramolecular H-bonded network is essential to position the pharmacophore into the active site of the tuberculosis drug target. In addition, it was determined that the number of H-bond donors and acceptors in the pharmacophore could not be extensively modified. As a result of these early studies, a novel bicyclic guanidinylated heterocyclic system was designed utilising a scaffold hopping approach to satisfy this criterion. The 8-guanidinylpyrido[2,3-b]pyrazin-6(5H)-one 47 was synthesised in 12 steps to give 4% (330 mg) of product. This represented the first time in the literature that an acylguanidine bioisostere was successfully developed as it maintained the tuberculocidal activity of the lead acylguanidine 4. In addition, 8-guanidinylpyrido[2,3-b]pyrazin-6(5H)-one 47 had superior pharmacokinetics (longer half-life and improved volume of distribution), pharmacodynamics (reduced intrinsic clearance) and physicochemical properties (improved aqueous solubility at pH 2 and pH 6.5 and reduction of gLogD7.4).

Another related pyrazine-fused bicyclic, 4-guanidinylpteridine 73 was developed and had equivalent tuberculocidal activity to lead acylguanidine 4 and 8-guanidinylpyrido[2,3-b]pyrazin-6(5H)-one 47. Eukaryotic cytotoxicity studies of 4, 47 and 73 revealed that 47 6-9 times less cytotoxic than 4 and 73. This satisfied the aim of the thesis, which was to develop a non-hydrolysable acylguanidine bioisostere that had superior drug properties and rendered 47 as the new lead compound.

When treated at 1 x MIC (8 μM) of 8-guanidinylpyrido[2,3-b]pyrazin-6(5H)-one and 5 x MIC of bedaquiline (a FDA-approved drug to treat multidrug resistant tuberculosis) replicating cultures of mc26230 mycobacteria were sterilised within two weeks. An animal model study is currently being prepared to assess the efficacy of the 8-guanidinylpyrido[2,3-b]pyrazin-6(5H)-one 47 when co-administered with bedaquiline to reduce the number of colony formation units per lung.