Synthesis and Biological Evaluation of Imidazo[1,2-b]pyridazines as Inhibitors of Mycobacterium Tuberculosis

Author: Kyle Farrell

Farrell, Kyle, 2021 Synthesis and Biological Evaluation of Imidazo[1,2-b]pyridazines as Inhibitors of Mycobacterium Tuberculosis, Flinders University, College of Science and Engineering

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This thesis discusses the synthesis and biological evaluation of a library of imidazo[1,2-b]pyridazines as inhibitors of Mycobacterium tubercuolosis (Mtb). Chapter 1 introduces the nature and impact of tuberculosis as well as the current methods used in treating the disease. Shortcomings of these methods, including a growing resistance to the drugs used in the current regimen, are highlighted. The drug discovery and development pipeline, a process which is used to find new antitubercular agents with the intention to alleviate these issues, is described. Compounds within the drug discovery and development pipeline, including Phase I clinical candidate Q203, are discussed. It is noted that Q203 possesses an imidazo[1,2-a]pyridine core structure which resembles the scaffold of interest, the imidazo[1,2-b]pyridazine. The antimycobacterial activity of these compounds became of interest after CSIRO (Commonwealth Science and Industrial Research Organisation) and GIBH (Guangzhou Institutes of Biomedicine and Health) screened 18,000 pre-existing CSIRO compounds against Mtb and discovered some to be highly active. High-throughput screening (HTS) results of a range of these imidazo[1,2-b]pyridazines from the CSIRO Compound Library are displayed and early structure-activity relationship (SAR) insights are discussed, providing the foundation for the design of the compounds synthesised in Chapter 2. Antitubercular imidazo[1,2-b]pyridazines were scarcely observed within the literature, which gave scope to claim intellectual property for these compounds and to move forward with this project.

Chapters 2 and 3 discuss the syntheses of a range of imidazo[1,2-b]pyridazines for screening against Mtb. Chapter 2 focuses on two synthetic pathways which converge at the condensation product, 2-phenylimidazo[1,2-b]pyridazin-3-ol, which can be methylated to form the desired 3-methoxy-2-phenylimidazo[1,2-b]pyridazine. Side-products discovered along these synthetic pathways are structurally elucidated and mechanisms for their production are proposed.

Chapter 3 discusses the syntheses of imidazo[1,2-b]pyridazines with other C3 substituents

other than a methoxy moiety to probe this position for SARs. The compounds synthesised in

this chapter include 3-ethoxy- (221 and 222), 3-dialkylaminomethyl- (230, 234 and 235) and

3-carboxylate(lactone)- (239) substituted-imidazo[1,2-b]pyridazines shown below.

Chapter 4 discusses the attempts made to scaffold hop from an imidazo[1,2-b]pyridazine to

an imidazo[2,1-b][1,3,4]thiadiazole core structure, in hope of finding a potent bioisostere to

inhibit Mtb. Furthermore, hybridisation of Q203 and TB47 (a GIBH compound structurally

similar to Q203 with comparable potency against Mtb but contains a pyrazolo[1,5-a]pyridine

core structure) and a scaffold hop to the imidazo[1,2-b]pyridazine led to the successful

syntheses of “Q203/TB47 hybrid” imidazo[1,2-b]pyridazines 270 – 273.

SARs of the imidazo[1,2-b]pyridazines synthesised in Chapters 2, 3 and 4 are discussed in

Chapter 5 on the basis of their in vitro antimycobacterial activities against Mtb and Mm

(Mycobacterium marinum). The IC50 and MIC values of imidazo[1,2-b]pyridazines from the

CSIRO library were also evaluated to gather further SAR data. Five compounds displaying

highly potent in vitro activity were evaluated in a mouse in vivo study conducted at GIBH,

discussed in Chapter 6. However, all five compounds exhibited no in vivo activity. Four of

these compounds were evaluated for their physicochemical and metabolic properties at the

CDCO at MIPS which provided reasons for their lack of in vivo activity.

In Chapter 7, conclusions from the work described in Chapters 2 - 6 are presented, followed

by a discussion of future directions. Chapter 8 contains the experimental protocols for the

compounds synthesised. The appendices are divided into two sections where Section 1

contains the NMR spectra, LC-MS and HRMS chromatograms/spectra for compounds tested

against Mtb and Section 2 contains the NMR and HRMS spectra for miscellaneous

compounds, mostly side-products, described throughout this thesis.

Keywords: tuberculosis, mycobacterium tuberculosis, imidazo[1,2-b]pyridazine, imidazopyridazine, inhibition, antitubercular activity, anytimycobacterial activity

Subject: Chemistry thesis

Thesis type: Doctor of Philosophy
Completed: 2021
School: College of Science and Engineering
Supervisor: Assoc. Prof. Michael V. Perkins