| dc.description.abstract | The metabolic breakdown of the essential amino acid L-tryptophan is controlled by three dioxygenase enzymes: indoleamine-2,3-dioxygenase 1 (IDO1), indoleamine-2,3-dioxygenase 2 (IDO2), and tryptophan-2,3-dioxygenase(TDO). The overexpression of these enzymes leads to immune dysregulation. IDO1 has evolved to suppress autoimmunity during periods of sustained inflammation but its activity is co-opted by cancer cells as a mechanism to evade immune detection by the host, commonly referred to as ‘tumour immunity escape’. Anti-tumour immunity is restored via IDO1 enzyme inhibition and thus the use of small molecule inhibitors as potential immunotherapeutic agents has been the subject of intensive study for more than two decades. Furthermore, much less work has been done on inhibitor cross-reactivity and enzyme selectivity. One of the great challenges in this field is to design less cross-reactive and more potent IDO1 inhibitors.
In recent years, boron-containing molecules have attracted much interest as Lewis acids and unique robust frameworks in drug design. Polyhedral boranes such as the carboranes, in particular, are low in toxicity and the closo forms are hydrophobic in nature. Such clusters can act as robust frameworks with the capacity to target hydrophobic regions of biological receptors such as enzyme active sites. Indeed, IDO1 is an excellent target for the study of carborane-enzyme interactions due to the presence of two hydrophobic pockets near its active site. Other boron-containing fragments, such as boronic acids, boronate esters, MIDA-boronate esters and benzoxaboroles, can also be exploited in drug design owing to the electron-deficient nature of the boron centre, which allows for target engagement involving covalent bond formation with amino acid residues such as serine.
In this thesis, we reported the design and synthesis of new boron-based, small-molecule inhibitors of IDO1 based on known (organic) inhibitors of this enzyme. The boron moieties exploited in this thesis include: boronic acid, boronic acid pinacol ester, benzoxaborole and the two carborane isomers, closo-1,2- and 1,7-carborane.
We report the design and synthesis of two series of boronated diaryl urea compounds and their preliminary IDO1 binding characteristics were evaluated by means of SPR. Those compounds bearing the boronic acid pinacol ester moiety, such as 70, 74, 79, and 80 exhibited IDO1 binding affinity in the low micromolar range. Furthermore, boronic acid functionalised compounds 73, 86 and 87 showed excellent binding to the enzyme (KD = 1.0 – 2.3 µM). Compounds 91 and 92, containing benzoxaborole fragments were also found to be potent binders of IDO1 enzyme (KD = 1.0 and 2.0 µM, respectively). The SPR binding studies indicated that the isomeric phenyl carboxylic acid linker leads to enhanced IDO1 binding compared to the 2,5-dimethylfuran-3-carboxylic acid linker. In the case of the two closo-carborane isomers 139 and 140, those compounds containing a closo-1,7-carborane (meta-carborane) cage were found to strongly bind the enzyme (KD = 1.7 and 1.0 µM) due to the increased hydrophobicity of these molecules compared to other boron derivatives. Substitution at the para- or meta-positions of the aryl ring also resulted in excellent IDO1 binding (KD = 1.2 – 2.3 µM) whereby the position of the carboxylic acid functionality was varied. Furthermore, derivatives with boronic acid pinacol ester or boronic acid group at the 4- or 3-position on the terminal phenyl ring had the most promising binding affinity towards IDO1 (KD = 1.2 – 2.3 µM). In summary, the investigation of this series of compounds indicated that the boronated diaryl urea is a compelling target for IDO1 binding.
In this study, we also demonstrated the synthesis and SPR binding of new boron-containing molecules based on the 1,4-naphthoquinone framework. Among these, compound 175 with meta-carborane substitution at the 2-position of the imidazole ring was shown to be the most potent binder of IDO1 in this particular series of compounds (KD = 1.9 µM), and is identified as a promising lead compound for further study. Amongst the 1,4-naphthoquinone derivatives, compound 180 bearing the 3-phenylboronic acid moiety at the 2-position of the thiazole ring exhibited moderate binding to the enzyme (KD = 75 µM).
Many diaryl hydroxylamine derivatives have been reported as excellent IDO1 inhibitors to date. Herein, a new series of boron-based diaryl hydroxylamine compounds were designed, synthesised and evaluated for their IDO1 binding by means of SPR. In this series of compounds, only those derivatives related to boronic acid pinacol ester fragment were evaluated. Most of the target compounds displayed only weak binding to the IDO1 enzyme (KD = 222 – 2186 µM), except for compounds 237 and 240 containing 3,4-dichloro phenyl linker, which showed moderate binding affinities (KD = 97 and 81 µM, respectively).
In summary, the incorporation of diverse boron functionalities into organic inhibitors led, in some cases, to a significantly enhanced IDO1 binding compared to their organic analogues. The most potent IDO1 binders from each class of boron fragment prepared in this work include the boronic acid compound 73 (KD = 1.2 µM), boronate ester compound 70 (KD = 1.2 µM), benzoxaborole compound 92 (KD = 1.0 µM), and meta-carborane compound 140 (KD = 1.0 µM). Selected compounds prepared in this work are worthy of further exploration as small molecule inhibitors of IDO1. This work demonstrates that boron functionalities can enhance the binding of known IDO1 inhibitors and provides further evidence that boron is a unique element that is worthy of exploration in future drug discovery programs. | en |
