Design and development of novel mTOR and SRC family kinase inhibitors via a phenotypic drug discovery approach
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Traditionally, drug discovery programs have focused on prioritising compounds by their affinity to a specific target in isolation, which was hypothesised to be the cause of a particular disease. Through chemical inhibition, the disease could, thus, be prevented or at the very least, controlled. These hypotheses require significant validation before drug screening can begin which relates to lengthy and expensive programs. Furthermore, drug screening against a single target in isolation is not a realistic model of cellular behaviour and is not appropriately tailored to more complex diseases such as cancer. Phenotypic drug discovery, on the other hand, bypasses any involvement of known targets, instead focusing on the desired outcome – the phenotype. In this way, drugs are biased by their potency on the phenotype and not against any particular targets. The molecular mechanism of action need not be known at all, however, it can be useful to later reveal the target(s) involved by various deconvolution methods. This thesis describes a cooperative ligand based phenotypic drug discovery approach, undertaken in order to develop more suitable small molecule drugs for cancer treatment. For this purpose, the promiscuous pyrazolopyrimidine inhibitor PP1 was chosen as a starting model compound. Modification of PP1 on the N1 position allowed a series of water solubilising groups to be incorporated into the pyrazolopyrimidine scaffold which created an initial 12-membered library. Testing against MCF7 breast cancer cells and looking at phenotypic end points such as cell proliferation, cell mobility and cell cycle, generated early target-agnostic structure/anti-proliferative activity relationships. These early results, along with compounds published in recent literature, were used to generate further libraries. Profiling lead compounds against a selection of 18 kinases known to be targeted by PP1, showed the compounds were inhibiting either SRC family or mTOR kinases which enabled the creation of two, structure specific, groups of inhibitors. Further lead optimisation led to the rapid discovery of preclinical candidates with excellent drug-like properties and potencies in both cellular assays and against their respective targets. Compounds also showed improved selectivity profiles compared to PP1 and commonly known inhibitors of SRC and mTOR kinases. Reported, herein, is the discovery of the first sub-nanomolar SRC inhibitor which does not inhibit the kinase ABL and shows excellent properties suitable for further preclinical development.