Converging approaches towards selective FLT3 inhibitors – BIODOL

The efficient treatment of chronic and neuropathic pain is a yet unsolved medical, economic and societal problem. It affects an increasing number of patients and severely impairs their daily life (weekly symptoms over ca. 7 years on average, 60% of patients obliged to work at home, 13% of unemployed patients). Current treatment of chronic pain is purely symptomatic (NSAIDs, weak opiates, antiepileptics, antidepressants, anxiolytics) and inefficient in 75% of the cases.
Among the many signaling pathways that are currently investigated to better define molecular mechanisms involved in the appearance and maintenance of chronic pain, our consortium is particularly interested in a cytokine (FL) and its receptor tyrosine kinase (FLT3) whose expression on primary sensory neurons has been shown to mediate mechanical as well as thermal hyperalgesia through the activation of different TRP channels. Administration of the FL cytokine in mice induces a FLT3-specific hyperalgesia that can be reversed by inhibiting the FLT3 receptor (FLT3 ko mice, anti-FLT3 siRNAs). Importantly, reversal of FL-induced hyperlgesia with anti-FLT3 agents is devoid of addiction and tolerance, as classically observed with opiates. FLT3 ko mouse displays normal response to acute pain stimuli, but almost failed to develop neuropathic pain syndrome after peripheral nerve injury. Intrathecal administration of anti-FLT3 siRNA suppressed both the development and the maintenance of tactile allodynia after sciatic nerve ligature. Taken together, our results indicate a previously unknown role for FLT3 expressed by sensory neurons in maintaining sensitization that has been implicated in persistent neuropathic pain. Thus blocking FLT3 signaling in somato-sensory neurons might be a new strategy for the therapy of chronic neuropathic pain induced by nerve injury.
Since mutations of the FLT3 gene is the most common genetic lesion in acute myeloid leukaemia (AML), many FLT3 inhibitors have been developed for the treatment of AML. However, they suffer from a lack of selectivity for the receptor tyrosine kinase FLT3 since they all target the conserved kinase catalytic ATP-binding site. Severe side effects associated with the therapeutic use of FLT3 inhibitors are tolerated in oncology but not in the perspective of a long-lasting treatment of chronic and neuropathic pains. Combining virtual screening of compound libraries and experimental binding/functional assays, our consortium has identified the first extracellular FLT3 receptor antagonist able to inhibit, at a low micromolar range, the binding of the cytokine FL to its FLT3receptor and further block FL-induced FLT3 autophosphorylation. When administered to rats, the inhibitor completely reverses neuropathic pain induced by a chronic constriction injury (CCI) of the sciatic nerve. This compound is however not directly usable in humans because of a still moderate affinity for its primary target, the existence of two off-targets that may lead to undesired effects, and a low oral bioavailability.
Having made the proof-of-concept that extracellular FLT3 inhibition by a low molecular weight compound is feasible and really reverses neuropathic pain in rodents, the BIODOL project is aimed at developing convergent approaches towards the identification of a potent and orally available selective FLT3 inhibitor along 3 main axes: (i) medicinal chemistry optimization of pharmacodynamic and pharmacokinetic properties of the existing initial lead; (ii) identifying novel hits by experimentally screening the French National Library on an already developed in vitro HTRF assay, (iii) evaluate the ability of peptides featuring the transmembrane segment of FLT3 and related receptor tyrosine kinases (e.g. PDGFR, c-kit, Fms) to block the receptor dimerization necessary for downstream signaling. The final deliverable of the project will be the development of a potential clinical candidate for treating neuropathic pain in humans.