Novel PI3K and mTOR selective inhibitors to deconvolute PI3K signaling

Phosphoinositide 3-kinase (PI3K) signaling has key roles in the regulation of cellular processes such as cell growth, proliferation, and metabolism. Constitutive activation of PI3K in tumors is frequent and drives cancer progression. Considering the contribution of aberrant PI3K signaling in cancer progression, pharmacological intervention strategies to inhibit PI3K-driven malformations have been broadly explored as a therapeutic target, but many pan-PI3K inhibitors displayed a low response rate in clinical trials mainly due to on target metabolic side effects. Acute pan-PI3K inhibition triggers a rapid increase in blood glucose and insulin level since PI3Kalpha and PI3Kbeta isoforms have redundant roles in insulin signaling in the hepatocyte, and acute inhibition of the both isoforms impairs glucose homeostasis. Given that, isoform selective PI3K inhibition may alleviate hyperglycemia and hyperinsulinemia. However, the selectivity of the claimed PI3Kalpha-specific drugs is currently limited at their physiologically effective concentrations.

Herein, this project aimed to develop a rational drug design approach to increase target selectivity of a pan-PI3K inhibitory scaffold, PQR514, by its covalent attachment to an isoform-specific non-conserved nucleophilic amino acid side chain, Cys862 in PI3Kalpha. PQR514 reversible scaffold was derivatized to attach an electrophilic moiety (warhead) after adjusted improvements in warhead stability and warhead intrinsic chemical reactivity. The warhead proximity and its orientation to the covalent anchor site were optimized through our “active volume scanning” strategy to promote isoform-selective covalent attachment on the target site.

In addition to the development of novel PI3K targeting pharmacological probes, a dual pan-PI3K/mTOR-selective inhibitor (PQR530) and an mTOR-selective inhibitor (PQR626) were developed to deconvolute PI3K and mTOR signaling and to evaluate novel treatment modalities against epileptic seizures occurring due to loss of tuberous sclerosis complex (TSC) function. TSC2 (tuberin) together with its binding partner TSC1 (hamartin) have key functions to integrate multiple inputs from PI3K, ERK, Wnt, and energy signals through the attenuation of mTORC1 activity. Given that, TSC1 and TSC2 function as tumor suppressors, and genetic mutations disrupting TSC function cause a malformation called tuberous sclerosis complex (TSC) disease, which is manifested by the formation of cysts and benign tumors in vital organs such as brain and kidney. Targeting mTOR in the treatment of epileptic seizures using blood-brain barrier (BBB) permeable, orally bioavailable, and mTOR selective drug-like small molecule inhibitor, PQR626, reduced the loss of TSC1-caused mortality in a TSC1 (GFAP) CKO mouse model and did not induce metabolic side effects including hyperglycemia and hyperinsulinemia. mTOR selective/PI3K sparing inhibition strategy with PQR626 introduced certain advantages over dual mTOR/pan-PI3K inhibition strategy with PQR530 in order to circumvent on target metabolic side effects of pan-PI3K inhibition.