Abstract 411: Reversibility of tumor adaptations to PI3K inhibition decreases as a function of time
Authors: Mukherjee Radha , de Stanchina Elisa , Chandarlapaty Sarat , Rosen Neal 
Published: June 2022

Abstract
The PI3K pathway is a key regulator of metabolism, cell proliferation and migration and some of its components (e.g. PIK3CA, HER2 and PTEN) are frequently altered in cancer by genetic events that deregulate its output. However, inhibitors of components of the pathway have only modest antitumor effects and, in some instances, have resulted in “tumor flares” in patients upon cessation of therapy. Low efficacy has been attributed to toxicity and to adaptive resistance. Inhibition of the PI3K pathway has been associated with relief of feedback inhibition of receptor signaling (RTKs) by a variety of mechanisms including increased receptor expression and decreased expression of the PTEN phosphatase, a major negative regulator of the pathway. Now we show that changes in expressions of RTKs, PTEN, anti-apoptosis proteins like Mcl1 and transcription factors known to drive oncogenic and survival programs in tumors like c-Myc, c-Jun, FOXOs are induced upon continuous long-term PI3K inhibition (2 weeks and longer). These changes increase as a function of duration of inhibition and become less reversible with time in drug. Unbiased clustering of RNA seq of cells treated with PI3K inhibitors revealed clusters of gene expression changes based upon short or long-term inhibition. Correspondingly the mRNA of the RTKs and transcription factors change with similar kinetics as that of the proteins and become less reversible with duration of inhibition. Moreover, the persister cells treated with PI3K inhibitors for 2 weeks or longer grow faster than the treatment naive cells upon drug removal both in vitro and in vivo and the degree of reversibility of the increased proliferation rate is inversely proportional to the duration of inhibition. Persister cells treated with PI3K inhibitors for more than two weeks have higher levels of expression of a set of transcription factors, RTKs, Mcl1 and of pAKT after one week of drug wash-out and are less sensitive to re-inhibition of PI3K. Combined inhibition of PI3K with Mcl1 inhibitors lead to greater reduction of cell proliferation and increased cell death.This suggests that continuous long-term inhibition of PI3K leads to a chronic adaptive state driven by relief of negative feedback which becomes less reversible with increasing duration of inhibition. Cells undergoing long term inhibition have higher steady state levels of components of the pathway leading to increased cell growth upon drug removal as compared to untreated cells. The data suggests that induction of adaptive resistance is in part responsible for the continued persistence of tumor masses after the initial response to targeted therapy. We hypothesize that pulsatile targeted inhibition with short intervals off therapy could avoid development of chronic adaptation. Intermittent administration of PI3K inhibitor combinations that cause potent induction of cell death may result in improved therapeutic outcomes.
Citation Format: Radha Mukherjee, Elisa de Stanchina, Sarat Chandarlapaty, Neal Rosen. Reversibility of tumor adaptations to PI3K inhibition decreases as a function of time [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 411.