Biological effects of combination therapy with oncolytic reovirus and palbociclib

Abstract

Reovirus type 3 Dearing (RtD3) is an oncolytic dsRNA virus with limited single-agent activity in clinical studies but has potential for use in combination regimens. Herein, I sought to discover synergistic drug-virotherapy combinations using an unbiased screening approach that highlighted the CDK4/6 inhibitor, palbociclib, as a leading hit. Cytoplasmic nucleic acid sensors for double-standard (ds) RNA (RIG-I/MDAS) and DNA (cGAS-STING) are pattern recognition receptors (PRRs) key to intracellular anti-viral responses. Recent research has highlighted roles for PRR agonists, including oncolytic virotherapy agents, in anti-tumour immunotherapy. Combined rt3D-palbociclib treatment potently increased expression of PRRs and interferon signalling. Additionally, this combination also induced an endoplasmic reticulum (ER) stress signature. Knockdown (siRNA) studies indicated key proteins involved in the unfolded response (UPR) and the RNA sensor, RIG-I, were essential to the phenotype observed. Mechanistically independent experiments, using synthetic RNA agonists and the ER stress inducer (thapsigargin), confirmed cross-talk between RNA sensing and ER stress pathways that augment cancer cell death and interferon production. Combined Rt3D-palbociclib increased innate immune activation and effector function. These findings demonstrate that UPR signalling and innate immune RNA sensor crosstalk can be exploited to enhance anti-cancer efficacy with pro-immunogenic consequences. This has implications for future clinical development of PRR agonists and oncolytic viruses, as well as broadening the therapeutic remit of CDK4/6 inhibitors to include their role as both an ER stress and dsRNA PRR sensitiser.