Abstract 4513: Investigation of bystander effects of hypoxia activated prodrugs using three dimensional cell cultures

Background: Hypoxia in solid tumors acts as the major barrier to anti-cancer therapies and promotes tumor progression. Hypoxia-activated prodrugs (HAPs) are designed to not only eliminate these resistant cells, but also to exploit hypoxia as a basis of tumor selectivity. A key concept in exploiting hypoxia with HAPs is that active drug metabolite should be able to diffuse from hypoxic zones to maximize effect in surrounding aerobic tumor tissue. There is some evidence that these ‘bystander effects’ contribute to the antitumor activity of HAPs, but currently there is a lack of robust methods for their detection. Objectives: To investigate bystander effects from HAPs in a novel spheroid co-culture system using PR104A which releases a diffusible DNA crosslinking mustard and the tirapazamine analogue SN30000 which is converted to a short lived radical that is not expected to diffuse out of cells. Methods: A new method for measuring the bystander effect with multicellular spheroids under hypoxic conditions was developed using co-cultures of HCT116 colon cancer cells with high metabolic capacity for HAP metabolism (activators, transfected to overexpress cytochrome P450 reductase [POR]) and cells that cannot activate HAPs efficiently (targets, with POR knocked out by zinc finger nuclease gene targeting) along with differential antibiotic resistance and fluorescent protein markers to enable their quantitation in mixed cultures. Sensitivity of activators and targets to PR104A and SN30000 was determined in spheroid co-cultures by dissociation and clonogenic cell survival assay. Results were compared to output of a novel agent-based mathematical model (ABM) of spheroid co-cultures. Results: In monolayer cultures, activator cells showed markedly higher sensitivity to the HAPs than did targets. Tagging of activators and targets with fluorescent proteins allowed visualization of their spatial distribution in spheroid co-cultures. When spheroids were treated with PR104A, killing of targets markedly increased with increasing proportion of activators, confirming a bystander effect. However, increasing proportions of activators reduced cell killing of targets after SN30000 treatment, suggesting that rapid metabolism by activators inhibits its penetration, which was confirmed by flux experiments in multicellular layer co-cultures. ABM simulations of cell killing by PR104A and SN30000 in spheroids using our published transport parameters agreed with experimental results indic