Abstract
Use of multicellular tumor spheroids (MTS) to investigate therapies has gained impetus because they have potential to mimic factors including zonation, hypoxia and drug-resistance. However, analysis remains difficult and often destroys 3D integrity. Here we report an optical technique using targeted nanosensors that allows in situ 3D mapping of redox potential gradients whilst retaining MTS morphology and function. The magnitude of the redox potential gradient can be quantified as a free energy difference (ΔG) and used as a measurement of MTS viability. We found that by delivering different doses of radiotherapy to MTS we could correlate loss of ΔG with increasing therapeutic dose. In addition, we found that resistance to drug therapy was indicated by an increase in ΔG. This robust and reproducible technique allows interrogation of an in vitro tumor-model's bioenergetic response to therapy, indicating its potential as a tool for therapy development.
Highlights
The microenvironment of cancer, and the generation and distribution of energy by cancer cells and stroma for growth and survival, is complex
For photothermal optical coherence tomography (PTOCT), nanosensors in the multicellular tumor spheroids (MTS) were photothermally heated by a modulated laser (785 nm), which created a modulation in the phase of the OCT signal over time
We have demonstrated that MTS can be grown such that nanosensors are targeted to distinct zones and we confirmed this targeting using photothermal OCT
Summary
The microenvironment of cancer, and the generation and distribution of energy by cancer cells and stroma for growth and survival, is complex. Many cancers exhibit the Warburg effect,[1] whereby metabolism shifts away from oxidative phosphorylation (which is optimized for ATP generation) towards glycolysis, a less energy efficient. Conventional 2D monolayer tissue culture techniques overlook microenvironmental effects, but 3D tissue culture techniques such as the formation of multicellular tumor spheroids (MTS) may provide a better model of tumor structure.[4,5,6,7,8] MTS develop a radial structure with a core that may be necrotic or quiescent, surrounded by quiescent cells and an outer layer that contains proliferating cells (Fig. 1); this structure has gradients in oxygen, nutrients and pH (Fig. 1) that mimic gradients present in vivo.[4,7] the analysis of MTS models is more difficult due to their 3D nature and often misses either temporal or spatial resolution (i.e. fast methods typically overlook 3D heterogeneity and methods with good spatial resolution are time intensive and may require fixing and sectioning).[9] Here we report a method for the measurement of redox potential and pH with spatial and temporal resolution that may be used to probe MTS metabolic phenotype and response to radiation and drug treatment in live MTS
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