Abstract
Three-dimensional in vitro tumor models are highly useful tools for studying tumor growth and treatment response of malignancies such as ovarian cancer. Existing viability and treatment assessment assays, however, face shortcomings when applied to these large, complex, and heterogeneous culture systems. Optical coherence tomography (OCT) is a noninvasive, label-free, optical imaging technique that can visualize live cells and tissues over time with subcellular resolution and millimeters of optical penetration depth. Here, we show that OCT is capable of carrying out high-content, longitudinal assays of 3D culture treatment response. We demonstrate the usage and capability of OCT for the dynamic monitoring of individual and combination therapeutic regimens in vitro, including both chemotherapy drugs and photodynamic therapy (PDT) for ovarian cancer. OCT was validated against the standard LIVE/DEAD Viability/Cytotoxicity Assay in small tumor spheroid cultures, showing excellent correlation with existing standards. Importantly, OCT was shown to be capable of evaluating 3D spheroid treatment response even when traditional viability assays failed. OCT 3D viability imaging revealed synergy between PDT and the standard-of-care chemotherapeutic carboplatin that evolved over time. We believe the efficacy and accuracy of OCT in vitro drug screening will greatly contribute to the field of cancer treatment and therapy evaluation.
Highlights
Three-dimensional in vitro tumor cultures restore many of these important variables, and have been shown to replicate many features of ovarian tumors found in vivo7. 3D in vitro culture models are of particular significance in studies of therapeutic response in ovarian cancer as their size and complexity are similar to that of ovarian metastatic lesions
Cytotoxic agents typically have limited uptake and penetration, and are usually not uniformly distributed within model tumor nodules. To avoid these potential limitations in this validation step, we employed a photodynamic therapy regimen using 5-aminoleuvaneutic acid (5-ALA), which has been shown in past studies to be uniformly distributed throughout the cells within 3D cancer model16. 5-ALA is a naturally occurring small molecule used in the heme biosynthesis pathway
When delivered exogenously at micromolar concentrations, the heme biosynthesis pathway can be overwhelmed, leading to the generation of excess protoporphyin IX (PpIX), a naturally occurring photosensitizer, within mitochondria20. 5-ALA was useful for this study as its uptake and conversion to PpIX occurs throughout spheroids at micromolar and higher incubation concentrations[21]
Summary
For example, 85% of patients will initially experience a complete therapeutic response, a significant majority of patients eventually succumb to recurrent, treatment-resistant metastatic ovarian cancer[2,3] This high rate of recurrence leads to poor quality of life and an overall low 5-year survival rate of 30%. A subpopulation of ovarian cancer cells are thought to have tumor-initiating or stem-like properties that allow even a small set of surviving cells to repopulate a patient with tumors[5,6] Much of this cellular heterogeneity is lost when tumor cells are plated on standard plastic culture dishes, which have stiff surfaces and lack biologically-relevant cell-cell and cell-matrix interactions. Important as they model the target tumor dimensions of interest for the majority of therapeutics currently under development
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