Mariette Heins and Ilona Vuist
Sampling The Tumor Microenvironment
A new tumor-guide cannula allows the use of microdialysis over non-consecutive days.
Microdialysis is a sampling technique used to continuously measure the extracellular fluid in tissue. Microdialysis probes are designed with a semi-permeable membrane, an inlet and an outlet, where the membrane allows the transport of small molecules, such as drugs, metabolites and signaling molecules. Historically, microdialysis was developed for neuroscience research to measure neurotransmitter changes in freely moving animals. However, its use has greatly expanded to other research areas, including the study of tumors and how they react to a drug.
In cancer therapeutics, it is important to understand what the concentration and corresponding efficacy of the drug is in the extracellular space of tumor tissue, the liquid environment present between the tumor cells. The tumor environment is quite different from healthy tissue as it is specifically suited to let the tumor grow and often unfavorable for a healthy immune response and compound penetration. As a result, the concentration of the therapeutic and its specific action within the TME are different from other tissues. Microdialysis helps scientists to probe this region because the tool is specifically designed to evaluate small molecules, signaling molecule and metabolites within tumor extracellular space.
This, in turn, enables us to glean more data about what’s going on in the tumor microenvironment (TME), a place where cancer cells not only interact with each other but with the host cells and immune system. For instance, we know that tumor cells can influence the TME by releasing signals to reduce the influx of immune cells.
Not surprisingly, perhaps, the extracellular space within the TME is a hot area of research. Scientists are studying it extensively to try and elucidate its role in cancer growth and treatment. Hopefully, we will gain a better understanding of the TME so we can design better cancer therapeutics. However, methods used most frequently to study this area, in vitro screening and histological assessment have their limitations.
Fortunately, microdialysis is able to help paint a more complete picture of the TME by providing in vivo measurements over several hours. This can help us to visualize tumor penetration of potential treatments, as well as track metabolic changes within the tumor, which together provide insights into the acute effects of treatment on tumor cells.
To address long term effects of treatment in the TME our lab used to require separate cohorts be
sampled at separate occasions. To reduce data variability that inevitably arises from measuring separate cohorts and reduce the number of animals, a tumor-guide was developed to allow for non-consecutive days of sampling. For experimental procedures using this cannula, tumor-bearing mice undergo surgical implantation of a guide cannula into the tumor. On two separate occasion, a microdialysis probe is placed in this cannula before sampling. When the desired samples have been collected, the probe is removed, and a dummy placed in the cannula until the next sampling day.
We found that this process worked in our studies of several commonly used tumors, up to a week after the cannula-placing surgery. However, this time frame may change depending on specific tumor type and treatment. With the first couple of samples as a baseline, it is possible to assess both the acute relative effects of compound administration as well as the subchronic effects of treatment. In this way, longitudinal sampling from the TME using in vivo microdialysis can provided powerful data about the mechanisms within the TME. It also enables us to meet our 3Rs commitments—in this case reducing animal use by collecting multiple samples at different days from the same animal and refining the use of animals by doing the samples in freely-moving animals.
In summary, the newly developed guide cannula for tumor microdialysis can elucidate mechanisms of tumor growth, as well as long-term drug effects within the same animals. The analysis of biomarkers from microdialysis samples from non-consecutive days shows that this technique is a powerful long-term monitor of the TME.
The analysis of a wide range of relevant biomarkers of TME can give insight into the energy consumption and metabolic activity of the tumor, and how this is altered in time upon treatment. Therefore, questions regarding the acute effects of treatment can be extended. In addition, treatments can be prolonged and followed over days. And this technique can be used in various tumors, including patient-derived-xenografts, to help develop cancer therapeutics for different cancers. This provides better insights into the mechanisms of cancer development and into the long-term effects of new therapeutics in the tumor.
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Mariette Heins, PharmD, is a Director in Discovery and Ilona Vuist is a Trial Manager at Charles River's Groningen site in The Netherlands, which specializes in microdialysis.