There are many types of microdialysis probes available – some labs even make their own microdialysis probes. This guide can help you choose the right microdialysis probe for your next project. To learn about how microdialysis works and how it can advance your research goals, please start with our overview: What Is Microdialysis?

If you will be doing chronic microdialysis to measure dopamine, serotonin, acetylcholine, amino acids or other small molecules in mouse and rat brains, then, our FZ and CX probes are the best choice. These offer industry-leading recovery rates (the ratio of the concentration of an analyte in a recovered sample to the concentration of the analyte in the area surrounding the microdialysis membrane) for small molecules.

All of the probes discussed above were designed for use in the brain, but we also have probes for peripheral tissues. The OP linear probe is designed for dermal use and has a needle at one end to make it easy to implant, and the TP flexible concentric probe can be used in blood vessels and the GI tract.

Many of our probes can be made with different membrane materials. If you would prefer a polyethersulfone (PES) or polyacrylonitrile (PAN) membrane instead of cuprophane (artificial cellulose), please ask us. All of our probes, stereotaxic adapters, and accessories are made to your custom size requirements in just two weeks – even for the smallest orders. We also maintain a large inventory of FZ, CX-i, and PEP probes ready to ship the day you place your order.

If you are planning a new project or having trouble with your current one, please give us a call and we can help you troubleshoot. Eicom has been manufacturing microdialysis probes and equipment for 30 years and we have an integrated support/sales team so you can rely on our expertise.

There are many ways to speed up your microdialysis experiments and ensure their success.

During surgery:

• Stereotaxic frames with digital readouts are much quicker to use than ones still equipped with vernier scales: the instant readout and ability to zero each axis speed up each movement of the manipulator. Digital readouts are available as a retrofit for many older stereotaxic frames. Additionally, dual manipulator stereotaxic frames can speed up each surgery by keeping multiple tools accessible at all times.
• After the initial incision, scratch the surface of the skull with a scalpel: cement will bond more tightly to the roughened surface.
• Wipe the skull with hydrogen peroxide to help dry it and then wipe with epinephrine to prevent bleeding. For implantation to be successful, the surface of the skull must be dry. If the surface is still damp when cement is applied, the trapped moisture can lead to infection and necrosis of the skull.
• Switching to UV cure cement lets you avoid mixing a new batch (powder plus solvent) of acrylic cement each time your previous batch hardens. A simple UV light hardens the cement when you need it to – not before.

For sample collection:

• Use 96 well plates to collect samples instead of individual sample tubes. This eliminates capping, labeling, and handling of individual tubes as samples are collected and moved to storage. Used in conjunction with well plates, self-closing plate seals such as silicone sealing mats and cap mats will protect samples from evaporation and contamination.
• Use the best fraction collector. The Amuza FC-90 collects from up to four animals/probes simultaneously and never skips a sample. Samples can be collected into 96 well plates or racks of individual tubes, and are kept refrigerated throughout.

And finally – Ask Amuza!
Amuza (formerly Eicom USA) has decades of experience in microdialysis and HPLC-ECD, and we are always ready to help.

The brain is a challenging target for therapeutic monoclonal antibodies (mAbs), nanobodies, antibody drug conjugates (ADCs), and other drugs. The blood brain barrier prevents many drugs with otherwise good absorption profiles from crossing into the brain, and also complicates attempts to model how drugs are distributed within the brain.

Prof. Dhaval Shah and PhD student Hsueh-Yuan Chang of the University at Buffalo study the pharmacokinetics/pharmacodynamics (PK/PD) of therapeutic antibodies and ADCs. Their lab recently used the Eicom AtmosLM (large molecule) microdialysis system as a way to quantitate a mAb in multiple brain regions simultaneously, generating data to underpin pharmacokinetic models for the disposition of mAbs in rats (1,2). They found that tissue homogenate and lumbar cerebrospinal fluid samples do not make good proxies for predicting mAb concentrations at their sites of action within the parenchyma of the brain. They also found that the lateral ventricles and the blood-CSF barrier may be an important route for mAb entry.

How Large Molecule microdialysis works

AtmosLM is a push-pull microdialysis system for measuring the levels of large proteins and peptides, as opposed to the catecholamines and other small molecules typically measured by microdialysis. AtmosLM features unique probes that incorporate vents to equalize the pressure inside the membrane of the probe with the outside atmosphere. This prevents ultrafiltration and yields more consistent analyte recovery rates than other push-pull systems. It has been widely used to study levels of Abeta, Tau, synuclein, lipidated ApoE particles, cytokines, and other molecules. Recently, the Derendorf lab
(U of Florida) used AtmosLM to determine tissue interstitial concentrations of mAbs in liver, skin, kidney, and muscle after IV dosing to aid in their development as anticancer drugs.

Microdialysis based PK modeling

Amuza spoke with Hsueh-Yuan (Luke) Chang about how he used AtmosLM in this project, and he also shared several tips for other users of AtmosLM.

Amuza: Could you explain how your PBPK (physiologically-based pharmacokinetic) model can be used by those studying the use of mAb based drugs in the brain?

Hsueh-Yuan: Our current version of the PBKP model was developed to capture nonspecific mAb distribution in the brain and different regions of the brain. Additionally, it can help to quantify the correlation between mAb concentrations in brain CSF and ISF. It may also help to quantify the amount of mAb entering brain parenchyma versus brain CSF compartments.
While the nonspecific mAb PBPK model has not incorporated target binding or receptor-mediated transcytosis yet, both novel delivery mechanisms and target binding kinetics can be mechanistically added into the current basic version of the PBPK model.
The final version of the PBPK model for mAbs may provide an a priori prediction of mAb distribution in the human brain once kon/koff of values of specific mAbs and receptor/target concentrations in the brain have been included. This prediction could be tested in rodents and primates.

Amuza: Central nervous system (CNS) concentrations of mAbs are often determined by taking whole brain homogenate samples or cerebrospinal fluid (CSF) samples from the lumbar region. What are these methods missing?

Hsueh-Yuan: They do not provide direct information of the mAb concentration at the site-of-action as we mentioned in the introduction of the paper. There are many studies suggesting mAb concentrations are different between CSF and ISF.
More importantly, some studies have reported mAb accumulation within brain capillary cells, which may utilize endogenous receptor binding to enhance brain uptake of mAbs.

Shah lab insights for using AtmosLM

Amuza: I’d also like to ask you a few questions about using our AtmosLM system.

You used siliconized sample tubes to prevent adsorption of antibodies in your samples to the plastic. Would blocking the tubes by rinsing with BSA work as well?

Hsueh-Yuan: Yes. However, the storage of low concentration IgG microdialysates requires 0.1-0.15% BSA. BSA is compatible with ELISA. For LC/MS, the BSA method should be replaced.

Amuza: Do you think endogenous IgG could be used similarly to an internal standard to suggest whether or not a microdialysis experiment is working correctly?

Hsueh-Yuan:  Yes, ELISA methods can quantify rat, mouse, or human IgG specifically. They can serve as an endogenous IgG reference, a proxy for calculating in vivo recovery of the mAbs themselves. Hemoglobin can be used, too.

Amuza: During in vitro tests, you were very careful when balancing the flow between the syringe pump (push) and the peristaltic pump (pull), adjusting the peristaltic pump until the ratio of fluid pumped in/out of the probe stayed in the range of 97 – 103%.

What happens if fluid recovery is outside of this range?

Hsueh-Yuan: Then convection [bulk flow of solutes and solvent across a membrane due to a pressure imbalance] will happen.

Amuza: This is indeed a problem. Your paper (1) found that recovery rates were strongly changed when the flow was not properly balanced. This data is available in the supplementary material.

How did you measure the amount of fluid recovered in each sample?

Hsueh-Yuan: By measuring their net weight.

Amuza: With AtmosLM probes, this can also be accomplished by visually monitoring the flow exiting the probe during in vivo experiments. If the peristaltic pump is pulling fluid out of the probe faster than the syringe pump is pushing fluid in, air will enter the system through the vent in the probe and be visible as bubbles in the tubing. If instead the syringe pump is pushing more fluid than the peristaltic pump is removing, the excess fluid will exit the probe through the vent hole. The vent hole is downstream from the membrane, and does not interfere with microdialysis.

What suggestions do you have for others using AtmosLM to study antibody concentrations?

Hsueh-Yuan: 

• Endogenous IgG or another internal reference should be measured to validate that the BBB is intact.
• Fresh CSF perfusion buffer should be used. BSA may precipitate if the CSF is left sitting at room temperature.
• Due to the instability of low concentrations of antibodies in microdialysate, samples should be analyzed ASAP. Alternatively, standards should be diluted to their final concentrations and stored together with the samples until analysis.
• Always check the inlet and outlet of the probe before connecting the probe to the push-pull system.

Amuza: Do you have future projects in mind for large molecule microdialysis?

Hsueh-Yuan: Yes. We have been working on several projects using AtmosLM microdialysis.

1. Chang, H. Y., Morrow, K., Bonacquisti, E., Zhang, W., & Shah, D. K. (2018, August). Antibody pharmacokinetics in rat brain determined using microdialysis. In MAbs (Vol. 10, No. 6, pp. 843-853). Taylor & Francis.https://doi.org/10.1080/19420862.2018.1473910
2. Chang, H. Y., Wu, S., Meno-Tetang, G., & Shah, D. K. (2019). A translational platform PBPK model for antibody disposition in the brain. Journal of pharmacokinetics and pharmacodynamics, 1-20.https://doi.org/10.1007/s10928-019-09641-8
3. Jadhav, S. B., Khaowroongrueng, V., Fueth, M., Otteneder, M. B., Richter, W., & Derendorf, H. (2017). Tissue distribution of a therapeutic monoclonal antibody determined by large pore microdialysis. Journal of pharmaceutical sciences, 106(9), 2853-2859. https://doi.org/10.1016/j.xphs.2017.03.033