Investigation of Microdialysis Sampling to Monitor Pharmacodynamics

Abstract

Microdialysis sampling is a technique commonly used to monitor a variety of processes in a tissue-targeted fashion. This sampling technique has several advantages over other methods since it allows continuous sampling throughout the entire experiment. Because microdialysis can be implanted in virtually any tissue, the application of the microdialysis sampling technique can be applied in many areas, including monitoring enzyme activity mechanisms or monitoring the effects of chemotherapy agents on brain activity. To expand the application of microdialysis sampling, two different enzymes were monitored and tested with this tissue-specific sampling technique. The use of microdialysis sampling to monitor the activity of 11beta-HSD1 in vitro and in vivo would increase the throughput for screening novel inhibitors. Microdialysis sampling was successful in monitoring the conversion of cortisone to cortisol in vitro human microsome experiments, but not so in rat microsome in vitro or in vivo experiments. This was because rat enzymes generate a different metabolite profile than humans. In a separate study, microdialysis was used to monitor the production of cGMP by the enzyme activation of GC-C in the colon submucosa. The location of the microdialysis probe was verified by comparing the concentration of cGMP in a probe implanted in the submucosa and lumen microdialysis probe. These results indicated a clear correlation with probe placement. Upon the activation of a GC-C agonist, a consistent increase in cGMP levels was seen in the submucosa probe. This allows further determination of the mechanisms responsible for pain reduction upon the administration of GC-C agonists in the submucosa, which has not been performed before due to the inability of obtaining this tissue using conventional methods. The second studies utilized microdialysis sampling to continuously monitor neurotransmitters and an oxidative stress biomarker (MDA) before, during and after the administration of anticancer compounds. Both short- and long-term studies were performed. Initial studies monitored the direct effects of these chemotherapy compounds in short-term experiments using direct perfusion. An increase in glutamate and GABA was seen upon the administration of doxorubicin and cyclophosphamide through the microdialysis probe. This demonstrated the toxicity of these compounds on brain activity. These anticancer compounds were then administered in long-term experiments to obtain a complete time-profile of the changes on several neurotransmitters and MDA upon chemotherapy administration. A detectable change was not seen in the amino acid neurotransmitters or MDA in the samples analyzed. An increase was seen in glutamate with IP administration; however this was also seen in the control rat. To alleviate an increase in glutamate from handling, several intravenous cannulations were attempted and no difference was seen in any of these experiments. During these experiments, the longest microdialysis studies were performed, to the best of our knowledge, and the technique was optimized to allow further investigation of these compounds or other disease states that occur over months instead of days.