its limited spatial resolution and the complexity of its image reconstruction and quantitation. These aspects make it difficult to draw ROIs precisely BCR-ABL Signaling Pathway around the tumor border. Chances are that different observers would draw ROIs differently. To evaluate whether this would influence the results of our study, 2 different sizes of ROIs were also drawn besides the medium ROI we used for the primary results : a very small ROI in the center of the tumor and a very large ROI around the entire tumor including some surrounding normal tissue. Average counts of all of these ROIs were calculated at all imaging time points. We found comparable differences in optical imaging signal post and pretreatment for all ROI sizes . This implies that the interpretation of the signal changes was not importantly influenced by the manner in which the ROI was drawn.
Another limitation of the relatively low spatial resolution is that partial volume effects can lead to inaccurate optical imaging signals in very small lesions Piroxicam compared with the system’s spatial resolution. Quantification of optical imaging signal is more complicated as compared with PET imaging in which percentage injected dose per gram of tissue can be calculated. Due to the fairly large background signal in vivo, the correlation between in vivo and in vitro results is relatively limited. However, our results support that relative signal quantification with the right optical imaging set up is achievable and in the range of Oude Munnink and colleagues and Kamer Marek and colleagues , and that it is thus feasible to semiquantitatively measure molecular changes over time using optical imaging.
In ongoing studies, we are evaluating other molecular imaging agents, such as engineered antibodies and peptides, in the same xenograft model to make cell nucleus better comparisons between the different imaging agents. We aim to translate one or more of these molecular imaging agents to clinical studies. For clinical applications, deeper light penetration is required and therefore it will be advantageous to conjugate the targeted agents to a nearinfrared dye with excitation wavelengths above 700 to 750 nm, for example, IRdye 800CW , which has already been registered with the European Regulatory Authorities and the U.S. Food and Drug Administration in anticipation of clinical trials.
Future preclinical studies will also include administering 17DMAG more than once to repeatedly monitor the transient effect on Her2 expression over time and investigating whether repeated probe injection within hours yields reproducible imaging results after preinjection background subtraction to adjust for residual probe levels. If possible, we will be able to show the reproducibility of the entire optical imaging procedure and not only from probe injection onward which we showed to be highly reproducible . This will give a better understanding of the magnitude of effects that can be measured with this optical imaging assay. In conclusion, optical imaging with an affibody can be used for noninvasive in vivo imaging of Her2 expression and for monitoring the changes in Her2 expression as a response to treatment. This makes optical imaging a promising molecular imaging tool for treatment monitoring in preclinical models and potentially.