Summary                         

Longitudinal studies on mouse models related to Alzheimer disease (AD) pathology play an important role in the investigation of therapeutic targets to help pharmaceutical research in the development of new drugs and in the attempt of an early diagnosis that can contribute to improving people’s quality of life. There are several advantages to enriching longitudinal studies in AD models with Positron Emission Tomography (PET); among these advantages, the possibility of following the principle of the 3Rs of animal welfare is fundamental. The goal of the 3Rs Principle is to avoid animal experiments (Replacement), to limit the number of animals (Reduction), and to limit their suffering in tests to an absolute minimum (Refinement). Improving the quality of life for experimental animals and reducing their stress and pain are among the goals that must be considered when it is not possible to replace animals, according to the principle of the 3Rs. In this manuscript, good daily experimental practice focusing on animal welfare is described and commented upon, based on the experience attained from studies conducted in the Nuclear Medicine department.

Results from nanoScan® PET/CT

Longitudinal studies were performed in different AD mouse models, in which each animal acted as its own control. The models consisted of PET/CT scans from 3 to 5 time points, with 2 to 4 different radiotracers characterizing changes in biological targets up to the age of 12 to 18 months.

Approximately 20 MBq of radioactive tracer was injected to the tail vein. PET/CT measurement was carried out up to 60 min post injection: for ¹⁸F-Ge-180 and ¹⁸F-Florbetaben, the measurement was carried out for 30 min, 60 and 30 min. after radiotracer uptake, respectively (static scan); those for ¹⁸F-D2-Deprenyl and ¹⁸F-UCB-H were carried out immediately post injection, for 60 min (dynamic scan).

Using a four-mouse bed it was possible to scan multiple mice (up to four) simultaneously. The use of this chamber is advantageous in many aspects:

1. The chance to perform more scans in the same day using radioactive isotopes with a short half-life. This is especially important in longitudinal studies with a large number of animals because all of the scans have to be performed respective of the age-related time point. 

2. There is a lower injected volume. This is important, as the ¹⁸F-based tracers decay quickly. Since the activity has to be the same for each animal, the volume of the injection needs to be increased according to the radioactive decay in sequential scans. Thus, the volume to be injected would be above the maximum limit allowed for these animal experiments. Using the 4-mouse bed, this problem is circumvented, since four mice are injected and scanned at the same time (instead of four individual scans one after the other). 

3. This particular bed for four animals is equipped with a heating system that keeps the temperature of the bed constant at about 37°C, which is very important during anesthesia, as hypothermia can arise, and also because the temperature can have an effect on the biodistribution of the injected radiotracer. In addition, there is a monitoring system for cardiac and respiratory function that allows continuous monitoring in each animal.

• PET/CT image of four mice scanned simultaneously using the four-mouse bed:

Figure 1. The four APPSL70 mice were scanned for 30 min after 30 min uptake with 20 MBq ¹⁸F-Florbetaben tracer for beta amyloid accumulation. Axial, sagittal, and coronal slices.

• Beta amyloid labeling in an APPSL70 mouse brain over time with ¹⁸F-Florbetaben:

Figure 2. Upper panel: example of beta amyloid in an APPSL70 mouse brain over time with ¹⁸F-Florbetaben: the same mouse was scanned at 6, 9, and 12 months of age. Lower panel: axial slices of PET scans of a transgenic AD-related mouse brain at 12 months of age after injection of four different radioligands

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