[18F]Fallypride is an established radioligand for human and small animal imaging of striatal as well as extrastriatal D2-dopamine receptor binding in brain. In the present study we developed [11C]fallypride which can be radiolabelled to a higher molar activity than [18F]fallypride. The unique combination of [11C]fallypride and [18F]fallypride allowed for examination of binding over a wide range of molar activity and mass injected. It is important to point out that between the body of work done in these experiments and its publication, an advancement has been made in the field of [18F]Fallypride production by Huhtala et al. to achieve much higher MA levels than before. This would have negated the need to include the C11 labelled version in this study [23], however we feel that this does not take away from the conclusions of this paper.

In our experiments in mice, we found that the striatal BPND decreased markedly when going from the high MA condition with [11C]fallypride to lower MA condition with [18F]fallypride. The most likely explanation is that the mass of injected [18F]fallypride is sufficient to approach saturation of the D2 dopamine receptor. The BPND calculated with Logan reference method dropped from 13.5 in the [11C]fallypride experiments to 5.3 BPND with [18F]fallypride at the lower MA condition. This reduction corresponds to at least 50% radioligand occupancy at the receptor.

The concept “tracer dose” has been suggested when the occupancy of the injected ligand is below 5%. At such occupancy, variations in the radiochemistry production quality have limited impact on BPND [1, 2, 24, 25]. However, at the demonstrated high radioligand occupancy of [18F]fallypride the BPND will be sensitive to the production quality which can be viewed as a confounder in applied studies.

A critical condition in our study is if the injected radioactivity and molar activity is representative of applied studies in literature. While the range of injected radioactivity in our study is in line with what has been applied by other centers [15, 16, 23, 26, 27], the actual injected mass or molar activity at injection is rarely reported in literature.

Worth noting is that the hereby discussed issue of injected mass effect is not a problem when using PET radiopharmaceuticals that explore non-saturable systems. like [18F]Fluoride for bone surface uptake imaging. Another example for which injected mass is a limited problem is the study of intermediate saturable systems such as with [18F]Fluoro-2-deoxyglucose (FDG) that measure changes in glucose consumption [1, 2].

A slight difference in radioligand washout from the cerebellum might still be observable at the end of data collection, when C11 and F18 curves are compared. This is most likely due to spillover from bone uptake and may have had a negligible effect on BPND estimation.

An interesting observation was that binding in extrastriatal regions was conspicuous after injection of [11C]fallypride having the highest molar activity concentration, and a significantly lower amount of injected mass. BPND measurements in low receptor density areas should thus be possible as well. This potential is less evident for [18F]fallypride.

An attempt was made to use Scatchard analysis to quantify fallypride binding to the D2-dopamine receptor in the striatum. Bmax was estimated to 62 pmol/g which is somewhat higher than the Bmax estimates of about 30 pmol/g seen in humans [28, 29] and 34 pmol/g in mice using [3H]spiperone in vitro [30]. It has been suggested that for fallypride binding in brain there may be a D3 component of around 20%, that could be subject to species differences [27].

Using data obtained in the Scatchard plot analysis (Fig. 4b) we estimate that the mass related occupancy in our experiments with [18F]fallypride was quite high: 41% and 64% at average, [18F]fallypride vs [18F]fallypride lower MA respectively. Importantly even with [11C]fallypride the average occupancy was 19% despite a molar activity of 153.3 GBq/µmol and an injected mass of 0.03 µg (Table 1).

There is a possibility to decrease injected mass by lowering the radioactivity injected. However, such approach might reduce image quality. Ideally, MA must be further improved to assure the tracer conditions required for small animal imaging.