The present study provides a first direct quantitative comparison of volume-based evaluation of in vivo imaging data and in vitro data on the uptake of SPECT tracers in murine brain and heart using [123I]IBZM and [99mTc]sestamibi, respectively. Thresholds of detection are calculated for tracers with different distribution patterns in murine brain and in the heart, respectively, for the conditions of the multi-pinhole SPECT system employed. Moreover it is shown that the multi-pinhole SPECT method is able to identify differences in the range of 20% of the maximal uptake in the dose range between 950 kBq/g and 440 kBq/g employed in this study. The in vivo uptake of [123I]IPPA in mice is visualised for the first time.
Brain investigations
Dopamine D
2 receptor ligands such as [
123I]IBZM or [
11C]raclopride belong to the preferred tools used to investigate the performance of small animal SPECT and PET imaging devices with respect to in vivo visualisation of brain structures and signal transduction [
26‐
28,
30]. That is due to the fact that dopamine D
2 receptors are found with a high density in the striatum as a compactly shaped telencephalic part of the brain, providing a well-defined volume of interest ranging from 30 to 60 mg (both striata in summary) in adult mice surrounded by a large zone of lower dopamine D
2 receptor density and relatively low non-specific binding.
Evaluations of PET and SPECT in vivo imaging data in small animal models with the aim of developing algorithms for tracer quantification or coregistration with other imaging data have been described previously [
24,
36‐
40]. The literature includes comparisons with autoradiographic measurements or other semi-quantitative reference tissue methods for brain, heart and some tumour models of different localisations [
39,
41]. In our MPS [
123I]IBZM measurements, the activity bound non-specifically in the whole brain decreases at the end of the observation period of 3 hs to 20% of the initial values, while the activity bound specifically reached an equilibrium during the fifth acquisition period. This is in agreement with the time course reported in a previous pinhole SPECT study of [
123I]IBZM in mouse models [
28] as well as with an ex vivo study [
40]. The imaging data evaluated with the InVivoScope and Vinci 2.3.1. data processing tools allowed us to distinguish differences upwards from 20% of maximal tracer amount accumulated in the brains in the dose range between 440 kBq/g and 950 kBq/g. The least-squares linear fit for the comparison of in vitro and in vivo uptake of [
123I]IBZM allowed us to reveal that this percentage corresponds to a total cerebral activity of 116 kBq/g. The estimate involves the assumption that approximately 50% of the tracer detected in the brain binds to receptors in the striata. A lower ratio of striatal to total cerebral activity would result in higher activity values.
There are only a few reports on measurements of [
123I]IBZM in the murine brain using SPECT or multi-pinhole SPECT, respectively. More attention has been paid to ligands of dopamine transporters using [
123I]FP-CIT or
99mTc-labelled ligands such as [
99mTc]TRODAT [
25,
30,
39,
42,
46]. This may be due to the fact that the preparation of [
123I]FP-CIT, which is commercially available, contains less ethanol than preparations of [
123I]IBZM on the one hand and shows higher specific activity on the other hand. This allows easier i.v. application and promises a favourable proportion of occupancy of the target structures at appropriate visualisation quality. A direct comparison of in vitro and in vivo measurements in murine models (or other rodents) neither exists for [
123I]IBZM nor for dopamine D
2 receptors labelled with positron-emitting isotopes. The fraction of the total cerebral [
123I]IBZM found in the striata during our experiments was calculated for the different observation times to be in the range between 25 and 50%. Studies analysing ligand binding following chemical impairment of the target region or with displacement by highly specific antagonists of receptors or transporters describe alterations in the receptor occupancy of 50–90% [
27,
43]. Chalon et al. reported in 1990 [
43] for in vivo/ex vivo studies using [
125I]IBZM a 45% increase of the binding of the tracer after pre-treatment with haloperidol. Singhaniyom et al. [
44] observed a displacement of [
125I]IBZM by about 72% following in vivo pre-treatment with antagonists of the dopamine D
2 receptor in mice. Under such conditions multi-pinhole SPECT already provides a suitable tool for in vivo characterisation of signal transduction units.
Using the InVivoScope data processing tool, we found 4.2 ± 0.27 of the injected dose of [
123II]BZM per g brain tissue. For an activity of 109 kBq/brain as measured in one of our experiments at a specific activity of 74 TBq/mmol and 50% of total cerebral activity in the striata (30 mg wet weight), 24.5 nmol/kg [
123I]IBZM are bound. That would be close to half of the B
max value in vitro but below the ED
50 described for IBZM previously with 37 nM [
47,
48]. The receptor occupancy calculated according to the equation given by Hume et al. [
23] with
\( occupancy = activity \div weight * ED50 * specific\;activity + activity \) was 18.5 ± 2.5% for all [
123I]IBZM experiments (range: 25–13%). This confirms our previous estimation using preliminary SPECT data of [
123I]IBZM uptake and the assumption that at activities of ≤ 20 MBq/animal (in mice weighing 50–25 g) the concentration of IBZM stays below ED
50, but for displacement studies with a low mass effect it is necessary to work with higher specific activities than the reference 74 TBq/mmol used in our calculations [
30]. This reference is the lower boundary of the specific activity guaranteed by the manufacturer. The concentration in non-striatal cerebral tissue in the sample described above was 1.62 nmol/kg at the same time resulting in a ratio of specific to non-specific binding of 15. Meyer et al. [
40] described a ratio of 17 in autoradiographic studies in mice if working with a ketamine/xylazine (without isofluorane) anaesthesia comparable to that which we applied. The ratio was markedly lower if isoflurane was included in the anaesthesia. An occupancy below 5% can be achieved with higher specific activities as described recently [
42].
The dose-dependent increase in the uptake of [123I]IBZM during the eight observation periods suggests that the activity doses applied are in the linear range of the dose-response curve. This allowed identification of the border of detection for the difference in the uptake in regard to different injection doses without impairment by saturation effects.
The activity threshold for the visualisation of in vivo uptake in the brain for a tracer like [
123I]IBZM with a small target region and a relatively high specific activity was 45 kBq (and 6 kBq ex vivo; data not shown; using 100 s acquisition time and 10 step rotation mode in our camera system) as revealed by the offset of the regression line for the comparison between in vivo and in vitro data at y = 0. The comparison of the doses recommended to be applied in different species for several visualisation levels is verified for ligands of dopamine transporters and receptors in mice and rats by our own data. The results agree also with the calculations by Kung et al. [
47,
48]. If measurements of species with larger geometric dimensions require a larger radius of rotation further correction factors are necessary. Weber et al. [
49] described for a triple-headed pinhole SPECT system how spatial resolution and sensitivity change with the increase in the distance between detectors and target region. Taking these alterations into account for a special multi-pinhole system, also the doses necessary for equipotent visualisation would need an additional correction factor. An increased role of attenuation in larger species would also require further modification of the data [
41]. For a tracer such as [
99mTc]HMPAO with homogeneous distribution through the target organ as well as a specific activity < 5% [
50] of that of the receptor ligand investigated in our study, the threshold of detection is 2–10-fold higher.
[
99mTc]HMPAO ([
99mTc]exametazime [(RR, SS])-4,8-diaza-3,6,6,9-tetramethylundecane-2,10-dione bisoxime) is an uncharged, low molecular weight, lipophilic complex that easily crosses the blood-brain barrier [
51]. It is known that in man 3–5 to 7% of HMPAO injected pass into the brain by passive diffusion during the first minutes after injection. In humans it remains for 24 h in the brain, probably trapped inside cells via a reaction with glutathione [
52], and is not preferentially bound to a specific region of the brain.
Cardiac investigations
Altered activity requirements were also revealed during the imaging of the perfusion tracer [
99mTc]sestamibi (4–10-fold of that necessary for the [
123I]IBZM experiments). The lipophilic compound [
99mTc]sestamibi is a cationic complex which is accumulated in tissues rich in mitochondria like liver and heart and trapped in the mitochondrial matrix [
53]. Accumulations of 1.2% of the dose were reported for silent myocardium and 1.5% under stress [
51]. The compound enters the heart by passive diffusion and correlates well with regional perfusion under conditions of normal load. It shows almost no redistribution [
54] under these conditions, while its extraction at high coronary flow or under hypoxic conditions is not proportional to coronary perfusion [
51].
Our model of cardiac overexpression of the EP
3 receptor, a subtype of the prostaglandin receptor, showed a decreased cardiac accumulation of [
99mTc]sestamibi in the transgenic animals compared to their wild-type littermates. This suggests a contribution of disturbances in perfusion or mitochondrial dysfunction [
55] to potential changes in the uptake of metabolic tracers—in our case the long chain fatty acid [
123I]IPPA [
14,
56].
Quantitative comparison of in vitro and in vivo uptake of [
99mTc]sestamibi in murine hearts observed with SPECT cameras is not available in the literature to our knowledge. However, semi-quantitative approaches have been reported by Zhou et al. [
57] for investigations of [
99mTc]sestamibi-labelled regionally ischaemic rat hearts included in dual isotope studies (140 and 247 keV) on the tracking of [
111In]-labelled stem cells [
41,
57]. Zhou et al. employed a triple-headed pinhole camera for the detection of [
99mTc]sestamibi (74 MBq/animal) and could show the grafting of the stem cells during a 96-h observation period. A bull’s eye plot was used for the evaluation of regional cardiac distribution of
99mTc and
111In. Left ventricular visualisation of the tracers was obtained in a similar quality as in our experiments. Beekman et al. [
58] were able to visualise murine heart perfusion with a submillimetre resolution after injection of 222 MBq [
99mTc]tetrofosmin. However, quantitative approaches to the correlation of in vivo SPECT data with in vitro measurements of the tracer uptake are rather available for sestamibi and other perfusion tracers in porcine and canine models [
59,
60]. Da Silva et al. [
60] drew an offset of 7–11 kBq/g heart weight from the correlation of in vivo SPECT data of [
99mTc]sestamibi uptake in porcine hearts with ex vivo data. That would give a detection threshold of 1 MBq total uptake for a 150-g heart. For our murine hearts in the multi-pinhole system we found a threshold of 200 kBq/heart. That corresponds for hearts with a size between 100 and 200 mg to 1–2 MBq/g. Compared to the offset found in the porcine model the threshold for the uptake per murine heart would be lower, but the offset per g heart weight would be 150–350-fold higher. However, the ratio of the body weights of mouse and pig would be 1:1,000.
Estimates of the whole-body dose in SPECT [
61] are given with a range from less than 2 cGy (160 MBq of
99mTc; biological half-life 1 h) in rats to 90 cGy (740 MBq in 30 g-mice assuming a residence time of 3.2 h) [
41]. The activity of [
99mTc]sestamibi applied in our experiment comprised maximally 30% of this amount and on average 15%. Funk et al. [
61] reported theoretical values of the whole-body doses from phantom measurements corresponding to the conditions of biological experiments in the range between 6 and 90 cGy for mice and 1–27 cGy for rats and a LD
50 of 7 Gy/30 d for mice.
Transgenic B6C3F1(EP3) mice show an increase of left ventricular mass, end-diastolic and end-systolic volumes as well as a significant decrease in left ventricular ejection fraction and dP/dt, as found recently [
29]. These alterations were already obvious in 5- to 7-week-old animals [
29]. Our data support the maintenance of the cardiomyopathic alterations in the 11- to 17-week-old mice with 8.6 ± 0.36 mg heart weight/g body weight in comparison to 4.6 ± 0.14 mg/g in wild-type littermates.
The analysis of the hearts excised in transgenic and wild-type littermates following the SPECT measurements revealed that the uptake of [99mTc]sestamibi was 30% lower in cardiomyopathic animals than in wild-type animals. This difference is confirmed if the SUV is calculated for in vivo conditions. The segmental analysis of ROIs reveals heterogeneity of [99mTc]sestamibi accumulation in both cardiomyopathic and wild-type hearts with lowest uptake in the apical regions, but reduced accumulation of [99mTc]sestamibi in the septal and lateral part of the left ventricle of transgenic mice compared to the wild-type animals.
[
123I]IPPA, in contrast to [
99mTc]sestamibi or [
99mTc]HMPAO, is accumulated in the target tissue achieving 39% of the injected dose per g heart weight [
56] by an active process via a CD36 protein acting as a carrier of long chain fatty acids [
14,
62]. It is not clear whether the paradoxical hypertrophic response of the heart to the overexpression of EP
3 receptors, which are known to mediate cardiac protection, is related to these processes. Although an increased phosphorylation of the S6 ribosomal protein has been observed, and it is known to correlate with an enhanced protein synthesis, the kind of relation between EP
3 overexpression and morphological and metabolic alterations associated with the cardiomyopathic properties of these hearts has not been characterised. In contrast, alterations in the parameters of contractility and cardiac function can be explained by mediation by the known signalling pathways via G
i-coupled receptors. The imaging of the uptake of long chain fatty acids by MPS methods provides the possibility to investigate such regulatory processes and their metabolic consequences in vivo.
The present investigations support the theory that the multi-pinhole SPECT device in combination with suitable data processing tools is capable of visualising such alterations and quantifying metabolic processes and ligand binding in different target organs of mice in a feasible dose range. Recently, the quantitative approach which can be used in experimental models has received further support by a plenitude of reports concerning attenuation [
63], algorithms for improving reconstruction image quality [
64,
65], further improvement of the pinhole collimator design [
66,
67] and multimodal approaches used for PET and SPECT investigations [
36,
68]. The small scale of murine models is not only a disadvantage for visualisation, but also offers advantages for example in relation to the attenuation [
41]. The multi-pinhole approach in contrast to small animal PET allows multi-isotope investigations and provides advantages by a more favourable whole-body dose during the use of SPECT isotopes in comparison to positron emitters, shown by favourable MIRD S values [
61], availability of SPECT isotopes and high specific activity for receptor and transporter ligands exceeding frequently those of PET radiopharmaceuticals. The rising availability of CT/SPECT as well as the use of a gated mode of measurements during cardiac investigations extend the access to in vivo information on biochemistry, molecular and cell biology as well as physiology.