Dual-radiotracer translational SPECT neuroimaging. Comparison of three methods for the simultaneous brain imaging of D2/3 and 5-HT2A receptors

Simultaneous SPECT imaging with 123I and 125I - a practical approach to assessing a drug and its carrier at the same time with dual imaging

Radiolabeling of a drug with radioactive iodine is a good method to determine its pharmacokinetics and biodistribution in vivo that only minimally alters its physicochemical properties. With dual labeling, using the two radioactive iodine isotopes ¹²³I and ¹²⁵I, two different drugs can be evaluated at the same time, or one can follow both a drug and its drug delivery system using a single photon emission computed tomography (SPECT) imager. One difficulty is that the two radioisotopes have overlapping gamma spectra. Our aim was therefore to develop a technique that overcomes this problem and allows for quantitative analysis of the two radioisotopes present at varied isotope ratios. For this purpose, we developed a simple method that included scatter and attenuation corrections and fully compensated for ¹²³I/¹²⁵I crosstalk, and then tested it in phantom measurements. The method was applied to the study of an orally administered lipid formulation for the delivery of fenofibrate in rats. To directly compare a traditional study, where fenofibrate was determined in plasma samples to SPECT imaging with ¹²³I-labeled fenofibrate and ¹²⁵I-labeled triolein over 24 h, the drug concentrations were converted to standardized uptake values (SUVs), an unusual unit for pharmaceutical scientists, but the standard unit for radiologists. A generally good agreement between the traditional and the radioactive imaging method was found in the pharmacokinetics and biodistribution results. Small differences are discussed in detail. Overall, SPECT imaging is an excellent method to pilot a new formulation with just a few animals, replaces blood sampling, and can very quickly highlight potential administration problems, the excretion pathways and the kinetics. Furthermore, dual labeling with the two radioisotopes ¹²³I and ¹²⁵I clearly shows if a drug and its drug delivery system stay together when traveling through the body, if slow drug release takes place, and where degradation/excretion of the components occurs.

Effect of 5-HT2A receptor antagonism on levels of D2/3 receptor occupancy and adverse behavioral side-effects induced by haloperidol: a SPECT imaging study in the rat

Several studies suggested that 5-HT_2A receptor (5-HT_2AR) blockade may provide a more favorable efficacy and side-effect profile to antipsychotic treatment. We hypothesized that a combined haloperidol (a D_2/3 receptor (D_2/3R) antagonist) and MDL-100,907 (a 5-HT_2AR antagonist) treatment would reverse the side effects and the neurochemical alterations induced by haloperidol alone and would potentialize its efficacy. We thus chronically treated male Mdr1a knock-out rats with several doses of haloperidol alone or in combination with a saturating dose of a MDL-100,907. Receptor occupancy at clinically relevant levels was validated with a dual-radiotracer in-vivo SPECT imaging of D_2/3R and 5-HT_2AR occupancy. Experimental tests of efficacy (dizocilpine-disrupted prepulse inhibition (PPI) of the startle reflex) and side effects (catalepsy, vacuous chewing movements) were performed. Finally, a second dual-radiotracer in-vivo SPECT scan assessed the neurochemical changes induced by the chronic treatments. Chronic haloperidol failed to reverse PPI disruption induced by dizocilpine, whilst administration of MDL-100,907 along with haloperidol was associated with a reversal of the effect of dizocilpine. Haloperidol at 0.5 mg/kg/day and at 1 mg/kg/day induced catalepsy that was significantly alleviated (by ~50%) by co-treatment with MDL-100,907 but only at 0.5 mg/kg/day dose of haloperidol. Chronic haloperidol treatment, event at doses as low as 0.1 mg/kg/day induced a significant upregulation of the D_2/3R in the striatum (by over 40% in the nucleus accumbens and over 20% in the caudate-putamen nuclei), that was not reversed by MDL-100,907. Finally, an upregulation of 5-HT_2AR after chronic haloperidol treatment at a moderate dose only (0.25 mg/kg/day) was demonstrated in frontal cortical regions and the ventral tegmental area. Overall, a partial contribution of a 5-HT_2AR antagonism to the efficacy and side-effect profile of antipsychotic agents is suggested.

Dopamine D2/3 Receptor Availabilities and Evoked Dopamine Release in Striatum Differentially Predict Impulsivity and Novelty Preference in Roman High- and Low-Avoidance Rats

BACKGROUND

Impulsivity and novelty preference are both associated with an increased propensity to develop addiction-like behaviors, but their relationship and respective underlying dopamine (DA) underpinnings are not fully elucidated.

METHODS

We evaluated a large cohort (n = 49) of Roman high- and low-avoidance rats using single photon emission computed tomography to concurrently measure in vivo striatal D2/3 receptor (D2/3R) availability and amphetamine (AMPH)-induced DA release in relation to impulsivity and novelty preference using a within-subject design. To further examine the DA-dependent processes related to these traits, midbrain D2/3-autoreceptor levels were measured using ex vivo autoradiography in the same animals.

RESULTS

We replicated a robust inverse relationship between impulsivity, as measured with the 5-choice serial reaction time task, and D2/3R availability in ventral striatum and extended this relationship to D2/3R levels measured in dorsal striatum. Novelty preference was positively related to impulsivity and showed inverse associations with D2/3R availability in dorsal striatum and ventral striatum. A high magnitude of AMPH-induced DA release in striatum predicted both impulsivity and novelty preference, perhaps owing to the diminished midbrain D2/3-autoreceptor availability measured in high-impulsive/novelty-preferring Roman high-avoidance animals that may amplify AMPH effect on DA transmission. Mediation analyses revealed that while D2/3R availability and AMPH-induced DA release in striatum are both significant predictors of impulsivity, the effect of striatal D2/3R availability on novelty preference is fully mediated by evoked striatal DA release.

CONCLUSIONS

Impulsivity and novelty preference are related but mediated by overlapping, yet dissociable, DA-dependent mechanisms in striatum that may interact to promote the emergence of an addiction-prone phenotype.

Crosstalk Reduction Using a Dual Energy Window Scatter Correction in Compton Imaging

Compton cameras can simultaneously detect multi-isotopes; however, when simultaneous imaging is performed, crosstalk artifacts appear on the images obtained using a low-energy window. In conventional single-photon emission computed tomography, a dual energy window (DEW) subtraction method is used to reduce crosstalk. This study aimed to evaluate the effectiveness of employing the DEW technique to reduce crosstalk artifacts in Compton images obtained using low-energy windows. To this end, in this study, we compared reconstructed images obtained using either a photo-peak window or a scatter window by performing image subtraction based on the differences between the two images. Simulation calculations were performed to obtain the list data for the Compton camera using a 171 and a 511 keV point source. In the images reconstructed using these data, crosstalk artifacts were clearly observed in the images obtained using a 171 keV photo-peak energy window. In the images obtained using a scatter window (176-186 keV), only crosstalk artifacts were visible. The DEW method could eliminate the influence of high-energy sources on the images obtained with a photo-peak window, thereby improving quantitative capability. This was also observed when the DEW method was used on experimentally obtained images.