## 3 Results and Discussions
### Biodistribution in Rats  
The radioactivity distribution in various organs after the injection of [\({}^{11}\)C]**2a** and [\({}^{18}\)F]**2b** in rats is summarized in Table 1. Both radiotracers displayed homogeneous distribution in the brain regions as shown in Figure 2A,B. For [\({}^{11}\)C]**2a**, the total brain uptake (%ID/gram) of radioactivity at five, 30 and 60 min post injection were 0.953 \(\pm\) 0.115, 0.287 \(\pm\) 0.046 and 0.158 \(\pm\) 0.014 respectively; in the peripheral tissues, liver had the highest uptake among the tissues analyzed; the uptake (%ID/gram) in liver reached 2.198 \(\pm\) 0.111 at 5 min and 1.116 \(\pm\) 0.024 at 60 min. For [\({}^{18}\)F]**2b**, the total brain uptake (%ID/gram) at five, 30, 60 and 120 min was 0.758 \(\pm\) 0.013, 0.465 \(\pm\) 0.018, 0.410 \(\pm\) 0.030 and 0.359 \(\pm\) 0.016, respectively; At 5 min post-injection, this compound also has a high liver uptake: (%ID/gram) of 1.626 \(\pm\) 0.221. However, after 30 min, the kidney has retained the highest radioactivity of all tissues that were analyzed. The bone uptake (%ID/gram) was very stable and no defluorination was observed for [\({}^{18}\)F]**2b**. More importantly, the _ex vivo_ rat biodistribution data revealed that both compounds readily crossed the BBB and entered the brain. Both tracers exhibit high initial brain  
\begin{table}
\begin{tabular}{c c c c c} \hline
**Radioligand** & **Organ** & **5 min** & **30 min** & **60 min** & **120 min** \\ \hline \multirow{8}{*}{[\({}^{11}\)C]**2a**} & blood & \(0.506\pm 0.040\) & \(0.369\pm 0.031\) & \(0.300\pm 0.015\) \\  & heart & \(0.758\pm 0.052\) & \(0.377\pm 0.046\) & \(0.245\pm 0.010\) \\  & lung & \(1.149\pm 0.058\) & \(0.740\pm 0.038\) & \(0.485\pm 0.036\) \\  & muscle & \(0.271\pm 0.005\) & \(0.325\pm 0.030\) & \(0.199\pm 0.016\) \\  & fat & \(0.155\pm 0.023\) & \(0.241\pm 0.042\) & \(0.293\pm 0.076\) \\  & pancreas & \(1.007\pm 0.262\) & \(0.506\pm 0.066\) & \(0.522\pm 0.036\) \\  & spleen & \(0.659\pm 0.049\) & \(0.400\pm 0.052\) & \(0.386\pm 0.027\) \\  & kidney & \(1.362\pm 0.054\) & \(0.807\pm 0.086\) & \(0.559\pm 0.053\) \\  & liver & \(2.198\pm 0.111\) & \(1.349\pm 0.116\) & \(1.116\pm 0.024\) \\  & brain & \(0.953\pm 0.115\) & \(0.287\pm 0.046\) & \(0.158\pm 0.013\) \\ \hline \multirow{8}{*}{[\({}^{18}\)F]**2b**} & blood & \(0.553\pm 0.047\) & \(0.589\pm 0.016\) & \(0.606\pm 0.035\) & \(0.585\pm 0.046\) \\  & heart & \(0.757\pm 0.033\) & \(0.505\pm 0.015\) & \(0.466\pm 0.040\) & \(0.410\pm 0.030\) \\  & lung & \(0.833\pm 0.053\) & \(0.561\pm 0.021\) & \(0.491\pm 0.030\) & \(0.436\pm 0.018\) \\  & muscle & \(0.430\pm 0.031\) & \(0.451\pm 0.018\) & \(0.376\pm 0.019\) & \(0.315\pm 0.015\) \\  & fat & \(0.255\pm 0.037\) & \(0.425\pm 0.023\) & \(0.371\pm 0.067\) & \(0.293\pm 0.048\) \\  & pancreas & \(1.004\pm 0.147\) & \(0.546\pm 0.068\) & \(0.409\pm 0.029\) & \(0.330\pm 0.021\) \\  & spleen & \(0.672\pm 0.064\) & \(0.509\pm 0.022\) & \(0.446\pm 0.038\) & \(0.398\pm 0.019\) \\  & kidney & \(1.070\pm 0.058\) & \(0.988\pm 0.090\) & \(0.678\pm 0.032\) & \(0.659\pm 0.027\) \\  & liver & \(1.626\pm 0.221\) & \(0.847\pm 0.027\) & \(0.561\pm 0.028\) & \(0.467\pm 0.023\) \\  & bone & \(0.340\pm 0.027\) & \(0.309\pm 0.020\) & \(0.407\pm 0.043\) & \(0.644\pm 0.071\) \\  & brain & \(0.758\pm 0.013\) & \(0.465\pm 0.018\) & \(0.410\pm 0.030\) & \(0.359\pm 0.016\) \\ \hline \end{tabular}
\end{table}
Table 1: Biodistribution of [\({}^{11}\)C]**2a** and [\({}^{18}\)F]**2b** in male Sprague-Dawley rats (%ID/gram).  
uptake and appropriate washout kinetics in the brain of normal rats. Rapid clearance of the radioactivity for both [11C]**2a** and [18F]**2b** was observed from brain, as well as other organs, such as lung, pancreas, spleen, kidney and liver. However, [11C]**2a** showed faster wash-out kinetics than [18F]**2b**, as shown in Figure 2. [11C]**2a** was chosen for subsequent microPET evaluation in an NHP.