## 2 Radiolabeled Tacrine and Its Derivatives Used in Alzheimer's Disease
### Radiotracers Based on Tacrine
The lack of transformation of tacrine into its metabolites in the brain and low ability of metabolites to cross the blood-brain barrier correlate perfectly with (observed in radioautographic studies in rats) a slight accumulation of [9-14C]1-OH-THA in the brain and a high level of [9-14C]1-OH-THA in the blood, which explains the significantly higher brain-to-plasma ratio determined for tacrine than for its 1-OH-THA metabolite. The results of the experiments conducted by the authors also indicate that the transport of THA and its metabolites to the brain tissue takes place through a simple passive process, and the excretion of these compounds occurs through the biliary and urinary tracts. Tritium-labelled tacrine, [7-3H]tacrine ([3H]THA, a very close analogue of tacrine, custom synthesized from 7-bromo-9-amino-1,2,3,4-tetrahydrooracridine, Figure 1C), was used in the study by [PERSON] et al. to locate tacrine binding sites in the rat brain [22]. Using P2  
Figure 1: Radiotracers based on tacrine: (**A**) [9-14C]tacrine [20,21]; (**B**) [9-14C]1-OH-THA [21]; (**C**) [7-3H]tacrine [22,23]; (**D**) [5,7-3H]tacrine [23]. membrane fractions prepared from rat brain, [PERSON] and [PERSON] performed a number of tests: AChE enzymatic test, localization of [3H]THA binding sites, kinetic parameters of [3H]THA binding using various concentrations of inactive THA and the ability of THA to inhibit AChE. In order to be able to compare the results obtained, experimental conditions (buffer, pH, concentration of the tested compound) as similar as possible were used in all studies. Based on the results obtained, the authors concluded that [3H]THA binds to the membrane almost rapidly but in a reversible manner--adding an inactive ligand completely removed [3H]THA from the membrane. They also showed that the binding of [3H]THA in the rat brain is not blocked by a number of other neurotransmitters/neuromodulators, so the binding site of [3H]THA is different from the sites of action (sites of receptors) of these compounds. Moreover, their research (autoradiography) also showed that [3H]THA binding sites are not located together with the activity of acetylcholine (and other acetylcholinesterase inhibitors); therefore, it can be concluded that the clinical effect of THA may also result from an action other than through the cholinergic neuronal system. The procedure for the synthesis of tritium-labeled tacrine was described by [PERSON] et al. [23]. The authors synthesized two radiopreparations, [7-3H]tacrine (Figure 1C) and [5,7-3H]tacrine (Figure 1D), using the catalytic tritium dehalogenation of 7-bromo-tacrine and 5,7-dibromo-tacrine compounds, respectively. The products of the individual stages of synthesis were analyzed using the proton and tritium NMR method and TLC and HPLC methods equipped with UV and/or liquid scintillation detectors. In 2007, [PERSON] et al. presented the results of studies on the intravenous and intranasal administration of tacrine labeled with technetium-99m [24]. The labeling reaction was performed directly using a solution of tacrine in propylene glycol, reducing agent SnCl\({}_{2}\) and pertechnate solution [99 mTc]TcJTcO\({}_{4}\) - (eluate from the \"99 Mo/\"99 mTc generator). The authors demonstrated the stability of the radiopreparation in normal saline solution and in mouse serum as well as in a challenge experiment with DTPA (diethylenetriamine pentaacetic acid). Unfortunately, the obtained radiopreparation [99 mTc]TcJacrine solution ([99 mTc]Tc-TS) was tested only by thin-layer chromatography (TLC) using silica gel-coated fiberglass sheets as a stationary phase and two different mobile phases: acetone and pyridine:acetic acid:water (3:5:1.5, \(v/v\)). The structure of [99 mTc]Tc-TS is also unknown. The paper presents the results of biodistribution studies in mice and \(\gamma\)-scintigraphy imaging studies (performed using single photon emission computerized tomography, SPECT) in rabbits. In both studies, intravenous (IV) and intranasal (IN) administrations of [99 mTc]Tc-TS were used.