Synthesis and characterization of a 11C-labelled derivative of S12968: an attempt to image in vivo brain calcium channels

Preparation and preclinical evaluation of 68Ga-DOTA-amlodipine for L-type calcium channel imaging

Aim:

In order to develop a possible tracer for L-type calcium channel imaging, we here report the development of a Ga-68 amlodipine derivative for possible PET imaging.

Materials and Methods:

Amlodipine DOTA conjugate was synthesized, characterized and went through calcium channel blockade, toxicity, apoptosis/necrosis tests. [⁶⁸Ga] DOTA AMLO was prepared at optimized conditions followed by stability tests, partition coefficient determination and biodistribution studies using tissue counting and co incidence imaging up to 2 h.

Results:

[⁶⁸Ga] DOTA AMLO was prepared at pH 4–5 in 7–10 min at 95°C in high radiochemical purity (>99%, radio thin layer chromatography; specific activity: 1.9–2.1 GBq/mmol) and was stable up to 4 h with a log P of −0.94. Calcium channel rich tissues including myocardium, and tissues with smooth muscle cells such as colon, intestine, and lungs demonstrated significant uptake. Co incidence images supported the biodistribution data up to 2 h.

Conclusions:

The complex can be a candidate for further positron emission tomography imaging for L type calcium channels.

Practical Radiosynthesis and Preclinical Neuroimaging of [11C]isradipine, a Calcium Channel Antagonist

In the interest of developing in vivo positron emission tomography (PET) probes for neuroimaging of calcium channels, we have prepared a carbon-11 isotopologue of a dihydropyridine Ca²⁺-channel antagonist, isradipine. Desmethyl isradipine (4-(benzo[c][1,2,5]oxadiazol-4-yl)-5-(isopropoxycarbonyl)-2,6-dimethyl-1,4-dihydropyridine-3-carboxylic acid) was reacted with [¹¹C]CH₃I in the presence of tetrabutylammonium hydroxide in DMF in an HPLC injector loop to produce the radiotracer in a good yield (6 ± 3% uncorrected radiochemical yield) and high specific activity (143 ± 90 GBq·µmol⁻¹ at end-of-synthesis). PET imaging of normal rats revealed rapid brain uptake at baseline (0.37 ± 0.08% ID/cc (percent of injected dose per cubic centimeter) at peak, 15–60 s), which was followed by fast washout. After pretreatment with isradipine (2 mg·kg⁻¹, i.p.), whole brain radioactivity uptake was diminished by 25%–40%. This preliminary study confirms that [¹¹C]isradipine can be synthesized routinely for research studies and is brain penetrating. Further work on Ca²⁺-channel radiotracer development is planned.

Radiosynthesis of dimethyl-2-[18F]-(fluoromethyl)-6-methyl-4-(2-nitrophenyl)-1,4-dihydropyridine-3,5-dicarboxylate for L-type calcium channel imaging

Dimethyl 2-(fluoromethyl)-6-methyl-4-(2-nitrophenyl)-1,4-dihydropyridine-3,5-dicarboxylate 4a, a fluorinated nifedipine analog, has been shown to elicit significant calcium channel blocker activity using a guinea pig ileal longitudinal smooth muscle model. In order to perform biological studies for detection of L-type calcium channel distribution, we decided to prepare the [ 18F]-labeled compound. The latter compound was prepared in no-carrier-added (n.c.a.) form from dimethyl 2-(bromomethyl)-6-methyl-4-(2-nitrophenyl)-1,4-dihydropyridine-3,5-dicarboxylate 2 in one step at 80 °C in Kryptofix[222]/K[ 18F]F and acetonitrile as a solvent in 15 min. Column chromatography afforded the radiochemically pure compound in 20 min. Radiochemical purity of the 18F-nifedipine was determined by RTLC and HPLC (>98%) and specific activity of 21–48 GBq/μmol (EOB).