Radiosynthesis of [123I]N-methyl-4-iododexetimide and [123I]N-methyl-4-iodolevetimide: in vitro and in vivo characterisation of binding to muscarinic receptors in the rat heart.

[123I]N-methyl-4-iododexetimide, [123I]MIDEX, and its pharmacologically inactive enantiomer [123I]N-methyl-4-iodolevetimide, [123I]MILEV were prepared via electrophilic iododesilylation using Chloramine-T as oxidising agent followed by N-methylation using methyl iodide. The radiotracers were purified with semi-preparative HPLC with radiochemical yields of 80 +/- 11% (n = 6). The average time of synthesis was 100 min with specific activity > 2000 Ci/mmol. In vitro, the binding of [123I]MIDEX, after addition of carrier, measured on homogenates of rat atrium was Bmax = 4.5 +/- 0.4 pmol/mg protein, Kd = 3.3 +/- 0.2 nM while in the ventricle Bmax = 2.3 +/- 0.2 pmol/mg protein, Kd = 4.0 +/- 1.4 nM. In vitro, the binding of [123I]MILEV was non-specific. The in vivo biodistribution of [123I]MIDEX showed high uptake in the atrium (3.2% ID/g) and left and right ventricles (2.2, 2.5% ID/g respectively) at 5 min followed by clearance. High heart-to-lung and moderate liver-to-lung ratios were obtained during 60 min. Radioactivity in the atrium and ventricles was reduced by pre-administration of the m-AChR antagonist MQNB (1 mg/kg). Pretreatment of rats with other m-AChR ligands, pirenzapine (M1), methoctramine (M2) and 4-DAMP (M3) also resulted in reduction of [123I]MIDEX uptake with methoctramine being the most potent. [123I]MIDEX distribution in the rat heart was not significantly inhibited by pre-administration of selective adrenergic drugs. The uptake was highly stereoselective since the inactive enantiomer, [123I]MILEV, demonstrated very low myocardial retention. The stability of [123I]MIDEX was evaluated by performing a metabolite study on atrium samples which revealed unchanged radiotracer 60 min postinjection. These results suggest that [123I]MIDEX may be a useful single photon agent for in vivo imaging of myocardial m-AChR in humans with [123I]MILEV offering the potential of assessing non-specific binding of the active tracer.