[11C]metaraminol, a false neurotransmitter: preparation, metabolite studies and positron emission tomography examination in monkey

Pharmacokinetic study of three cardiovascular drugs by high-performance liquid chromatography using pre-column derivatization with 9,10-anthraquinone-2-sulfonyl chloride

A new method was developed to analyze three cardiovascular drugs in rat plasma, Mexiletine hydrochloride (MXL), Methoxamine hydrochloride (MTX), and Metaraminol bitartrate (MTR), by high-performance liquid chromatography (HPLC) using 9,10-anthraquinone-2-sulfonyl chloride (ASC) as the derivatization reagent. The derivatization modes and conditions for this method were optimized. The quantitative analysis was achieved using a C18 column at room temperature (25 degrees C), with various volume ratios of methanol-water as the mobile phase and a detection wavelength at 256 nm. Analytical linearity was obtained for the method over the concentration range of 0.04-8.0 microg mL(-1) for all the three drugs. The lower limit of quantification (LLOQ) was 0.04 microg mL(-1). This method was successfully applied to the analysis of the three drugs in rat plasma and their pharmacokinetic studies. The t1/2 values of the three drugs in rats were found to be 5.38+/-0.61, 4.49+/-0.53, and 3.70+/-0.19 h for MXL, MTX, and MTR, respectively.

A novel electrophilic synthesis and evaluation of medium specific radioactivity (1R,2S)-4-[18F]fluorometaraminol, a tracer for the assessment of cardiac sympathetic nerve integrity with PET

(1R,2S)-4-[18F]fluorometaraminol (4-[18F]FMR), a tracer for cardiac sympathetic innervation, was synthesized by electrophilic aromatic substitution. A trimethylstannyl precursor, protected with tert-butoxycarbonyl protecting groups, was radiofluorinated with high specific radioactivity [18F]F2. Specific radioactivity of 4-[18F]FMR, in average 11.8 +/-3.3 GBq/micromol, was improved 40-800-fold in comparison to the previous electrophilic fluorinations. The biodistribution of 4-[18F]FMR in rat was in accordance with the known distribution of sympathetic innervation. 4-[18F]FMR showed no metabolic degradation in left ventricle of rat heart, where the uptake was high, rapid and specific.

Assessment of cardiac sympathetic nerve integrity with positron emission tomography

The autonomic nervous system plays a critical role in the regulation of cardiac function. Abnormalities of cardiac innervation have been implicated in the pathophysiology of many heart diseases, including sudden cardiac death and congestive heart failure. In an effort to provide clinicians with the ability to regionally map cardiac innervation, several radiotracers for imaging cardiac sympathetic neurons have been developed. This paper reviews the development of neuronal imaging agents and discusses their emerging role in the noninvasive assessment of cardiac sympathetic innervation.

Synthesis of high-specific-radioactivity 4- and 6-[18F]fluorometaraminol-PET tracers for the adrenergic nervous system of the heart

Fluorine-18- (t(1/2) 109.8 min) and carbon-11 (t(1/2) 20.4 min)-labeled norepinephrine analogues have been found previously to be useful positron-emission-tomography (PET) radioligands to map adrenergic nerve terminals of the heart. Metaraminol ((1R,2S)-2-amino-1-(3-hydroxyphenyl)-1-propanol) is a metabolically stable structural analogue of norepinephrine and possesses high affinity towards the norepinephrine transporter and the vesicular monoamine transporter. This paper presents the radiosynthesis of new positron-emission-tomography halogeno analogues of metaraminol labeled with high specific radioactivity. Firstly, fluorine-18-labeled 4-fluorometaraminol (4-[18F]FMR or (1R,2S)-2-amino-1-(4-[18F]fluoro-3-hydroxyphenyl)-1-propanol) and its three other stereoisomers were prepared based on the following key steps: (a) condensation of the corresponding no-carrier-added labeled fluorobenzaldehyde with nitroethane, and (b) HPLC (C18 and chiral) resolution of the diastereomeric product mixture into the four individual enantiomers. Secondly, the corresponding 6-fluoro analogues, fluorine-18-labeled 6-fluorometaraminol (6-[18F]FMR or (1R,2S)-2-amino-1-(2-[18F]fluoro-5-hydroxyphenyl)-1-propanol) and its three other enantiomers, were prepared in an analogous way. Typically, 0.48-0.55 GBq of 4-[18F]FMR and 0.14-0.15 GBq of 6-[18F]FMR could be obtained after 120-160 min total synthesis time, with a specific radioactivity of 56-106 GBq/micromol. Furthermore, the synthesis of racemic 4-fluorometaraminol and 6-fluorometaraminol as reference compounds was performed. as well as independent chiral syntheses of the optically active (1R,2S) enantiomers. For the chiral syntheses, the key step was an electrophilic fluorination with acetyl hypofluorite of (1R,2S)-configurated organometallic derivatives of metaraminol. Tissue distribution studies in rats suggested that both 4-[18F]FMR and 6-[18F]FMR display similar affinity towards the presynaptic adrenergic nerve terminal in the heart. From a practical point of view, 4-[18F]FMR appeared to be the more attractive candidate for future PET investigations, due to higher radiochemical yields.

High specific radioactivity (1R,2S)-4-[(18)F]fluorometaraminol: a PET radiotracer for mapping sympathetic nerves of the heart

The radiolabeled catecholamine analogue (1R, 2S)-6-[(18)F]fluorometaraminol (6-[(18)F]FMR) is a substrate for the neuronal norepinephrine transporter. It has been used as a positron emission tomography (PET) ligand to map sympathetic nerves in dog heart. 6-[(18)F]FMR could be only synthesized with low specific radioactivity, which precluded its use in human subjects. We have recently prepared (1R,2S)-4-[(18)F]fluorometaraminol (4-[(18)F]FMR), a new fluoro-analogue of metaraminol, with high specific radioactivity (56-106 GBq/micromol). In the present study, we demonstrate in rats that 4-[(18)F]FMR possesses similar affinity toward myocardial norepinephrine transport mechanisms as 6-[(18)F]FMR. When compared with control animals, an 80% and 76% reduction in myocardial uptake was observed in animals pretreated with desipramine (an inhibitor of the neuronal norepinephrine transporter) and with reserpine (a blocker of the vesicular storage of monoamines), respectively. The entire radioactivity in rat myocardium represented unmetabolized parent tracer as determined by high performance liquid chromatography analysis of tissue extracts. In dogs, myocardial kinetics of 4-[(18)F]FMR were assessed using PET. A rapid and high uptake was observed, followed by prolonged cardiac retention. A heart-to-lung ratio of 15 was reached 10 min after injection of the radiotracer. Pretreatment with desipramine reduced the heart half-life of 4-[(18)F]FMR by 90% compared with control. Moreover, an infusion of tyramine caused a rapid decline of radioactivity in the heart. This demonstrates that 4-[(18)F]FMR specifically visualizes sympathetic neurons in dog heart. High specific radioactivity 4-[(18)F]FMR is a promising alternative to 6-[(18)F]FMR for myocardial neuronal mapping with PET in humans.

Direct chromatographic resolution and isolation of the four stereoisomers of meta-hydroxyphenylpropanolamine

Methods for the direct chiral chromatographic separation of the four stereoisomers of meta-hydroxyphenylpropanolamine (MHPA) on an analytical and preparative scale are described. Separations were carried out on a Crownpak CR (+) chiral column with 113 mM aqueous perchloric acid as the mobile phase. Baseline resolution of the more retained (+)-stereoisomers (1S configuration) and partial resolution of the less retained (-)-stereoisomers (1R configuration) were obtained under these chromatographic conditions. Removal of the bulk of the (1R,2S)-stereoisomer (metaraminol) from the initial crude mixture by fractional crystallization as the (+)-bitartarate salt substantially improved the peak resolution factors (Rs) of the remaining three stereoisomers. Semipreparative chromatographic resolution of the latter isomeric mixture provided milligram quantities of each stereoisomer in >97% enantiomeric excess. Subsequent recrystallization of their bitartarate or fumarate salts gave enantiomeric purities >99%.