The Flourous Labelling Strategy: A New Paradigm in Preparing Radiolabelled Compounds in High Effective Specific Activity

Abstract

<p>A new radiolabelling strategy suitable for preparing radioiodinated compounds in high effective specific activity was developed. The core method employs fluorous chemistry in such a manner that unlabelled precursors can be separated from labelled compounds without the use of HPLC purification. The fluorous labelling strategy involved tin-aryl groups in which the substituents off the metal were fluorine rich. Upon reaction with radioiodide and an oxidant, the tin-aryl bond is cleaved. The non-fluorous product could then be separated from the fluorous starting material by fluorous solid-phase extraction (F-SPE).</p> <p>The initial goal of this thesis was to first establish the viability of the fluorous labelling strategy (FLS). To this end, fluorous benzoic acids, 3- and 4- (tris[2-perfluorohexylethyl]stannyl) benzoic acid (m- and p-2.1) were prepared via organozinc derivatives and subsequent hydrolysis. Following direct iodinolysis of m-2.1 using a sub-stoichiometric amount of iodine, the fluorous benzoic acids were treated with Na[1251]1. After purification via fluorous solid-phase extraction, the resulting iodinated benzoic acids (m- and p-2.5) were prepared in 92 and 93 %yield and excellent purity (>98% by HPLC).</p> <p>Next, to demonstrate that the FLS could be used to create libraries of both precursors and labelled compounds, a small collection of benzamides were prepared from fluorous tetrafluorophenyl active esters (m- and p-2.9). An excess of a series of simple amines were added to reaction wells containing one of the two fluorous active esters and the products purified by taking advantage of the non-fluorous nature of the reagents and reaction byproducts. All products, which were fully characterized and their purities verified by HPLC, were subsequently labeled with 1251 in high radiochemical yield (> 85 %), and the products were obtained in greater than 98% radiochemical purity.</p> <p>To demonstrate the applicability of this method clinically relevant radiopharmaceuticals, a fluorous precursor of the radiopharmaceutical metaiodobenzylguanidine (MIBG) was prepared via the fluorous benzylamine, 3-tris[2- perfluorohexylethyl]stannyl) benzylamine (3.3) upon reacting with 1H-pyrazole carboxamidine. Following radioiodination, high purity C251]- and [1231]MIBG were produced in 81±3% and 80% radiochemical yield respectively in less than 20 min without high-performance liquid chromatography (HPLC) purification. The purified product contained less than 1 ppm tin as determined by inductively coupled plasma-mass spectrometry (ICP-MS).</p> <p>Next, the FLS was expanded to include fluorous precursors suitable for reacting with groups common in peptides and proteins for use in bioconjugation reactions. Among these were a fluorous isocyanate (4.1) which was shown to be stable in fluorous solvents for up to 3 weeks, fluorous benzaldehydes (m- and p- 5.1) and a fluorous iodoacetamide (5.3). Following their purification and characterization, these compounds were each reacted with model systems (e.g. amines, amino acids) and were subsequently radioiodinated using Na[1251]1 in good yields ( > 80 %) and in very high purity(> 98 %) as determined by HPLC.</p>