<p> Nuclear medicine requires the use of radio labelled pharmaceuticals in order to carry out imaging and therapeutic protocols.1 Unfortunately, traditional radiolabelling approaches used in radiopharmaceutical synthesis often generate multiple products, which require exhaustive HPLC purification prior to use.2 Chromatographic purification reduces radiochemical yields, increases exposure, and can, in certain cases, preclude the use of shorter-lived isotopes.</p> <p> In light of the limitations of current radio labelling methods, we endeavored to develop a versatile and efficient radiolabelling strategy that would avoid the need for HPLC purification. To this end, the compounds to be labelled were first bound to a highly fluorinated stannylated precursor, which, when reacted with a radiohalogen, generated the corresponding radiolabelled compound. Unlike other halodestannylation reactions, however, the radiolabelled compound could be isolated from the stannylated precursor by elution through a fluorous Sep-Pak.</p> <p> As a model system, tris(perfluorohexylethyl)tin-3-benzoic acid (1), was synthesised and labelled. Compound 1 was prepared through a novel reaction which involved treatment of bromotris[(2-perfluorohexyl)ethyl]tin with the organozinc reagent 3-(ethoxycarbonyl)phenylzinc. Reaction of compound 1 with [18F]F2, followed by fluorous Sep-Pak purification, generated the corresponding labelled 3-[18F]fluorobenzoic acid in 27 min, in 30% radiochemical yield, and having a specific activity of 1966 mCi/mol. Alternatively, reaction of compound 1 with Na125I provided the corresponding product, 3-[125I[iodobenzoic acid, in <1 hr, in 75% radiochemical yield, and greater than 99% radiochemical purity. Prior tests clearly showed that any excess or unreacted substrate was fully removed from the product using a fluorous Sep-Pak.</p> <p> In addition to the initial validation studies, new synthetic methods were developed as a means of preparing more complex "fluorous" substrates. A coupling methodology was developed which permitted synthesis of a fluorous "tagged" benzamide, through reaction of 1 in the presence of HBTU with N,N-dimethylethylenendiamine. Subsequent labelling using F2 and I2 has been shown to generate the corresponding labelled benzamides, which are important agents for imaging melanoma and dopamine receptors.3 Another relevant radiopharmaceutical precursor, tris[(2-perfluorohexyl)ethyl]tin-3-benzylamine (2), was synthesized though reaction of bromotris[(2-perfluorohexyl)ethyl]tin with 1-(3-bromobenzyl)-2,2,5,5-tetramethyl-1,2,5-azadisilolidine. Compound 2 was successfully coupled to the chemotactic peptide, GFLM(f), and the product subsequently labelled with iodine. Compound 2 was also used to prepare the corresponding benzylguanidine (3), an important precursor to m-iodobenzylguanidine, which is used for imaging and therapy of neural crest tumors.4 Initial labelling results show that reaction of 3 with NaI and an oxidant generates the corresponding labelled m-iodobenzylguanidine.</p><p> Results suggest that the fluorous synthesis method will offer several advantages over traditional radiolabelling strategies. The radiolabelled products are generated in high yield, through rapid and facile reactions that avoid the need for HPLC purification.</p> / Thesis / Master of Science (MSc)

Thesis (MScMedSc)--Stellenbosch University, 2010. / ENGLISH ABSTRACT: Radiolabelled compounds have been widely used as investigative tools for psychiatric disorders using positron emission tomography (PET) or single photon emission tomography (SPECT) of the brain. In particular 123I-5-IR91150, a serotonin (5-HT) 2a antagonist, has been used for imaging the serotonergic system. The current study developed optimal radiolabelling and purification methods in our laboratory with the objective that it can provide 123I- 5-I-R91150 in sufficient quantity and of acceptable pharmaceutical quality for human use. Unlabelled R91150 was obtained from Janssen Pharmaceutica (Beerse, Belgium). Carrier free [123I]Iodine was produced by iThemba LABS, South Africa, via the 127I(p,5n)123Xe-123I reaction, providing Na[123I] in 0.05 N sodium hydroxide with a specific activity of 4000-6000 MBq/ml. A direct electrophilic radioiodination method of labelling was used in this study for labelling 123-I-5-IR91150 in glacial acetic acid. After radiolabelling, the product was purified using two different methods, namely a high performance liquid chromatography (HPLC) purification method and a solid phase extraction (SPE) method. The analyses of the purified product for both methods were done using HPLC. Methods were tested to reduce the volume of the purified product using C8 or C18 solid phase extraction cartridges. The average labelling efficiencies for SPE and HPLC purification methods were 76% ± 13.6% and 52% ± 11.2% respectively. The yields of 123I-5-I-R91150 were about 80%. Sep-Pak C8 and C18 were both unable to concentrate the HPLC purified product. Products from both purification methods were sterile and pyrogen free. Both SPE and HPLC purification methods have been shown to provide products meeting most criteria set for this study. However, both methods have advantages and disadvantages. The SPE purification method provided higher labelling efficiency and a much lower product volume. The stability of this product is however of concern as some free iodide was detected. If this purification method is used, the product should therefore be administered as soon as possible after completion of analysis. After HPLC purification, the undiluted product remained stable up to 4.15 hours after production but the product volume was relatively high, and purification time-consuming. In order to obtain a useful patient dose, labelling would have to start with at least 740 MBq 123I and the labelled product should be collected in fractions of 5 ml or less in order to obtain a fraction of sufficiently high specific activity. It was concluded that radiolabeling R91150 is possible at our institution, but that an improved HPLC system would be of value for routine production of a pure and safe product. / AFRIKAANSE OPSOMMING: Radioaktief gemerkte verbindings word baie gebruik as ondersoekmiddel vir psigiatriese afwykings met behulp van positron emissive tomografie (PET) of enkelfoton emissie tomografie (SPECT) van die brein. Die verbinding 123I-5-IR91150, ‘n serotonien (5-HT) 2a antagonis, is beskryf vir beelding van die serotonerge sisteem. Die huidige studie het ondersoek ingestel na optimale metodes vir radioaktiewe merking en suiwering vir ons laboratorium met die doel om 123I-5-I-R91150 in genoegsame hoeveelhed en van aanvaarbare farmaseutiese gehalte geskik vir menslike gebruik te verskaf. R91150 is van Janssen Pharmaceutica (Beerse, België) verkry. Draervry [123I]jodium is deur iThemba LABS, Suid-Afrika, via die 127I(p,5n)123Xe-123I reaksie geproduseer, om Na[123I] in 0.05 N natriumhidroksied met spesifieke aktiwiteit van 4000-6000 MBq/ml te lewer. ‘n Direkte elektrofiliese radioiodineringsmetode is in hierdie studie gebruik om 123-I-5-I-R91150 in ysasynsuur te merk. Na radioaktiewe merking is die radioaktiewe produk deur twee verskillende metodes gesuiwer, naamlik ‘n HPLC metode en ‘n soliede fase ekstraksie (SPE) metode. Vir beide metodes is die produk deur middel van HPLC analiseer. Metodes is getoets om die volume van die gemerkte produk met C8 of C18 SPE kolommetjies te verminder. Die gemiddelde bindingsdoeltreffendheid vir die SPE en HPLC suiweringsmetodes was 76% ± 13.6% en 52% ± 11.2% onderskeidelik. Die opbrengs van 123I-5-I-R91150 was ongeveer 80%. Sep-Pak C8 en C18 kon beide nie gebruik word om die HPLC gesuiwerde produk te konsentreer nie. Produkte van beide suiweringsmetodes was steriel en pirogeenvry. Daar is getoon dat beide suiweringsmetodes produkte lewer wat aan die meeste kriteria wat in hierdie studie gestel is, voldoen. Beide metodes het egter voor- en nadele. Die SPE suiweringsmetdode het tot hoër bindingsdoeltreffendheid gelei, asook ‘n baie laer produkvolume. Daar is egter ‘n mate van kommer oor die stabiliteit van die produk aangesien vry radiojodied waargeneem is. Indien hierdie suiweringsmetode gebruik word, moet die produk dus so gou as moontlik na voltooiing van analise toegedien word. Na HPLC suiwering was die onverdunde produk tot 4.15 uur na produksie stabiel maar die produkvolume was relatief hoog en suiwering tydrowend. Om ‘n bruikbare pasiëntdosis te verkry moet merking met ten minste 740 MBq 123I begin en die gemerkte produk moet na suiwering in fraksies van 5 ml of minder versamel word om ‘n fraksie met geskikte spesifieke aktiwiteit te verkry. Die gevolgtrekking is gemaak dat radioaktiewe merking van R91150 by ons instelling moontlik is, maar dat ‘n verbeterde HPLC sisteem vir roetineproduksie van ‘n suiwer en veilige produk van waarde sou wees.