Positron emission tomography imaging probes targeting chemokine receptors

Alzahrani, Seraj Omar

May 2016

Thesis or dissertation

© 2016 Seraj Omar Alzahrani. All rights reserved. No part of this publication may be reproduced without the written permission of the copyright holder.

Positron emission tomography (PET) is a highly sensitive nuclear medicine imaging technique. PET is used to accurately diagnose cancer and can detect early stage tumours. Molecular probes containing a positron emitting metal radioisotope (such as ⁶⁴Cu or ⁶⁸Ga) need to give a stable complex in vivo as well as targeting biomarkers or metabolic processes within the tumour. The roles of chemokine receptors in multiple disease stages have been demonstrated. The CXCR4 and CCR5 chemokine receptors have been implicated in cancer, as well as other disease states including HIV infection and chronic inflammatory diseases, including asthma and rheumatoid arthritis. Incorporation of a positron emitting radioisotope into a CXCR4 or CCR5 specific antagonist compound could allow visualisation of physiological locations with high expression levels of these receptors to characterise the disease.

The small molecule CXCR4 antagonist AMD3100 (Plerixafor) has been approved for clinical use as a haematopoietic stem cell mobilising agent and also exhibits anti-HIV, anti-inflammatory and anti-tumour activity. Configurationally restricted analogues of AMD3100 complexed to metal ions have improved binding characteristics compared to AMD3100 and its metal complexes. A synthetic pathway to obtain series of configurationally restricted macrocyclic compounds (analogues of AMD3100) fixed in the trans IV configuration, has been developed and the copper(II), zinc(II) and nickel(II) complexes characterised. Their biological properties (anti-HIV, cytotoxicity and Ca²⁺ signalling inhibition) were evaluated to allow selection of compounds to be radiolabelled with ⁶⁴Cu²⁺ for evaluation as a PET imaging agent targeting CXCR4. The most active trans IV complexes, bis(zinc(II))- 1,4-xylyl bis(methyl side-bridged cyclam and bis(zinc(II))-1,4-xylyl bis(benzyl sidebridged cyclam) have EC₅₀ values of 516 and 247 nM respectively in the anti-HIV assay with cyotoxicity (CC₅₀) values of 42800 and 39600 nM respectively. However, the novel mixed metal trans II complex (copper(II)zinc(II))-1,4-xylyl bis(sidebridged cyclam) has a higher binding affinity with an EC₅₀ value of 3 nM (four times more potent than AMD3100) and cytotoxicity CC₅₀ value greater than 10 µM. Bis(zinc(II))-1,4-xylyl bis(side-bridged cyclam) was successfully radiolabelled with ⁶⁴Cu²⁺ via transmetallation to form (⁶⁴Cu (zinc(II))-1,4-xylyl bis(side-bridged cyclam) with a crude radiochemical yield of 92%.

A derivative of known CCR5 antagonist TAK-779 containing a carboxylic acid functional group 2-(p-tolyl)-6,7-dihydro-5H-benzo[7]annulene-8-carboxylic acid 18 was prepared. A DO3A compound with a spacer terminating in a primary amine (tritert-butyl 2,2',2''-(10-(2-((2-aminoethyl)amino)-2-oxoethyl)-1,4,7,10- tetraazacyclododecane-1,4,7-triyl)triacetate) was successfully conjugated to compound 18 forming a potential CCR5 targeting compound that could be radiolabelled with gallium-68 for PET imaging applications. Preliminary in vitro affinity assays indicated that the modification of the structure had disrupted the CCR5 binding and some structural modification redesign may be required.
Radiolabelling of the conjugate compound 2-(p-tolyl)-6,7-dihydro-5Hbenzo[7]annulene-8-amidoethyl-DOTA with gallium-68 was carried out. A crude radiochemical yield of ca. 100% was achieved to give ⁶⁸Ga-(2-(p-tolyl)-6,7-dihydro-5H-benzo[7]annulene-8-amidoethyl-DOTA) which is stable in buffer and against transferring challenge for over four hours.

Department of Chemistry, The University of Hull
Archibald, Stephen J.
Qualification level
Qualification name
3 MB
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