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Title: Fluorinated 2-nitroimidazoles : non-invasive probes for detecting therapeutically relevant tumour hypoxia by magnetic resonance spectroscopy
Author: Aboagye, Eric Ofori
Awarding Body: University of Glasgow
Current Institution: University of Glasgow
Date of Award: 1995
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Tumour hypoxia is a well described phenomenon which is primarily created by an inadequate and non-uniform vascular network. Importantly, hypoxia within certain solid tumours has been linked to the failure of conventional therapy, including radiotherapy and chemotherapy. For this reason, many attempts have been made to identify, characterise, and quantify hypoxic cells within these tumours as a prelude to the design of rational therapies aimed to kill all the cancer cells present within tumours. The use of 2-nitroimidazoles containing appropriate labels as markers for the detection of hypoxic cells, is based on the ability of these compounds to undergo selective nitroreduction and adduct formation within hypoxic cells. The fluorinated 2-nitroimidazole, SR-4554, was rationally designed as a non-invasive hypoxia probe with low toxicity and high sensitivity for detection by magnetic resonance spectroscopy (MRS) and imaging (MRI). SR-4554 was demonstrated to be enzymatically reduced by mouse liver microsomes, SCCVII tumour homogenates and purified rat and human NADPH; cytochrome P450 reductase under hypoxia. In this regard, NADPH: cytochrome P450 reductase was found to be the major enzyme involved in this bioreduction of SR-4554. In a panel of murine and human tumour xenografts, only a 3 fold variation in NADPH: cytochrome P450 reductase activities was observed. These differences in NADPH: cytochrome P450 activities are unlikely to significantly influence SR-4554 adduct formation in vivo. Importantly, the reduction of SR-4554 by mouse liver microsomes showed a characteristic oxygen dependence with a half- maximal inhibition of 0.48%. Using a high resolution nuclear magnetic resonance (NMR) technique, soluble hypoxia-dependent metabolites of various fluorinated 2-nitroimidazoles, including SR-4554, were identified. The chemical shifts of these metabolites were found to be less than those required to give resolved peaks in in vivo MRS studies. Determination of the possible regions of SR-4554 adduct formation within A2780 human ovarian multicellular spheroids was achieved by electron energy loss spectroscopic imaging. Using this technique, an 8 fold differential between the fluorinated probe levels retained in the inner hypoxic regions of the spheroid vs the outer more aerobic regions was observed. SR-4554 was localised mainly to the nuclear periphery, nucleus and cytoplasm within cells from the hypoxic region of the spheroids. The toxicity and pharmacokinetics of SR-4554 was studied in mice as a prelude to non-invasive MRS/MRI studies. SR-4554 was found to be non-toxic in mice up to a dose of 1300 mg/kg. Pharmacokinetics of SR-4554 fitted either a one compartment or two compartment open model depending on the route of administration. The probe was characterised by a relatively short half-life, linear kinetics, high urinary excretion, high oral bioavailability and low plasma protein binding. Importantly, the brain to plasma ratio of SR-4554 was lower than previously reported trends in the literature, thus confirming its potentially low toxicity. As a consequence of the low toxicity and favourable pharmacokinetic properties of SR-4554, a non-invasive multi-tuned 19F MRS technique was implemented to detect this compound in tissues. Using this MRS technique, SR-4554 was found to be selectively retained in both murine and human tumour xenografts. Importantly the 19F retention index was found to correlate with the reported radiobiological hypoxic fraction of the tumours used. This 19F retention index was also found to be higher in larger tumours but, interestingly, did not correlate with the bioenergetic status of the tumours, as determined by 31P MRS. Manipulation of tumour microenvironment to affect the degree of hypoxia within these tumours was studied in vivo. In mouse tumours for instance, the 19F retention index increased 2 fold compared to control values, following hydralazine pre-treatment, and decreased by 6 fold compared to control values, following carbogen breathing. Important to the clinical development of SR-4554, 19F MRI techniques showed localisation of SR-4554 in the bladder, tumour and liver of mice. This technique was, however, limited by low sensitivity due to the extremely short transverse relaxation, time of the probe in tissues. Finally this thesis has demonstrated that SR-4554 has suitable metabolic, pharmacological and MRS properties for its use in the quantification of critical levels of hypoxia within human tumours. As a consequence, this compound has been approved by CRC phase I/II committee for clinical development.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available