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Title: Investigation of pharmacokinetics and thioether metabolites to assess bioactivation and toxicity of drugs
Author: Webb, Hayley Margaret
ISNI:       0000 0004 2734 7899
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2011
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ADRs are a major complication of drug therapy and are one of the main causes of attrition in drug development. Bioactivation of drugs to CRMs is believed to be a crucial step in the development of many direct and immune-mediated ADRs. FS, a furan containing diuretic drug, has been shown to produce massive hepatic necrosis in mice through bioactivation of the furan ring to a reactive epoxide intermediate. A thiophene analogue of FS (TP A) has been synthesized in order to compare the heterocyclic moieties in the same biological environment. NVP, used for the treatment of HIV -1 infection, can cause skin reactions and hepatotoxicity, which are thought to be mediated through CRM formation and subsequent induction of the immune system. FS, TPA and NVP can be used to assess current systems used to investigate the potential hazard of a drug. Substitution of the furan ring in FS to a thiophene ring (TP A) improved the potency towards the pharmacological target; the Na+/2CrlK+ co-transport system in the thick ascending limb of the loop of Henle. It was found that TPA was three times more potent than FS, with ICso values of 17 μM and 50 μM respectively. TPA displayed an improved safety profile in vivo in terms of serum AL T activity levels compared to FS (237.3 and 4441.4 U/L respectively, 1.21 mmol/kg i.p. for 24 hours). Evidence for the bioactivation of TP A, was provided through studies in rat and mouse liver microsomes. Neither FS nor TPA elicited hepatotoxicity or depleted hepatic GSH levels in the rat in vivo. The disposition of FS and TPA were found to be similar in the mouse; however, the plasma AUC increased supraproportionally (~~240-fold to 4300-fold) as the dose was increased from 3/5 to 400 mg/kg orally in the mouse and rat suggesting that, in both species, clearance for both compounds becomes saturated at high oral doses. The toxicokinetic studies reported highlight that the liver exposure in the mouse was twice that in the rat (1.12 mmol/kg i.p.), with free liver AUC values at 243 and 128 ug.h/ml respectively. It was also observed, an i.p. dose of FS induced hepatotoxicity in male mice, yet, at the same dose, orally administered FS failed to do so. Oral administration of FS in the mouse resulted in significantly reduced plasma and liver exposure compared to i.p. administration. NVP was tested in in vitro and in vivo systems for a toxicological end-point; NVP failed to induce cytotoxicity in both male Wistar and female Brown Norway rat hepatocytes, however, a NVP-induced skin rash developed over 3 weeks of daily NVP treatment in female Brown Norway rats. Urinary metabolites were quantified by mass spectrometry on day 7, 14 and 21 of the study; however, no time-dependant trend in any NVP metabolite was observed. No evidence of liver injury or elevation in serum HMGB-l was observed. It is shown, in the work presented, that 12-0H NVP is a substrate for bioactivation; 12-0H NVP was administered to male Wistar rats and only Ml was identified in the bile. Following administration ofNVP to male Wistar rats, both M1 and M2 were identified in the bile. The work presented here has shown that the development of an assessment framework that encompasses all aspects of drug disposition and metabolism, and their relationship with bioactivation and toxicity, would be a valuable tool in drug development. The construction of a decision tree that can be populated with quantitative data could be used in the assessment of NCEs and highlight properties that require optimisation. Outcomes of changes or interventions to a new therapy could then be measured quantitatively and mechanisms defined. A caveat to this framework would stipulate that the model systems employed would have to be carefully selected to best represent the clinical situation. This work highlights the need for improved preclinical hazard assessment and understanding of animal models. Development of more informative and translational biomarkers would allow better clinical hazard identification, as well as improving survivability of NeE during the drug development process.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral