Elucidation of the mode of action of a furanone based antituberculosis compound

Ngwane, Andile Happyboy (2012-12)

Thesis (PhD)--Stellenbosch University, 2012.

Thesis

ENGLISH ABSTRACT: The prevalence of multi-drug resistant (MDR) and extensively drug-resistant (XDR) Mycobacterium tuberculosis has been increasing to alarming levels globally. This has been exacerbated by tuberculosis (TB) co-infection with HIV where the epidemic is endemic. South Africa as a developing country is hit hard by TB and efforts to develop TB drugs that are compatible with anti-retroviral medication and also effective against MDR/XDR, could help shorten the treatment duration of the current TB treatment regimens. This thesis presents the identification and characterisation of a novel furanone based compound (F1082) and its derivatives as leads for anti-TB drug development. Furanones are generally known for an array of biological activities ranging from antibacterial, antifungal and antitumor. F1082 has an aromatic benzene structure and was identified from screening synthetic compounds against M. tuberculosis. It is potent against M. tuberculosis at minimum inhibitory concentration (MIC) of 8 μg/ml. It is selective for mycobacteria since it did not inhibit the growth of Gram-positive and Gram-negative bacteria at concentrations five times the MIC for M. tuberculosis. F1082 is generally bacteriostatic around MIC concentrations in its effects against M. tuberculosis however; it may be bactericidal at higher concentrations. It is as effective against MDR, XDR and clinical isolates of M. tuberculosis at the same concentration as the M. tuberculosis H37Rv reference strain. This suggests that F1082 may have a different mechanism of action compared to current TB drugs. It has been shown to have no antagonistic effect with the first-line anti-TB drugs and it has been shown to synergize with rifampicin by reducing the MIC of rifampicin. A drawback of F1082 is that it is cytotoxic to human cell lines, but this is presently being addressed through the synthesis of analogues that have shown improved activity and less cytotoxicity. The synthesis of more than 40 analogues has led to identification of 4 compounds that have more than five times higher activity and more than 100 times less cytotoxicity against human cell-lines. Microarray analyses have identified possible metabolic pathway/s in M. tuberculosis that is/are affected by F1082. One subset of genes which showed the most prominent alteration encodes the siderophores, which are involved with iron homeostasis in the M. tuberculosis bacillus. Of these genes, 7 were of interest (mbtB, mbtC, mbtD, mbtE, mbtF, mbtH and bfrB) as they all fall in the same cluster and are involved in iron acquisition. Due to the involvement of iron we also show that F1082 generates oxidative stress that is metal (iron) dependent. From the results we conclude that F1082 is a promising antituberculosis lead compound with unique target properties and also specificity against mycobacteria.

AFRIKAANSE OPSOMMING: Die voorkoms van veelvuldige middelweerstandige M.tuberculosis (MDR) en uiters middelweerstandige M.tuberculosis (XDR) is besig om toe te neem teen ‘n kommerwekkende tempo wêreldwyd. Hierdie situasie word vererger met die ko-infektering van M.tuberculosis en HIV. Suid- Afrika, as ontwikkelende land, word sleg benadeel met tuberkulose siekte. Antituberkulose middels wat kan saamwerk met bestaande antiretrovirale middels en ook effektief is teen MDR en XDR stamme, kan alles meewerk om die behandelingstyd van tuberkulose te verkort. In hierdie tesis identifiseer en karakteriseer ons ‘n furanoon-gebaseerde verbinding (F1082) en derivate daarvan as voorloper-middels vir anti-tuberkulose middelontwikkeling. Furanone is algemeen bekend vir ‘n verskeidenheid van biologiese aktiwiteite insluitende antibakteriële-, antifungale- en antitumor aktiwiteite. F1082 bevat ‘n aromatiese benseenstruktuur en is oorspronklik geïdentifiseer gedurende die skandering van sintetiese middels teen M.tuberculosis. Dit het ‘n sterk werking teen M.tuberculosis met ‘n minimum inhibitoriese konsentrasie (MIC) van 8ug/ml. Dit is baie selektief vir mikobakterieë aangesien dit nie gram-positiewe of gram-negatiewe bakterieë teen 5 maal die MIC, soos vir M.tuberculosis, geïnhibeer het nie. F1082 is bevind om, by laer konsentrasies, bakteriostaties te wees in sy aktiwiteit teen M.tuberculosis maar by hoër konsentrasies word ‘n meer bakteriosidiese effek waargeneem. F1082 is effektief teen MDR, XDR en kliniese isolate van M.tuberculosis en teen dieselfde konsentrasie soos vir die M. tuberculosis H37Rv verwysingstam waargeneem is. Dit impliseer dat F1082 dalk ‘n alternatiewe meganisme van werking het in vergelyking met die van die huidige TB teenmiddels. F1082 toon geen antagonistiese werking in kombinasie met die voorste anti- TB middels nie, maar toon wel sinergistiese werking in kombinasie met rifampisien. F1082 toon nog sitotoksiese aktiwiteit teenoor menslike sellyne, maar die sintese van derivate van F1082 toon tot dusvêr groter anti-TB aktiwiteit en verminderde sitotoksisiteit. Die sintese van meer as 40 homoloë het gelei tot die identifisering van vier verbindings met vyf keer hoër anti-TB aktiwiteit en honderd keer verminderde sitotoksisiteit teen menslike sellyne as F1082 self. “Microarray” ontledings het ‘n aantal metabolise paaie geïdentifiseer waar F1082 ‘n effek kan uitoefen. Een stel gene wat die mees uitstaande effek toon kodeer vir siderofore wat betrokke is by yster homeostase in M.tuberculosis. Van hierdie gene was daar sewe van belang omdat hulle in dieselfde groep voorkom en almal betrokke is by ysteropname (mbtB, mbtC, mbtD, mbtE, mbtF, mbtH, bfrB). Weens die rol wat F1082 in ysterhomeostase speel, toon ons ook dat F1082 intrasellulêre oksidatiewe stres bevorder wat yster afhanklik is. Al ons resultate dui daarop dat F1082 ‘n belowende ant-TB voorloper verbinding is met spesifisiteit teen M.tb en unieke teikeneienskappe in M. tuberculosis.

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