Combinatorial treatments of tamoxifen with SM6Met, a selective estrogen receptor subtype modulator (SERSM), from Cyclopia subternata are superior to current endocrine treatments in breast cancer cell models.

Thesis (MSc)--Stellenbosch University, 2018.ENGLISH ABSTRACT: Globally and in South Africa breast cancer is the most frequent malignancy amongst women. Most breast cancers are estrogen receptor (ER) positive and requires estrogen for growth and metastasis. Adverse side-effects associated with, and resistance to, the current standard of care (SOC) hormone therapies that target estrogen signalling, like tamoxifen, a selective estrogen modulator (SERM), and fulvestrant, a selective estrogen receptor down-regulator (SERD), have driven the recent development of using natural compounds or extracts as novel therapies, either alone or in combination with conventional chemotherapeutic agents, for the treatment and/or prevention of breast cancer. Previous work from our laboratory has suggested that a sequential methanol extract, SM6Met, prepared from the indigenous fynbos plant, Cyclopia subternata (honey bush), has several properties that may make it an effective chemopreventative and/or chemotherapeutic agent for breast cancer. The current study investigated the ability of SM6Met to prevent or treat breast cancer either as monotherapy or in combination with 4-OH-Tam (the active metabolite of tamoxifen) by evaluating its effects on the processes required for the development and progression of breast cancer such as proliferation, migration, invasion and colony formation. Firstly, I validated previous findings, characterizing SM6Met as a selective estrogen receptor subtype modulator (SERSM) that behaves as an ERα antagonist and ERβ agonist, which is able to inhibit estrogen-induced breast cancer cell proliferation. Importantly, I show that although SM6Met as monotherapy could not compete, in terms of efficacy or potency, with current SOC therapies like 4-OH-Tam and fulvestrant, with regard to inhibiting breast cancer cell proliferation, SM6Met showed potential in targeting two pro-metastatic processes, invasion and colony formation, to an extent equal to, if not greater than the SOC therapies and thus has the potential to be developed into a phytoestrogenic nutraceutical that can be beneficial in the prevention and treatment of breast cancer and metastasis. Secondly, I show, for the first time, that the effects of SM6Met could be replicated and enhanced by combining an ERα selective antagonist (MPP) and an ERβ selective agonist (liquiritigenin), thus validating the concept that a treatment with these ideal ER subtype selective properties may be more beneficial for the treatment and prevention of breast cancer and metastasis than current SOC therapies. Thirdly, I show, for the first time, that the combination therapy of 4-OH-Tam and SM6Met produced a strong synergistic effect in terms of antagonizing breast cancer cell proliferation and that a 20 times lower dose of 4-OH-Tam in combination with SM6Met is required to produce the same inhibitory effect on cell proliferation as 4-OH-Tam alone. Moreover, the best combination ratio (20:1) of SM6Met with 4-OH-Tam displayed greater anti-metastatic potential than the extract or the SOC therapies alone, suggesting that SM6Met together with 4-OH-Tam could be a viable drug combination for not only delaying resistance and ameliorating the negative side effects associated with current SOC therapies, like tamoxifen, but could also provide a novel, more affordable therapeutic alternative for treating or preventing breast cancer metastasis.AFRIKAANSE OPSOMMING: Wêreldwyd en in Suid-Afrika is borskanker die mees algemene kanker onder vroue. Meeste borskankers is estrogeen reseptor (ER) positief en benodig estrogeen vir groei en metastase. Die verskeideneid newe-effekte wat geassosieer is met, asook weerstand teen, die huidige standaard gebruikte hormoon terapie wat estrogeen-sein teiken, soos tamoxifen, 'n selektiewe estrogeen reseptor modulator (SERM) en fulvestrant, 'n selektiewe estrogeen reseptor af-regulator (SERD), het die onlangse ontwikkeling van die gebruik van natuurlike verbindings of ekstrakte as nuwe terapieë gedryf, hetsy alleen of in kombinasie met konvensionele chemoterapeutiese middels, vir die behandeling en/of voorkoming van borskanker. Vorige werk uit ons laboratorium het getoon dat 'n opeenvolgende metanol ekstrak, SM6Met, wat vanaf die inheemse fynbosplant, Cyclopia subternata (heuningbos) voorberei is, verskeie eienskappe het wat dit 'n effektiewe chemovoorkomings en/of chemoterapeutiese middel vir borskanker kan maak. Die huidige studie het die potensiaal van SM6Met ondersoek om borskanker te voorkom of te behandel as enkelterapie of in kombinasie met 4-OH-Tam (die aktiewe metaboliet van tamoxifen) deur die effekte daarvan te evalueer op die prosesse wat nodig is vir die ontwikkeling en progressie van borskanker soos proliferasie, migrasie, indringing en kolonie vorming. Eerstens, bevestig ons vorige bevindinge, wat SM6Met kenmerk as 'n selektiewe estrogeen reseptor subtipe modulator (SERSM) wat optree as 'n ERαantagonis en ERβ-agonis, wat estrogeen-geïnduseerde borskanker-proliferasie kan inhibeer. Verder, wys ons dat, alhoewel SM6Met as monoterapie, met betrekking tot die inhibering van borskanker-proliferasie, nie in terme van effektiwiteit of sterkte kon meeding met huidige standaard gebruikte hormoon terapieë soos 4-OH-Tam en fulvestrant nie, wys SM6Met potensiaal om twee pro-metastatiese prosesse, indringing en kolonie vorming, te inhibeer tot 'n mate gelyk aan, indien nie groter as die standaard gebruikte hormoon terapieë nie, en sodoende het SM6Met die potensiaal om as fitoëstrogeniese neutraseutiese middel ontwikkel te word wat voordelig kan wees in die voorkoming en behandeling van borskanker en metastase. Tweedens wys ons, vir die eerste keer, dat die effekte van SM6Met gerekonstruktureer kan word deur 'n ERα selektiewe antagonis (MPP) en 'n ERβ selektiewe agonis (liquiritigenin) saam te kombineer, wat die konsep bevestig dat 'n behandeling met hierdie ideale ER subtipe selektiewe eienskappe meer voordelig kan wees vir die behandeling en voorkoming van borskanker en metastase as huidige standaard hormoon terapieë. In die derde plek wys ons, vir die eerste keer, dat die kombinasie terapie van 4-OH-Tam en SM6Met 'n sterk sinergistiese effek toon in terme van inhibisie van borskanker-proliferasie en dat 'n 20 keer laer dosis 4-OH-Tam in kombinasie met SM6Met nodig is om dieselfde vlak van inhibisie as 4-OH-Tam alleen te produseer. Daarbenewens, het die beste kombinasie verhouding (20: 1) van SM6Met tot 4-OH-Tam groter anti-metastatiese potensiaal as die ekstrak alleen of die huidige standaard gebruikte hormoon terapieë alleen getoon, wat daarop dui dat SM6Met tesame met 4-OH-Tam 'n lewensvatbare geneesmiddelkombinasie kan wees, om nie net weerstand te vertraag en die negatiewe newe-effekte wat met huidige standaard gebruikte hormoon terapieë, soos tamoxifen, geassosieer word te verlaag nie, maar kan ook 'n nuwe, meer bekostigbare terapeutiese alternatief vir die behandeling of voorkoming van borskankermetastase voorsien