Glucocorticoids in acute myeloid leukaemia
While glucocorticoids are established agents in the treatment of lymphoblastic leukaemia, their use in myeloid leukaemia is debatable [18]. Steroids are not included in any standard treatment protocols for AML [19]. Health professionals are reluctant to use glucocorticoids due to an increased risk of invasive fungal infections, sterile site infections, bacteraemia, sepsis, and death [18, 20]. Moreover, in vitro studies have reported that in some cases, paediatric AML cells were prone to glucocorticoid-induced proliferation as opposed to induced differentiation [21]. If extrapolated to the in vivo situation, this might increase the risk of relapse. However, the addition of dexamethasone to intensive chemotherapy results in a significant reduction in relapse and overall better survival rate in already hyperleucocytic AML patients [18]. In such cases, it is recommended that patients also receive prophylactic antifungal treatment to prevent infections.
Glucocorticoids bind to GR in the cytoplasm, which can then either homodimerise and bind DNA or remain monomeric, acting independent of DNA binding. This may alter certain transcriptomic programmes, inhibiting the activity of transcription factors (such as AP-1 and NF-κB) and/or by modulating the early inflammatory response (which is associated with chemoresistance). Thus, dexamethasone may sensitise AML stem cells to chemotherapy-induced cell death, thereby limiting the risk of leukaemic regrowth and relapse [18]. Glucocorticoids and other steroids, such as progesterone, may also exert collateral sensitivity in multidrug resistance (MDR) cell lines, which may help improve survival rates [22]. AML cells treated with dexamethasone were also shown to express higher levels of the CD38 marker after 1 week [18]. This increased CD38 expression could serve as a positive prognostic factor in treatment, as it helps the immune system to target the malignant cells. Apart from this, since the monoclonal antibody daratumumab binds to the CD38 protein, inducing malignant cell apoptosis, dexamethasone can help provide a potential new cytotoxic therapy route [23]. Moreover, with long-term dexamethasone treatment, a greater amount of monocytic CD11b/CD14-positive cells can be noted. This demonstrates that dexamethasone also encourages differentiation of AML cells, thus reducing the proliferating leukaemia cell burden [18].
Corticosteroids have already been used to treat certain complications that may be encountered during the therapy of AML. In acute promyelocytic leukaemia (APL), steroids are used in patients who develop differentiation syndrome, an unpredictable, but frequent complication of all-trans retinoic acid (ATRA) administration [24].
Cardiotonic steroids
Cardiotonic steroids (CS) are natural compounds which are found in both the animal and plant kingdoms. They have long been used in the medical field to treat heart-related problems, namely heart failure and arrhythmias [22]. The cytotoxic properties of CS were first recognised when higher plasma concentrations of digitoxin were correlated with lower risks of leukaemia and urinary tract cancers [25]. A number of studies were then conducted regarding the effects that CS have on malignant cells. These drugs are capable of either binding to Na+/K+ ATPase pump [22] or inhibiting the ATP-binding cassette sub-family B member 1 (ABCB1) transporter (also known as P-glycoprotein) [25].
Genetic and epigenetic differences amongst various cancer cells lead to the acquisition of drug resistance [26]. Mechanisms of cancer resistance include increased efflux of the drug from the cell and decreased cytosolic intake of the drug. Other mechanisms include the inhibition of the apoptotic pathways, as well as the activation of cellular growth and DNA repair pathways. MDR cell lines are capable of keeping intracellular drug concentrations at very low levels by different mechanisms. Such MDR cell lines have enhanced expression of ABCB1 transporters, which pump out chemotherapeutic drugs, including anthracyclines, taxanes, and vinca alkaloids [22, 27]. Inhibition of P-glycoprotein results in inhibition of MDR, which leads to an overall better prognosis when treating concomitantly with chemotherapy [25].
MDR cells have also altered expression of other membrane proteins, including the Na+/ K+ ATPase pumps and Na+/H+ exchanger. MDR cell lines typically have downregulation of Na+/K+ ATPase pumps and upregulation of the Na+/H+ exchangers. The latter results in an increase in cellular pH, which is typically seen in malignant cells. Moreover, downstream signalling pathways associated with the Na+/K+ ATPase pumps are also commonly deregulated. The administration of correct doses of CS, such as ouabain and digitalis, will reverse MDR and increase intracellular acidity, which suppresses cancer cell growth [22].
Interestingly, the most potent CS compounds induce a greater cytotoxic effect on resistant cell lines than on normal, non-malignant white blood cells [22]. It is hypothesised that CS induce apoptosis in malignant cells through the phenomenon of collateral sensitivity due to P-glycoprotein inhibition. Collateral inhibition explains how a particular drug may exert greater cytotoxic effects in more resistant leukaemic cell lines compared to other cells. For example, with verapamil (a P-glycoprotein inhibitor), the rate at which this drug is removed from the cell by P-glycoprotein in MDR cell lines is much faster than in non-MDR cell lines. Pumping out verapamil from within the cell consumes high amounts of adenosine triphosphate (ATP), which has to be re-supplied via oxidative phosphorylation. This in turn generates high amounts of reactive oxygen species (ROS), which eventually leads to apoptosis [22].
A number of CS/CS derivatives have shown overall promising results, attributed to their low-resistance indices, when compared to the resistance indices of other established chemotherapeutic agents. The resistance index is a mathematical calculation which takes into consideration the respective half maximal inhibitory concentration (IC50) value of each drug in the resistant cells and the sensitive ones [22, 25]. Therefore, CS may possibly be considered in patients with leukaemias resistant to multiple chemotherapeutic drugs.
To date, clinical trials have been carried out to examine the effectiveness of CS on solid tumours, where mixed results were obtained [25]. In leukaemia, only in vitro studies have been carried out so far. These have shown promising results, but clinical trials are essential to establish their suitability for inclusion in potential future treatment regimens [22, 25].
Ecdysteroids
Ecdysteroids are a family of natural compounds, found in different organisms, including insects, other arthropods, and plants. In insects, they play a key hormonal role in moulting and development. In plants, they serve as protection from phytophagous animals. They have also been identified in fungal extracts, but their role there has not yet been deduced [28]. It is hypothesised that these compounds are unable to interact with the mammalian steroid-hormone receptors due to fundamental differences in their structure, polarity, bulk, and shape [29].
Despite this, the addition of ecdysteroids/ecdysteroid derivatives, namely 20-hydroxyecdysone; 20-hydroxyecdysone 20,22-acetonide; 20-hydroxyecdysone 2,3;20,22-diacetonide, with various chemotherapeutic agents, has yielded better results in vitro overall [28]. These ecdysteroids/ecdysteroid derivatives are known inhibitors of the ABCB1 transporter (similar to CS [25]). However, besides yielding positive results on MDR-cell lines, the least polar ecdysteroid derivatives also showed positive outcomes on non-MDR cell lines. The IC50 values of the chemotherapeutic agents (doxorubicin, paclitaxel, and vincristine) decreased considerably when used simultaneously with ecdysteroids/ecdysteroid derivatives. On the other hand, when used with cisplatin, these same ecdysteroids/ecdysteroid derivatives caused an increase in the IC50 of the same malignant cells. This indicates that collaborative cytotoxicity does not always occur and is probably dependent on the mechanism of action of the particular chemotherapy, with alkylating-like agents being rendered paradoxically less efficient.
Other studies suggest that the ecdysteroid, β-ecdysone (20-hydroxyecdysone) derived from Helleborus niger (a traditional medicinal plant native to large parts of Europe), targets the mitochondrial outer membrane permeabilisation pathway (MOMP), modulating the mechanism of apoptosis. The BCL-2 family of proteins are regulators of apoptosis. It was noted that β-ecdysone (20-hydroxyecdysone) counters the MOMP inhibition pathway mediated by the anti-apoptotic BCL-2 protein. Moreover, 20-hydroxyecdysone also stimulates CD2 presentation on T-lymphocytes and aids in the function of the immune system. Since Helleborus niger has other active components besides β-ecdysone (20-Hydroxyecdysone), it cannot be concluded that the plant extracts’ apoptotic properties are solely the result of ecdysteroids and this requires further studies [30].
In contrast to their pro-apoptotic effect on leukaemic cells [30], ecdysteroids exert an anti-apoptotic effect on non-cancerous cell lines. In this context, most refer to the phosphatidylinositol-3-kinase/protein kinase B signal transduction (PI3K/Akt) pathway, which is responsible for the anti-apoptotic effects of ecdysteroids [29, 31]. Thus, ecdysteroids appear to specifically target neoplastic cells and may yet be developed as an additional tool in the clinical armamentarium.
To date, only in vitro studies of ecdysteroids have been conducted in AML. Based on these encouraging results, it is hoped that clinical trials will be developed in the near future.
