As some of you may be aware, Perth is in a lockdown, which opens up a lot of time for thought. This is the result.
I have done some serious thinking after reading this post by
@RaceOncology and I would like to share my thoughts on why the EMD AML mouse model program is very important and provide an undated opinion piece on Bisantrene. This work is from my own understanding, which may be wrong. Remember: I am your average investor.
I think the preclinical EMD AML mouse model study is probably one of the most important preclinical programs that RAC are currently undertaking, as this work will uncover and establish the true mechanism(s) of action for Bisantrene. Clearly, I do not know what the preclinical model will include, but I assume that it will include a component that measures the Bisantrene-induced inhibition of topoisomerase II and FTO. I imagine this model will help us to get to the bottom of whether it is FTO or topoisomerase II inhibition, or a combination of the two driving the anti-cancer efficacy seen in humans. I believe that this data will dramatically change the way Australian and overseas investors will look at Bisantrene.
Opinion Piece
There has been a fair bit of discussion about Bisantrene's mechanism of action lately, which has been largely led by me reviewing the preclinical studies investigating Bisantrene as an anthracene and an FTO inhibitor. I appreciate the critical questions that come with challenging the prevailing dogma regarding Bisantrene's function, as this is what drives knowledge and understanding forward. Ahead of the data generated from preclinical and clinical programs currently underway, I would like to lay out my understanding of the in-human interactions of Bisantrene. This information comes from many hours reading and thinking, and although it is well-thought and referenced, it remains detailed speculation until unequivocal evidence is generated. Admittedly, I have limited the use of citations for topics I have covered previously.
Having looked through the historic and recent data countless times, I have always been interested in the discrepency between the efficacy in AML (extremely good) compared to solid tumors like breast, ovarian, and others (below). I think it is fair to say that Bisantrene has never been used in a such a way as to take full advanatage of its FTO inhibitory properties, particularly in solid tumors. This is because Bisantrene's main mechanism of action falls under precision oncology, which requires select patient populations and targeted and specific dosing regimens. Hence, I am not overly concerned by the lack of efficacy in solid tumors. One thing that has been front of mind for me is the difference between the dosing regimen used in each cancer type, with AML following a 7 day 250 mg/m2/d regimen and solid tumors using ~250-300 mg/m2 every 3 or 4 weeks.
Summary of phase II AML data
Summary of Single Agent Phase II data
If you are like me, then you may be asking yourself: How can Bisantrene show such amazing potential in AML, yet relatively very little efficacy in others?
I think I now understand why this may be the case.
The factors that I have addressed to generate an understanding of what I am seeing include the concentration and binding kinetics of Bisantrene required to inhibit FTO (142 nM; fast) and topoisomerase II (18,000 nM or 18 uM; slow) and pharmacokinetics of Bisantrene in humans. It's quite clear that Bisantrene will bind to FTO better and long before it has bound to topoisomerase II. The interaction of Bisantrene with FTO is quick and at very low concentrations, which means you don't need a lot of it in the cell for an effect to be seen quickly. The interaction of Bisantrene with topoisomerase II is slow and at very high concentrations, which means that you need a lot of it in the cell for a long period of time for an effect to be seen. Preclinical models indicate that a 10 uM concentration of Bisantrene loses its ability to function as a topoisomerase II inhibitor (anthracene).
Pharmacokinetics of Bisantrene
Pharmacokinetics is basically what the body does to a drug when it is in the system. I have looked through some of the literature that have explored how the body handles Bisantrene and will use it to demonstrate to you my deeper understanding of the drug. The following table is partly made of real human data (weight & BSA) taken from a pharmacokinetic clinical trial (1). The original paper used different doses between 200 and 280 mg/m2, which I have altered and made all of them 250 mg/m2. Multiplying dose (mg/m2) by BSA provides the amount of dose (mg) given to each patient. Assuming that 60% of the body is made up of water, you divide the dose (mg) by the total water weight to
estimate the concentration (ug/mL) of Bisantrene, which can be changed to uM by dividing by the molecular weight of Bisantrene (0.3985 kD). Based on the data below, we may assume that a dose of 250 mg/m2 produces an average maximal concentration of ~28.4 uM.
The figure below is an example of a patients plasma disappearance of Bisantrene (1). While the information contained isn't necessarily important to understand in complete detail, I have shared it because I want to highlight what is happening to the concentration of Bisantrene over time. After the initial dose of Bisantrene enters the system, the concentration drops rapidly and then curves toward flat. There is a point known as the terminal half-life (red circle), where half of the concentration of the drug remains in the plasma following the distribution phase (the large drop). The trial indicates 26-hours. In other words, there is an inverse relationship between time and concentration, where as time after infusion increases, concentration decreases. Keep in mind that this relates to plasma concentrations and not intracellular concentrations, which may decrease in concentration at a different rate(s).
1
https://pubmed.ncbi.nlm.nih.gov/6678875/
Bringing it together
Firstly, we need to compare the IC50 values for FTO (142 nM or 0.142 uM) and Topoisomerase II (18,000 nM or 18 uM), and then compare that to the estimated average total body water concentration of Bisantrene at 250 mg/m2 (28.4 uM). The estimated concentration of Bisantrene is higher than the IC50 values for FTO and topoisomerase II inhibition, which indicates that there is FTO and topoisomerase II inhibition within the cell. If we also then consider the inverse relationship between time following infusion and concentraiton, the concentration of Bisantrene will decrease over time. Based on this information we can then estimate what is happening in the body over time following the administration of Bisantrene.
Following administration of Bisantrene, the concentration rises beyond the IC50 values for FTO and topoisomerase II inhibition, which enables the drug to bind to these proteins and inhibit their function. Due to binding kinetics, Bisantrene will bind swiftly to FTO and slowly to topoisomerase. As time goes past, the concentration of the drug will decrease to a point where Bisantrene will no longer function as a topoisomerase II inhibitior (10 uM). However, at this point, the concentration will still be great enough to inhibit FTO 100%. Eventually, the concentration of Bisantrene will decease to a point where it is unable to inhibit FTO and the function of Bisantrene will stop. Due to not knowing the rate of decrease in intracellular concentrations, it is difficult to determine when anthracene or FTO inhibitory activity would stop.
Right. Now we have that covered, we can start to take another look at the clinical trials that have used Bisantrene and make an assumption as to what might be happening in humans. Because I do not know the rate at which Bisantrene decreases in the cells of the body over time, I will assume that the concentration decreases similarly to the elimination half-life, which means half concentration of Bisantrene (~14.2 uM) at 26 hours and very low concentrations by 60-hours. This assumption is not accurate and is instead used to illustrate a point. The true rate of intracellular decline may be more or less than this value, and I reserve the right to change my views on this if or when new data comes to light.
Bisantrene was historically used in a once every three or four week regimen in solid tumors. While doses varied, for the purpose of this exercise, I will use 250 mg/m2. For a single 250 mg/m2 dose of Bisantrene used once every three weeks, we may expect the following response (below). The intracellular concentration of Bisantrene is enough to inhibit FTO for 2-3 days and topoisomerase II for 1 day every three weeks.
Bisantrene was historically used in a 7-day 250 mg/m2/d dose with a 3-day 250 mg/m2/d consolidation. For the purpose of this exercise, we will only use the initial 7-day dose. For a 7-day 250 mg/m2/d regimen of Bisantrene, we may expect the following response (below). The intracellular concentration of Bisantrene is enough to inhibit FTO for 8-9 days consecutively and topoisomerase II roughly 7-8 days consecutively.
You can see from the figures above, that a dosing regimen of Bisantrene used every day provides substantially more opportunity to take advantage of each mechanism of action. Small, consistent concentrations of Bisantrene are required to inhibit FTO. Large, consecutive doses of Bisantrene are required to maintain intracellular concentrations above 10 uM (the point at which Bisantrene stops functioning as an anthracene), for topoisomerase II inhibition.
If you are like me, you'd be hoping that there was a study that had used the AML dosing regimen in solid tumors. Unfortunately, it is common practice to use a single bolus dose (ie 250 mg/m2) once every 3 or 4 weeks for solid tumors. Fortunately, there is a study that we can look at that have done something a little similar.
The recent phase II AML investigator initiated trial utilized a 7 day 250 mg/m2 dose of Bisantrene in heavily pretreated, poor prognosis AML patients. The study included four EMD AML patients (skin, breast, chloroma, and CNS), who all achieved a clinical response - 25% CR & 75% PR. Now, this is very important as EMD AML tumors adopt solid tumor characteristics (2). Also, FTO overexpression has been linked with EMD multiple myeloma (3). So, what we have are patients that have tumors that resemble solid tumors and who have recieved a dosing regimen that supports Bisantrene's mechanism(s) of action. I feel a little weird about doing this, as I feel
@wombat777 has some kind of ownership rights over this, but you can see the tumor regression in the breast of a patient that recieved Bisantrene (below). Importantly, this group of patients were very sick and the consolidation regimen (3 days @ 250 mg/m2/d) was not used, meaning that patients lossed approximately ~5 days and ~3 days of FTO and topoisomerase II inhibition, respsectively.
2
https://onlinelibrary.wiley.com/doi...OTFfDYoPizQ-xCFNDX-TcwWQfG5h1SWGSlmwz6_qvlNGo
3
https://www.researchgate.net/publication/337248777
In summary, I believe that when Bisantrene is used in selected patients that supports its mechanism(s) of action, we will start to see the true potential efficacy of this drug. I believe that the activity in EMD AML patients is a glimpse of what we may see in such a patient population and dosing regimen. However, as there are only 4 EMD AML patients, it is very challenging to draw strong conclusions and extrapolate these findings to efficacy in other solid tumors. Fortunately for shareholders, RAC have multiple clinical programs underway and a lot to be excited about. Based on the information that I have presented here, I believe that there is real potential for self-synergy - FTO and topoisomerase II inhibition - allbeit low for anthracene function.
Disclosures:
- Not paid by RAC ever.
- 100% my own work.
- Not an expert.
- Do your own research.
(20min delay)