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This article may be tough going for those without a deeper interest and knowledge about CLL cellular biology, but I hope by plowing through it, perhaps more than once, and watching the short educational videos, it will reward the reader with some understanding about what promises to be the next big breakthrough in CLL.
Due to my own lack of understanding for the number of drugs coming out of the important research into the Bcl-2 “death” signaling pathway and a recent CLL forum post on one of those drugs (ABT-263), I decided to write an overview of this focused arena of therapy and share what I have learned. Readers should know that I am without qualifications, medically or academically, to be regarded as an authority but I submit the following layman’s look into this fascinating area of CLL biology and therapeutic approach to CLL in hopes of enlightening others as well as myself. A shortcut video introduction to this subject was recently made by Dr. Brian Koffman who interviewed Dr. Jeff Sharman, a CLL specialist regarded as an authority on the subject. Therefore, you can have another option to gain an understanding of Bcl-2 antagonist drugs. I urge you to view this informative video.
A Look in the Treatment Toolkit – Bcl-2 Antagonists
I began my exploration of the Bcl-2 death signaling pathway by unraveling some of my ignorance for the cellular biology that is relevant to explain why Bcl-2 antagonist drugs are used and how they work. With the increasing number of these drugs, it is clear that problems exist from a perspective of efficacy and side effects so I wanted to know what are the reasons behind these problems? How important are these drugs to improving therapy outcomes? I needed to clarify the meaning of the medical jargon surrounding this emerging class of drugs so bear with me, for although the subject is very complex, I hope to leave you with a conceptual, if cartoon-like, overview of Bcl-2 antagonist drugs and their roles in the ongoing effort to put the CLL “BEAR” into permanent hibernation.
I have purposely employed repetition of function and term definition for the benefit of those new to CLL and medical jargon. If this is worth reading, let me know and I might do more on other drug development areas relevant to our community. Constructive criticism is welcome to improve future content presentation and for correcting any missed errors or omitted facts of interest.
Guardians of the Galaxy in a battle with the CLL Bear
When we have CLL/SLL we are dancing with a “Bear,” some in a slow waltz and some to a deadlier paced tempo. In either case, the dance won’t stop until the Bear has had enough of the dance. Our bodies are galaxies of tissues comprised of trillions of cells where our functional well-being is dependent on an ability to keep a balance or state of homeostasis between newly-minted cells coming online and cells that need to die due to defects of age or corruption. Bcl-2 antagonist drugs are potent agents that help pressure the Bear into choosing hibernation over “the dance”. It is interesting to know how these drugs work and in which part of the immune complex they interact. In the big picture of the immune system complex, there are overlapping and multilayered players so we will be exploring only a portion of that world as it relates to Bcl-2 antagonist drugs. In any understanding of the immune complex, tumor suppressor genes are critically important, with most patients aware that the TP53 gene, and to a slightly lessor extent, the ATM gene, if deleted or mutated, means the likelihood of a poorer prognosis. The TP53 gene is often called the guardian of the genome by researchers and molecular biologists. Tumor suppressor genes act in ways analogous to our law court judges, deciding when to repair cells or order cells to auto-destruct because of defects. This is why, in CLL, when you are cytogenetically tested using FISH (Fluorescence In Situ Hybridization) and found to have an ATM, TP53 or RB1 tumor suppressor gene deleted or mutated, it is worrisome because that is when an important function of maintaining homeostasis of healthy functioning cells maybe lost or compromised. Let’s pause to define a few important terms that will help us navigate in the world of CLL biology to better understand the function of these new Bcl-2 antagonist drugs.
“Homeostasis” – Homeostasis refers to the dynamic struggle of balance in keeping functionally healthy cells alive through actions of pro-apoptotic proteins in proportion to anti-apoptosis proteins in service of the continual need for our body’s tissues to remove dysfunctional or diseased cells by apoptosis. The means to engage the protein machinery of the Bcl-2 family of proteins that regulates homeostasis is an arena for manipulative advantage to the CLL Bear in keeping CLL cells alive and an opportunity for oncologists to intervene in efforts to kill CLL cells.
“Apoptosis” – is an important term referring to programed cell death or cellular suicide, a natural process as opposed to cell death by direct means of chemotherapy as one example. One of the two “clans” of all proteins that make up the Bcl-2 family of proteins are pro-apoptotic (death signaling) initiators of apoptosis. The other clan of Bcl-2 proteins are anti-apoptotic or pro-survival proteins. The terms “pro-survival” and “anti-apoptosis” mean the same thing and will be used interchangeably. As in all families, not everyone gets along harmoniously and this is the case when the CLL “Bear” co-opts the pro-survival or anti-apoptotic protein clan, thus subverting apoptosis of dysfunctional CLL cells. This allows CLL cells to proliferate uncontrolled and allows “the Dance” to go on and on. You can see a great video illustrating apoptosis here.
“Bcl” stands for B-cell lymphoma and signifies both a specific protein and the collective name for some 25 pro- and anti-apoptotic proteins composing the Bcl-2 family of proteins. The “2” in Bcl-2 distinguishes the family proteins of interest to this essay as separate from other Bcl-families, i.e. Bcl-3,5,6…etc. Drugs developed for interaction with this family of proteins/genes are referred to as “Bcl-2 antagonists” and as “BH3” mimetics,” meaning that these drugs target, in different ways, certain proteins in the Bcl-2 family and accomplish this by mimicking a common (homology 3rd domain) feature shared by those proteins as separate from several other homologous (same featured) groups, i.e. BH1, BH2 and BH4.
“Antagonists” – as in “Bcl-2 antagonists,” our drugs of interest are to be understood with the term “agonist,” its counterpart. Agonists can be pro-apoptosis proteins, producing the action of apoptosis signaling, causing the death of cells, whereas an antagonist (for our context) is a drug/chemical that will bind to a receptor on a protein/gene that will block its function. In this case the antagonist blocks the function of pro-survival/anti-apoptotic proteins in order to allow the agonist properties of the pro-apoptosis protein clan to initiate CLL cell death. These drugs are designed to dock at selected receptors on certain Bcl-2 anti-apoptosis proteins, inhibiting cell survival signaling. This allows the reinstatement or stimulation of apoptosis that shifts an imbalance of a high number pro-survival protein signaling, to pro-apoptosis protein signaling resulting in reduction of CLL cells, often dramatically as in the use of ABT-199 aka venetoclax.
When the CLL becomes established, it orchestrates an environment for its own survival, sabotaging homeostasis. CLL cells’ own selfish survival interests to avoid apoptosis distort the harmonious function of Bcl-2 proteins to become biased toward preventing or ignoring apoptosis signaling thus keeping alive dysfunctional cancerous B cells. In the bio-galaxy of tissues and cells that is us, the genetic court system of tumor suppressor genes like TP53 is not perfect and defective cells sometimes slip by to become oncogenes (corrupted genes usually from mutations) which can later become cancers. When we get a cancer like CLL, it is initially due to the failure of our immune system’s tumor suppressor genes that would normally discover and destroy abnormal cells, but these tumor suppressors are not the complete story. In the complex interplay with a host of many protein/gene guardians of our personal galaxy, we will now focus on the role of some select Bcl-2 guardian proteins who protect our cellular function by maintaining homeostasis and what happens when they are corrupted to perform that duty. The failure of Bcl-2 family protein function can also throw a light on why cancers become resistant to anti-cancer drugs.
Meet A Few Guardians of our Galaxy
All protein actors of the two Bcl-2 clans, when expressed in appropriate numbers and function, are good guys in keeping a healthy cell population viable through regulation. It is when we get CLL, or any cancer for that matter, that some of those proteins in the pro-survival clan end up “breaking bad” and become villainous accomplices of our CLL. Let’s take a look at some of those individual actors and put names and faces on them. While tumor suppressor genes might act like judges and juries of our biological courts, making decisions on which defective cells can be repaired and which must commit to apoptosis, it is up to other protein genes found in the Bcl-2 complex, often working together, to execute the verdicts of our bio-chemical “court judges.” These pro-apoptotic proteins of the Bcl-2 family are right out of central casting for movies like Guardians of the Galaxy, sporting names like Bim, Bid, Bad, Bax, Bak, Noxa and PUMA, to mention just a few. “Bim,” a Bcl-2 Interacting Mediator of cell death; “Bid,” the BH3 Interacting Domain Death Agonist; “Bad,” Bcl-2 Associated Death Promoter; “Bak,” Bcl-2 Antagonist Killer; and Noxa and PUMA who much speak for themselves. As you can tell by their names, these proteins, if functional, can kick butt in making CLL cells die. Herein lies a problem. Let’s shift our attention to the other clan of Bcl-2 actors. Pro-survival proteins are useful and necessary as exemplified by protein Bcl-xL that helps to keep platelets alive. However, when the CLL Bear hijacks cellular machinery for the purpose of its own survival, the up-regulation of pro-survival proteins leads to sequestering of pro-apoptotic clan members of the Bcl-2 family. It makes pro-survival proteins accomplices to the evil CLL empire caricatured by the CLL Bear’s compulsion to dance with us until our demise. Prominent pro-survival proteins in CLL cell survival are Bcl-2, Bcl-w, Bcl-xL, Bfl-1, Bcl-B and Mcl-1. To better appreciate the magnitude of incredible detail and complexity of this subject, see this overview, complete with drug list and color cartoon of the players here.
Surveying the battle ground
To save the Galaxy, which is us, involves the pro-apoptosis elements acting in sufficient numbers to defeat the anti-apoptotic protein minions of the CLL Bear. If the pro-apoptotic forces can be freed from confinement by anti-apoptosis protein sequestration, they can then function to signal the CLL cells, forcing them to obey a commandment to die. Getting some pro-apoptotic proteins out of molecular-bound “jail” is what much of the Bcl-2 antagonist drugs are about. When apoptosis signaling is sufficiently resumed or stimulated by pro-apoptotic Bcl-2 proteins, the action for CLL cell death occurs at the power plants of the CLL cells, which are called mitochondria. All cells are powered by mitochondria. Apoptosis is achieved through breaching the mitochondrial outer membrane to have its “guts,” containing cytochrome C, spill out into the intra-cellular space called cytosol. This action results in crippling mitochondrial function, which in turn causes the cell to die. Bcl-2 pro-apoptosis proteins do the job of creating pores in the mitochondrial membrane through which its contents leak out. Because of the number of actors in both camps of the Bcl-2 family, not to mention other protein players outside of that family, the apoptosis process is devilishly complicated. When patients lose major players like the TP53 gene or the ATM gene, correlated to 17p & 11q deletions respectively, it makes the task of pro-apoptotic genes more difficult due to interdependency of networking.
There is a great video of this process posted by Chris Dwyer on the Health Unlocked CLL forum here.
Drug Cavalry to the rescue
To facilitate CLL cell apoptosis, Bcl-2 antagonists can be potent with a range of targeting abilities that result in varied outcomes of efficacy and side effects dependent largely on which anti-apoptosis Bcl-2 proteins are expressed or up-regulated and the functioning state of tumor suppressor genes, predominantly TP53 associated with the short arm (p) of chromosome 17. Small molecular drugs that target specific vulnerabilities for cancer cell survival are becoming the future of controlling, if not curing, CLL. None of the Bcl-2 drugs are expected to be a silver bullet for a final cure, but rather think of them of as key components of drug combinations that may be unique options for each individual’s CLL. To this end, we will need to employ appropriate profiling for every patient given the extremes of CLL heterogeneity. (click on #4. below). The Bcl-2 antagonist strategy exploits natural mechanisms of cell destruction in order to quell the CLL Bear’s need to dance. I like that trend and we are only at the beginning of this journey. It is for these reasons and the fact that side effects can present themselves, that we need to be vigilant to know that our treating oncologists are knowledgeable and aware of known problems once these drugs come into use by the general medical community. The unknown unknowns we cannot worry about but the known knowns are for us to make sure our oncologists are cognizant of.
You might be asking “Why so many different drugs?” Cancer cells are like drug addicts and become “addicted” to certain proteins that enable them to proliferate without restriction and to achieve a degree of immortality. This addiction involves many proteins and in networks at that. One of the proteins of the pro-survival clan of the Bcl-2 family is called Bcl-xL and is an example of a protein that confers survival benefits to platelets so if your platelets are low it is good to take into consideration the effects of a Bcl-2 antagonist like ABT-263 or ABT-737 that inhibits Bcl-xL. Mcl-1 is another pro-survival/anti-apoptotic protein that may not be wise to block in all patients even though it has been found to play a key role in resistance to therapy. Knowing that there are other Bcl-2 antagonist drugs like ABT-199 that exclude the blocking of Bcl-xL, is valuable to patients with a low platelet profile. As these drugs become FDA approved, will your general oncologist know this?
I found 12 drugs engineered to inhibit the anti-apoptosis protein clan of the Bcl-2 family of proteins. Due to the large number of proteins in the Bcl-2 family and the interactions of these proteins with other protein networks, nothing in CLL is straight forward or simple.
ABT-263 is an example of an early drug in its class, designed to inhibit the pro-survival protein clan, specifically Bcl-2, Bcl-w and Bcl-xL, but due to its broader range of targets, led to low platelet counts and development was stopped.
Bcl-2 is both a target protein as well as a collective name for a diverse number of proteins that regulate cell survival and cell death. “Live Long And Prosper” is not what we want for our CLL cells but here is the catch; we need to “cure” CLL cells of their addiction to pro-survival proteins without causing unintentional side effects, and we need to be reassured that the cancer cells do not find alternative ways to survive by finding survival proteins a particular drug does not target. This becomes challenging when a patient relapses on any given therapy or develops intolerable side effects that necessitate a shift in drug therapies on the patient’s journey toward a remission or cure.
A snapshot review of a number of Bcl-2 antagonist drugs, what they target and some observations of interest.
|ABT-263 (Navitoclax)||Potent Bcl-2 family inhibitor targets Bcl-1, Bcl-xL & Bcl-w||These two drugs are early in their class and were of limited use due mainly to threats on platelet levels for certain patients. Navitoclax, however, has been effectively keeping the CLL Bear in hibernation for at least 5 years for some patients.|
|ABT-737||Bcl-2 inhibitor also targeting Bcl-xL and Bcl-w|
|ABT-199 (Venetoclax)||Bcl-2 inhibitor, potent and selective||This is currently the most likely patient-encountered Bcl-2 antagonist drug. It is engineered to diminish effects on platelets yet remaining potent in initiating CLL apoptosis. Development of drug resistance in a number of patients has limited, to some extent, use of this drug. The potency of ABT-199 was not initially anticipated by doctors and ended up causing the death of some brave patients who volunteered to participate in early clinical trials of this drug. In light of the potency of ABT-199 and the caution now used in its administration, the CLL Bear has found a way around this drug in a subset of patients. A June 2014 Blood journal article titled “Acquired mutations in BCL2 family proteins conferring resistance to the BH3 mimetic ABT-199 in lymphoma,” highlights the resilience and resourcefulness of CLL in spite of new drugs. “The BAX mutation was, however, unpredictable and, most importantly, led to cross-resistance to other antineoplastic agents, highlighting a dangerous mechanism of apoptotic resistance.” Nonetheless, this drug will likely be in many of our futures.|
|TW-37||Bcl-2 inhibitor, inhibits Bcl-2, Bcl-xL and Mcl-1||This drug appears to be effective in NOTCH1 pathway signaling. NOTCH1 is a refinement of molecular testing for poor outcome in Trisomy 12 CLL. Therefore, TW-37 is one to watch for those found to have NOTCH1 up-regulation if it can be correlated to other drug therapy resistance. Notch1 by itself may not mean much until there is an understanding of how it is related to the rest of CLL biology. Having a drug to target NOTCH1 will yield both a better understanding, and hopefully, a benefit to some patients in the short term. You can read more here.|
|Obatoclax Mesylate (GX15-070)||pan Bcl-2 family protein inhibitor||This drug is infused and has been associated with transient neurotoxicity as a dominant side effect. Response as a single agent has not been exciting. Read more here.|
|Marinopyrrole A (Maritoclax)||selective for Mcl-1||This drug is extracted from streptomycetes and may be of a drug type that could be paired with ABT-199 to ward off resistance when Mcl-1 needs to be inhibited.|
|HA14-1||Bcl-2 inhibitor||This one is cross-reacting with not just BH3, but also BH4 domain with possible efficacy of selective importance in DLBCL.|
|BM-1074 or BM-1197||Bcl-2/Bcl-xL selective inhibitor||When you see drugs such as these that belong to a class of drugs that inhibit Bcl-xL, platelets will likely be inhibited also.|
|Apo-G2 (Apogossypol)||Bcl-2 family protein inhibitor with exception of Bfl-1.||Apogossypol or its derivatives may extend usage to auto-immune or inflammatory conditions.|
Any of the above characteristics for the profiled drugs are meant to contrast and hi-light a few of their unique features and do not tell the whole story for how they might be used or how they work.
To sum up the current state of Bcl-2 antagonist drug development is to appreciate the dynamic nature of CLL biology in which these drugs must work. CLL biology is a complex arena of protein networking in which the druggable protein targets are in a complex “dance” of their own with many other cellular proteins. Not all Bcl-2 antagonist drugs are best used in CLL so efficacy for this class of drug will ultimately depend on the molecular profiling of each individual patient. It is well to remember that Bcl-2 antagonist drugs are not silver bullets but will likely be partnered with other drugs on the path for a cure.
It looks great for the future of the Galaxy. So, Mr. CLL Bear, see if you like dancing with these – Bim, Bid, Bad, – Bax/Bak and some NOXA/PUMA for good measure!!!;-)
Wayne Wells is a member of the CLL Society Patient Advisory Board: I was diagnosed with CLL on Sept. 11, 2006 and told I had a “good cancer” and as such, I might never need treatment. Testing using IGHV mutation status, FISH and CD38 all predicted an indolent disease course. The one test (ZAP-70), which CLL specialists said was inaccurate, was 58% positive contradicting the other good markers. My disease rate of increase marked by bulky lymphadenopathy indicated I was discordant to the favorable markers. I opted for a Phase Ib Clinical Trial with a new agent called PCI-32765 later renamed Ibrutinib. Since the end of June 2011 I have been on Ibrutinib with an ever deepening remission hovering just above a complete remission.
Originally published in The CLL Tribune Q3 2015.