What is BCL-2 and why should we inhibit it?

Another class of investigational treatments working their way through the system are the BCL-2 inhibitors (named as the "B-Cell Lymphoma gene number #2).  These investigational therapies will likely be relevant to patients with CLL or NHL.   They may actually be relevant to a number of cancers but CLL/NHL will be be where they get evaluated first. 

BCL-2 inhibitors have been a long sought goal for the pharmaceutical industry but they have had to try targeting the protein a number of ways before they found what looks to be the promising set of compounds that are moving forward now.

I thought a little biology might be fun - so let's talk about the mechanism by which these work.  Within a cell, there are a variety of "organelles."  In the same way you have a liver and kidneys, cells have their own organs - known as organelles.  These have names such as the golgi apparatus, endoplasmic reticulum, mitochondria, lysozymes, etc.  The mitochondria is the equivalent of the "power plant" for the cell.  It is where the vast bulk of energy is synthesized for the normal function of the cell.

In normal physiology, it is important to have a way to get rid of cells that you don't need anymore.  The main mechanism by which this "programmed cell death" occurs is called "apoptosis."  It turns out that the mitochondria plays an important role in apoptosis.  It contains a variety of enzymes known as caspases tucked away between two membranes.  When the caspases are released from the mitochondria into the cell, they function as the "cellular executioners" to bring about an orderly demise of the cell.

At the outside surface of the mitochondria, there is a constant battle between two sets of proteins.  These conflicting proteins create a balance between the pro-survival and pro-death signals.  Depending on environmental cues or external signals, the cells fate is played out.  The main "pro-survival" protein is known as BCL-2.

You might ask why your body doesn't just turn to mush when you take one of these drugs as it should release the death signal throughout mitochondria everywhere.  It turns out though that cancers are "addicted" to BCL-2 in ways that most other cells are not.  The inside of a cancer cell is not as friendly as other cells and they need some mechanism to protect themselves from the death signals they constantly receive.  In follicular lymphoma, pieces from two chromosomes (14:18) break apart to ensure that too much BCL-2 is produced.  In CLL the mechanism is less clear, but may involve genomic alterations in the 13q14 region (see my prior post on micro-RNA's).  In either case, there is too much BCL-2 protecting the mitochondria, so no matter what "death signal" (i.e.. chemotherapy, immunotherapy, etc) you send to the cell, it survives.

For this reason, pharma has long wished to create a good BCL-2 inhibitor.  The first attempt was a molecule known as Genasense.  This used a technology known as "anti-sense technology" which has not proven itself to be a good way of making a drug.  The next significant foray into BCL-2 inhibitors was a series of compounds generated by the folks at Abbott Labs.  Abt-737 was an IV formulation of what became the oral Abt-263.

Abt-263 has reported activity in CLL - but as is often the case with drugs, we discover things along the way we didn't expect.  In this case, there is a protein very similar to BCL-2 known as BCL-xL.  It is the main protein keeping your platelets going and when you inhibit BCL-xL platelets drop really fast.  This makes it unappealing to think about combining it with other types of CLL/NHL treatment and may prevent the drug from finding a home in our treatment choices.

The next research treatment coming down the pipeline is Abt-199 which also has the designation of GDC-0199.  This drug was designed by the same folks at Abbott that made 737/263.   Abstract 546 at the 2012 European Hematology Association details the clinical experience to date.  This link downloads a long PDF.  Best to use the search function after getting it open.

How I approach follicular lymphoma (part 1) - patients with low risk disease

A lot has changed in follicular lymphoma in the past few years. Quite a few items considered standard of care just a few years ago have been replaced by new standards. I thought it might be worth while putting forward my take on all of it. For the purposes of this post, I want to talk about patients with follicular lymphoma and "low risk" disease.

In order for us all to be on the same page about what constitutes "low risk" disease, I need to immediately introduce a scoring system called the FLIPI scale. It stands for "follicular lymphoma international prognostic index"

Follicular Lymphoma International Prognostic Index

This scale uses age > 60, stage III-IV, hemoglobin < 12, number of nodal areas > 4, elevated LDH. Patients with 0-1, 2, 3, 4-5 are grouped together in risk strata. Lower scores are better and higher scores are worse.

A good starting place for patterns of care in follicular lymphoma is the "lymphocare" study published in 2009. What is startling is how much has changed in the last three years.

Follicular Lymphoma in the United States: First Report of the National LymphoCare Study

The punch line from the paper is that in a mix of community practice sites (80%) and academic sites (20%), about 2800 patients were enrolled at diagnosis and followed in order to characterize treatment choices and outcomes. 18% were placed on "watch and wait," 14% were given rituximab alone, 52% were given rituxan with chemotherapy. When R-Chemo was given R-CHOP was given to about half, R-CVP to about a quarter, and R-Fludarabine to about 15%.

I find it unusual that so much has changed so quickly. For a disease like follicular lymphoma, this treatment mix has been turned on it's head by several high profile publications.

Let's start with "Watch and wait." For many patients, the idea of "watchful waiting" is a tough recommendation. I completely understand the sentiment that if you have cancer you ought to do something about it. The problem from a historical perspective is that we didn't necessarily have anything that really changed the long term prognosis. That historical precident was established for quite a few years by the BNLI study which started accruing patients a LONG TIME ago. In this study patients with advanced stage FL were randomized to watch and wait vs immediate chlorambucil.

Long-term effect of a watch and wait policy versus immediate systemic treatment for asymptomatic advanced-stage non-Hodgkin lymphoma: a randomised controlled trial.

This study found that with long term follow up, the folks who went on "watch and wait" actually did a little better than patients who got up front chlorambucil (keep in mind that chlorambucil is an "alkalating" agent which causes DNA damage). A generation of oncologists followed this paradigm for nearly 20 years. Unfortunately, chlorambucil is such a lousy drug (I only use it in the "very unfit patient population"). One point to highlight for later is that the average survival of either arm on this study was only between 5-6 years. Furthermore, this wasn't terribly different than the overall natural history of the disease.

The natural history of initially untreated low-grade non-Hodgkin's lymphomas.

Over time however, more effective regimens such as CVP, or CHOP came along. With more effective regimens, watch and wait remained a good option, but the pendulum moved back toward combination therapy. By the time of the lymphocare study in the mid 2000's watch and wait was a distinctly small minority. Having done my fellowship at Stanford with Ron Levy and even Saul Rosenburg, I have certainly watched and waited on quite a few patients.

The biggest change to this equilibrium has been the introduction of rituximab. Rituximab is not chemotherapy - it is "immunotherapy." It is an "engineered antibody." You make antibodies to flu, e. coli, etc. This is just an antibody against the lymphoma. I tell patients it coats the outside of the cancer cell and "focuses" the immune system. There may even be some degree to which it helps "train" the immune system to fight the lymphoma.

There are several important studies that have made this option more welcome in the minds of physicians. The first was a single arm phase 2 study of four doses of rituxan which showed an overall response rate of 70% with complete response of 30%.

Rituximab therapy for patients with newly diagnosed, advanced-stage, follicular grade I non-Hodgkin's lymphoma: a phase II trial in the North Central Cancer Treatment Group.

The second paper was a plenary session at ASH in 2010 which was a redo of the original chlorambucil study but was "watch and wait" versus rituxan. This study remains quite "young" so we do not have good long term data. We cannot yet say whether patients live longer etc. What we can conclude however is that giving rituximab certainly delays the need for subsequent chemotherapy.

An Intergroup Randomised Trial of Rituximab Versus a Watch and Wait Strategy In Patients with Stage II, III, IV, Asymptomatic, Non-Bulky Follicular Lymphoma (Grades 1, 2 and 3a). A Preliminary Analysis.

Finally, the Resort study presented at ASH 2011 was significant. This study took "low risk" patients and randomized to four doses of Rituximab versus four doses followed by one dose every three months forever. In the patients who only got four doses, they could get four more doses if their disease came back. Only those patients who had an initial response were followed long term (70%). So the study design is more about the best way to maintain long term benefit from rituximab with either "re-treatment" or "maintenance" in rituximab sensitive patients.

Results of Eastern Cooperative Oncology Group Protocol E4402 (RESORT): A Randomized Phase III Study Comparing Two Different Rituximab Dosing Strategies for Low Tumor Burden Follicular Lymphoma

The key findings was there was very little difference between the two arms in terms of how long a patient was sensitive to rituximab. There was a slight increase in patients needing chemotherapy in the re-treatment arm but not enough to really make thought leaders think that maintenance was the winner. Perhaps the most significant secondary finding though was that there were more patients doing well in either arm of this study than compared to historical controls by a long shot. There is danger in this sort of comparison because that is not what the study was designed to answer.

With these three studies however, you see more and more editorials asking if "watch and wait" is an outdated strategy. I don't think it is just yet, but I admit, my threshold for giving single agent rituximab is a lot lower than it was before and I think that is reflected in national practice patterns as well. If we swing back to the lymphocare study, I think the impact here has been that single agent rituximab use has gone up quite a bit. Instead of the 14% reported previously, I would wager it is somewhere between 25-35% of patients. This strategy has probably stripped out more of the "watch and wait" group as well as some of the other folks who would have previously gotten R-Chemo.

I think that "watch and wait" was probably underutilized in the lymphocare observational study and probably more so now. A lot of patients are less comfortable with extra CT scans due to the radiation concerns.

I think that rituxan monotherapy opens up a lot of interesting research options. It may prove to be a useful platform for new drug combinations in research. I am particularly excited by a study (which sadly I am not a part of) where it is revlimid-rituxan versus investigator choice of R-chemo regimen. I hope to see studies soon in which the design is rituxan versus rituxan / new agent.

In our next follicular lymphoma post, I will talk about what sorts or R-Chemo regimens I give when I need to reach for chemotherapy regimens.

13q Part Three - interpreting test results and understanding limitations

13q test results:

When FISH is ordered and a 13q abnormality is observed, there are a variety of ways it can be reported.  It is possible to have a deletion on one chromosome in many of the cells, a deletion on both chromosomes in some of the cells, or any combination of the two.  Typically the report will indicate the number of cells lacking one or both copies of 13q.

Up until recently, we were not totally clear how these differences influenced clinical outcome.  Several earlier papers seemed to indicate that having both copies of 13q missing was worse than having only one copy missing.  Unfortunately, some of these studies were limited by small sample sizes and may not have statistically accurately reflected what these results mean.

Genome-wide analysis of DNA copy number changes and LOH in CLL using high-density SNP arrays.

Biallelic deletion 13q14.3 in patients with chronic lymphocytic leukemia: cytogenetic, FISH and clinical studies.

More recently, several groups seem to have arrived at consensus that the actual percentage of cells lacking the 13q matters more than whether there is only one copy or two.  Different groups come up with different number thresholds, but that is because they need to make a categorical separation of data that is by nature a continuous variable.  It is probably safe to say that the more cells with 13q deletions the worse it is.

The prognostic difference of monoallelic versus biallelic deletion of 13q in chronic lymphocytic leukemia.

A comprehensive evaluation of the prognostic significance of 13q deletions in patients with B-chronic lymphocytic leukemia.

A high number of losses in 13q14 chromosome band is associated with a worse outcome and biological differences in patients with B-cell chronic lymphoid leukemia.

There are a number of things however that FISH simply doesn't tells us.  In some cases we know whether the missing data is important, in some cases, we have yet to figure it out.  FISH does not distinguish between the minimal deleted region (MDR), commonly deleted region (CDR) which contain Dleu7, or the less frequent larger 13q deletions that contain the RB protein (type II deletion).  Unfortunately FISH only tells us if there is a deletion or not - it doesn't tell us the size or what genes are included.

13q14 deletion size and number of deleted cells both influence prognosis in chronic lymphocytic leukemia.

The prognostic significance of various 13q14 deletions in chronic lymphocytic leukemia.

Functional dissection of the chromosome13q14 tumor-suppressor locus using transgenic mouse lines

Mutations have been found in the Rb protein in patients with CLL, but those are not detectable by FISH.  Other conditions where the cell tries to replace the missing material by making an extra copy of the remaining chromosome (aka uniparental disomy) are missed entirely.  Furthermore, the cancer cell can turn off the mRNA's through a process called methylation and even if you could figure all this out, it is not clear how these findings affect the biology.

For now we know that if you have a 13q deletion and NO OTHER FISH abnormalities, you are probably in good shape.  If you also have a 17p or 11q, those trump the 13q.  Hopefully we will learn how to handle the other information we know is there and create tests that let us sort it all out.