Monthly Archives: August 2013

Gilenya and PML: cause, effect and risk

Today the FDA followed up on a case of progressive multifocal leukoencephalopathy (PML) in a multiple sclerosis (MS) patient, first reported in July, by announcing an investigation. FiercePharma spelled it out as follows: 

Novartis’ MS drug Gilenya tied to rare brain infection FDA sends out alert
That’s a pretty scary headline, and the news drifted across twitter streams like a dry fly on a light tippet. Novartis quickly declared that there was no indication that Gilenya caused the rare brain infection, the patient was on co-medications, had been given intravenous steroid treatment, etc. etc. 

Lets back up. 

PML is caused by an infection of the JC virus within the central nervous system (CNS). PML is common and almost always benign, unless it occurs in a patient who in severely immunosuppressed. Of note, PML first came to public attention in the early days of the AIDs epidemic, appearing in HIV patients whose disease had progressed to the point that they had no more T cells in circulation. T cells are a critical arm of the immune system, that, along with B cells and other cells, control pathogen infections, including viral infections.   

MS, as readers know, is a chronic inflammatory autoimmune disease in which T cells, B cells and other components of the immune system engage in a deranged attack on the CNS, particularly targeting the myelin and other proteins that encase neurons in a sheath (thus, the myelin sheath). Neurons stripped of myelin do not signal properly and die off pretty quickly. The disease occurs in flares, geographically localized. The product of a flare is a “lesion” within the CNS, where the effect of demyelination and neuronal die back can be visualized by an MRI image. 

So, immune system cells, particularly T cells, are necessary for protection from JC-virus induced PML on the one hand, and cause MS flares and lesions on the other hand. It should be noted that more flares = more lesions = MS relapse or progression = more disability. 

The interplay of immune defense or immune surveillance and the pathology of MS first came to light with the development of two powerful immune modulating agents: Tysabritmand Rituxantm. Tysabri is a monoclonal antibody to the alpha4beta1 integrin (thus, a4b1) developed by Elan and licensed to Biogen. Tysabri is the single most potent agent developed to date for the treatment of MS, and its mechanism of action suggests why this is so. T cells use the a4b1 integrin to migrate out of circulation and across cell layers like endothelium. To gain access to the CNS, T cells must cross an endothelial cell membrane, a layer of basement membrane and then an epithelial cell membrane. This collection of barriers between the circulation and the CNS is called the blood-brain barrier. Importantly, a4b1 is necessary for efficient trafficking of T cells out of circulation and into the CNS (also into the lung, the gut mucosa and other tissues). When a4b1 is blocked by treatment with Tysabri, which binds and neutralizes a4b1, T cell movement out of circulation is effectively stopped. Indeed one of the first effects of Tysabri treatment is a rapid increase on the white blood cell count, as T cells excluded from tissues begin to accumulate in the bloodstream (and lymph). 

Tysabri treatment reduces signs and symptoms of MS, reduces relapses, number of flares, time to flare, number of lesions, and disease progression. Tysabri treatment also puts patients at a low, but still troubling, risk of developing PML, since the population of T cells excluded from the CNS also include those T cells that normally would control JC virus. In this case the mechanism of action (MOA) of Tysabri contributes to the risk of PML, and this can be seen in patients treated only with Tysabri (as monotherapy) as well as in patients taking multiple medications. The risk is handled in several ways: patients are monitored for the presence of antibodies to PML, that indicate exposure and likely the presence of quiescent virus in the patient, and by temporarily suspending Tysabri use for several months, usually after 2 years and intermittently thereafter. Its not a well defined art at this point, but because of the efficacy achieved with use of Tysabri, most patients and their physicians tolerate the risk, now estimated at about 1% across all patient groups. 

PML in Rituxan treated patients presents very differently. Rituxan is an antibody that binds to CD20 expressed on the surface of B cells, and this binding causes the B cells to be depleted. The antibody was developed by Idec and acquired by Biogen when those two companies merged to become Biogen Idec. Before Rituxan was essentially removed from the clinical MS landscape – a truly scandalous story for another time – it had undergone clinical trials in MS, showing excellent efficacy and limited toxicity, which did not include PML. Rituxan causes PML very rarely, and almost exclusively in lymphoma patients, who have been treated previously or concurrently with chemotherapy. In other words, PML was seen in heavily immunosuppressed cancer patients. Rituxan is also used in rheumatoid arthritis (RA) and a host of other inflammatory and autoimmune indications, and the risk of PML in these patients taking Rituxan is at least 10-fold lower than in lymphoma patients taking Rituxan. This is because RA patients are much less immunosuppressed than cancer patients. What we see then with the risk of PML from Rituxan is that the risk is really dependent on the prior immune status of the patient. In this sense you would not make the claim that Rituxan causes PML, but rather that it contributes to the risk of developing PML. IN the same way, Tysabri treatment doesn’t cause PML, but also contributes the risk of developing the disease. It is the case that the risk is higher with Tysabri than with Rituxan. 

Why Rituxan treatment allows JC viral escape and the occurrence of PML is a result of the depletion of the B cells that Rituxan targets. B cells make antibodies to pathogens, including anti-JCV antibodies, and this contributes to the control of virus by clearing both free viral particles and infected cells. Further, B cells supply “help” to T cells by secretion of cytokines and other factors that induce T cell survival, proliferation and function. So, T and B cells together are required for efficient JC virus control, although the available data suggest T cells are more directly critical. 

OK, back to Gilenya. I guess it was 4 or so years when the phase 3 data was coming out that I had a conversation with Jeff Browning, then at Biogen Idec and now at BU, about the results. He said something along the lines of, “sure, impressive results, but wait until they start to get PMLs.” 

Prescient, and certainly a well informed view. Jeff, myself, Paula Hochman (all from Biogen) and Reina Mebius (a key collaborator) along with a host of colleagues both inside and outside the company, had reinvigorated the study of lymph node biology in the mid-90s by unraveling the mechanisms by which these organs develop (search PubMed if interested), and as the field literally burst with new findings, attention turned to the control of lymphocyte (T cell and B cell) movement through the lymphatic system, within which lymph nodes are the critical organs. Lymph nodes (LN) provide the essential microenvironment for surveillance of tissues, including the CNS. Many labs unraveled the biology of cell traffic through LN, but it was Jason Cyster at UCSF who discovered that an agent in clinical trials called FTY720 functioned by shutting down the molecular pathway that allows T cells to exit LNs. The molecular pathway was driven by a bioactive lipid called sphingosine 1 phosphate (S1P). S1P bound and signaled through a class of receptors (the S1P receptors) and S1P receptor 1 was shown to be critical for T cell egress from LN. 

FTY720 became Gilenya. Thus, the mechanism of action of Gilenya is eerily similar to that of Tysabri – it controls the trafficking of T cells, in this case by trapping them inside of lymph nodes (B cells as well, although not so completely). The fact that Gilenya treated patients do not develop lymphadenopathy(LN enlargement) suggests that these trapped T cells die off. Gilenya treated patients experience a rapid drop in circulating white blood cell number – the opposite of what is seen in Tysabri treated patients – but the net effect is the same: T cells do not access the CNS, and therefore the signs and symptoms of MS are reduced, the relapse rate drops, the number of new lesions drops, etc, more or less similar to Tysabri treatment, albeit somewhat less effective. Gilenya has a host of non-immune system toxicities, which we won’t discuss here, but until July, Novartis, who developed the drug, had reported no patients with PML. 

Now, back to the headline. What Jeff Browning had recognized was that by its’ mechanism of action of Gilenya should increase the risk of PML, just as Tysabri increased that risk. Both stopped T cells from entering the CNS, by different mechanisms, and therefore both should prevent immune defense against JC virus in the CNS. Furthermore, the degree of risk should be proportional to the extent to which the exclusion of T cells was achieved. Since the efficacy of the drug should also be proportional to the extent to which t cells were excluded from the CNS . . . it follows that efficacy and risk of PML should be correlated. 

When Novartis states that Gilenya does not cause PML, they are being accurate, as accurate as when we state that Tysabri and Rituxan do not cause PML. More importantly and usefully, Gilenya is now hypothesized to be associated with the risk of developingPML, although it’s certainly a low risk. More specifically, the null hypothesis should be that Gilenya is associated with PML risk, based on its’ mechanism of action, and regulatory agencies and physicians should take note accordingly. 

We’ll watch out for other cases and comment as the clinical landscape develops. 

As always, stay tuned. 

 

Apoptosis Induction for the Treatment of B Cell Lymphomas: Update on AbbVie’s Bcl-2 Inhibitor ABT-199.

Lost in the fanfare surrounding the BTK inhibitor Ibrutinib and the PI3Kdelta inhibitor Idelalisib at this year’s American Society of Clinical Oncology meeting (ASCO 2013) was some rather stunning data from AbbVie on their Bcl-2 selective inhibitor ABT-199.
ABT-199 binds to Bcl-2 in such a manner as to prevent this protein from interacting with pro-apoptotic proteins such as BAX and BID, thereby allowing them to attach to the mitochondrial membrane, induce cytochrome-c release and trigger apoptotic cell death. Tumors disable the Bcl-2 pathway in a number of clever ways. Many tumor types delete p53, a protein that normally up-regulates expression of BAX and BID and down-regulates expression of Bcl-2 and related proteins (BclxL, Mcl-1, Bcl-w, etc). Tumors with this deletion phenotype are termed 17p(del) referring to the site of chromosomal alteration. Such tumors are generally very aggressive, resistant to chemotherapy and radiation, and associated with poor prognosis.
B cell lymphomas constitute a diverse collection of lymphomas and some leukemias (the distinctions have become blurred). B cell lymphomas have historically been classified as Hodgkin lymphomas or the very diverse Non Hodgkin lymphomas (NHL). The term NHL is confusing as this covers chronic lymphocytic lymphoma (CLL), mantle cell lymphoma (MCL), diffuse large B cell lymphomas of 2 subtypes (DLBCL-ABC and DLBCL-GCB), indolent Non Hodgkin lymphoma (iNHL) and a host of other tumor types. A baffling mix of biology and acronyms, but simply put, all of these tumor types are derived from various stages of B cell development, and most require signaling through the B cell receptor (BCR) to survive. The BCR triggers proliferative and survival (anti-apoptotic) signals, and therefore Bcl-2 becomes a relevant target in NHL.
Not surprisingly, genetic evaluation of B cell lymphomas has revealed mutations in Bcl-2 or downstream of Bcl-2 that foster lymphoma cell survival. Examples include constitutive activation of Bcl-2 in follicular lymphoma (FL) and CLL, amplification of Bcl-2 in MCL, down-regulation of NOXA (another pro-apoptotic protein) and BIM (a protein that regulates Bcl-2 activity) in iNHL and CLL, and down-regulation of expression of the caspases, part of the apoptotic machinery triggered by mitochondrial cytochrome-c release, in CLL.
ABT-199 was developed as a follow-on to the earlier Abbott compound navitoclax (ABT-263). Navitoclax inhibited multiple Bcl-2 family members, including BclxL, and was associated with severe thrombocytopenia (platelet loss) in clinical trials. Bcl-xL is an obligate pro-survival protein for platelets and thrombocytopenia became a dose limiting toxicity in navitoclax clinical trials. ABT-199 has a much lower affinity for BclxL than for Bcl-2, and spares other proteins in this family, as shown in this data reproduced from a recent AbbVie paper (Souers et al. 2013. Nat Med 19: 202-210):
                                 TR-FRET Ki (nM)
                      Bcl-2    Bcl-xL    Mcl-1    Bcl-w
navitoclax       0.044    0.055      > 224     > 7
ABT-199      < 0.01     48          > 444     245
What is important to note here is the relative difference in potency of ABT-199 against Bcl-2 (less than 10 picomolar, an unbelievable potency number) vs 48nM against BclxL. Taken at face value this is a 4800 fold difference in target potency. As noted below however, the number may be deceiving given the drug exposure achieved in patients.
AbbVie presented several studies at ASCO, including their Phase 1b trial in CLL. What is striking about the data is the response rates, as these compare favorably with similar data shown recently for BTK inhibitors and PI3K inhibitors, from Gilead (Idelalisib), Infinity (IPI-145), Celgene (CC-292) and J&J (ibrutinib). A snapshot of results from the CLL trials is shown below.
drug 

ABT-199 

Ibrutinib 

CC-292 

Idelalisib 

IPI-145 

pathway 

Bcl-2
BTK
BTK
PI3Kd isoform
PI3Kd/g isoforms
Partial response (PR) 

65%
68%
40%      (n = 5)
39%
55%
Complete response (CR) 

18%
2%
0%
0%
0%
Overall response (OR = PR + CR) 

84%
71%
60% (n=5)
39%
55%
OR in 17p(del)
81%
68%
NA
NA
50%
The table is adapted from results presented at ASH 2012, ASCO 2013 and this year’s European Hematology Association meeting (EHA 2013), as generously posted on Twitter by @andybiotech. ABT-199 differentiates from the competing drugs in several critical ways. First the OR rate reached 84% and was not statistically different between CLL patients having or not having the p53 deletion, ie.17p(del). Second, the CR rate was 18%, meaning that this many patients absolutely cleared disease from the bloodstream, lymphatic fluid, and lymph nodes. In addition AbbVie stated at ASCO that in the CR group, 11% had complete recovery of the bone marrow and 7% had partial bone marrow recovery – these are responses that are not seen with BTK or PI3Kg/d or PI3Kd inhibitors.
So why did the fanfare around NHL treatment coming out of Chicago in May during ASCO 2013 not include ABT-199? The answer was toxicity, and this toxicity came in 2 distinct forms. One has been addressed recently, the other is somewhat complex.
Tumor Lysis Syndrome (TLS) is a drastic physiological response to the sudden and acute destruction of massive numbers of tumor cells, more or less simultaneously. The disgorging of massive quantities of intracellular material triggers acute physiologic response as the heart, kidney, spleen and other organs cope with a titanic overload of potassium, calcium, phosphate, and uric acid. Organs fail and circulation collapses the patient drops quickly toward death. ABT-199 caused this syndrome in the first 3 patients that were dosed, each having been given 200mg of drug. One died. Faced with this spectacular disaster AbbVie immediately halted all ABT-199 clinical trials. That was back in January or February of this year. So the update at the end of May was tempered by this ongoing toxicity issue, and what AbbVie had to say at that point didn’t help. Starting with a lower dose of 50 mg they had restarted all trials, and gotten the drug into 34 CLL patients. Three experienced TLS, 1 died, 1 lost renal function (acute renal failure, generally meaning a lifetime of dialysis or a transplant). In addition, other toxicities were present, as seen in the next table showing severe toxicities (grade 3+) in CLL clinical trials. URI: upper respiratory infection. NA: data not available
drug 

ABT-199 

Ibrutinib 

CC-292 

Idelalisib 

IPI-145 

pathway 

Bcl-2
BTK
BTK
PI3Kd isoform
PI3Kd/g isoforms
Infections 

2% (URI)
18% (pneumonia
NA
19% (pneumonia)
NA
Liver damage (AST/ALT) 

NA
NA
NA
4%
6%
thrombocytopenia 

11%
5%
0%
NA
NA
neutropenia 

38%
18%
18%
NA
26%
anemia 

7%
5%
9%
NA
0%
The table is adapted from results presented at ASH 2012, ASCO 2013 and EHA 2013, as generously posted on Twitter by @andybiotech.
Where really jumps out here is at the level of thrombocytopenia and neutropenia seen in patients receiving ABT-199. Its worth noting that patients taking any of these new drugs are already at risk for decreased cellularity due to chemotherapy, and so adding to this burden complicates their subsequent care. Neutropenia is a bona fide pathway toxicity as demonstrated by defective expression of Bcl-2 in Kostmann syndrome (severe congenital neutropenia) and so there is nothing to do about this but lower drug exposure, potentially at the expense of efficacy. An interesting question in this regard is whether the thrombocytopenia and anemia seen are due to residual inhibition of BclxL. If you look at the PK data presented from the CLL trial, its pretty clear the effective concentrations of drug are far in excess of what is needed to inhibit BclxL. Taking data presented by Seymour et al. at ASCO 2013, abstract #7018, its clear that the maximum concentration achieved after multiple doses (Cmax(ss)) is up to 2mg/ml, which is going to give low mM exposure. So, at Cmax and for some time thereafter, drug concentration exceeds that required to inhibit BclxL by as much as 50-fold. Whether the duration of inhibition is sufficient to induce thrombocytopenia and anemia in patients is not known.
Lets go back to TLS, as this is the real show-stopper. On their July 26, 2013 earnings call, AbbVie stated that the dosing regimen has been further modified (details not clear). AbbVie further stated “With regard to tumor lysis syndrome which is a direct consequence of the explicit potency of 199 we have been enrolling CLL patients with a revised dosing schedule where we start at a lower dose and ramp up at a more slow rate, and so far so good with regard to the patients that we have been treating under that new protocol.” It is beginning to look as if AbbVie is learning how to manage TLS, and therefore have the potential to get their drug back in the fast lane.
Why does all this matter? Selective Bcl-2 inhibition is a unique strategy and ABT-199 illustrates compelling activity. In the era of rapidly advancing combination therapy for NHL, this could be an important component of the evolving treatment paradigm. With a lower dose they will try to avoid TLS and perhaps blunt thrombocytopenia and neutropenia. In the near term this drug may play a prominent role in patients who are at high risk (17p(del) or who are refractory to other targeted therapies. More interestingly, Bcl-2 inhibition in combination therapy may be a breakthrough treatment paradigm, and in this regard, AbbVie is swinging for the fences with trials combining ABT-199 with R-CHOP (Rituxan + chemotherapy) in pursuit of achieving outright cures. Combine this with genetic profiling of Bcl-2 and related proteins across many tumor types, and you have a very interesting story. 

This is a drug to watch, and just, perhaps, to improve upon.
As always, stay tuned, and follow us on Twitter @PDRennert.