Bluebird Bio's Lentiglobin - Is the Sky Falling? (ASH 2015 Abstract Review)

While I don't think the sky is falling for bluebird's LentiGlobin, sentiment is certainly changing, as the story "seems" to have shifted from "it can cure every patient" to "the therapy wears off over time and it doesn't work for everybody". However, I feel this is more a shift in sentiment than their data getting significantly weaker. I think people were way too optimistic if they were expecting cures for every patient, but the pendulum may have swung a bit too far to the negative side. So here I'll take a look at bluebird bio's most recently released data in their 2015 ASH abstracts and how the whole picture is coming together for the efficacy of LentiGlobin.

I've recently written a number of other posts on bluebird, mostly on LentiGlobin in sickle cell disease (SCD) and B-Thalassemia and their results presented at the EHA meeting in June. Check them out if you want additional context for this post:
Posts 1, 2, 3, 4.

The abstract that appears to be the main focus of people's attention is on further follow-up data on patients in the Northstar (HGB-204) study, which is enrolling patients with B-Thalassemia.

HGB-204 Abstract

So they've treated 10 patients with B-Thalassemia major by July 31st with LentiGlobin, 5 with the B0/B0 genotype, where no functional HbA hemoglobin is made, and 5 who are heterozygous for B0 and have one functional or hypomorphic copy of beta globin (BE etc.). The headline is that none of the 3 patients evaluable (>6 months follow up) who have B0/B0 became transfusion independent. However, the 4 evaluable patients with non-B0/B0 all have been transfusion free for >90 days. Up until now all patients treated (only 4 - 2 in the Northstar study, 2 in HGB-205) had achieved transfusion independence for at least 90 days of follow up. While perhaps some had expectations that every patient would achieve transfusion independence, that was always unlikely to be the case based on their previous data. Total hemoglobin needs to be at least >8 g/dl (probably better if 9-10 g/dl) to avoid transfusions, and looking back at their data from last ASH shows this might be right at the edge of what they're able to achieve with LentiGlobin for B0/B0 patients. Here's the data on their first 4 patients they presented last year:

3 patients had B0/BE (patients 1201, 1202 and 1102) and reached transfusion independence. You can see those patients are still getting a contribution in their total hemoglobin from HbE (from BE allele) and HbF (fetal hemoglobin). The B0/B0 patient, 1106, whom they also called transfusion free, only has HbF to add to their HbA-T87Q hemoglobin. While they were off to a good start with already 6.8 g/dl T87Q being made, it would probably need to get ~8 g/dl to achieve stable transfusion independence, assuming they continue to get some contribution from HbF.

It appears that this patient was one of the two B0/B0 HGB-204 patients that required a single transfusion, based on the new ASH abstract. I haven't heard many discussing this B0/B0 patient, and it would have potentially made for an even worse headline of having a previously "cured" patient no longer being cured. This was basically the headline a couple weeks ago when a patient treated with an older version of the T87Q expressing vector required a transfusion after 7 years of being transfusion free. I'll get back to my thoughts on that patient later.

Getting back to the more recent B0/B0 patient, I had hoped that her T87Q hemoglobin would increase enough to remain transfusion independent. You can see they still have some HbA (blue) from previous blood transfusions in their total hemoglobin, and this would need to be made up for by the patient's own hemoglobin after the transfused cells have completely gone. So it was probably premature to announce this patient as "transfusion free" (ASH 2014 PR), and it's probably best to wait until all the transfused HbA goes away. In any case, having a single transfusion over the course of about a year is still a significant reduction in transfusions considering the entry criteria for the study requires at baseline at least 8 transfusions per year or >100 mL/kg/year of transfusions. It's also possible the patient will become transfusion independent if their T87Q production goes up a bit more, which it has in some of their previously treated patients. So, I will be interested if their production of T87Q has increased, and by how much, since last year. Importantly, this patient had a relatively high vector copy number (VCN) in their CD34+ LentiGlobin product (1.5), so if they wanted to more consistently achieve >8 g/dl T87Q in B0/B0 patients they might need to achieve ~2 VCN, like in patient 1202, who has now been holding steady at around 9.7 g/dl T87Q for almost a year.

How tight a correlation between VCN, CD34+ cell dose, and the quantity of T87Q patients produce will become clearer as they continue to treat more patients. My general feeling is VCN will be more important for determining peak levels of T87Q. However, I could imagine low CD34+ cell doses might cause reduced chimerism - where a lower percentage of old hematopoietic stem cells get replaced by transplanted ones - and thus cause lower levels of T87Q production. So far, though, it seems like the VCN they see in the LentiGlobin product they administer is similar to the amount they see in patients' peripheral blood, so they are likely getting relatively good reconstitution:

Another thing I noted from the abstract was that a patient had relatively modest T87Q production (just 1.9 g/dl), as well as one patient having very low VCN in the CD34+ cell product they received (just 0.3 VCN). It will be interesting to see if the patient given the 0.3 VCN product is also the one producing the very low levels of T87Q, and if this is the B0/B0 patient who is still considered transfusion dependent. It's surprising a patient was treated with that low VCN, as I thought the original release criteria for LentiGlobin on this study was 0.5 VCN - page 43 here.

Turning, briefly, to the other 2 abstracts, which had less significant data updates, the abstract for the HGB-205 study includes the first sickle cell patient treated (previously reported data at this year's EHA meeting), and follow up data on patients 1201 & 1202, whom I mentioned above in the 2014 ASH data.

HGB-205 Abstract

All this data looks consistent with previous releases, both B-Thalassemia patients have maintained their T87Q production steady for over a year. The SCD patient's T87Q levels continued to increase now at 9 months compared to 6 month data at the EHA meeting. In SCD, the ratio of sickling hemoglobin (HbS) to anti-sickling hemoglobin (T87Q + HbF) is more important than the absolute quantity (g/dl) of T87Q produced. The levels bluebird believes will be sufficient to significantly reduce symptoms is 30% anti-sickling hemoglobin, which they have already achieved. I am a bit more conservative, as I had outlined in previous posts, about what percentage one might need to achieve, thinking it could be slightly higher than 30% depending on how evenly the T87Q hemoglobin is distributed throughout the patient's red blood cells. However, the ~50% anti-sickling hemoglobin achieved in this patient is impressive, and I would expect to be therapeutically beneficial. So far the patient does not have any reported no hospitalizations or blood transfusions. It is not surprising that the T87Q levels are leveling off somewhat. They're actually slightly higher than I would have expected at 48% now from 40% at 6 months. A major reason for the large increases in the %T87Q in the patient's blood from months 3-6 came from having their previously transfused blood cells (carrying HbA) die off, raising the relative amount of T87Q. So I am not expecting a significant increase in T87Q% when they present updated data at ASH. I will be looking to see the maintenance of T87Q levels, and if the other efficacy measures in this patient continue to look promising.

Lastly, the abstract for the HGB-206 trial contains only information on the LentiGlobin drug product that was infused into 2 patients with SCD.

HGB-206 Abstract

It's dangerous to mobilize hematopoietic stem & progenitor cells in SCD patients to enter the blood stream, which is how CD34+ cells were collected for the previously treated B-Thalassemia patients. So SCD patients need to have CD34+ cells harvested directly from bone marrow. This typically leads to lower amounts of CD34+ cells, as you can see the cell dose in these two patients is <3 x10^6 cells/kg, compared to 5.6 x10^6 cells/kg for the previous SCD patient, and normally 8 x10^6 cells/kg or higher in their previously treated B-Thalassemia patients. The first patient listed in this abstract also has a fairly low VCN of 0.5/0.6. It will be important to see how this low of a CD34+ cell dose affects the kinetics and total production of T87Q in these patients. For the patient with the 0.5/0.6 VCN dose (compared to 1.2/1.0 VCN for the SCD patient we have 9 month data on) they might produce significantly less T87Q. This patient may not make the 30% threshold bluebird has been using, however, an increase to 10-20% of anti-sickling hemoglobin could still have some therapeutic benefit.

Transfusions needed patient treated with original HPV569 vector
Another recent event was the disclosure that Subject 3, a B0/BE patient who had achieved transfusion independence after being treated with a previous version of bluebird's vector, required 2 transfusions after almost 7 years of transfusion independence. Since people were hoping for sustained transfusion independence, this news certainly sounded disappointing with potential implications for the duration of the efficacy for this approach. However, if we look at the levels of T87Q and total hemoglobin this patient had, they were always on the borderline of needing a transfusion, making only 2.7 g/dl T87Q and having total hemoglobin levels fluctuating between 8 and 9 g/dl over the past 5 years.

Importantly, bluebird noted in their press release that the levels of T87Q and VCN levels in peripheral blood have remained "largely unchanged". Unfortunately, the data have not been released to see exactly what "largely unchanged" means, but I doubt there was a sudden drop in T87Q based on the modest but relatively stable levels of T87Q the patient was producing. It does warrant keeping an eye on to see if T87Q levels do drop over long time periods. If there had been silencing of the viral construct, or perhaps an immune reaction against cells containing the transgene, one would have imagined there would have been a preferential drop in T87Q compared to total hemoglobin levels, which does not seem to be the case here. It's unclear how long viral gene expression can last in cells, but there is some data for transduced T cells that it can continue to last over 10 years, and potentially longer. 1st generation retrovirally transduced CAR-T cells still can be found with CAR expression more than a decade later. Additionally, there is another presentation at ASH where the authors retrovirally transduced a Thymidine Kinase safety switch into transplanted T cells, and they also saw that expression can be maintained for over 10 years. However, it will take further follow up in more patients before we get a better sense of the duration of efficacy for bluebird's LentiGlobin.

Sangamo also brought up in their 3rd quarter conference call concerns about the lentiviral integrations in this patient, and clonal dominance. Previously, cells containing a viral integration near HMGA2 grew in prevalence, a potential concern suggesting preferential growth of this clone. However, that clone ended up decreasing over time:

Other clones have increased in prevalence among the vector-modified cells recently, but bluebird suggests this is potentially due to random increases and decreases in prevalence over time, which could be exacerbated by the fact that so few cells were efficiently transduced such that, inherently, any given clone could grow or shrink considerably. There were only approximately 100 integration sites for the original patient treated, whereas there were many more in the more recently treated patients. An integration site analysis of their more recent patients suggests they are currently maintaining a very polyclonal population of engrafted cells:

 

From https://twitter.com/BioTerp/status/610431838151143425

Additionally, other self-inactivating (SIN) lentiviral vectors similar to bluebird's are also showing improved safety compared to earlier gamma-retroviruses so far, as well as signs of efficacy. A number of them are being presented at the same ASH session as bluebird:
- Safety and Clinical Benefit of Lentiviral Hematopoietic Stem Cell Gene Therapy for Wiskott-Aldrich Syndrome
- Gene Therapy Using a Self-Inactivating Lentiviral Vector Improves Clinical and Laboratory Manifestations of Wiskott-Aldrich Syndrome

- Lentiviral Hematopoietic Stem Cell Gene Therapy for Older Patients with X-Linked Severe Combined Immunodeficiency

Safety & efficacy data in Wiskott-Aldrich Syndrome was also published here with integration site analysis.

Continued safety, in particular the risk of insertional oncogenesis, is still a significant consideration, but I think the data so far suggests these SIN-lentiviral vectors are likely considerably safer than earlier gamma retroviral ones.

Conclusions:
In conclusion, I don't think the data presented here is out of line with bluebird's previously released data, but expectations were probably too high. LentiGlobin, with the average VCN and CD34+ cell dose given so far seems to be able to consistently induce transfusion independence in B-Thalassemia patients with B0/BE. In B0/B0 patients, the average VCN and cell dose do not seem to consistently be able to produce sufficient levels of T87Q to achieve transfusion independence, but will still likely be therapeutically beneficial and cause transfusion reductions. If bluebird can more consistently treat B0/B0 patients with VCNs on the higher side of what they've been able to achieve so far, it's possible they could achieve transfusion independence. It's also possible that currently treated patients will have T87Q levels increase over time to those sufficient for transfusion independence, but I'm not expecting large increases 6 months post-treatment. I think we will continue to learn considerably more about the relationship between VCN, CD34+ cell dose, and the kinetics of T87Q production from their upcoming presentations at ASH and in subsequent data updates. Bluebird has stated that only about 1/3rd of B-Thalassemia major patients are of the B0/B0 genotype, and their therapy will still probably be beneficial to those patients by reducing transfusion frequency, if not transfusion independence. Their data in the first SCD patient looks very promising, but the follow-up is still early, and subsequent SCD patients may have been treated with less efficacious doses. In total, I still think bluebird's approach appears to be of significant therapeutic benefit if the results they've seen so far continue.

None of my blog posts are intended as investment advice, and I intentionally don't typically go into valuation, I normally focus only on my interpretation of the data. There are, however, additional approaches for treatments in B-Thalassemia and SCD that are being presented at ASH that look promising as well. So I'll briefly look at a couple below:

Additional approaches in B-Thalassemia and SCD being presented at ASH
There are a number of other companies with interesting therapeutic approaches to B-Thalassemia or SCD that will be presenting pre-clinical and clinical data at ASH as well, they include:

Bellicum - Allogeneic HSCT is curative in patients with B-Thalassemia and SCD, in addition to other monogenic blood diseases and also is potentially curative in the treatment of hematologic malignancies. Bellicum is trying to make allogeneic transplants safer by adding back T cells containing their inducible caspase safety switch to T cell-depleted allogeneic transplants. Adding back T cells potentially can improve early immune reconstitution and hopefully reduce potentially lethal infections. Their safety switch is added to improve the safety of having donor T cells in the transplant, so T cells can be depleted if they begin to cause GVHD. Bellicum will be presenting at ASH on this approach, including in patients with B-Thalassemia. They have stated already that 4 patients with B0/B0 Thalassemia have been treated without complications and are transfusion independent. It's not surprising the transplant is curative, but the main things I will be looking for are levels of infection (and maybe measures of virus-specific T cells), as well as instances of GVHD and how well it's handled by activation of the suicide switch. In a number of patients from earlier studies, the approach has worked quite well in resolving GVHD rapidly, while surprisingly maintaining antiviral immunity post-depletion, published here.

For me, I have not researched enough to know for how many patients this would shift the risk:reward balance in favor of doing HSCTs, but I think how Bellicum's approach might effect this balance will become clearer as their data matures.

They have 3 ASH abstracts on this approach:
- BPX-501 Cells (donor T cells transduced with iC9 suicide gene) Are Able to Clear Life-Threatening Viral Infections in Children with Primary Immune Deficiencies Given Alpha/Beta T-Cell Depleted HLA-Haploidentical Hematopoietic Stem Cell Transplantation (haplo-HSCT)
- Clinical Outcome after Adoptive Infusion of BPX-501 Cells (donor T cells transduced with iC9 suicide gene) in Children Given Alpha/Beta T-Cell Depleted HLA-Haploidentical Hematopoietic Stem Cell Transplantation (haplo-HSCT): Preliminary Results of a Phase I-II Trial
- Immune Reconstitution after Adoptive Infusion of BPX501 Cells (donor T cells transduced with iC9 suicide gene) in Children Given Alpha/Beta T-Cell Depleted HLA-Haploidentical Hematopoietic Stem Cell Transplantation (haplo-HSCT): Preliminary Phenotypic and Functional Results of a Phase I-II Trial

Sangamo - Sangamo is targeting an erythroid-specific enhancer that controls BCL11A expression. BCL11A restricts fetal hemoglobin levels, and mutations that affect BCL11A, or an erythroid-specific enhancer, have been shown to cause higher levels of fetal hemoglobin in patients. Sangamo is using their zinc finger approach to try to disrupt one or both copies of the enhancer that controls BCL11A erythroid expression. BCL11A is not just expressed in erythroid cells, so targeting the erythroid-specific enhancer is probably a better therapeutic approach. This approach looks promising as an alternative way to induce therapeutically beneficial levels of fetal hemoglobin in gene-modified autologous transplants. They will be presenting a number of preclinical studies on the approach in oral presentations at ASH:
- Clinical-Scale Genome Editing of the Human BCL11A Erythroid Enhancer for Treatment of the Hemoglobinopathies
- Genome Editing of the Bcl11A Erythroid Specific Enhancer in Bone Marrow Derived Hematopoietic Stem and Progenitor Cells for the Treatment of Sickle Cell Disease

Some background papers on BCL11A and fetal hemoglobin:
2008: Human fetal hemoglobin expression is regulated by the developmental stage-specific repressor BCL11A.
2011: A functional element necessary for fetal hemoglobin silencing. 
2011: Correction of sickle cell disease in adult mice by interference with fetal hemoglobin silencing.
2013: An erythroid enhancer of BCL11A subject to genetic variation determines fetal hemoglobin level.

Global Blood Therapeutics - will also be presenting data at ASH on their oral small molecule drug for SCD.
GBT440, a Potent Anti-Sickling Hemoglobin Modifier Reduces Hemolysis, Improves Anemia and Nearly Eliminates Sickle Cells in Peripheral Blood of Patients with Sickle Cell Disease



Disclosure: I own shares of Bluebird and Bellicum, no position currently in Sangamo or Global Blood Therapeutics.