Patrick McGann, MD, MS  

Pat, can you describe the steps people go through to do a bone marrow transplant to gene therapy? Set the stage to help people understand why busulfan and why we’re talking about mutations. Can you walk us through the whole gene therapy process?

 With transplant or gene therapy, the term transplant could be autologous, meaning your own cell gene therapy transplant. It’s not as if you’re transplanting a solid organ or a kidney. Sometimes patients get confused about this. It’s looks like a blood transfusion hanging when it eventually goes in. For a bone marrow transplant, the donor—whoever that is—either gets a bone marrow aspiration, where they get bone marrow cells taken from their bone marrow, or a medicine to rev up their blood cells and get blood taken just from their vein. The patient needs to get prepared because they need to get rid of all their sickle cells. They need to suppress their immune system, so they don’t reject this foreign cell, which is someone else’s.

We use strong chemotherapy. If you have leukemia, as a comparison, you need to kill every last leukemia cell, and you get blasted with really strong chemotherapy agents and strong immunosuppressive agents. This is usually a week or so before; the days are counted backward. The cells that go in your body are most vulnerable to infection to everything. It’s a dangerous time. That’s when complications come in. If it’s a transplant, you get infused with that donor’s bone marrow cells and hope it takes. It takes some weeks’ time for your body to take those new cells, and you’re often receiving antibiotics and getting transfused and sustaining it, because your bone marrow is still not working. Basically, your immune system is suppressed. It’s a tough time.

Transplant conditioning, as this regimen is called, has gotten a little less toxic—reduced conditioning is the term. But that’s still serious conditioning. Even though it’s reduced from what it used to be, it’s a relative term. Gene therapy is a little different because you’re giving back your own cell. The way gene therapy happens is it’s ex vivo, meaning they take it out of your body. There are different ways that this is being done. Many patients need to have a bone marrow aspiration or many procedures to take enough cells out of their bone marrow to take them to the lab to fix them. There have been new ways to do this with peripheral blood and a medication called plerixafor, which is much better than going to the operating room for these horrible procedures. 

Those cells are then taken to the lab and edited, or whatever the mechanism of gene therapy is. You still need to ablate your bone marrow to get rid of all your sickle cells. Because if you have any or many sickle cells in there, when you give back your edited gene cells, those will just take over. You still need to suppress that bone marrow. Because it’s your own cells, the immune suppression isn’t as much of a problem as it is with transplant. Still, it’s a week of pretty serious medicine—busulfan, traditionally—and you’re in the hospital for less of a period of time than transplant. It’s quite an ordeal. Similarly, it takes or doesn’t, and you monitor over time if that gene therapy has worked and whether it’s sustainable—cross your fingers—in the long term.

Basically, you have 2 options. The first option, you have to kill off their own bone marrow cells using chemotherapy. Then you give them somebody else’s bone marrow, like a blood transfusion. The stem cells from the other person finds its way into their bone marrow and then grows. Then you wait and see what happens. Do you fight it? Do you accept it? We know if it takes or not. For gene therapy, we take out the patient stem cells, take it to a laboratory. It’s usually in New Jersey—don’t ask me why. They manipulate it to pick out the gene they don’t want. Then they give that patient back their own modified stem cells and wait for it to grow. But you still have to wipe out that person’s bone marrow, so you don’t have this fight going on. Even though they’re your cells, they’re a little different with the new gene change that has been made. It’s a complicated process, and it’s really the only way to cure your disease. Either stem cell or gene therapy. We have had some setbacks in the past and more recently, but I believe that science is going to prevail. Over time we’re going to get to the point where we figure out the way to do this in the safest way to make it available to the most people with sickle cell disease and other blood disorders.

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A discussion of the course of gene therapy compared with stem cell therapy.  

Stem Cell vs Gene Therapy Processes in Sickle Cell Disease

 There are other gene therapy approaches. One is using CRISPR-Cas9, which has been used for 2 purposes. There are trials that use CRISPR-Cas9 to inactivate BCL11A in red blood cells. As you know, BCL11A is the main factor in this switch. It decreases the fetal hemoglobin expression and switches to the adult. If you disrupt this in red blood cells only, and you can do that by deleting an enhancer, then the fetal hemoglobin expression continues to be high. That’s the approach used by David Williams at Boston Children’s Hospital.

There was a publication of 7 patients with thalassemia and 2 patients with sickle cell with the same approach. It was a plenary session paper, and it showed early-stage efficacy. Now there’s another gene therapy approach that attempts to correct the sickle cell mutation. It’s being developed by investigators at Stanford University. What they try to do is correct the sickle mutation. What I have been able to find out, the study has not started. But in vitro, they have found that 70% of the CRISPR-Cas9 export cells have a correction, some correction of both alleles. In other words, you take an SS and turn it to an AA, which is the most desirable thing. For the remainder, there’s a single correction, which means you turn an SS to an AS. It remains to be seen how effective it is. Certainly, the occurrence of these malignancies in gene therapy trials cast some doubt. I don’t know what the outcome will be. Certainly, that’s a much-needed therapeutic approach. Hopefully, some safety measures will be able to be put in place, because they work. 

She looked at 40 patients and put us on a roller coaster, right? She showed us that 25 of 40 had been treated and followed for 12 months. And their hemoglobin count was really good, an 11 g/dL. With 5 of 11 coming from that new hemoglobin that you just got through talking about. 

The other thing that I didn’t mention, Wally, is that she looked at the crisis numbers. These patients had a median VOC [vaso-occlusive disease] and acute chest syndrome of 4 per year in the previous 3 years. And after that it was 0.

 Everybody got excited. You’ve turned an SS into a sickle cell trait, essentially. It was a whole bunch of patients, and we got excited. And what you said happened just a couple of weeks ago. It’s as if they’re running to the exit. Everybody’s going off; forget that. But at this point, we don’t know whether it was the Busulfan, the conditioning agent, or if it was something else that caused these problems. The jury is still out.

Experts review data around emerging gene therapy approaches in sickle cell disease. 

Emerging Gene Therapy Methods in SCD 

 This is where we talk about gene therapy and new drugs under development. I want to throw this out to Abdullah. In broad strokes, can you talk about what gene therapy is? How are we going to use it for the layperson who doesn’t understand all the technical stuff?

There are different approaches to gene therapy. When you say gene therapy, the most understood version is you take this sickle mutation and correct it, and the patient is cured. Well, that is now being tried. Before that, there were other interventions for gene therapy. The 1 that has been most used is the bluebird bio, which has an antisickling recombinant hemoglobin…just like fetal hemoglobin with a very significant antisickling effect. They’ve been using this in transfusion-dependent thalassemias. There was a talk at FSCDR [Foundation for Sickle Cell Disease Research Scientific Meeting] 3 or 4 years ago. They started in 2007 in France with transfusion-dependent thalassemias. Their patients are still transfusion independent. Following that, they started a phase 1/2 trial in sickle cell in the United States, HGB-206, and there are more being planned.

There were reports in the last ASH [American Society of Hematology Annual Meeting] about the efficacy. Alexis Thompson had an abstract. She summarized the results of 25 patients who had been followed for 12 months or greater. There have been several advances in this. No. 1, the company has improved its manufacturing process to have more effective introduction of the lentiviral vector into the CD34 cells. The second point is that initially they could not use GCSF in sickle cell because of the known deleterious effects. So they were harvesting the patient’s cells only through bone marrow biopsies. Then they introduced plerixafor, which was found to be safe. According to the report that Alexis summarized, these 40 patients at 12 months had a mean hemoglobin of 11.5 g/dL. The percentage of the antisickling hemoglobin median was 45%. That translates into a great therapeutic effect. And patients continued to be randomized. However, in 2018, 1 of the patients who had received this gene therapy 5 years prior developed myelodysplastic syndrome with monosomy-7. 

Tell our audience what myelodysplastic syndrome is.

Myelodysplastic syndrome is a bone marrow disorder. Sometimes it’s the precursor of an acute leukemia, where you do not produce enough blood cells for a variety of reasons. The marrow is crowded with cells, but it doesn’t result in an efficient production of white cells, red cells, and platelets for the most part. As I said, sometimes it can be a precursor to an acute leukemia. In this patient, it was.

After the data were carefully looked at, it was thought that this was not related to the vector. But it was related more to the conditioning therapy with Busulfan, which has been known to result in myelodysplastic syndromes, and that was fine. As of last month, February, there were 2 more patients: 1 with MDS, myelodysplasia, the other with an acute leukemia, with trisomy chromosome 8. The analysis in these is still incomplete as far as I know. But when this happened, the company suspended all its gene therapy trials. We’re at that point now, awaiting further analysis of karyotypes. One of the things they do in monitoring the effects of gene therapy is to look at the integration sites of these vectors to ensure that they’re not disrupting any gene or activating any oncogene or that kind of thing. In the first case, in 2018, there was no evidence of any clonal predominance of any integration sites. But in 1 of these cases, it appears that about 60% of the cells have integrated in 1 site and nobody knows what that site does, the importance of that site. But analysis is progressing.

An overview of gene therapy in sickle cell disease as well as clinical trials underway in the space.  

Patrick McGann, MD, MS

Articles

Stem Cell vs Gene Therapy Processes in Sickle Cell Disease

Emerging Gene Therapy Methods in SCD

Gene Therapy Approaches in SCD