Engineering a broader bio-delivery system for gene therapy

The first gene therapies approved in the U.S. have undeniably transformed lives. Families describe the results with genuine wonder: Children who had been going blind can now see the moon for the first time, thanks to Spark Therapeutics’s Luxturna. Babies who would otherwise be paralyzed are crawling, rolling, and even learning to walk after treatment with Novartis’s Zolgensma.

These are truly accomplishments to celebrate, and they’re just the beginning. The accelerating pace of technological breakthroughs is dramatically increasing the number and kinds of diseases that are amenable to gene therapy. There’s a revolution afoot.

Luxturna and Zolgensma are both designed to deliver healthy genes to just a single location in each patient’s body: the eye (for patients with an inherited retinal disease) or motor neurons (for very small children with spinal muscular atrophy). The treatments are made using a highly modified adeno-associated virus (AAV) as a vector.

The challenge? Many diseases will not respond to a treatment delivered only to the eye, the liver, or one of the few other tissues that AAV vectors can target. For these diseases, we need a delivery system that can transport the transgene coding the therapeutic protein throughout the body – ideally even reaching hard-to-access areas, such as the brain and muscle.

"There’s a revolution afoot."

In recent years, we have seen compelling evidence that such a delivery system can be engineered.

The approach is called ex vivo gene therapy, meaning that the transgene incorporation takes place in the lab, rather than in the patient’s body. The process starts with the collection of the patient’s hematopoietic stem cells. Back in the lab, a highly effective vector, based on a significantly modified lentivirus, is used to transduce those cells with a transgene that encodes for a functional version of the enzyme that the patient lacks. Those engineered cells are then infused back into the patient.

This is an elegant solution for several reasons.

First, lentiviral vectors are excellent at working undercover. An AAV vector can effectively introduce transgenes into cells, as we’ve seen, but their genetic message (DNA) will remain floating in the nucleus -- an outsider, not integrated into the patient’s chromosomes. The lentiviral vector, on the other hand, seamlessly inserts its genetic message right into the patient’s DNA. That integration ensures durability: When the cell divides, each of the daughter cells will also carry the transgene.

As convenient as this sounds, there is a catch: For the treatment to take hold for the long-term, the engineered stem cells can’t just drift at random through the patient’s body -- they have to “engraft” in the bone marrow. In other words, they have to move in and take up residence.

Once installed, these engineered stem cells can mature into all the elements needed for healthy circulatory and immune systems: red blood cells, platelets, T cells, macrophages and more. The nucleated descendants of the engrafted modified stem cells become delivery trucks for the therapeutic protein by manufacturing, delivering and secreting it throughout the body, thereby providing deficient cells with a 24/7 supply of functional protein.

Even more exciting, there’s evidence that one kind of descendant, the macrophages, recognize and flock toward organs that have been damaged by the patient’s disease. If the heart needs extra help, the macrophages swarm to the cardiac tissue. If the kidneys are in trouble, they congregate there, producing the functional enzyme where it’s needed most.

Perhaps the best news of all is that cells carrying the transgene also engraft in the brain.

"The key question... Does it work? Data from a number of biopharma leaders and academic institutions suggest it does."

That is quite a feat considering that the human brain is protected by an intricate barrier of protective tissue. Known as the blood-brain barrier, it selectively moves key nutrients and selected proteins from the blood into the brain while blocking unwanted proteins, harmful toxins and immune cells that can interfere with brain function. This is a crucial protection in normal circumstances, but it makes it extremely difficult to get desperately needed protein-based therapies into the brain.

That’s why it’s so remarkable that the modified stem cells delivered via ex vivo lentiviral vector therapy have been shown to engraft not only in the bone marrow, but also to enable the sustainable repopulation of the microglia cell population in the brain. These new generations of modified microglia traffic throughout the brain, delivering functional enzyme directly to neural tissues.

The one tissue our ‘delivery trucks’ have a hard time reaching is muscle. That’s a common challenge for both gene therapy vectors as well as the enzyme replacement therapies (ERTs) typically used to treat lysosomal storage disorders, such as Pompe disease.

We at AVROBIO have developed innovative technology that we believe can overcome that limitation. In our preclinical development work for a potential Pompe therapy, we engineer the transgene to produce and secrete not just the functional protein the patient needs, but also to incorporate what’s known as a “GILT tag,” attached to the protein with a short linker. That GILT tag is designed to dock onto a receptor found on the surface of muscle cells in order to gain entry. Upon entry, the therapeutic protein does its essential work: it is designed to enable lysosomes in the cells to function properly, breaking down the toxic substrate that would otherwise accumulate and harm the patient’s health.

Thanks to the efficacy of this miniature manufacturing and trucking system, we believe that patients who receive lentiviral gene therapy treatment, if approved, can expect to have many billions of engineered cells in circulation for years – potentially decades – after treatment, their transgenes working around the clock to help produce the active enzymes the patients need to stay healthy.

This is the kind of delivery service we need to tackle more genetic diseases! It’s global, responsive – and a different way of thinking about gene therapy.

The key question, of course: Does it work?

Data from a number of biopharma leaders and academic institutions suggest it does.

Last spring, for instance, bluebird bio published strong interim data from two distinct ex vivo β-thalassemia programs in the New England Journal of Medicine. Fifteen of 22 patients followed for at least two years after treatment stopped needing regular red blood cell transfusions. The need for transfusions was significantly reduced in the majority of other patients.

Patients in these bluebird bio trials underwent “conditioning” before treatment with a drug called busulfan. This drug works by creating space in both the bone marrow and CNS compartments for the billions of infused stem cells that will soon be delivered. It’s an essential part of the process; the new cells need that space to engraft.

In another strong sign for this cutting-edge delivery system, Orchard Therapeutics this spring reported positive data from a trial evaluating a gene therapy for treatment of metachromatic leukodystrophy, or MLD, a life-threatening metabolic disease. Like the therapies mentioned above, this is an ex vivo, autologous, hematopoietic stem cell-based gene therapy. Children treated with the therapy scored substantially higher in gross motor function than children of the same age who were not treated; importantly, they also maintained cognitive performance scores within the normal range.

"We look forward to the day when the world will hear a new round of heartfelt stories from patients and families who have had their lives transformed by gene therapy."

Both bluebird bio and Orchard Therapeutics found evidence that the gene-corrected cells they delivered via their therapy had engrafted into the patients’ bone marrow. In other words, they had settled in, as intended. That suggests these gene therapies could prove durable, with new generations of the engineered cells continuing to circulate and secrete active proteins for many years to come.

If those two data points weren’t enough, here at AVROBIO, we recently announced interim data from ongoing trials of our investigational gene therapy for Fabry disease. Patients living with Fabry have a genetic mutation that interferes with the function of a crucial enzyme. When that enzyme does not work properly, substrates and toxic metabolites, including Gb3, a fatty substrate, build up in the patient’s cells. Initially the patients often experience pain and burning sensations, especially in the hands and feet, followed by an array of other symptoms. Our interim data included a kidney biopsy performed on the first patient in our Phase 2 study a year after his treatment with our gene therapy. It found an 87% reduction in Gb3 substrate. We also saw substantial and stable reductions in Gb3 in plasma in the first four patients in our Phase 1 trial.

You can see why we’re excited.

To be sure, we still have considerable work to do. Like our peers, we’re working hard to improve our lentiviral vectors, to streamline our manufacturing process and to fine-tune our conditioning regimen so patients can get maximum benefit with minimal side effects. We’re also continuing to advance our clinical trials, with the help of courageous patients and a terrifically dedicated group of physician investigators.

We firmly believe that we – and others in the industry – will be able to demonstrate the efficacy and durability of ex vivo lentiviral gene therapy built around transducing hematopoietic stem cells with transgenes encoding functional proteins. This delivery system, we believe, will prove a vital tool for expanding the reach of gene therapy to many more indications and many more patients in need.

We look forward to the day when the world will hear a new round of heartfelt stories from patients and families who have had their lives transformed by gene therapy.

Geoff MacKay is President and Chief Executive Officer of AVROBIO, Inc.  AVROBIO is currently conducting clinical trials to evaluate the safety and efficacy of its investigational ex vivo lentiviral gene therapies.  None of these investigational gene therapies have been approved by the U.S. Food and Drug Administration or any other regulatory agency.  For more information, go to www.avrobio.com.