Aaron T.T. Chuang Ph.D. Chief Scientific Officer Plasticell

From the Quagmire to Savior and Beyond

The recent regulatory approval of the first hematopoietic stem and progenitor cell gene therapy (HSPC-GT) signals the start of a new era for gene therapy and highlights the potential contribution by high-throughput cell culture technologies in propelling HSPC-GT from curing rare diseases to curing more common diseases.

Hematopoietic stem cells (HSCs) are the “fountain” for all blood cells that circulate in our bodies throughout life. Arguably, no other cell type has more profound and far-reaching influence on our well-being than HSCs. They reside in our bone marrow and continuously produce a variety of cells with vital tasks, for example, oxygenation via red blood cells, termination of bleeding via platelets, and immunity via leukocytes, which also provide immune defense to the central nervous system.

There is, however, a flipside to the pre-eminence of HSCs. When faulty HSCs emerge, devastating outcomes can ensue, such as autoimmune diseases like multiple sclerosis and blood cancers like leukemia. Thankfully, a solution to these life-threatening indications is well at hand because HSCs can be removed and replaced with healthy HSCs using HSC transplantation (HSCT)—a highly effective procedure pioneered by Nobelist E. Donnall Thomas over five decades ago.

Success with HSCT

To date, HSCT has already saved hundreds of thousands of lives, mainly of patients with life-threatening blood and bone marrow cancers. More recently, the therapeutic application of HSCT has been expanded to saving the lives of patients with hereditary disorders, such as lysosomal storage diseases, with the strong prospect of its application expanding further into the treatment of more common diseases. For example, a recent clinical study demonstrates the therapeutic benefits of HSCT in multiple sclerosis, thus pointing the way for the use of HSCT in autoimmune diseases.

Developing HSPC-GT

Built on the foundation of HSCT, HSPC-GT has been developing for over two decades. It is an approach in which a patient’s disorder, be it hereditary or sporadic, is corrected by ex vivo manipulation of the genetic content of HSCs to produce “therapeutic” HSPCs. These HSPCs are then administered to the patient, where they can engraft and remain in the recipient’s body to produce healthy blood cells throughout life. In this way, HSPC-GT can be curative through a one-off intervention.

In the current early phase of development, due to the high risk it carries, HSPC-GT has been approved for use in clinical trials of untreatable, rare, life-threatening diseases. Success in these “proof-of-principle” studies is anticipated to provide the springboard to the development of HSPC-GT for a wide range of more common diseases. Therefore, it is very encouraging that after about two decades of research and development, the first regulatory approval of an HSPC-GT has recently been granted by the European Regulatory agency to the ex vivo stem cell gene therapy Strimvelis , which was jointly developed by GlaxoSmithKline and the Telethon Institute for Gene Therapy in Italy. This is merely one in a list of HSPC-GT assets being developed by organizations such as Genethon, Bluebird Bio, and the recently formed Orchard Therapeutics.

Challenges for HSPC-GT

Despite such exciting advancements, significant improvement is needed to overcome hurdles that could prevent HSPC-GT from becoming available to the general public. A pertinent hurdle is the prohibitory economic burden of HSPC-GT. The current cost for a single treatment with Strimvelis is €594,000 ($672,675), a cost that faces a formidable challenge in obtaining reimbursement from payers. A priority in the continuing development of HSC-GT, therefore, is to reduce cost while simultaneously seeking to improve clinical safety and efficacy.

HSC-GT is a risky and complex multistep process. In its current form, namely autologous HSPC-GT, wherein the patient is the provider of the HSPCs for gene correction, the process can essentially be split into two parts. One part involves the preparation of the vectors for gene delivery into the recipient cells. This includes the genetic engineering of the vector for gene delivery, the creation of packaging cell lines for the production of the vectors, and the manipulation of the cells to maximize the yield of vectors. The other part involves the manipulation of the HSPCs, including the isolation of HSPCs from the patient, which are then treated ex vivo for gene transduction and subsequently returned to the patient for engraftment.

Improving HSPC-GT and HSCT

Cells are involved in most of the HSPC-GT process and opportunities for improvement exist at multiple steps of the process, some examples of which are discussed below.

The generation of vectors is performed in cell lines such as HEK293T, and it is feasible to optimize the cell culture conditions to maximize the process of vector generation. The step of gene transduction into HSPCs may also have room for improvement, considering that two rounds of transduction by vectors at multiplicity of infection (MOI) of 100 are required in the protocol for leading clinical-stage HSPC-GT assets.

Aside from the therapeutic gene, the quantitative aspect of HSPC engraftment in the patient is another critical factor for HSPC-GT and HSCT to be successful. It is well established from clinical studies that the effective cell dose is directly correlated with speed of hematopoietic recovery, immune reconstitution, long-term persistence of transplanted cells, and survival of recipients. Different methods for augmenting the effective HSPC cell dose are being attempted, including the enhancement of engraftment and the ex vivo expansion of HSPCs prior to transplantation. The ability of HSPCs to engraft could be enhanced, for example, by pretreatment with 16,16-dimethyl prostaglandin E2. Concomitantly, there has been intense research into the ex vivo expansion of HSPCs by companies that include Gamida Cell and Novartis, giving rise to reagents that induce varying multiples of HSC expansion.

These examples illustrate the opportunities and activities underway that are specifically aimed at improving HSPC-GT and HSCT. To help achieve these goals, suitable technologies are needed to identify best-in-class protocols for a variety of cell-based processes. Considering that the best protocols may be composed of multiple steps, flexible screening technologies with a dynamic range of capacity would be most suited to such a task. Potential platform technologies to meet such a need for high-throughput screening are beginning to emerge, including microfluidics, which provides computerized nanoscale culture systems, and combinatorial cell culture, which provides three-dimensional, bead-based cell cultures for screening thousands of protocols in a single experiment, with proven ability to identify improved cell manipulation protocols.

Such cell culture technologies bring new hope that improved cell culture protocols will be identified to streamline the complex process of HSPC-GT and so lower the cost, while also improving the safety and efficacy of the therapy, thus rendering this life-saving therapy a reality for all who can benefit from it.

Aaron T.T. Chuang, Ph.D. ([email protected]), is CSO at Plasticell.


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