How haematopoietic stem and progenitor cell gene therapy has gone from quagmire to saviour and beyond
The recent regulatory approval of the first haematopoietic 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 more common diseases.
Haematopoietic stem cells (HSCs) are the source of all the blood cells that circulate in our bodies throughout life. Arguably, no other cell types have a more profound and far-reaching influence on our well-being than HSCs. They reside in our bone marrow to continuously produce a variety of cells with vital tasks, such as the oxygenation of red blood cells, the termination of bleeding via platelets, and immunity via leukocytes that also provide immune defence to the central nervous system.
There is, however, a flipside to the pre-eminence of haematopoietic stem cells. When faulty HSCs emerge, devastating outcomes ensue, such as autoimmune diseases including multiple sclerosis and blood cancers like leukaemia. Thankfully, a solution to this situation is upon us: defective HSCs can be removed and replaced with healthy ones to tackle these life threatening indications by transplantation (HSCT) — a highly effective procedure pioneered by the late E.D. Thomas more than 50 years ago.
To date, HSCT has already saved hundreds of thousands of lives, mainly those of patients with life-threatening blood and bone marrow cancers. More recently, the therapeutic application of HSCT has 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.
Built on the foundation of HSCT, HSPC-GT has been developing for more than two decades. It’s an approach in which a patient’s disorder, be it hereditary or sporadic, is corrected by the ex vivo manipulation of the genetic content of the HSCs to produce ‘therapeutic’ HSPCs. These HSPCs are then administered into the patient, upon which 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, owing to the high risk it carries, HSPC-GT has only 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 for the development of HSPC-GT for a wide range of more common diseases. It is therefore 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 Medicines Agency to Strimvelis, 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 organisations such as Genethon, BluebirdBio and the recently formed Orchard Therapeutics.
Despite such an exciting advancement, 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 economical burden of HSPC-GT. The current cost for a single treatment with Strimvelis is €594,000, a cost that faces a formidable challenge in obtaining reimbursement from payers.1 A priority in the continuing development of HSC-GT is therefore to reduce cost of goods 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, it can essentially be split into two parts. One part involves the preparation of the vectors for gene delivery into the recipient cells. This can be broken down into 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 maximise the yield of the 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. 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 optimise the cell culture conditions to maximise the process of vector generation. The step of gene transduction into HSPCs may have room for improvement — considering that two rounds of transduction by vectors at an 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 the speed of haematopoietic recovery, immune reconstitution, the long-term persistence of transplanted cells and recipient survival.
Different ways to augment the effective HSPC cell dose are being attempted, including enhancing the engraftment and the ex vivo expansion of HSPCs prior to transplantation. The ability of HSPCs to engraft could be enhanced, for example, by pre-treatment with 16,16-dimethyl prostaglandin E2. Concomitantly, there has been intense research into the ex vivo expansion of HSPCs by companies including Gamida Cell and Novartis, giving rise to reagents that induce varying multiples of HSC expansion.
These examples illustrate the opportunities and activities under way 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 computerised nanoscale culture systems, and combinatorial cell culture, which provides 3D bead-based cell cultures to screen thousands of protocols in a single experiment, with proven ability to identify improved cell manipulation protocols.
Such technologies bring new hope that improved cell culture protocols will be identified to streamline the complex process of HSPC-GT to lower the cost, while improving the safety and efficacy of the therapy, thus rendering this life-saving therapy a reality for the general society.