Next Phase Newsletter

Juan Carlos Lopez

Andy Marshall

Ex vivo HSC lentiviral gene therapies have been on the market for nearly a decade, with six products approved and at least 55 now in clinical testing for rare inherited diseases, HIV infection or cancer. And yet, their commercial success remains in question. Bluebird Bio—which was valued at $10 billion only a few years ago and successfully shepherded to market Zynteglo against transfusion-dependent β-thalassemia, Skysona for early cerebral adrenoleukodystrophy, and Lyfgenia for sickle-cell disease (SCD)—was sold earlier this year to private-equity firms Carlyle and SK Capital for a measly $29 million. Last November, the company had treated only 57 patients (35 for Zynteglo; 17 for Lyfgenia and 5 for Skysona), with just 28 of 70 medical centers across the US ready to treat patients due to delays in accreditation and training of personnel. In Europe, Orchard Therapeutics halted marketing and production of a treatment for severe combined immunodeficiency caused by adenosine deaminase mutations (Strimvelis) after six years, forcing Fondazione Telethon to take over production. Even market uptake of Vertex’s much-heralded CRISPR/Cas9 BCL11a SCD therapy Casgevy has been sluggish.

These subpar commercial launches relate to the complexity of ex vivo lentiviral gene therapy: patient identification and qualification is lengthy; HSC mobilization and sourcing efficiencies vary due to patient heterogeneity; and manufacture and distribution processes remain lengthy and convoluted (sometimes requiring repetition if a poor quality product batch is generated). From first evaluation, patients are required to make several hospital visits over a period (of up to a year) and must undergo punishing conditioning regimes with lymphodepletive bisulfan before infusion, which itself carries infertility and cancer risks. All of these challenges have added impetus to the search for alternative and more efficient approaches for carrying out HSC gene therapy.

​A group led by Alessio Cantore and Luigi Naldini, from the San Raffaele Telethon Institute for Gene Therapy in Milan, Italy, report in Nature that it may be possible to obviate these challenges by delivering recombinant lentiviral vectors in vivo soon after birth, when HSCs continue to circulate in the bloodstream in large numbers and are beginning their transition from the liver (where they are located in the fetus) to bone marrow (where they remain through adulthood).

Cantore, Naldini and their colleagues started by measuring the number of circulating HSCs in neonatal, 1-, 2- and 8-week-old mice, looking at the peripheral blood, spleen, liver and bone marrow. They found that HSCs were present in the circulation right after birth and that their number immediately declined. These cells could be transduced with lentiviruses, successfully engrafted, and persisted in the mice for several months.

To show that these HSCs could be harnessed to treat genetic disorders, the team tried to correct three mouse models of disease — adenosine deaminase deficiency, autosomal recessive osteopetrosis and Fanconi anemia. Although the therapeutic effect of the cells varied depending on the disease, the results provided compelling evidence for the potential for in vivo gene transfer to HSCs.

The authors reported that human neonates also have circulating HSCs in high numbers. And although the therapeutic window in the mouse only existed during the neonatal period, it was possible to lengthen it by mobilizing the HSCs from their niche in two-week-old animals using protocols in clinical use (granulocyte-colony stimulating factor/CXCR4 antagonist Plerixafor) These observations raise the possibility of therapeutically targeting HSCs in newborns, potentially opening the gates to treatment of a variety of inherited conditions.

Compared with the headaches of ex vivo manipulation, the authors’ concept of simply injecting a lentiviral gene therapy into a newborn to bring about a genetic cure is certainly alluring. But reducing this to clinical practice will require optimization of many different factors. How to account for the heterogeneity and fragility of patient HSCs in a particular disease? How to measure the cellular activation/metabolic state of HSCs in newborns and assess the affect on amenability to lentiviral transduction in the hostile milieu of blood? What effect would shear stress in circulation have on lentiviral transduction efficiencies in situ? What would be the selective engraftment advantage provided to HSCs after engraftment of a particular gene? And what would be the potential safety implications of off-target transduction events in cells other than HSCs, given instances of dysplastic syndromes have been reported with ex vivo lentivectors?

Current ex vivo lentiviral gene therapy like Lyfgenia and Zynteglo infuse between 3–5×10⁶ gene-modified CD34+ HSCs/kg in a patient. The challenge for in vivo lentiviral gene therapy will be to achieve transduction efficiencies that transduce as many cells and obtain similar engraftment rates in the rapidly turning over HSC population. Beyond these issues, there are additional practical challenges: can genetic testing of an infant happen fast enough to take advantage of the short therapeutic window for which an in vivo lentiviral HSC therapy could work?

Clearly, the new work raises many intriguing questions for the lentiviral gene therapy space. And for newborns with genetic diseases, such as severe immunodeficiencies or Fanconi anemia, in vivo HSC gene therapy may open up new treatment options.

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