Two-year safety outcomes of iPS cell-derived mesenchymal stromal cells in acute steroid-resistant graft-versus-host disease

Mesenchymal stem cells (MSCs) may exhibit immunomodulatory activity useful for the treatment of several diseases, but their efficacy against graft-versus-host disease (GvHD), a potentially fatal complication of allogeneic hematopoietic stem cell transplantation (HSCT), has been remarkably inconsistent.¹This variability is believed to be due to the significant scalability and manufacturing variations associated with primary donor-derived MSC production, which can lead to unpredictable medicines and suboptimal clinical outcomes.

A fundamental challenge in traditional MSC production is the mismatch between the low numbers of MSCs isolated from a single tissue donation and the high cell numbers required for adult therapy. For example, bone marrow harvest yields an initial population of approximately 10,000-80,000 MSCs;²A typical dosing regimen for bone marrow-derived MSCs involves administering a total of at least 1 × 10⁸ Average adult MSCs³Therefore, extensive expansion in culture in vitro is required to generate enough cells to treat a single patient, and even more extensive expansion is required to manufacture batches of allogeneic MSCs. Although such ex vivo subcultures can generate large therapeutic doses from a single donation, they can result in functional changes in MSCs, which eventually enter replicative senescence.^2,4As a result, there are limitations to the extent to which donor-derived MSC populations can be expanded without adversely affecting cell function.

Theoretically, if the number of therapeutic doses produced from a single donation could be minimized, the need for culture expansion would also be minimized. However, such an approach would require the frequent use of new donations, which is problematic given the degree of variability in MSC populations derived from different donors. The immunomodulatory activity of MSCs is mediated in part by the expression of indoleamine 2,3-dioxygenase. This enzyme is produced when MSCs are activated by inflammatory cytokines such as interferon gamma (IFNγ) and tumor necrosis factor (TNF), which suppress T-cell proliferation. However, there is a large degree of variability between donors in the propensity of MSCs to be activated by IFNγ and TNF, and thus in their ability to express indoleamine 2,3-dioxygenase.^Five,67The gene expression, differentiation, proliferation, and colony formation capacity of MSCs are also donor and tissue source dependent.^2,8.

The use of iPS cells as a starting material offers an alternative approach that facilitates consistent, large-scale manufacturing of MSC-based therapies: iPS cells can replicate indefinitely without losing their pluripotency and can be further differentiated into any adult cell type.^9,Ten,11The Cymerus iPS cell-based platform facilitates large-scale production of consistent, allogeneic MSCs from a single cell bank derived from a single blood donation. This approach avoids both donor-to-donor variability and over-expansion of MSC cultures. The GMP-compliant Cymerus process uses xenogeneic, serum- and feeder-free conditions to reduce in-process variability and minimize the risk of contamination with zoonotic pathogens. Furthermore, the process and quality control tests are designed to ensure that no undifferentiated iPS cells remain in the final product, avoiding the risk of teratoma formation, a hallmark of undifferentiated pluripotent cells.¹².

First clinical trial (registered on ClinicalTrials.gov: NCT02923375A clinical trial of Cymerus MSC (CYP-001) was conducted in adult patients with steroid-refractory acute GvHD after allogeneic hematopoietic stem cell transplantation (SR-aGvHD) at seven centers in the UK and Australia.¹²Eligible subjects were required to have a diagnosis of grade II-IV aGvHD, as judged by the investigator, followed by steroid resistance. Steroid resistance was defined as failure to respond or progression after receiving steroid therapy and a period consistent with usual practice in the relevant clinical setting (minimum 3 days at a dose of ≥1 mg/kg/day). After consent, one subject experienced a myocardial infarction and then dropped out before receiving CYP-001, and was therefore excluded from the analysis. Participants were assigned sequentially to cohort A or cohort B. Participants in cohort A (yeah= 8) received two intravenous infusions of CYP-001, one on day 0 and one on day 7, and one at 1 × 10⁶ Up to a maximum absolute dose of 1 × 10 cells per kg body weight⁸ Cells. Participants in cohort B (yeah= 7) also received infusions of CYP-001 on days 0 and 7, but at a dose of 2 × 10⁶ cells/kg, maximum absolute dose 2×10⁸ All participants continued treatment with concomitant standard-of-care aGvHD medications in addition to CYP-001 as previously reported, but were not permitted to receive other investigational medications until at least 28 days after the first dose of CYP-001.

The primary study evaluation period ended 100 days after the first dose of CYP-001. With extended follow-up of up to 2 years, participants were required to undergo clinical evaluation visits every 6 months, during which vital status, GvHD grade assessment, details of any additional GvHD treatment received, malignancy grade, and adverse events were ascertained.

As previously reported, CYP-001 was safe and well tolerated during the primary evaluation period, with encouraging efficacy results (complete and overall response rates of 53% and 87%, respectively), representing the first reported safety and efficacy results from a completed human clinical trial using iPSC-derived cells in any disease anywhere in the world. We are now reporting results from a two-year follow-up study.

No serious adverse events, tumors, or other safety concerns related to CYP-001 treatment were identified during the follow-up period.

Nine of the 15 participants treated with CYP-001 (60%) survived for at least 2 years (Figure 1). Two deaths occurred during the previously reported primary evaluation period and four occurred during the extended follow-up period. The investigators did not consider any of these deaths to be related to CYP-001. The reported causes of death included common complications observed in recipients of allogeneic HSCT, such as recurrence of pre-existing malignancies (yeah= 2); Pneumonia (yeah= 2); GVHD (yeah= 1); and sepsis or multiple organ failure (yeah= 1).

Survival and GvHD status are summarized in Table 1. At the 6-month visit, three participants had ongoing aGvHD symptoms. Two of them had grade I aGvHD at the 6-month visit, which in both cases showed a partial response (at baseline, the participants had grades II and III aGvHD, respectively). One other participant had grade II aGvHD at the 6-month visit, which showed stable disease (this participant had grade II aGvHD at baseline and all visits up to 6 months, but was free of GvHD at the 12-, 18-, and 24-month visits). No participant had aGvHD symptoms after 12 months. Three participants had chronic GvHD (cGvHD) at the 12- and 24-month visits, and two participants had cGvHD at the 18-month visit. Participants who developed cGvHD received additional treatments, including corticosteroids, calcineurin inhibitors, protein kinase inhibitors, mycophenolate mofetil, and extracorporeal phototherapy.

Although caution is required when comparing results from different clinical trials, if the 2-year survival rate of 60% in participants treated with CYP-001 is confirmed in a larger study, it would compare favorably with previously reported results in patients with SR-aGvHD. For example, several studies of MSCs from other tissue sources in SR-aGvHD have reported 2-year survival rates ranging from 0% to 40% (ref. ^{13,14,15,16,17,18,19}Additionally, the Janus kinase inhibitor ruxolitinib has been approved for the treatment of SR-aGvHD by multiple regulatory agencies, including the U.S. Food and Drug Administration.²⁰A pivotal phase III study in patients with SR-aGvHD reported favorable response rates to ruxolitinib (34% complete response rate, 62% overall response rate). However, 2-year overall survival could not be assessed, and at 18 months, overall survival was 38% in the ruxolitinib arm and 36% in the best treatment control arm (treatment with antithymocyte immunoglobulin, extracorporeal photopheresis, MSC, low-dose methotrexate, mycophenolate mofetil, everolimus, sirolimus, etanercept, or infliximab).^{twenty one}Furthermore, a recent study of “real-world” experience with a bone marrow-derived MSC product reported that overall survival rates at 6, 12, and 24 months after MSC treatment in adults with aGvHD refractory to ruxolitinib were 47% (38–56%), 35% (27–44%), and 30% (22–39%), respectively.^{twenty two}.

In conclusion, CYP-001 was safe and well tolerated in this study, with sustained results over the planned 2-year follow-up. NCT05643638) will begin in 2023.