While mesenchymal stem cells (MSCs) show potential immunomodulatory activities that could be valuable in the treatment of several diseases, their efficacy in the context of graft-versus-host disease (GvHD), a potentially fatal complication of allogeneic hematopoietic stem cell transplantation (HSCT), has been markedly inconsistent1. This variability has been attributed to substantial scalability and manufacturing variations associated with primary donor-derived MSC production, potentially leading to unpredictable medicinal products and suboptimal clinical outcomes.

A fundamental challenge for conventional MSC manufacture lies in the disparity between the small number of MSCs isolated from a single tissue donation and the large number of cells required to treat adults. For example, while a bone marrow collection yields a starting population of approximately 10,000–80,000 MSCs2, a typical bone marrow-derived MSC dose regimen requires a total of at least 1 × 108 MSCs for an average adult3. Thus, extensive ex vivo culture expansion is necessary to generate enough cells to treat a single patient, with even greater levels of expansion required to manufacture batches of allogeneic MSCs. While such in vitro passaging can generate large numbers of therapeutic doses per donation, it can result in MSCs undergoing functional changes and ultimately entering replicative senescence2,4. Consequently, there is a limit to the extent that donor-derived MSC populations can be expanded without adversely affecting cell functionality.

In theory, if the number of therapeutic doses produced per donation could be kept to a minimum, then the need for culture expansion could also be minimized. However, such an approach would necessitate frequent use of new donations, which is problematic given the extent of variability in MSC populations derived from different donors. The immunomodulatory activity of MSCs is in part mediated by the expression of indoleamine 2,3-dioxygenase, which is produced when MSCs are activated by inflammatory cytokines, including interferon-γ (IFNγ) and tumor necrosis factor (TNF). These in turn lead to suppression of T cell proliferation. However, there is a high level of interdonor variability in the propensity of MSCs to be activated by IFNγ and TNF, and thus in their capacity to express indoleamine 2,3-dioxygenase5,6,7. MSC gene expression, differentiation, proliferation and colony-forming capacity are also donor-dependent and tissue source-dependent2,8.

The use of iPS cells as a starting material offers an alternative approach to facilitate consistent, large-scale manufacture of MSC-based therapies. iPS cells can replicate indefinitely without loss of pluripotency, in addition to the ability to differentiate into any adult cell type9,10,11. The Cymerus iPS cell-based platform facilitates large-scale production of consistent, allogeneic MSCs from a single cell bank, which in turn was derived from a single blood donation. This approach avoids both interdonor variability and excessive MSC culture expansion. The good manufacturing practice-compliant Cymerus process uses xenogen-free, serum-free and feeder-free conditions to reduce variability within the process and to minimize the risk of contamination with zoonotic agents. Additionally, the process and quality control tests were designed to ensure the absence of residual undifferentiated iPS cells in the final product to avoid the risk of teratoma formation, which is a defining characteristic of undifferentiated pluripotent cells12.

The first clinical trial (ClinicalTrials.gov registration: NCT02923375) of Cymerus MSCs (CYP-001) was conducted in adults with steroid-resistant acute GvHD (SR-aGvHD) after an allogeneic HSCT across seven centers in the United Kingdom and Australia12. Eligible individuals were required to have been diagnosed with grades II–IV aGvHD followed by steroid resistance in the opinion of the investigator. Steroid resistance was defined as failing to respond or progressing after receipt of a steroid regimen and duration consistent with normal practice at the relevant clinical site (minimum 3 days at a dose of at least 1 mg per kg per day). After providing consent, one individual withdrew before receiving CYP-001, after experiencing a myocardial infarction, and was thus excluded from the analysis. Participants were sequentially assigned to cohort A or cohort B. Participants in cohort A (n = 8) received two intravenous infusions of CYP-001, one on day 0 and one on day 7, at a dose of 1 × 106 cells per kg body weight, up to a maximum absolute dose of 1 × 108 cells. Participants in cohort B (n = 7) also received infusions of CYP-001 on days 0 and 7, but at a dose of 2 × 106 cells per kg, up to a maximum absolute dose of 2 × 108 cells. In addition to CYP-001, all participants continued treatment with concomitant standard of care aGvHD medications, as reported previously, but were not permitted to receive other investigational agents until at least 28 days after the first dose of CYP-001.

The primary trial evaluation period concluded at 100 days after the first dose of CYP-001. Extended follow-up of up to 2 years required participants to attend clinical assessment visits every 6 months, which consisted of survival status, GvHD grade assessment, details of any additional GvHD treatment received, malignancy status and adverse events.

As reported previously, CYP-001 was safe and well tolerated during the primary evaluation period, with encouraging efficacy outcomes (complete and overall response rates of 53% and 87%, respectively). This represented the first report of safety and efficacy in a completed human clinical trial using iPS cell-derived cells in any disease worldwide. We now report the results of the 2-year follow-up.

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 (60%) treated with CYP-001 survived for at least 2 years (Fig. 1). Two deaths occurred during the previously reported primary evaluation period and four occurred during the extended follow-up period. None of the deaths was considered by investigators to be related to CYP-001. Reported causes of death were commonly recognized complications observed in recipients of allogeneic HSCT (relapse of preexisting malignancy (n = 2); pneumonia (n = 2); GvHD (n = 1); and sepsis or multi-organ dysfunction (n = 1)).

Letters represent cause of death: G, GvHD; P, pneumonia; S, sepsis/multi-organ dysfunction; R, relapse.

Source data

Survival and GvHD status are summarized in Table 1. Three participants exhibited ongoing aGvHD symptoms at the 6-month visit. Two of those participants had grade I aGvHD at the 6-month visit, which represented a partial response in both cases (at baseline, the participants had grades II and III aGvHD, respectively). Another participant had grade II aGvHD at the 6-month visit, which represented stable disease (this participant had grade II aGvHD at baseline and every visit up to 6 months, but was free of GvHD at the 12-month, 18-month and 24-month visits). No participant had aGvHD symptoms at 12 months or later. Three participants had chronic GvHD (cGvHD) at the 12-month and 24-month visits, and two participants had cGvHD at the 18-month visit. Participants who developed cGvHD received additional treatment, including corticosteroids, calcineurin inhibitors, protein kinase inhibitors, mycophenolate mofetil and extracorporeal photopheresis.

Caution must be exercised when comparing outcomes from different clinical trials, but if the 60% 2-year overall survival rate in participants treated with CYP-001 is confirmed in larger studies, it would compare favorably with previously reported outcomes in patients with SR-aGvHD. For example, several studies of MSCs from other tissue sources in SR-aGvHD reported 2-year overall survival rates ranging from 0% to 40% (refs. 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 authorities, including the US Food and Drug Administration20. A pivotal phase III study in patients with SR-aGvHD reported encouraging ruxolitinib response rates (complete response: 34%; overall response: 62%). However, 2-year overall survival could not be evaluated and overall survival at 18 months was 38% in the ruxolitinib group and 36% in the ‘best available therapy’ control group (which involved treatment with antithymocyte immunoglobulin, extracorporeal photopheresis, MSCs, low-dose methotrexate, mycophenolate mofetil, everolimus, sirolimus, etanercept or infliximab)21. Furthermore, a recent study of ‘real-world’ experience with a bone marrow-derived MSC product reported that the probability of overall survival in adults with ruxolitinib-refractory aGvHD after MSC treatment at 6, 12 and 24 months was 47% (38–56%), 35% (27–44%) and 30% (22–39%), respectively22.

In conclusion, CYP-001 was safe and well tolerated in this study, with sustained outcomes at the planned 2-year follow-up. A global phase II clinical trial of CYP-001 in aGvHD (ClinicalTrials.gov registration: NCT05643638) commenced in 2023.