Umbilical Cord Blood Transplants: Current Status and Evolving Therapies

Various approaches have been used to expand HSCs and HPCs from UCB with the use of feeder stromal cells, epigenetic modifiers, and small molecules. Over the past decade, UCB expansion has been successfully achieved, demonstrating rapid recoveries of neutrophils and platelets, better rates of engraftment, and fewer days in the hospital as surrogate for reducing costs.

Several ex vivo expansion strategies used over the last few years show promising results today (). Irrespective of the technique, there is a robust increase in CD34+ stem cells and their progenitors, leading to much faster neutrophil recovery and myeloid engraftment after infusion as compared to historical controls (3944). A regimen containing different cytokines or small molecules is used to exponentially enhance the stem cell population, thereby influencing the kinetics of engraftment and reducing the duration of severe neutropenia.

Clinical trials for ex-vivo cord blood stem cell expansion.

In early clinical trials with ex vivo expanded UCB, investigators often first evaluated the engraftment potential of the expansion product in the context of double UCBT, where one unit was expanded and one was infused without prior manipulation. The notch signaling pathway has an important regulatory role in hematopoietic differentiation as well as in the proliferation of HSCs and HPCs (53). Using engineered notch ligand delta 1, the expanded UCB unit resulted in rapid neutrophil recovery, but long-term engraftment was most often from the unmanipulated UCB unit (39). This approach demonstrated safety and feasibility as well as early neutrophil recovery [16 days; range (r) 734 days] as compared to the median time of 26 days (r, 1648 days; p = 0.002) in historical controls with double UCBT without expansion (38).

Co-culture with mesenchymal stem cells (MSCs) to provide the necessary factors for HSC expansion was explored as an ex vivo expansion strategy (45). It used an initial co-culture with MSCs for 7 days, followed by culture with cytokines. This trial enrolled 31 adults who underwent dUCBT, one with expanded cord and another with an unmanipulated cord. About 30-fold higher CD34+ cell dose was noted in the expanded unit. This study also reported earlier neutrophil and platelet recovery as compared to the CIBMTR data for dUCBT. The expanded cord lasted for about a year, when entire donor chimerism was noted from the unmanipulated cord. This technique has been further refined by the addition of fucosylation during UCB expansion (clinical trial {"type":"clinical-trial","attrs":{"text":"NCT03096782","term_id":"NCT03096782"}}NCT03096782) to investigate further improvement in hematopoietic recovery.

While promising, the results of these two studies suggested that these culture methods preferentially expanded primitive progenitors at the expense of HSC, providing only a transient wave of hematopoietic recovery. However, in the absence of T cells after expansion culture, the expanded product is at an immunological disadvantage in the setting of dUCBT, where one unit actively rejects the other (54). Based on this observation, transient engraftment after expansion culture may have been due to the absence of T cells rather than the loss of HSC. Therefore, subsequent expansion trials re-cryopreserved the CD34-depleted fraction after the CD34-enriched population was placed in expansion cultures.

Copper chelation technique using tetraethylenepentamine (TEPA) was investigated, with pre-clinical evidence demonstrating the prevention of stem cell differentiation in an in vitro culture (55). The UCB graft is derived from a single unit, where a fraction of the UCB unit undergoes a 21-days expansion culture in the presence of TEPA, followed by infusion of expanded and unmanipulated fractions on transplant day. The phase I/II clinical trial enrolling 10 patients confirmed safety and feasibility and showed neutrophil engraftment at day 30 (r, 1646) and platelet engraftment at day 48 (r, 35105 days) (40). A larger international multi-center trial was then conducted and reported results from 101 patients, which showed a median nucleated cell expansion of about 400-fold, with CD34 expansion of 77-fold (41). The 100-days survival was superior to dUCBT in the contemporary period. Neutrophil and platelet engraftments were significantly earlier than the comparison group (21 vs. 28 days and 54 vs. 105 days, respectively).

More recent trials evaluated small molecules, including vitamin B derivatives, aryl hydrocarbon receptor antagonists (AHRa), and pyrimidoindole derivatives that impeded HSC differentiation in cultures containing stimulatory cytokines, like SCF, Flt-3L, and thrombopoietin, but also infused the unit's T cells, in contrast to prior trials. Nicotinamide, a vitamin B derivative, inhibits differentiation, thereby enhancing the expansion of HSC and HPCs expanded in ex vivo cultures with stimulatory hematopoietic cytokines. In the initial phase I trial, 11 patients were enrolled. Of the 11 patients, the median time to neutrophil and platelet recovery was 13 and 33 days, respectively, faster than the controls (25 days, p < 0.001 and 37 days, p = 0.085). However, it is most notable that sustained myeloid engraftment from the NiCord-derived unit was observed in eight patients (42). Subsequently, a phase II study was completed using NiCord as a stand-alone graft in 36 patients (median age, 44 years), with high-risk hematologic malignancies treated with myeloablative conditioning. The results were compared to those of 146 patients who received standard UCB transplantation, with data reported to the CIBMTR. In the recipients of NiCord, the cumulative incidence of neutrophil engraftment was 94% at day 42, and the median time to neutrophil recovery was 11.5 days (95% CI, 914) vs. 21 days (95% CI, 2023) for patients who received standard transplant (p < 0.001). Similarly, the median time to platelet recovery was 34 days (95% CI, 3242) with NiCord vs. 46 days (95% CI, 4250) with standard UCB (P < 0.001). The unadjusted probability of overall survival after 2 years was 51% (95% CI, 3367), and the 2-years disease-free survival was 43% (95% CI, 2460) (43).

Boitano et al. reported the first use of an AHRa, StemRegenin 1 (SR1), for purified CD34+ expansion when cultured in media with SCF, Flt3L, TPO, and IL6, resulting in about a thousand-fold expansion (56). In the initial phase I/II clinical trial, 18 patients were treated with the lower dose unit placed in expansion culture and the larger dose unit which was unmanipulated. All the patients demonstrated sustained engraftment. In the 12 that had unit predominance with the expanded unit, the median time to recovery was 10.5 days, in contrast to 23 days in those engrafted with the unmanipulated unit (44). With sustained engraftment in the 12 patients recovering with the expanded product, the subsequent study evaluated the safety and the efficacy of the expanded product as a stand-alone graft. In addition, because of the marked expansion with the AHRa, lower-dose UCB units containing a cell dose of 1 107 nucleated cells, rather than 3 107 nucleated cells, per kilogram of recipient body weight were considered, potentially increasing the chance of better HLA-matched units for adults. An interim analysis demonstrated CD34+ cell expansion of 421-fold (r, 2191,476), with the patients receiving a median of CD34+ cell dose of 2.6 107/kg (r, 0.913.5 107/kg) and CD34+CD90+ cell dose of 1.3 106/kg (r, 0.57.0 106/kg). Neutrophil recovery occurred in 100% of patients at a median of 13 days (r, 831) vs. 25 days in prior recipients of unmodified CB (p < 0.01). Similarly, platelet recovery and red blood cell transfusion independence occurred in 100% at a median of 36 days (r, 3056) and 48 days (r, 13196), respectively. Time to neutrophil and platelet recovery strongly correlated with CD34+CD90+ dose, and there had been no transplant-related mortality reported so far (57).

The pyrimidoindole derivative, UM171, was evaluated as another strategy for UCB HSC expansion which enhances the self-renewal potential of human long-term repopulating HSCs independently of AHR suppression (58). The clinical phases 1 and 2 trial using this compound was conducted in two parts (46). Part 1 enrolled four patients who received dUCBTone with unmanipulated UCB unit and the other expanded with UM171until the patients showed UM171 UCB unit-derived engraftment. In part 2 of this study, 22 patients received single UM171 expanded UCBT with a dose de-escalation design. The minimal UCB unit that achieved prompt engraftment as a single UM171-expanded UCBT was 0.52 105 CD34+ cells. The median time to neutrophil (ANC > 500/l) and platelet recovery was 18 (r, 12.520 days) and 42 days (interquartile range, 3547), respectively, with no incidence of graft failure.

Based on preclinical models, it is clear that relatively few HSCs make it to the hematopoietic niche. Therefore, investigators explored ways that might augment homing and engraftment as an alternative to expansion culture. The first studies evaluated direct intra-bone marrow injection (IBMI) of UCB stem cells (59). This phase I/II study enrolled 32 adult patients with acute leukemia. The median time to neutrophil and platelet recovery was 23 and 36 days, respectively, and early sustained donor-derived engraftment was noted among all patients. There was no incidence of grade III and IV acute GVHD on this study. In a subsequent study by Brunstein et al., a dUCBT platform was used for the IBMI of one of the UCB units, while the other was given intravenously (60). Ten adult patients were enrolled on this trial, and the median time to neutrophil and platelet recovery was 21 and 69 days, respectively. In nine out 10 patients that engrafted, four engrafted with IBMI UCB unit. The trial demonstrated the safety of the procedure, but the technique offered no advantage over the traditional intravenous route.

Alternatively, agents like the dimethylated form of prostaglandin E2 (dmPGE2) and fucosylation have been used to augment the homing of HSCs (61). In the earlier approach, dmPGE2 was used to augment the homing of stem cells by increasing the number of stem cells that reach the bone marrow niche. This was considered to deliver a greater number of stem cells to the target site without the need for in vivo or ex vivo expansion (61). A phase I safety and efficacy trial was conducted, evaluating this concept using co-transplantation of a dmPGE2-treated UCB with an unmanipulated cord in patients with hematologic malignancies (62). The trial initially enrolled nine patients, with median time to neutrophil and platelet engraftment at 24 and 72.5 days, respectively. Two of seven patients undergoing engraftment demonstrated prolonged hematopoiesis from the dmPGE2-UCB units. Given the lack of accelerated engraftment in the initial trial, dmPGE2 was optimized in the subsequent trial with a modulation protocol, and 12 additional patients were enrolled. The median time to neutrophil engraftment was 17.5 days (r, 1431 days) compared to 21 days for the historical cohort (p = 0.045). The median time to platelet engraftment was 43 days (r, 2660 days), and 10 of 12 patients had early and sustained engraftment of the dmPGE2-UCB unit.

Another approach was evaluated by exploring the role of complement 3a (C3a), which attaches to the HSCs and improves homing by its immunomodulatory properties, including stromal-derived factor I (SDF I)-mediated homing (63). Based on pre-clinical data, the phase I study was conducted in adults receiving non-myeloablative conditioning in a dUCT model. Engraftment was noted in two-thirds of the patients from the non-manipulated cord, thus failing to show earlier homing and engraftment with this technique (64). Another group investigated the effect on homing with inhibition of dipeptidyl peptidase (DPP)-4, which is a peptide cleavage protein that truncates the chemotaxis factor, SDF-1-alpha. A pre-clinical investigation in mice demonstrated that deletion or inhibition of DPP-4 enhanced engraftment of human CD34+UCB cells in mouse marrow (65, 66). In the subsequent clinical trial, an oral inhibitor of DDP-4, sitagliptin, was used to enhance the engraftment of single-unit UCB transplants in adults with high-risk hematological malignancies (67). In this feasibility trial, 24 patients received sitagliptin on days 1 and 2 at a dose of 600 mg daily and engrafted at a median of 21 days (r, 1350). Though sitagliptin was well-tolerated, a significant reduction in area under the curve was noted. After dose optimization, 600 mg every 12 h administered on days 1 to +2, another 15 adult patients were treated, and all engrafted by day 30, with 12 (80%) engrafting by day 21 (68). The median time to neutrophil engraftment was 19 days (r, 1230).

Slower homing and engraftment with UCB relative to bone marrow HSC had also been attributed to poor binding to adhesion molecules P- and E-selectins present on bone marrow endothelial cells (69). Pre-clinical models showed that both endogenous as well as ex vivo fucosylation of UCB HSCs increased the affinity for these adhesion molecules, resulting in earlier engraftment (70, 71). In the phase-I clinical trial using the dUCT model, one unit underwent fucosylation using fucosyltransferase-I enzyme, while the other unit was infused unmanipulated. Significantly faster neutrophil (17 vs. 26 days; p < 0.05) and platelet engraftment (36 vs. 46 days; p < 0.05) was noted from both units compared to the historical controls, suggesting that endogenous fucosylation benefited the engraftment of the unmanipulated cord as well (72).

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Umbilical Cord Blood Transplants: Current Status and Evolving Therapies