Chapter 14: Alternative Donor Transplants: Cord Blood Transplant

The numbers of allogeneic stem cell transplants (SCTs) performed in the United States have increased steadily, from about 7,500 in 1994-1995 to over 13,500 in 2010-2011, in patients above the age of 20 years (¹). Donor identification has been a constant challenge, and only 30% of patients who need allogeneic SCT have a matched sibling donor. The National Marrow Donor Program (NMDP) and its cooperative international registries have about 16 million volunteer donors. It is estimated that 75% of white patients, but only 16% of African Americans and other minority patients, will be able to find a suitably matched unrelated donor (MUD) and proceed to transplantation (²). Mismatched related (often haploidentical), cord blood (CB) or mismatched unrelated donors (MMUDs) with either peripheral blood (PB) or bone marrow (BM) graft sources are potential options for patients in need of a SCT but lacking a matched related donor (MRD) or unrelated donor.

Using CB as the graft source provides many advantages; CB units are easy to collect with little or no risk to the mother or newborn. Cord blood units can be rapidly obtained for 80% to 95% of the patients 20 years and older across all races and in almost 100% of younger patients (²). This is particularly advantageous in cases where urgent transplant is mandated. Owing to rapid procurement of CB units, patients can receive CB transplantation (CBT) 4 or 5 weeks earlier than those receiving SCT with a MUD (³). Further, CBT is associated with low risk of infection transmission, requires less-stringent human leukocyte antigen (HLA)–matching criteria, and has relatively lower risk of graft-versus-host disease (GVHD) with preserved graft-versus-malignancy effects. However, it is associated with a greater risk of graft rejection, delayed engraftment, and delayed immune reconstitution, leading to heightened risk of infection or nonrelapse mortality (NRM) (^4,5,6,7). Many of the adverse outcomes noted after CBT are attributed to the naïveté of CB T lymphocytes and the low numbers of total nucleated cells (TNCs) and CD34⁺ cells in the graft.

The outcomes of CBT depend on the impact of cell dose and the degree of HLA match (⁸). The TNC dose available for CBT is a fraction of what is typically used in the PB or BM setting. The median TNC dose obtained from a BM harvest is about 3 × 10⁸ TNCs/kg; the granulocyte colony-stimulating factor (G-CSF)–mobilized PB can yield a median of 7 × 10⁸ TNCs/kg (⁹). In contrast, about one-fourth of the CB units contain less than 0.25 × 10⁸ TNCs/kg, and two-thirds of the units have between 0.25 × 10⁸ to 1 × 10⁸ TNCs/kg (⁸). The recommended minimum cell dose is typically 2 × 10⁷ TNCs/kg for successful engraftment after a CBT.

The HLA matching criteria between the CB units and the recipient are less stringent compared with other donor sources. Therefore, while unrelated adult donors are selected to be closely matched to recipients at HLA-A, -B, -C, and -DRB1 by high-resolution testing (^10,11), CB units are selected using lower-resolution HLA typing (antigen level) for HLA-A and -B and at the allele level for HLA-DRB1 (¹²). In a study of single CBT, Barker et al showed that recipients of 6/6 matched CB units had the lowest transplant-related mortality (TRM) regardless of the dose, followed by 5/6 matched CB units with TNC dose greater than 2.5 × 10⁷/kg or 4/6 matched units with TNC dose greater than 5.0 × 10⁷/kg, and 5/6 matched units with lower TNC dose (<2.5 × 10⁷/kg) (^8,13). These findings support the notion that both TNC dose and HLA-matching level should be taken into account for CB unit selection.

Although the standard HLA-matching criteria do not require high-resolution typing at class I antigen in CBT, a recent study by the Center for International Blood and Marrow Transplant Research (CIBMTR) and the Eurocord reported better outcomes in single CBT after myeloablative conditioning (MAC) with improved allele-level matching for 4 HLA loci (-A, -B, -C, and -DRB1) (¹⁴). The investigators showed that the frequency of neutrophil recovery was lower for recipients of mismatches at three to five but not at one or two alleles compared with those of HLA-matched units. Nonrelapse mortality was higher with units mismatched at one to five alleles compared with matched units. This retrospective study confirmed the clinical importance of selecting better HLA allele-matched units for single CBT, an observation already well described for BM and PB progenitor cell transplantation. The effect of HLA matching by high-resolution testing is unclear after double CBT (dCBT) and should be investigated.

The relatively low number of progenitor cells in a single CB unit resulting in delayed hematopoietic recovery, and engraftment failure limited the use of CBT in adults. Most adults do not have access to a single CB unit containing the recommended nucleated cell dose of 2.5 × 10⁷ TNCs/kg (¹⁵). To overcome the cell-dose limitation, investigators pioneered an approach by which two partially HLA-matched CB units were used to augment the progenitor cell dose when a single unit was considered inadequate and confirmed its feasibility (¹⁵). A recent CIBMTR analysis investigated the relative risks and benefits of transplanting double CB units compared with an adequately dosed single CB unit. The investigators observed no differences in clinical outcomes after dCBT or adequately dosed single CBT. Both transplant approaches had comparable outcomes with 78% (95% CI, 72-83) versus 81% (95% CI, 74-88, P = .83) probabilities of neutrophil engraftment by day 42, and 68% (95% CI, 62-74) versus 63% (95% CI, 53-72; P = .34) probabilities of platelet recoveries at 6 months, respectively. There were no differences for grades III or IV acute GVHD (18% [95% CI, 11%-26%] versus 14% [95% CI, 10%-19%], P = .64), 2-year probabilities of chronic GVHD (31% [95% CI, 26-37] versus 24% [95% CI, 15-34], P = .27), treatment-related mortality (hazard ratio [HR] 0.91; P = .63), risk of relapse (HR 0.90, P = .64), and overall mortality (HR 0.93, P = .62) (¹⁶).

A unique feature after a dCBT is evidence of mixed chimerism from both the CB units observed during the initial posttransplant period.(¹⁷) In the early post-dCBT period (day +21), both CB units contribute to hematopoiesis in 40% to 50% of patients, but by day +100 one unit predominates in a vast majority (^18,19). The factors leading to unit dominance are not well defined. It is however recognized that there is no association with the TNC or CD34⁺ cell doses, HLA match, gender match, ABO typing, or the order in which CB units are infused (^{15,18,19,20,21}). This current lack of evidence is a major limitation to dCBT, and identifying predictive factors for unit dominance would optimize CB unit selection algorithms by allowing for the selection of two CB units with a high probability of long-term engraftment.

Myeloablative Conditioning Regimens

High-intensity MAC regimens are reserved for young and otherwise-fit patients who can tolerate the associated regimen morbidity. Such regimens lead to a low risk of relapse at the expense of a high TRM compared with reduced-intensity conditioning (RIC) regimens.

One of the largest registry studies comparing single-unit CB (n = 165) to PB (n = 888) or BM (n = 472) transplants in adults with acute leukemia using MAC regimens from 2002 through 2006 showed promising outcomes with CBT. Total body irradiation (TBI) constituted part of the preparative regimen in about half of patients in the CB group and about two-thirds in the comparative groups. Despite a significantly higher number of patients with fully HLA-matched PB or BM grafts (70%) compared to CB grafts (6%), the rates of disease-free survival (DFS) and relapse were similar among the groups, while the risks of acute or chronic GVHD were significantly lower with CBT. Also, TRM was similar with CBT compared with mismatched PB or BM grafts, but higher in contrast to fully matched PB (HR 1.62; 95% CI, 1.18-2.23; P = .003) or BM transplants (HR 1.69; 95% CI, 1.19-2.39; P = .003). This was offset by a significantly lower incidence of chronic GVHD compared with fully matched PB (HR 0.38; 95% CI, 0.27-0.53; P = .001) or BM transplants (HR 0.63; 95% CI, 0.44-0.90; P = .01) (⁴). Therefore, in the absence of matched PB or BM donors, CBT potentially offers better outcomes compared with mismatched alternative donor transplants.

Similar encouraging results were noted in a study that compared 4-6/6 matched dCBT exclusively (using the MAC regimen including fludarabine 75 mg/m², cyclophosphamide 120 mg/kg, and TBI 1,200 to 1,320 cGy [Flu/Cy/TBI]) to 8/8 MRD or MUD, or 1 allele–MMUD donors (⁷). This study also noted lower risk of relapse, higher TRM, lower GVHD, and comparable DFS after CBT compared to other groups. The risk of relapse was significantly lower after dCBT (15%, 95% CI, 9%-22%), compared with MRD (43%, 95% CI, 35%-52%) or MUD (37%, 95% CI, 29%-46%) transplants. Higher NRM was noted after dCBT (34%, 95% CI, 25%-42%) compared to MRD (24%, 95% CI, 17%-39%) or MUD (14%, 95% CI, 9%-20%) transplants, which resulted in comparable 5-year DFS between CB (51%, 95% CI, 41%-59%), MRD 33% (95% CI, 26%-41%), and MUD (48%, 95% CI, 40%-56%) transplants. The cumulative incidence of grades II to IV GVHD at day 100 after DCBT, MRD, and MUD was 60% (95% CI, 50%-70%), 65% (95% CI, 57%-73%), and 80% (95% CI, 70%-90%), respectively. The rate of chronic GVHD at 2 years was 26% (95% CI, 15%-35%), 47% (95% CI, 39%-55%), and 43% (95% CI, 34%-52%) respectively (⁷).

In adults with high-risk acute lymphoblastic leukemia (ALL), CBT leads to similar overall survival (OS), TRM, and relapse risk, with significantly lower risk of acute or chronic GVHD, compared with PB or BM transplants. This was demonstrated in a recent registry study that compared outcomes after single or double 4-6/6 matched CB (n = 116) and 7-8/8 matched PB (n = 546) or BM (n =140) transplants after MAC regimens (²²). More than half of the patients in the CBT group received Flu/Cy/TBI as the conditioning regimen, while about 75% of the patients in the PB or BM groups received TBI/Cy-based regimens. There were no differences in the 3-year OS rates (44%, 44%, and 43%, respectively); relapse risk (22%, 25%, and 28%, respectively); or TRM (42%, 31%, and 39%, respectively) among the groups. However, the risk of acute grades II to IV GVHD (27%, 47%, and 41%, respectively) or grades III and IV acute GVHD (9%, 16%, 24%, respectively) was appreciably lower after CBT compared with 8/8-matched and 7/8-matched PB or BM transplants, respectively (²²).

Reduced-Intensity Conditioning Regimens

The advent of RIC regimens extended the utility of CBT to older patients and those with comorbid conditions that otherwise restrict the use of the MAC regimens. It is noteworthy that a majority of trials in MRD or MUD transplants used an arbitrary age definition of greater than 55 to 60 years (to define “older patients”) as a threshold of using an RIC regimen. However, age greater than 40 to 45 years is generally chosen as a threshold for RIC in the CBT setting.

Barker et al reported that the RIC with fludarabine 200 mg/m², cyclophosphamide 50 mg/kg, and 2-Gy TBI (Flu/Cy/2Gy TBI) was well tolerated, with rapid neutrophil recovery, a sustained donor engraftment rate of 94%, and a low incidence of TRM (²³). This regimen was associated with significantly better DFS compared with other RIC regimens (51% vs 28%, P = .0002, HR 0.53) (²⁴). Multiple studies supporting the use of RIC CBT in patients who would not be able to tolerate more intensive preparative regimens have subsequently been reported (^{6,18,19,25,26,27}).

A retrospective single-center study compared the outcomes in patients older than 55 years who underwent CB and MRD SCT with RIC (primarily of Flu/Cy/2Gy TBI). There were no differences in TRM at 180 days (28%, 95% CI, 14%-41% vs 23%, 95% CI, 11%-36%); 3-year DFS (34%, 95% CI, 19%-48% vs 30%, 95% CI, 16%-44%); or 3-year OS (34%, 95% CI, 17%-50% vs 43%, 95% CI, 29%-58%) between the groups (⁶). These findings were confirmed by a registry analysis of patients with acute leukemia comparing the outcomes after CB (n = 161), 8/8-matched (313) and 7/8-matched PB (111) transplants with RIC regimens. Patients with CBT following a Flu/Cy/2 Gy TBI regimen had comparable results with 8/8 HLA-matched PB donors. However, higher TRM and lower OS and LFS were observed in recipients of CBT if they were treated with alternative RIC regimens, including busulfan plus melphalan, or cyclophosphamide with fludarabine and in vivo T-cell depletion with antithymocyte globulin (ATG) (²⁷). Similar findings were reported by Eurocord and the European Group for Blood and Marrow Transplantation (EBMT) in patients with lymphoid malignancies. When patients with CB units (n = 104) were compared with 8/8-matched PB MUD (n = 541) transplants, no difference was noted in NRM (29% vs 28%), PFS (28% vs 35%), or OS (56% vs 49%) at 3 years (Fig. 14-1). Further, the risk of chronic GVHD was significantly lower in the CBT group (26% vs 52% at 3 years; P = .0005) (²⁶). These studies supported the use of CBT with RIC as a suitable alternative for patients who may benefit from RIC SCT and who do not have a suitable related or unrelated volunteer donor in the time period transplantation is needed.

Disease relapse is the most common cause of mortality after allogeneic SCT, while GVHD and infections are the two leading causes of NRM. Treatment options after SCT relapse include withdrawal of immunosuppression, chemotherapy, or donor lymphocyte infusion (DLI). Although DLI is currently unavailable for CBT patients outside clinical trials, a study showed that in adults who have acute myelogenous leukemia relapse following CB or MRD transplants, DLI in the latter group did not have an impact on OS (19% CB vs 28% MRD at 1 year; P = .36), and relapsed patients had poor prognosis independent of the donor source (²⁸).

In the CBT setting, infections are the leading cause of early 100-day NRM (27%), while GVHD contributes to most (20%) of the delayed NRM (beyond 100 days) (⁴). The reported probabilities of acute grades III and IV GVHD at day 100 (10%-25%) and chronic GVHD (25%-35%), TRM (20%-50%), risk of relapse (20%-50%), DFS (30%-35%), and OS (30%-45%) at 2 to 3 years varied depending on the conditioning regimens and the study population (^{7,16,19,22,24,27}). The rates of posttransplant complications are comparable after the use of dCBT and single CBT (²⁹).

Early infections (within 100 days) are primarily due to neutropenia and mucosal damage caused by conditioning regimens. Delayed infections are related to the speed of cell-mediated immune reconstitution and use of immunosuppressants. Most early infections are bacterial; more than half of the invasive fungal infections and 45% of cytomegalovirus (CMV) infection occur beyond day +100 (³⁰).

The heightened risk of infections in CBT is partly explained by the naïveté of CB T cells and delayed T-lymphocyte immune reconstitution and neutrophil engraftment in contrast to other donor sources (^4,7,27,31,32). Although significant B-cell recovery starts within 3 to 4 months and may approach normal numbers by 6 months after transplant, T-cell reconstitution is substantially prolonged (³²). The CD4⁺ and CD8⁺ T-cell counts are strikingly reduced after CBT, remain low for up to 6 months, and approach normal values by 1 year. The PB T cells after CBT are more dysfunctional as compared with other types of allogeneic SCTs. Patients also fail to recover thymic function after CBT, in contrast to other allogeneic SCT recipients (³³) (Fig. 14-2).

A retrospective registry analysis from the CIBMTR comparing CB (n = 150) with matched (n = 367) or antigen-mismatched (n = 83) BM transplants reported higher risk of early (within 100 days) infection-related deaths after CBT compared to other groups (45%, 21%, and 24%, respectively; P = .01) (²⁹). In another study, the risk of severe infection, especially bacterial infections in the first 100 days, was significantly higher after CBT in contrast to BM or PB graft (85% vs 69%, P < .01). The risk of infection-related mortality did not differ at day 100 or at 3 years. In multivariate analysis for CBT, CMV-seropositive recipient, prolonged neutropenia beyond day 30, and low cell dose (<2 × 10⁷/kg) were the predictors for infection-related mortality (³⁰).

At the MD Anderson Cancer Center, patients with various hematological malignancies who do not have an MRD or MUD but require SCT are considered for CBT and a search for a CB unit is initiated (Fig. 14-3).

Unit Selection

Within the first days to weeks of the initial donor search, coordinators determine the likelihood of obtaining a suitably matched donor based on the patient’s ancestry, the preliminary search results, and review of the HLA typing. If the likelihood of finding a matched donor is deemed to be low, we proceed with confirmatory HLA typing of CB units. Alternatively, we may delay typing of CB units if multiple 10/10 HLA-A, -B, -C, -DRB1, -DQ allele-MUDs are probable or if the transplantation is not urgent. If an unrelated donor collection is delayed because of problems with donor health or availability, a prompt decision is made whether to abandon the unrelated donor search in favor of CB.

To maximize the chance of identifying optimal CB unit(s), we conduct a global search while being aware that there is no global regulatory oversight of CB-banking standards. It is important to know which banks are included in the NMDP consortium of banks, in the Netcord, and in the NMDP Cooperative Registries. We give equal consideration to domestic and international units as the primary units of the graft, whereas we prefer domestic units for backup.

Our current institutional policy is to use a double-unit graft in an effort to augment engraftment and reduce TRM. Given that either unit may engraft after a double-unit CBT, each unit of a double-unit graft is equally important. We give a strong priority to HLA match over precryopreservation TNC threshold of about 2.0 × 10⁷/kg for each unit of a double-unit graft. This approach gives strong priority to HLA match but augments the chance of engraftment by infusing two units with at least an adequate dose in each.

We use novel ex vivo graft manipulation techniques pioneered at the MD Anderson Cancer Center, such as ex vivo CB expansion with mesenchymal stromal cells (MSCs) (³⁴) and ex vivo CB fucosylation (³⁵). With these, remarkable improvement in engraftment is noted compared with historical controls. The median time to neutrophil engraftment is 15 (range, 9 to 42) days with MSC expansion (³⁴) and 14 (range 12-28) days with fucosylation, which is significantly faster than the 24 (range 12-52) days noted in the CIBMTR controls (P < .001) (³⁵). Similarly, platelet engraftment is 42 (range 15-62) days with MSC expansion and 33 (range 18-100) days with fucosylation, compared with 49 (range 18-264) days in the CIBMTR controls (P = .03) (^34,35). We are investigating the impact of CB unit KIR (killer immunoglobulin-like receptor) genotype on transplant outcomes with an aim to integrate KIR information into the CB unit selection algorithm.

Conditioning Regimens

We have investigated various MAC regimens for CBT, which include combinations of (a) melphalan, fludarabine, and thiotepa; (b) busulfan and fludarabine; (c) busulfan, clofarabine, fludarabine, and low-dose TBI given 9 days after chemotherapy; and (d) busulfan, clofarabine, fludarabine, and low-dose TBI given immediately after the chemotherapy (³⁶). The most favorable outcomes appear to be associated with the latest regimen, which is well tolerated and associated with prompt engraftment and effective disease control. This regimen consists of fludarabine 10 mg/m²/d (days -7 through -4), clofarabine 30 mg/m²/d (days -7 through -4), busulfan at a dose calculated to deliver a daily area under the curve (AUC) of 4,000 μmol/min for 4 days (days -7 through -4) based on an outpatient test dose of 32 mg/m², and 2-Gy TBI on day -3. Our preferred RIC regimen consists of fludarabine 40 mg/m²/d (days -5 through -2) and melphalan 140 mg/m² (day -2) in addition to the Flu/Cy/2Gy TBI for patients older than 50 years or not medically fit to tolerate an MAC regimen.

Prophylaxis

Prophylaxis against GVHD is provided with mycophenolate mofetil (MMF) and tacrolimus. We start MMF from day -3 at a dose of 15 mg/kg (maximum of 1 g orally twice daily) and continue it through day +100. Tacrolimus is started from day -2 and taper started at day 180 in the absence of GVHD. We use rabbit ATG 3 mg/kg infused over 2 days on days -4 and -3 in all patients. Patients on azole antifungals require appropriate dose adjustments for tacrolimus. Other drug interactions and creatinine clearance need be considered while calculating the dosing.

Filgrastrim is administered from day 0 until neutrophil engraftment and blood products are transfused as indicated. Standard infectious disease prophylaxis with antibacterial (levofloxacin), antiviral (valacyclovir), and antifungal (voriconazole, posaconazole, or caspofungin—the choice depending on risk factors) and against Pneumocystis jiroveci pneumonia are also provided for all CBT patients. The surveillance for CMV using quantitative polymerase chain reaction (qPCR) (or antigenemia assay if absolute neutrophil count is >1 × 10⁹/L) is performed routinely twice weekly for the first 100 days after CBT or longer if any complications are present. We routinely perform Epstein-Barr viremia testing using qPCR every 2 weeks from day +30 until day +100. Other viruses, including adenovirus, BK virus, respiratory syncytial virus, influenza, human herpes virus 6, and parainfluenza, are tested as clinically indicated.

Novel Strategies to Improve Cord Blood Transplantation Outcomes

To increase CB cell dose, a variety of ex vivo expansion techniques have been developed that yield significantly higher final numbers of TNCs. These include coculturing the CB cells with cytokine support or MSCs to simulate the BM “hematopoietic niche” ex vivo (³⁴) or using nicotinamide analogs (³⁷) or copper chelators (such as tetraethylenepentamine) (³⁸) and targeting the Notch signaling pathway (³⁹), all of which block the differentiation of early progenitor cells, leading to expansion of hematopoietic stem cells. Apart from augmenting the cell dose, ex vivo graft manipulation techniques are being explored to improve the inherent homing capacity of CB cells, with an aim to accelerate engraftment. Different methods are being tested in clinical trials, such as fucosylation of the CB progenitor cells (⁴⁰) or the use of prostaglandin E₂ derivatives (⁴¹) or dipeptidyl peptidase-4 inhibitors. Many of these techniques have demonstrated significantly improved time to neutrophil engraftment (13-17 days) comparable to other donor types (Fig. 14-4).

Ex vivo graft manipulation permitted the generation and clinical use of antivirus and antitumor adoptive immunotherapies as well as cellular therapies for GVHD prevention. A variety of “designer” CB lymphocytes can now be engineered and expanded ex vivo. For instance, these include T cells with specific cytotoxicity against tumors or viruses (^42,43,44), chimeric antigen receptor (CAR) T cells (⁴⁵), natural killer (NK) cells (⁴⁶), and regulatory T cells (T_Regs) (⁴⁷), which are being tested in clinical trials.

Cord blood transplantation is an attractive option for patients who lack a matched PB or BM donor. To overcome the limitation of low cell doses in single-CB units, dCBT has been adopted for many patients and is associated with outcomes comparable to those with other donor sources. There are promising strategies to improve engraftment with ex vivo expansion or homing and to enhance immune reconstitution with the infusion of CB-derived NK cells and cytotoxic T lymphocytes with antiviral and antileukemic specificities. Prospective multicenter clinical trials are needed to determine the efficacy of these promising technologies.