Single-Unit Umbilical Cord Blood Transplantation from Unrelated Donors in Adult Patients with Chronic Myelogenous Leukemia Academic research paper on "Clinical medicine"

ASBMI

American Society for Blood and Marrow Transplantation

Single-Unit Umbilical Cord Blood Transplantation from Unrelated Donors in Adult Patients with Chronic

Myelogenous Leukemia

Jaime Sanz,1'2 Pau Montesinos,1 Silvana Saavedra,1 Ignacio Lorenzo,1 Leonor Senent,1 Dolores Planelles,3 Luis Larrea,3 Guillermo Martín,1 Javier Palau,1 Isidro Jarque,1 JesUs Martínez,1 Javier de la Rubia,1 Federico Moscardo,1 David Martinez,1 Ines Gomez,1 María López,1 Miguel A. Sanz,1 Guillermo F. Sanz1

Clinical studies focused on outcomes of umbilical cord blood transplantation (UCBT) for patients with chronic myelogenous leukemia (CML) in need of allogeneic stem cell transplantation and lacking an HLA-matched adult donor are limited. We analyzed the outcome of 26 adults with CML receiving single-unit UCBT from unrelated donors after myeloablative conditioning at a single institution. Conditioning regimenswere based on combinations of thiotepa, busulfan, cyclophospamideorfludarabine, and antithymo-cyte globulin. At the time of transplantation, 7 patients (27%) were in first chronic phase (CP), 11 (42%) were in second CP, 2 (8%) were in accelerated phase (AP), and 6 (23%) were in blast crisis (BC). The cumulative incidence (CI) of myeloid engraftment was 88% at a median time of 22 days and was significantly better for patients receiving higher doses of CD341 cells. The CI of acute graft-versus-host disease (GVHD) grade II-IV was 61%, that of acute GVHD grade III-IV was 39%, and that of chronic extensive GVHD was 60%. Treatment-related mortality (TRM) was 41% for patients undergoing UCBT while in first or second CP and 100% for patients in AP or BC (P <.0i). After a median follow-up of 8 years, none of the patients relapsed, giving an overall disease-free survival (DFS) at 8 years of 41%. The DFS for patients undergoing UCBT while in any CP was 59%. These results demonstrate that UCBT from unrelated donors can be a curative treatment fora substantial number of patients with CML. Advances in supportive care and better selection of cord blood units and patients are needed to improve TRM.

Biol Blood Marrow Transplant 16: 1589-1595 (2010) © 2010 American Society for Blood and Marrow Transplantation KEY WORDS: UCBT, Transplantation, CML

INTRODUCTION

Allogeneic hematopoietic stem cell transplantation (HSCT) is no longer the first-line treatment for most patients with chronic myelogenous leukemia (CML) because of the high efficacy of tyrosine-kinase inhibitors (TKIs), such as imatinib [1]. However, a number of patients treated with imatinib will eventually need second-line treatment [2]. For those patients progress-

From the 'Department of Hematology, Hospital Universitario La Fe, Valencia, Spain; Department of Medicine, Universitat Autónoma de Barcelona, Barcelona, Spain; and Centro de Trasfusion de la Comunidad Valenciana, Valencia, Spain. Financial disclosure: See Acknowledgments, page 1595. Correspondence and reprint requests: Dr Jaime Sanz, Hospital Universitario La Fe, Servicio de Hematología, Avda Campanar 21, Valencia, Spain (e-mail: sanz_jai@gva.es). Received March 21, 2010; accepted May 18, 2010 © 2010 American Society for Blood and Marrow Transplantation 1083-8791/$36.00 doi:10.1016/j.bbmt.2010.05.014

ing to a more advanced phase of the disease and those in first chronic phase (CP) who fail second-line TKI therapy or harboring a T315I mutation, allogeneic HSCT is recommended [3]. Umbilical cord blood (UCB) could be used as an alternative stem cell source for patient in whom allogeneic HSCT is indicated and lack an HLA-matched adult donor.

Very little information exists on the outcome of patients undergoing UCB transplantation (UCBT) to treat CML. Only 2 previous studies—a preliminary report of a small series of patients from our institution testing feasibility [4] and a recent analysis from the Japan Cord Blood Bank Network (JCBBN) registry [5]—have focused on patients with CML. In addition, some early registry-based studies of patients undergoing UCBT for a variety of hematologic malignancies suggested a particularly poor outcome for patients with CML [6,7].

We report the outcome of a series of adult patients with CML with prolonged follow-up who underwent UCBT with a graft from an unrelated donor at a single

institution. Apart from confirming the feasibility and efficacy of the procedure in this specific disease, an additional aim of the study was to identify variables influencing short-term and long-term outcomes.

PATIENTS AND METHODS

Eligibility Criteria

This report constitutes a retrospective review of all 26 consecutive adult patients with CML who underwent UCBT from unrelated donors at our institution between May 1997 and July 2009. Patients were eligible for enrollment if they met the following criteria: (1) Allogeneic HSCT from an unrelated donor was considered the best therapeutic option; (2) there was no suitable related donor (HLA-identical or one-antigen-mismatched); (3) there was a need for urgent transplantation or lack of an HLA-identical unrelated donor after a search of the international registries; and (4) an there was an available UCB unit fulfilling the minimum established criteria for both HLA compatibility between donor and recipient and cell dose. In this regard, UCB units had to share at least 4 HLA antigens with the recipient (HLA class I antigens [A and B] determined by serologic or low-resolution DNA typing and class II antigens [DRB1] by highresolution DNA typing). In terms of cell dose, a nucleated cell (NC) dose >1.5 x 107 per kg of recipient body weight was required until May 2006. Thereafter, institutional policy required a NC dose >2 x 107 per kg of recipient body weight and a CD341 cell dose >1 x 105 per kg of recipient body weight because of our observation of the impact of cell dose on outcome. Our Institutional Review Board approved the protocol, and written informed consent was obtained from all patients according to the Declaration of Helsinki.

UCB Unit Selection and Management

The search for UCB units was conducted by the Spanish Registry of Bone Marrow Donors (Registro Espaíiol de Donantes de Medula (Osea). Among the available UCB units fulfilling the minimum established criteria, units with higher cell dose (considering both NCs and CD341 cells) and greater HLA compatibility were selected across the study period. ABO compatibility and year of storage were considered as well. All UCB units tested negative for human immunodeficiency virus, hepatitis B and C viruses, and human T cell lymphotropic virus type I. All UCB units and mothers were negative for immunoglobulin M antibody to cytomegalovirus (CMV).

UCB units were thawed according to the method of Rubinstein et al. [8], with minor modifications as described elsewhere [9]. Before infusion, a sample was drawn directly from the final bag for cell counts,

including CD34+ cells [10], cell viability, clonogenic assays [11], and microbiology.

Conditioning Regimen

All patients received a myeloablative conditioning regimen based on a combination of thiotepa, busulfan (Bu), cyclophospamide (Cy), or fludarabine (Flu), and antithymocyte globulin (ATG) (Table 1). Until November 2004, 21 patients received thiotepa 5 mg/kg/ day on days —9 and —8, oral Bu 1 mg/kg/6 hours or i.v. Bu 0.8 mg/kg/6 hours or on days —7 to —5, Cy 60 mg/kg/day on days —4 and -3, and ATG on days —5 to —2. Thereafter, Cy was substituted by Flu in 5 patients, who received thiotepa 5 mg/kg/day on days —7 and —6, i.v. Bu 3.2 mg/kg as a single daily dose on days —5 to —3, Flu 50 mg/m2/day on days —5 to —3, and ATG on days —5 to —2.

In terms of the type of ATG used, the first 15 patients received horse ATG (Lymphoglobuline; Merieux, Lyon, France; total dose, 60 mg/kg), and the subsequent 11 patients received rabbit ATG (Thy-moglobulin; Sangstat/Genzyme, Lyon, France; total dose, 8 mg/kg).

Graft-versus-Host Disease Prophylaxis and Treatment

All patients received cyclosporine 1.5 mg/kg/12 hours i.v., followed by 3-5 mg/kg/12 hours orally when oral intake was possible, with slow tapering starting between days +90 and +180 and discontinuation on day +180 or before if feasible. Cyclosporine was combined with prednisone in the first 22 patients (0.5 mg/kg/day on days +7 to +14 and 1 mg/kg/day on days +14 to +28, with slow tapering to discontinuation on day +180) [9]. Three patients patients received mycophenolate mofetil 15 mg/kg/12 hours until day +28 and prednisone 1 mg/kg/day on days + 14 to +28 in the remaining patient.

Patients who developed acute graft-versus-host-disease (aGVHD) received high-dose methylpredniso-lone as initial therapy (20 mg/kg/day, with the dose halved every 3 days until it reached 1 mg/kg/day, and tapered gradually thereafter).

Supportive Care

Patients were nursed in high-efficiency particulate air (HEPA)-filtered rooms. Intravenous access was achieved with a double-lumen tunneled central venous catheter. Granulocyte colony-stimulating factor 5 mg/ kg/day was administered s.c. from day +7 untilneutro-phil engraftment occurred. All patients received oral ciprofloxacin 500 mg/12 hours as antibacterial prophylaxis. Prophylaxis against Pneumocystis jiroveci consisted of cotrimoxazole (320/1600 mg trimethoprim/ sulfamethoxazole daily) from day -10 to day -2, and then restarted after engraftment and maintained for

Table 1. Graft- and Transplantation-Related Characteristics*

HLA compatibility, n (%)

6 of 6 0(0)

5 of 6 9(35)

4 of 6 17(65)

ABO blood group mismatch, n (%)

Major 5(19)

Minor 5(19)

None 16 (62)

Donor-recipient sex match, n (%)

Male-male 7(27)

Male-female 7(27)

Female-male 8(31)

Female-female 4(15)

Year of transplantation, n (%)

1997-2001 17(65)

2002-2009 11 (35)

Conditioning regimen, n (%)

TT + Bu + Cy + ATG 21 (81)

TT + Bu + Flu + ATG 5(19)

GVHD prophylaxis, n (%)

Cyclosporine A + prednisone 23 (88)

Cyclosporine A + MMF l07/kg 3(12)

Number of NCs before freezing, x

Median 2.5

Range 1.5-6.9

Number of NCs infused, x l07/kg

Median 1.9

Range 1.1-4.9

Number of CD34+ cells before freezing, x 10 /kg

Median 1.2

Range l05/kg 0.3-2.2

Number of CD34+ cells infused, x

Median 0.9

Range 0.3-2.2

TT indicates thiothepa; Bu, busulfan; Cy, cyclophosphamide; Flu, fludarabine; MMF, mycophenolate mofetil. *Percentages may not sum to 100 because of rounding.

a minimum of 1 year or until the cessation of immuno-suppresion (2 days a week). Fluconazole 100 mg/day orally was administered as antifungal prophylaxis at the beginning of the study period. Starting in November 2003, prophylaxis with fluconazole was substituted by i.v. itraconazole 200 mg/day. All blood products were irradiated and leukocyte-depleted. CMVprophylaxis, infection surveillance, and treatment have been described in detail elsewhere [12]. Nonspecific i.v. immunoglobulin was administered at a dose of 500 mg/ kg weekly up to day +100 and then monthly during the first year after transplantation.

Definitions

Myeloid engraftment was defined as an absolute neutrophil count (ANC) of $0.5 x 109/L on 3 consecutive days. Platelet engraftment was defined as a platelet count of $20 x 109/L, without transfusion support, for 7 consecutive days. Patients who survived for more than 28 days after transplantation and who failed to achieve myeloid engraftment were considered graft failures. Time to myeloid or platelet engraftment was defined as the time required to reach the first day of engraftment. aGVHD and chronic GVHD (cGVHD)

were defined and graded according to standard criteria [13-15].

Statistical Analysis

The probabilities of engraftment, treatment-related mortality (TRM), GVHD, and relapse were estimated by the cumulative incidence (CI) method (marginal probability) [16]. For CI analysis of engraftment, GVHD, and relapse, death while in complete remission was considered a competing cause of failure, whereas for TRM, relapse was considered the competing event. Unadjusted time-to-event analyses were performed using the Kaplan-Meier estimate [17], and, for comparisons, the log-rank test [18]. Disease-free survival (DFS) was calculated from the date of UCBT. In the analysis of DFS, relapse and death while in complete remission, whichever occurred first, were considered uncensored events. Variables considered for prognostic factor analysis were age, sex, recipient CMV serology, disease status at transplantation, degree of HLA match, donor-recipient sex match, and NC and CD34+ cell dose before freezing and at infusion. Continuous variables were dichotomized at the most discriminative cutoff point for each outcome. The follow-up of the patients was updated on February 1,2010. The median follow-up for surviving patients was 99 months (range, 7-153 months). All P values reported are 2-sided. Except for the cumulative incidence method, computations were performed using the appropriate programs from the BMDP statistical library (BMDP Statistical Software, Los Angeles, CA) [19].

RESULTS

Patient and Disease Characteristics

Table 2 shows the characteristics of the 26 adult patients with CML who underwent UCBT from unrelated donors. In brief, 14 patients were males and 12 females with a median age of 33 years (range, 16-48 years). Seven patients (27%) were in first chronic phase (CP), 11 (42%) were in second CP, 2 (8%) were in accelerated phase (AP), and 6 (23%) were in blast crisis (BC) at the time of transplantation.

All 7 patients in first CP were transplanted without any cytogenetic response; 5 had stable disease, and 2 were in complete hematologic remission. Only one of these patients had received previous treatment with imatinib. This patient achieved complete hema-tologic response, but had no cytogenetic response after 6 months of treatment. The remaining patients who underwent transplantation while in first CP did so before imatinib became available. Three patients had received a mobilization regimen with mini-ICE as described previously [20], with successful collection

Table 2. Characteristics of Patients with CML Undergoing UCBT*

Number of patients 26

Age, years

Median 33

Range 16-48

Age group, n (%)

16-20 years 3(12)

21-30 years 7(27)

31-40 years 9(34)

>40 years 7(27)

Sex, n (%)

Male 14(54)

Female 12(46)

Weight, kg

Median 63

Range 45-95

Disease status at transplantation, n (%)

First chronic phase 7 (27)

Second chronic phase 11 (42)

Accelerated phase 2 (8)

Blast crisis 6(23)

Previous ASCT, n % 4(15)

CMV serologic status before transplantation, n (%)

Positive 21 (81)

Negative 5(19)

Time from diagnosis to transplantation

for patients in first CP months

Median 19

Range 6-40

CML indicates chronic myelogenous leukemia; UCBT, umbilical cord blood transplantation; ASCT, autologous stem cell transplantation; CP, chronic phase.

*Percentages may not sum to 100 because of rounding.

of peripheral blood stem cell progenitors that were stored and used as a backup. The median time from diagnosis to transplantation for patients undergoing transplantation while in first CP was 19 months (range, 6-40 months).

Of the 11 patients undergoing transplantation while in second CP, 2 had a previous AP (1 at diagnosis and 1 after imatinib failure) and received an autologous stem cell transplantation (ASCT) to achieve a second CP. The remaining 9 patients had a previous BC, 5 of lymphoid lineage and 4 of myeloid lineage (in 5 patients at initial diagnosis and in 4 after a first CP). To restore second CP, 4 patients received imatinib, 3 patients received acute myelogenous leukemia (AML)-type chemotherapy, and 1 patient received a combination of AML-type chemotherapy and imatinib.

Two patients underwent UCBT while in AP (1 at presentation and 1 progressing from a CP after ASCT). The remaining 6 patients underwent transplantation while in BC. All of these patients had received chemotherapy in an attempt to revert to CP. In 3 of these patients, BC was present at diagnosis; 2 patients relapsed to second BC after initial response to chemotherapy, and 1 patient had refractory disease. In the other 3 patients, BC developed during the course of the disease, after ASCT in 1 patient and after hydroxyurea and interferon therapy in 1 patient. The third patient, who had a T315I mutation, developed

BC on imatinib and relapsed to second BC after initial response to chemotherapy.

Cord Blood Unit and Transplant Characteristics

Table 1 shows the characteristics of UCB units and those related to the transplantation procedure. All patients received an HLA-mismatched UCB unit. Donor-recipient disparity in 1 and 2 of 6 HLA antigens was present in 9 patients (35%) and 17 patients (65%), respectively. The median number of NCs and CD34 cells infused was 1.9 x 107/kg (range, 1.14.9 x 107/kg) and 0.9 x 105/kg (range, 0.3-2.2 x 107/kg), respectively. Five patients (19%) received UCB units with a major ABO blood group mismatch. Eight male patients (31%) received a UCB unit from a female donor.

Myeloid and Platelet Engraftment

One patient died on day 10 after UCBT without evidence of myeloid engraftment. Two other patients required special considerations; 1 patient who underwent transplantation while in CP died on day 25 without evidence of myeloid engraftment with complete lack of donor chimerism and was considered a graft failure, and 1 patient died on day 47 with an ANC of 0.4 x 109/L and detection of full donor chimerism and was considered to have a competing event for engraftment. The remaining 23 patients experienced myeloid engraftment at a median time of 22 days (range, 13-52 days). The CI of myeloid engraftment was 88% at 52 days (Figure 1). All patients with myeloid engraftment demonstrated full donor chimerism at the time of reconstitution.

The only variable influencing the time to myelogenous engraftment was CD341 cell dose at infusion, with a best cutoff at 0.6 x 105/kg (Figure 1). The CI rates of myeloid engraftment for patients receiving

CD34+ > 0.6 x 105/kg 0,9 (N =21)

0 8- CI: 95%

a, Median: 22 days

g 0,7 d)

« 0,5 >

ni 0,4

0 10 20 30 40 50 60

Days after transplant

Figure 1. Cumulative incidence of neutrophil recovery after UCBT, overall and according to the number of CD34 cells at infusion (cutoff, 0.6 x 105/kg).

Overall CI: 88% Median: 22 days

CD34+S0.6 x 105/kg (N =5) C!: 60% Median: 39 days

UCB units with CD341 cells above and below 0.6 x 105/kg were 95% and 60%, respectively (P = .02).

Twelve patients died between 10 and 154 days after UCBT without platelet engraftment. The remaining 14 patients had platelet engraftment at a median of 67 days (range, 29-188 days). The cumulative incidence of platelet engraftment was 54% at 188 days.

All patients developed some grade of aGVHD (grade I in 7 patients, grade II in 7, grade III in 5, and grade IV in 5). The median time to the development of acute GVHD grade II-IV was 14 days (range, 838). The CI of grade II-IV and grade III-IV aGVHD at day 100 after UCBT was 61% and 39%, respectively. Skin involvement was observed in 23 patients (grade II in 14, grade III in 2, and grade IV in 1), intestinal involvement was seen in 9 patients (grade II in 2 and grade IV in 4), and liver involvement was seen in 14 patients (grade II in 1, grade III in 3, and grade IV in 4). Four patients died of acute GVHD grade IV.

Eleven of 16 patients at risk developed cGVHD (limited in 2 patients and extensive in 9 patients). The median time to development of cGVHD was 214 days (range, 96-385 days), and the 2-year CI of extensive cGVHD was 60%. Four of 9 patients with extensive cGVHD (45%) had complete resolution and discontinuation of immunosuppressive therapy.

0,9 . °,8

Si °,7

Ö 0,6 c

Ö) 0,5 >

o 0,2 0,i 0

Accelerated phase/blast crisis (N = 8) TRM = 100%

Chronic phase (N = 18) TRM = 41%

P < .01

0 12 24 36 48 60 72 84 96 108 120 132 144 156 Months after transplant

Figure 2. Cumulative incidence of TRM according to disease stage at UCBT.

patients remained alive and leukemia-free after UCBT at last follow-up. The overall DFS at 8 years was 41% (95% confidence interval, 21%-61%).

For patients in first CP, 3 of 7 remained CML-free after 129, 130, and 135 months of follow-up, with an 8-year DFS of 43%. Eight of 11 patients who underwent transplantation while in second CP remain alive and leukemia-free, with an 8-year DFS of 69%. Overall, DFS at 8 years was 59% (95% confidence interval, 37%-81%) for patients undergoing UCBT while in any CP, compared with 0% for those undergoing transplantation while in AP or BC (P <.001) (Figure 3).

TRM and Causes of Death

Fifteen transplantation-related deaths occurred at a median time of 56 days after transplantation (range, 10-681 days). The causes of death were infection in 6 patients, GVHD in 5 patients (4 aGVHD and 1 cGVHD), hemorrhage in 2 patients, encephalitis of unknown cause in 1 patient, and veno-occlusive disease in 1 patient. Three of the 6 deaths attributable to infection were bacterial infections (2 with Acinetobacter bau-manii and 1 with Staphylococcus aureus), 1 was an adenovirus infection with invasive aspergillosis, and 1 was interstitial pneumonia without microbiological isolation. The distribution of causes of death did not differ between patients who underwent transplantation while in CP and those who underwent transplantation while in AP or BC.

The CI of TRM was 38% at day 100, 50% at 1 year, and 59% at 8 years. The only variable influencing TRM was disease stage at time of transplantation; the CI ofTRM at 8 years was 41% for patients undergoing UCBT while in first or second CP and 100% for those undergoing UCBT while in AP or BC (P <.01) (Figure 2).

Relapse and DFS

No hematologic, cytogenetic, or molecular relapses were observed during the study period. Eleven

DISCUSSION

This single-center study with long-term follow-up confirms that single-unit UCBT from unrelated donors after myeloablative conditioning is a feasible alternative for adults with CML undergoing transplantation while in CP. The absence of relapses in our patients confirms the high efficacy of the procedure, although it is hampered by high TRM. We also were able to identify CD341 cell dose at time of infusion

Chronic phase (N = 18) DFS = 59%

Accelerated phase/blast crisis (N = 8) DFS = 0%

P < .01

0 12 24 36 48 60 72 84 96 108 120 132 144 156 Months after transplant

Figure 3. Kaplan-Meier estimates of DFS according to disease stage at UCBT.

as the main factor associated with myeloid recovery. Although the relatively small sample size and the retrospective nature of this study are limitations, these data may provide clinically useful information to improve the outcome of UCBT in adults with CML in need of allogeneic HSCT through better selection of both UCB grafts and recipients.

Apart from a previous report from our group describing the preliminary data from a series of 9 patients undergoing UCBT [4], only 1 study using data from the JCBBN registry has focused on patients with CML [5]. Thus, we discuss our results in the context of this limited information on UCBT in patients with CML. Moreover, because of the special characteristics of the sample population, our results cannot be extrapolated to those obtained after HSCT using other stem cell sources or donors.

In contrast with the sole previous report on UCBT in CML by Nagamura-Inoue et al. [5], which was based on registry data, our study analyzed a homogenous population of adults undergoing busulfan-based myeloablative conditioning with closely similar GVHD prophylaxis and supportive care at a single institution. It should be taken into account that the vast majority of transplantations were performed before 2002, the early period of UCBT activity, when quality programs for processing and cryopreserving UCB units, as well as the criteria for unit selection and optimization of clinical management, were not yet well established. It should also be considered that, similar to previous studies, most of the patients were heavily pretreated and underwent transplantation in a very advanced phase of disease.

Our study found a high rate of myeloid engraftment with rapid neutrophil recovery using single-unit UCB that compares favorably with other registry data [5-7]. Rubinstein et al. [6] reported a significantly reduced likelihood of successful engraftment in patients with CML, comparable to that seen in patients with Fanconi's anemia and severe aplastic anemia. The study by the JCBBN also reported a worse-than-expected myeloid engraftment, with a CI ofneutrophil recovery of 68% at a median time of 24 days and a graft failure rate of 17% [5]. However, in the current series, the CI of neutrophil recovery was 88% at a median of 22 days, with only 1 patient demonstrating graft failure. Again, CD341 cell dose was significantly associated with the speed and rate of engraftment, as reported previously by us and others [21-23].

The apparently higher incidence of aGVHD in this series compared with that previously reported by us in patients with AML undergoing UCBT [22] merits some comment. A possible explanation could be the substantially higher proportion of patients with CML who received horse ATG instead of rabbit ATG in the conditioning regimen, because of the fact that most of them underwent transplantation at the be-

ginning of the UCBT program at our center. In fact, unpublished data from our institution suggest that the type of ATG has a clear impact on the risk of aGVHD. Nevertheless, an intrinsic susceptibility to the development of GVHD in patients with CML compared with patients with other malignant diseases cannot be ruled out, given that this has been reported after UCBT [7] and bone marrow (BM) HSCT [24].

Although we possibly could have underestimated the risk of relapse because of the elevated TRM, it is noteworthy that all 11 patients who survived UCBT (8 of whom underwent transplantation while in second CP after previous BC) remained CML-free after a median follow-up of around 8 years. This finding suggests a powerful graft-versus-leukemia effect after UCBT, as has been observed after unrelated donor allogeneic BM or peripheral blood HSCT [25]. However, the relapse rate seems to be significantly different than that reported for T cell-depleted grafts [26]. This is a particularly important observation, because the low lymphocyte content of UCB grafts and the lower incidence of GVHD after UCBT compared with transplantation with other stem cell sources led to early concerns regarding a possible increased risk of relapse.

Our overall results in terms of DFS were influenced by TRM and the associated risk factors. TRM was relatively high, particularly in patients undergoing transplantation while in BC or AP, as has been described in all series including patients with CML [5-7]. A Eurocord analysis of 171 adults undergoing UCBT for a variety a hematologic malignancies found a significantly higher 2-year CI of TRM (76%) in patients with CML compared with those with other malignant diseases [7]. Moreover, the New York Blood Center reported that the overall incidence of transplantation-related events other than relapse were negatively correlated with the recipient's diagnosis of CML [6]. A possible explanation for this finding is the advanced phase of the disease and the cumulative drug-related toxicity at the time of transplantation for the vast majority of patients. In addition, we should consider that most transplantations were performed in the early period of UCBT, before the imatinib era. Thus, those results should be interpreted cautiously, because they do not necessarily reflect current practice. It is likely that pretransplantation imatinib mesylate will reduce the TRM rates in more recent years by reducing the tumor burden at the time of transplantation and by avoiding exposure to more toxic treatments, as has been described in the unrelated donor BM transplantation setting [27,28].

In conclusion, our study shows that single-unit UCBT from an unrelated donor after myeloablative conditioning can be a curative treatment for a substantial number of patients with CML in need of HSCT. Because of its impact on engraftment, CD34+ cell

dose should be considered when selecting UCB units. Because TRM is the most important limitation of the procedure, advances in supportive care and better selection of UCB units and patients are needed. More specific studies in adults with CML undergoing UCBT from unrelated donors are warranted to definitely establish the role of the procedure in the TKI era.

ACKNOWLEDGMENTS

The authors thank Shirley Weiss for data collection and management.

Authorship Statement: Jaime Sanz and Guillermo F. Sanz conceived the study and interpreted the data; Jaime Sanz wrote the manuscript and performed the statistical analyses; and Pau Montesinos, Silvana Saavedra, Ignacio Lorenzo, Leonor Senent, Dolores Planelles, Luis Larrea, Guillermo Martín, Javier Palau, Isidro Jarque, Jesus Martínez, Javier de la Rubia, Federico Moscardoí , David Martinez, Ineís Gómez, María López, and Miguel A. Sanz reviewed the manuscript and contributed to the final draft.

Financial disclosure: The authors have nothing to disclose.

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