Analysis of Hematopoietic Cell Transplants Using Plasma-Depleted Cord Blood Products That Are Not Red Blood Cell Reduced

Article Outline

• Abstract
• Introduction
• Patients, Materials, and Methods
  • Standards and Regulations
  • Collection, Donation, Plasma Depletion Processing, Testing, Cryopreservation, and Storage
  • Thaw Procedure, Conditioning, and GVHD Prophylaxis Regimens
  • Comparisons of Postprocessing Cell Dose between PD and Red Cell Depleted/Reduced (RD) CB Inventories
  • Outcome Data Collection Rate, Source, Audits, Data Subsets, and Statistical Analysis
    • Outcome Data Collection Rate and Sources
    • Data Audit
    • Data Subsets
  • Statistical Analysis
• Results
  • CB Inventory
  • Postprocessing and Postthaw Recovery Rates for Plasma Depletion Processing
  • Comparisons of PD and RD CB Inventories
  • Outcome Data Audits
  • Characteristics of Transplanted PD CB Product
  • Characteristics of Patient Population and Disease Indications
  • Infusion Associated Adverse Reactions of PD CB Product
  • Outcome Data—Engraftment, TRM, GVHD Rate, Relapse Rate, RFS, and OS
• Discussion
• Acknowledgments
• References
• Copyright

Abstract 

Limited cell dose hampers wider use of cord blood transplantation (CBT). By depleting plasma but not RBC during processing, nucleated cell (NC) loss is reduced to <0.1% which increases significantly the proportion of high cell dose products—3-fold for products with NC ≥200 × 10⁷. Clinical outcome for plasma depleted (PD) CBT was previously unavailable. A retrospective audited analysis was performed on 118 PD CBT, with mean and median NC doses of 7.6 × 10⁷/kg and 5.6 × 10⁷/kg, respectively, for this mostly pediatric population. The median times to engraftment and engraftment rates for ANC 500 and platelet 20K were 22 and 50 days, respectively, and 90% ± 3% and 77% ± 5%, respectively. The incidences of grade III-IV acute graft-versus-host disease (aGVHD) and extensive chronic GVHD (cGVHD) were 13% ± 4% and 17% ± 6%, respectively. Relapse rate for malignancies was 25% ± 6% and 100-day treatment-related mortality (TRM) was 16% ± 3%. With a median follow-up of 557 days, the 1-year overall survival and relapse-free survival are 65% ± 5% and 51% ± 6%, respectively. These results demonstrate that PD CBT is safe and effective, and that eliminating RBC reduction or depletion improves cell recovery during CB processing, resulting in a larger proportion of the inventory with high NC number.

Key Words: Cord blood transplantation, Cord blood banking, Plasma depletion, Cord blood processing, Volume reduction, Postthaw wash

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Introduction 

Many patients in need of a hematopoietic cell transplant (HCT) do not have a matched-related or unrelated bone marrow or peripheral blood donor, although there are currently over 10 million volunteers registered worldwide [1]. If a patient belongs to an ethnic minority, the probability of not finding an HLA-matched unrelated donor is even greater. Moreover, because the search process for an adult donor often takes months, a significant proportion of patients will expire, or become higher risk, or ineligible for transplantation while waiting for the completion of a donor search [2]. For patients who cannot find a suitably HLA-matched adult donor, cord blood (CB) is a viable option [1]. As CB products are HLA-typed, tested for lack of infectious agents, and stored, they are immediately available upon request and can be shipped to transplant centers around the world. Cord blood transplantation (CBT) has been reported to be associated with a lower incidence of acute graft-versus-host disease (aGVHD), and partial HLA match between the donor and recipient is tolerable [1].

Favorable results following CBT have contributed to changes in clinical practice. In the pediatric setting, CBT is now an established practice and may soon surpass the number of unrelated volunteer donor transplants, as is already the case in Japan. In the adult setting, adaptation has been slower due to the cell-dose limitation. Strategies focused on overcoming this limitation are under intense investigation including the utilization of double CB grafts [3, 4, 5, 6, 7], avoidance of postthaw wash [8, 9, 10, 11, 12], and the combination of unrelated CB with haploidentical peripheral blood stem cells [13], all of which have shown promising engraftment and survival results.

Because stem and progenitor cells are inevitably lost during red cell depletion/reduction processing (RD) of CB products with nucleated cell losses of 14% to 42% [14, 15, 16, 17], we have developed and evaluated the use of CB products from which plasma but not red cells have been removed to minimize cell loss. A large CB inventory of 26,000 CB products processed with this novel technology has now been established, with 88% from minority donors.

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Patients, Materials, and Methods 

Standards and Regulations 

All applicable state, federal, national, and international regulations and standards were followed, including standards for Foundation for the Accreditation of Cellular Therapy and AABB.

Collection, Donation, Plasma Depletion Processing, Testing, Cryopreservation, and Storage 

CB was collected under institutional review board approval and donor informed consents. Donors with genetic or behavioral risks on medical history were excluded from donation. Trained collectors harvested CB by closed system collection blood bag containing citrate-phosphate buffer-dextrose. Collected CB units were shipped at room temperature (15°C to 26°C) to processing laboratories using validated containers. Plasma depletion processing was performed and completed in a closed system within 48 hours of collection. CB units were centrifuged in the original collection bag at 1680 × g for 10 minutes at room temperature followed by variable amounts of plasma removed into an attached empty bag using a plasma expressor to reach a final CB volume of 60-84 mL before adding cryoprotectant solution. For larger units, the products were stored in 2 bags. Postprocessing, samples were taken for complete blood count with differential and nucleated red blood cell enumeration with a Sysmex-9600, trypan blue viability, hemoglobinopathy screening, ABO/Rh typing, HLA typing using DNA-based methods and CD34⁺ cell enumeration with the Pro-Count Method using a Becton Dickinson (Franklin Lakes, NJ) FAC-SCAN. Colony-forming unit (CFU) assays were added since 2004, respectively. The collection bag(s) containing the remaining cellular portion was then connected in a sterile manner to a Baxter (Deerfield, IL) Cryocyte® freezing bag via a sterile docking device. The plasma depletion (PD) CB product was then precooled at 4°C for at least 30 minutes prior to cryoprotectant addition. The cryopreservation solution consisting of 50% DMSO and 10% Dextran Sulfate (Gentran-40), prechilled to 4°C, was then added to a final concentration of 5% to 10% DMSO using a syringe pump, with the CB and the cryoprotectant wrapped between cool packs during this 20-minute period. After cryoprotectant addition, CB products were subjected to controlled rate freezing from 4°C to −50°C at −1°C per minute, and then from −50°C to −90°C at −10°C per minute in MVE CRF 2100 controlled rate freezers. All control rate freezing curves were reviewed to ensure that at no point from 4°C to −50°C did the product experience a cooling rate in excess of −5°C per minute. After attaining −90°C, the CB products were immediately transferred from the controlled rate freezer to liquid nitrogen tanks.

Products with maternal or CB reactive for HIV-1/2, HTLV-I/II, HCV, and HBsAg serology, positive HIV or HCV NAT results, positive bacterial or fungal culture, homozygosity or double heterozygosity for hemoglobinopathy mutations were excluded from transplantation inventory.

Thaw Procedure, Conditioning, and GVHD Prophylaxis Regimens 

Prior to infusion of the thawed CB, the product was washed in 67 instances (56.8%), whereas 50 products were infused without washing (42.4%) per transplant center’s decision (information unavailable for 1 unit). When washing was performed, transplant centers either used their own protocol or followed the StemCyte recommended wash protocol. As a CB bank study with many transplant centers (TC), various conditioning and GVHD prophylaxis regimens were used.

Comparisons of Postprocessing Cell Dose between PD and Red Cell Depleted/Reduced (RD) CB Inventories 

Postprocessing and postthaw recovery rates were determined for plasma depletion. Although different recovery rates for RD have been reported [14, 15, 17, 18], no data exist for a rigorous comparison of actual CB inventories. The National Marrow Donor Program (NMDP) CB inventory was chosen to study if there exists any difference in cell dose between the 2 types of inventories, because the NMDP CB banks have the largest national inventory of CB, with 2 banks that practice PD processing and 16 that practice RD processing. As of June 31, 2006, there were 10,912 PD CB and 38,819 RD CB for a combined inventory of 49,731 units [19]. Comparison was performed between the PD CB and RD CB inventories to assess any differences in proportion of high total nucleated cell (TNC) CB units.

Outcome Data Collection Rate, Source, Audits, Data Subsets, and Statistical Analysis 

A retrospective analysis was performed that included all patients with engraftment and/or survival data transplanted with PD CB from November 2001, the time of the first transplant performed with plasma-depleted CB, to the cutoff date of April 28, 2005. Two CB banks (StemCyte US and Taiwan CBB) provided all the PD units for the analysis. The full cohort of all 118 CBT using PD CB constituted the “All Patients” group. When patients with prior transplants or transplanted during relapse were excluded in the engraftment and survival analysis to assess outcome without the most advanced risk patients, the remaining subset had 98 such patients and are identified as “Remission First Transplant Patients,” and this cohort contained both patients with benign indications and patients with malignant indications transplanted during remission. Transplants using CB products that were washed postthaw were compared against products that were infused without wash. Transplants for malignant indications were also analyzed separately from transplants for benign indications. Patients were also divided by body weight, <50 kg and ≥50 kg. Last, transplants using single CB were compared with transplants using double cords.

Data for CBT facilitated by the National Marrow Donor Program (NMDP) were supplied and audited by NMDP. For non-NMDP facilitated transplants, outcome data were obtained directly from the respective transplant centers. Additional data not contained in the NMDP forms were elicited directly from all transplant centers. Data regarding each patient were then sent to the respective transplant centers to audit accuracy. StemCyte U.S. and Taiwan subsets were analyzed individually and compared to the NMDP subset to assess the impact of the difference in data collection rates on outcome data and to evaluate the possibility of favorable reporting bias. Last, the auditing services of the Center for International Blood and Marrow Transplant Research (CIBMTR) were retained to perform on-site audits for 85 patients transplanted with PD CB products.

Double CBT, nonmyeloablative or reduced-intensity CBT, and combination CB and haploidentical peripheral blood transplant accounted for 21, 19, and 1 of the total number of transplants. Malignant cases were assigned as early or intermediate/advanced risk for the overall population and for the Remission/First Transplant patients based on IBMTR classifications.

Statistical Analysis 

Kaplan-Meier estimates were used for probabilities of engraftment, relapse, treatment-related mortality (TRM), relapse-free survival (RFS) for malignant cases, and overall survival (OS), with standard definitions used for neutrophil ANC500, platelet 20K, and platelet 50K engraftments. Myeloid engraftment was defined as the first day of 3 consecutive days of an absolute neutrophil count (ANC) of ≥0.5 × 10⁹/L. Platelet engraftment was defined as the first day of 7 consecutive days of a platelet count of ≥20 × 10⁹/L maintained without transfusion for platelet 20K engraftment, or ≥50 × 10⁹/L for platelet 50K engraftment maintained without transfusion. The “unevaluable” classification was not used for any patient. Donor chimerism analysis was performed routinely at most transplant centers to confirm engraftment. Log rank test was used to compare different study populations.

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Results 

CB Inventory 

With the focus on minorities CB collection, the combined inventory of 25,970 banked CB units at StemCyte U.S. and Taiwan (as of 5/31/2006) consisted mostly of non-Caucasian donor units with 63% Asians, 19% Hispanics, 13% Caucasians, 2% African-Americans, and 3% donors of mixed heritage.

Postprocessing and Postthaw Recovery Rates for Plasma Depletion Processing 

The effect of PD processing on postprocessing nucleated cell recovery was examined on validation CB products. There was <0.1% total nucleated cell (TNC) loss (mean = 0.072% and median 0.055%, range: 0-0.21%, standard deviation: 0.060%) incurred by processing as measured by recovery in the discarded plasma fraction (n = 27); however, due to sampling for various tests and archival purposes and clumping after processing, an average of 98% nucleated cell recovery was observed in the cryopreserved units (n = 25,970) in the inventory. After thawing, average recovery of TNC, CD34+ and viability were 85.81% (n = 617), 81.88% (n = 500), and 90.50% (n = 617). Postthaw CFU testing yielded 78.99% postthaw recovery for CFU concentration on 121 consecutive products selected for confirmatory typing that have precryopreserved values. Due to lower postthaw recovery for postthaw washed PD products (median 75% versus 95%), the postthaw TNC dose as reported by transplant centers was lower for washed products (median 5.3 × 10⁷/kg and mean 7.8 × 10⁷/kg) than unwashed products (median 5.6 × 10⁷/kg and mean 8.5 × 10⁷/kg), although precryopreservation, washed CB products had higher median and mean TNC dose (6.1 × 10⁷/kg and 8.0 × 10⁷/kg) than unwashed products (median TNC dose of 5.3 × 10⁷/kg, mean TNC dose of 7.8 × 10⁷/kg).

Comparisons of PD and RD CB Inventories 

The effect of CB processing techniques on postprocessing cell dose among the NMDP banks were studied (Table 1) [14, 15]. Using data derived from the NMDP Cord Blood Bank Performance data as of 6/30/2006 [19], the percentage of high cell dose units among the 2 types of inventories in the NMDP were compared. At all 3 TNC count levels (≥125, ≥150, and ≥200 × 10⁷), the difference in proportions of inventories with high cell dose was significantly in favor of PD inventory (P < .0001). Even after exclusion of CB products with TNC count of <90 × 10⁷ from the analysis to rule out potential contribution of different TNC thresholds set by different CB banks, the difference between PD CB and RD CB inventories remained highly significant at these TNC levels (P < .0001).

Table 1. Comparisons of Proportion of High Postprocessing Prefreeze TNC CB Units between the NMDP PD and RD CB Inventories as of June 30, 2006

Comparison of Postprocessing Prefeeze TNC Number For Plasma-Depleted and Red Cell-Depleted Cord Blood Products
CB with TNC (×107)	NMDP PD CBB # (% of 10,912 CB)	NMDP RD CBB # (% of 38,819 CB)	Test for Diff. in Proportions
At least 125	3772(35)	8172(21)	P<.0001
At least 150	2157(20)	3871(10)	P<.0001
At least 200	703(6)	822(2)	P<.0001

NMDP indicates National Marrow Donor Program; PD, plasma depleted; CB, cord blood; RD, red cell depleted/reduced.

Outcome Data Audits 

The audit conducted by transplant centers was performed for the purposes of double checking the submitted data and uncovering and correcting data submission or data entry errors. The NMDP consistently maintains a high data collection rate and finds high accuracy rates on their audits. Further, for data accuracy rate assessment, CIBMTR was employed. Of the 2021 total PD CB data points for the 85 transplants selected by CIBMTR for audit, outcome data by the 2 PD CB banks in this study attained a 97.3% overall data accuracy rate.

Characteristics of Transplanted PD CB Product 

The cell dose and the number of HLA A/B/DR matches between donor and recipient for all patients and for patients who were in remission and receiving a first HCT are presented in Table 2. The mean and median nucleated cell doses for this series were 7.6 × 10⁷/kg and 5.6 × 10⁷/kg, respectively. The number of HLA A/B/DR matches were similar to other series reported by CB banks [16, 20, 21, 22, 23, 24, 25]. What is most striking is the high TNC number and TNC dose of the transplanted PD CB (Table 2, Table 3), which highlights the impact of a CBB inventory of high TNC dose. In fact, the reported cell doses underrepresented the combined cell dose used for the 11 double CBT that employed both PD and RD CB, because transplant centers did not provide information on the RD CB unit, so that the cell dose of the second red cell-depleted CB unit was not included in the TNC of these 11 transplants. The median and range of preprocessing nucleated red blood cell percentage for the transplanted products are 6.9% and 0%-30.9%; the postprocessing nucleated red blood cell percentage median and range are 7.9% and 0%-31.7%.

Table 2. Postprocessing Prefreeze TNC Number and Dose of Transplanted PD CB Product

Transplanted CB Products
Postprocessing Prefreeze TNC Dose	N	%
Transplants with TNC Dose below 2 ×107/kg	9	8
Transplants with TNC Dose between 2.0 and 2.9 ×107/kg	15	13
Transplants with TNC Dose between 3.0 and 3.9 ×107/kg	16	14
Transplants with TNC Dose between 4.0 and 4.9 ×107/kg	12	10
Transplants with TNC Dose between 5.0 and 5.9 ×107/kg	12	10
Transplants with TNC Dose above 6.0 ×107/kg	54	46

TNC indicates total nucleated cell; CB, cord blood.

Table 3. Patient Characteristics, CB Characteristics, Diagnosis, and Disease Risk Factors

	n	%	n	%
Adult patients (≥16 years)	32	27	24	24
Larger patients (≥50 kg)	37	32	27	28
Male recipients	72	61	59	60
Non-U.S. transplant center patients	42	36	36	37
Double PD cord transplants	10	8	10	10
Double PD/RD cord transplants	11	9	6	6
Reduced-intensity conditioning	19	16	15	15
6/6 HLA A/B/DR matches⁎	18	15	17	17
5/6 HLA A/B/DR matches⁎	35	30	30	31
4/6 HLA A/B/DR matches⁎	50	42	39	40
2-3/6 HLA A/B/DR matches⁎	15	13	12	12
Malignant indications	87	74	68	69
ALL	38	32	29	30
AML	22	19	16	16
CML	8	7	7	7
MDS	8	7	7	7
JMML	2	2	2	2
Others	9	8	7	7
Early risk	21	24	21	31
Intermediate/advanced risk	46	53	29	43
Risk unknown or not applicable	20	23	18	26
Prior transplant	7	6	NA	NA
Transplanted during relapse	16	14	NA	NA
Benign indications	31	26	30	31
Hemoglobinopathies	14	12	14	14
AA	5	4	4	4
WAS	5	4	5	5
Others	7	6	7	7

Characteristics of Patient Population and Disease Indications 

Table 3 provides information on the total patient population and the subset of patients who were in remission and receiving a first HCT, as well as a breakdown of the indications. These characteristics appear to be similar to other series by CB banks [16, 20, 21, 22, 23, 24, 25] or transplant centers and registries [1, 6, 21, 22, 23, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37].

Infusion Associated Adverse Reactions of PD CB Product 

Analysis of infusion-associated adverse reactions were separated by patients who received postthaw washed products (W) and those who received CB that were not washed after thaw (NW). Events occurring during or after infusion include hypertension (6NW, 4W), hives (1NW, 1W), nausea/vomiting (2NW), and dyspnea (1NW, 1W). Hemoglobinuria (9NW, 1W) is an expected occurrence for PD CB because most red blood cells lyse after thawing. Most symptoms attributed to DMSO administration were self-limiting or easily managed. One patient developed seizure and encephalopathy, although relationship to infusion was uncertain.

Outcome Data—Engraftment, TRM, GVHD Rate, Relapse Rate, RFS, and OS 

Table 4 summarizes the median age, weight, TNC dose, CD34⁺ dose, number of HLA matches, neutrophil (ANC500), platelet 20K and 50K engraftment, relapse rate for malignant cases, 1-year TRM, 1-year RFS, and OS for all patients and for the various subsets of patients.

Table 4. Summary of Transplant Characteristics and Outcome for the Various Cohorts

TRM indicates transplant related mortality; OS, overall survival; RFS, relapse free survival for malignant indications; All, All PD CBT; Remission 1st Xsplant, PD CB transplanted during remission as first transplant; NMDP, NMDP mediated PD CB transplant; Non-NMDP, PD CB transplant outside of the NMDP network; US CBB, PD CBT sourced from U.S. PD cord blood bank; TW CBB, PD CBT sourced from Taiwan PD cord blood bank; 1X PD CBT, single cord blood transplants using PD CB; 2X PD CBT, double cord blood transplants using one or two PD CB; <50 kg, PD CBT for patients under 50 kg; ≥50 kg, PD CBT for patients 50 kg or over; Benign, PD CBT performed for nonmalignant indications; Malignant, PD CBT performed for malignant indications.

Neutrophil engraftment for all patients and for remission/first transplant patients are illustrated in Figure 1A and B, respectively. The platelet 20K engraftment for all patients and for remission/first transplant patients are shown in Figure 1C and D, respectively. ANC500 and platelet 20K and 50K engraftment for all patients were 90% ± 3%, 77% ± 5%, and 75% ± 5%, respectively. For the remission/first transplant patients, ANC500 and platelet 20K and 50K engraftment were 94% ± 3%, 81% ± 5%, and 80% ± 5%, respectively.

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• Figure 1. 

  Kaplan-Meier estimates of the probability of neutrophil engraftment. For A, all transplants (top panel); B, remission first transplants (second panel from top) and platelet 20 K engraftment for C, all transplants (third panel from top); and D, remission first transplants (bottom panel).

The ANC500 engraftment rates for the NMDP-mediated PD CBT and non-NMDP-mediated PD CBT were similar (P = .55). CBT sourced from the U.S. and Taiwan PD CB banks were also similar (P = .81). Single PD CBT had similar engraftment rates as double CBT with 1 or 2 PD CB units (P = .97). Unwashed products engrafted faster for ANC500 (20 days versus 27 days) (log rank test; P < .02) and platelets (47 days versus 54 days; log rank test; P = .0003).

Unlike most previous studies [22, 23, 26, 27, 29, 30, 31, 32, 33, 34, 35, 36, 37], transplants for adult weight recipients (≥50 kg) had similar engraftment rates as pediatric recipients weighing <50 kg (P = .58 for ANC500 and .52 for Platelet 20K). However, transplants for patients with benign indications showed better engraftment than transplants for malignancies (ANC500 P = .01; Platelet 20K P = .008).

TRM at 100 days was 16% ± 3% for all patients (n = 118) and 10% ± 3% for remission first transplant cases (n = 98). The 1-year TRM are summarized in Table 4. Primary and secondary reported causes of death include infection (50% of the cases), relapse (25%), graft failure (7%), GVHD (7%), and other causes (25%). One-year TRM for NMDP mediated PD CBT and non-NMDP mediated PD CBT were similar (P = .89). Single PD CBT had similar 1-year TRM as double CBT with 1 or 2 PD CB unit (P = .77). CBT sourced from StemCyte U.S. and StemCyte Taiwan had similar 1-year TRM (P = .53). Transplants with washed and unwashed PD CB had similar 1-year TRM (P = .76). However, TRM for transplants for recipients weighing under 50 kg trended toward a lower rate than those recipients weighing 50 kg or above (P = .09).

The incidences of grade III-IV aGVHD and extensive chronic GVHD (cGVHD) among all patients were 13% ± 4% and 17% ± 6%, respectively.

For malignancies, 1-year relapse rate for the various subgroups were all similar: NMDP-mediated transplants and non-NMDP-mediated transplants (P = .86), single PD CBT and double CBT with 1 or 2 PD CB unit (P = .56), recipients under 50 kg and recipients 50 kg or above (P = .64), transplants with washed and unwashed PD CB (P = .40), and CBT sourced from StemCyte U.S. and Taiwan (P = .29).

The 1-year RFS of all malignant patients (n = 85) was 51% ± 6% (Figure 2A), and that of the remission/first transplant patients with malignancies (n = 67) was 59% ± 7% (Figure 2C). The 1-year OS rate for all patients (n = 118) was 65% ± 5% (Figure 2B), and that for remission/first transplant patients (N = 98) was 73% ± 5% (Figure 2D).

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• Figure 2. 

  All patients—Kaplan-Meier estimates of the probability of A, relapse-free survival of all malignant patients (top panel); B, overall survival of all patients (second panel from top); C, relapse-free survival for remission first transplant patients (third panel from top); and D, overall survival for remission first transplant patients (bottom panel).

Transplants for recipients under 50 kg had a 1-year OS of 72% ± 6%, whereas those recipients weighing 50 kg or above had a lower 1-year OS at 55% ± 9% (P = .05). In contrast, RFS for malignant indications were similar at 51% ± 8% for patients weighing <50 kg and 52% ± 9% for patients weighing 50 kg or more (P = .78).

For the other subsets, 1-year RFS and OS were similar: for the NMDP-mediated PD CBT versus non-NMDP-mediated PD CBT (P = .27 for RFS and P = .63 for OS), single PD transplants versus double transplants with 1 or 2 PD CB unit (P = .39 for RFS and P = .57 for OS), and CB from StemCyte U.S. versus Taiwan (P = .35 for RFS and P = .72 for OS).

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Discussion 

Although there have been several published reports on the clinical use of PD CB for HCT [8, 28, 38, 39, 40, 41, 42, 43, 44], this is the first large patient series. The NMDP, CIBMTR, and transplant centers audited the outcome data in this report. To our knowledge, this is the first outcome report by a CB bank that has employed external audits. This series is also unusual in terms of the high proportion of international and minority transplants as well as the high mean and median cell doses that were observed. The high neutrophil and platelet 20K engraftment probability for both pediatric (92% ± 4% and 80% ± 5%) and adult (90% ± 5% and 71% ± 10%) patients observed in this study is also worth noting.

High cell dose is recognized as a critical determinant of success for CBT [36], and limited cell doses have hampered the use of CB for transplantation in adults. Although recommendations as recent as 2003 [26] suggested that a minimum acceptable cell dose should be 1.5 × 10⁷ nucleated cells/kg to reduce time to myeloid recovery and increase the probability of engraftment, other recommendations suggest that CB units should be selected on the basis of a minimum nucleated cell dose of 3 × 10⁷ [27] to 4 × 10⁷/kg [29] recipient body weight to obtain acceptable clinical outcomes. It has been reported that only 12% of the current inventory in established public CB banks contains sufficient cells to deliver such cell doses to patients weighing >60 kg; therefore, alternative strategies to increase cell dose for larger patients need to be explored [27].

To minimize cell loss during processing, we have eliminated the step of red blood cell reduction from CB prior to cryopreservation and plasma depletion processing was utilized instead. This process minimizes nucleated cell loss during processing to <0.1% as measured by cell content of the discarded plasma. This minimal level of cell loss is unattainable using current red cell reduction techniques [14, 15, 17, 18]. To test if plasma depletion processing makes a significant improvement in cell dose to actual inventories of CB banks, the 2 types of CB inventories of almost 50,000 CB of the 18 CB banks of the NMDP network were compared. As the largest analysis comparing cell doses of different inventories, it showed that PD CB inventory has significantly higher proportions of high cell dose CB products than the RD CB inventory in the NMDP network. The possibility that the PD CB inventories started off with a higher collection or preprocessing cell dose can be excluded because most of the products were collected during a period when the minimal volume threshold was 40 to 50 mL. We conclude that utilizing plasma depletion allows for cryopreservation of CB units with higher cell doses, and therefore has the potential of making a higher percentage of units collected available for transplantation. The high cell dose of the PD inventory appears to be directly correlated with the high prefreeze nucleated cell dose of the transplanted units observed in this study. In parallel processing studies using identical samples, the nucleated cell, CD34⁺ cell, and CFU recoveries appear to be 24%, 23%, and 34% higher with plasma-depleted products than red cell-depleted products (manuscript in preparation); however, this study cannot address whether the resultant higher cell dose of plasma depleted products translates into improved clinical outcome without rigorous comparisons with patients transplanted with red cell-depleted units.

Although anecdotal reports and abstracts have been published on the use of plasma-depleted CB units [8, 28, 38, 39, 40, 41, 42, 43, 44], adequate data have not previously been available to evaluate the safety and effectiveness of units stored after plasma depletion. A rigorous comparison of results using PD CB versus RD units was not possible in the current study because outcomes using RD units were not available to us. Because of potential center effects, we have refrained from comparing our data against individual transplant center studies. However, the results of CBT using PD CB that are presented in this study are at least comparable to those presented in other reviews of CBT outcomes from CB banks [7, 16, 20, 21, 22, 23, 24, 25, 37] or transplant registries [1, 22, 23, 24, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 45]. This includes incidence and speed of neutrophil and platelet engraftment, TRM, relapse rate, OS, and disease-free survival rates, the incidence and severity of aGVHD and cGVHD.

Various other potential explanations for the favorable outcomes observed in this study were systematically examined and excluded. For example, the potential for favorable reporting bias by transplant centers is a theoretic problem that all registries or CB banks face when collecting and analyzing outcome data. The way to exclude significant favorable reporting bias is with extensive on-site audits to ensure accuracy or high data collection rate. In this study, data accuracy was ensured through audits by both transplant centers and by external agencies—CIBMTR and NMDP. Half of the transplants in this study were facilitated by NMDP, which collected, provided, and audited this data subset. Because the NMDP data subset is 91% complete, the potential of withholding unfavorable outcome data is minimal; therefore, for this study, the NMDP data were considered the “gold standard,” and precludes significant favorable reporting bias. Because the non-NMDP transplant outcome was not different from those of the NMDP transplants, significant favorable reporting bias can be ruled out in the non-NMDP cohort and in the combined dataset.

A beneficial influence of double CBT [3, 4, 5, 7] to the outcome in this study was considered unlikely due to the low percentage of double cords and because engraftment and survival outcomes of single and double CBT were comparable in this series (Table 4). The results of the double CBT using PD CB were similar to those reported by other groups that used RD CB [3, 4, 5]. However, similarity in engraftment between double and single CBT appears to differ from earlier reports [3, 4, 5, 7] and to be consistent with the most recent Minnesota experience (Wagner and Brunstein, 2007 International CB Transplantation Symposium). The high cell dose for single CBT may have contributed to the favorable outcome for single CBT observed in both studies. A positive contribution of a second RD CB from another CB bank in the double CBT setting was considered unlikely because the number of mixed PD/RD double CBT were limited to 9%. Moreover, engraftment and survival of double PD CB transplant were significantly better (ANC500 and platelet 20K engraftment) or trended better (1-year OS) than double CBT with the combination of a PD CB and an RD CB (data not shown).

The patient characteristics in this series appear to be comparable to other series reported by CB banks in the literature [1, 6, 7, 16, 21, 22, 23, 24, 25], including sex, age, weight, percentage of malignancies, and number of HLA mismatches. This is expected, because CB banks do not have the option to select patients or centers, so in relatively large studies, most patient, disease, HLA, and other characteristics would conform to standard distributions. One significant difference appears to be the very high percentage of minority patients and procedures performed at non-U.S. centers, both related to the minority and international focus of the 2 CB banks employing plasma depletion processing. Given this difference and the outstanding CBT results by the University of Tokyo group in Japan [45], the notion that Asians may experience better outcomes after CB transplantation was examined. The hypothesis was rejected as a significant contributor to this study because, unlike the 100% Asian patient composition of the University of Tokyo study [45], only 33% of the transplants in this study were for Asian patients. Moreover, transplants for Asians experienced outcomes similar to those for other patients in this study (data not shown).

The most noticeable difference of this series from previous studies was the unusually high prevalence of both high prefreeze and postthaw cell doses, which, as shown earlier, may be attributed to the generally higher cell dose in the inventory. This higher cell number of units in the PD inventory increases the probability of a CB unit being picked having a high cell dose, contributing to the high prevalence of excellent prefreeze cell doses used in transplants (Table 2). Moreover, in centers that have opted not to wash, the additional 10%-30% loss due to wash is avoided [8, 9, 10, 11, 12], contributing to the minimal loss in the postthaw numbers with those transplants. Similar to our earlier reports [8], we confirm here safety of PD CB and significantly improved engraftment speed and probability using PD units that are not washed versus PD units that are washed after thawing. The outcome of this study shows that, like other cryopreserved stem cell products, with proper premedication and hydration prior to infusion [46, 47, 48, 49, 50], PD CB with or without postthaw wash, is safe with occasional infusion reactions that are mostly self-limited and easily treated or resolved without treatment. Although no cryopreserved stem cell products can be used without rare occurrences of serious reactions, with attention to DMSO dose and comorbid conditions, such untoward incidents can be minimized. Like all cryopreserved products, the recommended threshold of 1 g DMSO per kilogram of recipient body weight minimizes the possibility of adverse reactions and should not be exceeded [35, 44, 46, 47, 48, 49]. For this reason, all PD products are supplied with information regarding the volume or mass of DMSO in the product.

There were no clear observed benefits of performing postthaw wash for PD CB. Transplants with PD CB not washed were at least as efficacious as washed PD CB with respect to TRM, relapse rate, 1-year OS, and RFS. Moreover, to our knowledge, this is the first report that washing appears to have a negative impact on the incidence and speed of neutrophil and platelet engraftment. Stiff et al [9] concluded that, when using a nonwash postthaw procedure, few viable cells are lost, allowing successful CBT at lower postthaw nucleated cell doses than would otherwise be possible. The St. Louis group also reported satisfactory viability and CFU recovery with unwashed and undiluted CB within an hour after thawing [50]—within the usual period when CB infusion would be completed after thawing. Therefore, for the PD CB unit, we highly recommend that in most instances, cell loss be minimized at the transplant center by eliminating the washing of the product before infusion. Appropriate exceptions may include patients with histories of sensitivity to DMSO, small children (≤15 kg) resulting in unwashed DMSO dose of 1 g/kg recipient weight, and patients with compromised renal function [46, 47, 48, 49].

In conclusion, transplantation using PD CB units is safe and effective for diverse patients; eliminating red cell depletion increases the nucleated cell recovery of CB processing and contributes to a higher percentage of units collected available for transplantation. The low TRM, high engraftment, OS, and RFS are not surprising given the high cell doses used in this study. Although high cell dose appears to be the most logical hypothesis to explain our findings, rigorous prospective trials or matched pair retrospective analysis with and without adjustment for cell dose between plasma-depleted CB and red cell-depleted CB will be necessary to prove a causal relationship.

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Acknowledgments 

The support of NMDP is acknowledged. Some of the data used in this study were obtained in the course of performance of HRSA Contract HHSH 234-2004-37003C.

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