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01.11.2019 | Immune Deficiency and Dysregulation(C Kuo, Section Editor) | Ausgabe 11/2019 Open Access

Current Allergy and Asthma Reports 11/2019

Conditioning Regimens for Hematopoietic Cell Transplantation in Primary Immunodeficiency

Zeitschrift:
Current Allergy and Asthma Reports > Ausgabe 11/2019
Autoren:
S. H. Lum, M. Hoenig, A. R. Gennery, M. A. Slatter
Wichtige Hinweise
This article is part of the Topical Collection on Immune Deficiency and Dysregulation

Publisher’s Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Introduction

Primary immunodeficiency (PID) comprises a large, heterogeneous group of disorders that result from defects in immune system development and/or function. Long considered as rare diseases, recent studies show that one in 2000–5000 children younger than 18 years is thought to have a PID. There are now around 350 single-gene inborn errors of immunity and the underlying phenotypes are as diverse as infection, malignancy, allergy, autoimmunity, and autoinflammation. Therefore, presenting features, severity, and age of diagnosis vary immensely. Hematopoietic cell transplantation (HCT) is a well-recognized curative therapy for many of these PIDs. Since the first transplant took place in 1968, utility of HCT was initially limited by high rates of graft failure and transplant-related morbidity and mortality; however, transplant survival and graft outcomes have significantly improved, particularly since 2000 [ 1, 2]. Many factors have contributed to this improvement including earlier diagnosis, a detailed graft selection hierarchy, superior HLA matching technology, improved methods for graft manipulation, greater availability of grafts, improved supportive care, vigilant infection surveillance and pre-emptive treatment, and more effective antimicrobial therapy. In the modern era, graft engineering, additional cellular therapy, and pharmacokinetic-guided conditioning regimens enable precise personalized transplant care including prescription of graft components, better cell-dosed grafts, and a patient-tailored conditioning regimen [ 3, 4•, 5••].
Short-term transplant survival outcomes must be carefully distinguished from long-term disease outcomes and late effects of transplant. As survival from transplant has improved, more attention is now given to long-term disease outcomes and quality of life. Therefore, the goal of conditioning is to give the least toxic regimen with minimal short- and long-term side effects but still achieve cure of the underlying condition. This review will focus on newer conditioning regimens, how they have changed, and possible future directions. It is important to note that success does not simply depend on which conditioning chemotherapeutic agents are employed but on a combination of factors such as additional serotherapy, timing and dosage, and stem cell source. In almost all cases, preparative conditioning with a combination of chemotherapeutic agents, with or without monoclonal antibodies, is required for successful engraftment and stable robust long-term immune reconstitution.

Definition

The intensity of the conditioning regimen can vary substantially and has been classified as myeloablative conditioning (MAC), reduced toxicity conditioning (RTC), reduced intensity conditioning (RIC), and minimal intensity conditioning (MIC) in decreasing order (Fig.  1). MAC, consisting of alkylating agents with or without total body irradiation (TBI), is expected to myeloablate the recipient’s hematopoiesis which does not allow for autologous hematological recovery. This aims to prevent rejection by the use of supralethal chemotherapy to remove host-versus-graft reaction and create marrow niche space for donor stem cells. Newer myeloablative chemotherapy agents are being explored to reduce toxicity and enable safer HCT. These reduced toxicity conditioning (RTC) regimens, including pharmacokinetic targeted busulfan-fludarabine (Bu-Flu) and treosulfan-fludarabine, have a comparable myeloablative effect with conventional MAC but reduced organ toxicities. Compared to MAC, RIC has been traditionally characterized by reversible myelosuppression in the absence of stem cell rescue, reduced regimen-related toxicity, and a higher incidence of mixed chimerism. MIC is strictly non-myeloablative, does not eradicate host hematopoiesis, and allows relatively rapid autologous hematopoietic recovery without a transplant, but adequately myelosuppresses the recipient to enable at least partial donor engraftment.

Myeloablative Conditioning Regimens in PID

Historically, conditioning therapy prior to HCT in PID was based on the combination of alkylators busulfan and cyclophosphamide. However, many children with PID have significant comorbidities at the time of HCT, and these conventional myeloablative preparative regimens are associated with significant toxicity and a relatively high incidence of transplant mortality, as well as long-term sequelae. While initial results may have been acceptable, appreciation of acute conditioning toxicities and recognition of long-term sequelae mean that few centers now approach transplantation of PID patients with conventional myeloablative preparative regimens (Table 1) [ 69].
Table 1
Outcome of HCT in PID after myeloablative conditioning regimens
Author, Year
Year of HCT
No. of patients/diagnosis
Median age at HCT (range), years
Donor and stem cell source
Conditioning regimen
OS
Fisher, 1994 [ 6]
1977–1991
149 non-SCID PID received 171 transplants
Range 0.1–16
65 MSD/MFD
6 MUD
78 MMUD
Bu+Cy
12 additional TBI
Before 1985, 51.7%
After 1985, 81.5%
Klein, 1995 [ 7]
1981–1993
19 MHC class II deficiency (7 s HCT)
1.4 (0.5–9.5)
8 MFD marrow
1 MMFD marrow
10 HID marrow
All 7 s HCT used HID
MFD
Bu20mg/kg + Cy 200 mg/kg or Cy 50 mg/kg + ALG or
Cy 50 mg/kg + CCNU 300 mg/m 2 + procarbazine 280 mg/kg + ALG
MMFD
Bu 16 mg/kg + Cy 200 mg/kg or
Bu 20 mg/kg + Cy 200 mg/kg + anti-LFA-1 antibody or
Bu 20 mg/kg + Cy 200 mg/kg + anti-LFA-1 antibody + anti-CD2 antibody
47%
Antoine, 2003 [ 8]
1968–1999
1082 HCT in 919 PID patients
566 HCT in 475 SCID patients
512 HCT in 444 non-SCID PID patients
SCID: 5.5 months
Non-SCID: 34.6 months
88% marrow
12% PBSC
0.7% CB
T cell depletion:
91% MD
41% UD marrow
205 SCID: unconditioned
361 SCID: Bu 8 mg/kg + Cy 200 mg/kg
512 non-SCID; Bu 16 mg/kg + cy 200 mg/kg
SCID: 77% MD vs 54% in MMD
Non-SCID: 71% MFD vs 42% MUD vs 59% MMD
Renella, 2006 [ 9]
1981–2004
15 MHC class II deficiency
1.5 (0.3–5.4)
13 MFD marrow
2 MUD marrow
Bu 16-20 mg/kg + Cy 200 mg/kg + ATG in MUD
53%
ALG antilymphocyte globulin, Bu busulfan, CB cord blood, CCNU lomustine, Cy cyclophosphamide, HID haploidentical donor, MD matched donor, MFD matched family donor, MMD mismatched donor, MSD matched sibling donor, MMUD mismatched unrelated donor, MUD matched unrelated donor, OS overall survival, PID primary immunodeficiency, SCID severe combined immunodeficiency, TBI total body irradiation, UD unrelated donor, WAS Wiskott-Aldrich syndrome

RTC Regimens in PID

The use of reduced toxicity conditioning regimens are now generally preferred for patients with PID as there is no malignant disease to eradicate, stable mixed chimerism achieves cure for many diseases, and many patients enter HCT with chronic infections and end-organ comorbidities. Additionally, many patients are infants at the time of transplant and may be more susceptible to toxicity [ 10]. Less toxic regimens may reduce early and late adverse effects, particularly infertility [ 4•]. There are several reduced toxicity regimens that have been utilized by investigators in PID (Table 2) [ 14•, 49, 50].
Table 2
Outcome of HCT in PID according to reduced toxicity conditioning regimens
Author, year
Year of HCT
No of patients/diagnosis
Median age at HCT (range), years
Donor and stem cell source
Conditioning regimen and GvHD prophylaxis
Median day of N engraftment
VOD, n
aGvHD %
cGvHD %
OS %
ES %
Graft failure %
Second procedure, n
Latest donor chimerism/remarks
Fludarabine and treosulfan
  Slatter, 2018 [ 11]
2006–2013
160
39 SCID
20 WAS
17 CGD
18 HLH
66 Other PID:
1.36
(0.1–18.3)
29 MSD/MFD
73 MUD
54MMUD
4 HID
49 marrow
70 PBSC
41 CB
Flu 150 mg/m 2 + Treo 42 g/m 2 (36g/m 2 if < 1 year; 30 g/m 2 for SCID) + alemtuzumab 0.3 to 1.0 mg/kg
GvHD prophylaxis: CSA/MMF
NA
0
I–IV: 46
III–IV: 9
15
2-year OS: 88
5-year OS: 78
2-year ES: 88
5-year ES: 78
3
4 s HCT for graft loss or poor immune reconstitution
5 unconditioned boost
3 DLI
PBSC was associated with better donor myeloid chimerism without an increased risk of GvHD
  Morillo-Gutierrez, 2016 [ 12]
2006–2015
70 CGD
8.9 (IQR 3.8–19.3)
13 MSD/MFD
44MUD
12 MMUD
1 HID
36 marrow
32 PBSC
1 TCR ∝β/CD19 depleted PBSC
1 CB
46 Flu150mg/m 2 + Treo 42 g/m 2 (36g/m 2 if < 1 year)
Alemtuzumab ( n = 39) or ATG ( n = 18) or no serotherapy ( n = 13)
15 Flu + Treo + TT + alemtuzumab or ATG
9 other Treo-based conditioning regimen
GvHD prophylaxis:
CSA ± MMF or MTX
17 (IQR 15–35)
0
I–II: 39
III–IV: 12
13
91.4
81.4
12
8 (2 unconditioned boost; 3 DLI; 5 conditioned 2nd HCT [2 had DLI])
Myeloid ≥ 95%: 80% surviving patients
  Slatter, 2015 [ 13]
2005–2010
316
144 PID
39 IMD
70 H-globinopathy
32 histiocytic disorders
24 marrow failure
2 autoimmune disease
5 others
< 1 year, n = 95
1–12 years, n = 189
> 12 years, n = 32
94 MSD/MFD
29 MMRD
39 MUD
16 MMUD
138 undefined UD
167 marrow
8 marrow + CB
3 marrow + PBSC
87 PBSC
1 PBSC + CB
50 CB
106 Flu 150 mg/m 2 + Treo 42 g/m 2
98 Cy 200 mg/kg + Treo 42 g/m 2
104 Flu 150 mg/m 2 + Treo 42 g/m 2 + TT 8 mg/kg
8 Flu 150 mg/m 2 + Treo 42 g/m 2 + melphalan
GvHD prophylaxis:
284 CSA alone
100 CSA + MMF
101 CSA + MTX
NA
0
I–IV: 38
III–IV: 10
NA
83
76
5.1
NA
NA
  Burroughs, 2014 [ 14]
2009–2013
31
6 IPEX
5 CGD
2 other PID
6 HLH
6 BM failures
6 RBC disorders
10.7 (0.4–30.5)
4 MSD
27 MUD
29 marrow
2 PBSC
Flu 150 mg/m 2 + Treo 42 g/m 2
Serotherapy:
22 ATG
GvHD prophylaxis:
Tacrolimus + MTX
21 (range, 12–46)
0
II–IV: 62
III–IV: 10
21
90
NA
3
2 s HCT
ATG patients:
19 (86%) full or high level of mixed CD3 chimerism
3 (14%) low-level mixed donor CD3 chimerism
No ATG patients:
6 full/high level of mixed CD3 chimerism
2 low-level mixed donor CD3 chimerism
1 graft failure
  Dinur-Schejter, 2015 [ 15]
2009–2013
45 HCT in 44 patients
12 SCID
5 severe congenital neutropenia
2 WAS
2 CGD
1 HLH
10 PID
5 thalassaemia
5 osteopetrosis
3 IMD
4 others
1.5 (0.1–15.1)
19 MSD/MFD
3 MMFD
14 MUD
9 unrelated CB
19 Flu + Treo
6 Cy + Treo
20 Flu + Treo + TT
(Treo 36 g/m 2 for < 1 year; 42 g/m 2 for > 1 year)
Serotherapy:
9 no serotherapy
26 ATG
8 Alemtuzumab
1 OKT3
Flu/Treo/TT: 18.4
Flu/Treo: 25.3
Cy/Treo: 19.5
1
I–IV: 44.4
III–IV: 27
18.9
71
55
14
3 s HCT (one had a further 3rd HCT)
Full: 31 (72%)
Mixed: 6 (28%)
  Lehmberg, 2014 [ 16]
2010–2012
19 HLH
3.9 (0.2–22)
1 MRD
6 MUD
9 MMUD
HID 1
17 marrow
1 PBSC
1 CD34 selected PBSC for HID
16 Flu150mg/m 2 (3 Flu 160-180 mg/m 2) + Treo 42 g/m 2 (36g/m 2 if < 12 kg)
Alemtuzumab 0.3 mg—1.0 mg/kg
14 additional TT 10 mg/kg (7 mg/kg if < 12 kg)
GvHD prophylaxis:
2 CSA alone
7 CSA + MMF
9 CSA + MTX
1 Tacrolimus + MMF
20 (range 11–62)
1
I–II: 21
III–IV: 1 patient after DLI
No
100
NA
11 ( n = 2)
2 s HCT
(1 1° graft failure after HID; 1 2° graft failure)
6 DLI
WB > 95%: 10
WB 75–95%: 2
WB 20–74%: 4
  Beier, 2013 [ 17]
2003–2009
53 non-malignant patients
10 SCID
4 CGD
2 HLH
2 WAS
11 other PID
3 osteopetrosis
9 H-globinopathy
9 BM failure
1 IMD
2 0thers
4.8 (0.1–20.1)
16 MSD/MFD
1 MMFD
1 MUD
25 MUD
I HID
2 CB + HID
36 marrow
11 PBSC
1 CB
2 CB + PBSC
2 NA
15 Flu + Treo (1 additional radioimmunotherapy)
32 Flu + Treo + TT
5 Flu + Treo + melphalan
Serotherapy
4 None
19 ATG
3 ATG + OKT3
1 ATG + alemtuzumab
16 alemtuzumab
1 alemtuzumab + rituximab
1 rituximab
5 OKT3
20
0
I–IV: 32
III–IV: 4
6 ( n = 3)
87
NA
4
NA
Full: 46 (87%)
  Slatter, 2011 [ 18]
2006–2009
70
26 SCID
7 Omenn syndrome
7 WAS
4 HLH
4 LAD
4 CGD
2 IPX
16 other PID
0.7 (0.1–14.6)
21 MSD/MFD
45 MUS
4 HID
40 Flu150mg/m 2 + Treo 42 g/m 2
30 Flu150mg/m 2 + Cy 200 mg/kg
53 alemtuzumab 0.3 to 1.0 mg/kg
NA
2 in Cy group
I–IV: 26
III–IV: 10
6
81
Flu: 85%
Cy: 77%
NA
3 ( n = 2_
1 had both top-up and second conditioned HCT
57% full donor chimerism
43% stable mixed chimerism
Busulfan ± fludarabine
  Dvorak, 2019 [ 19]
2011–2017
10
4 typical SCID
6 leaky SCID
5 mos (range, 2–108 mos)
2 MUD
2MMUD
6 HID
Marrow for MUD/MMUD
CD34 selected PBSC or HID
Bu with target AUC 30 mg*hr./L ATG or alemtuzumab
For patients with any T cells:
Additional Flu 160 mg/m 2
For patients with NK cells:
Additional TT 10 mg/kg
2 had plerixafor 9 h prior to each dose of Bu
16 (range, 14–23)
0
II–IV: 2 patients
0
100
NA
10
1 additional HCT
Median myeloid at one-year post HCT
14% (range, 2–100%)
6 had full T- and B cell reconstitution
3 had no B cell recovery (2 had rituximab for autoimmunity post-HCT)
3 had B cell autoimmunity
  Güngör, 2015 [ 20]
2003–2015
56 CGD
12.7 (IQR 6.8–17.3)
21 MSD/MFD
25 MUD
10MMUD
45 marrow
11 PBSC
Flu 150 mg/m 2
Bu with target AUC 45–65 mg*hr./Lxh
Serotherapy
ATG for MFD
Alemtuzumab for MUD
19 (IQR 16–22)
0
III–IV: 4
7
93
89
5
3 s HCT
Myeloid > 90%: 52 (93%)
  Jacobsohn, 2004 [ 21]
2000–2004
13
6 PID
4 H-globionopathy
3 IMD
5.2 (IQR, 0.6–11.1)
4 MSD
1MMFD
6 MUD
2 unrelated CB
11 PBSC
Flu 150 mg/m 2
Bu with target AUC 3800 to 4200umol x min
ATG
GvHD prophylaxis
CSA ± MMF
18 (IQR, 14–25)
0
II–IV: 8
25
84
NA
15
(2 h-globinopathy)
none
72% full donor chimerism
Fludarabine and melphalan
  Allen, 2018 [ 22]
2013–2015
34 HLH
12 PID
2.3
(0.4–28)
7 MSD
1 MMRD
25 MUD
13 MMUD
All had marrow
Flu 150 mg/m 2
Melp 140 mg/m 2
Alemtuzumab 1 mg/kg
GvHD prophylaxis
CSA and steroid
13
0
II–IV: 17.4
III–IV: 10.9
26.7
18-month OS: 66.9%
60.9% with second procedure
39.1% without intervention
Primary: 4
Secondary: 4
2 s HCT
57% had full chimerism in all cell lines
42% had stable mixed chimerism
  Fox, 2018 [ 23]
2004–2014
29 PID
24 [ 11, 12, 16, 1848]
11 MFD
13 MUD
5 MMUD
Non-CGD
Flu 150 mg/m 2
Melp 140 mg/m 2
Alemtuzumab 100 mg
CGD
Flu 150 mg/m 2
Meph 10 mg/m 2 or Bu 9.6 mg/kg
Alemtuzumab or ATG
GvHD prophylaxis
CSA
13 (IQR, 11–17)
0
I–II: 45
III–IV: 3
Limited: 34
Extensive: 1
1-yr: 85.2
1-year: 85.7
None
None
85% full chimerism
  Marsh, 2010 [ 30]
2003–2009
40 HLH
1 (0.1–16)
7 MFD
33 MUD
36 marrow
2 PBSC
2 CB
26 RIC
Flu 150 mg/m 2 (5mg/kg if < 10 kg)
Melp 140 mg/m 2 (4.7 mg/kg if < 10 kg)
Alemtuzumab
14 MAC
Bu 14 mg/kg
Cy 200 mg/kg
12 additional etoposide 30 mg/kg
GvHD prophylaxis
CSA or tacrolimus + steroid/MTX
MAC: 14.5
RIC: 10
NA
II-III
MAC: 14
RIC: 8
( p = 0.317)
MAC: 0
RIC: 12% limited
MAC: 43%
RIC 89%
( p = 0.0036)
NA
None
3 CD34+ boost
14 DLI
MAC: 18% mixed
RIC: 65% mixed
Mixed chimerism in RIC was less in patients who received distal aleumtuzumab (29%) vs 79% in proximal alemtuzumab ( p = 0.02)
  Rao, 2005 [ 49]
1998–2001
33
6 SCID
27 non-SCID
5.9 (0.19–18)
22 MUD
11 MMUD
All marrow
Flu 150 mg/m 2
Melp 140 mg/m 2
Alemtuzumab 1 mg/kg
CSA
13 (range, 8–34)
0
II–IV: 9
Limited: 0
Extensive: 3
94%
NA
NA
NA
55% had full chimerism
32% had high level mixed chimerism
6.5% had low level mixed chimerism
6.5% very low mixed chimerism
  Amrolia, 2000 [ 31]
NA
8
3 SCID
1 XLP/HLH
2 CID
2 CD40 ligand def
6.5 (range, 0.75–18)
2 MSD
6 MUD
All marrow
Flu 150 mg/m 2
Melp 140 mg/m 2
ALG 10 mg/kg
GvHD prophylaxis
CSA and steroid
13 (range, 9–17)
0
I: 50
II–IV: 0
limited cGvHD, n = 1
88
NA
1 patient
None
4 had 100% donor chimerism
3 had mixed chimerism
Fludarabine and low-dose TBI
  Burroughs, 2010 [ 36]
NA
2 IPEX
0.75, 16
2 MUD
1 marrow
1 PBSC
Flu 90 mg/m 2
TBI 4Gy
GvHD prophylaxis
CSA and MMF
16, 17
0
2 had grade II
1 severe
Both alive
Both engrafted
None
None
Full immune function and normal FOXP3 protein expression
  Burroughs, 2007 [ 35]
1998–2006
14 PID
Range 0.5–30
8 MFD
8 MUD
8 marrow
5 PBSC
1 CB
Flu 90 mg/m 2 ( n = 13)
TBI 2Gy ( n = 14)
GvHD prophylaxis
CSA and MMF
15 (range 5–23)
0
II: 71
III–IV: 7
Extensive: 47
62
62
1
1 unconditioned PBSC for slipping myeloid chimerism
1 conditioned HCT for persistent thrombocytopenia
1 DLI for low donor CD4 and CD8 chimerism
1 conditioned HCT for graft failure
5 mixed chimerism
8 full donor chimerism
Antibody-based conditioning
  Schulz, 2011 [ 44]
2003–2007
14 non-malignant
4 SCID
2 CGD
2 Hyper IgM
2 other PID
4 H-globinopahty
7.5 (range, 1–20)
3 MFD
1 MMFD
8 MUD
2 HID
8 marrow
4 PBSC
2 TCD-PBSC
90Y-labeled anti-CD66 antibody at Day −14
Fludarabine 160 mg/m 2
Melphalan 70-140 mg/m 2
ATG for mismatched donor and unrelated donor
NA
0
II: 36
III–IV: 0
limited, n = 2
extended, n = 3
88
81
n = 1
1
9 had 100% chimerism
2 had mixed chimerism
  Straathof, 2009 [ 24]
1999–2002
16
8 SCID
1 MHC class II def.
1 IPEX
1 HLH
1 DKC + SCID
1 Ligase 4 def.
1 CD40 ligand def.
2 Other PIDs
0.7 (range, 0.4 to 11.4)
5 MSD
9 MUD
2 MMUDD
40 marrow
12 PBSC
1 PBSC + marrow
17 CB
Anti-CD45 1.6 mg/kg (day − 5 to − 2)
Flu 150 mg/m 2
Alemtuzumab 0.3 to 0.6 mg/kg
GvHD prophylaxis
CSA and MMF
9.5 (range 1–15)
0
II–IV: 38
III–IV: 19
31
81
95
3
1 s HCT
Median myeloid: 100% (range, 41–100%)
Median lymphocyte: 100% (range, 54–100%)
primary, secondary, aGvHD acute graft-versus-host disease, ALG antilymphocyte globulin, ATG anti-thymocyte globulin, AUC area under curve, BM bone marrow, BU busulfan, CB cord blood, CGD chronic granulomatous disease, cGvHD chronic graft-versus-host disease, CSA ciclosporin, def deficiency, DLI donor lymphocyte infusion, ES engrafted survival, Flu fludarabine, H-globinopathy hemoglobinopathy, HID haploidentical donor, HLH hemophagocytic lymphohistiocytosis, IMD inherited metabolic disease, IQR interquartile range, MMF mycophenolate mofetil, MMRD mismatched related donor, MMUD mismatched unrelated donor, MSD matched sibling donor, MUD matched unrelated donor, MTX methotrexate, N neutrophil, NA not available, OS overall survival, PID primary immunodeficiency diseases, SCID severe combined immunodeficiencies, Treo treosulfan, TT thiotepa, vs versus, WAS Wiskott-Aldrich syndrome, WB whole blood

Fludarabine and Treosulfan

Treosulfan (L-treitol-1,4-bis-methanesulfonate) is a prodrug and a water-soluble bifunctional alkylating agent which has been used for many years as treatment for various neoplasms, but more recently as part of conditioning for HSCT. In addition to myeloablative properties, it has marked immunosuppressive properties which contribute to the achievement of stable engraftment posttransplant. It causes relatively low organ toxicity compared to high-dose busulfan and cyclophosphamide leading to fewer complications such as veno-occlusive disease of the liver.
The first successful allogeneic transplant in a child using treosulfan was performed in 2000 and since then many reports have confirmed its efficacy and safety in both malignant and non-malignant disorders [ 11••, 12•, 13, 14•, 1518]. Slatter et al. first published results of 70 children with PID who received treosulfan in combination with either cyclophosphamide ( n = 30) or fludarabine ( n = 40) with an overall survival of 81% (median follow-up 19 months) equivalent in those aged less or greater than 1 year at time of transplant [ 13]. Toxicity was low but worse after cyclophosphamide, and T cell chimerism was significantly better after fludarabine [ 18]. Slatter et al. more recently reported 160 patients who had received conditioning with treosulfan and fludarabine achieving a probability of 2-year survival of 87.1% with a high level of complete or stable mixed chimerism in the diseased cell lineage, sufficient to cure disease [ 11••]. There was a high survival rate in children transplanted under 1 year of age in whom toxicity can be a problem with conventional and other reduced intensity conditioning regimens [ 24, 25]. A 100-day survival of 94% demonstrated the low toxicity of this regimen making it suitable for patients with PID who often have infection and organ damage prior to HCT. In this series, a higher level of myeloid chimerism was found in recipients of PBSC compared to CB and BM, without an increased risk of grade III/IV acute or chronic graft-versus-host disease (GvHD). This highlights the importance of the whole transplant package including stem cell source and serotherapy when tailoring therapy [ 26].
Excellent results were reported by Lehmberg et al. in 19 patients with hemophagocytic lymphohistiocytosis (HLH) following HCT with treosulfan, fludarabine, alemtuzumab, with or without thiotepa, all of whom survived with a median follow-up of 16 months [ 16].
Haskologlu et al. reported 15 patients with PID who had a high risk of developing transplant-related toxicity due to previous lung and liver damages and were given treosulfan-based conditioning [ 27]. At 32 months follow-up, the overall survival was 86.7% with excellent chimerism and low conditioning associated morbidity despite the high-risk population.
Mixed chimerism is sufficient to achieve cure in some non-malignant disorders, but the specific diagnosis and level of chimerism needed to achieve cure must be taken into account when balancing the need for increased myeloablation against short- and long-term toxicities from the conditioning regimen. The addition of thiotepa is common in order to increase the intensity of the regimen, but there are few reports of any comparison in outcomes comparing treosulfan and fludarabine with or without additional thiotepa. Yael Dinur-Schejter et al. reported 44 patients with non-malignant diseases: 19 received treosulfan with fludarabine 66.7% of whom achieved complete engraftment compared to 94.7% of 20 patients who received additional thiotepa, but this did not translate into any significant difference in overall or event free survival [ 15].

Fludarabine and Busulfan

Traditionally, busulfan (Bu) was used in combination with cyclophosphamide (Cy) as the standard myeloablative conditioning regimen for HCT for both malignant and non-malignant disorders in both adult and pediatric patients. Cyclophosphamide is increasingly being substituted with fludarabine (Flu), a nucleoside analogue with immunosuppressive properties, to provide a less toxic but equally effective regimen [ 19, 21, 28].
Harris et al. compared 1400 children who received Bu-Cy to 381 who received Bu-Flu. Busulfan doses were comparable between the 2 groups and the majority had pharmacokinetic monitoring. Eight hundred and three had non-malignant disorders including 195 with PID who received Bu-Cy and 86 who received Bu-Flu. Nine hundred and seventy-eight had malignant disorders. Children receiving Bu-Flu for non-malignant conditions experienced less toxicity than those receiving Bu-Cy, but survival was comparable. Children with malignancy had shorter postrelapse survival with Bu-Flu than Bu-Cy although transplant-related mortality and relapse were similar [ 29].
The pharmacokinetics of busulfan have been studied extensively and the use of a lower target area under the curve (45–65 mg/L × h) combined with fludarabine has been pioneered by Tayfun Güngör and colleagues in Zurich. Particularly impressive results have been seen using this regimen for patients with chronic granulomatous disease (CGD). Fifty-six children and young adults with CGD were reported, many of whom had high-risk features such as intractable infections and autoinflammation. Twenty-one HLA-matched related-donor and 35 HLA-matched unrelated-donor transplants were done. The 2-year probability of overall survival was 96% (95% CI 86∙46–99∙09), and of EFS was 91% (79∙78–96∙17). Graft-failure occurred in 5% (three of 56) of patients. The cumulative incidence of acute GvHD of grade III–IV was 4% (two of 56) and of chronic GvHD was 7% (four of 56). Stable (≥ 90%) myeloid donor chimerism was documented in 52 (93%) surviving patients [ 20••].
Dvorak et al. have recently reported the result of the use busulfan at a lower target area under the curve (30 mg/L × h) alone or in combination with fludarabine or thiotepa in 10 patients with severe combined immunodeficiency. All the patients survived, one patient required second HCT, and 3 had no B cell reconstitution [ 19].

RIC in PID

Fludarabine and Melphalan

Increasing recognition of the significant toxicities associated with conventional doses of busulfan and cyclophosphamide, particularly in very young infants and especially in those with pre-existing end organ damage, led to the adoption of immunosuppressive-based, rather than myelo-ablative-based regimens, with fludarabine and melphalan. The results, principally in those with significant preexisting comorbidities, were striking with significantly improved early survival [ 22, 23, 30, 31, 49]. However, donor chimerism was not always optimal, and there was a high incidence of late viral reactivation, and late onset acute GvHD. Furthermore, toxicities in infants < 1 year of age remained significant [ 25]. Melphalan in particular has been associated with cardiac toxicities [ 32]. Good results have been reported for patients with hemophagocytic lymphohistiocytosis [ 33]. Patients with X-linked inhibitor of apoptosis protein (XIAP) deficiency, which is difficult to transplant, also have good outcomes reported using fludarabine and melphalan-based regimens [ 34]. It has been used in adults with PID with good transplant survival [ 23]
While the approach remains attractive in terms of reduced toxicities, concerns regarding late graft failure and high mortality in the < 12-month-aged infants remain.

Minimal Intensity Conditioning for PID

Fludarabine and Low-Dose TBI

Burroughs et al. from the Seattle group have reported the transplant outcome of using fludarabine and low-dose TBI in 14 PID patients with significant preexisting organ dysfunction and infections. All received posttransplant GvHD prophylaxis with cyclosporin and mycophenolate mofetil but no serotherapy. Overall survival at 3 years was 62%, but there were high rates of acute (79%) and extensive chronic GvHD (47%) [ 35]. One had graft failure and an additional three patients required a second procedure for decreasing chimerism. Of 10 evaluable patients, 8 had correction of immune deficiency with stable chimerism. However, the high rate of GvHD has limited the broader use of this conditioning regimen in children with PID [ 35, 36].

Antibody-Based

While conditioning regimens have undoubtedly become less toxic, the ability to achieve donor chimerism without the use of chemotherapeutic agents, particularly in patients with non-malignant disease, is extremely attractive. Furthermore, some primary immunodeficiencies have significant toxicities associated with the administration of alkylating agents, due to the nature of the molecular defect, leading to serious long-term effects or early mortality [ 3739]. A number of different strategies have been employed to minimize the exposure to chemotherapeutic agents by the use of antibodies to aid stem cell engraftment, with or without adjunct chemotherapy.

Anti-CD45 Antibodies

CD45 is selectively expressed on all leucocytes and hematopoietic progenitors but is absent on non-hematopoietic tissues. Straathoff and colleagues studied 16 patients with PID who were less than 1 year of age or had significant preexisting comorbidities and were felt not suitable for conventional reduced intensity conditioning [ 24]. The conditioning regimen was comprised of alemtuzumab 0.2 mg/kg daily for 3 days for unrelated donors, or 0.1 mg/kg daily for 3 days for matched sibling donors on day − 8 to day − 6, clinical grade rat anti-CD45 (YTH24·5and54·12) 0.4 mg/kg on day − 5 to day − 2, fludarabine (30 mg/m 2 daily for 5 days on day − 8 to day − 4) and cyclophosphamide (300 mg/m 2 daily for 4 days on day − 7 to day − 4). Twelve patients were alive and well at the end of the study, one failed to engraft and was successfully re-transplanted, and 3 died—none of conditioning toxicity. Donor chimerism was variable but high level and sufficient to cure disease in the survivors.

Radioimmunotherapy

Radioimmunotherapy is an attractive concept for conditioning of patients with PIDs as it exploits of the physical cytotoxic effect of radiation and reduces the toxicity to other organ systems by its internal application and the conjugation of radioisotopes to specific antibodies [ 40]. Radioisotopes emitting α, β or γ-radiation of calculated intensity can be brought in direct proximity to the cells of interest. This enables malignant cells to be eradicated or benign hematopoietic cells to be depleted as part of conditioning before autologous or allogeneic HSCT. The method was developed to allow better and more specific control of malignant cells in the setting of HSCT without an increase in non-relapse mortality. Considerable clinical data was accumulated with conjugates of 90Yttrium or 131Iodine to anti-CD20 antibodies in the treatment of patients with refractory or recurrent B cell non-Hodgkin lymphoma (B-NHL). The drugs were used in combination with chemotherapy to prepare patients for autologous and allogeneic stem cell transplantation. This experience resulted in the approval of two drugs (Zevalin® and Bexxar®) by the FDA at the beginning of the century [ 40].
The use of RIT for the treatment of leukemias or for myeloablation in non-malignant disease until present is limited to clinical studies. A conjugate of 131Iodine to anti-CD45-antibody was explored in the treatment of patients with AML and high-risk MDS, again a combination of RIT with conventional myeloablative or immunosuppressive drugs was used for conditioning before allogeneic HSCT [ 41, 42]. CD45 is expressed on most AML and ALL blasts as well as on virtually all developing and mature cells of normal hematopoiesis. Radiolabeled anti-CD45 antibody doses up to 43 Gy were administered to the bone marrow in combination with RIC and allogeneic transplantation with good tolerance and without additional toxicity in younger adult patients with AML and MDS [ 43]. For children, limited published data exists for the use of RIT for pretransplant conditioning. A conjugate of 90Yttrium to an antibody targeting CD66 was used in combination with melphalan and fludarabine or TBI for the treatment of children with considerable comorbidities with malignant and non-malignant disease. 90Yttrium emits pure β-radiation with a maximum range of 11 mm and a half-life of 2.7 days [ 44]. With these qualities, no isolation of the pediatric patients was necessary, but the dosimetry had to be performed with another isotope, emitting γ-radiation to be detected in a γ-camera. CD66 is abundantly present on mature myeloid cells but usually not expressed on malignant blasts. The therapeutic principle of RIT with this antibody in malignant disease therefore relies on the so-called cross-fire effect, which describes the indirect depletion of blasts by binding of the antibody to cells in close proximity [ 40]. In order to avoid graft rejection in unrelated or mismatched grafts, recipients received serotherapy with ATG in this setting. Fifteen of 16 children with non-malignant disease survived the procedure, 13/15 with complete donor chimerism. The Kaplan-Meier estimation for disease-free survival at 24 months was 94%. This clearly documented feasibility of and reliable myeloablation by RIT in children and young adults with non-malignant disease.

Anti-CD117 Antibodies

The molecule CD117 (c-Kit receptor) is expressed on hematopoietic stem cells at all stages of development. Interactions with the ligand of CD117, stem cell factor, are crucial for hematopoietic stem cell survival, and this signaling pathway plays a critical role in the homing, adhesion, maintenance, and survival of hematopoietic stem cells in the hematopoietic niche. Preclinical studies demonstrated that using an antibody against CD117 to impede CD117-stem cell factor signaling selectively depleted hematopoietic stem cells with no effect on differentiated progenitor or mature cell lineages, and enabled engraftment of donor cells [ 45]. A clinical trial is currently in progress using anti-CD117 antibody alone to treat patients with primary immunodeficiencies (AMG191 Conditioning/CD34 + CD90 Stem Cell Transplant Study for SCID Patients, ClinicalTrials.​gov Identifier: NCT02963064). The early results of this dose finding study show that some donor stem cell chimerism, leading to donor T and B lymphocyte chimerism can be achieved [ 46]. These preliminary data are extremely exciting and potentially lead the way to a step change in approaches to conditioning in patients with PIDs.

Conditioning for Haploidentical Donor Transplant

As the outcomes of HCT using newer T cell depletion methods have improved, there is an increasing number of haploidentical transplants performed for both SCID and non-SCID PID. Various non-myeloablative conditioning regimens have been used in T-deplete and T-replete haploidentical transplant (Table 3) [ 5••, 47, 48, 51]. The Great North Children’s Hospital (GNCH) group in Newcastle has used fludarabine, treosulfan, ATG (Grafalon), and ritixumab for patients who received CD3 TCR ab/CD19 depleted peripheral blood stem cells. Patients with non-SCID PID received additional thiotepa. The overall survival was comparable with family and unrelated donor transplant using a similar conditioning regimen [ 18, 51]. Neven et al. reported the outcome of Bu-Flu in 22 patients with PID received haploidentical transplant using posttransplant cyclophosphamide. The overall survival and donor chimerism were good, but 48% had acute GvHD and 24.2% had chronic GvHD.
Table 3
Outcome of haploidentical donor transplant in PID using modern T lymphocyte depletion strategies and various conditioning regimens
Author, year
Year of HCT
No of patients/diagnosis
Median age at HCT (range), years
Donor and stem cell source
Conditioning regimen and GvHD prophylaxis
Median day of N engraftment
VOD %
aGvHD %
cGvHD %
OS %
ES %
Graft failure %
Second procedure, n
Latest donor chimerism/remarks
Fludarabine and treosulfan
  Neven, 2019 [ 48]
2014–2017
22 PID
5 osteopetrosis
21 first HCT
6 s HCT
1.5 (0.2–17)
27 HID
All marrow
20 MAC with Bu-pk + Flu 160 mg/m 2 (4 received additional Cy 28 mg/kg)
Serotherapy: rituximab plus alemtuzumab/ATG
7 had RIC (1 first HCT and 6 s HCT)
GVHD prophylaxis
CSA
MMF
PTCy 50 mg/kg on day 3 + 4
19 [ 1113, 1534]
11
II–IV: 48
II: n = 10
III: n = 2
24.2
77.7
77.7
n = 2
1
24 full chimerism
1 mixed chimerism
  Shah, 2018 [ 5]
2012–2016
25 PID
3 for refractory GvHD
1.75 (0.28–10.3)
23 HID
2 MMUD
TCR ab/CD 19 depleted PBSC
Flu 150 mg/m 2
Treo 36-42 mg/m 2
TT 10 mg/kg
24 had serotherapy (ATG/alemtuzumab)
6 had rituximab
3 SCID: unconditioned
GvHD prophylaxis: CSF/MMF
25 [ 1019, 21, 2428, 49, 50]
0
II–IV: 22
None
83.9
80.4
n = 1
1
76.1% full donor chimerism
5 had high T cell but mixed myeloid chimerism (2 unconditioned)
  Rastogi, 2017 [ 47]
2013–2016
8 PID
4.9 (0.8–12)
7 HID
1 MUD
Unmanipulated marrow/PBSC
5 Flu 160 mg/m 2 + Cy 29 mg/kg + TBI 2Gy (3 had additional TT) + ATG/alemtuzumab
2 Flu 160 mg/m 2 + Treo 42 mg/ 2
1 Flu 160 mg/m 2 + Bu 3.2 mg/kg
GVHD prophylaxis
Tacrolimus
MMF
PTCy 50 mg/kg on Day 3 + 4
Mean 17
NA
I–II: 3 patients
II–IV: none
2 limited
75
75
None
None
All full donor chimerism
  Balashov, 2015 [ 51]
2012–2014
37 PID
5 SICD
32 non-SCID PID
2.6 (0.2–17)
27 MUD
10 MMRD
TCR ab/CD 19 depleted PBSC
Flu 150 mg/m 2
Treo 36-42 mg/m 2
8 had Melphalan 140 mg/m 2 for high risk graft rejection
14 had rituximab
1 unconditioned
Serotherapy
35 ATG
2 alemtuzumab
16 (range 11–28)
NA
Max grade 2 in 7 patients
Only one had grade IV (no conditioning)
1 patient (unconditioned)
96.7
67.7
27%
HID: 36%
MUD: 28%
10
NA
aGvHD acute graft-versus-host disease, BU busulfan, cGvHD chronic graft-versus-host disease, CSA ciclosporin, ES engrafted survival, Flu fludarabine, HID haploidentical donor, MAC myeloablative conditioning, MMF mycophenolate mofetil, MMUD mismatched unrelated donor, MSD matched sibling donor, MUD matched unrelated donor, N neutrophil, NA not available, OS overall survival, PID primary immunodeficiency diseases, RIC reduced intensity conditioning, SCID severe combined immunodeficiencies, Treo treosulfan, TT thiotepa, WAS Wiskott-Aldrich syndrome

Pharmacokinetic Studies

Although levels of busulfan have been measured for many years, to target the narrow myeloablative therapeutic window, minimize toxicity from supra-therapeutic levels and avoid sub-myelo-ablation and rejection, it is only recently that the importance of pharmacokinetic monitoring of other agents of the conditioning cocktail has been appreciated.

Fludarabine Pharmacokinetics

Ivaturi et al. prospectively studied the pharmacokinetics and pharmacodynamics of 133 children undergoing HCT for a variety of disorders with a variety of conditioning regimens but all included fludarabine. Young age and renal impairment were found to lead to an increased exposure. In the setting of malignancy, disease-free survival (DFS) was highest 1 year after HCT in subjects achieving a systemic fludarabine plasma (f-ara-a) cumulative area under the curve (cAUC) greater than 15 mg*hour/L compared to patients with a cAUC less than 15 mg*hour/L (82.6% versus 52.8%, p = 0.04) [ 52]. Further development of model-based dosing may minimize toxicity and maximize efficacy, resulting in superior outcomes for malignant and non-malignant patients.

Treosulfan Pharmacokinetics

Relatively high variability of treosulfan pharmacokinetics in pediatric patients may raise the need for implementing therapeutic drug monitoring and individual dose adjustment in this group. Vander Stoep et al. and Mohanan et al. recently published the first results of a relationship between the exposure of treosulfan and early toxicity, as well as clinical outcome, in children undergoing conditioning prior to HSCT. In the former study, patients with an AUC > 1650 mg h/L demonstrated a statistically higher incidence of mucosal and skin toxicity than those with an AUC 1350 mg h/L (odds ratio 4.4 and 4.5, respectively). The odds of developing hepato- and neurotoxicity were also higher in the former group, but the difference did not reach statistical significance. No association was found between treosulfan exposure and early clinical outcomes, i.e., engraftment, donor chimerism, acute graft-versus-host disease, treatment-related mortality, and overall survival. PK parameters were shown to be age-dependent, with higher AUC values in younger children (< 1 year old) and corresponding lower treosulfan clearance. A challenge in therapeutic monitoring of treosulfan within conditioning prior to HCT is a very brief course of treatment, consisting of three doses administered on 3 consecutive days. This allows personalization of only the second and third dose of the prodrug unless a test dose is applied prior to starting the actual regimen.
Since pharmacokinetic studies of treosulfan began, it has been assumed that plasma (serum) concentrations of the prodrug are a good representation of the alkylating activity of its epoxy transformers. However, for years, a correlation between treosulfan concentrations in plasma and levels of specific DNA adducts in tissues, for example the bone marrow, or clinical effects, have not been investigated. Therapeutic drug monitoring of not only prodrug but also its active epoxide might be needed. In addition blood pH, body temperature, and intravenous fluid delivery may influence glomerular filtration, tubular reabsorption, and nonenzymatic epoxy transformation of the prodrug [ 53].

Serotherapy Levels

It is now well recognized that type of serotherapy, dose and timing in relation to the transplant all have an impact on outcome of transplant in terms of occurrence of GVHD, immune reconstitution importantly in terms of viral reactivation, clearance of infection, and chimerism. Marsh RA et al. collected data from 105 patients to examine the influence of peritransplant alemtuzumab levels on acute GVHD, mixed chimerism, and lymphocyte recovery. Significantly higher levels of aGVHD but higher levels of donor chimerism, lymphocyte counts at D+30 and T cell counts at D+100 were associated with lower alemtuzumab levels at day 0 [ 54].
In a recent report, the clearance of the active components of the 2 widely used types of ATG (Fresenius/Grafalon and Genzyme) was studied in 38 children with malignant hematological disorders. They found that ATG Fresenius was cleared rapidly and uniformly from the circulation whether they received 60 mg/kg or 45 mg/kg, but there were significant differences in patients who received a high dose of ATG Genzyme (10 mg/kg) who had significantly slower reconstitution for CD3, CD4, and CD8 T cells compared to patients who received a low dose of ATG Genzyme (6–8 mg/kg) or ATG Fresenius [ 55].

Stem Cell Source in Non-MAC Conditioning

Historically bone marrow has been the preferred stem cell source for HCT in children due to concerns that peripheral blood stem cell products led to an increased risk of GVHD. In Slatter et al.’s report of 160 PID patients who received uniform conditioning with treosulfan and fludarabine, a higher level of myeloid chimerism was found in recipients of PBSC compared to CB and BM, without an increased risk of grade III/IV acute or chronic GvHD [ 26]. This is an important finding particularly for patients with diseases where a high level of chimerism is required to achieve complete cure.

Conclusions

The use of RTC and RIC has been a major paradigm shift in HCT for PID and may have contributed to improved survival through a reduction in early post-HSCT toxicities. Almost certainly, long-term toxicities will be reduced, although further data are required to confirm this. However, the use of antibody-based conditioning regimens is likely to transform the field in the future. The drive for this has been that PID can be completely cured by HCT, and as malignancy is rarely a feature of the disease, toxicity from the curative procedure should be minimized. More recently, newborn screening for severe combined immunodeficiencies has meant that these patients are now being identified by 2–3 weeks of age [ 56]. Rapid transplantation is preferred, as survival and neurological outcome results are best in patients with no preexisting infection [ 57, 58]. As gene therapy approaches become mainstream treatment, then a non-toxic conditioning approach followed by an autologous gene-corrected stem cell procedure should almost eliminate short- and long-term treatment-related morbidities for patients with SCID [ 59, 60]. These conditioning approaches will have to be modified for combined immunodeficiencies and gain-of-function diseases where high-level or complete donor chimerism is required to abolish disease manifestations [ 6164]. However, combinations of antibody-based regimens and pharmacokinetically targeted reduced low-toxicity agents may help resolve these issues. The future for patients with PID looks extremely encouraging.

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Conflict of Interest

The authors declare no conflicts of interest relevant to this manuscript.

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This article does not contain any studies with human or animal subjects performed by any of the authors.
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