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\begin{document}

\title{Busulfan Fludarabine and Melphalan is effective conditioning for
pediatric and young adult patients with myeloid malignancies underdoing
matched sibling or alternative donor transplantation}



\author[1]{Emmanuel Katsanis}%
\author[1]{Laurel Truscott}%
\author[1]{Holly Pariury}%
\author[1]{Santosh Hanmod}%
\author[1]{Monica Davini}%
\author[1]{Michelina de la Maza}%
\author[1]{Lauren Sapp, MSN, CPNP}%
\author[1]{Kyleigh Staples}%
\author[1]{Maria Proytcheva}%
\affil[1]{The University of Arizona}%


\vspace{-1em}



  
  \date{May 27, 2024}


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\selectlanguage{english}
\begin{abstract}
\textbf{Background:} Allogeneic hematopoietic cell transplantation
(allo-HCT) remains a curative option for patients with high-risk myeloid
malignancies. \textbf{Procedure:} We present our ten-year experience
(October 2012-October 2021) of consecutive allo-HCT in patients with
myeloid malignancies treated on the pediatric HCT service and
conditioned with myeloablative targeted dose-busulfan (BU), fludarabine
(FLU) and melphalan (MEL). Twenty-three children, adolescents, and young
adult patients (CAYA) (median age 15.4yr) with acute myeloid leukemia
(AML, n=17), myelodysplastic syndrome (MDS, n=4), or chronic myeloid
leukemia (CML, n=2) underwent allo-HCT post-BU-FLU-MEL. Four patients
had treatment-related AML/MDS. Donor/stem cell source was MSD PBSC
(n=7), MUD PBSC (n=2), UCB (n=3) or haploidentical-BMT (n=11). Risk
stratification was low (n=2), intermediate (n=15), high (=3) and very
high risk (n=1). The two patients with CML had failed tyrosine kinase
inhibitor therapies. \textbf{Results:} With a median follow-up of 39.6
months the relapse rate is only 4.5\% with an OS 100\%, PFS 95.5\% and
graft-versus-host-free-relapse-free survival (GRFS) 67.8\%. The donor
source and the GvHD prophylaxis regimen significantly impacted grades
II-IV aGvHD 66.7\% versus 19.2\% ( \emph{P=0.039}) and cGvHD 66.7\%
versus 0\% ( \emph{P=0.002}) in the patients receiving matched sibling
(MSD) or matched unrelated donor (MUD) PBSC compared to haplo-BMT
respectively, resulting in improved GRFS in haplo-BMT, 83.3\% compared
to 40\% matched donor PBSCT ( \emph{P=0.025}). \textbf{Conclusions:} Our
results demonstrate that BU-FLU-MEL is efficacious conditioning for
disease control in young patients with myeloid malignancies undergoing
MSD or alternative donor allo-HCT but in the setting of PBSC grafts with
CSA-MTX GvHD prophylaxis it results in an unacceptably high incidence of
GvHD.%
\end{abstract}%



\sloppy


\textbf{Introduction}

Standard treatment for children, adolescents, and young adult patients
(CAYA) patients with acute myeloid leukemia (AML) includes up to five
cycles of chemotherapy containing cytarabine and anthracyclines.
Improvements in supportive care, including oral and skin care regimens,
empiric broad spectrum antibiotics and prophylactic antifungals have had
a meaningful impact on the treatment related toxicity associated with
AML therapy\textsuperscript{1, 2}. Despite this, more than 40\% of
patients ultimately die of refractory or relapsed disease or treatment
related toxicity\textsuperscript{3}. Patients with high-risk AML are
candidates for allogeneic hematopoietic cell transplantation (allo-HCT)
which similarly carries regimen related toxicities in addition to risk
for graft-versus-host disease (GvHD). Advancements in supportive care
have also led to improvement in survival for patients with myeloid
malignancies undergoing allo-HCT\textsuperscript{4}.

The choice of conditioning regimen plays a major role in a patient's
likelihood of achieving a graft-versus-host-free-relapse-free survival
(GRFS), however there is no consensus on the optimal conditioning
regimen. Historically, the most common preparative regimen used for
patients with AML was total body irradiation (TBI) with cyclophosphamide
(CY) which was later largely replaced by busulfan (BU) and
CY\textsuperscript{5}. BU-CY was associated with comparable disease
control without the long-term secondary effects of
irradiation\textsuperscript{6}. BU-fludarabine (FLU) gained significant
traction to reduce the toxicity of high dose CY, however it led to
inferior progression-free-survival (PFS), when compared to BU-CY in
adults\textsuperscript{7}. To improve the efficacy of pre-transplant
conditioning while keeping non-relapse mortality (NRM) low, three drug
regimens were introduced with an additional alkylator. These included
thiotepa (TT)-BU-FLU, BU-CY-melphalan (MEL) or BU-FLU-MEL. In an adult
study, TT-BU-FLU demonstrated superior survival as compared to BU-FLU
due to a reduced risk of relapse, with comparable
NRM\textsuperscript{8}. BU-FLU-Mel was also associated with better
overall (OS) in adult patients without increase in non- NRM compared to
FLU-BU due to decreased relapse\textsuperscript{9}. There is limited
pediatric data on the comparison of three drug regimens to the more
conventional BU-CY or TBI-CY. A multi-center study by the European Group
for Blood and Marrow Transplantation (EBMT) reported on AML patients
between ages 2-18 years. Patients receiving BU-CY-MEL showed a lower
incidence of relapse, higher OS and PFS with a comparable NRM when
compared to those conditioned with BU-CY or TBI-CY\textsuperscript{10}.
We previously demonstrated that BU-FLU-MEL conditioning was safe and
effective in a limited series of six pediatric patients with high-risk
myeloid malignancies\textsuperscript{11}. Herein, we expand our
experience utilizing this regimen effectively in CAYA with myeloid
malignancies undergoing matched or alternative donor allo-HCT.

\textbf{Methods}

\emph{Patients:} This is a single-center retrospective review of
consecutive pediatric and young adult patients with myeloid malignancies
who underwent allo-HCT on the pediatric service from October 2012 to
October 2021. All patients received myeloablative (MAC) conditioning
with BU-FLU-MEL, which has been our pediatric hematopoietic transplant
program's standard of care conditioning for myeloid leukemias for the
last ten years. Allo-HCT recipients met organ criteria allowing for MAC
and had no evidence of active untreated infection. University of Arizona
institutional review board approval was obtained to review and report
our findings. Four of the eleven patients receiving haploidentical bone
marrow transplantation (haplo-BMT) were treated as part of an
institutional review board (IRB)--approved Phase Ia single-institution
trial\textsuperscript{12, 13}.

\emph{Conditioning regimens:} Allo-HCT conditioning for matched sibling
donor (MSD) and unrelated allo-HCT consisted of BU at 0.8 mg/kg IV every
6 hours for a total of 16 doses (days -9 to -6). BU pharmacokinetics of
the first dose were performed at the Seattle Cancer Care Alliance
laboratory. The seventh and remaining doses were modified to achieve an
average area under the curve (AUC) of 3.9 to 4.4 mg x hr/L for the
duration of the course. The estimated median exposure was 4.0 for MSD
and 4.2 mg x hr/L for unrelated allo-HCT. FLU was given at 30
mg/m\textsuperscript{2} on days -5 to -2 and MEL 180
mg/m\textsuperscript{2} divided into three equal doses and administered
on days -4 to -2. One patient with AML who underwent matched unrelated
donor (MUD) peripheral blood stem cell transplant (PBSCT) also received
rabbit anti-thymocyte globulin (rATG) 1 mg/kg on days -3 to -1. The
other patient with AML (evolving from myelodysplastic syndrome
{[}MDS{]}) who received a MUD PBSCT had 32\% myeloblasts prior to
conditioning and was therefore given a higher total dose of MEL at 200
mg/m\textsuperscript{2} split between days -3 and -2 without the
addition of ATG\textsuperscript{11}.

All eleven patients undergoing T-replete haplo-BMT received BU at 0.8
mg/kg IV every 6 hours for a total of 12 doses (days -8 to -6). BU
pharmacokinetics of the first dose were performed with the seventh and
remaining doses modified to achieve average AUC exposure of 4.0 to 4.5
mg x hr/L for the duration of the course (median was 4.3). BU was
followed by FLU 30 mg/m\textsuperscript{2} for our first seven patients
and FLU 40 mg/m\textsuperscript{2}, for the later four, given on days -5
to -2 and a single dose of MEL 100 mg/m\textsuperscript{2} on day
-2\textsuperscript{12-15}.

\emph{Graft-versus-host disease prophylaxis:} All MSD and MUD PBSCT
patients (n=9) received GvHD prophylaxis with methotrexate (MTX) 10
mg/m\textsuperscript{2} IV infused on days +1, +3, +6 and +11 and
cyclosporine A (CSA) starting on day -2 and targeting levels of 150-200
ng/ml. The three umbilical cord blood (UCB) patients received
mycophenolate mofetil (MMF) 15 mg/kg q 8 h IV between day -3 and +28 and
CSA also starting on day -3. In all patients and in the absence of GvHD,
CSA taper occurred between days + 90 and +180. Seven haplo-BMT patients
received PT-CY 50 mg/kg IV on days +3 and +4 and another four patients
that were part of an IRB--approved phase I single institution clinical
trial through the University of Arizona Cancer Center (NCT02996773)
received PT-CY/bendamustine (BEN). One patient (cohort 2) received PT-CY
50 mg/kg on day +3 and PT-CY 20 mg/kg followed by PT-BEN 60
mg/m\textsuperscript{2} on day +4. Another patient (cohort 3) was
treated with PT-CY 50 mg/kg IV on day +3 and PT-BEN 90
mg/m\textsuperscript{2} on day +4. Two patients (cohort 4) received
PT-CY 40 mg/kg on day +3 followed by PT-BEN 20 mg/m\textsuperscript{2}on
day +3 and PT-BEN 90 mg/m\textsuperscript{2} on day +4. All patients
were started on MMF on day +5 until day +28 and tacrolimus from day +5,
targeting levels of 6-10 ng/ml. In the absence of GvHD, tacrolimus was
weaned starting day +70 to +90 and discontinued by day +120 to +180.
GvHD was graded according to the consensus criteria for grading acute
and chronic GVHD\textsuperscript{16, 17}.

\emph{Supportive care:} Levofloxacin bacterial prophylaxis was started
when the absolute neutrophil count (ANC) dropped below 0.5 x
10\textsuperscript{9}/L. Antifungal prophylaxis with voriconazole was
initiated following completion of BU administration.
For\emph{Pneumocystis jirovecii} prophylaxis IV pentamidine was given on
day -1 and then twice monthly until day +45 and monthly afterwards until
one year after BMT. Acyclovir was also started on admission for herpes
simplex and varicella virus prophylaxis. Bi-weekly polymerase chain
reaction (PCR) monitoring for cytomegalovirus (CMV) and weekly for
adenovirus, Epstein-Barr virus (EBV) and human herpes virus-6 (HHV-6)
were performed until discharge from hospital and subsequently at least
every other week during first 100 days and at least monthly for another
6 months. All patients were transplanted in HEPA filtered rooms on a
HEPA filtered unit and encouraged to walk laps on the unit daily.

\emph{Donor selection:} MSD and MUD PBSC donors were 10/10 HLA-A, -B,
-Cw, -DRB1, and -DQB1 antigen matched at high resolution and -DPB1
matched or permissive. The UCB units were a 5/6 HLA-A, -B, -DRB1 antigen
match (single) with all units being 4/6 in the two double UCB
transplants. Haploidentical donors were first degree relatives who were
HLA-haploidentical based on high-resolution typing at HLA-A, -B, -Cw,
-DRB1, and -DQB1. Five of the donors were 5 of 10 antigen matches, five
were 6 of 10 and one was 7/10 \textbf{(Table 1)} . None of the patients
had anti-donor HLA antibodies.

\emph{Engraftment and Donor chimerism monitoring.} For UCB transplants
and haplo-BMT granulocyte-colony stimulating factor (G-CSF) was started
on day +1 and +5 respectively at 5 \selectlanguage{greek}µ\selectlanguage{english}g/kg/day until an ANC of 2.5 x
10\textsuperscript{9}/L was achieved for three consecutive days. Day of
myeloid engraftment was defined as the first of three consecutive days
with an ANC of 0.5 x 10\textsuperscript{9}/L. Day of platelet
engraftment was considered the first of three consecutive days with
platelet counts of \emph{\textgreater{}} 20 x 10\textsuperscript{9}/L
with no platelet transfusions administered in the previous 7 days.

Donor chimerism was evaluated on days +28 from bone marrow (BM) and
+100, +180 and +365 from peripheral blood (PB) by short tandem repeats
(STRs). Engraftment testing was performed using labeled primers to
PCR-amplify STR polymorphic DNA markers followed by capillary
electrophoresis to distinguish between the DNA contributed by the
recipient versus the donor and estimate the percentage of the
contribution. The Promega GenePrint 24 System which includes 24
polymorphic markers was used (Promega Corporation, Madison,
WI)\textsuperscript{18}.

\emph{Statistical analysis:} Time to event endpoints were estimated
using cumulative incidence curves and Kaplan-Meier curves, with
comparisons using log-rank tests\textsuperscript{19, 20}. Cox
proportional hazards models for aGvHD II-IV, aGVHD III-IV, cGVHD and
severe cGVHD also were adjusted for competing risks due to disease
relapse and graft failure. Analyses and graphs were performed using
GraphPad Prism version 9.4.0.

\textbf{Results}

\emph{Patient, disease and transplant characteristics:}Twenty-three
pediatric and young adult patients with myeloid malignancies underwent
allo-HCT following BU-FLU-MEL conditioning. The clinical characteristics
of the patients are outlined in \textbf{Table 1} . The median age at
transplant was 15.4 years (0.6 to 27.2 years). Seventeen patients were
male. Ethnic and/or racial minorities constituted 48\% of all patients,
the majority of whom (35\%) were Hispanic.

Seventeen patients had AML with fourteen being \emph{de novo} , one
evolving from MDS and two were treatment related (t-AML) secondary to
osteogenic and Ewings sarcoma therapy. The remaining patients included
four patients with MDS, two of which were treatment related secondary to
acute lymphoblastic leukemia (ALL) therapy, and two patients had chronic
myeloid leukemia (CML). Most AML patients were transplanted in first
remission (76\%) while five MDS patients (including an MDS patient
evolving to AML) did not receive conventional chemotherapy prior to
conditioning \textbf{(Table 1)} . Cytogenetics were unfavorable in the
majority of AML/MDS patients \textbf{(Table 1)} while the pediatric
disease risk index (DRI) was intermediate in 71\%, and high or very high
in 19\%\textsuperscript{21}. Two patients with CML are included in this
analysis, with one undergoing allo-HCT following accelerated phase while
the other had T315I mutation in the BCR-ABL1 kinase domain.

Seven patients received PBSCT from an HLA-matched sibling. Unrelated
donor allo-HCT included two patients who received a MUD PBSC graft, one
patient who received a UCB unit and two patients who were infused with
double UCB units. Eleven patients underwent T-cell replete haplo-BMT.

\emph{Engraftment and chimerism:} All patients had trilineage
engraftment with complete donor chimerism on their day +28 bone marrows
biopsies. One patient who received haplo-BMT experienced secondary graft
failure after early CMV reactivation and treatment with ganciclovir. He
was salvaged 46 days after the initial transplant using a single day
conditioning regimen followed by infusion of unmanipulated G-CSF
mobilized PBSC from his original maternal donor and engrafted promptly
(day +14)\textsuperscript{22, 23}. All patients when re-assessed by
peripheral blood chimerisms on days +100, +180 and +365 remained
complete donor chimeras.

\emph{Graft-versus-host disease:} The cumulative incidence of grades
II-IV and III-IV acute GvHD (aGvHD) was 44.8\% \textbf{(Figure 1A)}
(MSD/MUD-PB 66.7\% vs haplo-BMT 19.2\%; \emph{P=0.039} )\textbf{(Figure
1B)} and 17.8\% respectively (MSD/MUD-PB 33.3\% vs haplo-BMT 10\%;
\emph{P=0.196} ) \textbf{(Figure 1B)} . The cumulative incidence of
chronic GvHD (cGvHD) was 27.3\% \textbf{(Figure 1C)}(MSD/MUD-PB 66.7\%
vs haplo-BMT 0\%; \emph{P=0.002} ) \textbf{(Figure 1D)} and severe cGvHD
was 9.1\% respectively (\textbf{Figure 1C} ). Of the six patients with
cGVHD (1 mild, 3 moderate, 2 severe) three previously had grade III-IV
aGvHD, two had grade II aGvHD and one developed \emph{de novo} cGvHD
seven months after MSD PBSCT while off immunosuppression. Five of six
patients required systemic steroid therapy and three were also given
ruxolitinib with resolution/improvement of their cGvHD symptoms. Two
patients continue on cGvHD therapy.

\emph{Survival:} Two patients (8.7\%) required admission to the
intensive care unit (ICU) within their first 100 days post-allo-HCT. One
patient developed hyperammonemia due to \emph{Ureaplasma parvum} with
metabolic encephalopathy necessitating intubation and mechanical
ventilation\textsuperscript{24}. The other patient was transferred to
ICU because of mucositis needing brief intubation for airway support.
Only one patient with infant AML who underwent haplo-BMT (paternal
donor) in CR1 with 1.8\% minimal residual disease (MRD)+ relapsed 13.6
months following transplantation resulting in a cumulative incidence of
relapse of 4.5\%. This patient received a second haplo-BMT (maternal
donor) 16.6 months after the first and remains in remission without GvHD
8.2 months post second haplo-BMT. With a median follow-up of 39.6 months
(range 11 to 115 months) the OS is 100\% and PFS 95.5\% (\textbf{Figure
1E} ). Taking into consideration both relapse and grade III-IV acute or
chronic GvHD requiring systemic treatment, the GRFS is 67.8\%
(\textbf{Figure 1E} ) with haplo-BMT recipients having significantly
improved GRFS 83.3\% compared to 40\% in patients receiving MSD or MUD
PBSCT (\emph{P=0.025} ) (\textbf{Figure 1F} ).

\textbf{Discussion}

We describe our ten-year institutional experience in CAYA patients with
myeloid malignancies undergoing allo-HCT on our pediatric service. This
includes twenty-three consecutive patients of all risk categories who
were conditioned with BU-FLU-MEL and received grafts from a variety of
donor and stem cell sources. Thirty-nine percent of our patients were
young adults (19 to 27 years). At a median follow-up of greater than 37
months, we have a remarkable OS of 100\% and PFS of 95.5\%. Irrespective
of age, risk index, or treatment related disease, our OS and PFS is
noticeably higher that previous reports of AML/MDS in CAYA undergoing
allo-HCT.

There are several indications for allo-HCT in AML, including poor
cytogenetics such as the presence of high allelic \emph{FLT3/ITD} ratio,
monosomy 5, monosomy 7, or an \emph{MLL/KMT2A} rearrangement. A
pediatric DRI has been developed to risk stratify children and
adolescents with leukemia prior to transplant\textsuperscript{21}. For
AML, stratification into low, intermediate, high, and very high-risk
groups is based upon cytogenetic risk, but also age and disease status
complete remission (CR) and MRD. Using this stratification, 5-year
leukemia free survival (LFS) was 78\%, 53\%, 40\%, and 25\% respectively
with OS of 80\%, 64\%, 50\%, and 33\%\textsuperscript{21}. Applying this
DRI to our AML/MDS patient cohort, we had 2 low risk patients, 15
intermediate risk, 3 high, and 1 very high for which we would anticipate
OS and PFS of 64\% and 54\% respectively at 36 months. Four of our
patients had treatment related MDS/AML. A multicenter study of 273
pediatric patients with treatment-related MDS/AML undergoing MAC
allo-HCT found the 5-year OS and EFS to be 49.9\% and
41.2\%\textsuperscript{25}. A retrospective review of pediatric patients
with poor cytogenetics who underwent allo-HCT (55 MSD and 55 unrelated
donor {[}URD{]}), demonstrated a 5-year OS of 46\% with MSD, 50\% with
URD\textsuperscript{26}. These results are in line with what would be
predicted based on the DRI. In a review of Center for International
Blood and Bone Marrow Transplant Research (CIBMTR) data comparing
allo-HCT outcomes of pediatric (\textless{}15 years) and adolescent and
young adult (AYA) patients (15-40 years), similar survivals were
noted\textsuperscript{27}. For patients receiving MSD allo-HCT in first
complete remission (CR1) the 5-year OS was 69\% for pediatric patients
compared to 52\% for AYAs. The PFS was 65\% and 51\% respectively. For
those receiving URD allo-HCT, decreased OS was seen with 34\% for
pediatric patients and 41\% for AYAs. More recent CIBMTR data have shown
improvement in 3-year probabilities of OS at 70\% for AML patients
\textless{}18 years receiving MSD in CR1 and 67\% in those in CR2 or
greater and 61\% following MUD allo-HCT.

As there is still no consensus on optimal conditioning regimen for
allo-HCT in CAYA with myeloid malignancies there have been several
reports examining various regimens. A large multicenter study by
Pediatric Disease Working Party of the European Group for Blood and
Marrow Transplantation reported on pediatric patients with AML receiving
TBI-CY (n=109), BU-CY (n=389) or BU-CY-MEL (n=133)\textsuperscript{10}.
The addition of MEL to BU-CY resulted in a reduced incidence of relapse
of 14.7\% when compared to 31.5\% with BU-CY and 30\% with TBI-CY as
well as improved OS, 76.6\% vs 64\% with BU-CY, and 64.5\% with TBI-CY
as well as PFS 74.5 vs 58\% vs 61.9\% respectively with comparable NRM.
There have been limited studies applying BU-FLU-MEL as conditioning in
pediatric transplantation. We have previously reported on six patients
receiving alternative donor allo-HCT conditioned with BU-FLU-MEL that
was found to be safe and effective\textsuperscript{11}. No grade 4
toxicities were seen, with grade 3 mucositis being the most common
adverse effect. In a single institution study, pediatric (n=17) and
adult patients with AML/MDS received BU-MEL-FLU and ATG followed by a
T-depleted PBSC graft. The dose of BU and MEL was lower, 8-9.6 mg/kg and
140 mg/m\textsuperscript{2} when compared to our regimen of 12.8 mg/kg
and 180 mg/m\textsuperscript{2} respectively for MSD and URD
transplants\textsuperscript{28}. The PFS was only 47.2\% for pediatric
patients. The incidence of grade III-IV aGvHD and cGvHD was low as one
would expect from a T-cell depleted graft at 3.9\% and 5.9\%
respectively with a relapse rate of 28\%\textsuperscript{28}. In a large
multicenter retrospective report of 846 adult patients compared FLU-BU
to FLU-BU-MEL conditioning in myeloid and lymphoid malignancies and
found a significant improvement in the 5-year OS of 34.2\% vs 30.1\% and
a decrease in relapse 30.8\% vs 38\%\textsuperscript{9}. Similarly, a
retrospective single-center review of 39 adult patients reported on
their use of FLU-BU-MEL in myeloid and lymphoid malignancies with BM, PB
and UCB as stem cell sources, with a 2-year OS of 62\% for their entire
cohort and 44\% in their high risk to very high-risk subset. The authors
concluded that this was a safe and successful regimen considering the
highest risk group included many chemo-resistant patients with active
disease at the time of transplant\textsuperscript{29}.

We observed a significantly higher incidence of grades II-IV aGvHD in
MSD and MUD PBSCT compared to patients undergoing
haplo-BMT\textbf{(Figure 1B).} As expected with PBSC grafts, we saw a
significantly increased cumulative incidence of cGvHD (1 mild, 2
moderate and 1 severe) and MUD allo-HCT (1 moderate, 1 severe) when
compared to the anticipated lower occurrence in haplo-BMT with PT-CY,
with none of the haplo-BMT patients developing cGvHD \textbf{(Figure
1D)} . Similarly, none of the three unrelated UCB transplant patients
showed signs of cGvHD. Consequently, GRFS was superior in haplo-BMT
patients when compared to our MSD/MUD PBSCT recipients \textbf{(Figure
1F)} . While aGvHD, and especially cGvHD, is well-known to occur more
frequently with the use of PBSC grafts, the addition of MEL has also
been associated with increased aGvHD when compared to non-MEL containing
conditioning regimens. In an adult report in patients with hematologic
malignancies receiving MUD or MSD PBSC, BM, or UCB grafts and either
tacrolimus or cyclosporine GvHD prophylaxis, an increase in grade II-IV
aGvHD was seen with BU-FLU-MEL compared to FLU-BU (52.6\%
\emph{vs}40.8\%) with a analogous protection from relapse (30.8\% vs
39\%)\textsuperscript{9}. Moreover, pediatric AML patients receiving
BU-CY-MEL had a higher incidence of grade III-IV aGvHD (17.6\%) compared
to those conditioned BU-CY alone (9.6\%) with a parallel reduction in
relapse of 14.7\% and 31.5\% respectively. Chronic GvHD was higher in
PBSCs (34.9\%) vs BM recipients (25.6\%) as was observed in our series.

Our OS and PFS were similar, regardless of donor and stem cell source.
In a recent single institution retrospective pediatric study, the
outcomes of patients with hematologic malignancies (40\% AML/MDS)
undergoing MSD BMT (n=31) and MUD BMT (n-47) where compared to their
haplo-PBSCT with PT-CY (n=26) recipients\textsuperscript{30}. The OS
(80.9\%, 83.3\% and 80.7\%), PFS (66.5\%, 70.2\% and 73.8\%) and the
GRFS (62.1\%, 44.6\% and 61\%) were not different between MSD, MUD and
haplo respectively. However, cGvHD was significantly higher in MUD BMT
recipients 31.7\% compared to MSD BM 10\% and haplo 9.2\% despite use of
PBSC grafts in the latter group. We have recently updated our experience
of 31 CAYA patients with hematologic malignancies receiving haplo-BMT,
which compares favorably to the above study with OS 85.6\%, PFS 76.1\%
and GRFS 58.2\% with our median follow-up of 32 months compared to only
12 months in the above study\textsuperscript{13}.

Our current study along with our previous reports and those of others
confirm that haplo-BMT in CAYA is a viable alternative to MSD and MUD
allo-HCT\textsuperscript{12-15, 30, 31}. This study expands our
experience with myeloablative BU-FLU-MEL in allo-HCT for myeloid
malignancies. As we are a smaller pediatric HCT center, this report is
limited by the relatively small number of patients, their heterogeneous
transplant characteristics in terms of stem cell source, type of donor
and GvHD prophylaxis regimen used and the retrospective nature of our
study. Nonetheless, the favorable outcomes remain striking considering
the disease risk in this ethnically and racially diverse population with
myeloid malignancies undergoing allo-HCT. Of caution is the unacceptably
high aGvHD and cGvHD seen in patients receiving MSD and MUD PBSC
transplants with MTX + CSA GvHD prophylaxis. It remains to be determined
whether the evolving application of PT-CY in the non-haplo setting will
offer better control of GvHD in CAYA PBSCT recipients following
BU-FLU-MEL without increasing their risk of relapse.

\textbf{Acknowledgements}

The authors wish to thank the inpatient and outpatient nursing and other
staff on the pediatric HCTT unit at Banner University Medical Center in
Tucson for their outstanding care of our patients.

\textbf{Conflict of interest disclosure}

There are no conflicts of interest, financial or otherwise, involving
any of the authors regarding the submission or publication of this
manuscript.

\textbf{Authorship}

LT, HP, MD, MM, SH treated the patients before HCT and co-wrote the
manuscript. LS and KS co-managed the patients during HCT and edited the
manuscript. MP reviewed the blood and bone marrow results and edited the
manuscript. EK managed the patients throughout their HCT course,
collected and analyzed the data and co-wrote the manuscript.



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\textbf{Legends to figures}

\textbf{Figure 1. Kaplan-Meier estimates of acute and chronic
graft-versus host disease and survival. (A)} Cumulative incidence of
grades II-IV and III-IV aGvHD; \textbf{(B)} Comparisons of cumulative
incidence of grades II-IV and III-IV aGvHD between MSD and MUD PBSCT
versus haplo-BMT; \textbf{(C)} Cumulative incidence of all cGvHD and
severe cGvHD; \textbf{(D)} Comparisons of cumulative incidence of cGvHD
and severe cGvHD between MSD and MUD PBSCT versus haplo-BMT;\textbf{(E)}
Probability of OS, PFS, and GRFS; \textbf{(F)} Comparison of probability
of GRFS between MSD and MUD PBSCT versus haplo-BMT\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Figure-1/Figure-1}

\end{center}
\end{figure}

\textbf{Hosted file}

\verb`Table copy.docx`  available at \url{https://authorea.com/users/356726/articles/585820-busulfan-fludarabine-and-melphalan-is-effective-conditioning-for-pediatric-and-young-adult-patients-with-myeloid-malignancies-underdoing-matched-sibling-or-alternative-donor-transplantation}

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