Clinical experience with fetal hemoglobin induction therapy in patients with β-thalassemia

online January 11, 2013 originally publisheddoi:10.1182/blood-2012-10-408021

2013 121: 2199-2212

Khaled M. Musallam, Ali T. Taher, Maria Domenica Cappellini and Vijay G. Sankaran

-thalassemiaβwith Clinical experience with fetal hemoglobin induction therapy in patients

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Review Article

CME Article

Clinical experience with fetal hemoglobin induction therapy in patientswith b-thalassemiaKhaled M. Musallam,1,2 Ali T. Taher,2 Maria Domenica Cappellini,1 and Vijay G. Sankaran3,4

1Department of Medicine and Medical Specialties, Fondazione IRCCS Ca’ Granda, Ospedale Maggiore Policlinico, University of Milan, Milan, Italy; 2Department

of Internal Medicine, American University of Beirut Medical Center, Beirut, Lebanon; 3Division of Hematology/Oncology, Boston Children’s Hospital, Dana-Farber

Cancer Institute, Harvard Medical School, Boston, MA; and 4Broad Institute and Whitehead Institute for Biomedical Research, Cambridge, MA

Recent molecular studies of fetal hemo-

globin (HbF) regulation have reinvigorated

the field and shown promise for the

development of clinical HbF inducers to

be used in patients with b-thalassemia

and sickle cell disease. However, while

numerous promising inducers of HbF have

been studied in the past in b-thalassemia

patient populations, with limited success

in some cases, no universally effective

agents have been found. Here we exam-

ine the clinical studies of such inducers

in an attempt to systematically review the

field. We examine trials of agents, in-

cluding 5-azacytidine, hydroxyurea, and

short-chain fatty acids. This review high-

lights the heterogeneity of clinical studies

done on these agents, including both the

patient populations examined and the

study end points. By examining the pub-

lished studies of these agents, we hope

to provide a resource that will be valuable

for the design of future studies of HbF

inducers in b-thalassemia patient popu-

lations. (Blood. 2013;121(12):2199-2212)

Introduction

Increased production of fetal hemoglobin (HbF) can ameliorate theseverity of both b-thalassemia and sickle cell disease (SCD), themajor disorders of b-hemoglobin. The defective production ofthe b-globin molecule in patients with b-thalassemia can be

compensated for by an increase in the production of the b-likeglobin molecule, g-globin, which pairs together with a-globinchains to form HbF.1,2 The increased g-globin production decreasesthe a/b-chain imbalance that is a hallmark of b-thalassemia. As

Continuing Medical Education onlineThis activity has been planned and implemented in accordance with the Essential Areas and policies of the Accreditation Council forContinuing Medical Education through the joint sponsorship of Medscape, LLC and the American Society of Hematology.Medscape, LLC is accredited by the ACCME to provide continuing medical education for physicians.Medscape, LLC designates this Journal-based CME activity for a maximum of 1.0 AMA PRA Category 1 Credit(s)™. Physiciansshould claim only the credit commensurate with the extent of their participation in the activity.All other clinicians completing this activity will be issued a certificate of participation. To participate in this journal CME activity:(1) review the learning objectives and author disclosures; (2) study the education content; (3) take the post-test with a 70% minimumpassing score and complete the evaluation at http://www.medscape.org/journal/blood; and (4) view/print certificate. For CMEquestions, see page 2372.DisclosuresThe authors, Associate Editor Narla Mohandas, and CME questions author Charles P. Vega, Associate Professor and ResidencyDirector, Department of Family Medicine, University of California-Irvine, declare no competing financial interests.Learning objectivesUpon completion of this activity, participants will be able to:

1. Assess the pathophysiology of b-thalassemia.2. Analyze the efficacy of hydroxyurea in the management of b-thalassemia.3. Analyze the safety of hydroxyurea in the management of b-thalassemia.4. Evaluate the use of other inducers of HbF among patients with b-thalassemia.

Release date: March 21, 2013; Expiration date: March 21, 2014

Submitted October 7, 2012; accepted December 26, 2012. Prepublished

online as Blood First Edition paper, January 11, 2013; DOI 10.1182/blood-

2012-10-408021.

© 2013 by The American Society of Hematology

BLOOD, 21 MARCH 2013 x VOLUME 121, NUMBER 12 2199

For personal use only.on November 9, 2014. by guest www.bloodjournal.orgFrom

http://www.medscape.org/journal/blood



a result, there is improvement in the ineffective erythropoiesis seenin the disease, decreased hemolysis, and increased total hemoglo-bin levels from the improved survival of red cells containing higherlevels of HbF.3 The earliest clinical observations suggestingthat this was the case came from patients with rare forms ofb-thalassemia, particularly those with deletions that result in hereditarypersistence of fetal hemoglobin, who express high levels of HbF andhave a relatively benign clinical course.2 Additionally, infants withb-thalassemia only become symptomatic following the decrease inHbF production as the normal developmental fetal-to-adult hemoglo-bin switch occurs. More-recent clinical studies have substantiated thequantitative ameliorating effect of increased HbF production on theclinical course in a variety of patients with b-thalassemia.4-9

These observations have prompted a 3-decade search for inducersof HbF that can therapeutically recapitulate what occurs in b-thalassemia patients who have naturally higher levels of HbF. Theseefforts started almost immediately after the cloning of the globingenes and began after the initial recognition that DNA meth-ylation occurred at the silenced g-globin genes. While these initialefforts have subsequently generated a large body of evidence inSCD that led to the successful use of HbF inducers, as exemplifiedby the widespread use of hydroxyurea, experience in patients withb-thalassemia remains much more limited.

Here we review all clinical studies evaluating HbF inducers thathave been performed in b-thalassemia patients. We attempt to besimultaneously comprehensive and succinct in our analysis of theavailable evidence. Of note, there have been no large randomizedclinical trials performed to date in this area of clinical investigation.This review serves a dual purpose. First, these studies have never beencomprehensively reviewed, and therefore we have written this reviewto serve as a resource for those interested in examining the literatureon clinical efforts to induce HbF in b-thalassemia. Second, recentbasic science studies have identified extremely promising molecularregulators of HbF, including BCL11A, MYB, and KLF1.10-15 Whileno therapeutics currently exist to target these molecules or relatedpathways, it is likely that further work prompted by these molecularfindings may lead to the discovery of promising candidates for clinicalHbF inducers in the near future. Additionally, epigenetic modifierpartners of these transcription factors, such as histone deacetylases(HDACs), already have inhibitors available in clinical use and thusmay represent more-immediate targets for HbF induction trials.10,16

However, given the heterogeneous clinical studies of HbF inducersthat have been performed to date, the best approaches to carry out suchstudies remains enigmatic. By carefully revisiting clinical studies thathave been performed to date, we hope to provide a resource for allsuch future clinical studies in b-thalassemia patient populations.

The earliest attempts to induce fetalhemoglobin: DNA methylation inhibition

Soon after the initial cloning of the globin genes in the late 1970s,it became apparent that DNA methylation was present at thesilenced g-globin genes in adult erythroid cells, but was absentfrom the active g-globin genes in fetal erythroid cells.17 Aroundthis time, the mechanism of action of the DNA methylationinhibitor 5-azacytidine was being elucidated.18,19 This ledDeSimone et al20 to examine whether 5-azacytidine could induceHbF in phlebotomized primates. The results were stunning andled Ley et al21 to test 5-azacytidine in a patient with severe b-thalassemia. After 7 days of therapy, g-globin synthesis increased

7-fold, normalizing the globin chain imbalance and leading to anincrease in total hemoglobin level from 80 to 108 g/L. Due toconcerns and debates about the safety of this agent (cytotoxicity,mutagenicity, immuno- and myelosupression, activation of latentviruses),22,23 subsequent use was limited to severe cases for whomconventional therapy was unfeasible. Dunbar et al24 reported aremarkable improvement in total hemoglobin from 62 to 92 g/L inan alloimmunized b-thalassemia patient after 5 days of 5-azacytidinetherapy. Similar findings of increases in total hemoglobin level ofapproximately 30 g/L, transfusion independence, and improvementin cardiac function were described in a report on 3 patients withend-stage b-thalassemia.25 Myelotoxicities requiring dose mod-ifications were the main concern. Similar reports from patients withSCD confirmed the rapid and favorable effects on HbF productionand the hematological outcomes.26-29 Of note, a 2008 studysuggested that 5-azacytidine induction of HbF was not the resultof global DNA demethylation or changes in differentiationkinetics, but rather may be due to a localized demethylation of theg-promoter, although other results suggest that posttranscriptionalregulation could also play an important role.30

Decitabine (5-aza-2’-deoxycytidine) was also shown to deme-thylate and reactivate the expression of the g-globin gene. At lowconcentrations, it has a favorable safety profile without causingsignificant DNA damage or cytotoxicity. Small studies in SCDsuggest that decitabine can substantially increase HbF and totalhemoglobin in the majority of patients treated.31-34 A pilot study in2011 showed that subcutaneous decitabine given at 0.2 mg/kg 2times per week for 12 weeks increased total hemoglobin from 78.8to 90.4 g/L (2 patients had elevations >15 g/L), and it increasedabsolute fetal hemoglobin from 36.4 to 42.9 g/L in 5 patients withb-thalassemia intermedia. Favorable changes in red blood cellindices were also noted. Treatment was well tolerated, with the mainadverse event being an elevation in platelet counts.35

Hydroxyurea therapy in b-thalassemia: thelargest body of evidence

Hydroxyurea (or hydroxycarbamide) is a cytotoxic, antimetabolic,and antineoplastic agent known for its use in the management ofpatients with myeloproliferative disorders36 and human immunode-ficiency virus infection,37 where the drug acts as a potent inhibitor ofribonucleotide reductase, an enzyme required for DNA synthesis andrepair. After being identified as a potent HbF inducer,38 hydroxyureabecame 1 of the key therapeutic agents for the management ofpatients with SCD. However, the exact mechanisms by whichhydroxyurea induces HbF production are not fully understood.A cytotoxic effect resulting in stress erythropoiesis, with increasedHbF levels occurring as a result, is most commonly proposed.39

More-complex effects involving the production of nitric oxide andthe soluble guanylyl cyclase and cyclic guanosine monophosphate–dependent protein kinase pathway gene have been proposed asbeing responsible for this activity.40-43 Hydroxyurea therapyexerts a 2- to 9-fold increase in g-mRNA expression in b-thalassemiapatients,44-48 leading to improvement in the a/non–a-chain im-balance and more-effective erythropoiesis.49 There is good cor-relation between in vitro g-mRNA fold increase and the in vivoHbF fold increase;47,50 however, increases in HbF level did notalways correlate with increases in total hemoglobin level in clinicalstudies, as described in subsequent sections. This may be bestexplained by findings from earlier studies showing increases in the

2200 MUSALLAM et al BLOOD, 21 MARCH 2013 x VOLUME 121, NUMBER 12




a/b butnot thea/gbiosynthetic ratio inb-thalassemiapatients receivinghydroxyurea.51,52 Thus, in addition to its known effects in stimulatingg-globin production, hydroxyurea may have a more general role inaugmenting globin synthesis, including b-globin in some patientswho maintain the capacity to express normal b-globin chains.51

Evidence from patients with SCD suggests that the benefits ofhydroxyurea may not be limited to the increase in HbF levels,because improved erythrocyte morphology and deformability, alowering of circulating leukocyte and reticulocyte counts, a re-duction in hemolysis, and a potentially local release of nitric oxidealso appear to contribute to improved clinical outcomes.53 Similarstudies in patients with b-thalassemia are limited. Hydroxyureatherapy is not associated with considerable or steady effects onerythrocyte deformability in b-thalassemia,54 which may explainthe reduced response to the drug in some patients.55 However, insplenectomized patients with hemoglobin E/b-thalassemia, thereis evidence that hydroxyurea diminishes phosphatidylserine exter-nalization on the red cell,54 an observation previously establishedin patients with SCD.56 Whether this is attributed to HbF inductionand an associated decrease in a-globin aggregates remains to beelucidated.54 Regardless, phosphatidylserine membrane exposureis not only associated with reduced red cell survival but also withincreased thrombin generation, leading to hypercoagulability andsubsequent morbidity in patients with b-thalassemia.57

Hematological outcomes

After early case reports documented hematological improvementsin b-thalassemia patients treated with hydroxyurea,51,58-64 severalstudies evaluated the efficacy and safety of the drug in this patientpopulation (Table 1).44,49,50,65-85 These primarily included smallsingle-arm trials or retrospective cohort studies. Reported eleva-tions in HbF level from baseline showed substantial variability,ranging between 1% and 90% and averaging 20%. Such increasesare generally higher than those reported in SCD trials,43,53 althoughpatients with b-thalassemia recruited in the available studies havehigher baseline HbF levels. An association between the degree ofHbF level increase and improved hematological outcomes wasnoted in some studies,70,73 while others failed to document such anassociation.49,65,82 In fact, some reports noted a reduction in HbFlevel, especially at low doses,86 despite observed hematologicalresponses,51,72 further supporting the idea that the effects ofhydroxyurea in b-thalassemia patients could extend beyond HbFinduction.

In the available studies, two main hematological outcomes werecommonly evaluated, depending on patients’ transfusion statusbefore hydroxyurea therapy. In transfusion-dependent phenotypes(b-thalassemia major, severe b-thalassemia intermedia, or severehemoglobin E/b-thalassemia), patients were considered respondersto therapy if they became transfusion independent. The approxi-mate response rate ranged between 30% and 70% in b-thalassemiamajor patients and between 60% and 100% in b-thalassemiaintermedia patients, although the latter group was less frequentlytransfused before hydroxyurea therapy. The response rate inpatients with hemoglobin E/b-thalassemia was around 50%. Theevaluation of reported elevations in total hemoglobin level in thistransfusion-dependent group of patients is challenging consider-ing the variable time of baseline measurement with respect totransfusion therapy. However, responding patients usually hadhigher increases in total hemoglobin level than nonrespondingpatients did. Partial response, usually defined as a decrease intransfusion requirements, ranged between 15% and 50% (Table 1).

Although these findings remain promising, they should be interpretedwith caution, especially in patients with b-thalassemia intermedia.The confidence intervals in studies reporting the highest responserates are expected to be wide considering the small samplesizes. Moreover, interpretation of the observed benefits may bechallenging, as the indications for transfusion therapy with respectto target total hemoglobin levels vary considerably between studiesand centers. The documented final total hemoglobin level achievedupon transfusion independence ranged between 60 and 100 g/L.Finally, recent evidence supports a benefit of transfusion therapyfor the prevention or management of several morbidities in patientswith b-thalassemia intermedia,87,88 and the role of hydroxyurea asan alternate therapy should be evaluated against similar end points,particularly given that analogous transitions in patients withSCD have not consistently met with success.89 However, it isnoteworthy that improvements in transfusion requirements inmost b-thalassemia patients were also associated with a favor-able reduction in iron overload and hemolytic indices.

In studies including transfusion-independent patients withb-thalassemia, the primary hematological outcome was improve-ment in total hemoglobin level. Mean increases within studiesranged approximately between 5 and 25 g/L, with an average around15 g/L (Table 1), which is comparable to findings in patients withSCD.43,53 However, a high variance is noted in total hemoglobinresponse in most studies, indicating that although some patientsachieve considerable elevations, others have minimal or no change.The proportion of patients having total hemoglobin increases .10g/L ranged between 40% and 70%.50,65,76,77,85 Improvement inanemia was usually associated with better exercise tolerance,appetite, and sense of well-being. Elevations in mean corpuscularvolume and hemoglobin were consistently noted along withimprovements in total hemoglobin level.

Predictors of response

Responses in b-thalassemia patients were observed at hydroxyureadoses ranging between 10 and 20 mg/kg per day, with mostinvestigators opting to use a fixed low dose (10 mg/kg per day),while others escalated the dose according to toxicity (maximaltolerated dose) up to a maximum of 20 mg/kg per day (Table 1).These doses remain lower than those used in patients with SCD,which are often in excess of 20 mg/kg per day.43,53 Whether doseincrements above 20 mg/kg per day could lead to more favorableresponses warrants further study; however, a 2009 report suggeststhat a dose increase to 30 mg/kg per day in a small group ofnonresponsive patients did not provide any additional benefit.44

Experience from patients with SCD established the long-termefficacy of hydroxyurea.43,53 In b-thalassemia, most studies eval-uated outcomes after 6, 12, or 24 months of therapy, althoughresults from longer follow-up were also reported (Table 1).Response to hydroxyurea therapy was commonly noted in the first 3to 6 months of therapy, with further improvements noted up to 12months, and sustained responses observed over long-termfollow-up.65,69,73,78-80,82,85 However, some studies noted a declinein hematological response beyond 12 months.50,76 Because of theseobservations, it has been theorized that long-term treatment withhydroxyurea may result in an impairment in the ability of certainhematopoietic stem cells to give rise effectively to erythroid lineagecells. Basic science studies suggest that peripheral blood hematopoieticstem and progenitor cells from b-thalassemia patients under long-termtreatment with hydroxyurea lose the ability to undergo erythroiddifferentiation, which may lend some support to such theories.50

BLOOD, 21 MARCH 2013 x VOLUME 121, NUMBER 12 HbF INDUCTION IN THALASSEMIA 2201




Table

1.Clinicalstudiesevaluatinghydroxyureatherapyin

patients

withb-thalassemia

Reference

Sample

Age(y)*

HU

dose

(mg/kgperd)*

Duration(m

o)

TotalHb(g/L)*

FetalHb*

Comments

Karimietal65

126TM

22.3

10

96-156

Baseline:85-100

—TI:NTD

orTD

[interm

ittent]

Final:$90(n

551[40%])

TM:86[68.3%]TD

→NTD,25[19.8%]

TD

→1or2transfusions

Final:$80to

,90(n

544[35%])

Final:$70to

,80(n

516[13%])

Final:,70(n

515[12%])

106TI

18.1

10

96-156

Δ:1$20(n

512[11%])

—

Δ:110to

,20(n

544[42%])

Δ:1,10(n

547[44%])

Δ:nochange(n

53[3%])

Amoozgaretal66

33TI

20

10

12

Baseline:98.4

Baseline:79.3

g/L

TI:NTD

orTD

[interm

ittent]

Δ:113.4†

Δ:18.0

g/L‡

51TI

17

None

12

Baseline:92.8

—

Δ:14.7‡

Ansarietal67

119TM

2.6-25

16

24

-Baseline:75

Baseline:11.2%

TI:NTD

Δ:11‡(responders,n5

33[27.7%])

Δ:188.4%

(responders)

Responders:TD

→NTD

-Baseline:82

Partialresponders:.50%

↓in

transfusionneed

Δ:214†(partialresponders,n5

57[47.9%])

-Baseline:79

Nonresponders:,50%

↓in

transfusionneed

Δ:213†(nonresponders,n5

29[24.4%])

27TI

3-13

16

24

Baseline:67

Baseline:98.5%

Δ:111†

Δ:11.4%

Italia

etal68

13HbE/b-thalassemia

8-34

15-20

17-23

-Baseline:61

-Baseline:22.3%

Responders:TD

→NTD

(median20)

Δ:111†(responders,n5

6[46.2%])

Δ:116%†(responders)

Partialresponders:↓in

transfusion

need(30%

to50%)

-Baseline:63

Fcells:125.8%†

Δ:0‡(partialresponders,n5

4[30.8%])

-Baseline:15.2%

Nonresponders:nochangein

transfusionneed

-Baseline:76

Δ:116.6%‡(partialresponders)

Δ:212‡(nonresponders,n5

3[23.1%])

Fcells:118.5%‡

-Baseline:2.9%

Δ:15.8%†(nonresponders)

Fcells:113.3%†

Karimietal69

143TI

21

10.7

18-120

Baseline:74

—TI:NTD

orTD

(median68.4)

Δ:123†

Riganoetal50

24TI

37

14.6

12

Baseline:78

Baseline:46.5%

TI:NTD

Δ:115†

Δ:110.7%†

17patients

withΔH

b.

10g/L

followedfor.12mo(m

ean:68mo);

9maintainedresponse,while

8had

reduction

Ehsanietal70

16TI

10.7

20

6Δ:

116

Δ:117g/L

TI:NTD

Hb,hemoglobin;HU,hydroxyurea;NTD,nontransfusiondependent;TD,transfusiondependent;TI,b-thalassemia

interm

edia;TM,b-thalassemia

major.

*Allvaluesrepresentthecentraltendency(m

eanormedian)unlessotherw

iseindicated.ForHU

dose

,thefixedoractualmean/m

edianmaxim

altolerateddoseis

indicatedwhenreported;otherw

ise,thedosingschemeis

described.

†Statistically

significant(P

,.05).

‡Statistically

insignificant(P

..05).





Table

1.(continued)

Reference

Sample

Age(y)*

HU

dose

(mg/kgperd)*

Duration(m

o)

TotalHb(g/L)*

FetalHb*

Comments

Italia

etal44

41TM

12-28

15-20

20-24

-Baseline:81

-Baseline:5.4%

TI:TD

[interm

ittent]orTD


13[31.7%])

Δ:132%†(partialresponders)

Responders:TD

→NTD

-Baseline:76

Fcells:140.9%†

Partialresponders:.50%

↓in

transfusionneed


28[68.3%])

-Baseline:3.7%

Δ:18.2%†(nonresponders)

Doseincreasesto

30mg/kgperdayin

nonresponders

hadnobenefit

Fcells:113.8%†

38TI

5-40

15-20

20-24

-Baseline:76

-Baseline:51.8%

Δ:115†(responders,n5

22[57.9%])

Δ:135.4%†(responders)

-Baseline:79

Fcells:127.4%†


6[15.8%])

-Baseline:6.3%

-Baseline:63

Δ:131.1%†(partialresponders)


10[26.3%])

Fcells:133.3%†

-Baseline:25.9%

Δ:120%†(nonresponders)

Fcells:132.8%†

Zamanietal71

49TM

18.4

10

12

Baseline:85.2

—Meannumberoftransfusedunits↓from

22.8

→6.0†

Δ:20.7†

12[24.5%]becameNTD

Korenetal72

11TM

18

10.9

5-60

Responders,n5

9[81.8%]

Baseline:83%

TI:NTD

orTD

[interm

ittent]

(median24)

Δ:21%

(1y)‡,25%

(2y)‡

Responders:TD

→NTD

(maintaininganHbof

70g/L

inTM

and60g/L

inTI)

7TI

18

10.9

36-96

-Baseline:67

Baseline:15%

(median60)

Δ:12(5

TD,responders,n5

5[100%])

Δ:113%‡(1

y),18%

(2y)‡

-Baseline:66

Δ:118(2

NTD)

Mtvarelidzeetal73

6TM

9.8

15

60

Baseline:94

Baseline:23%

3[50%]patients:TD

→NTD

Δ:11(12mo),13(60mo)

Δ:18.3%

(12mo),120.4

(60mo)

2[33.3%]patients:↓in

transfusionneedafter1y

Ansarietal74

23TM

8.7

16

24

Baseline:86

—42.8%

↓in

bloodvolumetransfused

Δ:214

68.7%

↑in

transfusioninterval

Bradaietal75

45TM

10

17.4

12

Δ:115(goodresponders,n5

20[44.4%])

—TI:TD

Δ:17(partialresponders,n5

9[20%])

Goodresponders:.70%

↓in

transfusionneed

Δ:12(nonresponders,n5

16[35.6%])

Partialresponders:40%

to70%

↓in

transfusionneed

9TI

12.5

17

12

Δ:137(8

goodresponders

[88.9%],

1nonresponder[11.1%])

—Nonresponders:,40%

↓in

transfusionneed


interm


major.



iseindicated.ForHU

dose,thefixedoractualmean/m

edianmaxim

altolerateddoseis



described.

†Statistically

significant(P

,.05).

‡Statistically

insignificant(P

..05).





Table

1.(continued)

Reference

Sample

Age(y)*

HU

dose

(mg/kgperd)*

Duration(m

o)

TotalHb(g/L)*

FetalHb*

Comments

Mancusoetal76

18TI

37

14.6

12

Δ:115†

—TI:NTD

11patients

withΔH

b.

10g/L

were

followedfor.12mo

(mean:66mo);7maintained

response,while

4hadreduction

Taherand

Sheikh-Taha84

7TI

24

10-20

6-46

Baseline:70

—TI:NTD

(median12)

Δ:15

Singeretal85

45HbE/b-

thalassemia

13

18-20

24

Baseline:68

Baseline:29.8%

Patients

weanedofftransfusions

before

study

Δ:16

Δ:16.7%

16[38%]patients

hadΔH

b.

10g/L

Dixitetal77

37TM

1TI

10

10-.

20

6-Baseline:65

-Baseline:67%

-TI:NTD,TD

[interm

ittent],orTD

Δ:126†(m

ajorresponders,n5

17[45.9%])

Δ:19%†(m

ajorresponders)

Majorresponders:TD

→NTD

(Hb.

80g/L),orHb↑.20g/L

-Baseline:63

-Baseline:70.2%

Δ:118†(m

inorresponders,n5

9[24.3%])

Δ:18.5‡(m

inorresponders)

Minorresponders:TD

→NTD

(Hb,

80g/L),.50%

↓in

transfusionneed,orHb

↑10to

20g/L

-Baseline:65

-Baseline:40.9%

Δ:15†(nonresponders,n5

11[29.7%])

Δ:11%‡(nonresponders)

Nonresponders:,50%

↓in

transfusionneed,orHb↑,10g/L

Karimietal78

163TI

13.5

10

72

Baseline:86.8

Nonsignificantchanges

TI:group1(n

5120)TD,group2

(n5

16)TD

[interm

ittent],and

group3(n

527)NTD

Δ:19.6†(group21

3,12mo)

Group1:83[69.2%]TD

→NTD,

23[19.2%]TD

→1or2transfusions

Group2:14[87.5%]TD

→NTD

Alebouyehetal79

36TM

16.3

20

.6

Baseline:100

—TI:NTD

Δ:17

25[69.4%]TM

patients

TD

→NTD

9TI

14.7

20

34-58

Baseline:93

—

(median40)

Δ:111

Yavarianetal80

133TM

17.1

10-15

24-60

Goodresponders:n5

81[60.9%]

—Goodresponders:TD

→NTD

(Hb.

95g/L)

(median42)

Moderate

responders:n5

31[23.3%]

Moderate

responders:transfusion

interval.6mo(H

b75to

96g/L)

Nonresponders:TD

after12mo

Bradaietal81

7TM

1TI

12

18.4

13-21

-Baseline:45

Baseline:90.9%

TI:TD

(median19)

Δ:134(TM)

Δ:16.8%

5[71.4%]patients:TD

→NTD

-Baseline:65

2[28.6%]TD

→1or2transfusions

Δ:140(TI)


interm


major.



iseindicated.ForHU


edianmaxim

altolerateddoseis



described.

†Statistically

significant(P

,.05).

‡Statistically

insignificant(P

..05).





Alongside dose and duration of therapy, several other factorswere assessed for their association with hematological response inpatients with b-thalassemia. Findings regarding the roles of ageand HbF level at the start of treatment are conflicting.49,65,70,73,77,85

Moreover, although some studies found certain b-globin genotypesto be predictors of a favorable response,50,70,79 others failed toestablish such an association.44,65,75,77,85 Similar discrepancies arenoted for b-globin haplotypes.44,49 Patients with Lepore ordb-thalassaemia genotypes usually showed a better response.50,61,90

Of note, several reports confirm that the effects of hydroxyurea inpatients with hemoglobin S/b-thalassemia may be better than thoseeven reported for homozygous hemoglobin S disease, because thesynthesized g-chains not only inhibit the sickling process, butthey also neutralize the noxious effects of the excess a-chains andcut down on ineffective erythropoiesis.91-95 Co-inheritance ofa-thalassemia was described as a predictor of good response insome studies,44,77,96 but it was found to have no effect inothers.49,68,85 Homozygosity for the XmnI polymorphism (2158C.TGg) was a strong predictor of favorable responses,44,72,75,79,80,97

although the case was different in some studies, especially thoseincluding patients with hemoglobin E/b-thalassemia.65,68,70,77 A2012 study97 also showed that the rs766432 polymorphism atintron 2 of the BCL11A gene correlates strongly with a response tohydroxyurea therapy; further studies in this direction are en-couraged. However, it is important to bear in mind that when geneticmarkers are studied, the markers themselves may not be causallylinked to such effects, but rather variants in linkage disequilibriumwith these markers may be the mediators of such effects.

There is some basic evidence that response to hydroxyurea maybe affected by the degree of iron overload.98 However, clinicalstudies evaluating such an association are lacking.

Other clinical outcomes

Hydroxyurea therapy was also found to decrease the frequency ofcertain morbidities in patients with b-thalassemia. A beneficial rolein patients with pulmonary hypertension was suggested, especiallyupon combination with the antioxidant L-carnitine, although thesefindings primarily relied on echocardiographic findings.66,99,100 Insmaller studies, hydroxyurea therapy was also associated withimprovements in endocrine function,80 leg ulcers,101 and extra-medullary hemtopoietic tumors.102 These findings are further con-firmed through a 2010 cross-sectional study of 584 b-thalassemiaintermedia patients from the Middle East and Italy, wherehydroxyurea therapy was associated with reduced adjusted oddsof extramedullary hemtopoietic tumors (0.52, 95% CI, 0.30-0.91),pulmonary hypertension (0.42, 95% CI, 0.20-0.90), leg ulcers(0.10, 95% CI, 0.02-0.43), hypothyroidism (0.05, 95% CI, 0.01-0.45), and osteoporosis (0.02, 95% CI, 0.01-0.09).87 These effectswere independent of total hemoglobin level or transfusion status,which further suggests that the benefit from hydroxyurea couldextend beyond HbF induction and subsequent improvement ofanemia. It should be noted that hydroxyurea’s effects, especiallyon phosphatidylserine exposure and hypercoagulability, are mostnotable in splenectomized b-thalassemia patients,54,81 which rep-resent the subgroup of patients with a considerably high risk formorbidity from thrombotic events.87

Safety of hydroxyurea in b-thalassemia patients

Hydroxyurea therapy was generally well tolerated at the doses usedin b-thalassemia studies, with some studies reporting no adverseevents at all, even with long-term therapy.66,73,76,81,83,84 The rate ofT

able

1.(continued)

Reference

Sample

Age(y)*

HU

dose

(mg/kgperd)*

Duration(m

o)

TotalHb(g/L)*

FetalHb*

Comments

dePaula

etal82

4TM

18

10-.

20

6-96

Δ:130in

1patientafter3mo,

nochangein

3patients

—TI:TD

[interm

ittent]

1TM

patientbecameNTD

after3mo

7TI

35

10-.

20

6Baseline:77

Baseline:44.9%

ΔHb.

10g/L

notedin

3TIpatients

at6mo,1patientshowedfurther

responseat12mo,noadditional

responsesnotedafter.12mo

Δ:110†

Δ:112.3%†

Choudhry

etal83

15TM

3-6

50

35-d

courses

Δ:18‡

2-fold

risein

8[53.3%]patients

Responsecontinuedfor6moin

9[60%]patients

Fucharoenetal49

13HbE/b-thalassemia

34

15

5Baseline:65

Baseline:42%

—

Δ:17.4†

Δ:132.5%†

Fcells:↑by1/3


interm


major.



iseindicated.ForHU


edianmaxim

altolerateddoseis



described.

†Statistically

significant(P

,.05).

‡Statistically

insignificant(P

..05).





myelotoxicity ranged between 2% and 30%,44,49,71,74,75,77,82,85

while some studies did not report any hematological toxicities.65,69,70,72

Myelotoxicity was usually dose dependent, especially when doses.20 mg/kg per day were used, and it could be reversed upon dosereduction. The bone marrow of b-thalassemia patients may be moresensitive to myelosuppression by hydroxyurea than occurs in otherdisorders, possibly due to medullary inflammation.86,103 There isonly 1 report of leukemic transformation in a b-thalassemiaintermedia patient following 3 years of hydroxyurea therapy at 19mg/kg per day.75 The rate of gastrointestinal adverse eventsranged between 1% and 30%.50,65,67,69,71,74,77,81 Some studiesalso reported dermatological (hyperpigmentation, alopecia,maculopapular rash, or facial erythema)65,69 and neurological(headache or dizziness)65,69 adverse events on long-term therapy,although others did not observe such symptoms or attributedthem to other disease-related risk factors.67,74,104 No renal orhepatic side effects were reported with hydroxyureatherapy.67,69,71,74,75,82,85 Although some reports suggested thathydroxyurea may adversely affect gonadal function,87 othersfailed to document such an association even on long-termtherapy.105 Interestingly, 2 patients became pregnant whileon hydroxyurea and delivered normally, without any congenitalmalformations in the infants.72

Short-chain fatty acid induction offetal hemoglobin

In the middle of the 1980s, as the initial clinical trials of 5-azacytidine and hydroxyurea were being reported, Perrine et al,106

as well as Bard et al,107 reported that the infants of diabetic mothershave a delayed fetal-to-adult hemoglobin switch. While the exactunderlying mechanisms were not clear, hypotheses were put forthabout potential mechanisms. Because it was known that hydrox-ybutyrate is elevated in mothers with diabetes, Perrine andcolleagues108 tested the idea that butyrate or other similar short-chain fatty acids may be effective as inducers of fetal hemoglobinin sheep. An initial trial involving a 2- to 3-week infusion ofarginine butyrate (at a dose of 500 mg/kg per day) in 3 SCD and 3b-thalassemia patients (2 who were transfusion dependent)showed promise. Nearly all the patients in the trial showed anincrease (2- to 6-fold) in g-mRNA levels and in the synthesis ofthis globin chain, leading to improvement in the globin chainratios.109,110 The 2 transfusion-dependent patients showed lowerlevels of plasma-free hemoglobin, which is an indicator ofineffective erythropoiesis. In addition, 1 of these patients showed asteady rise in total hemoglobin level from 45 to .100 g/L over thecourse of 7 weeks of therapy. No other markers of disease progressionwere measured, and no further assessment of ineffective erythro-poiesis was done. A follow-up trial extended such therapy to 9 to13 weeks, with doses of arginine butyrate escalating from 500 to2000 mg/kg per day for 6 days per week, and it studied theresponses in 5 SCD and 5 b-thalassemia patients.111 This trial hadprimary end points involving hematologic responses, as defined byan increase in total hemoglobin concentration of 20 g/L in patientswith b-thalassemia. The 5 b-thalassemia patients had variabletransfusion requirements, with some being transfusion independent(2 patients) and others requiring intermittent (2 patients) or regulartransfusions (1 patient). Unfortunately, the primary hematologicend points were not achieved in this longer-term trial. It has beensuggested that the loss of response to butyrates over long-term

therapy may be a result of the antiproliferative effects on the bonemarrow.112 This hypothesis is supported by data confirming thatintermittent or pulse-butyrate therapy is associated with well-tolerated, marked, and sustained response in patients with SCD.112

A separate cohort study of oral sodium phenylbutyrate therapy ata dose of 20 g per day over the course of 41 to 460 days showedthat 4 of 11 b-thalassemia patients had increased levels of totalhemoglobin .10 g/L (mean increase: 21 g/L), along with anincreased production of HbF.113 All these responders had higheraverage levels of erythropoietin at baseline (all .120 mU/mL) andwere transfusion independent. Changes in the percent of HbF,absolute HbF levels, or a/non–a-globin ratios did not correlatewith response to treatment, nor did the b-genotype. In addition, thereduction of markers of ineffective erythropoiesis and hemolysis,including lactate dehydrogenase and indirect bilirubin, was alsonoted in the patients who showed responses. Two cohort studieshave examined the efficacy of the oral butyrate derivativeisobutyramide to induce HbF. The first study was an open-label,phase 2 trial on a group of 12 patients with transfusion-independentb-thalassemia intermedia (mean age: 31 years) for a period of 28days.114 There was some increase in the percent of HbF followingtreatment with 150 mg/kg per day of isobutyramide (P 5 .06, withwide variability among patients). No change in globin chainimbalance or in markers of ineffective erythropoiesis was noted inthis study. Another study examined 8 patients with transfusion-dependent b-thalassemia and involved treatment of these patientswith 350 mg/kg per day of oral isobutyramide for 126 to 384days.115 HbF increased from 3% to 6%, while a drop in plasma-free hemoglobin was noted. Two of the 8 patients showed adecreased frequency of transfusion requirements in the setting ofreceiving this treatment, and a reduction in iron burden was alsonoted in some patients. Response to treatment was associated withhigh pretreatment HbF (.4.5%), high parental HbF, and increasederythropoietin levels. Treatment with short-chain fatty acids wasgenerally tolerable in published trials, with side effects beingminimal or limited to gastrointestinal disturbances. It has beensuggested that the relatively poor response to butyrate therapy inpatients with b-thalassemia compared with patients with SCD maybe attributed to the effects of these agents on other globin genes. Itwas shown that butyrate exposure increases a-globin expression inprogenitor-derived erythroid cells from patients with b-thalassemia,while it decreases a-globin mRNA levels in patients with SCD. Thus,the favorable effects of the butyrate-induced increase in g-globinexpression on a/b-chain imbalance in b-thalassemia may be reducedas a result of the associated increase in a-globin expression.116

Trials of other derivatives of these agents, such as 2,2-dimethylbutyrate, were reported in 2012 in patients with SCD,and data from patients with b-thalassemia are awaited.117 Theseshort-chain fatty acids are thought to work as inhibitors of HDACs,and specific inhibitors of these molecules have been produced, whichmay also be promising for future trials aimed at inducing HbF.10,16

Combination therapy

The use of recombinant human erythropoietin or the newererythropoietic-stimulating agent darbepoetin alfa in patients withb-thalassemia is associated with increases in total hemoglobinlevel.118-123 A correlation between serum erythropoietin and HbFlevels also exists.124 Earlier studies from primates and frompatients with SCD suggested that erythropoietin augments the HbF





response attributed to hydroxyurea therapy.125-127 More-prominenteffects were also noted when hydroxyurea was combined withsodium phenylbutyrate.126 Consequently, several trials evaluatedthe value of combinations of these 3 agents for the management ofb-thalassemia patients. Loukopoulos et al128 demonstrated that thecombination of hydroxyurea and erythropoietin (50 000 U 3 timesa week) is associated with higher increments in total hemoglo-bin level than hydroxyurea alone (17 vs 2 g/L after 6 months oftherapy) in patients with b-thalassemia intermedia. Lower erythro-poietin doses (10 000 U) did not achieve such effects. Long-termtransfusion independence in a b-thalassemia major patient was alsoreported with this combination.129 In the late 1990s, Olivieri et al90

reported 2 b-thalassemia major siblings (homozygous for hemo-globin Lepore) who showed remarkable total hemoglobin re-sponses and transfusion independence from HbF induction with thecombination of sodium phenylbutyrate and hydroxyurea. However,further observations in this direction were not always promising. In2 patients with b-thalassemia intermedia, the addition of sodiumphenylbutyrate to hydroxyurea treatment failed to produce anincrease in total hemoglobin, despite increasing HbF levels.64

Similarly, in another study on 45 patients with hemoglobin E/b-thalassemia treated with hydroxyurea, the addition of sodiumphenylbutyrate had no benefit, while the addition of erythropoietindid benefit selected patients.85 In addition, there may exist some biasin the literature toward the reporting of positive results.

Other approaches

Thalidomide, a drug known for its immunomodulating and anti-angiogenic properties, has been suggested to induce g-globin geneexpression and to increase the proliferation of erythroid cells usingin vitro culture models.130 Two case reports reported that thalidomidetherapy at 75 to 100 mg/kg per day caused a progressive and rapidincrease in total hemoglobin and HbF levels in b-thalassemiamajor patients.131,132 Promising roles of thalidomide derivatives

(pomalidomide and lenalidomide) for the induction of HbF alsowere reported in 2008 and 2011 from in vitro and animalstudies.133,134 These findings support the evaluation of such agentsas a potential new therapy that may not have the cytotoxic sideeffects associated with other HbF inducers.

Concluding remarks: lessons from the pastand hope for the future

For HbF induction therapy to become part of the standardmanagement for patients with b-thalassemia, there needs to bea great deal of work from both basic scientists and clinicalresearchers. Ideally, efforts to understand the exact mechanismsthrough which current and future agents exert their effects shouldgo in parallel with the establishment of large randomized clinicaltrials and formal clinical development programs evaluating theefficacy and safety of these agents.

Several lessons can be learned from the pitfalls and successes ofthe available studies (Table 2), which should help the design offuture trials in b-thalassemia cohorts. We herein provide somerecommendations.

Study population

One of the main limitations in previous studies is the inclusion ofa heterogeneous group of patients, especially with regard totransfusion dependence, which precludes the interpretation ofstudy outcomes. Although there exists a variety of genotypes thatcan lead to a b-thalassemia syndrome, the distinction of variousphenotypes is primarily attained through clinical parameters.1,135

As the hallmark of disease in these syndromes is ineffectiveerythropoiesis and subsequent anemia, transfusion dependence hasclassically been an essential factor in characterizing the variousphenotypes and their severity. It should be noted, however, thatcommitment to transfusion therapy may be the consequence of

Table 2. Summary of findings and limitations of fetal hemoglobin inducer studies in patients with b-thalassemia

Agent Main positive findings Limitations

DNA methylation inhibitors

5-azacytidine Marked hematological responses achieved. Few studies.

Small sample sizes.

Safety concerns.

Decitabine Hematological responses achieved. Few studies.

Favorable effects on red cell indices noted. Small sample sizes.

Treatment was well tolerated.

Hydroxyurea Hematological responses achieved. Heterogonous phenotypes studied together.

Favorable effects on red cell, hemolysis, and hypercoagulability

indices noted.

Heterogeneous study end points evaluated together.

Favorable effects on clinical morbidities noted. Ideal dose and duration of therapy still controversial.

Treatment was well tolerated. Lack of efficacy on long-term therapy.

Data on predictors of response still inconsistent.

Short-chain fatty acids Hematological responses achieved. Small sample sizes.

Favorable effects on red cell and hemolysis indices noted. Lack of efficacy on long-term therapy.


Erythropoietic-stimulating agents Hematological responses achieved. Few studies.

Favorable effects on combination with hydroxyurea noted. Small sample sizes.

Treatment was well tolerated. High doses required.

No additive effects with short-chain fatty acids.

Thalidomide and derivatives Hematological responses achieved.


Few studies.

Small sample sizes.





physician, patient, or family preferences rather than a reflection ofdisease severity. For the purpose of selecting a patient cohort forinclusion in an HbF inducer trial, patients should essentially bedivided into 2 distinct groups: (1) transfusion-dependent patients,and these include phenotypes where patients require regular-transfusion therapy for survival, such as b-thalassemia major or severehemoglobin E/b-thalassemia, and (2) nontransfusion-dependentthalassemia (NTDT) patients, and these include patients not requiringregular-transfusion therapy for survival, such as b-thalassemiaintermedia or mild/moderate hemoglobin E/b-thalassemia.136

Study end points for the 2 distinct groups may be dissimilar, asoutlined below. One challenge, however, is that some patients withNTDT may still require occasional blood transfusions duringinfection or pregnancy or before surgery. They may also requiremore regular, yet temporary, transfusions in the case of poorgrowth or development during childhood or in the managementof specific complications in adulthood where the benefit oftransfusion therapy has been established.137-139 For this lattergroup, we recommend that patients should be off transfusions for atleast 6 months before inclusion in an HbF inducer trial.

Study design

The ideal design would be that of a randomized, placebo-controlledtrial, although data from well-conducted, single-arm, phase 2studies would also be valuable. Once the benefit is established inan evidence-based manner, agents may be evaluated in compar-ative trials against each other or against other conventional ther-apies. HbF-inducing agents may also be trialed in combination,either together or with conventional therapies. Although, as outlinedin “Primary study end points,” a reduction in the regular-transfusionrequirement is a study end point in transfusion-dependent patients,the effect of a short-term treatment strategy that includes an HbF-inducing agent alongside transfusion therapy may still be worthevaluating for the management of certain morbidities in NTDTpatients.87

Primary study end points

Another challenge in the available studies is the definition of endpoints and response criteria, which does not allow for a systematiccomparison of results. An essential question to answer is whatresponse is expected from an HbF inducer in a b-thalassemiapatient. In untreated patients with b-thalassemia, ineffectiveerythropoiesis and premature red cell death are the hallmarks ofdisease leading to chronic anemia and hypoxia, increased intestinaliron absorption, intra- and extravascular hemolysis, and a hyperco-agulable state. These mechanisms collectively lead to a varietyof clinical morbidities involving almost every organ system.3,140

The ineffective erythropoiesis in patients with b-thalassemia isestimated to be 10 to 20 times the normal basal erythropoieticlevel. Thus, the main objective of intervention would be to enablean effective supply of normal erythrocytes and partially suppressineffective erythropoiesis and subsequent pathophysiologic mech-anisms that lead to clinical morbidity. Erythroid activity decreasesto 1 to 2 times normal levels with total hemoglobin values between100 and 110 g/L, 1 to 4 times normal levels with values between90 and 100 g/L, and 2 to 6 times normal levels with valuesbetween 86 and 90 g/L.141 Achieving a total hemoglobin level.90 g/L has been associated with significant reduction inmorbidity in b-thalassemia patients,87,142 although lower levels(.70 g/L) may be equally beneficial in children with hemoglobinE/b-thalassemia who have a remarkable facility for adaptation to

low hemoglobin levels.143,144 Moreover, total hemoglobin eleva-tions of 10 to 20 g/L have been associated with improvement indisease severity in some studies.145

In light of these observations, in patients with NTDT, response toHbF induction therapy may be defined as follows: Excellent Response,achieving an elevation in total hemoglobin level of 10 to 20 g/L andreaching a final total hemoglobin level .90 g/L; Good Response,achieving an elevation in total hemoglobin level of 10 to 20 g/L orreaching a final total hemoglobin level .90 g/L; Poor Response,achieving an elevation in total hemoglobin level ,10 g/L; and NoResponse, no elevation in total hemoglobin level. For transfusion-dependent patients the following definitions may be used: ExcellentResponse, transfusion independence and reaching a pretransfusion totalhemoglobin level .90 g/L; Good Response, >50% reduction inpretreatment transfusion requirement and reaching a pretransfusion totalhemoglobin level .90 g/L; Poor Response, ,50% reduction inpretreatment transfusion requirement and reaching a pretransfusion totalhemoglobin level.90 g/L; and No Response, no change in transfusionrequirement to reach a pretransfusion total hemoglobin level .90 g/L.

Elevations in HbF level should not be used as a primary studyend point. As described earlier in this review, several studies reporteddiscordance between changes in HbF and total hemoglobin levels.This may be a particular problem given that HbF inducers may exerttheir effects through pathways other than g-chain expression.

Secondary study end points

Alongside changes in HbF level and safety measures, it would beworthwhile evaluating alterations in indices of pathophysiologic mech-anisms (eg, globin chain ratios, hemolysis, iron overload, hypercoag-ulability), which could shed more light on the specific mechanisms ofthe action of HbF-inducing agents. Moreover, the evaluation of theeffects of HbF inducers on the incidence of clinical morbidities (eg,pulmonary hypertension, leg ulcers, extramedullary hematopoieticpseudotumors) remains an area of extreme importance.

Dose and duration of therapy

Previous studies have mainly used 2 dosing strategies. For ex-ample, in the hydroxyurea studies reviewed in this review, mostused a fixed starting dose ,20 mg/kg per day, while some studiesescalated the dose to the maximal tolerated dose. We recommendthe latter approach for the following reasons: (1) There is a lack ofpharmacokinetic or dynamic studies to identify the optimal dose inb-thalassemia patients. (2) Such studies were not always helpful indosing hydroxyurea for patients with SCD. (3) Data on the efficacyor safety of high dosing in b-thalassemia patients is limited tosingle case reports or small case series. The duration of therapyshould rely on the evaluated drug and the anticipated outcomes(end points). Total hemoglobin levels should generally beevaluated after 6 and 12 months of therapy. In light of the fewreports, reviewed herein, that showed diminished response after 12months of therapy, it may be necessary to reevaluate long-termresponse through cohort studies. This is also especially relevant inpatients with b-thalassemia who show worsening of anemia anddisease severity as they advance in age, suggesting that alterationsin the dosing or course of management may be necessary.146 Sec-ondary study end points can be evaluated pre- and posttherapy and,potentially, early on for markers of hemolysis and ineffectiveerythropoiesis. When the incidence of clinical morbidities isa study end point, it may be difficult to evaluate outcomes in short-term studies, except in cases when the trial is designed for themanagement of a specific morbidity.





Predictors of response

There is great controversy from the available studies on whatfactors predict good response to therapy. Conducting studies withlarge samples will allow for association analysis, and the role of thefollowing factors should be evaluated: age, splenectomy status,b-globin genotype, a-globin genotype, molecular determinants ofincreased HbF production, and baseline hematologic profile (eg,total hemoglobin level, HbF level). Although through randomiza-tion the confounding effects of such risk factors are diminished,assessment through stratification remains essential.

Conclusions

Optimal design of future studies should assist with the improvedallocation of therapy to those patients who demonstrate clearevidence of benefiting from treatment. While the efforts to induceHbF in b-thalassemia patients by using the available agents havebeen ongoing for over 3 decades, recent molecular studies suggestthat there is great promise that more-effective targeted therapies toinduce HbF can be developed. For clinical investigators interested

in testing these potential therapies, many challenges lay ahead. Wehope that the lessons from prior trials in this field can serve asa guide for these future efforts.

Acknowledgment

This work was not supported by a funding source.

Authorship

Contribution: Conception and design: K.M.M. and V.G.S. Literaturereview and interpretation: K.M.M., A.T.T., M.D.C., and V.G.S.Manuscript drafting: K.M.M. and V.G.S. Manuscript review forimportant intellectual content: K.M.M., A.T.T., M.D.C., and V.G.S.All authors gave final approval of the manuscript for submission.

Conflict-of-interest disclosure: The authors declare no compet-ing financial interests.

Correspondence: Vijay G. Sankaran, Division of Hematology/Oncology, Boston Children’s Hospital, 300 Longwood Ave,Boston, MA 02115; e-mail: [email protected].

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Clinical experience with fetal hemoglobin induction therapy in patients with β-thalassemia

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