Ci a ion: Valen , P.; Akin, C.; A ock,
M.; Gleixne , K.V.; G einix, H.;
He mine, O.; Ho ny, H.-P.; I ano , D.;
O ao, A.; Rabi sch, W.; e al.
An ibody-Based and Cell The apies
o Ad anced Mas ocy osis:
Es ablished and No el Concep s. In .
J. Mol. Sci. 2023,24, 15125. h ps://
doi.o g/10.3390/ijms242015125
Academic Edi o : Paolo Colombo
Recei ed: 15 Sep embe 2023
Re ised: 3 Oc obe 2023
Accep ed: 3 Oc obe 2023
Published: 12 Oc obe 2023
Copy igh : © 2023 by he au ho s.
Licensee MDPI, Basel, Swi ze land.
This a icle is an open access a icle
dis ibu ed unde he e ms and
condi ions o he C ea i e Commons
A ibu ion (CC BY) license (h ps://
c ea i ecommons.o g/licenses/by/
4.0/).
In e na ional Jou nal o
Molecula Sciences
Re iew
An ibody-Based and Cell The apies o Ad anced Mas ocy osis:
Es ablished and No el Concep s
Pe e Valen 1,2,* , Cem Akin 3, Michel A ock 4, Ka oline V. Gleixne 1,2, Hildega d G einix 5, Oli ie He mine 6,
Hans-Pe e Ho ny 7, Daniel I ano 1,2 , Albe o O ao 8, We ne Rabi sch 9, And eas Rei e 10,
Axel Schulenbu g 9, Ka l So la 11, Wol gang R. Spe 1,2 and Celale in Us un 12
1Depa men o In e nal Medicine I, Di ision o Hema ology and Hemos aseology,
Medical Uni e si y o Vienna, 1090 Vienna, Aus ia
2Ludwig Bol zmann Ins i u e o Hema ology and Oncology, Medical Uni e si y o Vienna,
1090 Vienna, Aus ia
3Di ision o Alle gy and Clinical Immunology, Uni e si y o Michigan, Ann A bo , MI 48106, USA
4Depa men o Hema ological Biology, Pi ié-Salpê iè e Hospi al, So bonne Uni e si y, 75013 Pa is, F ance
5Di ision o Hema ology, Medical Uni e si y o G az, 8010 G az, Aus ia
6
Se ice d’Héma ologie, Imagine Ins i u e Uni e si éde Pa is, INSERM U1163, Cen e Na ional de Ré é ence
des Mas ocy oses, Hôpi al Necke , Assis ance Publique Hôpi aux de Pa is, 75015 Pa is, F ance
7Ins i u e o Pa hology, Ludwig-Maximilians Uni e si y, 80539 Munich, Ge many
8Se icio Cen al de Ci ome ia, Cen o de In es igacion del Cance (IBMCC; CSIC/USAL) Ins i u o
Biosani a io de Salamanca (IBSAL), CIBERONC and Depa men o Medicine, Uni e si y o Salamanca,
37007 Salamanca, Spain
9Depa men o In e nal Medicine I, S em Cell T ansplan a ion Uni , Medical Uni e si y o Vienna,
1090 Vienna, Aus ia
10 Depa men o Hema ology and Oncology, Uni e si y Hospi al Mannheim, 68135 Mannheim, Ge many
11 Ins i u e o Pa hology, Uni e si y Hospi al Salzbu g, Pa acelsus Medical Uni e si y, 5020 Salzbu g, Aus ia
12 Depa men o Medicine, Di ision o Hema ology, Oncology, and Cell The apy, Coleman Founda ion Blood
and Ma ow T ansplan Cen e a Rush Uni e si y Medical Cen e , Chicago, IL 60612, USA
*Co espondence: pe e [email p o ec ed]; Tel.: +43-1-40400-54880 o +43-1-40400-60850
Abs ac :
Ad anced sys emic mas ocy osis (SM) is a he e ogeneous g oup o myeloid neoplasms
cha ac e ized by an uncon olled expansion o mas cells (MC) in one o mo e in e nal o gans,
SM-induced issue damage, and poo p ognosis. Ad anced SM can be ca ego ized in o agg essi e
SM (ASM), MC leukemia (MCL), and SM wi h an associa ed hema ologic neoplasm (SM–AHN).
In a as majo i y o all pa ien s, neoplas ic cells display a KIT mu a ion, mos ly D816V and a ely
o he KIT a ian s. Addi ional mu a ions in o he a ge genes, such as SRSF2,ASXL1, o RUNX1,
may also be iden i ied, especially when an AHN is p esen . Du ing he pas 10 yea s, imp o ed
ea men app oaches ha e led o a be e quali y o li e and su i al in pa ien s wi h ad anced
SM. Howe e , despi e he a ailabili y o no el po en inhibi o s o KIT D816V, no all pa ien s en e
emission and o he s elapse, o en wi h a mul i-mu a ed and some imes KIT D816V-nega i e disease
exhibi ing mul i-d ug esis ance. Fo hese pa ien s, (poly)chemo he apy, an ibody-based he apies,
and allogeneic hema opoie ic s em cell ansplan a ion may be iable ea men al e na i es. In his
a icle, we discuss ea men op ions o pa ien s wi h d ug- esis an ad anced SM, including no el
KIT- a ge ing d ugs, an ibody-based d ugs, and s em cell-e adica ing he apies.
Keywo ds:
mas ocy osis; s em cell ansplan a ion; CAR-T; CAR- he apy; immuno he apy; mas
cell leukemia
1. In oduc ion
Sys emic mas ocy osis (SM) is a g oup o s em cell-de i ed, hema opoie ic neoplasms
cha ac e ized by uncon olled expansion and accumula ion o issue mas cells (MC) in
a ious in e nal o gans, including he bone ma ow (BM), spleen, li e , and gas oin es inal
(GI) ac [
1
–
3
]. In a subse o pa ien s, he skin may be in ol ed [
4
]. Howe e , unlike
In . J. Mol. Sci. 2023,24, 15125. h ps://doi.o g/10.3390/ijms242015125 h ps://www.mdpi.com/jou nal/ijms
In . J. Mol. Sci. 2023,24, 15125 2 o 14
pa ien s wi h ypical indolen SM, some pa ien s wi h ad anced SM may only ha e a
ew o no skin lesions. In pa ien s wi h ad anced SM, MC in il a ion in a ious o gans
is associa ed wi h o gan damage [
1
–
3
]. Due o imp o ed diagnosis, he p e alence o
(ad anced) SM has inc eased o e he pas ew yea s. The es ima ed p e alence o adul
SM is app oxima ely 1 pe 10,000 in he Wes e n wo ld.
Acco ding o he classi ica ion o he Wo ld Heal h O ganiza ion (WHO), ad anced
SM can be ca ego ized in o agg essi e SM (ASM), SM wi h an associa ed hema ologic
neoplasm (SM–AHN), and MC leukemia (MCL) [
1
–
3
,
5
–
9
]. ASM can u he be spli in o a
non- ans o med a ian and ASM in ans o ma ion o MCL (ASM- ) whe e MC in BM
smea s accoun o 5–19% o all nuclea ed cells [
8
,
10
]. SM–AHN is u he di ided in o
pa ien s wi h indolen SM (ISM–AHN), ASM–AHN, and MCL–AHN [8,10]. Fu he mo e,
SM–AHN is also classi ied based on he ype o AHN [1,5–8].
MCL can be di ided in o a leukemic a ian (MC in pe iphe al blood
≥
10%) and an
aleukemic a ian (MC in blood < 10%) [
5
–
8
,
10
]. In addi ion, MCL is spli in o a p ima y
o m o MCL (de no o MCL) and seconda y MCL a ising om ano he o m o mas ocy osis,
such as smolde ing SM (SSM), ASM, SM–AHN, o MC sa coma (MCS) [
8
,
10
]. Especially in
ASM- and MCS, he a e o ansi ion in o MCL is high [
10
–
13
]. The e o e, al hough no an
SM as pe SM c i e ia, MCS is a high-g ade MC neoplasm whe e cy o educ i e he apy
and o he in ensi e ea men app oaches ha e o be conside ed.
In mos pa ien s wi h ad anced SM, neoplas ic cells display ans o ming mu a ions
in he KIT oncogene (Table 1) [
14
–
17
]. These mu a ions lead o cy okine-independen
au onomous g ow h o neoplas ic MC p ogeni o s. The mos p e alen mu an o m o
KIT is D816V. This mu a ion is ound in all ca ego ies o ad anced SM, and o en also in
he AHN po ion o he disease. Howe e , in some pa ien s wi h ASM, MCL, o MCS,
o he KIT-ac i a ing mu a ions may be de ec ed [
11
–
17
]. The same holds ue o pedia ic
SM, amilial SM, and he well-di e en ia ed sub ypes o SM. Mo eo e , in pa ien s wi h
ad anced SM, especially SM–AHN, neoplas ic cells o en exhibi addi ional mu a ions in
clinically ele an genes [
15
–
23
]. These include mu a ions in SRSF2,ASXL1,RUNX1, o
DNMT3A, which may be associa ed wi h disease e olu ion and p og ession as well as d ug
esis ance in neoplas ic cells [
19
–
22
]. Some imes, mo e malignan subclones, especially
AHN- ela ed clones ha a e d ug esis an and lack KIT D816V, de elop ( h ough selec ion)
in hese pa ien s, e en i he ini ial dominan sub-clone displayed KIT D816V.
Table 1.
Es ima ed p e alence o KIT D816V, o he KIT mu a ions, and mu a ions in o he genes in
ad anced mas cell neoplasms.
Disease Va ian P e alence o KIT D816V O he KIT Mu a ions O he Gene Va ian s *
SSM >90% <10% <10%
SM–AHN >90% <10% >70%
ASM 80–90% 10–20% ** 10–20%
Ad SMWD 20% 50–80% ** <10%
MCL 70–80% 20–30% ** 10%
MCS *** <10% *** <10% 10–30% ***
* O he gene ic abno mali ies (apa om KIT mu a ions) a e o en de ec ed in pa ien s wi h SM wi h associa ed
hema ologic neoplasm (SM–AHN) and include, among o he s, mu a ions in RUNX1,TET2,SRSF2,RAS, o ASXL1.
** Some o he esul ing KIT mu an o ms a e sensi i e agains ima inib, whe eas KIT D816V con e s esis ance
agains ima inib. *** In ue (p ima y) MCS wi hou ea u es o SM, no KIT D816V o o he KIT mu a ions a e
ound. Abb e ia ions: SM, sys emic mas ocy osis; SSM, smolde ing SM; ASM, agg essi e SM; Ad SM, ad anced
SM; SMWD, well-di e en ia ed SM; MCL, mas cell leukemia; MCS, mas cell sa coma.
Ano he majo p oblem in SM is MC ac i a ion. In ac , hese pa ien s o en su e
om ecu en , media o -induced symp oms o e en anaphylaxis, especially when a con-
comi an , IgE-dependen alle gy is also p esen [
3
–
8
]. Media o - ela ed symp oms include,
among o he s, lushing, i ching, c amping, dia hea, and hypo ension [
3
–
8
]. In many cases,
alle gens a e inducing hese symp oms. Howe e , pa ien s wi h indolen o ad anced SM
may also de elop hese symp oms upon d ug exposu e.
In . J. Mol. Sci. 2023,24, 15125 3 o 14
2. S anda d T ea men Op ions o Pa ien s wi h Ad anced SM
In he pas 10 yea s, he ea men o pa ien s wi h ad anced SM has imp o ed sub-
s an ially. Fi s , a numbe o KIT D816V- a ge ing y osine kinase inhibi o s (TKI) ha e been
de eloped and ha e been applied success ully in pa ien s wi h ad anced SM [
24
–
33
]. Two o
hese TKI ha e ecen ly been app o ed o he ea men o ad anced ( esis an ) SM by majo
heal h au ho i ies in he US and EU, namely midos au in and a ap i inib. Bo h agen s a e able
o supp ess disease- ela ed symp oms and he expansion o neoplas ic MC in a conside able
numbe o pa ien s [
26
–
33
]. A ap i inib is a supe io d ug ha induces hema ologic emission
in a subs an ial subse o pa ien s wi h ad anced SM [
30
–
33
]. Based on hei clinical e icacy,
midos au in and a ap i inib a e conside ed i s -line agen s in he ea men o ad anced SM.
Howe e , despi e imp essi e esul s, some pa ien s con inue o p og ess o elapse, o en in he
o m o a KIT D816V-nega i e disease [
30
–
33
]. Fo such cases, al e na i e ea men op ions,
including expe imen al d ugs, ha e o be conside ed. Apa om KIT D816V- a ge ing d ugs,
also o he an i-neoplas ic agen s a e a ailable o pa ien s wi h ad anced SM. Some o hese
pa ien s may espond o clad ibine (2CdA) [
34
–
38
]. In o he pa ien s, especially hose wi h
apid p og ession o MCL o ano he (acu e) leukemia, poly-chemo he apy o e en allogeneic
hema opoie ic s em cell ansplan a ion (alloHSCT, e e ed o as HSCT in his pape ) a e
ecommended [
1
,
2
,
6
]. Hyd oxyu ea is commonly used as a pallia i e d ug in ad anced SM.
Table 2p o ides a summa y o ea men op ions o pa ien s wi h ad anced SM.
Table 2. T ea men op ions o pa ien s wi h ad anced mas ocy osis.
T ea men Indica ions
In e e on-alpha (IFN-A) *
T ea men - e ac o y os eopo osis (low dose IFN-A).
ASM wi h li e in ol emen and asci es
(pa ien no eligible o TKI he apy o 2CdA).
IFN-A esponsi e AHN.
Clad ibine (2CdA) Pa ien no eligible o KIT TKI he apy o pa ien esis an
agains KIT TKI o no KIT TKI a ailable.
Ima inib KIT D816V-nega i e ad anced SM, including ASM wi h KIT
K509I.
Masi inib KIT D816V-nega i e ad anced SM.
Midos au in ASM, ASM- , ASM–AHN, MCL, MCL–AHN.
ISM–AHN whe e he AHN is a KIT-d i en (KIT D816V+)
agg essi e neoplasm.
A ap i inib ASM, ASM- , ASM–AHN, MCL, MCL–AHN.
ISM–AHN whe e he AHN is a KIT-d i en (KIT D816V+)
agg essi e neoplasm.
Hyd oxyu ea (HU) Mul i-d ug- esis an ad anced SM, including ASM, MCL and
SM–AHN. HU is a s anda d pallia i e d ug.
Local adia ion
Mas cell sa coma (MCS) o MCS-like p og ession o ASM.
Huge splenomegaly: as debulking p io o CT.
Skele al disease (huge os eolysis wi h local umo mass).
Poly-chemo he apy
D ug- esis an ad anced SM.
Debulking as p epa a ion o HSCT.
SM–CMML wi h p og essing CMML o SM–AML in pa ien s
no eligible o HSCT.
Mono-chemo he apy:
deme hyla ing agen s and o he
d ugs, such as ene oclax
Pa ien s wi h ad anced, KIT TKI- esis an SM no eligible o
poly-chemo he apy o HSCT (and/o esis an agains 2CdA).
SM–AHN pa ien s no eligible o HSCT in whom he AHN
may be esponsi e: (example: azaci idine in MDS o AML).
Allogeneic HSCT
D ug esis an ad anced SM, ASM- o MCL.
ASM wi h apid p og ession.
SM–CMML, SM–AML.
* Du ing he ini ial ew weeks o IFN-A, he d ug is o en applied oge he wi h o al p ednisolone (s a ing
a 1 mg/kg/day). Abb e ia ions: TKI, y osine kinase inhibi o ; AHN, associa ed hema ologic neoplasm; SM,
sys emic mas ocy osis; ASM, agg essi e SM; ASM- , ASM in ans o ma ion; MCL, mas cell leukemia; MDS,
myelodysplas ic synd ome; ISM, indolen SM; CT, poly-chemo he apy; HSCT, (allogeneic) hema opoie ic s em
cell ansplan a ion; CMML, ch onic myelomonocy ic leukemia; AML, acu e myeloid leukemia.
In . J. Mol. Sci. 2023,24, 15125 4 o 14
When conside ing ea men op ions in a pa ien wi h ad anced SM, he i s impo -
an ques ions a e whe he he disease is apidly p og essing, whe he an AHN is p esen ,
and whe he mos o all disease componen s exhibi he KIT D816V mu a ion (Figu e 1).
In addi ion, i is impo an o know whe he he pa ien is eligible o in ensi e he apy
(Figu e 1). In hose wi h KIT D816V-posi i e ad anced SM, KIT D816V- a ge ing d ugs (mi-
dos au in o a ap i inib) a e usually ecommended as i s -line he apy [
1
,
2
,
25
–
33
]. When
TKI a e no a ailable o he pa ien is in ole an , clad ibine (2CdA) may be conside ed.
Howe e , al hough 2CdA is e ec i e, esponse a es a e lowe compa ed o hose seen wi h
midos au in o a ap i inib [
34
–
38
]. In pa ien s who ha e KIT D816V-nega i e ad anced
SM, o he KIT- a ge ing TKI, such as ima inib, may be conside ed, especially when KIT
sequencing e eals a sensi i e mu a ion o wild- ype KIT (Figu e 1, Table 2) [
39
–
42
]. Mi-
dos au in o a ap i inib may also be applied in such pa ien s (Figu e 1). I is impo an o
no e in his ega d ha in abou 80% o pa ien s wi h well-di e en ia ed ad anced SM, in
abou 20–30% o all cases wi h MCL, and in abou 90% o all pa ien s wi h ue (p ima y)
MCS, neoplas ic cells lack KIT D816V (Table 1). In se e al o he pa ien s wi h MCL, MC
display o he mu a ions in codon 816, such as D816H o D816Y. I is also impo an o no e
ha es ing o he KIT D816V mu a ion should be pe o med wi h a highly sensi i e PCR
es in o de o a oid alse-nega i e esul s. In pa ien s wi h well-di e en ia ed SM, a KIT
mu an o m ha is esponsi e o ima inib, may be de ec ed [
39
–
43
]. In mos pa ien s wi h
ue MCS, no KIT mu a ions a e ound, and neoplas ic cells a e usually esis an agains
KIT- a ge ing and con en ional d ugs, including chemo he apy [11–13].
In .J.Mol.Sci.2023,24,xFORPEERREVIEW5o 15



Figu e1.T ea men algo i hm o pa ien swi had ancedsys emicmas ocy osis(SM).A e ha ing
es ablished hediagnosiso ad ancedSM, he a ian o disease, heclinicalbeha io (agg essi e
wi h apidp og essiono indolen ),and hep esenceand ypeo AHNneed obede e minedusing
WHOc i e iaand/o c i e iap o idedby hein e na ionalconsensusclassi ica ion(ICC).Inaddi-
ion, hep esenceand ypeo molecula  a ge sneed obede ined.Inpa ien swi hKITD816V+
ad ancedSMo o he simila mu a ionsa codon816(KIT816-m), y osinekinaseinhibi o s(TKI)
di ec edagains KITD816Va eusuallyappliedas i s -line he apy.TwosuchTKIha ebeenap-
p o edby heFDAandEMA:midos au inanda ap i inib.Inpa ien swhoha ea apidlyp o-
g essingASMo MCLo SM–AHNwhe e heAHNisanacu emyeloidleukemia(AML)o  apidly
p og essingch onicmyelomonocy icleukemia(CMML),in ensi epoly-chemo he apy(CT)should
beconside edasanal e na i e i s -line ea men op ion.Inpa ien swhoa eeligibleand espond
oTKIo CT,subsequen allogeneichema opoie ics emcell ansplan a ionshould henalsobe
conside ed.Inaddi ion,pa ien swho elapsea e d ug he apyshouldbeconside ed o CTand
subsequen HSCT.Howe e ,suchpa ien smayalsobecandida es o expe imen ald ugso palli-
a i ed ugs.Inpa ien swi had ancedSMwhoha enoKITmu a iono amu an  o msensi i e o
ima inib,ima inibmaybeconside edas i s line he apy.Finally,inpa ien swi hSM–AHNin
whom heAHNisanagg essi ediseaseexhibi ingce ainmolecula  a ge s, a ge edd ug he a-
piesa eusually ecommended.Thes anda dpallia i ed ug o pa ien swi hd ug- esis an disease
who elapseda e CTo HSCT, emainshyd oxyu ea(HU).Abb e ia ions:SM,sys emicmas ocy-
osis;ASM,agg essi eSM;SM–AHN,SMwi hanassocia edhema ologicneoplasm;MCL,mas 
cellleukemia;CCR,con inuouscomple e emission;WHO,wo ldheal ho ganiza ion;FDA,Food
andD ugAdminis a ion;EMA;Eu opeanMedicinesAgency.
Fo pa ien swi h apidlyp og essingASMo MCL,poly-chemo he apywi ho 
wi hou subsequen HSCTiso en ecommended(Figu e1)[1,2,44–47].Fo  hosewhoa e
eligible o HSCT, heop imalway op oceedmaybe os a debulking(byaTKIand/o 
chemo he apy)and oin oduceHSCTasea lyaspossible[44–46].Thisalsoholds ue
o pa ien swi hMCS,asmanyo  hesecases ans o m oMCL[11–13,48].
3.SpecialConside a ions o  heT ea men o Pa ien swi hSM–AHN
Inpa ien swi hSM–AHN, ea men o  heAHNiso en equi ed.Indeed,insuch
cases, heAHNmaybeanagg essi emalignancy.In hesepa ien s,i isessen ial oknow
Figu e 1.
T ea men algo i hm o pa ien s wi h ad anced sys emic mas ocy osis (SM). A e ha ing
es ablished he diagnosis o ad anced SM, he a ian o disease, he clinical beha io (agg essi e
wi h apid p og ession o indolen ), and he p esence and ype o AHN need o be de e mined using
WHO c i e ia and/o c i e ia p o ided by he in e na ional consensus classi ica ion (ICC). In addi ion,
he p esence and ype o molecula a ge s need o be de ined. In pa ien s wi h KIT D816V+ ad anced
SM o o he simila mu a ions a codon 816 (KIT 816-m), y osine kinase inhibi o s (TKI) di ec ed
agains KIT D816V a e usually applied as i s -line he apy. Two such TKI ha e been app o ed by
he FDA and EMA: midos au in and a ap i inib. In pa ien s who ha e a apidly p og essing ASM
o MCL o SM–AHN whe e he AHN is an acu e myeloid leukemia (AML) o apidly p og essing
ch onic myelomonocy ic leukemia (CMML), in ensi e poly-chemo he apy (CT) should be conside ed
In . J. Mol. Sci. 2023,24, 15125 5 o 14
as an al e na i e i s -line ea men op ion. In pa ien s who a e eligible and espond o TKI o CT,
subsequen allogeneic hema opoie ic s em cell ansplan a ion should hen also be conside ed. In
addi ion, pa ien s who elapse a e d ug he apy should be conside ed o CT and subsequen HSCT.
Howe e , such pa ien s may also be candida es o expe imen al d ugs o pallia i e d ugs. In pa ien s
wi h ad anced SM who ha e no KIT mu a ion o a mu an o m sensi i e o ima inib, ima inib may be
conside ed as i s line he apy. Finally, in pa ien s wi h SM–AHN in whom he AHN is an agg essi e
disease exhibi ing ce ain molecula a ge s, a ge ed d ug he apies a e usually ecommended. The
s anda d pallia i e d ug o pa ien s wi h d ug- esis an disease who elapsed a e CT o HSCT,
emains hyd oxyu ea (HU). Abb e ia ions: SM, sys emic mas ocy osis; ASM, agg essi e SM; SM–
AHN, SM wi h an associa ed hema ologic neoplasm; MCL, mas cell leukemia; CCR, con inuous
comple e emission; WHO, wo ld heal h o ganiza ion; FDA, Food and D ug Adminis a ion; EMA;
Eu opean Medicines Agency.
Fo pa ien s wi h apidly p og essing ASM o MCL, poly-chemo he apy wi h o
wi hou subsequen HSCT is o en ecommended (Figu e 1) [
1
,
2
,
44
–
47
]. Fo hose who a e
eligible o HSCT, he op imal way o p oceed may be o s a debulking (by a TKI and/o
chemo he apy) and o in oduce HSCT as ea ly as possible [
44
–
46
]. This also holds ue o
pa ien s wi h MCS, as many o hese cases ans o m o MCL [11–13,48].
3. Special Conside a ions o he T ea men o Pa ien s wi h SM–AHN
In pa ien s wi h SM–AHN, ea men o he AHN is o en equi ed. Indeed, in
such cases, he AHN may be an agg essi e malignancy. In hese pa ien s, i is essen-
ial o know whe he mos AHN cells (subclones) ca y KIT D816V. Likewise, in SM wi h
ch onic myelomonocy ic leukemia (SM–CMML), neoplas ic monocy es a e o en KIT D816V-
posi i e, which may a o ea men wi h a KIT D816V- a ge ing d ug [
49
,
50
]. By con as ,
in acu e myeloid leukemia (AML), some o e en mos AML subclones may lack KIT D816V,
so ha ea men wi h a KIT D816V- a ge ing d ug alone may e en lead o selec ion o
mo e KIT D816V-nega i e sub-clones [
51
–
53
]. In gene al, pa ien s wi h SM–AHN should be
ea ed o hei AHN as i no SM was diagnosed, and he SM po ion o he disease should
be ea ed as i no AHN was ound [
1
–
3
,
5
–
7
,
45
,
47
]. This s a egy implies ha se e al o
hese pa ien s ecei e combina ions o an i-neoplas ic d ugs o sequen ial d ug he apies.
Fo example, in a pa ien wi h ASM–AML exhibi ing KIT D816V and a FLT3-ITD mu a ion,
he disease may be ea ed wi h midos au in, oge he wi h chemo he apy (di ec combina-
ion). Thus, i is impo an ha bo h he SM and he AHN po ion o he disease a e ea ed
in a a ge -speci ic manne . The e o e, i is also o c ucial impo ance o es ablish he co ec
inal diagnosis in each case, and o de ine bo h disease componen s by adding he molecula
signa u e in he epo [
1
–
3
,
5
–
7
,
45
,
47
]. Fo example, in a pa ien wi h ISM exp essing KIT
D816V wi h an associa ed JAK2 V617F-posi i e p ima y myelo ib osis (PMF), he inal
diagnosis is ISM–PMF exhibi ing KIT D816V (in SM cells) and JAK2 V617F (in MPN cells).
In such pa ien , no KIT D816V- a ge ing d ug is equi ed, bu when PMF- ela ed symp oms
occu , he pa ien may be a candida e o ea men wi h a JAK2 V617F- a ge ing d ug.
In some pa ien s wi h SM–AHN, i may be di icul o in e p e po en ial C- indings
(like cy openia), especially when bo h he SM po ion and he AHN could cause cy openia,
o example, ASM wi h associa ed myelodysplas ic synd omes/neoplasm (MDS). In hese
pa ien s, he AHN may be an ad anced KIT D816V-nega i e MDS (o seconda y AML)
equi ing ea men wi h 5-azaci idine (AZA) o AZA plus ene oclax. In such cases, he
AHN po ion o he disease may o may no espond o hese s anda d ea men s in
he same way as in pa ien s wi hou SM [
54
–
56
]. Midos au in and a ap i inib may also
be e ec i e in hese cases, especially when mos o all AHN cells a e KIT mu a ed, o
a e d i en by KIT-induced oncogenic pa hways [
57
,
58
]. Howe e , in SM–AML, some o
e en mos AML-subclones may lack KIT D816V [
51
,
52
]. The same holds ue o pa ien s
wi h MCS and some wi h MCL. O e all, i seems as i mo e ad anced, p og essi e SM is
associa ed wi h he de elopmen o mul iple d ug- esis an subclones, and some o e en
mos o hese subclones lack KIT mu an o ms including KIT D816V.

In . J. Mol. Sci. 2023,24, 15125 6 o 14
In pa ien s in whom he KIT D816V mu a ion s a us is unknown o no KIT mu a ion
was ound, a KIT inhibi o may s ill be a he apeu ic op ion. Indeed, midos au in and
a ap i inib supp ess he ac i i y o wild- ype KIT, and bo h may exe an i-neoplas ic
e ec s in SM ega dless o he KIT mu a ion s a us. Some imes, KIT D816V is no de ec ed
in neoplas ic cells based on he low sensi i i y o he assay. The e o e, we ecommend
es ing o KIT D816V wi h a highly sensi i e PCR assay be o e s a ing he apy in all
pa ien s [15–17,59].
As men ioned, when bo h he SM and he AHN equi e speci ic an i-neoplas ic he a-
pies, d ug combina ions ha e o be conside ed. In pa ien s wi h an agg essi e malignancy,
such as MCL and/o AML, in ensi e he apy (plus TKI he apy) is in oduced o achie e
emission be o e he pa ien is p epa ed o allogeneic HSCT [
1
,
2
,
45
–
47
,
58
]. In hose who
a e no eligible o HSCT, con inuous ea men wi h a ap i inib may be an al e na i e
ea men op ion.
4. T ea men Op ions o Pa ien s wi h D ug-Resis an Ad anced SM
The p ognosis o pa ien s wi h ad anced SM who a e esis an agains KIT- a ge ing
d ugs and/o o he an i-neoplas ic d ugs, including 2CdA, is usually dismal. Some o
hese pa ien s may no ha e ye been ea ed wi h a ap i inib because he d ug was no
a ailable. These pa ien s may bene i om a ap i inib he apy [
32
,
33
]. O he pa ien s may
bene i om expe imen al d ugs o d ug combina ions using TKI (midos au in, a ap i inib)
and o he a ge ed d ugs such as ene oclax, o TKI and con en ional an ineoplas ic d ugs
such as 2CdA. Howe e , mos esponses a e only ansien and a e ollowed by a elapse.
The e o e, when eligible, hese pa ien s a e p epa ed o HSCT. I HSCT is no a iable
op ion, pa ien s a e ea ed wi h expe imen al chemo he apy o pallia i e d ugs such as
hyd oxyu ea (HU) [
1
,
2
,
45
]. In hose wi h d ug- esis an SM–AML who a e no eligible o
HSCT, chemo he apy egimens con aining AZA, 2CdA, ene oclax, o o he d ugs may be
o e ed [1,2,45,60]. Howe e , mos o hese pa ien s ha e a sho su i al ime.
5. Allogeneic HSCT
HSCT emains he only po en ially cu a i e ea men o pa ien s wi h ad anced
SM who a e esis an agains mul iple d ug he apies. Especially young and i pa ien s
wi h mul i- esis an ad anced SM may bene i om HSCT [
44
–
46
,
56
,
58
,
61
–
64
]. In hese
pa ien s, g a e sus mas ocy osis e ec s ha e been documen ed [
44
,
62
,
63
]. Howe e , so
a , no con olled clinical s udies explo ing he de ini i e alue o HSCT in pa ien s wi h
ad anced, d ug- esis an SM ha e been pe o med. In one s udy, pa ien s wi h ad anced
SM who we e ea ed wi h HSCT (ei he i s -line o a e d ug he apy) we e examined
in a e ospec i e mul i-cen e s udy [
44
]. In his, s udy, pa ien s wi h ASM, MCL, and
SM–AHN we e included. The ou comes a e HSCT conce ning su i al and elapse- ee
su i al we e ound o be be e o hose who had ASM o SM–AHN compa ed o hose
wi h MCL [
44
]. The ou comes a e HSCT we e also ound o be mo e a o able when
myeloabla i e condi ioning was pe o med compa ed o dose- educed (non-myeloabla i e)
condi ioning [
44
]. The e a e addi ional ac o s ha may impac on ou come and p ognosis
o HSCT in pa ien s wi h ad anced SM. Fo example, he ou come may be be e when
success ul debulking can be pe o med p io o HSCT. Indeed, in pa ien s in whom de-
bulking wi h a ge ed d ugs o chemo he apy ailed, HSCT is di icul o pe o m, and
pos -HSCT ou comes a e usually poo . The e o e, mos expe s ecommend ea ly HSCT
and ea ly p e-HSCT he apy in eligible pa ien s wi h ad anced SM. The ype o debulking
he apy depends on he ype o AHN, he p esence o KIT D816V, and o he molecula
a ge s displayed by neoplas ic cells. In hose wi h a high a ian allele equency o KIT
D816V, a ap i inib may be app op ia e and su icien o induce debulking [
58
]. In he case
o SM–AML, poly-chemo he apy should be conside ed as he mos e ec i e and mos
p omising app oach o achie e subs an ial debulking be o e HSCT.
The e is also a g owing discussion abou he use o KIT- a ge ing d ugs (midos au-
in, a ap i inib) a e success ul HSCT. Indeed, based on case epo obse a ions, such
In . J. Mol. Sci. 2023,24, 15125 7 o 14
ea men , when in oduced a e hema ologic egene a ion ollowing HSCT ( o example
om day +100), may keep he neoplas ic disease p ocess unde con ol [
28
,
64
]. Howe e ,
no esul s om con olled clinical ials a e a ailable, and i emains unknown whe he
such pos -HSCT he apy exe s majo e ec s on esidual neoplas ic s em cells (NSC) as a
use ul main enance he apy. In addi ion, i emains unce ain how long such pos -HSCT
he apy wi h KIT- a ge ing d ugs should be pe o med. In mos cen e s, such pa ien s
ha e been ea ed wi h midos au in o 1 o 2 yea s pos -HSCT. In pa ien s wi h measu -
able minimal esidual disease (by KIT D816V PCR), longe ea men wi h KIT- a ge ing
d ugs may be conside ed. A ap i inib has ecen ly been applied pos -HSCT in pa ien s
wi h RUNX1-RUNX1T1-posi i e KIT-mu a ed AML wi h minimal esidual disease who
ailed immuno he apy wi h in e e on alpha o dono lymphocy e in usions [
65
]. In hese
pa ien s, cy openia occu ed equen ly. Howe e , no clinical s udies using a ap i inib
pos -HSCT in ad anced SM ha e been conduc ed o da e.
6. An ibody-Based T ea men o D ug-Resis an Ad anced SM
Al hough an ibody-based d ug he apies a e a ailable o pa ien s wi h high- isk
AML and he apy- e ac o y AML, no an ibody-based he apy o pa ien s wi h MCL
o o he o ms o ad anced SM is a ailable. Since mos MC and mos NSC, including
AHN- ela ed NSC, display CD33 in mos SM pa ien s (Table 3) [
66
–
69
], he CD33- a ge ed
d ug conjuga e gem uzumab ozogamicin (GO) has been es ed in p eclinical s udies. Based
on i s an i-neoplas ic e ec s on neoplas ic MC and NSC in
in i o
s udies [
67
,
68
], GO
has been p oposed o he ea men o ad anced SM. Howe e , only a ew anecdo al
epo s ha e been published so a . In one epo , ea men o a pa ien wi h d ug- esis an
SM–AHN esul ed in disease debulking and sus ained emission [
70
]. In o he pa ien s
wi h ad anced SM, ea men wi h GO and poly-chemo he apy esul ed in a subs an ial
dec ease in he numbe s o NSC [
68
]. The e a e also o he su ace a ge s ha a e exp essed
on neoplas ic MC and/o NSC, and may he e o e se e as molecula a ge s in ad anced
SM (see below).
Table 3.
Cell su ace a ge exp ession p o iles o neoplas ic mas cells and neoplas ic s em cells in
pa ien s wi h ad anced SM.
CD An igen,
Ta ge
Cell Su ace Exp ession De ec ed by Flow Cy ome y * on
Neoplas ic Mas Cells CD34+/CD38−S em Cells
ASM SM–AHN MCL ASM SM–AHN MCL
CD2 LFA-2 + + +/− − − −
CD13 A-Pep -N + + + + + +
CD25 IL-2RA + + + +/−+/- +/-
CD30 Ki-1 +/−+/−+/− −/+ −/+ −/+
CD33 Siglec-3 + + + + + +
CD44 He mes + + + + + +
CD47 IAP + + + + + +
CD52 Campa h-1 + + + −/+ −/+ −/+
CD117 KIT + + + + + +
CD123 IL-3RA +/−+/−+/−+++
CD184 CXCR4 + + + + + +
CD274 PD-L1 + + + n.k. n.k. n.k.
CD327 Siglec-6 + + + −−−
* Su ace exp essions o a ge s we e analyzed wi h luo och ome-labeled an ibodies and mul i-colo low cy ome-
y. Exp essions o a ge s on KIT
++
mas cells and CD34
+
/CD38
−
s em cells we e quan i ied using he ollowing
sco e: +, clea ly exp essed on >75% o all cells; +/
−
, exp essed weakly o only on a subse (10–50%) o all cells;
−
/+, exp essed only in ace amoun s o in a e y small sub- ac ion o cells (<10%). Abb e ia ions: SM, sys emic
mas ocy osis; ASM, agg essi e SM; SM–AHN, SM wi h an associa ed hema ologic neoplasm; MCL, mas cell
leukemia; A-Pep -N, aminopep idase N; IL-2RA, in e leukin-2 ecep o alpha chain; IAP-1, in eg in-associa ed
p o ein; IL-3RA, in e leukin-3 ecep o alpha chain; PD-L1, p og ammed dea h ligand-1; n.k., no known.
In . J. Mol. Sci. 2023,24, 15125 8 o 14
7. Ta ge Exp ession P o iles o NSC in Ad anced SM
Recen da a sugges ha NSC in ad anced SM (including ASM, SM–AHN, and MCL)
eside in a small CD34
+
/CD38
−
ac ion o he malignan clone [
68
]. These cells ha e a
selec i e po en ial o ini ia e and p opaga e he disease
in i o
in NSG mice exhibi ing hu-
man memb ane-bound s em cell ac o (NSG
hSCF
mice) [
68
]. Al hough no all obse a ions
can be ansla ed om mouse models o he human sys em, hese NSC may also p opaga e
he disease in pa ien s wi h ad anced SM o unlimi ed ime pe iods. By con as , he mo e
ma u e cells in he same disease, including CD34
+
/CD38
+
p ogeni o cells and he bulk
o neoplas ic MC, a e unable o ini ia e and p opaga e he malignancy
in i o
[
68
]. As a
consequence, any he apy can only ac as a cu a i e app oach when elimina ing mos o all
CD34
+
/CD38
−
NSC in a gi en pa ien . In mos pa ien s wi h ad anced SM, CD34
+
/CD38
−
NSC exhibi he key s em cell ma ke s CD13 (aminopep idase-N), CD123 (IL-3 ecep o
alpha), and CD133 (AC133) [
68
]. These an igens a e also exp essed on neoplas ic MC in
SM, and o en also on AHN cells. In addi ion, NSC as well as MC exp ess a numbe o
cell su ace a ge s, such as CD33 (Siglec-3), CD44 (He mes), and CD117 (KIT) (Figu e 2,
Table 3) [
67
–
75
]. In con as , o he immunological a ge s, such as CD30 (Ki-1) o CD327
(Siglec-6), a e only de ec able on neoplas ic MC, bu a e no de ec able (o only ound
in ace amoun s) on NSC in ad anced SM (Table 3) [
76
–
78
]. These a ge s may be less
a ac i e, since he apies di ec ed agains hese an igens would only lead o an e adica ion
o MC, bu no o an elimina ion o NSC. Ano he impo an aspec is ha almos all su ace
a ge s iden i ied on NSC in ad anced SM a e also exp essed on no mal hema opoie ic
s em cells. This holds ue o CD33, CD44, CD117, and CD123. As a consequence, a ge ed
ea men app oaches can lead o he e adica ion o no mal hema opoie ic s em cells, and
hus p olonged cy openia. This is a majo issue when conside ing he de elopmen o
an ibody-based he apies o cell-based he apies such as CAR-T o CAR-NK cell he apies.
On he o he hand, se e al o hese a ge s a e exp essed on NSC a much highe le els
compa ed o no mal s em cells, so ha ea men wi h an ibody-based a ge ed d ugs may
be a easible app oach because o he he apeu ic window. This may hold ue o CD33,
CD44, and CD123. Indeed, he ea men o AML wi h he CD33-based oxin conjuga e GO
a ecommended doses is o en associa ed wi h p olonged cy openia, bu usually does no
lead o i e e sible aplasia [
79
]. Whe he his is also he case wi h an ibody-based d ugs
di ec ed agains CD117 o CD123 emains unknown. Fo example, no use ul he apeu ic
window has been iden i ied o CD117 on neoplas ic MC o NSC in SM, as he su ace
exp ession is some imes e en lowe on NSC compa ed o no mal s em cells [
68
]. Howe e ,
he e is s ill some hope ha CD117- a ge ed d ug he apies may no lead o comple e s em
cell exhaus ion. In ac , in pa ien s wi h Ph+ ch onic myeloid leukemia, long- e m ea men
wi h s ong inhibi o s o KIT such as ima inib does no lead o se e e aplasia, e en when
he pa ien s a e ea ed o se e al decades [
80
]. Finally, some o he cell-based he apies,
such as CAR-T cell he apies, may be combined wi h HSCT in o de o sol e he p oblem
o e adica ion o no mal s em cells.
In . J. Mol. Sci. 2023,24, 15125 9 o 14
In .J.Mol.Sci.2023,24,xFORPEERREVIEW9o 15


since he apiesdi ec edagains  hesean igenswouldonlylead oane adica iono MC,
bu no  oanelimina iono NSC.Ano he impo an aspec is ha almos allsu ace a -
ge siden i iedonNSCinad ancedSMa ealsoexp essedonno malhema opoie ics em
cells.Thisholds ue o CD33,CD44,CD117,andCD123.Asaconsequence, a ge ed
ea men app oachescanlead o hee adica iono no malhema opoie ics emcells,and
husp olongedcy openia.Thisisamajo issuewhenconside ing hede elopmen o an-
ibody-based he apieso cell-based he apiessuchasCAR-To CAR-NKcell he apies.
On heo he hand,se e alo  hese a ge sa eexp essedonNSCa muchhighe le els
compa ed ono mals emcells,so ha  ea men wi han ibody-based a ge edd ugsmay
bea easibleapp oachbecauseo  he he apeu icwindow.Thismayhold ue o CD33,
CD44,andCD123.Indeed, he ea men o AMLwi h heCD33-based oxinconjuga e
GOa  ecommendeddosesiso enassocia edwi hp olongedcy openia,bu usuallydoes
no lead oi e e sibleaplasia[79].Whe he  hisisalso hecasewi han ibody-based
d ugsdi ec edagains CD117o CD123 emainsunknown.Fo example,nouse ul he a-
peu icwindowhasbeeniden i ied o CD117onneoplas icMCo NSCinSM,as he
su aceexp essionissome imese enlowe onNSCcompa ed ono mals emcells[68].
Howe e , he eiss illsomehope ha CD117- a ge edd ug he apiesmayno lead o
comple es emcellexhaus ion.In ac ,inpa ien swi hPh+ch onicmyeloidleukemia,
long- e m ea men wi hs onginhibi o so KITsuchasima inibdoesno lead ose e e
aplasia,e enwhen hepa ien sa e ea ed o se e aldecades[80].Finally,someo  he
cell-based he apies,suchasCAR-Tcell he apies,maybecombinedwi hHSCTino de 
osol e hep oblemo e adica iono no mals emcells.

Figu e2.Exp essionso cellsu acema ke sonneoplas ics emcellsinSM.Bonema owmononu-
clea cellswe eob ained omapa ien wi hsys emicmas ocy osis(SM)wi hanassocia edhema-
Figu e 2.
Exp essions o cell su ace ma ke s on neoplas ic s em cells in SM. Bone ma ow mononu-
clea cells we e ob ained om a pa ien wi h sys emic mas ocy osis (SM) wi h an associa ed hema o-
logic neoplasm (SM–AHN), and s ained wi h phycoe y h in (PE)-conjuga ed monoclonal an ibodies
di ec ed agains CD2, CD25, CD30, CD33, CD44, and CD52, as well as luo och ome-labeled an i-
bodies agains CD34 and CD38. Exp essions o a ge an igens on CD34
+
/CD38
−
s em cells we e
assessed ia mul i-colo low cy ome y, and a e indica ed by he blue his og ams. The iso ype-
ma ched con ol an ibody is also shown (black open his og ams). Rep esen a i e his og ams show
exp essions o CD25, CD33, and CD44 on neoplas ic s em cells, whe eas hese cells exp essed only
ace amoun s o CD30 and CD52, and s ained nega i e o CD2. S em cells also displayed CD123
(no shown), con i ming he neoplas ic na u e o hese cells.
8. No el App oaches o Ta ge NSC in Ad anced SM
A numbe o he apeu ic app oaches di ec ed agains ce ain su ace an igens ( a ge s)
exp essed on NSC ha e been de eloped. An ibody-based he apies include an ibody–
oxin conjuga es, bi-speci ic o i-speci ic linke -cons uc s, o an ibodies ha kill a ge
cells h ough complemen -dependen mechanisms [
68
,
77
,
81
]. Whe eas he an ibody– oxin
conjuga e GO has been applied in a ew cases, bi-speci ic o i-speci ic linke -cons uc s
ha e so a no been applied in clinical p ac ice in pa ien s wi h ad anced SM. Recen ly,
howe e , a i-speci ic kille engage CD16xIL15xCD33 ha induces NK cell ac i a ion
and cy o oxici y agains neoplas ic MC has been p esen ed [
81
]. Whe he such engage
cons uc s will be able o elimina e neoplas ic MC and NSC in pa ien s wi h ad anced
SM emains a p esen unknown. In addi ion, i emains unknown whe he CAR-T cell
o CAR-NK cell he apies will be de eloped a enough o each clinical applica ion in
in e en ional ials. Cu en ly, majo e o s a e being unde aken o es ablish CAR-T cell
app oaches di ec ed agains CD33, KIT, and a ew o he a ge s, wi h he aim o elimina e
NSC in pa ien s wi h ad anced SM, including SM–AHN. One speci ic aspec o conside
wi h hese he apies is ha an ibodies and CAR cells (CAR-T o CAR-NK) a acking MC
may no only induce a umo lysis synd ome, bu also a MC media o elease synd ome,