Larotrectinib followed by selitrectinib
in a novel DCTN1–NTRK1 fusion
undifferentiated pleomorphic sarcoma
Xue Na Goh1 , Michaela Su-Fern Seng2,3,
Amos Hong Pheng Loh3,4, Achint Gupta5
Kenneth Tou En Chang3,6 and Prasad Iyer2,3
Abstract
Introduction: Neurotrophic receptor tyrosine kinase fusions cause overexpression or activation of kinase and are
believed to confer oncogenic potential in some non-rhabdomyosarcoma soft tissue sarcomas. TRK inhibitors have
recently been shown to induce responses in these tumours though current experience with these agents is still limited.
Case report: We report a case of an adolescent with treatment-refractory non-rhabdomyosarcoma soft tissue sar￾comas, carrying a novel DCTN1–NTRK1 gene fusion whose progressive disease was treated with multi-kinase and TRK
inhibitors.
Management and outcome: Our patient was started on pan-TRK inhibitor larotrectinib, as his disease progressed
after chemotherapy, radiation therapy and surgery, based on next-generation sequencing test showing DCTN1–NTRK1
gene fusion. He responded quickly to larotrectinib with the improvement of symptoms and reduction of masses.
However, this response was short-lived due to the development of acquired solvent front resistance mutation. This
patient did not respond to next-generation TRK inhibitor selitrectinib and eventually succumbed to his disease.
Discussion: The initial rapid and drastic response of our patient to larotrectinib was not sustained due to the devel￾opment of acquired resistance. This case emphasizes the need for upfront and periodic next-generation sequencing
testing to guide treatment of patients with refractory non-rhabdomyosarcoma soft tissue sarcomas.
Keywords
Neurotrophic receptor tyrosine kinase fusions, TRK inhibitor resistance, larotrectinib, selitrectinib, non￾rhabdomyosarcoma soft tissue sarcomas
Date received: 19 May 2020; revised: 4 June 2020; accepted: 9 June 2020
Introduction
Non-rhabdomyosarcoma soft tissue sarcomas
(NRSTSs) are a heterogeneous group of mesenchymal
tumours and encompass several histological variants.1,2
These tumours account for less than 10% of all child￾hood cancers.2 Survival rate of NRSTS depends largely
on the histologic subtype, size, grade and extent of
metastasis, and is reported to be dismal at 15% for
metastatic diseases.2,3 The treatment of majority of
NRSTS involves a multi-disciplinary approach; com￾prising of surgery, radiation therapy and chemotherapy
used individually or in combination.3
Neurotrophic receptor tyrosine kinase (NTRK)
gene fusions have recently been implicated as
Department of Pharmacy, KK Women’s and Children’s Hospital,
Singapore, Singapore
Department of Paediatric Subspecialties, Haematology-Oncology
Service, KK Women’s and Children’s Hospital, Singapore, Singapore
Duke-NUS Medical School, Singapore, Singapore
Department of Paediatric Surgery, KK Women’s and Children’s Hospital,
Singapore, Singapore
Department of Diagnostic and Interventional Imaging, KK Women’s and
Children’s Hospital, Singapore, Singapore
Department of Pathology and Laboratory Medicine, KK Women’s and
Children’s Hospital, Singapore, Singapore
Corresponding author:
Goh Xue Na, KK Women’s and Children’s Hospital, 100 Bukit Timah
Road, Singapore 229899, Singapore.
Email: [email protected]
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DOI: 10.1177/1078155220938849
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oncogenic drivers of several tumours, and a treatment
strategy that targets these fusions has been found to be
effective.4-6 The most common mechanism of oncogen￾ic TRK activations is fusions involving NTRK1,
NTRK2 or NTRK3, which encode TRKA, TRKB
and TRKC, respectively.5 Alterations involving
NTRK genes are identified at high frequency in some
rare paediatric cancers but at low frequencies in a wide
range of common cancers such as high grade gliomas
and NRSTSs.1,6 The incidence of TRK fusions cannot
be estimated in paediatric NRSTSs as most reports are
anecdotal in nature.1
Next-generation sequencing (NGS) is a high￾throughput method used to discover the nucleotide
sequence of an individual’s genome.7 NGS is utilised
to study cancer specimens for relevant genomic alter￾ations and/or biomarkers to help to identify personal￾ized treatment options.8 The discovery of NTRK gene
fusions by NGS aids the identification of patients who
may benefit from treatment strategies targeting this
genomic alteration. Tyrosine kinase inhibitors active
against TRK fusions are broadly divided into multi￾kinase inhibitors or more selective TRK inhibitors.6
Multi-kinase inhibitors have differing degrees of inhi￾bition against TRK.6 Crizotinib, which is an ALK/
ROS1 inhibitor, exhibits modest activity against TRK
but at much lower affinity compared to MET, ALK
and ROS1.1,6 Larotrectinib is the first oral pan-TRK
inhibitor in clinical development and it is active against
all three TRK proteins with 50% inhibitory concentra￾tion of 5 to 11nM in vitro.6,9 It is highly selective,
inhibiting no other tested kinases at concentrations
up to 500 to 1000nM.1 It has been granted accelerated
approval by the Food and Drug Administration10
based on a recent phase I/II study showing that laro￾trectinib is well tolerated, with marked and durable
responses in patients with tumours harbouring a
NTRK gene fusion regardless of age and tumour
type.9 Next-generation TRK inhibitors, such as seli￾trectinib, are developed to overcome acquired muta￾tions to first-generation TRK inhibitors as they have
activities against TRK mutants at low nanomolar con￾centration range.6 Here we report experience with the
treatment of undifferentiated pleomorphic sarcoma
harbouring a novel DCTN1–NTRK1 gene fusion with
multi-kinase inhibitor crizotinib and TRK inhibitors
larotrectinib and selitrectinib.
Case report
A 14-year-old male presented with fever and a rapidly
increasing right abdominal wall mass. Imaging showed
a large lobulated mass centred in the right lateral
abdominal wall with areas of necrosis and haemor￾rhage with no evidence of distant spread. He was
started on neoadjuvant chemotherapy using the
European Paediatric Soft Tissue Sarcoma Study
Group’s 2005 protocol for localized NRSTSs.
A repeat scan after three cycles of 3-weekly ifosfa￾mide and doxorubin at doses of 3 g/m2 for 3 days and
37.5 mg/m2 for 2 days respectively, showed stable dis￾ease. As his disease did not show shrinkage on chemo￾therapy, he received radiotherapy at 50.4 Gy with
intensity-modulated radiation therapy followed by a
wide local resection of right flank sarcoma, lateral
abdominal wall and 10th to 12th ribs with ipsilateral
free anterolateral thigh and anteromedial thigh flap
reconstruction. This resection was complicated by
flap ischaemia and wound site infections, which
resulted in multiple skin grafts and flap reconstructions
over the right abdominal and chest wall areas. The
pathological features of this tumour were of a high￾grade pleomorphic sarcoma; pan-TRK immunohisto￾chemistry was positive in a cytoplasmic pattern.
FoundationOne HemeTM test, which is a targeted
Figure 1. Serial post-contrast axial T1-weighted MRI images, acquired at: (a) baseline, before commencement of treatment with
larotrectinib, showed large heterogeneously enhancing, partly necrotic right paraspinal mass (arrows), along with multiple enhancing
masses (arrowheads) in the right posterolateral abdominal wall, adjacent to prior resection site; (b) two months post initiation of
larotrectinib, showed smaller paraspinal mass with homogeneous signal and minimal enhancement, and there was resolution of the
peripheral masses; (c) six months post initiation of larotrectinib, demonstrated necrotic changes in the right paraspinal mass, with
internal enhancing nodules, and reappearance of enhancing abdominal wall masses, consistent with overall interval progression.
2 Journal of Oncology Pharmacy Practice 0(0)
next-generation sequencing (NGS) based assay, was
done using tumour tissue and identified two genomic
alterations, namely DCTN1–NTRK1 fusion and
CDKN2A/B loss.
A magnetic resonance imaging scan performed after
six weeks showed disease progression with significant
interval increase in size of the right paraspinal mass
and development of two smaller masses in the right
hemipelvis (Figure 1(a)). As early phase trials of laro￾trectinib (VitrakviVR
, Bayer Healthcare
Pharmaceuticals) in Singapore did not include paediat￾ric patients, an application was made for compassion￾ate use of larotrectinib at 100 mg twice daily. Crizotinib
(XalkoriVR
, Pfizer Private Limited) was used off-label
for a month until larotrectinib was available as his
tumour was rapidly progressing and causing intracta￾ble symptoms. The family declined palliative radiother￾apy and patient was referred to palliative care for
symptom management.
The patient was significantly better clinically within
a week of commencing larotrectinib with the complete
resolution of fever. He was bed bound, breathless on
oxygen therapy due to malignant pleural effusions and
completely dependent for all activities of daily living.
His fever was probably malignant fever as he was not
having active infections or receiving antibiotics at that
time. Within a month after commencing larotrectinib,
his Lansky score improved from 20 to 90 and he was
able to recommence schooling. Prior regular doses of
oral morphine were weaned and stopped by the fourth
week. No adverse effects attributable to larotrectinib
were seen on routine blood tests and clinical monitor￾ing. Surveillance scans done two and four months after
the initiation of larotrectinib showed an ongoing reduc￾tion in size of the loco-regional masses (Figure 1(b)) as
well as decrease in the size and number of pulmonary
nodules.
Unfortunately, routine surveillance imaging scans at
six months after initiation of larotrectinib demonstrat￾ed disease progression. New lobulated enhancing
masses were seen at the periphery of the prior resection
site, with interval increase in size and number of nod￾ular enhancing foci within the right paravertebral mass
and right psoas muscle, and increase in the size of
masses in the right hemipelvis (Figure 1(c)). A core
biopsy showed sarcoma that was less pleomorphic.
Pan-TRK immunohistochemistry remained positive in
a cytoplasmic pattern. Sequencing was performed and
the acquired NTRK1 c.1999G>T (p.Gly667Cys)
solvent front resistance mutation was identified in the
specimen (Figure 2).
Patient was continued on larotrectinib for two
months despite progressive disease until selitrectinib
(LOXO-195, Bayer Healthcare Pharmaceuticals) was
available on compassionate basis. Larotrectinib was
stopped and he was started on escalating doses of seli￾trectinib at 50 mg once daily for 14 days, 50 mg twice
daily for 14 days and reached target dose of 75 mg twice
daily with no reported adverse events. Unfortunately,
he succumbed to rapidly progressive disease seven
weeks from initiation of selitrectinib.
Discussion
We report a significant though unsustained response to
a first-generation TRK inhibitor, following the use of a
multi-kinase inhibitor, in a patient with a novel
DCTN1–NTRK1 gene fusion. Patient was initiated
with crizotinib whilst awaiting supply of larotrectinib
due to its modest activity in pre-clinical studies in chil￾dren with tumours harbouring NTRK fusions.1,11,12
Consistent with experiences in early phase I/II trial,
response to larotrectinib occurred within the previously
described median of 1.7 months and the patient also
showed good tolerance at the recommended
phase 2 dose.9
We observed the development of treatment resis￾tance to first-generation TRK inhibitors after initial
response. It has been postulated that on-target resis￾tance is due to amino acid substitution in the TRK
fusion protein involving 3 major regions: the solvent
front, gatekeeper residue or xDFG motif.13 Next￾Figure 2. Sequencing chromatogram showing acquired NTRK1 c.1999G>T (p.Gly667Cys) solvent front resistance mutation.
Goh et al. 3
generation TRK inhibitors have been developed to
overcome this acquired resistance.14,15 A phase I/II
trial showed preliminary efficacy of selitrectinib in
patients with acquired NTRK resistance mutations
during treatment with a prior TRK inhibitor.15
Although almost half of the patients with solvent
front mutations achieved response after the commence￾ment of selitrectinib,15 this patient unfortunately failed
to respond to selitrectinib.
Off-target resistance to TRK inhibitors involves
genomic alterations of other tyrosine kinase receptors
or downstream pathway mediators.13,16 Treatment
with mitogen-activated protein kinase pathway inhibi￾tors, either alone or in combination with TRK inhib￾itors, has been shown to re-establish disease control.14
Upfront inhibition of both TRK and MEK may also
delay resistance and progression based on experimental
models.14
Neoadjuvant larotrectinib has been tried in paediat￾ric patients with locally advanced TRK fusion sarco￾mas. This method has been shown to render tumours
resectable thus avoiding the need for morbid or radical
surgical resections.17 The integration of NGS early in a
patient’s diagnosis may be useful in patients who have
extensive disease requiring radical surgeries.
In conclusion, this case illustrated a rapid and dras￾tic response to larotrectinib which was short-lived due
to the development of acquired resistance. It under￾scores the importance of NGS for refractory patients
with NRSTSs and clinically guided re-testing to iden￾tify interval development of on-site and off-site resis￾tance mutations and their subsequent treatment
options. However, early integration of NGS testing
and the use of targeted therapy for the treatment of
NRSTSs, and the management of resistance to next￾generation TRK inhibitors and off-site mutations war￾rant further research.
Acknowledgements
Verbal consent was provided by the parent for the publica￾tion of this case. Compassionate access to larotrectinib and
selitrectinib was provided by Bayer Pte Ltd.
Declaration of Conflicting Interests
The author(s) declared no potential conflicts of interest with
respect to the research, authorship, and/or publication of this
article.
Funding
The author(s) disclosed receipt of the following financial sup￾port for the research, authorship, and/or publication of this
article: The VIVA-KKH PBST program kindly funded test￾ing for resistance mutation.
ORCID iD
Xue Na Goh https://orcid.org/0000-0001-9414-7983
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