Rapid, complete and sustained tumour response
to the TRK inhibitor larotrectinib in an infant with
recurrent, chemotherapy-refractory infantile
fibrosarcoma carrying the characteristic ETV6-NTRK3
gene fusion

Pediatrics 5 (Oncology, Hematology, Immunology), Center for Pediatric, Adolescent and Women’s Medicine, Stuttgart Cancer Center, Klinikum Stuttgart –
Olgahospital, Stuttgart; 2
Department of Pediatric Hematology and Oncology, University Children’s Hospital Muenster, Muenster, Germany; 3
Loxo Oncology, a wholly
owned subsidiary of Eli Lilly and Company, South San Francisco, USA; 4
Department of Pediatric Hematology and Oncology, Klinikum Kassel, Kassel; 5
Radiologic
Institute, Center for Pediatric, Adolescent and Women’s Medicine, Stuttgart Cancer Center, Klinikum Stuttgart – Olgahospital, Stuttgart; 6
Institute for Pediatric
Radiology, Klinikum Kassel, Kassel; 7
Institute of Pathology – Section Pediatric Pathology, University Hospital Schleswig-Holstein, Campus Kiel, Kiel, Germany
*Correspondence to: Dr Stefan S. Bielack, Pediatrics 5 (Oncology, Hematology, Immunology), Center for Pediatric, Adolescent and Women’s Medicine, Stuttgart Cancer Center,
Klinikum Stuttgart – Olgahospital, Kriegsbergstr. 62, 70174 Stuttgart, Germany. Tel: þ49-711-2787-2460; E-mail: [email protected]
Background: The ETV6-NTRK3 gene fusion is present in the majority of cases of infantile fibrosarcoma (IFS) and acts as a potent
oncogenic driver. We report the very rapid, complete, and sustained response of an advanced, chemotherapy-refractory,
recurrent IFS to targeted treatment with the oral tropomyosin receptor kinase (TRK) inhibitor larotrectinib.
Patient and methods: A male infant born with a large congenital IFS of the tongue had the tumour surgically resected at age
4 days. Within 2 months, he developed extensive lymph node recurrence that progressed during two cycles of vincristine￾doxorubicin-cyclophosphamide chemotherapy. At screening, a large right cervical mass was clinically visible. Magnetic
resonance imaging (MRI) revealed bilateral cervical and axillary lymph node involvement as well as infiltration of the floor of the
mouth. The largest lesion measured 5.54.54.4 cm (ca. 55 cm3
). The patient started outpatient oral larotrectinib at 20 mg/kg
twice daily at age 3.5 months.
Results: After 4 days on treatment, the parents noted that the index tumour was visibly smaller and softer. The rapid tumour
regression continued over the following weeks. On day 56 of treatment, the first scheduled control MRI showed the target
lesion had shrunk to 1.21.20.8 cm (ca. 0.6 cm3
), corresponding to a complete response according to the Response Evaluation
Criteria In Solid Tumors version 1.1. This response was maintained over subsequent follow-up visits, and on day 112 at the
second control MRI the target lymph node was completely normal. At last follow-up, the disease remained in complete
remission after 16 months on larotrectinib, with negligible toxicity and no safety concerns.
Conclusion(s): Selective TRK inhibition by larotrectinib offers a novel, highly specific and highly effective therapeutic option for
IFS carrying the characteristic ETV6-NTRK3 gene fusion. Its use should be considered when surgery is not feasible. (NCT02637687)
Key words: infantile fibrosarcoma, tropomyosin receptor kinase inhibition, larotrectinib
VC The Author(s) 2019. Published by Oxford University Press on behalf of the European Society for Medical Oncology.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in
any medium, provided the original work is properly cited.
Annals of Oncology 30 (Supplement 8): viii31–viii35, 2019
doi:10.1093/annonc/mdz382
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Background
Infantile fibrosarcoma (IFS), the most frequent sarcoma of in￾fancy, is one of few malignancies in which the vast majority of
tumours carry NTRK rearrangements [1–3], with the ETV6-
NTRK3 gene fusion occurring in about 70% of cases of IFS [4].
While surgery alone or in combination with chemotherapy is
often curative, there are cases when such treatment is either un￾successful or would lead to major mutilation and disability [5, 6].
Previously published cases have demonstrated the rapid and
robust efficacy of the selective tropomyosin receptor kinase
(TRK) inhibitor larotrectinib in patients with IFS, including
when used as neoadjuvant therapy. However, these reports did
not provide data on the long-term administration of this agent
[7, 8]. Here, we present the case of a young infant with
chemotherapy-refractory, recurrent IFS who experienced rapid,
pronounced and durable tumour regression during long-term
treatment with larotrectinib.
Patient and methods
This otherwise healthy male infant was born with a large congeni￾tal IFS that was located on the anterior portion of the tongue and
protruded from the mouth. The tumour was surgically resected
with close margins (R1/RX) when the patient was aged 4 days.
Pathological workup, including break-apart fluorescence in situ
hybridisation, revealed IFS with ETV6-breakage, and PCR con-
firmed the presence of the characteristic ETV6-NTRK3 re￾arrangement. At age 7 weeks, the parents noted a rapidly growing
mass of the right cervical region. Contrast-enhanced magnetic
resonance imaging (MRI) revealed a bilateral cervical lymph
node recurrence. The largest, right-sided cervical mass measured
4.5 4.2 2.6 cm (ca. 26 cm3
) and there was a contralateral
pathological lymph node measuring 2.3 1.3 1.7 cm.
A contrast-enhancing mass of the right submandibular region
measuring 2.5 1.7 1.6 cm infiltrated the musculature of the
floor of the mouth. In addition, left axillary lymph node involve￾ment was suspected. Given the extent of regional tumour involve￾ment, surgery was not considered feasible. A trial of cytotoxic
chemotherapy was therefore initiated, and the patient received
two cycles of cyclophosphamide-doxorubicin-vincristine, but the
tumour did not respond. Instead, the clinical impression was
progression of the right-sided cervical mass (Figure 1A).
Consequently, the patient was invited to participate in a multi￾centre, open-label, phase I/II study investigating the efficacy and
safety of larotrectinib for the treatment of advanced paediatric
solid or primary central nervous system (CNS) tumours
(NCT02637687) [9]. After informed consent was obtained, the
patient was screened for enrolment and found to be eligible.
Screening MRI confirmed disease progression, with the right￾sided cervical index lesion now measuring 5.5 4.5 4.4 cm (ca.
55 cm3
; Figure 2A). Outpatient treatment with oral larotrectinib
at a dose of 100 mg/m2 body surface area twice a day, correspond￾ing to 1.5 ml larotrectinib 20 mg/ml solution twice daily, was ini￾tiated at age 3.5 months (cycle 1 day 1; C1D1).
Results
The parents noticed a change in tumour consistency within
4 days, which was confirmed during the next clinic visit at C1D8
when the index lesion appeared softer, more mobile upon palpa￾tion and reduced in size. The rapid tumour regression continued
over the following days and weeks (Figure 1B–D). At the first
scheduled control MRI carried out on day 56 of larotrectinib
treatment (C3D1), the only abnormality remaining was a solitary,
slightly enlarged right cervical lymph node which measured
1.2 1.2 0.8 cm (ca. 0.6 cm3
; Figure 2B). According to the
Figure 1. Clinical impression of the right-sided cervical mass (A) before initiation of larotrectinib, (B) at week 3 of treatment, (C) at week 9 of
treatment and (D) at week 25 of treatment.
Original article Annals of Oncology
viii32 | Bielack et al. Volume 30 | Supplement 8 | November 2019
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Response Evaluation Criteria in Solid Tumours version 1.1 for
lymph node response, in which target nodes are measured in the
axis perpendicular to the longest diameter [10, 11], this corre￾sponded to a complete response (CR). CR was maintained at the
second control MRI on C5D1, with the previously described
lymph node now completely normal (Figure 2C), and at last
follow-up on C18D1 (16 months after initiation of larotrectinib).
Except for a brief period of grade 3 increased alkaline phos￾phatase, which was considered unrelated to larotrectinib, the pa￾tient experienced no adverse events of grade 3 or higher (coded
according to the National Cancer Institute’s Common
Terminology Criteria for Adverse Events version 4.3). His growth
was consistent with his percentiles and he met his developmental
milestones, learning to walk at age 11 months. Vaccinations were
carried out as recommended by the German Steady Vaccination
Committee without severe or unexpected complications (1 day
of fever following the measles-mumps-rubella vaccine).
Discussion
The presented IFS case illustrates that targeted therapy of TRK fu￾sion cancer with the selective TRK inhibitor larotrectinib can re￾sult in rapid, dramatic responses even after failure of
chemotherapy. The majority of IFS cases carry NTRK gene
fusions, similar to a number of other malignancies, including
mesoblastic nephroma, secretory breast cancer and the mammary
analogue secretory carcinoma of the salivary glands [1, 12]. This
characteristic gene fusion renders them potentially vulnerable to
Figure 2. Visualisation of the largest diameters of the right cervical lesion upon coronal and planar magnetic resonance imaging (A) at
screening, (B) on day 56 of treatment and (C) on day 112 of treatment.
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treatment with TRK inhibitors, such as larotrectinib, which
received accelerated US Food and Drug Administration (FDA)
approval based on data from three multicentre, open-label, sin￾gle-arm clinical trials [9, 13, 14].* This is only the second tissue￾agnostic drug approved by the FDA for the treatment of cancer
[14]. Larotrectinib has demonstrated durable responses in a wide
variety of NTRK gene fusion-positive tumours in both children
and adults [9, 15, 16]. Of note, all eight patients with NTRK gene
fusion-positive IFS treated in the phase I portion of the larotrecti￾nib paediatric trial responded to treatment: six patients with a
partial response and two patients with a CR [8, 15]. Five of the
patients with partial response underwent surgical resection, with
R0 in three patients, R1 in one patient and R2 in one patient. At
the time of surgical resection, two patients had complete patho￾logical responses.
Larotrectinib is generally well tolerated in both adults and chil￾dren [9, 15]. To date, most adverse events reported have been
grade 1 or 2, with no grade 4 or 5 treatment-related adverse events
observed [9, 15]. However, although acute toxicities seen with
larotrectinib are usually minor and rarely cause treatment inter￾ruption or dose reduction, little is known about the long-term
effects of this drug, particularly when administered over pro￾longed periods to young infants and children who are still grow￾ing. It is therefore reassuring that our patient seems to have
developed appropriately for his age over the 11 months that he
received larotrectinib treatment. It is also of note that he was vac￾cinated with toxoid, inactivated and live-attenuated vaccines
with no unexpected side effects. It remains to be seen whether his
development and that of other young children will remain un￾eventful if larotrectinib treatment is continued for many years. In
normal physiology, TRKs are involved in the neuronal develop￾ment, function and maintenance of the CNS and peripheral ner￾vous system, with important roles in neuronal cell survival,
morphology and differentiation [17, 18], as well as in the regula￾tion of sensation, movement, behaviour and cognition [19].
As the long-term effects of TRK inhibition on the CNS and
peripheral nervous system are currently unknown, it may be ad￾visable to include neurophysiological and neuropsychological
testing in the long-term follow-up of patients treated with TRK
inhibitors, particularly when they have been exposed to these
agents as infants or young children. The potential for TRK inhib￾ition to be used in paediatric upfront settings, without prior
chemotherapy, may only be fully exploited once studies confirm
the long-term safety of this approach.
Even with the limited clinical use of larotrectinib to date,
remissions lasting more than 2 years have already been reported
for patients whose inoperable localised or metastatic tumours
responded to the drug [9, 15]. However, the proportion of
tumours that will permanently respond to continued TRK inhib￾ition is unknown as secondary resistance has been observed in a
minority of patients treated with larotrectinib, possibly resulting
from mutations altering the kinase domain of TRK [9, 20].
Research is ongoing to elucidate resistance mechanisms to TRK
inhibitors and identify strategies for overcoming resistance
mutations [20]. In addition, too few patients with responding
malignancies have terminated larotrectinib in the absence of
complete surgery to determine whether and when treatment may
be interrupted, or even stopped, when enduring drug-induced re￾mission is achieved [7]. Hence, further research is warranted to
determine the role of TRK inhibition in the long-term manage￾ment of NTRK gene fusion cancers.
Conclusion
In summary, larotrectinib offers a novel, well-tolerated and often
highly effective treatment for patients with IFS and other NTRK
gene fusion-positive tumours. The present case report is consist￾ent with previous evidence and provides a graphic example of a
dramatic clinical and radiological response in an infant with re￾fractory infantile fibrosarcoma.
Acknowledgements
We thank the boy’s parents for allowing us to present his case
and acknowledge the dedicated support of the team members at
the authors’ institutions. Medical writing and editorial assist￾ance were provided by Cindy Cheung, MBBS, and Annabel Ola,
MSc, both of Scion (London, UK), funded by Bayer HealthCare
according to Good Publication Practice guidelines (https://
annals.org/aim/fullarticle/2424869/good-publication-practice￾communicating-company-sponsored-medical-research-gpp3).
Funding
This study is supported and funded by Loxo Oncology Inc., a
wholly owned subsidiary of Eli Lilly and Company, Stamford,
Connecticut, USA, and Bayer AG, Berlin, Germany (no grant
numbers apply). This paper was published as part of a supple￾ment financially supported by Bayer AG and Loxo Oncology,
Inc., a wholly owned subsidiary of Eli Lilly and Company.
Disclosure
SSB is the principal investigator for the LOXO-15003-TRK
study for Germany and has acted as a consultant and/or advis￾ory board member for Bayer, Celgene, Chugai, Clinigen, Eli
Lilly, Ipsen, Isofol, Novartis, Pfizer, Roche and Sensorion. MCC
is an employee of Loxo Oncology, a wholly owned subsidiary of
Eli Lilly and Company, holds a patent issued to Loxo Oncology
(62/318 041), and owns stock in Bayer AG. MR is a paid con￾sultant of Loxo Oncology, a wholly owned subsidiary of Eli Lilly
and Company, and holds a patent issued to Loxo Oncology (62/
318 041). MS has acted as an advisory board member for Eli
Lilly. RT is an employee of Loxo Oncology, a wholly owned sub￾sidiary of Eli Lilly and Company. All remaining authors have
declared no conflicts of interest.
*Note added in proof: The European Medicines Agency granted marketing authorisation for larotrectinib on 23 September 2019 as monotherapy for the treatment of adult and
paediatric patients with solid tumours that display a neurotrophic tyrosine receptor kinase (NTRK) gene fusion, and who have disease that is locally advanced, metastatic or
where surgical resection is likely to result in severe morbidity, and who have no satisfactory treatment options.
Original article Annals of Oncology
viii34 | Bielack et al. Volume 30 | Supplement 8 | November 2019
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