The Road Less Traveled: A Guide to Metastati

ROS1-Rearranged Non–Small-Cell Lung Cancer

Daniel Almquist, MD1, and Vinicius Ernani, MD1

abstract
Over the past decade, significant advances have been achieved in the diagnostic testing, treatment, and prognosis of advanced non–small-cell lung cancer (NSCLC). One of the most significant developments was the identification of specific gene alterations that define subsets of NSCLC. In 2007, ROS1 rearrangements were first described and observed in approximately 1%-2% of patients with NSCLC. Currently, crizotinib remains the
therapy of choice for advanced ROS1-rearranged NSCLC without CNS metastases, while entrectinib has emerged as the preferred option for those with CNS metastases. The next-generation inhibitors under de- velopment are more potent, have better CNS efficacy, and can overcome important resistance mutations. In this review, we focus on the management of patients with advanced NSCLC harboring a ROS1 rearrangement. We aim to provide insight into the diagnosis, treatment approach, and emerging treatments in this subgroup of NSCLC.
JCO Oncol Pract 16. © 2020 by American Society of Clinical Oncology

Author affiliations and support information (if
applicable) appear at the end of this article.
Accepted on September 25, 2020 and published at ascopubs.org/journal/ op on November 19, 2020: DOI https://doi. org/10.1200/OP.20. 00819

INTRODUCTION
Lung cancer is the leading cause of cancer-related mortality in the United States, with 135,720 esti- mated deaths in 2020. Non–small-cell lung cancer (NSCLC) accounts for approximately 85% of all lung malignancies, and . 50% of these patients present with metastatic disease at the time of initial diagnosis.1 There have been major advancements in the diagnostic testing, treatment, and prognosis of advanced NSCLC
over the past decade. Perhaps the most significant development arose from the identification of specific genetic alterations that define subsets of NSCLC, es- pecially within the adenocarcinoma histology.

ROS1 rearrangements in NSCLC were first described in 2007.2 Approximately 1%-2% of patients with NSCLC harbor this rearrangement, which has become
a successful target of multiple tyrosine kinase in- hibitors (TKIs).3 In 2016, crizotinib was the first ap- proved TKI by the US Food and Drug Administration (FDA) for the treatment of advanced ROS1-rearranged
NSCLC. Since then, entrectinib and lorlatinib have emerged as treatment options. More potent TKIs with better intracranial activity are currently under development.

In this review, we focus on the management of patients with advanced NSCLC harboring a ROS1 rearrange- ment. We aim to provide insight into the diagnosis, treatment approach, and emerging treatments in this subgroup of NSCLC.
CLINICAL CHARACTERISTICS OF ROS1-REARRANGED NSCLC ROS1 fusions have been described in 22 diverse adult and pediatric malignancies.4 In NSCLC, the reported frequency of ROS1 rearrangements has ranged from 0.9% to 2%.5 This fusion is typically associated with younger age, light smoking history (or never-smokers), and adenocarcinoma histologic subtype.3,6 On oc- casion, ROS1 rearrangements have been seen in large-cell and squamous cell carcinoma.7,8 Venous throm- boembolism has been reported at higher rates in this population.9 There are conflicting data with regard to
the frequency of brain metastases, with incidences ranging from 3.2% to 36%.10,11 Approximately 30% of the patients experience CNS metastases during the first-line treatment with crizotinib, making the CNS the most common site of disease progression in this small NSCLC population.12

DIAGNOSIS OF ROS1-REARRANGED NSCLC
Similar to other targetable driver mutations, ROS1 testing must be performed on all advanced-stage NSCLC adenocarcinomas, irrespective of clinical characteristics. It should also be evaluated for mixed- histology tumors and in limited specimens, where an adenocarcinoma component can be identified orcannot be completely excluded.13 For patients with squamous histology, the data are less clear, and testing can be considered in patients who are nonsmokers.14 Abbreviations: DCR, disease control rate; mOS, median overall survival; mPFS, median progression-free survival; NA, not available; NR, not reached; ORR, objective response rate.

Accurate identification of ROS1 fusions is critical to ensure that patients receive the appropriate treatment. In some circumstances, more than 1 diagnostic test needs to be performed to make the correct diagnosis. Immunohisto- chemistry (IHC) uses D4D6 monoclonal rabbit anti- body–stained slides to identify ROS1 protein positivity. IHC is inexpensive and fast, but it is operator dependent and difficult to interpret. ROS1 staining patterns vary secondary to intracellular localization of ROS1 fusions and ROS1
expression by benign tissues. Benign tissue expression generates more background staining, which potentially leads to false positives.7,15,16 Most studies demonstrate nearly 100% sensitivity, while specificity ranges from 70% to 100%.15-20 The International Association for the Study of Lung Cancer has recommended the use of IHC only as a screening tool, which should be confirmed with either fluorescence in situ hybridization (FISH) assay or next-generation sequencing.17,21

The FISH assay is considered the gold standard in terms of testing for ROS1 rearrangements. FISH positivity was re- quired in the registration trials of crizotinib. ROS1 FISH utilizes a dual break-apart probe design (39 and 59 por- tions).3 A rearranged ROS1 gene yields a split red and
green signal, and an atypical rearrangement will show an isolated 39 red signal. A tumor is considered FISH positive if at least 15% of evaluated tumor cells contain a split or isolated 39 signal. There should be at minimum 50 tumor cells counted within the specimen to yield a true result. To reduce false-negative and -positive results, a large amount of tissue is required for testing.15,17,21 The diagnosis and interpretation of a FISH ROS1 result can be complex. False- positive results may occur as a result of the detection of nonfunctional ROS1 fusions, and false-negative results can be observed with complex genomic rearrangements.

Reverse transcription polymerase chain reaction uses specific ROS1 gene fusion primers to confirm ROS1 rearrangements. This process has a significant limitation because it requires previous knowledge of fusions for
primer design to detect ROS1 rearrangements.22 There- fore, this technique is unable to identify rearrangements
involving unknown fusion partners.15,17,22 In 1 study, the sensitivity and the specificity of this assay was 100% and 85.1%, respectively.23
In light of multiple new actionable molecular drivers in NSCLC, single-gene assays, such as IHC, FISH, and re- verse transcription polymerase chain reaction, are no longer sufficient. Next-generation sequencing is becoming a well-established methodology to identify ROS1 fusions; it can simultaneously identify fusion partners and multiple other genomic biomarkers. This test can be performed on tumor tissue DNA or circulating cell-free DNA, also known as liquid biopsy. Of note, Guardant360 CDx (Guardant Health, Redwood City, CA) and FoundationOne Liquid CDx (Foundation Medicine, Cambridge, MA) were recently approved by the FDA for the detection of EGFR-mutated NSCLC. Cell-free DNA testing is particularly helpful in tissue-limited and time-limited circumstances. It is also an important tool to monitor resistance mechanisms to TKIs.24,25 Nevertheless, a nondiagnostic liquid biopsy requires tissue confirmation.

ROS1-TARGETED THERAPIES IN NSCLC Crizotinib
Crizotinib, a first-generation TKI, was the first TKI in- vestigated for ROS1-rearranged NSCLC. Initially de-
veloped as a MET inhibitor, crizotinib had activity in anaplastic lymphoma kinase (ALK) fusions and ROS1 rearrangements.12,26 ALK and ROS1 share significant ho- mology, which allows crizotinib to bind to both with high affinity.12,27,28 The first prospective phase I study, PROFILE 1001, evaluated crizotinib in an expansion cohort of pa- tients with ROS1-rearranged NSCLC.3,8,27,29 PROFILE 1001
included 50 patients in the ROS1-rearranged NSCLC co- hort. The objective response rate (ORR) to crizotinib was 72%, with a disease control rate of 90%. The median progression-free survival (mPFS) reached 19.2 months, with an overall survival (OS) rate at 12 months of 85%.12,30 The most recent update from PROFILE 1001 included 53 patients and demonstrated an mPFS of 19.3 months and a median OS of 51.4 months.30 This trial showed that

2 © 2020 by American Society of Clinical Oncology
ROS1-Rearranged Non–Small-Cell Lung Cancer crizotinib could produce early, marked, and durable re- sponses in ROS1-rearranged NSCLC. On the basis of the efficacy and safety of PROFILE 1001, crizotinib was granted full FDA approval for the treatment of ROS1-fused NSCLC in 2016.
There are also a number of retrospective and phase II studies that support and confirm crizotinib’s efficacy in ROS1-rearranged NSCLC. Table 1 lists these studies.11,31-36

Mechanisms of Resistance to Crizotinib
Despite crizotinib’s efficacy, there are a number of ways ROS1-rearranged NSCLC can develop resistance to it. ROS1 rearrangements can be pressured to develop secondary ROS1 resistance mutations (on-target) or al- ternative bypass pathways (off-target) that can lead to downstream signaling and cell proliferation. The in- cidence of these secondary resistance mutations has been evaluated in a retrospective analysis of 17 patients with ROS1-fused NSCLC who progressed after crizotinib therapy. Approximately 53% of patients developed sec- ondary resistance mutations, which is much higher than on-target resistance mutations observed in ALK-rearranged NSCLC. The most common resistance mutations in ROS1- rearranged NSCLC were G2032R (41%), D2033N (6%), and S1986F (6%).37-39 As of today, there are no FDA- approved TKIs that can overcome the G2032R mutation. Although multiple next-generation TKIs are under in- vestigation, chemotherapy is the current treatment of choice for these patients. Off-target mechanisms of resistance are much less com- mon than ROS1 resistance mutations. Some off-target mechanisms include epithelial-mesenchymal transition and alterations in EGFR, HER2, MET, BRAF, KRAS, KIT, and MEK.40-44 This subset of patients is so rare that no evidence-based treatment can be recommended. Never- theless, chemotherapy or a combination of targeted ther- apies are reasonable approaches.

Ceritinib
Ceritinib is a potent, second-generation ALK inhibitor with overlapping activity in ROS1-rearranged NSCLC. A phase II Korean trial enrolled 32 patients with metastatic ROS1- rearranged NSCLC. Of the 32 patients, 30 were crizotinib naive. Of note, the two patients who received crizotinib previously had no clinical response to ceritinib. However, among the crizotinib-naive patients, the ORR was 62%. The mPFS for this subgroup was 19.3 months compared with 9.3 months for all patients. This trial showed an in- creased rate of adverse events; 68% of patients required dose adjustments, and 72% required dose interruptions. The most common grade 1 and 2 adverse events were diarrhea, nausea, and anorexia. The most common grade 3 and 4 adverse event was fatigue.45 Ceritinib is currently not FDA approved for the treatment of ROS1-rearranged NSCLC. Crizotinib and entrectinib are FDA-approved TKIs in the first-line setting for patients harboring a ROS1 rearrange- ment. Ceritinib is active in crizotinib-naive ROS1-rearranged NSCLC; however, its toxicity profile and lack of regulatory approval make it a less compelling option. We do not rec- ommend ceritinib in the crizotinib resistance setting in light of its limited activity in the second-line space.

Lorlatinib
Lorlatinib is a potent, highly selective, third-generation ALK and ROS1 TKI. It is currently FDA approved as second-line therapy for patients with ALK-rearranged NSCLC. It has demonstrated at least a 10-fold improved potency against ROS1 compared with other TKIs. Lorlatinib is able to achieve higher cerebrospinal fluid-to-plasma ratios, translating into
improved CNS penetration.39 Shaw et al46 conducted a phase I study that evaluated lorlatinib in patients with advanced ALK and ROS1 rear- rangements. Of the 12 patients with a ROS1 fusion, 7 had received prior ROS1 TKI. The ORR and PFS in the ROS1 cohort were 50% and 7 months, respectively. The most common grade 1-2 adverse events were hypercholester- olemia, hypertriglyceridemia, peripheral edema, pe- ripheral neuropathy, and cognitive effects. The most common grade 3 toxicities were hypercholesterolemia and hypertriglyceridemia.

A subsequent phase I/II study enrolled 69 patients with ROS1-rearranged advanced NSCLC. Twenty-one patients (30%) were TKI naive, 40 (58%) had received prior cri- zotinib, and 8 (12%) had received 1 non-crizotinib TKI or 2 or more ROS1 TKIs. The ORR was 62% in TKI-naive pa- tients, 35% in crizotinib-resistant patients, and 13% in patients who received other TKIs. The overall ORR was 41% in patients with ROS1 rearrangement. The mPFS was 21 months for TKI-naive patients, whereas the mPFS for those who received prior TKIs was 8.5 months.46 One of the secondary end points of the study was to evaluate bio- markers of response and resistance to lorlatinib. In the TKI- naive patients, no ROS1 resistance mutations were found; however, among those who had received any previous ROS1 TKI, G2032R was the most common resistance mutation identified. Of note, none of the patients who
harbored this mutation responded to lorlatinib.46

Lorlatinib seems to have at least comparable efficacy to other ROS1 inhibitors in TKI-naive patients. In the crizotinib-resistance setting, off-label lorlatinib is the cur- rent treatment of choice. For patients who develop the
ROS1 G2032R resistance mutation after crizotinib, we favor chemotherapy.

Entrectinib
Entrectinib targets multiple tyrosine kinases and has shown benefit in ALK-, ROS1-, and NTRK-fused NSCLC. In ROS1- rearranged NSCLC, it has shown to be 30 times more potent than crizotinib. Nevertheless, its activity may be limited by Abbreviations: icRR, intracranial response rate; Mets, metastases; mOS, median overall survival; mPFS, median progression-free survival; NA, not available; NR, not reached; NSCLC, non–small-cell lung cancer; ORR, objective response rate; TKI, tyrosine kinase inhibitor. aStudy included 2 patients previously treated with crizotinib.
bALKA-372-001, previous cancer therapy was allowed (excluding previous ROS1 inhibitors); STARTRK-1, previous cancer therapy was allowed, including crizotinib, ceritinib, and investigational drugs; and STARTRK-2, previous anticancer therapy was allowed, excluding approved or investigational ROS1 inhibitors. cStudy included previous chemotherapy, crizotinib, and other TKIs. dmPFS was 21.0 months for TKI-naive patients, whereas the mPFS for those who received prior TKIs was 8.5 months. eStudy included previous chemotherapy and 0-3 TKIs. fORR for TKI-naive and TKI-pretreated patients were 90% and 28%, respectively. gSeven patients with measurable CNS lesions (3 TKI naive and 4 TKI pretreated). hORR for TKI naive, 100%; ORR of 50% for TKI pretreated. crizotinib-resistance mutations, such as G2032R.47,48 An integrated analysis of 3 ongoing phase I and II trials of entrectinib (ALKA-372-001, STARTRK-1, and STARTRK- 2) evaluated 53 TKI-naive patients with ROS1. The ORR, intracranial ORR, and mPFS were 77%, 55%, and 19 months, respectively. The most common grade 1-2 tox- icities were dysgeusia, constipation, and dizziness. The most common grade 3 adverse events were weight gain and neutropenia.49 On the basis of these results, entrectinib was approved by the FDA for the first-line treatment of advanced ROS1-rearranged NSCLC. Post- progression biopsies were not required in this study; thus, the profile of acquired resistance has yet to be in- vestigated. In the crizotinib resistance setting, entrectinib does not seem to be active.50

Entrectinib has shown encouraging systemic and in- tracranial activity in patients with NSCLC harboring a ROS1 rearrangement. In treatment-naive patients with CNS dis- ease, we recommend entrectinib. Although data are lim- ited, entrectinib seems to lack efficacy after crizotinib
failure. Repotrectinib Repotrectinib is a next-generation ALK, ROS1, and NTRK fusion inhibitor with a potency of at least 90-fold greater than crizotinib. In preclinical models, repotrectinib showed activity against the G2032R resistance mutation.51-53 TRI- DENT-1 is an ongoing phase I/II clinical trial that is evaluating repotrectinib in TKI-naive and TKI-refractory (1 or more TKIs) patients with ROS1, NTRK, and ALK fusions. The study enrolled 28 patients with ROS1-rearranged NSCLC. The ORR for TKI-naive and TKI-pretreated patients were 90% and 28%, respectively. The most common grade 1-2 adverse events were dysgeusia, dizzi- ness, paresthesia, nausea, anemia, constipation, fatigue, and oral numbness. Two patients had grade 3 adverse events (dizziness and dyspnea), which were dose-limiting toxicities.54,55 The FDA granted 2 fast-track designations for repotrectinib: first for patients with 1 prior line of platinum- based chemotherapy and 1 prior ROS1 TKI, and second for patients with ROS1 TKI-naive disease.

The data on repotrectinib are still evolving. However, the efficacy of this inhibitor is promising and suggests a po- tential new therapy on the horizon, especially for those with a ROS1 G2032R resistance mutation. Taletrectinib Taletrectinib is another next-generation TKI with activity against NTRK and ROS1 fusions. Similar to repotrectinib, preclinical studies have shown activity against the ROS1 G2032R resistance mutation.56 A Japanese phase I trial evaluated taletrectinib in NSCLC harboring ROS1 rear- rangements, and the ORR was 66.7% in crizotinib-naive patients.57,58 A phase I trial conducted in the United States enrolled 46 patients who had either neuroendocrine tumors or tumors that harbored ROS1/NTRK rearrangements. Six patients were previously treated with crizotinib; the ORR and mPFS were 33% and 4.1 months, respectively, in this small number of crizotinib-resistant patients. The 3 most common adverse events were nausea, diarrhea, and vomiting. Grade 3 adverse events included diarrhea, ab- dominal pain, and fatigue.59
4 © 2020 by American Society of Clinical Oncology
ROS1-Rearranged Non–Small-Cell Lung Cancer Treatment algorithm. NSCLC, non–small-cell lung cancer.

Taletrectinib is another next-generation TKI with preliminary efficacy against the ROS1 G2032R resistance mutation. Like repotrectinib, the data are still maturing, and phase II trials are currently ongoing. Table 2 lists the treatment efficacy of TKIs used in ROS1-rearranged NSCLC.
CNS METASTASES IN ROS1-REARRANGED NSCLC CNS metastases remain a major cause of morbidity and mortality in patients with NSCLC. Approximately 30% of patients are expected to develop CNS disease during cri- zotinib treatment. Furthermore, CNS disease has been reported as the first and sole site of progression in 47% of ROS1-rearranged NSCLC treated with crizotinib.10,31 The largest studies of crizotinib in this population did not evaluate intracranial ORR, but it is well established that this TKI has limited CNS penetration.30,37,60 Ceritinib also has poor CNS penetration, which was demonstrated in a phase II trial with an intracranial ORR of 25%.45 Entrectinib demonstrated CNS activity, irrespective of previous ther- apy. In a pooled analysis (ALKA-372-001, STARTRK-1, and STARTRK-2) of 20 patients with baseline CNS metastases, entrectinib demonstrated a CNS ORR of 55%. Of the 7 patients with measurable CNS disease who had not re- ceived radiotherapy within 2 months of starting a TKI, 71% achieved an intracranial response. Four (80%) of 5 patients who received radiotherapy within 2 months of starting the TKI had an intracranial response. Given its tolerability and intracranial efficacy, entrectinib is the preferred first-line option for patients with ROS1-rearranged NSCLC who present with brain metastases.49,50
JCO Oncology Practice 5Almquist and Ernani Significant intracranial activity has also been demonstrated with lorlatinib and repotrectinib. In a phase I study eval- uating lorlatinib, 5 patients had measurable CNS disease. The intracranial ORR was 60%.61 In the phase II study, the intracranial ORR for TKI-naive and crizotinib-resistant pa-tients was 64% and 50%, respectively.46,61 Repotrectinib demonstrated efficacy in a subgroup analysis of TRIDENT-1. This study included 7 patients with measurable CNS at baseline. In TKI-naive patients, the intracranial ORR was 100%, and in TKI-pretreated patients, the intracranial ORR was 50%.55

SUGGESTED APPROACH
The FDA has approved crizotinib and entrectinib in the first- line setting for patients with metastatic NSCLC harboring a ROS1 rearrangement. In patients with no CNS disease, first-line crizotinib is preferred. Nevertheless, for patients with asymptomatic CNS metastases, we favor starting with entrectinib. This TKI is slightly more toxic but has superior intracranial activity. For large or symptomatic CNS lesions, we recommend local control with either surgery or ste- reotactic radiosurgery followed by entrectinib.

After progression on first-line crizotinib or entrectinib, we recommend obtaining a new tissue biopsy specimen and/or circulating cell-free DNA to evaluate for resistance mechanisms. Additional management will, most of the time, be based on the status of the ROS1 G2032 mutation. In those with systemic progression and non-G2032R mutation, we recommend second-line lorlatinib. In patients with sys- temic progression and G2032R resistance mutation, we favor a clinical trial or pemetrexed-based chemotherapy. In the rare population with an off-target resistance mechanism, options may include chemotherapy or a combination of targeted therapies.

In patients with asymptomatic, CNS-only progression, we recommend lorlatinib. In symptomatic, CNS-only progres- sion, we recommend local therapy followed by lorlatinib. CNS and/or systemic oligoprogression can be managed with local therapy. Figure 1 summarizes our approach.
In conclusion, the treatment landscape of metastatic NSCLC is changing at an incredibly rapid pace, and ROS1 rearrangements are not the exception. Currently, there are at least 6 ROS1 TKIs showing clinical benefit in this small subset of NSCLC. The newer inhibitors are more potent,
have better CNS penetration, and can overcome the G2032R resistance mutation. Therefore, it is critical to perform broad molecular testing at the time of initial di- agnosis and post-ROS1 TKI progression to help to tailor the subsequent treatment.

As the armory of ROS1 TKIs continues to evolve, treatment sequencing is becoming more challenging. Several factors will need to be weighted in the treatment decision, such as CNS activity, resistance mechanisms, and tolerability. As of today, crizotinib and entrectinib are the standard first-line
treatments for these patients. Lorlatinib has an off-label indication in the second-line setting; however, it lacks activity against the G2032 resistance mutation. The iden- tification of therapeutic strategiesto overcome this mutation is critically important. Repotrectinib and taletrectinib are promising next-generation inhibitors with the potential to target this mutation and change the treatment landscape of ROS1 NSCLC.
Finally, it is our responsibility to encourage these patients to participate in clinical trials. This way, we can further advance the understanding of this disease, optimize drug discovery, and ultimately improve the lives of our patients.

AFFILIATION
1Division of Hematology and Medical Oncology, Mayo Clinic Cancer Center, Phoenix, AZ

CORRESPONDING AUTHOR
Vinicius Ernani, MD, Division of Hematology and Medical Oncology, Mayo Clinic Cancer Center, 5777 E Mayo Blvd, Phoenix, AZ 85054; e-mail: [email protected].

AUTHORS’ DISCLOSURES OF POTENTIAL CONFLICTS OF INTEREST
Disclosures provided by the authors are available with this article at DOI https://doi.org/10.1200/OP.20.00819.

AUTHOR CONTRIBUTIONS
Conception and design: All authors
Provision of study material or patients: All authors
Collection and assembly of data: All authors Data analysis and interpretation: All authors Manuscript writing: All authors
Final approval of manuscript: All authors
Accountable for all aspects of the work: All authors

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