Pralsetinib

Current management of RET rearranged non-small cell lung cancer

Abstract: The identification of oncogenic drivers, and the subsequent development of targeted therapies established biomarker-based care for metastatic non-small cell lung cancer (NSCLC). Biomarker testing is standard of care in NSCLC patients with adenocarcinoma because multiple targeted therapies are available. Rearranged during transfection (RET) rearrangements were identified as oncogenic drivers in NSCLC, and are more common among younger patients, adenocarcinoma histology, and patients with a history of never smoking. The prevalence is estimated to be 1–2% among patients with adenocarcinoma histology. The most common rearrangement is between intron 11 of the RET gene and intron 15 of the KIF5B gene, and the next most frequent rearrangement is with the CCDC6 gene. RET rearrangements lead to constitutive activation of the RET tyrosine kinase and increased cell proliferation, migration, and survival. Phase II studies investigated the activity of multi-targeted tyrosine kinase inhibitors in patients with NSCLC with a confirmed RET rearrangement. These agents have limited potency against RET, and activity against the epidermal growth factor receptor and vascular endothelial growth factor pathways. These agents revealed modest activity, and were poorly tolerated due to the off-target toxicities. These struggles contributed to the development of more potent and specific RET tyrosine kinase inhibitors. Preliminary results from early phase trials of selpercatinib (LOXO-292) and pralsetinib (BLU-667) revealed promising efficacy and improved tolerability. The availability of these agents will make routine testing for RET rearrangements a priority.

Lung cancer is the leading cause of cancer-related mortality in the United States, and one of the lead- ing causes globally.1,2 The majority of cases of lung cancer are the non-small cell lung cancer (NSCLC) subtype, and the majority of patients present with locally advanced or metastatic disease.3 NSCLC is further subdivided based on histology (e.g. adeno- carcinoma, squamous, or large cell carcinoma). Historically, patients with metastatic NSCLC were treated with a platinum-doublet and the spe- cific histology did not influence treatment selec- tion. However, in an early trial of bevacizumab a higher rate of pulmonary hemorrhage was observed among patients with NSCLC with squamous his- tology, and the use of bevacizumab was restricted to patients with non-squamous histology.4,5 The activity of pemetrexed was found to be restricted topatients with non-squamous histology.6 The development of these agents contributed to the development of histology-based selection of therapy for NSCLC.Epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors (TKIs) were initially developed in unselected patients, but patients with a history of never or light smoking, Asian ethnicity, and adeno- carcinoma histology were observed to have a higher response rate. These clinical characteristics were associated with a higher prevalence of EGFR acti- vating mutations, and patients with EGFR mutant NSCLC experienced greater clinical benefit from EGFR TKIs. Conversely patients without an EGFR mutation experience limited benefit from EGFR TKIs.7 The development of EGFR TKIsestablished a biomarker-driven care for NSCLC. Subsequently, anaplastic lymphoma kinase (ALK) rearrangements were observed to be oncogenic drivers in NSCLC, and ALK TKIs were developed only in patients with an identified ALK rearrange- ment.

The activity observed in early phase trials led to the rapid approval of this class of agents. The development of targeted therapies in a biomarker selected patient populations became the preferred paradigm for novel targeted therapies.The success of these targeted therapies combined with the increased availability and affordability of comprehensive genomic profiling changed the clinical research focus in thoracic oncology to developing novel agents in specific molecular subgroups. Targeted therapies were subsequently developed for patients with ROS1 rearrange- ments, BRAF V600E mutations, MET exon 14 alterations, and NTRK rearrangements.10–14 These agents were approved based on a single arm phase II trials which only enrolled patients with the specific biomarker of interest. The preva- lence of these molecular alterations is approxi- mately 1–3%, and the successful development of targeted therapies encouraged drug development in other rare molecular subtypes. The clinical impact of having multiple ‘actionable alterations’ is that many centers have adopted broad testing panels rather than using single gene tests.15Rearranged during transfection (RET) gene rear- rangements were identified as oncogenic drivers in NSCLC, and there has been a long-standing interest in developing a targeted therapy for this molecular alteration.16,17 RET rearrangements are more com- mon in patients with a history of never or light smok- ing, adenocarcinoma histology, and younger patients. RET rearrangements are mutually exclu- sive with other oncogenic drivers.18,19 The estimated prevalence of RET rearrangements observed in NSCLC with adenocarcinoma is 1–2%, and when patients without another oncogenic driver mutation are examined the prevalence of RET rearrangements is approximately 5%.

The initial research focus investigated the activity of multi-targeted TKIs which had shown activity in other malignancies. Recently, early phase trials have demonstrated activ- ity of RET-specific TKIs which will fundamentally change the clinical management of RET + NSCLC.The RET tyrosine kinase is a transmembrane glyco- protein, and RET does not bind directly to thereceptor ligands. The glial-derived neurotrophic factor ligands (GFLs) bind to glial-derived neuro- trophic factor family receptors (GFR), which act as co-receptors for RET. The GFL–GFR complex leads to RET homodimerization and subsequent autophosphorylation of the intracellular domain of the tyrosine kinase (Figure 1).22 The oncogenic event in NSCLC is a chromosomal rearrangement which produces a RET fusion protein.17,20,21,23 The RET gene is located on chromosome 10, and the pathogenic event is an intrachromosomal rearrange- ment. The most common partner genes are KIF5B, CCDC6, and NCOA4. KIF5B is the most common rearrangement observed in NSCLC (approximately 70% of cases), and the most common fusion is intron 11 of the RET gene and intron 15 of KIF5B.17,20,21,24,25 Numerous other gene rearrange- ments have also been reported (e.g. MYO5C, EPHA5, TRIM24, and TRIM33).23,26 The RETrearrangement is a combination of the RET intra- cellular kinase domain and the coiled coil domain of the partner gene, which results in ligand independ- ent homodimerization and activation of the RET tyrosine kinase by autophosphorylation.26,27 This leads to constitutive activation of the RET tyrosine kinase and increased cell proliferation, survival, migration and differentiation by activation of the phosphoinositide 3-kinases (PI3K)/AKT, mitogen- activated protein kinase (MAPK), and Signal transducer and activator of transcription 3 (STAT3) pathways (Figure 1).22,23,28A variety of molecular testing methods has been employed to detect RET rearrangement in retro- spective studies including whole exome sequencing, next generation sequencing (NGS), reverse tran- scription polymerase chain reaction (RT–PCR), fluorescence in situ hybridization (FISH), and immunohistochemistry (IHC).

IHC testing is effi- cient and convenient, but the sensitivity and speci- ficity are inadequate for routine clinical use.17,26,29 In order to improve the performance IHC future studies will need to develop a more specific anti- body and define the optimal cut-off for positive val- ues. Most RET FISH tests use a dual color break apart probe and examine 50 tumor cells, and clas- sify positive cases as having split signals or an iso- lated 3′ prime signal.19,30 In previous studies, the RET FISH break apart probe-positive cases under- went an additional FISH break apart probe for KIF5B and CCDC6 to confirm a RET rearrangment.19,30 The cut-off of tumor cells demonstrating a signal to be considered positive has varied, withmost studies using a cut-off between 10% and 20% of cells.19,29–31 FISH testing is highly sensitive, but the concerns are the technical expertise required and the test is not widely available. Reverse tran- scriptase polymerase chain reaction (RT–PCR) was used in RET screening studies, and used predefined primers to detect RET fusions.19,25,32 The strength is the ability to identify the specific RET fusion; how- ever, RT–PCR will not detect unknown fusion part- ners or variants, and poor preservation of RNA in the tumor sample can reduce the sensitivity. Many of the studies which used RT–PCR also used a sec- ond test to confirm the presence of a RET rearrangement.19,20,26Hybrid capture-based NGS allows the assess- ment of multiple molecular alterations andpotential concurrent mutations in a single test. Overall the last several years NGS testing has become more widely available for routine clinical care. The disadvantages are the detection of alter- ations of uncertain clinical significance, and con- cerns about the variation in the testing methods (i.e. whether RET is included in the testing panel).

RET rearrangements can also be detected using circulating tumor DNA (ctDNA).33 The primary advantages of ctDNA testing are the ability to test for a broad panel of molecular alterations simul- taneously and the shorter turnaround time com- pared to tumor testing. The primary concern is the lower sensitivity, especially in patients with fewer extra-thoracic metastatic lesions or lower disease burden.33 If a ctDNA test reveals a RET rearrangement or another oncogenic driver (e.g.EGFR, BRAF V600E, ALK, ROS1 molecularalterations) then the results can be acted on clini- cally. However, if there is no detectable DNA (i.e. an ‘uninformative’ test result) or an oncogenic driver is not identified (i.e. ‘mutation negative’) a tumor biopsy should be performed.Preliminary studies suggest RNA sequencing when integrated with DNA sequencing on tumor samples in which DNA fails to detect an onco- genic driver has the potential to detect additional gene fusions.34,35 However, RNA sequencing is not widely available for clinical use.RET FISH testing and NGS panels are the most commonly used clinical tests to identify RET rearrangements.Clinical data with multi-targeted TKIsThe initial attempts to develop a targeted therapy for RET rearrangements focused on the use of multi-targeted TKIs with indications in others solid tumors such as renal cell carcinoma, hepato- cellular carcinoma, or thyroid cancer. These drugs inhibit the RET tyrosine kinase but had limited potency for RET as they were not devel- oped as RET-specific inhibitors, and had activity against the vascular endothelial growth factor (VEGF) receptors and/or the EGFR pathway.36 Many of the adverse events observed were related to the activity of these agents on the EGFR (e.g. dermatologic toxicities and diarrhea) or VEGF pathways (e.g. hypertension), and these adverse events frequently required dose reduction, inter- ruption, or treatment discontinuation.

These off-targeted toxicities may have led to dose reduction below the dose required to effectively inhibit RET. The combination of the low potency and frequent dose reduction contributed to the lower objective response rate (ORR) and progres- sion-free survival (PFS) with these agents com- pared to targeted therapies in patients with EGFR, ALK, and ROS1 alterations.The clinical data available on the activity of multi- targeted TKIs are a combination of retrospective cases series, and small prospective single arm phase II trials. The prospective studies provide the most accurate data about the drug activity, adverse events, and tolerability (Table 1). Vandetanib is a multi-targeted TKI with activity against the VEGF receptors, EGFR, and RET. The study by Yoh et al. of vandetanib screened patients with EGFR mutation-negative NSCLC using RT–PCR and FISH testing was used to confirm RT–PCR posi- tivity.18 Of the 1536 patients who were screened, 34 patients were RET positive, and 19 patients were enrolled in the trial. Vandetanib demon- strated activity (Table 1), but grade 3 or 4 adverse events were common. The most common grade 3 or 4 adverse events were hypertension (n = 11, 58%), diarrhea (n = 2, 11%), rash (n = 3, 16%), dry skin (n = 1, 5%), and QT prolongation (n = 2, 11%). Four of the 19 patients discontinued ther- apy related to adverse events. A second trial by Lee et al. recruited patients with NSCLC without EGFR mutations or ALK rearrangements, and screened patients with FISH; FISH RET positive results were confirmed with IHC, RT–PCR, or NGS.37 Of the 315 patients recruited, 26 patients (8.3%) were positive by FISH testing, and 18patients were enrolled in the study. Vandetanib demonstrated activity (Table 1), and the grade 3 adverse events observed were hypertension (n = 3, 17%) QT prolongation (n = 2, 11%) and elevatedliver tests (n = 1, 6%).Cabozantinib is a multi-targeted TKI with activity against RET as well as ROS1, MET, VEGF receptors, AXL, TIE2 and KIT.

A single arm phase II trial enrolled patients with RET rear- rangement detected on FISH or NGS testing (Table 1).38 The most common grade 3 treat- ment-related adverse events were asymptomatic elevation of lipase (n = 4, 15%), increased alanine aminotransferase (ALT) (n = 2, 8%) increased aspartate aminotransferase (AST) (n = 2, 8%), thrombocytopenia (n = 2, 8%), and hypophos- phatemia (n = 2, 8%). Nineteen patients (73%) required a dose reduction related to an adverse event. Lenvatinib is a multi-targeted TKI with activity against VEGF receptors, fibroblast growth factor receptors, platelet-derived growth factor receptor alpha, RET and KIT. Tumor samples underwent tumor testing at the central laboratory using NGS, and 536 patients were screened and 25 (5%) were eligible. A single arm phase II trial revealed modest activity (Table 1).39 The most common grade ⩾ 3 treatment-emergent adverse events were hypertension (n = 14, 56%), hypona- tremia (n = 5, 20%), proteinuria (n = 4, 16%),pneumonia (n = 4, 16%), and nausea (n = 3, 12%).Six patients experienced treatment-related adverse events leading to treatment discontinuation.RXDX-105 differs from the other multi-targeted TKIs because it has RET activity, and limited activity against the VEGF receptors.40 A phase I/Ib trial investigated the activity of RXDX-105 in patients who were RET TKI naive and in patients previously treated with multi-targeted TKI. In the RET TKI naive patients, the drug revealed modest activity (Table 1). A subset analysis revealed that the response rate varied by fusion partner. The ORR in the RET-KIF5B rearrange- ment subset, the most common rearrangement, was 0% (0/20), and the RET-non-KIF5B rear- rangement subset was 67% (6/9) (p < 0.001). The reason for the difference in the ORR based on the type of RET rearrangement is unclear.40 In the patients who were previously treated with a multi-targeted (vandetanib or cabozantinib) TKI the response rate was 0% (0/9). The median duration of response, PFS and overall survival (OS) are not available. The most common adverse events at the recommend phase II dose (n = 74)were rash (n = 7, 10%), elevated ALT (n = 6, 8%), hypophosphatemia (n = 5, 7%), and elevated AST (n = 4, 5%). At this time no further trials are planned with this agent.42A case series of four patients revealed activity of alec- tinib, an ALK TKI, at a dose of 600 or 900 mg twice daily.43 The activity of alectinib was investigated in patients who were RET TKI treatment naive in a phase I/II trial. The phase II dose was 450 mg BID, and alectinib had limited activity (Table 1).41 The grade ⩾ 3 adverse events at the 450 mg BID dose level were increased creatinine phosphokinase, increased bilirubin, diarrhea, hyponatremia, neutro- penia, and pneumonitis (all in one patient, 4%).Sorafenib was investigated in a study which enrolled three patients, and no responses were observed, and in the global registry two patients were treated and the best response was stable disease.25,44 Ponatinib is a multi-targeted TKI with RET activ- ity, and demonstrated activity in xenograft models of RET rearrangements.45 In the global registry two patients were treated with ponatinib and the best response was stable disease, and a clinical trial was initiated and enrolled nine patients, and the results are not available at this time.25Additional clinical information is available from a global registry of RET + NSCLC patients treated outside a clinical trial.25 Patients were required to have a RET rearrangement based on RT–PCR, FISH, NGS and the individual patient data were collected. This registry collected data on 165 RET + NSCLC from 29 centers. The majority of patients were never smokers (63%), had adenocar- cinoma (98%), and KIF5B-RET rearrangement (72%). Fifty-three TKI naive patients received a multi-targeted TKI as part of their therapy. The best response rate with cabozantinib (n = 21), van- detanib (n = 11) and sunitinib (n = 10) was 37% (7 of 19 evaluable patients), 18% (2 of 11 evalua-ble patients), and 22%, (2 of 9 evaluable patients), respectively. As a formal response assessment was not possible, an ORR was not calculated. No dif- ferences in response or PFS-related RET rear- rangement type were observed (KIF5B versus other partner). In all patients the median PFS and OS were 2.3 and 6.8 months, respectively. The median PFS observed with cabozantinib, vandetanib, and sunitinib was 3.6 months, 2.9 months, and2.2 months, respectively and the median OS observed with cabozantinib, vandetanib, and suni- tinib was 4.9 months, 10.2 months, and 6.8 months, respectively.In summary, the multi-targeted TKIs demon- strated modest activity with poor tolerability due to off-targeted activity. With the exception of RXDX-105 the specific RET rearrangement has not been associated with efficacy. However, these analyses were subset analyses, and the number of patients with non-KIF5B rearrangements and benefiting from the therapy were small. Consequently, the relationship between the spe- cific RET rearrangement and efficacy cannot be definitively determined.Chemotherapy and chemotherapy and immunotherapy combinationsMany patients with RET + NSCLC are initially treated with first-line chemotherapy because molec- ular testing for RET rearrangements is not routine at the time of diagnosis, and there is not an estab- lished targeted therapy for RET + NSCLC. Patients with RET + NSCLC have clinical charac- teristics associated with better outcomes with chem- otherapy. A retrospective study of pemetrexed alone (n = 1) or in combination (n = 17) in patients with RET + NSCLC (n = 18) revealed an ORR of 45% (5 of 11 patients) and a median PFS of 19 months.46 In the global registry, 84 patients received platinum- based chemotherapy in the first line setting, 65 were evaluable for response, and the best response was 51% (33 of 65 patients). The median PFS was 7.8 months, and median OS was 24.8 months. In the subset of 66 patients who received platinum and pemetrexed the best response was 49% (27 of 55 patients), the median PFS was 6.4 months, and the median OS was 23.6 months.Single agent immunotherapy is available as a first- line option for patients with a programmed death- ligand 1 (PD-L1) expression ⩾1% or as second line therapy. Unfortunately, we do not have the out- comes specific to RET + NSCLC from these trials because RET status was not prospectively collected. A retrospective registry included 16 patients (3%) with RET rearrangements who were treated with single agent immunotherapy in the second or third line.47 The best response in the RET + NSCLC cohort was 6% (1 of 16 patients), and the median PFS was 2.1 months suggesting limited activity of single agent immunotherapy. Six patients under- went testing for PD-L1 expression and the median percentage of cells that expressed PD-L1 was 26, and the small sample size and low response rate limit the analysis of PD-L1 as a predictive biomarker for response in RET + NSCLC. A retrospective study identified 74 patients with RET + NSCLC.48Twenty-six patients had sufficient tumor samples available for PD-L1 testing, and PD-L1 expression was 0%, 1–49%, and ⩾50% in 58% (n = 15), 23%(n = 6), and 19% (n = 5), respectively. Forty-four patients had sufficient tumor for tumor mutational burden (TMB) testing and the median TMB was1.75 mutations/Mb. Fourteen patients were treated with immunotherapy, and 13 patients were evaluable for responses and no responses were observed. The median PFS was 3.4 months, and no associa- tion with PD-L1 or TMB and PFS was observed. These data suggest limited efficacy for single agent immunotherapy in RET + NSCLC.The combination chemotherapy and immuno- therapy is a first-line option without regard to PD-L1 status.49–51 Patients with RET + NSCLC were not excluded from the phase III trial of car- boplatin, pemetrexed and pembrolizumab as patient with EGFR mutant and ALK rearranged NSCLC were, so patients can receive this combi- nation. Patients with RET + NSCLC have many of the clinical characteristics (younger age and history of never smoking) and tumor characteris- tics (adenocarcinoma and single oncogenic driver) of patients with EGFR mutant NSCLC and ALK rearranged NSCLC. Patients with an EGFR mutation or ALK rearrangement who were previously treated with TKIs were eligible for the phase III trial of carboplatin, paclitaxel, bevacizumab (the standard arm) compared to carboplatin, paclitaxel, bevacizumab, atezoli- zumab or carboplatin, paclitaxel and atezoli- zumab. In a retrospective subset analysis patients with EGFR mutations (n = 124), patients treated with carboplatin, paclitaxel, bevacizumab and atezolizumab compared to carboplatin, pacli- taxel, bevacizumab experienced a numerically higher response rate, longer PFS and longer OS.52 The outcomes of patients with an EGFR mutation treated with carboplatin, paclitaxel, atezolizumab compared to carboplatin, paclitaxel and bevacizumab were similar. Forty patients with a ALK rearrangement were enrolled in the three treatment arms, and the small number of patients in each arm limited the interpretation of the efficacy results. The results of this retrospec- tive subset analysis suggest the combination of carboplatin, paclitaxel, bevacizumab and atezoli- zumab may be the preferred chemotherapy and immunotherapy combination for patients with oncogenic driver alterations with disease progres- sion of TKI. However, we do not have specific efficacy data on patients with RET + NSCLC from this trial.The struggles with the multi-targeted TKIs spurred the development of TKIs which were more specific and potent RET inhibitors. Selpercatinib (LOXO- 292) and pralsetinib (BLU-667) are highly selective for the RET tyrosine kinase, have activity against mul- tiple RET rearrangements, and have central nervous system (CNS) activity in mouse models.53–55 These agents also have activity against acquired RET gate- keeper resistance mutations that have been observed after multi-targeted TKIs.53,56–58 Importantly, in pre- clinical studies mechanisms of resistance other than RET resistance mutations have been observed, including the development of the NRAS mutation and increased expression of the EGFR and AXL.59 Thus, these agents may not have activity against RET independent mechanisms of acquired resistance.Selpercatinib was investigated in a phase I/II trial, which enrolled 253 patients with RET + NSCLC, and the primary analysis set is 105 patients who received prior platinum-based therapy.60 The median age in the primary analysis set was 61 years (range 23–81), 103 patients (98%) had a perfor-mance status of 0 or 1, 50 patients (48%) had received a multi-targeted TKIs previously, and 37 patients (35%) had brain metastases. The most common RET fusion partner was KIF5B in 85 patients (59%), followed by CCDC6 in 32 patients (22%). The recommend dose for phase II trials was 160 mg twice daily. The efficacy results for the primary analysis set and the subset of patients who were treatment naive (n = 39) revealed clini- cally significant activity (Table 2). The safety pro- file included data from 531 patients treated, and the grade 3 or 4 treatment-related adverse events observed in ⩾5% were hypertension (8% grade 3,<1% grade 4), increased AST (4% grade 3, 1%grade 4), and increased ALT (6% grade 3, 1%grade 4). Nine patients (1.7%) discontinued ther- apy due to treatment-related adverse events. Preliminary evidence suggests that selpercatinib has activity in patients with acquired resistance mutations from previous therapies.Pralsetinib was investigated in a phase I/II trial, which enrolled patients with RET + NSCLC who were treated with prior platinum-based therapy and were platinum naive.55 The recommended dose for phase II trials was 400mg daily. At the time of the analysis 120 patients with RET + NSCLC were included, and 91 patients had received previous therapy with platinum-based therapy. The median age was 60years (range 28–87), 46 patients (38%) had performance status of 1, 21 patients (18%) had received a previous multi-targeted TKI, and 48 patients (40%) had brain metastases. The most common RET fusion partner was KIF5B in 79 patients (66%), followed by CCDC6 in 16 patients (13%). Among the 48 patients evalua- ble for response the ORR was 58% [95% confidence interval (CI) 43–72%], and among the 35 patients who had received prior platinum-based therapy the ORR was 60% (95% CI 42–76%). Seven of the nine patients with measurable CNS disease experienced a decrease in the size of the brain metastases. The median duration of response data is not available. The grade ⩾3 treatment-related adverse events observed in ⩾5% of patients were neutropenia (n=16, 13%), and hypertension (n=12, 10%). Eight patients (7%) discontinued due treatment-related adverse events (pneumonitis, respiratory distress/hypoxia, mucositis/colitis, myelosuppression, gait disturbance, and anemia).In addition to these agents several other RET- specific TKIs are in development. TPX-0046 is a selective RET/SRC inhibitor which has revealed preclinical activity in a RET-driven cell line andpatient-derived xenograft tumor models, and a clinical trial has been initiated.61 BOS172738 is a novel RET inhibitor and a phase I trial has been initiated with this agent.62The development of selpercatinib and pralsetinib represents a fundamental change in the treatment of RET + NSCLC as these agents are highly active and well tolerated. The first critical step will be to increase the rate of molecular testing for RET rearrangements, ideally at the time of diag- nosis. Clinicians can use RET FISH testing, which represents a single marker testing strategy or as a panel using NGS, by either tumor testing or ctDNA. If testing panels are not available then RET testing could be performed in patients who have tested negative for EGFR, ALK, ROS1 and BRAF V600E because the prevalence RET rear- rangements is higher in this clinical situation. Currently, if a RET rearrangement is identified referral to a clinical trial of selpercatinib and pral- setinib is the preferred option, and if trials with these agents are not available a referral to another RET-specific inhibitor trial is an option (Table 3).63 Many of these trials enroll patients with dis- eases other than NSCLC, and include multiplecohorts depending on disease and previous treat- ment. Once selpercatinib and pralsetinib become available outside of clinical trials they could be considered for first line therapy. The multi-tar- geted TKIs remain a potential option if the next generation RET TKIs are not available.The optimal treatment at the time of disease progres- sion remains ambiguous. Carboplatin and peme- trexed has demonstrated activity and some clinicians may opt to use chemotherapy alone since RET + NSCLC patients have clinical characteristics associated with less benefit from immunotherapy. Patients with RET + NSCLC were not excluded from the trial of carboplatin, pemetrexed and pem- brolizumab so this combination is also an option. Some clinicians may extrapolate from the results of the EGFR mutation subset analysis, and preferen- tially use the combination of carboplatin, paclitaxel, bevacizumab, and atezolizumab. Most likely all three options will be used, and the selection of therapy will be based on physician and patient preference. One inherent challenge with this rare molecular subset is that performing prospective phase III tri- als is difficult and time consuming, and once anagent has shown promising activity patients and physicians may not have equipoise. In EGFR mutant and ALK rearranged NSCLC phase III tri- als demonstrated the superiority of targeted ther- apy compared to platinum-based chemotherapy. A phase III trial of pralsetinib compared to chemo- therapy with carboplatin and pemetrexed alone or with pembrolizumab, has been initiated.63 Global registries may have a critical role in this situation because they can be designed to collect specific clinical data, and participation would be less labor intensive than participation in a clinical trial. These registries would provide a better assessment of out- comes than retrospective studies.With the use of more potent RET inhibitors it is inevitable that mechanisms of acquired resistance will develop, and based on other targeted thera- pies the prevalence of resistance mutations will increase.64 Some patients may have upregulation of bypass tracks and develop ‘RET independent’ mechanisms of resistance. Biopsies or ctDNA at the time of disease progression will be important to assess the mechanisms of resistance, and develop the next generation Pralsetinib RET TKIs.