A Phase I/Ib Trial of the VEGFR-Sparing Multikinase RET Inhibitor RXDX-105

RET is an established proto-oncogene (1). Genomic alterations involving RET, such as fusions and activating point mutations, are oncogenic drivers of a variety of tumors. RET fusions were initially identified in papillary thyroid carcinomas, and later in non–small cell lung cancers (NSCLC) where these alterations are found in 1% to 2% of unselected cases (2). An increase in comprehensive molecular profiling subsequently resulted in the identification of RET fusions at lower frequencies in other solid tumors such as gastrointestinal malignancies, including colorectal cancer (3, 4). Activating RET point mutations such as RET^M918T are detected in medullary thyroid cancers (2).

RXDX-105 is an orally bioavailable, VEGFR-sparing, multikinase inhibitor with activity against RET. It inhibits wild-type RET, select mutant proteins (e.g., RET^M918T), and chimeric oncoproteins generated by RET fusion (KIF5B–RET, CCDC6–RET, NCOA4–RET, and PRKAR1A–RET). The drug is active in xenografts harboring the most common fusions in NSCLC (KIF5B–RET) and thyroid cancers (CCDC6–RET and NCOA4–RET). RXDX-105 is also active against wild-type BRAF and BRAF^V600E, albeit to a lesser degree compared with RET (5).

RXDX-105 was selected for its potent preclinical activity against RET and its ability to spare VEGFR2/KDR and VEGFR1/FLT compared with other multikinase RET inhibitors such as cabozantinib, vandetanib, and lenvatinib (5–8). The inhibition of angiogenesis by other multikinase inhibitors with anti-RET activity can result in clinically significant side effects that require dose modification and/or discontinuation, resulting in suboptimal RET target inhibition (1). The VEGFR-sparing nature of RXDX-105 was hypothesized to allow the clinical titration of the drug to a dose that would more optimally inhibit RET in comparison with other multikinase inhibitors, thus potentially improving outcomes for patients with RET fusion–positive or RET-mutant cancers.

This article presents the results of a phase I/Ib trial of RXDX-105 and describes the safety and antitumor activity of this agent. Although the drug was tested in a variety of solid tumors initially, including those harboring BRAF alterations, later-phase testing focused on RET fusion–positive lung cancers.

A total of 152 patients were treated with RXDX-105; 55 were treated in the phase I dose-escalation portion of the study, and 97 were treated in the phase Ib dose-expansion portion of the study (Table 1 and Supplementary Fig. S1). The median age was 63 (range, 27–90) years, and the majority (89%) of patients were pretreated and received one or more prior systemic therapies. The most common tumor type was NSCLC (54%), followed by colorectal cancer (18%) and thyroid cancer (11%). All patients had an Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1 at study entry. Patient disposition is summarized in Supplementary Table S1.

In the dose-escalation phase of the trial, patients were treated in nine dose-level cohorts (Supplementary Table S2). In the first seven cohorts, RXDX-105 was administered at doses that ranged from 20 mg daily up to a dose of 275 mg daily without dietary restrictions. In the last two cohorts, RXDX-105 was administered at 275 mg daily and 350 mg daily in the fed state.

In patients treated with any dose of RXDX-105 (n = 152), the most common treatment-related adverse events observed in more than 10% of patients were fatigue (25%), diarrhea (24%), hypophosphatemia (18%), maculopapular rash (18%), nonmaculopapular rash (17%), nausea (15%), elevated alanine (14%) or aspartate (13%) aminotransferase, muscle spasms (13%), decreased appetite (11%), and vomiting (10%). These are summarized in Table 2.

The most common grade 3 or higher treatment-related adverse events observed in ≥5% of patients were as follows: hypophosphatemia (9%), elevated alanine aminotransferase (8%), maculopapular rash (7%), elevated aspartate aminotransferase (5%), and diarrhea (5%). No QT interval/corrected QT interval prolongation was observed. Drug-related toxicities commonly associated with VEGFR2/KDR inhibition such as hypertension (3%) and proteinuria (1%) of any grade were uncommon.

In the dose-escalation portion, four dose-limiting toxicities (DLT) were reported. These included rash (grade 3, 200 mg daily), fatigue (grade 3, 275 mg daily fasted), diarrhea (grade 3, 275 mg fed daily), and hyperbilirubinemia (grade 3, at 350 mg fed daily). These toxicities resolved after treatment interruption and dose reduction.

Dose reduction was required in 28% (n = 43/152) of patients in the safety data set and 31% (n = 19/62) of patients who were treated at 275 mg daily in the phase Ib portion. The most common adverse events resulting in dose reduction in the safety data set of 152 patients were liver function test abnormalities (increased alanine/aspartate aminotransferase or bilirubin) in 9% (n = 13/152) and cutaneous disorders (maculopapular rash, nonmaculopapular rash, generalized rash, acneiform dermatitis, or skin discoloration) in 8% (n = 12/152). Dose discontinuation secondary to a treatment-emergent adverse event occurred in 16% (n = 25/152) of patients in the safety data set, and 13% (n = 8/62) of patients treated at 275 mg daily in the phase Ib portion.

Three cases of treatment-related cutaneous hypersensitivity to RXDX-105 were observed. All three had select features consistent with a differential diagnosis of drug rash with eosinophilia and systemic symptoms (DRESS). A 64-year-old woman with a metastatic RET fusion–positive lung cancer was previously treated with chemoradiation, durvalumab, stereotactic radiosurgery for brain metastases, pemetrexed and bevacizumab, and atezolizumab and an adenosine-A2A receptor antagonist. She developed a full-body rash 12 days after the initiation of RXDX-105 at 275 mg daily. This required hospitalization and steroid administration. Her course was also marked by the development of a transaminitis. Peripheral eosinophilia was not noted. RXDX-105 was discontinued, and the patient was taken off study. These side effects thereafter resolved.

A 58-year-old woman with a BRAF^D594G-mutant lung cancer was previously treated with cisplatin and pemetrexed, atezolizumab and cobimetinib, and gemcitabine and vinorelbine. She also developed a full-body rash, bilateral conjunctivits, facial swelling, fevers, hypotension, and thrombocytopenia 12 days after starting RXDX-105 treatment at 275 mg daily. These adverse events likewise resolved after study drug discontinuation and steroids.

A 71-year-old woman with a metastatic RET fusion–positive lung cancer was previously treated with carboplatin, pemetrexed, and bevacizumab, palliative radiation to the lung, rib, spine, and brain, and finally pembrolizumab. Twelve days after RXDX-105 was initiated at 350 mg daily, she developed a full-body erythematous maculopapular rash as shown in Supplementary Fig. S2. This was accompanied by facial swelling, oral mucositis, and hoarseness from suspected vocal cord edema. She was hospitalized, study drug was held, and steroids were initiated.

Despite this, the patient developed fulminant multiorgan dysfunction with respiratory failure requiring intubation, kidney failure, transaminitis, pancytopenia, and atrial fibrillation. A bronchoscopy revealed bleeding consistent with diffuse alveolar hemorrhage. A skin biopsy revealed interface dermatitis, lymphocytic exocytosis, and perivascular eosinophils, although eosinophilia in the peripheral blood was not observed. She died secondary to these complications. In addition to RXDX-105, her medication list at study entry included drugs that have also been associated with DRESS (etoricoxib, pregabalin, esomeprazole, and tramadol).

The steady-state pharmacokinetics (PK) of RXDX-105 following once-daily dosing at various dose levels are depicted in Fig. 1. Dose-dependent increases in RXDX-105 plasma exposures were observed. For the fed cohorts, patients were instructed to take RXDX-105 with breakfast (including solid food) or within 30 minutes after eating breakfast. Steady-state exposures of RXDX-105 were only slightly higher in the fed versus the food-uncontrolled states, suggesting the absence of a substantial food effect.

Based on an analysis of the toxicity profile and PK of RXDX-105 at all dose levels, a recommended phase II dose (RP2D) of 275 mg daily in the fed state was chosen. At steady-state levels (cycle 1, day 15) on this dose, the median T_max was 4 hours. The estimated mean effective t_1/2 was 45 hours. The mean C_max was 3,890 ng/mL [47.7% coefficient of variation (CV)]. The AUC_0–24 (day 15 AUC_0–24/day 1 AUC_0–24) was 69,600 ng·h/mL (47.7% CV). The accumulation ratio was 3.19 (54.6% CV).

At steady-state levels, the mean plasma concentration at the RP2D was about 2-fold above the target threshold for effective RET inhibition (1,500 ng/mL, estimated based on data generated in RET fusion–containing patient-derived xenograft models). This calculated value was presumed to apply to both wild-type RET and select RET chimeric oncoproteins, extrapolating from previously generated biochemical data that showed similar activity for both (i.e., IC₅₀ values of 0.33 nmol/L for wild-type RET and 0.33 nmol/L for CCDC6–RET). Of note, this target threshold was not expected to effectively cover select RET mutations. Specifically, RXDX-105 was not substantially active against the V804M and V804L gatekeeper substitutions preclinically; the biochemical IC₅₀ values of 266 nmol/L for RET^V804M and 319 nmol/L for RET^V804L were approximately 1,000-fold higher than 0.33 nmol/L for wild-type RET.

Steady-state plasma concentrations were also above the calculated threshold for effective BRAF inhibition (>2,500 ng/mL, estimated from a nude mouse model bearing A375 human melanoma BRAF^V600E-mutant tumor xenografts). Notably, the calculated threshold for BRAF inhibition was much higher than the calculated threshold for RET inhibition. This suggests that RXDX-105 was poised to more effectively target RET compared with BRAF in the clinic.

Finally, a wide therapeutic window between calculated RET inhibition and the much higher threshold for the inhibition of angiogenesis was observed. Plasma concentrations in patients were approximately one third of the estimated PK threshold for VEGFR2/KDR inhibition (∼10,000 ng/mL), estimated from the in vitro IC₅₀ of RXDX-105 for VEGFR2/KDR with correction for protein binding and tissue distribution.

Early signals of antitumor activity were observed during the phase I dose-escalation portion of the study. In the 55 patients treated with RXDX-105 in this phase, the best overall response to RXDX-105 was as follows: 0 (0%) complete responses, 2 (4%) partial responses, 20 (36%) stable disease, 22 (40%) progressive disease, and 11 (20%) unevaluable.

The two confirmed partial responses were observed in a patient with medullary thyroid cancer with a RET^M918T mutation (50% tumor regression) and in a patient with NSCLC with a KRAS^G12C mutation (40% tumor regression). The exact mechanism driving response in the latter patient whose tumor did not harbor a RET or BRAF alteration remains unclear. Tumor regression was also observed in a patient with an ovarian cancer harboring a BRAF^V600E mutation (26% reduction) and in a patient with an NSCLC harboring a BRAF^D594G mutation (28% reduction). Additionally, clinical benefit was noted in 2 of 4 patients with squamous NSCLC. Stable disease for more than 6 months was achieved in both cases, with a 27% reduction in tumor burden observed in one patient.

The phase Ib portion of this study was designed with this activity in mind (Supplementary Fig. S3). Eight cohorts of patients were treated with RXDX-105. Six cohorts were molecularly enriched, and two were enriched by histology: (i) tyrosine kinase inhibitor (TKI)–naïve RET fusion–positive lung cancers, (ii) TKI pretreated RET fusion–positive lung cancers, (iii) TKI-naïve RET-altered non-lung solid tumors, (iv) TKI-naïve BRAF^V600E-mutant lung cancers, (v) TKI-naïve BRAF^V600E-mutant colorectal cancers, (vi) BRAF^V600E-mutant non-lung and non-melanoma solid tumors, (vii) squamous cell lung cancers, and (viii) other cancers.

The activity of RXDX-105 in each of these cohorts is summarized in Table 3. In RET fusion–positive lung cancers, no responses were observed in 9 patients who previously received a RET inhibitor. The multikinase RET inhibitors that patients received prior to RXDX-105 included cabozantinib and vandetanib and are listed in Supplementary Table S3. No patient received a selective RET inhibitor such as LOXO-292 or BLU-667 prior to RXDX-105. In all 9 patients, disease progression occurred within the first two to four treatment cycles. One complete response was achieved in a RET inhibitor–naïve patient with a colorectal cancer that harbored a CCDC6–RET fusion. This patient was treated in a separate cohort (the non–molecularly enriched “other cancers” cohort). Response was not observed in BRAF-mutant cancers. This included 7 patients with BRAF^V600E-mutant NSCLC and 9 patients with BRAF^V600E-mutant colorectal cancers. No additional responses were observed in squamous cell lung cancers.

RXDX-105 was most active in patients with RET inhibitor–naïve RET fusion–positive lung cancers. A total of 31 patients were treated in this cohort. The objective response rate (ORR) was 19% (95% CI, 8%–38%, n = 6/31). No complete responses were observed. Confirmed partial responses were observed in 6 patients (19%), stable disease in 12 patients (39%), and progression of disease in 10 patients (32%). Therapy was discontinued in 2 patients for drug-related toxicity, and 1 patient had a noncomplete response and nonprogression (non-CR/non-PD). A waterfall plot of best objective response to RXDX-105 in 27 evaluable patients is shown in Fig. 2. The median duration of response was not reached (range, 5 to 18+ months); the median follow-up was 21.7 months. In all RET inhibitor–naïve patients with RET fusion–positive NSCLCs, response to therapy occurred early, after 4 to 8 weeks on treatment, and was durable in several patients, as shown in Fig. 3.

Although 14% (n = 21/152) of patients in the safety data set had a history of brain metastases, these lesions were all previously treated (untreated brain metastases were exclusionary) and thus the intracranial outcomes of RXDX-105 could not be assessed. At the time of the data cutoff, of the 152 patients treated with RXDX-105 on this trial, 140 patients discontinued therapy for the following reasons: 96 for disease progression, 23 for toxicity, 11 after consent withdrawal, 5 for death, and 5 for other reasons (e.g., investigator discretion for protocol noncompliance). Twelve patients remained on therapy with RXDX-105.

In an analysis of cancers that harbored known upstream partners, the ORR of RXDX-105 was 21% (95% CI, 8%–40%, n = 6/29). A significant difference in activity was noted between RET fusions involving a KIF5B versus a non-KIF5B upstream partner (P < 0.001, Fisher exact test) as shown in Fig. 2. Although no objective responses were observed in KIF5B–RET-containing tumors, in tumors harboring RET fusions with a non-KIF5B partner, the ORR was 67% (95% CI, 30%–93%, n = 6/9). Despite this difference in response, prolonged disease control was observed in select patients with cancers harboring both KIF5B–RET and non-KIF5B–RET fusions (Fig. 3).

A similar pattern of decreased activity in KIF5B–RET-containing compared with non–KIF5B–RET-containing lung cancers was found when the activity of RXDX-105 was compared with that of the multikinase inhibitors cabozantinib, vandetanib, and lenvatinib by upstream partner as shown in Fig. 4. Data on the activity of cabozantinib (6), vandetanib (7, 8), and lenvatinib (9) were derived from prospective trials of these agents in RET fusion–positive lung cancers. Within each of these trials, the ORRs and/or median progression-free survival (when available) in KIF5B–RET-containing tumors were lower than in non–KIF5B–RET-containing tumors.

To investigate possible reasons for this discrepancy, in vitro data on the activity of RXDX-105 and other multikinase inhibitors (cabozantinib, vandetanib, sitravatinib, and alectinib) were generated. About a 2-fold increase in IC₅₀s was observed in KIF5B–RET-containing compared with non–KIF5B–RET-containing Ba/F3 cell lines, as shown in Supplementary Fig. S4. For RXDX-105, IC₅₀s for CCDC6–RET and NCOA4–RET were 103 and 87 nmol/L, respectively, compared with 190 nmol/L for KIF5B–RET.

A number of multikinase inhibitors with activity against RET have been explored as treatments for RET fusion–positive solid tumors and RET-mutant thyroid cancers (1). These include cabozantinib, vandetanib, lenvatinib, ponatinib, sunitinib, and sorafenib. The goal of the RXDX-105 development program was to surpass the activity of these agents, considering that the relative sparing of VEGFR2/KDR by RXDX-105 might allow titration of the drug's dose to a level where RET would be more optimally inhibited (5).

In this phase I/Ib trial, the activity of RXDX-105 was most notable in patients with RET fusion–positive lung cancers who had not previously received a RET inhibitor. PK analyses revealed good calculated RET target coverage, and the frequency of VEGFR2/KDR-mediated adverse events was lower compared with other multikinase inhibitors with more potent antiangiogenic activity (e.g., hypertension of any grade was 3% with RXDX-105 compared with 33% with cabozantinib; ref. 10). Despite these factors, the overall activity of RXDX-105 did not differ substantially from the activity of other multikinase inhibitors in RET fusion–positive lung cancers. The ORR with RXDX-105 was 19% compared with an ORR of 16% to 53% with cabozantinib, vandetanib, or lenvatinib (1).

Although the activity of RXDX-105 in RET fusion–positive lung cancers was modest, a differential response to RXDX-105 was observed that seemed to be dictated by the gene fusion partner. Specifically, responses were observed only with non-KIF5B upstream partners. An analysis of prior clinical trials of other multikinase inhibitors also showed lower response rates and/or median progression-free survival when KIF5B–RET-containing tumors were compared with non–KIF5B–RET-containing tumors, consistent with the results of this trial (1, 11).

The exact reasons for this discrepancy remain unclear, although several factors are potentially contributory. First, KIF5B–RET may be somewhat more challenging to target with multikinase inhibition in preclinical models. Early in the development of multikinase inhibitors for RET, cell lines containing KIF5B–RET were not widely available. Although the preclinical activity of RXDX-105 did not substantially differ between select non–KIF5B–RET fusions that were initially tested (biochemical IC₅₀ values of 0.33 nmol/L for CCDC6–RET, 0.41 nmol/L for NCOA4–RET, and 0.81 nmol/L for PRKR1A–RET; cell-free kinase assay platform), we subsequently tested RXDX-105 and other multikinase inhibitors in Ba/F3 cells harboring KIF5B–RET, CCDC6–RET, and NCOA4–RET. In these experiments, we observed at least a 2-fold increase in cellular IC₅₀ values in KIF5B–RET-containing cell lines compared with CCDC6–RET- or NCOA4–RET-containingcell lines with RXDX-105; a shift was likewise observed with other multikinase inhibitors.

It is unclear if this fold change is responsible for the differential activity observed in the clinic; however, the plasma exposures of multikinase RET inhibitors have been shown to be suboptimal at effectively inhibiting RET, especially considering that dose modifications are frequent secondary to treatment-related toxicities with select drugs (1). If RET target coverage is already suboptimal, even a small window between the activity of multikinase inhibitors against KIF5B–RET and non-KIF5B–RET might meaningfully affect outcomes. Furthermore, although independent investigators have also shown suboptimal activity of RXDX-105 and other multikinase agents in KIF5B–RET-containing models (IC₅₀ values of 129 nmol/L for RXDX-105 and 833 nmol/L for vandetanib for the inhibition of RET autophosphorylation in Ba/F3 cells; ref. 12), other series have not observed a difference compared with non–KIF5B–RET-containing models (13). This underscores the need to further explore the fusion-specific activity of various RET inhibitors in the laboratory.

Second, KIF5B is postulated to result in a high level of RET expression in KIF5B–RET-containing tumors (i.e., 30-fold higher RET expression compared with noncancerous lung tissues; ref. 14). In contrast, other upstream partners such as CCDC6 and NCOA4 are thought to result in lower levels of expression in RET fusion–positive tumors harboring these partners (15). This implies that KIF5B–RET-containing tumors may contain higher total levels of chimeric RET oncoproteins that need to be overcome by targeted therapy. Finally, signaling and functional differences between RET fusions of different upstream partner types have been identified. This was initially demonstrated in Drosophila models with CCDC6–RET and NCOA4–RET when these fusion genes were expressed in the epithelia during development (16). A study also using Drosophila models in addition to engineered human bronchial epithelial cells further revealed that the kinesin domain of KIF5B and the kinase domain of RET act together to form a multikinase (RET/EGFR/FGFR/SRC) signaling hub (17). This suggests that, rather than targeting RET alone, multiple kinase components of the KIF5B–RET signaling hub may need to be simultaneously targeted for optimal effect. In our Ba/F3 models, for example, single-agent MEK inhibition (trametinib) and single-agent pan-PI3K/mTOR inhibition (omipalisib) were active in KIF5B–RET-containing cells, raising the question of the utility of combinatorial therapy in RET fusion–positive lung cancers, as is already being explored in the clinic (18).

Beyond the activity of RXDX-105, the drug's safety profile is informative for the design and development of kinase inhibitors with activity against RET and BRAF. Although the most common drug-related adverse events, such as grade 1 or 2 rash, diarrhea, fatigue, and liver function test abnormalities, were not uncharacteristic of inhibitors in this class, several other features were unique. In addition to the relative VEGFR sparing described above, drug-induced hypersensitivity syndrome with features of DRESS was identified as a rare but important adverse event for patients treated with RXDX-105. Notably, in all three cases presented in this series, rash occurred within the first 2 weeks of dosing and patients received prior treatment with an immune-checkpoint inhibitor. DRESS was previously observed with other agents that inhibit BRAF (e.g., vemurafenib), especially after prior immunotherapy (19). This raises the possibility that this idiosyncratic reaction represents a class effect of BRAF inhibitors that can be potentiated by prior immune-checkpoint inhibition. In general, an increase in drug-related toxicity has also been observed after immunotherapy with crizotinib in ALK fusion–positive cancers (20) and EGFR tyrosine kinase inhibitors in EGFR-mutant lung cancers (21).

Finally, newer RET inhibitors, specifically those that are more selective for RET, such as LOXO-292 (22) and BLU-667 (12), are currently in clinical testing. Preliminary data have shown that these drugs have increased activity and improved tolerability compared with multikinase inhibitors, including RXDX-105 (23). Although these drugs are likely to ultimately replace multikinase inhibition as the first line of targeted therapy in TKI-naïve patients, multikinase inhibitors remain viable treatment options after progression of disease on a selective RET inhibitor. Strategies to increase the activity of multikinase inhibitors should thus continue to be pursued, particularly for tumors harboring KIF5B–RET.

In conclusion, the multitarget, VEGFR-sparing RET inhibitor RXDX-105 is active in patients with RET inhibitor–naïve RET fusion–positive lung cancers, although overall clinical outcomes were not different from those of prior multikinase inhibitors. The activity of RXDX-105 was largely observed in non–KIF5B–RET-containing as opposed to KIF5B–RET-containing tumors, consistent with other trials of multikinase inhibitors. This exposes a potential biological difference between chimeric RET oncoproteins that is dictated by the upstream fusion partner and requires further exploration. Finally, RXDX-105 has a unique safety profile that is characterized by a lower frequency of VEGFR2/KDR-related adverse events, and rare but substantial cases of cutaneous hypersensitivity that may be mediated by BRAF inhibition, especially in the wake of prior immunotherapy treatment.

RXDX-105-01 (NCT01877811) was a first-in-human, multicenter, open-label, phase I dose-escalation and phase Ib dose-expansion study of RXDX-105, an oral VEGFR-sparing multikinase inhibitor with potent activity against RET and BRAF. The phase I portion of the study enrolled patients with any solid tumor. The phase Ib portion was designed as a basket study and enrolled patients with solid tumors, including those who harbored RET or BRAF alterations. Written informed consent was obtained from patients. This study was conducted in accordance with recognized ethical guidelines: the Declaration of Helsinki, CIOMS, the Belmont Report, and U.S. Common Rule. The protocol was approved by institutional review boards at each institution. Data were anonymized to protect the identities of patients involved in the research.

This phase I study followed a traditional 3 + 3 design for the dose-escalation portion. Dose levels were expanded on the basis of the occurrence of DLTs during the first cycle. The primary objective of this portion was to determine the maximum tolerated dose or recommended phase II dose of RXDX-105. The phase Ib portion was a dose-expansion phase in patients with advanced solid tumors with specific molecular alterations of interest or histologies. Treatment in the phase Ib portion was administered at the RP2D.

Patients received once-daily oral doses of RXDX-105 in 28-day treatment cycles until disease progression, unacceptable toxicity, withdrawal of consent, or protocol-specified parameters. Patients for whom there was no available approved and/or alternative therapy were allowed to continue treatment with RXDX-105 post-progression if the patient was felt to be deriving clinical benefit. Dose modification was permitted via a prescribed algorithm. In the phase I dose-escalation portion, patients who required dose reductions were treated at a prior dose cohort deemed to be safe. In the phase Ib portion, a maximum of two dose reductions (∼25% decrease per dose reduction) was permitted.

In the phase I dose-escalation portion, all patients had histologically or cytologically confirmed evidence of an advanced solid tumor for which curative therapy was not available, ECOG ≤ 1, any number of prior therapies, were at least 18 years of age at screening, and had adequate bone marrow, liver, and renal function. In the phase Ib basket study, all patients had histologically or cytologically confirmed advanced solid tumors that harbored a RET fusion or mutation or BRAF mutation, detected by FISH or RNA/DNA-based methods (e.g., next-generation sequencing) performed locally using a lab-developed test or through third-party commercial diagnostic labs in a Clinical Laboratory Improvement Amendments (CLIA) environment or equivalent. A basket for patients whose tumors harbored squamous cell NSCLC was also included. Patients with untreated central nervous system metastases was ineligible.

Safety assessments consisted of monitoring and recording adverse events, measurement of protocol-specified safety laboratory assessments, vital signs, and other protocol-specified tests deemed critical to an evaluation of safety. Safety was assessed from the first dose until 30 days after the last dose of RXDX-105 until resolution or stability of any drug-related toxicity (or until the patient was lost to follow-up or withdrew consent). In the dose-escalation portion, clinical and laboratory assessments were performed at least once weekly during the first cycle of treatment, biweekly in each cycle, and at the end of treatment. In the phase Ib portion, clinical and laboratory assessments were performed on day 1 of each cycle and at the end of treatment. Laboratory assessments included routine hematology and chemistry panels, cortisol analysis, and urinalysis. Twelve-lead single ECGs were performed at baseline, days 1 and 15 of cycles 1 and 2 (predose and approximately 2–4 hours after study drug administration), day 1 of all subsequent cycles (predose), and at the end of treatment. In the phase Ib portion, ECGs were performed predose at baseline, day 1 of each cycle, and at the end of treatment. An ophthalmologic examination was performed at screening and at the end-of-treatment visit, and at any other times when the patient reported vision and/or ocular abnormalities.

Serial blood samples for PK analyses were obtained for all patients at various time points throughout cycle 1. Samples were analyzed to determine RXDX-105 plasma concentrations. Full plasma PK profiles were generated using a validated assay based on high performance liquid chromatography with tandem mass spectrometry. PK analysis for all parameters was performed using Phoenix WinNonlin software version 6.4.0.768 (Pharsight Corp.). Parameters analyzed included maximum observed plasma drug concentration (C_max), time of maximum observed plasma drug concentration (T_max), area under the plasma drug concentration versus time curve from time 0 to the last measurable drug concentration (AUC_0–t), and area under the plasma drug concentration versus time curve from time 0 to 24 hours after study drug administration (AUC_0–24).

CT or magnetic resonance imaging of anatomic sites based on cancer type were performed at baseline, the end of cycle 2, and every 8 weeks thereafter. In the phase Ib portion, all patients had CT scans of the thorax and abdomen as part of tumor assessments. Imaging of the pelvis was also required for patients with colon cancer. Additionally, due to the high frequency of brain metastasis in patients with NSCLC, brain imaging was performed during screening in all patients with NSCLC. Untreated brain metastases rendered patients ineligible. If treated brain metastases were present, brain imaging was performed at each tumor assessment. Brain imaging was not required in all patients, however.

Patients who received at least one dose of RXDX-105 were included in an analysis of safety. Demographics, baseline characteristics, adverse events, vital signs, and clinical laboratory evaluations were summarized with descriptive statistics. Patients with evidence of RET fusion–positive NSCLCs were analyzed as a subset of the safety population for efficacy. Best objective response was derived according to RECIST version 1.1 based on investigator assessment and classified as a complete response, partial response, stable disease, progressive disease, or unevaluable. A confirmed response was defined as a complete or partial response that was confirmed upon repeat imaging ≥4 weeks after initial documentation of response. Separately, the maximal response in measurable disease at any time on study was reported using waterfall plots. The data cutoff date for safety and efficacy analyses was May 2, 2018.

Ba/F3 cell lines were purchased from DSMZ (2016; German Collection of Microorganisms and Cell Culture). Ba/F3 cells were not authenticated. Cell lines were confirmed to be Mycoplasma-free (Biomiga) and were used between 3 and 10 passages. The fusion genes, CCDC6(exon 1)–RET(exon12), NCOA4(exon6)–RET(exon12), and KIF5B(exon15)–RET(exon12), were synthesized at GenScript and cloned into pCDH-CMV-MCS-EF1-Puro plasmid (System Biosciences, Inc.). The corresponding cell lines were generated by transducing Ba/F3 cells lentivirus containing the desired fusion gene. In addition to RXDX-105, four other RET inhibitors (cabozantinib, vandetanib, sitravatinib, and alectinib), a MEK inhibitor (trametinib), and a pan-PI3K/mTOR inhibitor (omipalisib) were tested to determine IC₅₀ values against each fusion-containing cell line.

A. Drilon is a consultant/advisory board member for Ignyta, Genentech, BeiGene, Hengrui Therapeutics, Exelixis, Bayer, Roche, Loxo Oncology, TP Therapeutics, AstraZeneca, Pfizer, Blueprint, Takeda/Ariad, and Helsinn Therapeutics and has received Pocket Oncology royalties from Wolters Kluwer. M. Fakih reports receiving commercial research grants from AstraZeneca, Novartis, and Amgen, has received honoraria from the speakers bureaus of Amgen, Taiho, Genentech, and SIRTEX, and is a consultant/advisory board member for Array, Genentech, Seattle Genetics, and Bayer. D. Morgensztern is a consultant/advisory board member for AbbVie, Bristol-Myers Squibb, Takeda, and PharmaMar. S.V. Liu reports receiving commercial research grants from Genentech, AstraZeneca, Molecular Partners, OncoMed, Pfizer, Threshold, Bayer, Blueprint, Clovis, Corvus, Esanex, Lilly, Lycera, and Merck and is a consultant/advisory board member for AstraZeneca, Bristol-Myers Squibb, Celgene, Genentech, Heron, Lilly, Pfizer, Regeneron, Taiho, and Takeda. L. Bazhenova is a consultant/advisory board member for Genentech. C.P. Rodriguez reports receiving commercial research grants from Bristol-Myers Squibb, AstraZeneca, Ignyta, Merck, Acerta, Genentech, Pharmacyclics, Portola, and Seattle Genetics and is a consultant/advisory board member for AstraZeneca and Merck. R.C. Doebele reports receiving a commercial research grant from Ignyta, has received honoraria from the speakers bureau of Guardant Health, has ownership interest (including stock, patents, etc.) in Rain Therapeutics, is a consultant/advisory board member for Rain Therapeutics, Ignyta, Loxo, Pfizer, Torvagene, Ariad, AstraZeneca, Takeda, Spectrum Pharmaceuticals, and Bayer, and has received other remuneration from Abbott Molecular. A. Wozniak reports receiving commercial research support from Boehringer Ingelheim and is a consultant/advisory board member for AstraZeneca, Boehringer Ingelheim, and Takeda. K.L. Reckamp reports receiving commercial research grants from Eisai and Loxo and is a consultant/advisory board member for Boehringer Ingelheim, Euclises, Exelixis, Guardant, Genentech, Loxo, Seattle Genetics, Takeda, and Tesaro. P. Nikolinakos has received honoraria from the speakers bureau of Boehringer Ingelheim and is a consultant/advisory board member for AstraZeneca, Boehringer Ingelheim, Genentech, and Tesaro. Z. Hu is a Scientist II at Ignyta. P.S. Multani is chief medical officer at Ignyta, Inc. M.-J. Ahn has received honoraria from the speakers bureaus of Takeda and AstraZeneca and is a consultant/advisory board member for Takeda, AstraZeneca, Merck, Alpha Pharmaceutical, Bristol-Myers Squibb, and Boehringer Ingelheim. No potential conflicts of interest were disclosed by the other authors.

Conception and design: A. Drilon, M. Fakih, A.J. Olszanski, T. Seery, J.W. Oliver, R. Patel, P.S. Multani

Development of methodology: A. Drilon, J.W. Oliver, R. Patel, P.S. Multani

Acquisition of data (provided animals, acquired and managed patients, provided facilities, etc.): A. Drilon, S. Fu, M.R. Patel, M. Fakih, D. Wang, A.J. Olszanski, D. Morgensztern, S.V. Liu, B.C. Cho, L. Bazhenova, C.P. Rodriguez, R.C. Doebele, A. Wozniak, K.L. Reckamp, T. Seery, P. Nikolinakos, P.S. Multani

Analysis and interpretation of data (e.g., statistical analysis, biostatistics, computational analysis): A. Drilon, S. Fu, M.R. Patel, M. Fakih, S.V. Liu, B.C. Cho, L. Bazhenova, R.C. Doebele, K.L. Reckamp, Z. Hu, J.W. Oliver, D. Trone, R. Patel, P.S. Multani

Writing, review, and/or revision of the manuscript: A. Drilon, S. Fu, M.R. Patel, M. Fakih, D. Wang, A.J. Olszanski, D. Morgensztern, S.V. Liu, B.C. Cho, L. Bazhenova, C.P. Rodriguez, R.C. Doebele, A. Wozniak, K.L. Reckamp, P. Nikolinakos, Z. Hu, J.W. Oliver, D. Trone, R. Patel, P.S. Multani, M.-J. Ahn

Administrative, technical, or material support (i.e., reporting or organizing data, constructing databases): A. Drilon, S.V. Liu, L. Bazhenova, J.W. Oliver, K. McArthur, R. Patel

Study supervision: A. Drilon, S. Fu, M.R. Patel, L. Bazhenova, K.L. Reckamp, P. Nikolinakos, J.W. Oliver, K. McArthur, R. Patel, P.S. Multani, M.-J. Ahn

Other (site principal investigator, participated in discussions on protocol development, amendment, patient enrollment, and treatment along with their safety monitoring): D. Wang

A. Drilon is supported by the NIH award P30 CA008748. This study was sponsored by Ignyta.