Mechanisms of resistance to selective RET tyrosine kinase inhibitors in RET fusion-positive non-small-cell lung cancer.

BACKGROUND: Rearranged during transfection (RET) gene fusions are a validated target in non-small-cell lung cancer (NSCLC). RET-selective inhibitors selpercatinib (LOXO-292) and pralsetinib (BLU-667) recently demonstrated favorable antitumor activity and safety profiles in advanced RET fusion-positive NSCLC, and both have received approval by the US Food and Drug Administration for this indication. Insights into mechanisms of resistance to selective RET inhibitors remain limited. PATIENTS AND METHODS: This study was performed at five institutions. Tissue and/or cell-free DNA was obtained from patients with RET fusion-positive NSCLC after treatment with selpercatinib or pralsetinib and assessed by next-generation sequencing (NGS) or MET FISH. RESULTS: We analyzed a total of 23 post-treatment tissue and/or plasma biopsies from 18 RET fusion-positive patients who received an RET-selective inhibitor (selpercatinib, n = 10; pralsetinib, n = 7; pralsetinib followed by selpercatinib, n = 1, with biopsy after each inhibitor). Three cases had paired tissue and plasma samples, of which one also had two serial resistant tissue specimens. The median progression-free survival on RET inhibitors was 6.3 months [95% confidence interval 3.6-10.8 months]. Acquired RET mutations were identified in two cases (10%), both affecting the RET G810 residue in the kinase solvent front. Three resistant cases (15%) harbored acquired MET amplification without concurrent RET resistance mutations, and one specimen had acquired KRAS amplification. No other canonical driver alterations were identified by NGS. Among 16 resistant tumor specimens, none had evidence of squamous or small-cell histologic transformation. CONCLUSIONS: RET solvent front mutations are a recurrent mechanism of RET inhibitor resistance, although they occurred at a relatively low frequency. The majority of resistance to selective RET inhibition may be driven by RET-independent resistance such as acquired MET or KRAS amplification. Next-generation RET inhibitors with potency against RET resistance mutations and combination strategies are needed to effectively overcome resistance in these patients.

Selective RET kinase inhibition for patients with RET-altered cancers.

Background: Alterations involving the RET kinase are implicated in the pathogenesis of lung, thyroid and other cancers. However, the clinical activity of multikinase inhibitors (MKIs) with anti-RET activity in RET-altered patients appears limited, calling into question the therapeutic potential of targeting RET. LOXO-292 is a selective RET inhibitor designed to inhibit diverse RET fusions, activating mutations and acquired resistance mutations. Patients and methods: Potent anti-RET activity, high selectivity, and central nervous system coverage were confirmed preclinically using a variety of in vitro and in vivo RET-dependent tumor models. Due to clinical urgency, two patients with RET-altered, MKI-resistant cancers were treated with LOXO-292, utilizing rapid dose-titration guided by real-time pharmacokinetic assessments to achieve meaningful clinical exposures safely and rapidly. Results: LOXO-292 demonstrated potent and selective anti-RET activity preclinically against human cancer cell lines harboring endogenous RET gene alterations; cells engineered to express a KIF5B-RET fusion protein -/+ the RET V804M gatekeeper resistance mutation or the common RET activating mutation M918T; and RET-altered human cancer cell line and patient-derived xenografts, including a patient-derived RET fusion-positive xenograft injected orthotopically into the brain. A patient with RET M918T-mutant medullary thyroid cancer metastatic to the liver and an acquired RET V804M gatekeeper resistance mutation, previously treated with six MKI regimens, experienced rapid reductions in tumor calcitonin, CEA and cell-free DNA, resolution of painful hepatomegaly and tumor-related diarrhea and a confirmed tumor response. A second patient with KIF5B-RET fusion-positive lung cancer, acquired resistance to alectinib and symptomatic brain metastases experienced a dramatic response in the brain, and her symptoms resolved. Conclusions: These results provide proof-of-concept of the clinical actionability of RET alterations, and identify selective RET inhibition by LOXO-292 as a promising treatment in heavily pretreated, multikinase inhibitor-experienced patients with diverse RET-altered tumors.

Phase I dose-finding study of paclitaxel with panitumumab, carboplatin and intensity-modulated radiotherapy in patients with locally advanced squamous cell cancer of the head and neck.

BACKGROUND: Panitumumab has the potential to improve the therapeutic ratio of concurrent chemoradiotherapy for squamous cell carcinoma of the head and neck (SCCHN). PATIENTS AND METHODS: This phase I dose-finding study investigated escalating doses of paclitaxel (Taxol) given concurrently with panitumumab, carboplatin and intensity-modulated radiotherapy (IMRT) for stage III-IVB SCCHN. Untreated patients with oral cavity, oropharynx, larynx, hypopharynx or unknown primaries were eligible. Additional eligibility criteria included measurable disease, good performance status and no contraindication to therapy. Patients received weekly fixed doses of panitumumab and carboplatin plus escalating doses of paclitaxel with IMRT. RESULTS: Nineteen patients were enrolled on to two dose levels (DLs): weekly paclitaxel 15 mg/m(2) (n = 3) and 30 mg/m(2) (n = 16). One dose-limiting toxicity occurred in DL 2, which was declared the maximum tolerated dose. All patients experienced mucositis, primarily grade 3 or more. Oral pain, xerostomia, dysphagia, weight loss, dermatitis, nausea and acneiform rash were frequent. All patients had partial response according to RECIST, whereas the overall complete clinical response rate was 95%. At median follow-up of 21 months, 18 of 19 patients (95%) remained disease free. CONCLUSIONS: Panitumumab, carboplatin, paclitaxel and IMRT are well tolerated and appear highly active in the treatment of SCCHN. Further study of this regimen in SCCHN is warranted.

Sequential therapy for the locally advanced larynx and hypopharynx cancer subgroup in TAX 324: survival, surgery, and organ preservation.

BACKGROUND: Locally advanced laryngeal and hypopharyngeal cancers (LHC) represent a group of cancers for which surgery, laryngectomy-free survival (LFS), overall survival (OS), and progression-free survival (PFS) are clinically meaningful end points. PATIENTS AND METHODS: These outcomes were analyzed in the subgroup of assessable LHC patients enrolled in TAX 324, a phase III trial of sequential therapy comparing docetaxel plus cisplatin and fluorouracil (TPF) against cisplatin and fluorouracil (PF), followed by chemoradiotherapy. RESULTS: Among 501 patients enrolled in TAX 324, 166 had LHC (TPF, n = 90; PF, n = 76). Patient characteristics were similar between subgroups. Median OS for TPF was 59 months [95% confidence interval (CI): 31-not reached] versus 24 months (95% CI: 13-42) for PF [hazard ratio (HR) for death: 0.62; 95% CI: 0.41-0.94; P = 0.024]. Median PFS for TPF was 21 months (95% CI: 12-59) versus 11 months (95% CI: 8-14) for PF (HR: 0.66; 95% CI: 0.45-0.97; P = 0.032). Among operable patients (TPF, n = 67; PF, n = 56), LFS was significantly greater with TPF (HR: 0.59; 95% CI: 0.37-0.95; P = 0.030). Three-year LFS with TPF was 52% versus 32% for PF. Fewer TPF patients had surgery (22% versus 42%; P = 0.030). CONCLUSIONS: In locally advanced LHC, sequential therapy with induction TPF significantly improved survival and PFS versus PF. Among operable patients, TPF also significantly improved LFS and PFS. These results support the use of sequential TPF followed by carboplatin chemoradiotherapy as a treatment option for organ preservation or to improve survival in locally advanced LHC.

Identification of a novel protein with GDP dissociation inhibitor activity for the ras-like proteins CDC42Hs and rac I.

We have recently cloned the human cDNA for a gene, denoted D4, that encodes a protein 67% identical to the bovine rhoGDI protein, a GDP dissociation inhibitor (GDI) for the ras-related rho-subtype proteins. We now present data on the cloning and structural analysis of the murine D4 cDNA and confirm its preferential expression in hematopoietic tissues. The predicted murine and human D4 proteins are almost 90% identical, indicating that D4 and rhoGDI are different genes and that they are probably members of a related family of genes. Functional studies with the human D4 protein demonstrate that D4 has GDI activity against the CDC42Hs and rac I proteins, but binds to these proteins with a significantly weaker affinity than does the rho-subtype GDI. These data suggest that D4, which will in subsequent communications be denoted as GDI.D4, might be a GDI for other known or as yet unidentified ras-like GTP-binding proteins. Alternatively, D4 could have other biochemical functions. During murine embryogenesis, D4 transcripts are detected in yolk-sac cells, where the earliest hematopoietic precursors are found. When these precursors undergo proliferation and differentiation in vitro, a dramatic increase in D4 expression is seen. D4 probably has a significant function during the growth and development of hematopoietic precursors.