Somatic RET Indels in Sporadic Medullary Thyroid Cancer: Prevalence and Response to Selpercatinib

CoHIT: a one-pot ultrasensitive ERA-CRISPR system for detecting multiple same-site indels

Genetic testing is crucial for precision cancer medicine. However, detecting multiple same-site insertions or deletions (indels) is challenging. Here, we introduce CoHIT (Cas12a-based One-for-all High-speed Isothermal Test), a one-pot CRISPR-based assay for indel detection. Leveraging an engineered AsCas12a protein variant with high mismatch tolerance and broad PAM scope, CoHIT can use a single crRNA to detect multiple NPM1 gene c.863_864 4-bp insertions in acute myeloid leukemia (AML). After optimizing multiple parameters, CoHIT achieves a detection limit of 0.01% and rapid results within 30 minutes, without wild-type cross-reactivity. It successfully identifies NPM1 mutations in 30 out of 108 AML patients and demonstrates potential in monitoring minimal residual disease (MRD) through continuous sample analysis from three patients. The CoHIT method is also competent for detecting indels of KIT, BRAF, and EGFR genes. Integration with lateral flow test strips and microfluidic chips highlights CoHIT's adaptability and multiplexing capability, promising significant advancements in clinical cancer diagnostics.

Timing and Ideal Patient for an Appropriate Search for Somatic RET Mutation in Medullary Thyroid Cancer

RET somatic mutation analysis in sporadic MTC should be guided by postoperative evaluation results.

Spontaneous and Treatment-Related Changes of Serum Calcitonin in Medullary Thyroid Cancer: Long-Term Experience in a Patient With Multiple Endocrine Neoplasia Type 2B

PURPOSE

Medullary thyroid carcinoma (MTC) in MEN2B syndrome is associated with germline RET mutation. Patients harboring de novo mutations are usually diagnosed at more advanced disease stages. We present a young woman with Met918Th mutation diagnosed with stage IV MTC at age 10 years.

METHODS

The disease progressed despite total thyroidectomy and multiple surgical interventions for cervical lymph node recurrences, leading to distant metastases in the fifth year after the initial diagnosis. Subsequently, she underwent five different types of tyrosine kinase inhibitor (TKI) treatments. The 17-year disease course was divided into periods defined by four surgical interventions and sequential treatment intervals with four multikinase (sunitinib, vandetanib, cabozantinib, and lenvatinib) and one RET-selective TKI (selpercatinib). Tumor growth for different phases of spontaneous development and drug treatment intervals was characterized by changes in serial log-transformed calcitonin measurements (n = 114).

RESULTS

Three operations (one for calcitonin-producing adrenal pheochromocytoma) were associated with drops in calcitonin levels. All of the nonselective TKIs were stopped due to adverse effects. As reflected by the negative calcitonin doubling rate, the best treatment response was observed with selpercatinib, which was associated with an initial large drop followed by a decreasing calcitonin trajectory over 514 days without any major side effects.

CONCLUSION

This case of MEN2B medullary thyroid cancer with long-term survival presents how the effectiveness of different treatment modalities can be estimated using log-transformed calcitonin levels. Furthermore, our experience supports the view that serial calcitonin measurements may be more sensitive than radiological follow-up in advanced MTC. Our patient also represents a new case of rarely reported calcitonin-producing pheochromocytomas.

Next-Generation Sequencing in Sporadic Medullary Thyroid Cancer Patients: Mutation Profile and Disease Aggressiveness

Context

Next-generation sequencing (NGS) analysis of sporadic medullary thyroid carcinoma (sMTC) has led to increased detection of somatic mutations, including RET M918T, which has been considered a negative prognostic indicator.

Objective

This study aimed to determine the association between clinicopathologic behavior and somatic mutation identified on clinically motivated NGS.

Methods

In this retrospective cohort study, patients with sMTC who underwent NGS to identify somatic mutations for treatment planning were identified. Clinicopathologic factors, time to distant metastatic disease (DMD), disease-specific survival (DSS), and overall survival (OS) were compared between somatic mutations.

Results

Somatic mutations were identified in 191 sMTC tumors, including RET M918T (53.4%), other RET codons (10.5%), RAS (18.3%), somatic RET indels (8.9%), and RET/RAS wild-type (WT) status (8.9%). The median age at diagnosis was 50 years (range, 11-83); 46.1% were female. When comparing patients with RET M918T, RET-Other, and RET WT (which included RAS and RET/RAS WT), there were no differences in sex, TNM category, systemic therapy use, time to DMD, DSS, or OS. On multivariate analysis, older age at diagnosis (HR 1.05, P < .001; HR 1.06, P< .001) and M1 stage at diagnosis (HR 3.17, P = .001; HR 2.98, P = .001) were associated with decreased DSS and OS, respectively, but mutation cohort was not. When comparing RET M918T to RET indels there was no significant difference in time to DMD, DSS, or OS between the groups.

Conclusion

Somatic RET mutations do not portend compromised DSS or OS in a cohort of sMTC patients who underwent clinically motivated NGS.

Molecular Basis and Natural History of Medullary Thyroid Cancer: It is (Almost) All in the RET

Medullary thyroid cancer (MTC) is a rare disease, which can be either sporadic (roughly 75% of cases) or genetically determined (multiple endocrine neoplasia type 2, due to REarranged during Transfection RET germline mutations, 25% of cases). Interestingly, RET pathogenic variants (mainly M918T) have also been reported in aggressive forms of sporadic MTC, suggesting the importance of RET signalling pathways in the pathogenesis of MTC. The initial theory of RET codon-related MTC aggressiveness has been recently questioned by studies suggesting that this would only define the age at disease onset rather than the aggressiveness of MTC. Other factors might however impact the natural history of the disease, such as RET polymorphisms, epigenetic factors, environmental factors, MET (mesenchymal-epithelial transition) alterations, or even other genetic alterations such as RAS family (HRAS, KRAS, NRAS) genetic alterations. This review will detail the molecular bases of MTC, focusing on RET pathways, and the potential mechanisms that explain the phenotypic intra- and interfamilial heterogeneity.

Medullary Thyroid Cancer: Updates and Challenges

A personalized approach to the management of medullary thyroid cancer (MTC) presents several challenges; however, in the past decade significant progress has been made in both diagnostic and treatment modalities. Germline rearranged in transfection (RET) testing in multiple endocrine neoplasia 2 and 3, and somatic RET testing in sporadic MTC have revolutionized the treatment options available to patients. Positron emission tomography imaging with novel radioligands has improved characterization of disease and a new international grading system can predict prognosis. Systemic therapy for persistent and metastatic disease has evolved significantly with targeted kinase therapy especially for those harboring germline or somatic RET variants. Selpercatinib and pralsetinib are highly selective RET kinase inhibitors that have shown improved progression-free survival with better tolerability than outcomes seen in earlier multikinase inhibitor studies. Here we discuss changes in paradigms for MTC patients: from determining RET alteration status upfront to novel techniques for the evaluation of this heterogenous disease. Successes and challenges with kinase inhibitor use will illustrate how managing this rare malignancy continues to evolve.

D898_E901 RET Deletion Is Oncogenic, Responds to Selpercatinib, and Treatment Resistance Can Arise Via RET-Independent Mechanisms

PURPOSE

We analyzed the oncogenic potential of RET Δ898-901 mutant and its response to selpercatinib, vandetanib, and cabozantinib in vitro and in a clinical case.

MATERIALS AND METHODS

A 35-year-old man with a medullary thyroid cancer (MTC) harboring a somatic D898_E901 RET deletion was sequentially treated with vandetanib, selpercatinib, cabozantinib, and fluorouracil (5-FU)-dacarbazine. Functional study of RET Δ898-901 mutant was performed in HEK-293T, NIH-3T3, and Ba/F3 cells. RET C634R and wild-type cells served as positive and negative controls, respectively.

RESULTS

The patient showed primary resistance to vandetanib and secondary resistance to selpercatinib after 12 months. Comprehensive next-generation sequencing of a progressing lesion during selpercatinib showed no additional RET mutation but an acquired complete genetic loss of CDKN2A, CDKN2B, and MTAP genes. Subsequent treatment with cabozantinib and 5-FU-dacarbazine had poor efficacy. In vitro, RET Δ898-901 showed higher ligand-independent RET autophosphorylation compared with RET C634R and similar proliferation rates in cell models. Subcutaneous injection of Δ898-901 NIH 3T3 cells in nude mice produced tumors of around 500 mm3 in 2 weeks, similarly to RET C634R cells. Selpercatinib inhibited cell growth of Ba/F3 RET Δ898-901 and RET C634R with a similar half maximal inhibitory concentration (IC50) of approximately 3 nM. Vandetanib was five-fold less effective at inhibiting cell growth promoted by RET Δ898-901 mutant (IC50, 564 nM) compared with RET C634R one (IC50, 91 nM). Cabozantinib efficiently inhibited Ba/F3 RET C634 proliferation (IC50, 25.9 nM), but was scarcely active in Ba/F3 RET 898-901 (IC50 > 1,350 nM).

CONCLUSION

D898_E901 RET deletion is a gain-of-function mutation and responds to tyrosine kinase inhibitors in MTC. RET Δ898-901 mutant is sensitive to selpercatinib and vandetanib, and acquired resistance to selpercatinib may develop via RET-independent mechanisms.

Looking for RET alterations in thyroid cancer: clinical relevance, methodology and timing

PURPOSE

Thyroid carcinoma (TC) is a rare neoplasia of the endocrine system and account for about 2-3% of all human tumors. According to their cell origin and histological features, different histotypes of thyroid carcinoma are described. Genetic alterations involved in the pathogenesis of thyroid cancer have been described and it has been shown that alterations of the RET gene are common events in all TC hystotypes. Aim of this review is to give an overview of the relevance of RET alterations in TC and to provide indications, timing and methodologies, for RET genetic analysis.

METHODS

A revision of the literature has been performed and indications for the experimental approach for the RET analysis have been reported.

CONCLUSIONS

The analysis of RET mutations in TC has a very important clinical relevance for the early diagnosis of the hereditary forms of MTC, for the follow-up of TC patients and for the identification of those cases that can benefit from a specific treatment able to inhibit the effect of mutated RET.

Progress and challenges in RET-targeted cancer therapy

The rearranged during transfection (RET) is a receptor protein tyrosine kinase. Oncogenic RET fusions or mutations are found most often in non-small cell lung cancer (NSCLC) and in thyroid cancer, but also increasingly in various types of cancers at low rates. In the last few years, two potent and selective RET protein tyrosine kinase inhibitors (TKIs), pralsetinib (BLU-667) and selpercatinib (LOXO-292, LY3527723) were developed and received regulatory approval. Although pralsetinib and selpercatinib gave high overall response rates (ORRs), < 10% of patients achieved a complete response (CR). The RET TKI-tolerated residual tumors inevitably develop resistance by secondary target mutations, acquired alternative oncogenes, or MET amplification. RET G810 mutations located at the kinase solvent front site were identified as the major on-target mechanism of acquired resistance to both selpercatinib and pralsetinib. Several next-generation of RET TKIs capable of inhibiting the selpercatinib/pralsetinib-resistant RET mutants have progressed to clinical trials. However, it is likely that new TKI-adapted RET mutations will emerge to cause resistance to these next-generation of RET TKIs. Solving the problem requires a better understanding of the multiple mechanisms that support the RET TKI-tolerated persisters to identify a converging point of vulnerability to devise an effective co-treatment to eliminate the residual tumors.

Initial Experiences of Selective RET Inhibitor Selpercatinib in Adults with Metastatic Differentiated Thyroid Carcinoma and Medullary Thyroid Carcinoma: Real-World Case Series in Korea

Recently, selpercatinib, a highly selective inhibitor of RET receptor tyrosine kinase, has been used for RET-altered thyroid cancer. We present four cases of patients with advanced thyroid cancer who were treated with selpercatinib. The first patient was a 63-year-old male with advanced medullary thyroid cancer (MTC) treated with vandetanib. Six months ago, he had an intracranial hemorrhage and swallowing difficulty. He started selpercatinib with percutaneous endoscopic gastrostomy (PEG). For 11 months, a partial response (PR) was observed stably with PEG administration without any more cardiovascular events. The second patient was a 67-year-old female with advanced MTC treated with vandetatib. After selpercatinib treatment, a PR was observed for most metastatic sites, including choroidal metastasis. The third patient was a 32-year-old female with advanced papillary thyroid cancer (PTC) without history of systematic treatment. For six months, a PR was observed at her metastatic site with manageable adverse events. The last patient was a 59-year-old female with advanced PTC treated with lenvatinib. She suffered from a panic disorder and pleural pain due to metastasis during lenvatinib treatment. After selpercatinib treatment, her pain and panic symptoms were improved. Facing varying clinical obstacles of the real world, selpercatinib safely proved remarkable therapeutic efficacy regardless of previous treatment or metastatic site.

A Case Report of Poor Response to Selpercatinib in the Presence of a 632_633 RET Deletion

Background: Genomic deletions in medullary thyroid cancer (MTC) are rare. Selpercatinib is a highly selective RET inhibitor for treatment of metastatic RET-altered MTC. We report a 35-year-old male with an aggressive metastatic MTC harboring p.632_633del RET that was poorly responsive to RET kinase inhibitor selpercatinib. Objective: Our objective was to understand the clinical phenotype of p.632_633del RET in MTC in the context of novel RET kinase inhibitor treatment. Methods: Wild-type and p.632_633del RET sequences were modeled using a lighter version of the AlphaFold2 (AF2) software. Functional studies were performed on transfected HEK 293 cells (pCMV6-Entry, pCMV6-RET, or pCMV6-RET(p.632_633del) treated with inhibitors for 24 hours and analyzed on luciferase assays. Results: Structural modeling revealed a paucity of disulfide bridge between Cys630-Cys634 in p.632_633del RET sequences, apparent in wild-type, while forming an intermolecular disulfide bridge between two Cys656. Proximity of juxtamembrane segments of each dimer may impede Tyr687 phosphorylation and stable conformation of intracellular RET that hosts selpercatinib. In vitro experiments confirmed a reduction in efficacy of selpercatinib upon p.632_633del RET compared with wild-type RET control. Conclusion: Clinical presentation together with structural modeling and functional studies suggests that p.632_633del RET results in poor response to selpercatinib.

LIBRETTO-531: a phase III study of selpercatinib in multikinase inhibitor-naïve RET-mutant medullary thyroid cancer

Selpercatinib is a first-in-class, highly selective and potent, central nervous system-active RET kinase inhibitor. In the phase I/II trial, selpercatinib demonstrated clinically meaningful antitumor activity with manageable toxicity in heavily pre-treated and treatment-naive patients with RET-mutant medullary thyroid cancer (MTC). LIBRETTO-531 (NCT04211337) is a multicenter, open-label, randomized, controlled, phase III trial comparing selpercatinib to cabozantinib or vandetanib in patients with advanced/metastatic RET-mutant MTC. The primary objective is to compare progression-free survival (per RECIST 1.1) by blinded independent central review of patients with progressive, advanced, multikinase inhibitor-naive, RET-mutant MTC treated with selpercatinib versus cabozantinib or vandetanib. Key secondary objectives are to compare other efficacy outcomes (per RECIST 1.1) and tolerability of selpercatinib versus cabozantinib or vandetanib.

Precision oncology for RET-related tumors

Aberrant activation of the RET proto-oncogene is implicated in a plethora of cancers. RET gain-of-function point mutations are driver events in multiple endocrine neoplasia 2 (MEN2) syndrome and in sporadic medullary thyroid cancer, while RET rearrangements are driver events in several non-medullary thyroid cancers. Drugs able to inhibit RET have been used to treat RET-mutated cancers. Multikinase inhibitors were initially used, though they showed modest efficacy and significant toxicity. However, new RET selective inhibitors, such as selpercatinib and pralsetinib, have recently been tested and have shown good efficacy and tolerability, even if no direct comparison is yet available between multikinase and selective inhibitors. The advent of high-throughput technology has identified cancers with rare RET alterations beyond point mutations and fusions, including RET deletions, raising questions about whether these alterations have a functional effect and can be targeted by RET inhibitors. In this mini review, we focus on tumors with RET deletions, including deletions/insertions (indels), and their response to RET inhibitors.