RET V804*

Our analysis showed results in line with existing studies, except for a considerably lower-than-predicted occurrence of PCC in patients with V804M/L mutations. This study supports the current recommendation regarding the pathogenic variant-dependent management of this rare cancer-associated syndrome.

Multi tyrosine kinase inhibitors (mTKIs) cabozantinib and vandetanib are good first line options, even vandetanib prescription is currently limited to RET mutated patients. Selective RET inhibitors such as pralsetinib could be a preferred upfront treatment in case of RET mutated MTC presenting common or gatekeeper RET mutations (e.g. M918T; V804L/M). Selpercatinib, otherwise, can be prescribed as the second line after disease progression to mTKIs. The best option for subsequent lines is to consider inclusion in clinical trials or alternatively other mTKIs such as sunitinib, sorafenib, lenvatinib, or pazopanib could be evaluated. New perspectives include next-generation RET inhibitors able to overcome resistance mechanisms responsible for disease progression to standard mTKIs and RET inhibitors, and immunotherapy for MTC presenting with high tumor mutational burden.

Coexistence of MTC and PTC in germline RET mutation is an uncommon association and more information is required to understand common features, risk factors, phenotype and cancer aggressiveness in these patients. At this point it cannot be concluded that the association is a mere coincidence.*Unless otherwise noted, all abstracts presented at ENDO must not be released to the press or the public until the date and time of presentation. For oral presentations, the abstracts are embargoed until the session begins.

The V804M RET variant has been often associated with MTC or MEN2A phenotypes [1]. However, the variable expression of this variant may lead to a mild clinical phenotype or even absence of MTC/MEN2A manifestations. Hence, active surveillance as opposed to radical surgery like prophylactic thyroidectomy would be more beneficial in patients with no clinical or biochemical evidence of disease.

Background: Vepafestinib (TAS0953/HM06, Vepa) is a 2nd generation RET-selective inhibitor that effectively penetrates the brain, and inhibits the wildtype RET kinase domain (KD) and RET KD mutants (G810, V804, Y806, L730) (presented at AACR-NCI-EROTC 2021 meeting)...Vepa was more effective than vandetanib and similar to the FDA-approved RET inhibitors, selpercatinib (Selp) and pralsetinib (Pral), in all in vitro assays... Our preclinical results suggest that vepafestinib has the potential to more effectively manage CNS metastasis compared to selpercatinib, representing a promising new therapeutic option for patients with RET-driven sarcomas. Vepafestinib is currently in a phase 1/2 trial for adult patients with advanced solid tumors harboring RET alterations (margaRET, NCT04683250).

Pts received RETi after a median number of 1 treatment line (range 0‐6) and 83% had received vandetanib (V) prior to RETi... By‐pass resistance MA may be the most frequent mechanism of progression under RETi. More aggressive histology may arise following proghression; further explorations are warranted.

Vepafestinib (TAS0953/HM06) is a brainpenetrant 2nd generation RET-selective inhibitor that is also effective againstRET solvent front (G810) and gatekeeper (V804) mutations...In all in vitro assays vepafestinibwas more effective than vandetanib and as effective as the FDA-approvedRET inhibitors selpercatinib and pralsetinib... Our preclinical results suggest that vepafestinib is moreeffective than selpercatinib in the brain and represents a therapeutic strategyfor RET-driven sarcomas. Vepafestinib is currently in phase 1/2 clinical trialsfor adult patients with advanced solid tumors with RET alterations(margaRET, NCT 04683250).

Tumor-bearing mice were treated with TAS0953/HM06 (50 mg/kg BID), selpercatinib (10 mg/kg BID) or vandetanib (multi-kinase RET inhibitor, 50 mg/kg QD), and assessed weekly for tumor growth via bioluminescence imaging. Our data in animal models suggest that TAS0953/HM06 penetrates the CNS more effectively than selpercatinib, and is superior at decreasing CNS disease and extending survival. TAS0953/HM06 represents a promising new therapeutic option for patients with RET fusions with acquired resistance mutations, including those with brain metastasis and those resistant to first-generation selective RET inhibitors. TAS0953/HM06 is currently undergoing a biomarker-driven phase 1/ 2 clinical trial for patients with solid tumors driven by RET alterations (NCT04683250).

Selective RET inhibitors selpercatinib and pralsetinib were recently approved by the FDA and EMA for patients with RET-dependent NSCLC and thyroid cancers. APS03118 is a novel highly selective next-generation RET inhibitor that possesses potent in vitro and in vivo activity against a diverse range of RET alterations, including SFMs-mediated resistance. A first-in-human phase 1 trial for patients with RET-driven solid tumors with activating RET alterations is planned for 2022.

Despite the controversy and the heterogeneous presentation of patients with RET p.V804M mutation, our study and review of the literature suggest that this seemingly "low" risk mutation is associated with late-onset but potentially aggressive MTC. This indicates the need for follow-up and timely intervention based on calcitonin level elevation.

This study is active and recruiting, with planned enrollment of 400 participants at ~150 sites in 21 countries. In response to the global COVID-19 pandemic, mitigations for patient safety are included in the protocol.

Currently, several approved multikinase inhibitors such as vandetanib and cabozantinib demonstrate clinical activity in patients with RET-rearranged or RET-mutant cancers...Herein, we designed and synthesized a series of RET inhibitors based on the structure of selective RET inhibitor BLU-667 and investigated their biological activities...In mouse xenograft models, compound 9 repressed tumor growth driven by KIF5B-RET-Ba/F3 cells in a dose-dependent manner. Based on its exceptional kinase selectivity, good potency and high exposure in tumor tissues, compound 9 represents a promising lead for the discovery of RET directed therapeutic agents and the study of RET-driven tumor biology.

The potent, RET-selective tyrosine kinase inhibitors (TKIs) pralsetinib and selpercatinib, are effective against the RET V804L/M gatekeeper mutants, however, adaptive mutations that cause resistance at the solvent front RET G810 residue have been found, pointing to the need for the development of the next-generation of RET-specific TKIs. Also, as seen in EGFR- and ALK-driven NSCLC, the rising of the co-occurring amplifications of KRAS and MET could represent other escaping mechanisms from direct inhibition. In this review, we summarize actual knowledge on RET fusions, focusing on those involved in NSCLC, the results of main clinical trials of approved RET-inhibition drugs, with particular attention on recent published results of selective TKIs, and finally, pre-clinical evidence regarding resistance mechanisms and suggestion on hypothetical and feasible drugs combinations and strategies viable in the near future.

Previous studies in pts treated with selpercatinib or pralsetinib have reported the shared emergence of recurrent RET G810 mutations at the solvent front of the ATP pocket, which lead to a steric clash and loss of binding potency for both drugs. These data suggest that LOX-18228, as well as closely related compounds, represent promising next-generation RET inhibitor candidates that could be used to further extend durable disease control for pts with RET-altered cancers following the development of acquired resistance to current agents. An IND is planned for 2021.