Targeting RET-driven cancers: lessons from evolving preclinical and clinical landscapes

Abstract

The gene encoding the receptor-tyrosine kinase RET was first discovered more than three decades ago, and activating RET rearrangements and mutations have since been identified as actionable drivers of oncogenesis. Several multikinase inhibitors with activity against RET have been explored in the clinic, and confirmed responses to targeted therapy with these agents have been observed in patients with RET-rearranged lung cancers or RET-mutant thyroid cancers. Nevertheless, response rates to RET-directed therapy are modest compared with those achieved using targeted therapies matched to other oncogenic drivers of solid tumours, such as sensitizing EGFR or BRAF^V600E mutations, or ALK or ROS1 rearrangements. To date, no RET-directed targeted therapeutic has received regulatory approval for the treatment of molecularly defined populations of patients with RET-mutant or RET-rearranged solid tumours. In this Review, we discuss how emerging data have informed the debate over whether the limited success of multikinase inhibitors with activity against RET can be attributed to the tractability of RET as a drug target or to the lack, until 2017, of highly specific inhibitors of this oncoprotein in the clinic. We emphasize that novel approaches to targeting RET-dependent tumours are necessary to improve the clinical efficacy of single-agent multikinase inhibition and, thus, hasten approvals of RET-directed targeted therapies.

Figure 1 |. Timeline of key developments in therapeutically targeting RET in the clinic.

Milestones in our understanding of the pathobiology and prevalence of RET-activating germ-line and/or somatic alterations — RET mutations and RET rearrangements — in cancers (yellow) are shown. Key advances in the development of RET-targeted therapies (purple), including landmark clinical trials performed to evaluate the efficacy of multikinase inhibitors with activity against RET in patients with thyroid cancer (green) or non-small-cell lung cancer (blue)^–, are also depicted.

Figure 2 |. Mechanisms of RET activation in cancers.

a | In-frame RET rearrangements that result in fusion proteins containing the RET kinase domain can lead to the activation of oncogenic RET signalling. Activating RET fusions maintain the tyrosine-kinase domain of the 3’ RET gene. Breakpoints commonly occur within intron 11, however, introns 10 and 7 (not shown) are occasionally involved, resulting in the inclusion of the RET transmembrane (TM) domain. A variety of upstream, 5’ gene partners contribute different domains, such as coiled-coil, LIS1 homology (LisH), tryptophan–aspartate repeat (WDR), and sterile alpha motif (SAM) domains, to RET-fusion proteins. These motifs mediate ligand-independent dimerization of the chimeric oncoprotein and, thus, autophosphorylation of the RET kinase domain, resulting in the activation of downstream signalling pathways that drive tumour-cell proliferation. 5’ gene partners that contain two dimerization domains are listed twice. In general, 5’ partners are not thought to exclusively pair with specific 3’ partners with defined breakpoints at introns 7, 10, and 11. RET rearrangements are largely thought be somatic events, as opposed to RET mutations that can occur in the germ-line or be acquired somatically. b | Activating RET mutations can result in substitutions of extracellular cysteine residues for alternate amino acids, which disrupt intramolecular disulphide bridges, enabling the formation of novel intermolecular covalent disulphide bonds that lead to ligand-independent dimerization; Such mutations are identified in the germ-line of patients with multiple endocrine neoplasia type 2A (MEN2A) and familial medullary thyroid cancer (FMTC), which are associated with cancer predisposition. Mutations in the intracellular kinase domain of RET occur in multiple endocrine neoplasia type 2B (MEN2B), and occasionally in FMTC, leading to monomeric (depicted) and/or dimeric (not depicted, as is the case with the RET^M918T mutation) activation of RET. In cases of ligand-independent RET dimerization and activation, downstream signalling may be enhanced by ligand binding. Select extracellular and intracellular domain substitution mutations are listed, along with the exon in which the mutation occurs.

Figure 3 |. Consequences of inactivating RET mutations or Ret knockout.

The inactivation of RET leads to a variety of consequences that can affect organs including the genitourinary (green and yellow), gastrointestinal (blue), respiratory (orange), and haematopoietic (light blue) systems. Shown here are the human conditions associated with germ-line inactivating RET mutations (CAKUT, CCHS, and HSCR), and the related pathobiological effects observed in Ret-null (Ret^−/−) or tissue-specific Ret-knockout (Ret^fl/fl or Ret^lx/lx) mice. These findings illustrate that the deficiency or absence of RET can have substantial consequences on embryonic development; however, the phenotypic effects of RET deficiency in adult animals are typically mild, with a less-severe symptomatology, indicating that potent and selective RET inhibitors might have a favourable safety profile in older children or adult patients. HSC, haematopoietic stem cell.

Figure 4 |. Multikinase inhibitor activity against RET and other kinases.

The half-maximal inhibitory concentrations (IC₅₀s) of select multikinase inhibitors with varying levels of activity against RET are shown. The various colours represent a range of IC₅₀ values, from <5 nM to >200 nM. Unless otherwise indicated, the IC₅₀ values shown reflect the results of in vitro kinase assays. The presence of two or more colours within a given box indicate different IC₅₀ values reported in separate publications. A white box indicates that biochemical data was not available. The activity of select agents in cellular models is indicated by an asterisk; the inclusion of all reports of activity in RET-mutant tumour cell models was not feasible. Many agents potently inhibit wild-type RET; however, fewer drugs have documented activity against RET V804L or V804M substitution variants. In addition, many of these drugs potently inhibit non-RET targets, such as VEGFR1/2/3, EGFR, KIT, BRAF, and FGFR1, which increases the risk of ‘off-target’ toxicities in patients. These drugs have been explored preclinically in models of thyroid or lung cancer, and several have been explored clinically in patients with these cancer types.

Figure 5 |. Comparative efficacy of RET-directed targeted therapy in RET-rearranged lung cancers.

The efficacy of kinase-inhibitor therapy in molecularly-enriched subgroups of tyrosine kinase-inhibitor treatment-naïve patients with non-small-cell lung cancers (NSCLCs) is depicted as a bubble plot showing the objective response rate (ORR) on the x-axis and median progression-free survival (PFS) duration on the y-axis. Each circle on the plot represents a clinical trial of kinase-inhibitor therapy directed at a specific driver of oncogenesis. Circle sizes represent different phases of clinical testing, as indicated in the key on the right. In comparison with the outcomes with single-agent targeted therapy in patients with EGFR-mutant^,–, ALK-rearranged^,,, or ROS1-rearranged NSCLCs^,, and recognizing the limitations of cross-trial comparisons, relatively lower ORRs and median PFS durations have been achieved with single-agent multikinase inhibition in patients with RET-rearranged NSCLCs^–. Notably, however, the ORRs achieved in the latter group of patients still exceed the historical ORRs achieved with single-agent chemotherapy administered after progression on platinum-doublet chemotherapy (for example, the ORR with second-line, single-agent docetaxel is <10%).