Patient stratification

Importance of patient stratification for RET inhibition

In the following video Professor Fernando Lopez-Rios from San Pablo CEU University in Madrid, Spain, highlights the importance of molecular testing for rearranged during transfection (RET) gene fusions in non-small cell lung cancer (NSCLC) and thyroid cancers, and the real-world challenges of molecular testing.

Since its discovery in the 1980s, the altered RET gene has been detected in a variety of malignancies, including thyroid, lung, breast and colon cancer¹.

RET alterations occur in approximately 2% of all human cancers²

Alterations in the RET gene occur via two main mechanisms: RET point mutations and RET rearrangements³. These alterations lead to the expression of an overactive RET protein that drives tumorigenesis¹.

Mechanisms of RET alterations in cancer

Point mutations: Involve modification of the DNA sequence by the addition, deletion or substitution of a single nucleotide, which can change the amino acid sequence of the protein it encodes (Figure 1A)⁴.

RET point mutations can arise spontaneously, but they can also be inherited. They can occur in extracellular residues, leading to aberrant receptor dimerisation, or in the tyrosine kinase domain, leading to ligand-independent activation⁵

Gene rearrangements: Gene rearrangements involve formation of a hybrid gene through the fusion of two previously independent genes⁶.

RET fusions occur following incorrectly repaired double-strand breaks in DNA, which can be caused by oxidative stress or radiation⁷. Rearrangement of the RET gene leads to a fusion protein with a constitutively activated dimer (Figure 1B)⁸

Figure 1. Schematic representation of (A) a RET point mutation and (B) RET fusion (Adapted^4,8,9). KIF5B, kinesin family member 5B; TK, tyrosine kinase; TM, transmembrane domain; RET, rearranged during transfection.

The RET inhibitors selpercatinib and pralsetinib are indicated for patients with non-small cell lung cancer (NSCLC) and thyroid cancer harbouring RET alterations^10,11. As detailed in the previous section, RET inhibitor efficacy and tolerability for both cancer types were demonstrated in clinical trials; however, their utility is dependent on identifying patients with RET alterations.

Appropriate molecular screening for RET alterations is therefore essential to identify patients who are most likely to benefit from RET inhibitors^12–15.

Not only does the presence of a RET alteration inform the therapy options available for patients, it also indicates prognosis. For example, a somatic mutation at codon 918 occurs in approximately 25% of patients with sporadic MTC and is associated with worse prognosis¹⁶. This may also impact treatment decision making should the patient need additional management strategies alongside RET inhibition, such as surgery¹⁶.

Patients harbouring RET mutations associated with poor outcomes might also require more frequent screening and longer monitoring periods than counterparts with RET alterations deemed lower risk for these outcomes¹⁶.

RET alterations in NSCLC

In the following video Dr Alexander Drilon, a non-small cell lung cancer (NSCLC) expert, outlines how rearranged during transfection (RET) gene alteration status impacts on treatment decisions for patients with NSCLC.

The main types of RET alterations found in non-small cell lung cancer (NSCLC) are gene rearrangements, in the form of gene fusions. Somatic RET fusions occur in 1–2% of NSCLC patients, predominantly in patients with lung adenocarcinoma (Figure 2)^1,5.

Figure 2. Prevalence of different driver oncogenes in lung adenocarcinoma patients from East Asia (Japan, Korea and China) and from America and Europe (Adapted¹⁷). ALK, anaplastic lymphoma kinase; BRAF, B-Raf proto-oncogene, serine/threonine kinase; EGFR, epidermal growth factor receptor; HER2, human epidermal growth factor receptor 2; KRAS, Kirsten rat sarcoma viral oncogene homolog; RET, rearranged during transfection; ROS1, ROS proto-oncogene 1.

The most common RET fusion partner in NSCLC is the KIF5B gene, which is found in 70% of cases^8,18.

There are seven different variations of KIF5B-RET, which differ in the portions of the genes that are combined⁸. All combinations contain the tyrosine kinase domain of RET and the coiled-coil domain of KIF5B, resulting in ligand-independent dimerisation of RET and constitutive activation of downstream signalling (Figure 1B)⁸.

RET alterations in thyroid cancer

Thyroid cancer comprises differing subtypes, many of which are associated with rearranged during transfection (RET) gene alterations¹. Approximately 80% of thyroid cancer cases are papillary thyroid cancer (PTC), and <10–20% of these cases are driven by RET fusions (Figure 3)^1,7. Common RET fusion partners in PTC are CCDC6 and NCOA4, which are found in approximately 90% of RET fusion-positive cases and lead to constitutively active RET proteins⁷.

Figure 3. Incidence of RET alterations in thyroid cancers (Adapted¹⁹). fMTC, familial medullary thyroid cancer; MTC medullary thyroid cancer; PTC, papillary thyroid cancer; sMTC, sporadic medullary thyroid cancer.

Medullary thyroid cancer (MTC) is a less common subtype of thyroid cancer than papillary thyroid cancer (PTC), accounting for 5–10% of all cases²⁰. Approximately 75% of MTC cases are sporadic, but the remaining 25% of cases of MTC occur as part of a hereditary disorder known as multiple endocrine neoplasia type 2 (MEN2)^1,20. MEN2 comprises two subtypes, MEN2A (95% of all MEN2 cases) and MEN2B (<5% of all MEN2 cases)^1,21. 

MEN2 is a rare hereditary cancer syndrome driven by RET germline mutations, which is associated with tumours of the adrenal gland, thyroid and parathyroid^1,21

Somatic RET point mutations are found in approximately 50% of patients with sporadic MTC, while germline activating RET point mutations are found in virtually all cases of MEN2^1,3.

Activating point mutations are the primary RET alterations in MTC¹

The link between RET mutation type and disease severity is pronounced in MEN2, with mutations often stratified into three risk levels: moderate, high, and highest. The risk is based on the penetrance, aggressiveness and latency of the MTC and it informs the age at which thyroidectomy should be conducted in patients with RET alterations (Table 1)¹. Patients harbouring a mutation at either codons 918 or 883 are considered to be at the highest level of risk and require thyroidectomy as soon as possible, ideally before one year of age^1,16.

Table 1. Risk stratification in MEN2 syndrome* (Adapted¹). MEN2, multiple endocrine neoplasia type 2; RET, rearranged during transfection.

Point mutations in the cysteine rich domain (CRD) of the extracellular portion of RET are found in approximately 95% of patients with MEN2A and are classified as moderate risk¹. The common outcome of these mutations is the production of a constitutively active kinase¹. Familial MTC is also associated with RET mutations in the CRD or at intracellular locations¹.

MEN2B is associated with the highest risk category RET mutation, which is at codon 918, and results in the altered RET kinase being capable of activation independent of dimerisation¹. A somatic mutation at codon 918, whereby methionine is substituted for threonine (M918T) is also a common in patients with sporadic MTC, where it is found in up to 40% of patients and is associated with aggressive disease progression^1,20.

Emerging challenges in stratifying patients

Experts Dr Alexander Drilon and Professor Lori Wirth describe the challenges associated with stratifying patients for rearranged during transfection (RET) inhibition treatment in both thyroid cancer and non-small cell lung cancer (NSCLC).

The RET inhibitors, selpercatinib and pralsetinib, were shown to improve the outcomes of patients harbouring RET-altered NSCLC and thyroid cancer^12–15. The key to identifying patients most likely to benefit from these agents lies with molecular testing to detect RET alterations.

Patient stratification is vital for determining which patients may be eligible for targeted RET inhibitor therapy

Patient stratification requires cooperation between laboratories and care providers so that biomarkers such as RET can be integrated into diagnostic and care pathways²². This does not just depend on there being a validated detection method; it is important for physicians to have the necessary knowledge and awareness of molecular testing relevant to their patients. Physicians also require access to diagnostic facilities capable of performing the appropriate tests and support with the interpretation of results²³.

For RET alterations in particular, the lack of a gold standard assay is a major limitation. There are a few different approaches for detecting RET mutations, but they might not all have the capacity to determine whether the resulting RET protein is functional and overactive. The ARROW study, which assessed the efficacy and safety of pralsetinib in patients with RET-altered thyroid cancer and NSCLC, highlighted this issue as a key limitation of the trial¹⁵.

Similarly, patients recruited to the LIBRETTO-001 study of selpercatinib were tested locally for RET alterations using a variety of different techniques¹³.

Barriers impeding the clinical uptake of molecular testing

There are many potential barriers that might impede the clinical uptake of molecular testing in practice, delaying access to optimal care (Figure 4)²³.

Figure 4. Common barriers to molecular testing in clinical practice (Adapted²³).

Test request

Molecular tests must be requested by the acting physician. This requires physicians to be aware of novel biomarkers like RET and the tests best suited to detect them²³. They must also have an understanding of molecular profiling, how to interpret the results, and incorporate the findings into treatment decisions. Without this, physicians are unlikely to request the tests needed to stratify their patients and act on the outcomes. Indeed, in a survey of cancer physicians, 22% were not confident in their understanding of genomics, while 26% of respondents were not confident in their ability to use genomic data to make treatment recommendations²⁴. This highlights the need for education for clinicians to improve their understanding of genomics as this has been shown to increase the use of molecular testing techniques²⁵.

Sample issues

In addition to the acting physician requesting the test, the appropriate sample is also required²³. Different tests have certain sample requirements, and samples must be of sufficient quality and quantity to ensure the accuracy of results as unreliable data may lead to suboptimal or harmful treatment²³.

Costs

Finally, high costs and lack of reimbursement could restrict the use of molecular testing, so testing for RET alterations might be limited in less-developed countries^22,23.

Overcoming these barriers is vital for ensuring that patients with RET-altered cancers can receive optimal treatment. Care is also needed in selecting the best suited molecular testing technique to detect RET alterations, whether they are point mutations or gene fusions.

Learn more on the molecular testing techniques used to identify RET alterations

References

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11. Pralsetinib. Highlights of Prescribing Information. FDA. https://www.accessdata.fda.gov/drugsatfda_docs/label/2020/214701s000lbl.pdf. Accessed 27 August 2021.
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