Efficacy and Safety of RET-Specific Kinase Inhibitors in RET-Altered Cancers: A Systematic Review

Abstract RET proto-oncogene encodes receptor tyrosine kinase. Selpercatinib and pralsetinib are the only RET-specific tyrosine kinase inhibitors approved by FDA in RET-altered tumors. We searched PubMed, Embase, Cochrane, WOS, and Clinicaltrials.gov. Objective-response, complete-response, and partial-response were 60–89%, 0–11%, and 55–89%, respectively, with the use of RET-specific drugs. ≥Grade 3 adverse events were seen in 28–53% of the patients, with hypertension, change in ALT, QT prolongation, neutropenia, and pneumonitis among the common side effects. Hence, selpercatinib and pralsetinib were effective and well tolerated by most of the patients with RET-altered tumors.


Introduction
In 1985, transfection of NIH 3T3 cells with human lymphoma DNA was the first RET (Rearranged during Transfection) proto-oncogene identified (1).Glial Cell Line-Derived Neurotrophic Factors (GDNF) family of ligands bind with the ET encoded Tyrosine Kinase Receptor (2).This signaling is necessary for the survival, development, and regeneration of many neuronal populations (i.e., in the enteric and sympathetic nervous systems) and the kidneys.There are two common types of RET alterations: fusion (between gene sequences) and gene mutation.In fusion type alteration, the sequences that encode for the kinase domain are fused with different upstream gene sequences leading to the ligand-independent constitutional activation of signaling (3).1-2% of patients with non-small cell lung cancer (NSCLC) have RET fusions (4,5).RET gene fusion is also commonly associated with papillary and Hurthle-cell thyroid cancer (6).
Constitutional activation of RET proto-oncogene with germline mutation leads to hereditary multiple endocrine neoplasias (MEN 2A and MEN 2B).Medullary thyroid cancer (MTC), parathyroid hyperplasia, and pheochromocytoma are seen in MEN 2A.MTC, mucosal ganglioneuroma, and pheochromocytoma are seen in MEN 2B (7).About half of the patients with medullary thyroid cancer have somatic RET gene mutation (8).
Before the discovery of RET-specific inhibitors, multitargeted kinase inhibitors (i.e., vandetanib, cabozantinib) were approved for use in RETaltered tumors.However, the durability and safety of these agents were limited by the adverse events attributed to non-RET kinase inhibition (9,10).
Selpercatinib (LOXO-292) is the first RET-specific kinase inhibitor approved by the FDA for use in RET-altered thyroid and NSCLC tumors (11).Selpercatinib is a highly selective ATP-competitive small-molecule inhibitor of RET kinase.It has activity against multiple types of RET-alterations, including activating point mutation, fusions, and other predicted acquired resistance mutations.It spares other non-RET kinases (12).Pralsetinib is another RET-specific kinase inhibitor approved by the FDA after selpercatinib for RET-altered thyroid and NSCLC (13).
This systematic review assessed the efficacy and safety of RET-specific kinase inhibitors (selpercatinib and pralsetinib) in RET-altered tumors in adult patients and subgroup studies based on prior treatment and type of alteration in the RET gene.We discussed these results in comparison with other nonspecific RET-kinase inhibitors.

Material and methods
This systematic review was performed according to the Cochrane and PRISMA guidelines (14).

Search strategy
The literature search was performed on PubMed, Web of Science (WOS), Embase, and Cochrane databases and Clinicaltrials.govregistry with MESH and Emtree terms "selpercatinib" OR "pyrazoles" OR "pralsetinib" and "proto oncogene proteins c ret."The search was performed from the inception of the literature till 12/16/2021 by following the PICO (population, intervention, comparison, and outcome) framework (15) (Supporting Information Table 1).

Inclusion and exclusion criteria
All clinical trials, case series, and observational studies that provided safety and efficacy data in clinical terms, i.e., clinical responses, survival rates, and adverse events were included.We excluded all preclinical studies, case reports, meta-analyses, review articles, pediatric studies, and clinical studies with irrelevant disease/population/studied outcomes.

Study selection
Two researchers (RA and SFB) independently reviewed the articles based on inclusion and exclusion criteria.The differences were later addressed by a third researcher (MAA).

Data extraction
Data was extracted by 2 authors (SR and AA) and included characteristics of the study, baseline characteristics of participants, treatment drugs, measures of efficacy, including objective response (OR), complete response (CR), partial response (PR), durable response (DR), stable disease (SD), progressive disease (PD), progression-free survival (PFS), overall survival (OS), and toxicity (�grade 3 adverse effects).

Risk of bias assessment
Two researchers (MYA and MS) assessed the risk of bias in studies selected for final inclusion using the ROB-2 tool (16).The third researcher (NM) addressed the differences.

Risk of bias
The overall risk of bias was high in clinical trials as all clinical trials were nonrandomized singlearm studies.However, the risk of bias was low in missing data, measurement parameters of the outcome, and selected results (Figure 2).

Selpercatinib in RET-altered cancers
In 2 phase I/II clinical trials (N ¼ 306) conducted by Dirlon et al. and Wirth et al., selpercatinib was used to treat RET-altered thyroid (N ¼ 162) and non-small cell lung cancer (NSCLC) (N ¼ 148).The range of age of participants was 15-86 years.RET gene mutation was present in 143 participants, while 167 participants had RET gene fusion.One hundred seventy-nine patients were previously treated, while 127 patients were treatment naïve Table 1.
The disease progressed in 0-4% of the participants treated with selpercatinib.The disease was stable in 10-29% of the participants treated with selpercatinib, Table 2.

Selpercatinib with crizotinib in RET-altered tumor
In a case series (N ¼ 4) conducted by Rosen et al. (21), selpercatinib was used in combination with crizotinib Table 1.Two patients showed a brief response, one patient showed a durable response, and one patient showed a mixed response.Nausea, lower extremity edema, and gastroesophageal reflux were treatment-related adverse events seen in these patients.

Pralsetinib in RET-altered cancers
In 2 phase I/II clinical trials (N ¼ 216) (18,19), pralsetinib was used to treat RET-altered thyroid (N ¼ 95) and NSCLC (N ¼ 121).The range of age of participants was 49-70 years.RET gene mutation was present in 84 participants, while 132 participants had RET gene fusion.One hundred sixty-four patients were previously treated, while 52 patients were treatment naïve Table 1.
The disease progressed in 0-11% of the participants treated with pralsetinib.The disease was stable in 11-33% of the participants treated with pralsetinib Table 2.

Survival rates
In previously treated NSCLC patients (N ¼ 92), the median PFS was 17.1 months.In treatment naïve NSCLC patients (N ¼ 27), mPFS was 9.1 months.mPFS for thyroid cancer patients was not reached at the time of reporting the results Table 2.

Ongoing clinical trials
Another RET selective kinase inhibitor (BOS172738) has shown promising results in RET-altered NSCLC (N ¼ 30) and MTC (N ¼ 16) on an interim analysis of an ongoing phase 1 study.OR was reported in 33% of the patients and was well tolerated by most of the patients (22).There are 2 ongoing RCTs (N ¼ 650) to compare the efficacy and safety of selpercatinib vs. cabozantinib/vandetanib/chemotherapy in RET-altered thyroid and NSCLC cancers.There is an ongoing phase II clinical trial (N ¼ 49) to assess the safety and efficacy of selpercatinib in RET-altered lymphomas and histiocytic disorders along with ret-altered solid tumors.There are two ongoing phase III RCTs (N ¼ 424) to compare pralsetinib with the standard of care treatment in RET-altered NSCLC and MTC Table 3.

Discussion
The estimated number of new cases of lung cancer attributed to RET fusion is more than ten thousand each year (4,5).Similarly, RET mutation is the cause of most medullary thyroid cancers (8).Before the approval of RET-specific inhibitors, nonspecific kinase inhibitors were the only treatment option for RET-altered tumors.However, the results have shown a promising response of selpercatinib and pralsetinib treatment in cancer patients already treated with multikinase inhibitors with an OR in 60-89% of the patients, including the durable response.The response to RET-specific inhibitors was shown by patients with both types of RET-alteration.These drugs were effective in previously treated, treatment naïve, RET-mutated, and RET-fusion thyroid/NSCL cancer patients (23).Brain metastasis is a well-known complication of RET-altered tumors, especially NSCLC, and is considered a poor prognostic factor (24). 121/522 (23%) of the patients in the clinical trials had brain metastasis, and RET-specific inhibitors were effective in most patients regardless of CNS metastasis status.The objective intracranial response was reported in 19/20 (95%) of the patients with measurable intracranial metastatic NSCLC treated with RETspecific drugs.These results were consistent with the preclinical studies on blood-brain barrier penetration and intracranial activity of selpercatinib and pralsetinib (12,23).Several patients (>95%) experienced tumor shrinkage as well.Multiple case reports also supported the use of RET-specific inhibitors in these tumors with metastasis (25)(26)(27)(28)(29).
KIF5B-RET fusion is the most common type of fusion seen in 60-70% of RET-fusion tumors (36).In phase I/II trials on older drugs, vandetanib and RXDX-105 did not show any response in patients with KIF5B-RET fusion (32,35), and cabozantinib only showed a response in 20% of the patients with KIF5B-RET fusion (30).Contrary to that, treatment with RET-selective drugs showed a response in all types of RETfusion tumors.
MET amplification is the most common type of resistance to RET-specific inhibitors in patients treated with selpercatinib or pralsetinib (37).In the case series by Rosen et al. (N ¼ 4), Met inhibitor, crizotinib, was added to selpercatinib for patients with resistance to RET inhibitors due to MET amplification.The response was seen in all patients, including a durable response in one patient.Large-scale clinical trials are needed to confirm the efficacy and tolerability of combination therapy (21).
The adverse effects seen with multikinase inhibitors in RET-tumors are mostly attributed to the RET-nonselective profile of these drugs, especially inhibition of VEGFR2, as it has a similar kinase domain as RET.The previous drugs, cabozantinib and vandetanib, are more potent inhibitors of VEGFR2 than RET inhibition (38,39).Although most of the patients completed the trials on treatment with vandetanib, dose reduction was required in 35-60% of the patients, and the discontinuation rate was 12% (32,34).Similarly, in the trial on cabozantinib, dose reduction was required in 73-79% of the patients due to adverse events, and the discontinuation rate was 8-16% (30,40).In contrast, in the trials on selpercatinib treatment in RET-altered tumors (N ¼ 531), only 30% of the patients required dose reduction due to adverse events, and 2% of the patients discontinued treatment due to adverse effects (17,20).Similarly, in trials on pralsetinib, adverse events leading to dose reduction and discontinuation were reported in 38-46% and 4-6% of the patients, respectively.Hypertension, palmerplanter erythrodysesthesia, proteinuria, and so forth, are the adverse events attributed to VEGFR2 blockade with multikinase inhibitors and were far less common with the newer RET specific inhibitors as compared to nonspecific inhibitors (30,32).The safety profile of selpercatinib and pralsetinib was similar in general.However, QT prolongation was only reported with selpercatinib, and pneumonitis was only reported with pralsetinib.To summarize, the newer RET-specific inhibitors, selpercatinib, and pralsetinib, were better tolerated than older multikinase inhibitors.
Other than the drugs mentioned above, preliminary results of an ongoing trial on BOS172738 (zeteletinib) have shown significant efficacy with manageable toxicity.However, longer-term followup is needed to confirm these results.The initial trials of selpercatinib and pralsetinib were conducted on RET-altered NSCLC and medullary thyroid cancer.Now, trials are in progress to assess the efficacy and safety of these drugs in RETaltered solid tumors, lymphomas, and other RETaltered tumors (NCT04320888, NCT03037385).Two-phase III trials are in progress to assess the efficacy and safety of selpercatinib vs. chemotherapy/pembrolizumab/ multikinase inhibitors (NCT04211337, NCT04194944) and two phase III trials are in progress to assess the efficacy and safety of pralsetinib vs. standard of care (NCT04222972, NCT04760288).

Conclusion
RET-specific inhibitors were well tolerated by patients with RET-altered tumors, and the adverse effects were less than that seen with previous multikinase inhibitors in the cross-trial comparison.Selpercatinib and pralsetinib were effective in both RET-fusion and RET-mutated types of medullary and NSCL cancers.A significant response was seen in patients previously treated with multikinase inhibitors and in treatment naïve patients.On cross-trial comparison, the response with RET-specific inhibitors was more than that of multikinase inhibitors.Selpercatinib has similar efficacy and safety as pralsetinib.Selpercatinib combination with crizotinib improved response in patients with MET-amplification-related resistance to selpercatinib.Early trial results on BOS172738 have shown promising efficacy and safety in RETaltered tumors.More multicenter, randomized, double-blind, long-term clinical trials are needed to confirm these results and explore more RETspecific inhibitors for these patients.

Limitations
There was no randomized, double-blind clinical trial available on RET-specific inhibitors in RET tumors.Therefore, the risk of bias was high in the included studies.Another limitation was the follow-up length, and no long-term results were available.Comparisons between drugs were based on cross-trial comparisons, and randomized trials for direct comparison are needed to provide more reliable results.Despite this limitation, our analysis suggests better efficacy and tolerability of RET-specific inhibitors in RET-altered tumors than other RET-inhibitor drugs.

Figure 1 .
Figure 1.PRISMA flow sheet for included studies.

Figure 2 .
Figure 2. Risk of bias assessment in included clinical trials using ROB-2 tool.
5Patients in trials were divided into cohorts A, B, or C due to differences in baseline characteristics.

Table 2 .
Efficacy and safety data of RET specific inhibitors (selpercatinib and pralsetinib) in RET-altered tumors in clinical trials.

Table 3 .
Ongoing clinical trials on RET-specific inhibitors.