Insights into fourth generation selective inhibitors of (C797S) EGFR mutation combating non-small cell lung cancer resistance: a critical review

1.1. Overview

A broad spectrum of malignancies are included under the umbrella term “cancer”.¹ Unrestricted cell growth and proliferation, as well as uncontrollable cell migration and organ/tissue spreading, are the major hallmark characteristics of these malignancies.^2,3 A normal cell may become a tumor-affected cell due to the disruption of multiple regulatory, signal transduction, and apoptotic pathways by a number of different variables.^4–6 In 2020, with more than 1.38 million deaths worldwide, lung cancer was the most frequent type of cancer that causes death, with non-small cell lung cancers (NSCLC) representing about 80% of lung cancer cases.^7–9 Evenly NSCLC is reported to be less responsive to chemotherapy and radiation treatment compared to small cell lung cancers (SCLC).^10,11 Chemotherapy has not significantly enhanced survival for the majority of lung cancer patients, and it is clear that chemotherapy's effectiveness in the treatment of NSCLC has reached a plateau.¹² Targeted small molecules block signaling pathways that influence the development and advancement of cancer. As these pathways are preferentially activated in cancer cells compared to normal cells, targeted therapies are likely more tolerable than conventional cytotoxic agents.¹³

1.2. Epidermal growth factor receptor (EGFR) family and their inhibitors

Targeted therapy, a unique type of chemotherapy, is a helpful mechanism to stop cancer cells from growing and spreading by targeting specific genes or proteins.^14,15 Advancing understanding of the cellular and molecular biology will improve chemists' abilities to target specific function in cancer cells. The trans-membrane glycoprotein known as the epidermal growth factor receptor (EGFR) is a cytoplasmic tyrosine kinase; its physiological function is to regulate the development and homeostasis of epithelial tissue.^16,17 The cytoplasmic region of the kinase includes the kinase domain, which mediates the transfer of a phosphate group to the kinases with an ATP binding pocket found between the N- and C-lobes of the EGFR.^18,19

Epidermal growth factor (EGF) and transforming growth factor (TGF), the two ligands for the EGFR, cause the activation of dormant receptors through homo- or hetero-dimerization. Numerous downstream signaling cascades, including RAS/RAF/ERK, STAT, and PI3K/AKT/mTOR, are triggered by activation and auto-phosphorylation of the intracellular EGFR tyrosine kinase domain of receptors.^20,21 This is followed by a cascade of processes in the cytoplasm, including cell growth and apoptosis inhibition, represented in (Fig. 1).

EGFR is a crucial player in carcinogenesis and is highly expressed in breast and lung cancers.^22,23 Accordingly, EGFR inhibitors are recommended by international guidelines as the first-line treatment in patients with advanced EGFR mutations in breast and lung cancers, regarding the higher efficacy and safety of these agents rather than standard chemotherapy.²⁴ Two therapeutically effective pharmacological strategies for EGFR inhibition therapy are reported; monoclonal antibodies and small-molecule tyrosine kinase inhibitors (TKIs).²⁵ In order to prevent ligand-induced EGFR tyrosine kinase activation, EGFR monoclonal antibodies can compete for binding to the inactive extracellular domain of EGFR receptors.²⁶ In the same context, small-molecule EGFR TKIs exerted their mechanism of action via decreasing EGFR auto-phosphorylation and downstream signaling by reversibly competing with adenosine 5′ triphosphate (ATP) for binding to the intracellular tyrosine kinase domain of EGFR.²⁷

It was reported that adenylylimidodiphophate (AMPPNP) is a form of ATP that cannot be hydrolyzed and linked to the wild-type EGFR (EGFRWT) according to the later crystal structure (PDB ID: 3VJO); thus, AMPPNP has to typical hydrogen bonds with Met793 and Gln791, this activates the downstream signaling and cascade of process in the cytoplasm include cell growth, and apoptosis inhibition^28–31 (Fig. 2).

Since the development of EGFR TKIs for the treatment of NSCLC, hundreds of small molecules have been designed and evaluated as EGFR TKIs; these compounds are divided from first-to fourth-generation EGFR TKIs^32,33 (Fig. 3).

1.3. Role of EGFR-TKIs in NSCLC and their resistance mechanisms

EGFR plays an important role in the development and progression of lung cancer.³⁴ The progression of lung cancer begins with abnormal cellular multiplication, which prompts normal cells to transition into malignant forms.³⁵ In both NSCLC and SCLC, EGFR plays a significant role in the initiation and activation of signaling events. There are three different types of sub-categories incorporated within NSCLC: (i) adenocarcinomas, (ii) squamous cell carcinomas, and (iii) large cell carcinomas. Overexpression of EGFR has been found in all of these subgroups of NSCLC, with the greatest rates seen in squamous tumors (90%) and the lowest rates seen in adenocarcinomas (40%), and very little rates seen in large cell adenocarcinoma.³⁶ According to the literature, EGFR has been shown to be a potential therapeutic target in the treatment of NSCLC. Targeting the suppression of EGFR function can be accomplished using of two primary methods: (i) the deactivation of intracellular kinase signaling through TKIs; and (ii) the neutralization of EGFR and ligands through the use of antibodies.³⁷

Nowadays, mutations in the EGFR gene and various resistance mechanisms have been extensively studied in patients with lung cancer.³⁸ Both intrinsic (primary) and acquired (secondary) chemo-resistance reduce the response rate to EGFR TKIs therapy.³⁹ Inappropriate EGFR activation in cancer is mainly caused by genomic locus amplification, and that leads to full mutation.⁴⁰ The primary resistance mechanisms include point mutations in exon 18, deletions or insertions in exon 19, insertions, duplications and point mutations in exon 20 and exon 21 of EGFR gene, that mutation affected 4–10% of NSCLC patients.⁴¹ As a result of this primary mutation, NSCLC may develop primary innate resistance to reversible and irreversible EGFR-TKIs.⁴²

Since the secondary mutation or amplification of the EGFR has been observed to increase in tumors under therapeutic pressure, it is also widely recognized as a biomarker of drug resistance.⁴³ The secondary mutation is characterized by the ATP-binding cleft of EGFR containing the gatekeeper mutant T790M, which is associated with acquired resistance to first- and second-generation EGFR TKIs in 50% to 60% of NSCLC patients.⁴⁴ The T790M mutation reduces the affinity of EGFR TKIs while it increases the affinity of ATP for the EGFR, even though it causes steric hindrance in the interaction of TKIs with EGFR.⁴⁵ This enhances the ability of mutant NSCLC cells to survive by preventing EGFR inhibition by first- and second-generation EGFR-TKIs.⁴⁶ The T790M mutation-mediated resistance to first- and second-generation EGFR-TKIs in NSCLC has been overcome by third-generation EGFR-TKIs, which have been developed and produced.^46,47

The 797 location of the ATP binding pocket of the EGFR-tyrosine kinase domain is where the third-generation EGFR-TKIs can covalently engage with a cysteine residue.⁴⁸ After some time, individuals using third-generation EGFR-TKIs begin to acquire heterogeneous resistance to these medications.⁴⁹ Third-generation EGFR-TKI resistance is primarily caused by the appearance of a tertiary point mutation (C797S) in the ATP binding cleft. Deleting the secondary T790M mutation is another known factor in this secondary resistance.^50,51

More recently, the fourth-generation of EGFR TK inhibitors has been introduced to the clinical evaluation to fight against EGFR tertiary mutation (C797S).⁵² The studies are devoted to the use of multi-target agents and combination therapy to overcome epigenetic mutation and boost the effectiveness of EGFR TK inhibitors.⁵³ As well as the design and discovery of specific inhibitors targeting the mutant EGFR. In this regard, small molecule EGFR TKIs degraders have been evaluated to deal with these mutations in the tyrosine kinase domain of EGFR for combating resistance NSCLC.⁵⁴

This review article aims to categorize the current EGFR TKIs and provides a summary based on their structures. Additionally, it highlighted how the most significant protein–drug interactions influenced selectivity that may encourage the development of more potent selective fourth-generation EGFR TKIs that can fight against EGFR tertiary mutation (C797S).

1.4. EGFR modulation pipeline to overcome EGFR genetic code mutations in NSCLC

2.1. Purines based derivatives

A group of 8-phenylamino-9H-purine compounds had been reported as potent and irreversible EGFR inhibitors by Y. Hei and his coworkers. Compound (14) was the first one of these chemotype derivatives to inhibit EGFR^{L858R/T790M/C797S}, exhibiting moderate inhibitory activity (IC₅₀ = 114 nM) against this mutant enzyme.⁹³ In 2020, H. Lei and his colleagues created a series of 9-heterocyclyl substituted 9H-purine derivatives, which serve as EGFR^{L858R/T790M/C797S} TKIs with a fruitful modification that can be summarized in the following points: (i) maintaining the purine as a main scaffold, (ii) replacing the sulphide linker by –NH– linker to afford a hydrogen bond donor moiety, (iii) adding an extra hydrogen bond acceptor moiety and finally, (iv) cyclization of terminal acyclic aliphatic nitrogen containing moiety to piperazine one. Among these compounds, compounds (15) and (16) exhibited greatly anti-proliferative activities and mutant EGFR^{L858R/T790M/C797S} inhibitory activities with IC₅₀ equal to 22 nM and 18 nM, respectively⁹⁴ (Fig. 11).

Compound (16) showed potent anti-proliferative effects against both the HCC827 and H1975 cell lines, significantly suppress the mutant EGFR phosphorylation and induced cell apoptosis.⁹⁴ The authors believed that two hydrogen bond interactions were formed with Met793 in the hinge region between the nitrogen atom at position C-1 and the hydrogen atom from amino at position C-2 of the 9H-purine scaffold which are responsible for its activity. Also an extra–hydrogen bond interaction was observed with Thr854 when the –NH– linker replaced the –S– linker in the previously reported compound (14).

Additionally, adding a methylsulfonyl pyrrolidine group adds increases the number of hydrogen bond acceptor moieties to the mutant Ser797 residue. These findings could explain why compound (16) exhibited potent enzyme inhibitory activity with mutant EGFR^{L858R/T790M/C797S}. Therefore, adding an additional hydrogen bond interaction with mutant Ser797 can be considered an effective design strategy for fourth-generation EGFR-TKIs (Fig. 12).

2.2. Pyrimidine based derivatives

Z. Su and his research associates designed and synthesized a variety of pyrimidine-indole derivatives with the terminal hydroxyl of the alkyl chain to expand an extra binding interaction with the Asp855 in the conservative DFG site to overcome the resistance mutation 19D/T790M/C797S. Compound (17) stood out among them due to its notable inhibitory effect against EGFR^{19D/T790M/C797S} (IC₅₀ = 15.3 nM) and high selectivity over EGFR^WT (IC₅₀ > 1000 nM).⁹⁵

The 2D interaction diagram explained the good inhibitory effect for compound (17); it binds to Lys745 and Asp855 located in the allosteric binding site of the EGFR TK as well as aminopyrimidine scaffold forms a bidentate formation with Met793 residue and the indole moiety occupied the hydrophobic back pocket⁹⁵ (Fig. 13).

Additionally, compounds (18) and (19) were developed as representative of a novel category of N₂,N₄-diphenylpyrimidine-2,4-diamine derivatives bearing amino/methyl sulfonyl moieties (Fig. 14). Both compounds exhibited a potent inhibitory concentration (IC₅₀) of 0.2 nM for the mutant EGFR^{del19/T790M/C797S 96}. These compounds retain the aminopyrimidine scaffold to form a bidentate interaction with Met793 in addition to amino/methyl sulphonyl moiety as a strong hydrogen bond acceptor that bound to Lys745 and Asp855 residues (Fig. 15). So these findings offered interesting prospects for further research and development for new candidates served as fourth-generation EGFR inhibitors.

Furthermore, a series of WZ4002 (20) (a third-generation EGFR TKI) optimized analogs have been developed, and their antiproliferative effects on various cell lines and their EGFR kinase inhibitory efficacy against six different EGFR kinases have been evaluated. At a concentration of 10 μM, the ortho-hydroxyacetamide derivative (21) completely inhibited all six kinases, including the mutant EGFR^{L858R/T790M/C797S}, with 99% inhibition⁹⁷ (Fig. 16).

2.3. Pyrrolopyrimidine/pyrimidopyrimidine/pyridopyrmidine derivatives

Merging the pyrimidine ring with another nitrogen-containing five or six-membered ring showed a good progression in designing the fourth-generation EGFR TKIs. Jonas' research team analyzed the X-ray structure of the T790M mutant EGFR TK to develop the lead compound (22), which inhibits mutant EGFR^L858R/T790M with an IC₅₀ of 176 nM.⁹⁸ Lead optimization of compound (22) via O-alkyl substitution variation and addition of methyl piperazine moiety resulted in the synthesis of pyrrolopyrimidines (23–25) as potent EGFR^{L858R/T790M/C797S} inhibitors (Fig. 17).

These compounds exhibited a high level of efficacy, around 9 nM against the resistant EGFR^{L858R/T790M/C797S}. Molecular docking analysis showed the core pyrrolopyrimidine of these compounds could form a bidentate hydrogen bond with Met793, while the addition of piperazine moiety provides interaction with mutant Ser797. As well as, O-alkyl substitution with a bigger contact surface with the gatekeeper Met790 as in compound (25) having the isobutyl moiety showed relatively potent activity, with an IC₅₀ of 8.5 nM (ref. 98) (Fig. 18).

On the other hand, pyrimidopyrimidinone derivatives (26) and (27) were reported by Lu et al. to be effective as EGFR^C797S inhibitors. These compounds were found by a random screening of a kinase inhibitor library that contained about 3000 different compounds.^99–101 Compound (26) showed significant activity of EGFR^{L858R/T790M/C797S} with an IC₅₀ value of 5.8 nM. As well as compound (27) showed a potent inhibitory effect in the enzyme-linked immunosorbent assay (ELISA) against the kinase activity of EGFR^{L858R/T790M/C797S} with an IC₅₀ value of 3.1 nM. The “hinge” residue Met793 of the protein and the dihydropyrimido[4,5-d]pyrimidinone nucleus interact via a bidentate hydrogen bond. The 2-chlorophenyl group is located in a hydrophobic back pocket that contains numerous significant amino acid residues. Meanwhile, a hydrogen bond is formed between the nitrogen of Lys745 and the carbonyl of the main scaffold. Also, the propionamide group extended in the solvent-exposed region, and the NH moiety formed a hydrogen bond with Leu718 (ref. 101) (Fig. 19).

Also, compound (28) is a representative compound of a series of 5-methylpyrido[2,3-d]pyrimidine derivatives that were deployed as selective EGFR^{L858R/T790M/C797S} inhibitors. This compound's IC₅₀ for the mutant EGFR^{L858R/T790M/C797S} was 27.5 nM, compared to EGFR^WT (IC₅₀ > 1.0 μM), which was clearly less potent on EGFR^wt102 (Fig. 20). Additionally, based on the Ding group research,¹⁰¹ Zhang et al. developed a series of 2,4,6-trisubstituted pyrido[3,4-d]pyrimidine derivatives as potential fourth-generation EGFR-TKIs. Compound (29) was the most effective one in this study, with IC₅₀ values that suppressed the proliferation of HCC827 and H1975 cell lines by 0.044 μM and 0.40 μM, respectively. Also, it significantly inhibited EGFR^{L858R/T790M/C797S} with IC₅₀ = 7.2 nM.¹⁰³ It was noticed that methyl substitution of the main scaffold reduces the potency of this family with a negative impact on mutation selectivity (Fig. 20).

2.4. Quinazoline/quinoline based derivatives

The structure of EAI045 (13) inspires the design and the synthesis of many quinazoline-4-one derivatives, which were synthesized by replacing the 2-aminothiazole amide group with a more rigid, non-hydrolyzed amidic group embedded in the core of the quinazoline scaffold. The designed candidates were considered constrained analogs of EAI045 (13) with enhanced pharmacokinetic, pharmacodynamic, and safety features. According to the findings of the biochemical testing, compound (30) had an inhibitory effect against EGFR^{L858R/T790M/C797S}, with an IC₅₀ value = 5.3 μM (ref. 104) (Fig. 21).

Furthermore, a molecular hybridization between vandetanib (31), a quinazoline-based member of the first-generation of EGFR TKIs, and EAI0045 (13) were combined through chemical modification to produce compound (32), which strongly inhibited the activity of the EGFR^{L858R/T790M/C797S} (IC₅₀ = 2.2 nM). The hybridized molecule was found to bind to the mutated EGFRT790M, and it occupied both the ATP binding site and the allosteric binding site, according to the X-ray crystal structure, as it carried both the required pharmacophores.¹⁰⁵ Unfortunately, fast plasma clearance, low oral bioavailability, and oral exposure precluded further research on this derivative (Fig. 22).

Additionally, compound (33) was identified as the fourth-generation triple mutant EGFR^{del746-750/T790M/C797S} inhibitor based on the virtual screening strategy, with potent kinase inhibitory enzymatic activity (IC₅₀ = 149 nM) and 163-fold selectivity over EGFR^{WT 106}. Substitution variation of the 2-phenolic moiety resulted in the development of compounds (34) and (35).

With EGFR^{del19/T790M/C797S} IC₅₀ = 46 nM, compound (34) bearing the methyl ester may provide a hydrogen bond acceptor bind to Ser797; however, it may suffer from hydrolysis to corresponding acid analog, which may affect its pharmacodynamics proprieties. The 5-cyano derivative (35) exhibited about 1000 times more selectivity against the triple mutant EGFR^{del19/T790M/C797S} (IC₅₀ = 17.9 nM) than EGFR^WT. This could be explained by the forming of strong interactions with the mutant residue Ser797 by cyano moiety as hydrogen bond acceptor¹⁰⁶ (Fig. 23).

Also, a series of substituted quinoline compounds were designed and synthesized, according to K. S. Karnik and his coworkers, as mutant L858R/T790M/C797S EGFR-TKIs.¹⁰⁷ Significantly anti-proliferative and enzyme-based inhibitory efficiency of EGFR ^{L858R/T790M/C797S} was demonstrated by the substituted quinolone compounds (36–38). With an IC₅₀ value of 124 nM for compound (37), it exhibited a higher EGFR^{L858R/T790M/C797S} inhibitory effect (Fig. 24).

2.5. Oxoisoindoline phenyl-acetamide-piperazine based derivatives

A ligand extension strategy was applied to the lead compound EAI045 (13) by adding a phenyl-piperazine moiety to the para substitution of isoindolinone ring. With an IC₅₀ value of 0.1 μM, the newly developed drug (39) was relatively the most potent against the mutant EGFR^{L858R/T790M/C797S}.¹⁰⁸ The phenyl-piperazine extension could provide better exposure to the allosteric pocket to have better binding (Fig. 25).

Furthermore, it exhibited good in vivo efficacy after monotherapy and excellent results if it is administrated in combination therapy with osimertinib (8) in mice leading to a much less amount of residual tumor.¹⁰⁸

2.6. Trisubstituted imidazoles/benzoimidazole derivatives

Tri-substituted imidazole derivatives containing an aliphatic alcohol chain moiety were evaluated as EGFR^{L858R/T790M/C797S} mutant reversible inhibitors via selective p38 MAP kinase inhibition.¹⁰⁹ Among these derivatives, compounds (40) and (41) showed significant inhibitory activities against the mutant EGFR^{L858R/T790M/C797S} with IC₅₀ values of 7.64 nM and 8.83 nM respectively, unfortunately without selectivity over EGFR^WT (IC₅₀ less than 0.5 nM).¹¹⁰ In addition, the synthesized compounds shared the following standard pharmacophoric features: (i) para-fluoro-phenyl ring pointed towards mutant Met790 residue, (ii) central imidazole ring, (iii) pyrrolopyridine moiety provides H-bond acceptor and H-bond donor moiety to form bidentate H-bond to Met793, and (iv) aliphatic chain with H-bond donor targeted towards Asn842 residue (Fig. 26).

Moreover, another series of compounds (43) and (44) were designed via optimization compound (42). (Fig. 27) Both compounds successfully inhibit the mutant EGFR^{L858R/T790M/C797S} with IC₅₀ values 8 nM and 7 nM, respectively, if compared to the parent molecule compound (42), which inhibits the mutant EGFR^{L858R/T790M/C797S} with IC₅₀ more than 149 μM.¹¹¹ These compounds are modified with three carbons aliphatic alcohol chain, which was long enough to form a strong hydrogen bond with Asn842 residue. As well as, N-(4-methoxyphenyl)acrylamide or N-(4-methoxyphenyl)propionamide may afford interaction with mutant Ser797 residue of EGFR TK, as illustrated in the 2D diagram in Fig. 28.

Also, a survey through the literature demonstrated that the benzimidazole skeleton has widespread use in synthesizing EGFR inhibitors. H. Engelhardt et al. discovered a potent benzimidazole derivative (45) with highly selective but modest action that inhibits EGFR^{del19/T790M/C797S} at IC₅₀ = 790 nM through a library of selective kinase inhibitors.¹¹²

After that, compound (45) was macro cyclized to rigidize the molecule, yielding the target compound (46) with potent inhibitory activity against EGFR^{del19/T790M/C797S} with IC₅₀ = 0.6 nM. Compound (46) suppressed triple cis mutations in the presence of EGFR^del19 and complicated EGFR mutations containing T790M and C797S mutations. Simultaneously, compound (46) caused tumor regression in anti-osimertinib EGFR^{del19/T790M/C797S} mice xenograft models. The macrocyclization of the target compounds generally results the compounds having good in vivo and in vitro effectiveness¹¹² (Fig. 29).

2.7. Spiro/aryl/phosphorous/oxygen based derivatives

A series of spiro/aryl/phosphorous/oxygen derivatives were reported as fourth-generation kinase inhibitors of the mutant EGFR^{del19/T790M/C797S}. The IC₅₀ values for compounds (47–49) were less than 100 nM, demonstrating remarkable anti-proliferative activity against all tested cell lines (Ba/F3, A431, and NCI-H1975). Notably, this group of spiro/aryl/phosphorous/oxygen derivatives also exhibited potent anticancer activity in xenograft mouse models carrying the C797S mutation. Moreover, the experiments' results indicated that these molecules appointed a new scaffold for the inhibition of EGFR TK with C797S mutation⁹⁶ (Fig. 30). All these analogs shared an amino pyrimidine scaffold alongside phosphorus oxygen moiety, which acts as a strong H-bond acceptor pharmacophore, as well as a spiro aryl tail.

2.8. Natural products as EGFR inhibitors

One of the most plentiful sources for current drug discovery hits or leads is natural products.¹¹³ In 2003, Facompré et al. discovered the marine alkaloid lamellarin N (50) as a topoisomerase I inhibitor.¹¹⁴ Nishiya et al. developed Lamellarin derivative (51) via modification of ring C of the original natural product's scaffold with two tails attached to its phenolic group and methoxy substituent. This compound was tested against the EGFR^C797S and showed effective inhibition.¹¹⁵

The double or triple mutations of EGFR^C797S resistant to osimertinib (8) were inhibited by the bis(N,N-dimethylethanamine)ether derivative compound (51) in a specific manner. It also reduced downstream signal transduction and EGFR phosphorylation in triplet mutant EGFR PC-9 cells. Therefore, compound (51) can serve as a new EGFR-TKI structural skeleton to stop the development of cross-resistance to well-known medications like osimertinib (8)¹¹⁵ (Fig. 31).

2.9. Drug design based on PROTAC

Proteolytic targeting chimeras (PROTACs), which cause targeted protein degradation, have been a popular drug discovery approach in the last ten years.¹¹⁶ PROTAC is a heterobifunctional small molecule that may remove particular undesirable proteins. It consists of a linker and two active domains; the first is a ligand for the E3 ubiquitin ligase, and the second is a ligand bound to the protein of interest. Target proteins are induced to become ubiquitinated by PROTAC and then selectively destroyed by 26S proteolytic enzymes. Increased selectivity for protein degradation and escape mechanisms, such as acquired resistance mutations and target protein overexpression, could result from PROTAC.^117,118

Based on brigatinib (52), an ATP-competitive EGFR inhibitor with high affinity to the ATP binding site and multi-kinase activities. Zhang et al. developed and synthesized powerful and specific EGFR^{Del19/T790M/C797S} triple-mutated PROTAC degraders. According to mechanistic research, molecular linking of brigatinib (52) with VHL (E3 ubiquitin ligase) (53) provide compound (54) that exclusively mediated the degradation of EGFR protein via proteasome pathway with a DC₅₀ value of 8 nM; it decreased EGFR protein levels in Ba/F3 cells containing the triple mutation EGFR^{del19/T790M/C797S} in a dose- and time-dependent manner¹¹⁹ (Fig. 32).

Compound (54) has an IC₅₀ of 20 nM for suppressing Ba/F3 EGFR^{del19/T790M/C797S} cells, and it can successfully prevent the activation of intracellular EGFR downstream, signaling ERK and AKT. The resistance caused by the EGFR^C797S mutation can be overcome by developing new medications in the clinic using compound (54) as a lead candidate.¹¹⁹