RAS network
Not only alterations in receptors are associated with malignant transformation and tumor progression, but abnormal activation is also observed in members of signaling pathways that are generally triggered by these tyrosine kinase receptors and regulate proliferation, cell survival, apoptosis, migration and cell differentiation. Numerous transduction signaling pathways have been dissected, which are activated in different tumors, and many target therapies have been developed; however, many challenges still need to be circumvented, among them the existence of crosstalks between the intracellular circuitry activated by these different receptors.
The membrane-bound small guanosine triphosphatases (GTPases) comprise a family of four members (HRAS, KRAS4A, KRAS4B and NRAS), that although related, have different functions. RAS proteins are found in two states: inactive when GDP bound and active when GTP bound. Activation of RAS recruits guanine nucleotide exchange factors (GEFs; e.g., SOS1) to the plasma membrane, promoting nucleotide exchange and formation of the RAS-GTP active form. GTPase-activating proteins (GAPs: e.g., neurofibrimin) induce the hydrolysis of RAS bound GTP, leading to the formation of inactive RAS-GDP [
52]. Active RAS in turn triggers intracellular cascades of phosphorylation of downstream effectors, controlling energy metabolism, cell survival, proliferation, migration and invasion. In non-tumorigenic mammalian cells, the main and best studied RAS protein effectors are in the MAPK signaling pathway, comprising RAF/MEK/ERK and PI3K/AKT/mTOR transduction pathways. These mitogenic signaling cascades are hyperactivated in many neoplasias especially due to activating mutations [
53].
Mutations in the three
RAS genes have been described in more than 30% of human cancers and consist in the most common mutated oncogene family in neoplasias.
RAS genes are mutated in different frequencies,
KRAS being mutated in 85% of all RAS-driven cancers,
NRAS in 12% and
HRAS in 3% (COSMIC v82).
RAS mutations are frequently found in pancreatic ductal adenocarcinoma (69–95%), colorectal adenocarcinoma (40–45%) and NSCLC (16–40%). However, in breast, melanoma, brain and ovarian tumors, these mutations are less common [
54,
55]. All the mutations described result in high GTP loading, which in turn increases RAS activity, leading to uncontrolled cell proliferation, abnormal cell survival and apoptosis resistance, therefore, showing the involvement of
RAS oncogene in malignant transformation and cancer development. In spite of the intensive research in this field, the development of effective drugs that inhibit
RAS oncogenes has not been successful so far, because RAS isoforms have distinct properties and functions. Although the translocation and association of RAS proteins with plasma membrane is fundamental for its activation and to trigger downstream signaling pathways, the mechanisms that regulate these interactions among the isoforms through post-translational modifications and lipid processing are different. While HRAS is attached to the membrane by a farnesyltransferase catalyzed reaction, KRAS4B and NRAS undergo further modification by related geranylgeranyl isoprenoid formation [
56]. Consequently, farnesyltransferase inhibitors (FTIs) were more efficient in preclinical studies comprising HRAS-driven cancers [
57] and failed to demonstrate the same efficiency in tumors that harbor mutations in
KRAS [
58]. Tipifarnib and lonafarnib were the only FTI which advanced to Phase III clinical trials, but with poor clinical outcomes. The treatment with tipifarnib was evaluated in refractory advanced colon cancer, metastatic pancreatic cancer and advanced NSCLC, however it demonstrated minimal clinical activity and did not improve OS [
59‐
61]. Tipifarnib was also tested in combination with gemcitabine, the standard chemotherapy agent used in advanced pancreatic cancers. Although the combination of gemcitabine and tipifarnib demonstrated antiproliferative activity in preclinical and in phases I and II clinical studies, the OS of patients was not increased when compared with the administration of gemcitabine as a single agent in phase III trials [
62]. Lonafarnib was used in combination with paclitaxel and carboplatin in patients with metastatic, taxane-refractory/resistant NSCLC and the authors observed that the treatment was well tolerated and presented minimal toxicity, however without improving OS [
63]. The failure in anti-RAS drug discovery decreased the studies in this field and promoted the development of alternative strategies to inhibit RAS activation. In the last years, a significant effort has been made to develop low-molecular-weight chemical inhibitors of the downstream effectors of RAS, notably the RAF-MEK-ERK and PI3K-AKT-mTOR signaling pathways and some of them have already been approved by the FDA while others are in different clinical trial phases. Although some of the downstream effectors of RAS are not tyrosine kinases, they are activated by such proteins, as is the case of EGFR, PDGFR and VEGFR, and because of that they are discussed in this article.
RAF/MEK/ERK signaling pathway
The first kinase activated by RAS-GTP in the MAPK cascade is the serine/threonine-specific protein kinase RAF, comprising three tissue-specific isoforms: ARAF, BRAF and CRAF/RAF1. RAF activates MEK1 and MEK2 dual-specificity kinases, the only RAF known substrates, which in turn phosphorylate the effectors ERK1 and ERK2 related serine/threonine kinases. Activation of this signaling pathway culminates in the phosphorylation of cytoplasmic and nuclear targets regulating cell proliferation, survival, differentiation, apoptosis and in some circumstances negative feedback regulators of the RAF-MEK-ERK pathway [
64]. Activation of RAF-MEK-ERK transduction circuit is sufficient to induce proliferation and migration of normal fibroblasts independent on upstream RAS signaling, reinforcing the participation of these effectors in cancer progression [
65].
Mutations associated with RAF family are frequently associated with
BRAF and even though
BRAF mutations are genetic drivers in a wide range of tumors, they are mainly found in cancers that harbor
RAS mutations, such as malignant melanoma, colorectal and thyroid carcinomas. Mutations in
BRAF are found in up to 66% of melanoma patients, 18% of colorectal carcinomas and in 66% of papillary thyroid carcinoma cases and are associated with poor prognosis [
66‐
69]. All the mutations are in the kinase domain, almost all of which are a single substitution of valine for glutamic acid at codon 600 (V600E) [
66]. These mutations increase the kinase activity of BRAF and stimulate the phosphorylation of downstream effectors ERK1 and ERK2, increasing cell proliferation and survival and its identification provides new therapeutic opportunities [
66]. On the other hand, mutations of
CRAF,
ARAF or
MEK1/2 are uncommonly described in human tumors [
70]. However, in some lung cancer models that harbor
KRAS mutations, CRAF is mediating oncogenic signaling from KRAS [
71,
72], suggesting it would be a target for pharmacological inhibition. Moreover, as MEK is the only kinase that activates ERK and ERK is the only known substrate for MEK, the development of inhibitors for this signaling pathway is an attractive strategy in cancer therapy.
The participation of BRAF in tumor progression was reported in many studies. Overexpression of mutated
BRAF into immortalized melanocytes induces anchorage-independent growth, mediates melanoma cell invasion and the development of tumors in mice [
73‐
75]. On the other hand, inactivation of BRAF by RNA interference or small molecules leads to ERK phosphorylation inhibition, cell cycle arrest and apoptosis in preclinical models [
76,
77] exclusively in BRAF-V600E-positive cells, indicating BRAF as a promising druggable target.
Sorafenib, the first RAF inhibitor developed, was designed to inhibit CRAF, but it also decreases the activity of wild-type BRAF and the oncogenic
BRAF V600E mutant and is an antiangiogenic tyrosine kinase VEGFR/PDGFR-targeting drug. It was approved by the FDA in 2007 for advanced hepatocellular carcinoma, increasing OS and in 2013 for the treatment of locally recurrent or metastatic, progressive differentiated thyroid carcinoma refractory to radioactive iodine treatment. Although the treatment increases PFS, OS was not improved [
78,
79]. Moreover, it has been reported that sorafenib treatment causes the development of skin lesions, including keratoses, keratocanthomas (KA) and squamous cell carcinomas (SCCs), suggesting that the molecule may not be efficient in RAS-driven tumors, since it induces a feedback activation of this signaling pathway, increasing proliferation of epithelial cells [
79,
80].
Vemurafenib and dabrafenib, approved by the FDA in 2011 and 2013, respectively, improved OS and PFS of metastatic or unresectable melanoma patients when compared with dacarbazine and preferentially inhibit the V600E mutant form of
BRAF over the wild-type form [
81‐
84]. However, almost all patients relapsed due to development of drug resistance, in patients treated with vemufarenib the median time to progression being 7 months and with dabrafenib being 5 months [
68]. This occurs due to the paradoxical activation of ERK signaling in tumor cells with wild-type
BRAF or the ones that harbor
RAS and
BRAF mutations mutually [
82]. Innumerous mechanisms have been proposed to explain the BRAF-target therapy acquired resistance, including increased PDGFR receptor tyrosine kinase-mediated activation of alternative oncogenic pathways, secondary mutations in
NRAS [
85], formation and transactivation of BRAF-CRAF heterodimers [
86], upregulation of the
BCL2A1 anti-apoptotic gene [
87], hyperactivation of CRAF driven by oncogenic
RAS [
86], increased migration capability conferred by CD271 overexpression [
88] or activation of the other MAPKK COT [
89]. Furthermore,
HRAS mutations were detected in 60% of tumor samples from patients who developed KA and cutaneous SCCs after vemurafenib treatment [
90].
Vemurafenib and dabrafenib were also evaluated in innumerous clinical trials for NSCLC and colorectal cancer, however, as a monotherapy it did not overcome the tumor strategies to progress. In lung adenocarcinomas,
BRAF V600E mutant is found in only 1-2% of patients, conferring aggressiveness and resistance to currently available therapies including chemotherapy and radiotherapy [
91]. Dabrafenib treatment could represent an option for patients with advanced NSCLC, but studies demonstrate only partial response. Moreover, such as in melanoma patients, drug resistance was observed and 30% of the treated group relapsed.
These studies reinforce the importance of identifying mutated genes and consequently activated signaling pathways in clinical practice and before administration of BRAF-target drugs improving patient’s response and avoiding side effects.
The observation that
RAS oncogene overexpressed with BRAF
V600Erenders ERK signaling vemurafenib resistant and the essential participation of CRAF in lung cancers with mutations in
KRAS leads to development of pan-RAF inhibitors, named LY3009120 and PLX8394, which do not activate MAPK signaling in tumors that harbor
RAS mutations [
92,
93]. These inhibitors block signals from RAF homo and heterodimers, including CRAF-containing dimers, therefore, overcoming paradoxical MAPK activation.
LY3009120 inhibited the proliferation of melanoma cells with either
BRAF or NRAS and colorectal cancer cells with
BRAF and
KRAS mutations by inducing G0/G1 cell cycle arrest. Moreover, the treatment with LY3009120 inhibited the growth of melanoma cells that harbor
NRAS mutations xenografts and KRAS-driven colorectal tumors
in vivo [
92,
94]. However, the continuous treatment of HCT 116 cells with LY3009120 leads to development of resistance as showed by the reactivation of RAF/MEK/ERK cascade, possibly by the crosstalking with AKT signaling pathway [
94]. LY3009120 is in a phase I clinical trial for the treatment of advanced or metastatic melanoma, NSCLC and colorectal carcinomas (NCT02014116).
The other pan-RAF inhibitor developed, PLX8394, decreased the proliferation of vermurafenib resistant metastatic colorectal cancer cell lines by preventing RAF dimer formation and paradoxal MAPK signaling pathway activation [
93,
95]. PLX8394 is being evaluated in phase I/IIa clinical trial for safety, pharmacokinetics and pharmacodynamics in patients with advanced
BRAF mutated melanomas, thyroid carcinoma, colorectal cancer and NSCLC (NCT02428712).
Potent and highly selective allosteric MEK1/2 inhibitors were also developed for the treatment of oncogenic BRAF and RAS driven cancers and two of them, trametinib and cobimetinib, were approved as a single-agent therapy by the FDA for the treatment of V600E mutated metastatic melanoma [
96,
97]. However, acquired resistance was developed within 6 to 7 months after treatment with trametinib monotherapy in nearly 50% of the patients, in part because of reprogramming of protein kinase network, leading to expression and activation of multiple RTKs, which in turn, stimulate the RAF-MERK-ERK pathway, circumventing MEK abrogation [
98].
To overcome the development of resistance observed in patients treated with BRAF or MEK inhibitors as a single agent, it was believed that a more complete inhibition of the MAPK signaling pathway was required, so the combined therapy with trametinib and dabrafenib was approved by the FDA for the treatment of patients with BRAF V600E/K-mutant unresectable or metastatic melanoma in 2014.
The approval for the combination was based on results from an open-label phase I/II trial, which showed that trametinib combined with dabrafenib nearly doubled the duration of response and significantly improved ORR when compared with dabrafenib alone. The BRAF and MEK inhibitor combination was found to significantly reduce the incidence of secondary cutaneous squamous cell carcinoma. The approval of the agents in combination marks the first for a targeted therapy combination in advanced melanoma.
Uncountable phase III clinical trials, evaluating the combination of dabrafenib and trametinib in previously untreated melanoma patients with unresectable or metastatic disease harboring a
BRAF V600E or V600K mutation, showed the improvement in PFS and OS compared with conventional chemotherapy or placebo, establishing the combined therapy as a standard treatment in melanoma harboring
BRAF Val 600 mutations [
99‐
101].
The decreased response to platinum-based chemotherapy and acquired resistance to vemurafenib and dabrafenib in patients with NSCLC harboring
BRAF V600E mutations led to the development of a more effective targeted therapy combining dabrafenib and trametinib, which was approved by the FDA in 2015. That approval was based on results from a 3-cohort, multicenter, non-randomized, open-label study of patients with stage IV NSCLC. The combination of BRAF and MEK inhibitors demonstrated higher overall response and median PFS than dabrafenib monotherapy, establishing the combined therapy as a standard treatment in patients with advanced or metastatic NSCLC with
BRAF V600E driver mutations. The safety profile was manageable, decreasing toxicity with thorough dose modification [
102].
More recently, it has been shown that the combination of dabrafenib and trametinib treatment decreased ERK activation, cell proliferation and induced apoptosis in human cancer cell lines harboring non-V600
BRAF mutations, which accounts for approximately half of
BRAF-mutated NSCLC [
103]. This study shows evidences for the clinical use of these drugs for neoplasias harboring other
BRAF mutations.
Another approach approved by the FDA for the treatment of metastatic melanomas with BRAF mutations is the combination of cobimetinib with vemurafenib. Cobimetinib is a highly specific selective, ATP-non-competitive inhibitor of MEK1/2 in neoplasias harboring
BRAF V600E mutations. In human xenograft models, cobimetinib decreased tumor growth of colon and melanoma tumors containing
BRAF mutations [
104]. The combined therapy using cobimetinib and vemurafenib improved the median OS, PFS and the ORR in unresectable stage IIIC or stage IV melanoma patients harboring
BRAF V600E mutations when compared with vemurafenib monotherapy [
105,
106], demonstrating the clinical benefit of this treatment. Moreover, other MEK and BRAF inhibitors have been developed and several clinical trials are ongoing. Binimetinib is an allosteric selective, ATP-non-competitive inhibitor of MEK1/2 that demonstrated anti-tumoral activity by abrogating the growth of
NRAS- and V600E
BRAF-mutated melanomas in preclinical studies using in vitro and in vivo models [
107]. In a non-randomized, open-label phase II study of advanced melanoma patients harboring
NRAS or VAL600
BRAF mutations, binimetinib showed a partial response, providing the first target therapy to treat patients with
NRAS-mutated melanomas [
108]. Binimetinib has also been evaluated in combination with encorafenib, a highly selective BRAF inhibitor, in patients with advanced or metastatic melanoma with
BRAF driver mutations. In this phase III clinical trial, the combined therapy with binimetinib plus encorafenib improved PFS and objective response rate by local and central review when compared with vemurafenib in
BRAF mutant melanoma patients [
109].
Furthermore, uncountable therapeutic strategies using MEK inhibitors in combination with other drugs to target tumors harboring
BRAF and
RAS mutations are under investigation. The efficiency of the combination of binimetinib and encorafenib plus cetuximab in the treatment of colorectal cancers harboring
BRAF V600E mutations is in a phase III clinical development (NCT02928224). Biological evidence for the combination of binimetinib with erlotinib in the treatment of
KRAS mutated NSCLC to overcome erlotinib acquired resistance was also evaluated, providing a personalized treatment based on the identification of signaling pathway dysregulations [
110].
Network modeling analysis using Transcriptional Regulatory Associations in Pathways (TRAP) suggested CDK4 as an efficient target to be associated with MEK inhibitors in the treatment of melanoma harboring
NRAS mutations which remains without effective therapy [
111]. Cyclin-dependent kinases (CDKs) are a family of serine-threonine kinases that bind a regulatory protein called cyclin and the complex CDK-cyclin regulates the progression through the cell cycle, promoting cell proliferation. The complex cyclinD-CDK4 phosphorylates and inhibits members of the retinoblastoma (RB) protein family, including RB1, regulating the cell-cycle during G1/S transition. Biological and clinical evidence have showed that combination of ribociclib with MEK inhibitors as binimetinib or trametinib have increased anti-tumoral activity in neoplasias harboring
NRAS mutations, including melanoma, NSCLC and colorectal carcinomas in preclinical models in vitro an in vivo [
111‐
113].
Regarding the combination of BRAF and MEK inhibitors with immunomodulatory agents as pembrolizumab, durvalumab or atezolizumab, antibodies that target programmed cell death receptors (PD-1) or programmed cell death-ligand 1 (PD-L1), several trials are also in clinical development [
114‐
118].
PI3K/AKT/mTOR signaling pathway
Downstream to RAS there are the lipid kinases known as PI3Ks. These are heterodimeric proteins with one catalytic subunit of which there are three isoforms, each of them related to a specific gene: p110α/PIK3CA, p110β/PIK3CB, p110δ/PIK3CD, plus a regulatory subunit associated with cancer development by increasing cell survival, cell proliferation and conferring apoptosis resistance [
35]. They phosphorylate phosphatidylinositol (4,5)-bisphosphate (PIP-2) to phosphatidylinositol (3,4,5)-triphosphate (PIP-3) on the plasma membrane, which in turn, recruits and activates phosphoinositide-dependent protein kinase 1 (PDK1). PDK1 phosphorylates the serine/threonine kinase at AKT/PKB Thr308 which then translocates to the plasma membrane, resulting in partial activation. AKT is completely activated upon its phosphorylation at Ser473 by mTOR complex 2 (mTORC2), a serine/threonine kinase, when it targets many proteins associated with cell survival or cell death depending on the cellular context, including mTORC1 [
52]. PI3K pathway is negatively regulated by Phosphatase and Tensin Homolog (PTEN), which dephosphorylates PIP3, abrogating AKT activation. Innumerous genetic abnormalities associated with oncogenic transformation have been described in PI3K/AKT/mTOR pathway, including gain-of-function mutations and amplifications in
PIK3CA,
AKT1 and
mTOR oncogenes, and loss of function mutations, deletions or epigenetic inactivation in the tumor gene suppressor
PTEN [
52,
53]. Activating mutations in
PIK3CA oncogene are found in around 30% of different tumors, including breast, colon, endometrium and prostate carcinomas [
119].
AKT1 mutations were described in breast, colorectal, ovarian and endometrial carcinomas and cause AKT1 constitutive activation [
120]. The detailed knowledge of the PI3K/AKT/mTOR pathway leads to the development of several specific drugs some of which are currently in different phases of clinical trials.
Since PI3K/AKT signaling pathway is one of the mechanisms underlying hormonal therapy resistance in advanced breast carcinoma, PI3K inhibitors were used in combination with fulvestrant or tamoxifen. Buparlisib, an inhibitor of a pan-isorform class I PI3K, taken orally, increased PFS in association with fulvestrant in postmenopausal women with advanced or metastatic estrogen receptor (ER) positive HER-2 negative breast cancer harboring
PIK3CA mutations in a phase III clinical trial [
121,
122]. Buparlisib, is already being studied (phase IB) in association with lapatinib, a dual tyrosine kinase inhibitor which abrogates the HER-2/neu and EGFR pathways, in HER-2 positive advanced breast cancer that is resistant to trastuzumab, since the PI3K cascade is involved in trastuzumab resistance, and early conclusions demonstrate that this association is feasible for this kind of breast cancer [
123].
When
PIK3CA is mutated, the association of alpelisib, another alpha-specific PI3K inhibitor and fulvestrant showed good results in a phase I study of patients with advanced ER positive breast cancer on standard therapy [
124]. There is a phase III study ongoing about the association of alpelisib or placebo with fulvestrant, and it aims to evaluate the PFS in two cohorts, one on mutated
PIK3CA and the other with the wild type gene, and both stratified by the presence of lung and/or liver metastases, and prior CDK4/6 inhibitors treatment [
122]. Other associations are being tested and in early phases of trials, as alpelisib and exemestane and letrozole, both antitumoral combinations, alpelisib and letrozole being tested for the safety and tolerability in patients with ER+ and HER-2 negative metastatic breast cancers that do not respond to endocrine therapy [
122].
Another oral drug that is being studied in phase I is taselisib, a PI3K inhibitor with selectivity for the alpha isoform and preference for tumors that harbor
PIK3CA mutations. The data showed that taselisib was effective on metastatic or locally advanced solid malignancies that progressed or failed standard therapy, showing antitumor activity at low doses [
125]. When associated with other inhibitors such as fulvestrant, taselisib has demonstrated a higher antitumoral response in HER-2 negative and ER positive breast cancers with
PIK3CA mutations if compared with the wild type [
122].
PI3K/AKT signaling pathway is also hyperactivated in many B-cell malignancies being associated with tumor progression. A first-in-human phase IIa trial showed that copanlisib, a PI3K inhibitor with predominant inhibitory activity against both PI3K-α and PI3K-δ isoforms, has an antitumor effect as a single therapy in relapsed/refractory non-Hodkin’s lymphoma (NHL) and chronic lymphocytic leukemia [
126]. Two phase III studies are in progress in indolent NHL and one additional Phase II study in diffuse large B-cell lymphoma (DLBCL), an aggressive subtype of NHL. The phase III clinical trials are randomized, double-blind, placebo-controlled study of copanlisib in rituximab refractory indolent NHL patients who have previously been treated with rituximab and alkylating agents (NCT02369016) or to evaluate the safety and efficacy of copanlisib plus rituximab versus rituximab single therapy in patients with relapsed NHL who have received at least one prior line of treatment, including rituximab and an alkylating agent (NCT02367040). The phase II is open-label, single arm study in patients with relapsed or refractory DLBCL to evaluate the efficacy and safety of copanlisib (NCT02391116). It is important to know that most of the tumors that were more affected by copanlisib had less activity of PTEN, and there was no association to
PIK3CA mutation, despite the number of patients was not the best to conclude it definitely [
127].
In metastatic castration resistant prostate cancer (mCRPC), it was shown that AKT1 activation induces resistance to docetaxel and prednisolone chemotherapy [
128]. Preclinical studies demonstrated the antitumoral activity of AZD5363, a pan-AKT inhibitor, as a monotherapy. Moreover, the combination of AZD5363 with hormonal therapy improved efficacy of PI3K/AKT-targeted treatment in PTEN-negative prostate carcinoma models, implicating this pharmacological strategy in this type of cancer [
129]. There is an ongoing phase I/II trial in mCRPC that evaluates the association of AZD5363 with androgen receptor antagonist enzalutamide (NCT02525068). There are many studies about combination of AZD5363 with other drugs to potentialize its effect [
130‐
132], but just a few clinical trials, which means that there is a long way to FDA approved treatments involving AKT inhibition when it is super activated.
mTOR inhibitors are also being studied, and they seem to be a good treatment option for some kinds of cancers, including gynecological ones, since their use alone or in combination with other hormonal drugs are good strategies that need further studies [
133]. An example is the everolimus, an mTOR inhibitor approved by the FDA for the treatment of many types of cancer, including kidney cancer and some neuroendocrine tumors. Association of everolimus with endocrine therapy showed a good option for HER-2- and ER+ metastatic breast cancer [
134]. In renal cell carcinoma it was observed that everolimus associated with other drugs, as levantinib, cabozantinib and nivolumab, has a better antitumoral effect than everolimus alone [
135].
All these evidences show that altered PI3K/AKT/mTOR altered pathway may induce tumorigenesis, and treatments that focus on these mutations and dysfunctions are targets of further studies, moreover, association of drugs can interrupt tumor progression in more than one point and avoid resistance caused by pathway crosstalk.
ABL1 kinase
The
ABL1 (Abelson murine leukemia viral oncogene homolog 1) proto-oncogene encodes tyrosine kinases that can be found both in the cytoplasm and the nucleus of different cell types and that shuttle between the two compartments. Activation of ABL1 is mediated by different receptor tyrosine kinases, including EGFR, PDGFR and VEGFR [
136]. Furthermore, ABL is also activated by intracellular signals such as DNA damage and oxidative stress, leading to p73 phosphorylation and apoptosis induction [
137]. Activated ABL1 phosphorylates a large number of substrates, such as adaptors, other kinases, cytoskeletal proteins, transcription factors and chromatin modifiers, which in turn, activate innumerous signaling pathways, including RAS/RAF/MEK, PI3K/AKT and lipids and protein phosphatases, thereby regulating cell differentiation, cell proliferation, cell survival, cell migration, cell invasion and stress response [
138]. BCR-ABL1 is associated with the increased expression of cytokines as granulocyte colony-stimulating factor and granulocyte-macrophage colony-stimulating factor (GM-CSF) [
139].
Oncogenic activation of the ABL1 kinase is induced as a consequence of the t(9;22)(q34;q11) chromosome translocation in Philadelphia-positive human leukemia, generating the new fusion gene
BCR-ABL1, a cytoplasmic-target tyrosine kinase with constitutive activity, leading to abnormal cell proliferation and increased resistance to apoptosis [
136]. The presence of the BCR-ABL1 protein is a genetic hallmark of CML, characterized by the neoplastic transformation of haematopoietic stem cells. The requirement of BCR-ABL1 to the development of CML, renders ABL1 an attractive pharmacological target. In 2001, FDA approved imatinib, as the first-line treatment for Philadelphia chromosome-positive CML, both in adults and children. Imatinib is a potent inhibitor of the tyrosine kinases ABL, ARG, PDGFR and KIT, inducing apoptosis of BCR-ABL positive cells [
140]. The FDA has also approved imatinib for use in adults with relapsed or refractory Philadelphia chromosome-positive acute lymphoblastic leukemia (Ph + ALL) [
141]. It was reported that imatinib induced complete cytogenetic response as analyzed by in situ hybridization in more than 80% of the patients newly diagnosed with CML in chronic phase (CP), however, in patients with more advanced phases, the complete remission was less frequent [
142]. Acquired resistance to imatinib was observed in 40% to 60% of the patients since BCR-ABL positive cells persists after the target therapy and one of the mechanisms described was the emergence of point mutations in the kinase domain of
BCR-ABL gene that prevent drug interaction [
142]. More than 90 different mutations have been described in
BCR-ABL gene, conferring variable degrees of resistance to imatinib treatment.
Dasatinib, another BCR-ABL and also a Src family tyrosine kinase inhibitor was approved by the FDA as an important strategy for the treatment of patients with newly diagnosed chronic-phase CML and for imatinib-resistant or -intolerant patients with CP or advanced-phase CML or Ph + ALL [
143].
Nilotinib was also developed and approved by the FDA in 2007 for the treatment of adult patients with newly diagnosed Ph + CML-CP and patients with imatinib-resistant or imatinib-intolerant Ph + CML in CP or accelerated phase (AP). Nilotinib is a selective BCR-ABL kinase inhibitor, structurally related to imatinib and exhibited 10–30 fold more potency than imatinib in inhibiting BCR-ABL tyrosine kinase activity and proliferation of BCR-ABL expressing cells. It was showed that treatment with nilotinib is more effective because it induces less diverse
BCR-ABL mutations than imatinib in patients with chronic myeloid leukemia in CP, however, the incidence of the T315I mutation was similar with nilotinib and imatinib. Moreover, the progression to accelerated phase/blast crisis was lower with nilotinib than imatinib in patients with emergent
BCR-ABL mutations [
144].
More recently, ponatinib was developed and approved by the FDA in 2016 to treat patients with Ph + CML and Ph + ALL carrying T315I mutation, which was resistant to imatinib or nilotinib [
145]. Ponatinib was designed applying ARIAD’s computational and structure-based drug design platform to inhibit the kinase activity of BCR-ABL protein with more potency and specificity. Ponatinib was designed to target the mutated BCR-ABL isoforms that render leukemia cells resistant to treatment with existing tyrosine kinase inhibitors, especially including the T315I mutation for which no effective therapy exists [
146]. Drugs being tested for tumors other than the FDA approved scenarios in the case of signalling pathways downstream molecules are listed in table 2 and examples of inhibitors of each downstream molecule can be seen in figure 1 (Table
2, Fig.
1).
Table 2
Examples of drugs targeting downstream effectors of tyrosine kinase receptors that are in clinical development
BRAF;CRAF/RTK | Sorafenib + Vinorelbine | Breast Cancer | I/II | Completed | NCT00828074 |
BRAF;CRAF/Microtubule/DNA replication | Sorafenib + Paclitaxel + Carboplatin | Ovarian Cancer | II | Completed | NCT00390611 |
Pan RAF | LY3009120 | Advanced or Metastatic Cancer | I | Active | NCT02014116 |
Pan RAF | PLX8394 | Advanced Cancers | I/II | Terminated | NCT02012231 |
Pan RAF | PLX8394 | Advanced Unresectable Solid Tumors | I/II | Recruiting | NCT02428712 |
MEK1/2 | Trametinib | Oral Cavity Squamous Cell Cancer | II | Completed | NCT01553851 |
MEK1/2 | Trametinib | Cancer | II | Active | NCT01072175 |
MEK1/2 | Dabrafenib | Papillary Thyroid Carcinoma | Unknown | Completed | NCT01534897 |
MEK1/2/BRAF/BRAF | Binimetinib + Encorafenib + Vemurafenib | Melanoma | III | Active | NCT01909453 |
MEK1/2/BRAF/EGFR | Binimetinib + Encorafenib + Cetuximab | Metastatic Colorectal Cancer | III | Recruiting | NCT02928224 |
MEK1/2/CDK4/6 | Binimetinib + Palbociclib | NSCLC | I/II | Recruiting | NCT03170206 |
BRAF/EGFR | Encorafenib + Cetuximab | Metastatic Colorectal Cancer | I/II | Active | NCT01719380 |
BRAF | Encorafenib | Melanoma and Metastatic Colorectal Cancer | I | Active | NCT01436656 |
BRAF + PD-1/PD-L1 axis | Vemurafenib + Pembrolizumab | Melanoma | I | Recruiting | NCT02818023 |
BRAF + PD-1/PD-L1 axis | Vemurafenib + Atezolizumab | Melanoma | | Active | NCT01656642 |
PI3K/Estrogen Receptor | Buparsilib (BKM120) + Fulvestrant | Metastatic Breast Cancer | III | Active | NCT01633060 |
PI3K/EGFR, HER2neu | Buparsilib (BKM120) + Lapatinib | Breast Cancer | I/II | Suspended (data analysis) | NCT01589861 |
PI3K | Alpelisib + Fulvestrant | Breast Cancer | III | Recruiting | NCT02437318 |
PI3K | Alpesilib + Fulvestrant | Breast Cancer | II | Not yet recruiting | NCT03386162 |
PI3K/Aromatase | Alpelisib + Fulvestrant + Letrozole | Breast Cancer | II | Recruiting | NCT03056755 |
PI3K/mTOR/Aromatase | Alpelisib + Everolimus + Exemestane | Breast Cancer, Kidney Cancer, Pancreatic Neuroendocrine Cancer | I | Active | NCT02077933 |
PI3K | Taselisib + Fulvestrant | Breast Cancer | III | Active | NCT02340221 |
PI3K/Aromatase | Taselisib + Letrozole | Breast Cancer | II | Completed | NCT02273973 |
PI3K | Taselisib | Recurrent Squamous Cell Lung Carcinoma, Stage IV Squamous Cell Lung Carcinoma | II | Active | NCT02785913 |
PI3K | Copanlisib (BAY80-6946) | Non-Hodgkin Lymphoma | III | Active | NCT02369016 |
PI3K/CD20 | Copanlisib (BAY80-6946) + Rituximab | Non-Hodgkin Lymphoma | III | Recruiting | NCT02367040 |
PI3K | Copanlisib (BAY80-6946) | Diffuse Large B-cell Lymphoma (DLBCL) | II | Active | NCT02391116 |
AKT/Androgen Receptor | AZD5363 + Enzalutamide | Adenocarcinoma of the Prostate | II | Recruiting | NCT02525068 |
AKT/Microtubule | AZD5363 + Paclitaxel | Advanced Gastric Cancer | II | Recruiting | NCT02451956 |
AKT | AZD5363 | Metastatic Castrate-Resistant Prostate Cancer (mCRPC) | I | Completed | NCT01692262 |