A patent update on PDK1 inhibitors (2015-present)
Introduction: 3-Phosphoinositide-dependent kinase 1 (PDK1), the “master kinase of the AGC protein kinase family”, plays a key role in cancer development and progression. Although it has been rather overlooked, in the last decades a growing number of molecules were developed to effectively modulate PDK1 enzyme.
Areas covered: This review collects different PDK1 inhibitors patented from October 2014 to December 2018. The molecules have been classified on the basis of the chemical structure/type of inhibition and for each general structure examples have been discussed in extenso.
Expert opinion: The role of PDK1 in cancer development and progression as well as in metastasis formation and in chemoresistance has been confirmed by a large number of studies. Therefore, the pharmaceutical discovery in both public and private institutions is still ongoing despite the plentiful molecules already published. The majority of the new molecules synthetized interact with binding sites different from the ATP binding site (i.e. PIF pocket or DFG-out conformation). However, many researchers are still looking for innovative PDK1 modulation strategy such as combination of well-known inhibitory agents or multitarget ligands, aiming to block, together with PDK1, other different critical players in the wide panorama of proteins involved in tumor pathways.
Keywords: allosteric inhibitors, chemoresistance, DFG-out inhibitor, drug discovery, metastasis, 3-phosphoinositide-dependent kinase 1, PDK1 inhibitor, PIF-pocket inhibitor, PH-domain inhibitor, PI3K/PDK1/Akt pathway
Article highlights:
PDK1, generally referred to as the master kinase of the AGC family, is considered a compelling target in oncology; nevertheless, nowadays no selective PDK1 inhibitor is available for cancer therapy;
PDK1 inhibitors patented from October 2014 to December 2018 are collected and classified according to the chemical structure/type of inhibition;
Although the majority of patents concern newly synthetized chemical molecules for cancer therapy, innovative strategies of PDK1 inhibition are mentioned;
The combined and simultaneous inhibition of other targets could represent the future strategy to better exploit the potential of PDK1 inhibition in anticancer therapy.
1. Introduction
The fight against cancer is one of the major issue in Health: as WHO reposted on its website, the estimated number of deaths due to cancer in 2018 is approximately 9.6 million, about 1 in 6 deaths (http://www.who.int/news-room/fact-sheets/detail/cancer).
Cancer could be defined as a class of diseases, which encompasses a wide and heterogeneous range of different pathology affecting diverse tissues and cell types. Each tumour subtype has distinctive cell-of-origin, risk factors and clinical phenotypes, that generate unique combinations of mutation types [1].
Despite the wide heterogeneity, it is possible to define some common features, strongly related with each other and shared by most types of cancer such as the abnormal proliferation, the ability to migrate and adapt in other tissues, the chemoresistance and the recurrence. All these features depend by the gene/protein patterns, specific among tumor subtypes but also between each patient. In this complex scenario, oncology needs a continuous help by research to find innovative tools effective to fight this devastating disease.
In the last two decades, kinases have been intensively investigated as the most promising targets for the development of new anticancer drugs [2] due to their intimate involvement in pivotal cancer processes such as cell proliferation and survival.
Since its discovery [3], the phosphoinositide 3-kinase (PI3K)/PDK1/Akt pathway has been recognized as central in cellular physiology; rapidly, the increasing knowledge around its
implication in cellular processes such as proliferation, survival, angiogenesis, metabolism etc., expanded the interest as research topic in oncology [4, 5]. Epidemiological and experimental studies have validated the abnormal activation of this pathway as an essential step toward the initiation and maintenance of human tumors [6, 7] .
The PI3-Kinase, activated by extracellular stimulating signals, starts a cascade of phosphorylation: the first one transforms the second messenger phosphatidylinositol 4,5-bisphosphate (PIP2) in phosphatidylinositol 3,4,5-trisphosphate (PIP3), increasing its level. The growing amount of phosphoinositides (PI) facilitates the interactions of Akt and PDK1 with the plasma membrane through their respective PH domains. This signalling cascade allows PDK1 to phosphorylate Akt on Thr308; the second phosphorylation by mTOR on Ser473 fully activates Akt, which, in turn, interacts with a plethora of different downstream effectors.
For a long time, the vast majority of studies have focused on the protein kinase AKT as the dominant effector of PI3K signaling [8]. Both PI3K and Akt kinases have been widely investigated as key targets to develop new anticancer drugs. Only in the last two decades, many papers and registered patents are highlighting the intense efforts made in developing valid PDK1 inhibitors. This is mainly due to (a) overexpression of this target in many aggressive tumors (e.g acute myeloid leukemia, melanoma and esophageal squamous cell carcinoma, glioblastoma, lung cancer and breast cancer) [9, 10, 11] and (b) to the key role played in the regulation of at least 23 other members of the AGC family. Due to this relevant regulatory activity, PDK1 has been renamed the “master kinase of the AGC family” [12, 13, 14].
Notably, the mechanism described for PDK1-mediated Akt activation differs to the phosphorylation of the other substrates. Indeed, it critically depends on the translocation of both proteins to the plasma membrane by binding to PIP3 via their respective PH domains [15, 16] and involves the formation of a Akt-PDK1 heterodimer complex [17, 18].
On the contrary, different downstream kinases interact with the PIF-binding pocket of PDK1 through their phosphorylated hydrophobic motif (HM), to be then phosphorylated on their activation loop and activated [19]. Recently, several mechanisms involved in the regulation of PDK1 such as serine/threonine as well as tyrosine phosphorylation, subcellular localization, regulator binding and conformation status have been emerged [18]. Even if many of them remain to be clarified, such mechanisms are responsible for PDK1 modulation
of important kinases of AGC subfamily such as p70 ribosomal S6 kinase (S6K) [20], p90 ribosomal S6 kinase (RSK2) [21], serum/glucocorticoid regulated kinase (SGK) [22] and protein kinase C (PKC) [23]. The aberrant activation of these and other PDK1 downstream effectors contribute to increase the effects induced by Akt pathway activation and are thus responsible for the abnormal cell replication, metabolic reprogramming, apoptosis escape, atypical angiogenesis, invasion and metastasis formation [24]. Recently Castel et al.[25] have proved the existence of a PI-independent phosphorylation of Akt by PDK1. Such a mechanism involves the axis PDK1-SGK1, which is responsible for the mTORC1 activation, a Akt-downstream effector and promoter of tumor progression. The study shows that SGK1 activation requires both PDK1 kinase activity and the PIF-binding pocket involvement. These findings suggest that the simultaneous inhibition of PI3K and PDK1 could represent a strategy to overcome resistance to PI3K inhibition, thus opening up new therapeutic options for cancer therapy.
PDK1 is a soluble and globular medium-sized kinase composed by 556 amino acids. This protein could be divided into two domains: the N-terminal kinase catalytic domain and the C-terminal Pleckstrin homology (PH) domain [26]. The PH domain includes a sequence of approximately 120 amino acids shared by a wide range of proteins involved in intracellular signalling or as constituents of the cytoskeleton.
Three distinct ligand binding sites could be identified in the kinase domain: the substrate binding site, the ATP binding site, and the docking site (also known as PIF pocket) each one targeted by different classes of modulators.
The two N-terminal small lobe and the C-terminal large lobe, contain, as a “sandwich”, one ATP molecule needed for the subsequent substrate phosphorylation [27]. Around the ATP-binding site there are three regions, the Glycine-rich loop, the C-helix and an activation loop; all of them are essential for the activity since they arrange the ATP-binding site in the PDK1 active form [28].
Notably, in the past years, the existence of diverse allosteric sites have been demonstrated.
The most investigated one is the PIF-pocket binding site, through which small molecules proved to modulate the PDK1 activity [29, 30]. The pocket is formed by the conserved αC helix, β strands and helix αB; the residues T148, R131, Q150 and L155 has been identified as the key aminoacids involved in the substrate-interaction [31].
Another allosteric site, namely DFG-out pocket, has been detected. It contains mostly hydrophobic residues including Met 134, Leu 137, Phe 142, Leu 196 and Ile 221 [32]. Moreover, this pocket results to be accessible only when the DFG motif (e.g. Asp223-Phe224-Gly225), located at the N-terminus of the activation loop, assumes a particular conformation. The DFG motif flips into an “in” or “out” pose and then turns the kinase in its catalytically active (DFG-in) or inactive (DFG-out) conformation[33, 34, 35].
PDK1 is considered a constitutively active enzyme, due to the autophosphorylation on Ser-241 [36] and it is located both in the cytoplasm and in the mitochondrial matrix.
Beside the promotion of cancer development and progression, several studies also enlighten the role of PDK1 in the onset of chemoresistance in different types of malignancy [14]. More recently, the function of PDK1 in metastatisation has been investigated [13, 37].
Indeed, a crucial role of several kinases in the regulation of each phase of cell migration has been shown. This process is often regulated by the abnormal migratory signals which include the activation of several proteins such as Akt itself [38, 39, 40], myotonic dystrophy-related CDC42-binding kinases alpha (MRCK) [41], Rho associated coiled-coil containing protein kinase 1 (ROCK1)[42], phospholipase C gamma 1 (PLC1) [43] and integrins [44, 45]. PDK1 regulates cell migration through all these signalling transducers [37]. Additionally, PDK1 overexpression has been shown to enhance the invasiveness of tumor cells [41, 46].
It should be mentioned that the PI3K/PDK1/Akt pathway interacts with other relevant cancer promoter/inhibitor kinases such as the phosphatase and tensin homolog protein (PTEN)
[47], and downstream effectors such as p53 [48] and SGK [49] and establishes sideward connections with many other important regulator pathways like Ras/Raf/MEK/ERK [50] and KRAS [51]. With this knowledge in mind, it is quite evident that PDK1 is a key target with multiple roles in cancer and therefore, represents a worthwhile tool for the rational design of innovative therapies for cancer diseases, particularly for those tumors with a high degree of aggressiveness, invasiveness and metastatisation.
2. New patented PDK1 inhibitors for the treatment of cancer
Although it has been rather neglected in the past, the role for PDK1 - as an actor but also as a drug target - in cancer has recently emerged as pivotal. However, no selective PDK1 inhibitor is currently available in cancer therapy. Many molecules have been synthetized and they could be generally classified as: ATP binding site inhibitors, PH domain inhibitors,
DFG-out conformation inhibitors and PIF pocket inhibitors [31, 52]. Comprehensive reviews have been published [53, 54, 55, 56] and one of the most recent focuses on PDK1 disruptors and modulators patented within September 2014 [57].
Herein, we deal with patents published from October 2014 to December 2018. To ease the reading, we maintained the classification criterions adopted by Hossen et al [57]. Thus, we grouped the molecules according with the chemical structure, the inventors and the applicants.
2.1 Heterocyclic compounds
In a patent published in 2015, Sunesis Pharmaceuticals developed a series of heterocyclic compounds with the general structure indicated in Table1 (entry #1) [58]. The series of compounds is proposed as molecules endowed of a dual mechanism of action. In particular, many compounds described in this patent have been characterized as both inhibitors of PIF-mediated substrate binding and as blockers of ATP binding site. Indeed, these ligands seems to be able to promote a conformational change of PDK1 which induces (a) a block of PIF binding, thus preventing the binding and the phosphorylation of PI-indipendent substrates and (b) the inhibition of PDK1 activity through the blockage of ATP binding site [58]. The dual-mechanism of action could make these molecules useful in treatment of Akt-independent cancers or in tumors with already established resistance to Akt inhibitors . Moreover, such compounds can find therapeutic applications in cancer types in which growth is related to PIF-dependent downstream substrates such as S6K and RSK2, independently from Akt activation [59, 60].
Among the compounds patented, Cmpd 1, Cmpd 2 and Cmpd 3 (see Table 1) were deeply investigated. The selected molecules showed a good inhibition of target but also a good selectivity profile, with inhibition values ≥90% for ≈20 versus 270 kinases tested. In vitro assays in hematologic tumor cell lines showed a good inhibition of proliferation with EC50 from 3 to 853 nM and a dose-dependent capacity to induce apoptosis. Western blot analysis confirmed the phosphorylation inhibition of PDK1 downstream effectors. In particular, the phosphorylation status of Akt and RSK2 was analyzed.
Currently, known inhibitors of PDK1 typically affect both PDK1-mediated Akt phosphorylation and PDK1-mediated PKC phosphorylation, giving rise to doubts regarding the onset of side effects [61]. Since the RSK2 activity does not depend on Akt, targeting RSK2
has been suggested as a strategy to overcome resistance mechanism in patients treated with Akt inhibitors [62].
RSK2 is a PDK1 substrate and its activation occurs through a PIF- dependent mechanism; the kinase is known to be involved in cell growth of various cell types but also in hematopoietic transformation [63] and in several cancers such as myeloid leukemia [64].
In vitro results were confirmed by in vivo investigations in MV4-11 xenograft tumor where Cmpd 1 and Cmpd 2 reduced tumor volume and both pRSK2 and pAkt levels. Crystallographic complex and FRET- based peptide binding assay confirmed the binding to
the inactive form (DFG-out conformation). This kind of interaction causes a perturbation of PIF-pocket which negatively modulated the binding of PI-indipendent substrates [58].
These compounds are indicated for patients affected by RSK2-dependent or Akt independent solid tumor or hematological cancers, in a suitable pharmaceutical formulation, alone or in combination with other appropriate anticancer drugs able to exert a synergic or at least additive effect, including other PDK1 inhibitors, In particular, the authors , suggested the use of alkylating agents (e.g., chlorambucil, cyclophosphamide), antimetabolites (e.g., methotrexate), aurora kinase inhibitors (e.g., ZM447439, VX-680, AZD1152); purine and pyrimidine antagonists (e.g., 5-fluorouracil (5-FU), cytarabine (Ara-C), gemcitabine), spindle poisons (e.g., vinblastine, paclitaxel), podophyllotoxins (e.g. irinotecan, topotecan), antibiotics (e.g., doxorubicin, daunorubicin), inorganic ions (e.g., platinum complexes such as cisplatin, carboplatin), hormones (e.g., tamoxifen, leuprolide, flutamide, and megestrol), topoisomerase II inhibitors, EGFR inhibitors (e.g., gefitinib), antibodies (e.g., bevacizumab, rituximab), and various targeted agents such as HDAC inhibitors (e.g. vorinostat), Bcl-2 inhibitors, VEGF inhibitors, and cyclin-dependent kinase (cdk) inhibitors [58] as second active chemotherapeutic anticancer agents to be used in the combination.
Moreover, the patent reports the possibility to apply the invention in a variety of tumors scarcely sensitive to inhibitors targeting other kinases within the same pathway. Therefore, these compounds have been indicated potentially useful as first line therapeutics, or as second line treatments in cases of resistance to other PI3K/Akt pathway inhibitors.
Sunesis Pharmaceuticals recently filed a new patent [65] with 443 molecules included in the same general structure. The compounds were classified in 9 substructures showed in Table 1 (entry #2). The molecules herein described have been evaluated for their ability to inhibit phospo-PDK1 isolated enzyme and phospho-Akt (Thr308) levels in PC-3 cell line. The
structures and the in vitro data collected for some selected compounds belonging to different chemical substructures have been reported in Table 2.
2.2 Pyridonyl derivatives
In 2016, the International Society for Drug Development (ISDD, Milan) patented a series of 2-oxo-1,2-dihydropyridine-3-carboxamides as PDK1 inhibitors [66]. Even if a crystallographic study is still missing, docking and molecular dynamics studies performed with one of them, namely IB35, suggested the DFG-out conformation as the most plausible binding mode [67].
In this patent were included a number of molecules (general structure = entry #3 in table 1) described as pyridonyl-based derivatives. IB35, SA16 and SA23 (IC50 values on PDK1 = 112, 416 and 1520 nM respectively) were selected on the basis of the good selectivity profile and further investigated on U87MG cell lines and stem cells isolated from U87MG cells. Results collected from cells screening reflect the ranking of IC50 values on recombinant PDK1 (IC50= 449.7 nM for IB35, 2050 nM for SA16 and 1170 nM for SA23), thus confirming the involvement of the target. Together with the reduction of cell viability, the new compounds also trigger apoptosis and significantly inhibit cell migration, coherently with the role of PDK1 in tumour cell migration. Notably, the new compounds inhibit also proliferation of glioma stem cells (GSCs) isolated from U87MG cells (IC50 = 3.36 nM for IB35, 5.47 nM for SA16 and 0.30 nM for SA23). GSCs are a subpopulation of multipotent tumor-initiating cells displaying stem cell-like characteristics [68] and strongly correlated to aggressiveness and unresponsiveness of gliomas [69]. Administration of these compounds (or co-administration with other active substances) has been suggested in the treatment of pathologies that require PDK1 inhibition, such as cancer, but also diabetes and neurodegenerative diseases such as Alzheimer's and Prion Diseases.
In 2017, the same authors patented the combination of both PDK1 and Aurora-A kinase inhibitors as a cutting-edge therapy to treat GBM [70]. The authors demonstrated that this pharmaceutical combination achieves remarkable results in treating tumors when compared to the treatment with each inhibitor alone, particularly in glioblastoma [71]. Moreover, in this patent the series of pyridonyl-based derivatives has been enlarged and the molecules described here have been classified as dual PDK1/AurA inhibitors (entry #4, Table 1).
Data collected in this patent showed SA16 as a prototype of the new class of dual PDK1-AurA inhibitors. SA16 pharmacological profile was investigated and compared to the reference drugs MP7 (selective PDK1 inhibitor) [72, 73] and Alisertib (AurA kinase inhibitor) [74]. The dual inhibitor showed an antiproliferative activity against GBM and GSCs comparable to the one exerted by the two PDK1 and AurA inhibitors combined together. In particular, SA16 induced neurosphere differentiation toward both neuronal and glial phenotypes, thus increasing tumor sensitivity and decreasing the potential onset of chemoresistance.
This pharmaceutical strategy has been suggested for pathologies requiring a simultaneous inhibition of both PDK1 and AurA enzymes, such as human solid tumors (primary colorectal carcinoma, gliomas, breast, ovarian, pancreatic cancer) and hematologic malignancies (multiple myeloma, Non-Hodgkin lymphoma, and chronic lymphocytic leukemia). Since the investigated targets are also involved in other cellular processes, these molecules/combination could be also applied in neurodegenerative pathologies such as Alzheimer's disease (AD), Parkinson's disease (PD) and Huntington's disease (HD) or in cardiovascular diseases as well as in diabetes.
2.3 Anti PDK1 antibody
In 2016, Dessain and Heimbach patented an invention consisting in a new antibody targeting extracellular PDK1 [75]. The idea was mainly based on the discovery that PDK1 is located at the plasma membrane and in the cytosol, but is excluded from the nucleus [76] .
The antibody (or antigen-binding fragment) has been proposed as a tool to detect PDK1 in a biological sample, such as tumor tissue, cells, serum or plasma, by the use of a conventional immunoassay, but also as potential inhibitor of PDK1 activity in patients affected by PDK1-related disorder, such as a cancer. Coherently with the key regulator role of PDK1 in cell migration and cancer dissemination [77], the anti-PDK1 antibody could be also used to verify metastasis formation. Therefore, the administration of this antibody has been suggested in metastatic or non-resectable cancer, in particular cervical, breast, lung and prostate cancer, in acute myeloid leukemia (AML) and in squamous cell carcinoma. As well as other types of PDK1 modulators/inhibitors, the administration is preferred in combination with at least another drug such as a cytotoxic agent, an anti-angiogenic molecule or a chemotherapeutic drug. New therapies directed towards cancer-specific antigen targets are needed for
treatment of metastatic cancers, aiming to improve survival outcomes. PDK1 represents a valuable target also for this purpose, but, unfortunately, no paper related to this kind of application has been reported till now. In 2018, Waniczek et published a paper on the use of quantum dot-conjugated antibodies in colorectal cancer. The aim of the study was to assess the validity of the Akt- pathway activation as a prognostic factor or a predictor of response to targeted therapies [78]. Notably, the concentrations of phosphorylated/activated forms of PDK1 and AKT were assessed by the use of specific antibodies conjugated with different quantum dots [78]. Therefore, this study lay the basis for the development of new antibodies as an innovative strategy to visualize the activity state or the concentration of signaling proteins in specific compartments of cancer cells.
2.4 Combined inhibition with other kinase
In 2015 Sloan-Kettering Institute for Cancer Research filed a patent that includes combination of PDK1 and PI3K inhibitors, or PI3K and serum and glucocorticoid-regulated kinase 1 (SGK1) inhibitors. These kind of combinations have been proposed as alternative treatments in tumors resistant to PI3K inhibition [79].
The rationale behind this innovation is related to investigations revealing that resistance to PI3K inhibition triggers a PDK1-dependent mTORC1 activation [25]. This activation occurs through a PI3K-independent mechanism mediated by SGK1 [80, 81].
SGKs belong to a kinase family implicated in cell proliferation and survival regulation, but also in cancer development via an AKT-independent signalling pathway [49]. Combined inhibitions of PI3K/PDK1 or PI3K/SGK1 pathways showed to re-sensitize cells to PI3K inhibition and to reduce xenografts size in vivo.
The combinations proposed herein include: (a) compounds able to reduce PI3K expression (i.e. micro RNA (miRNA), interfering RNA (RNAi) molecules), or PI3K inhibitors (BYL719 (Apelisib), BAY80-6946 (Copanlisib), CH5132799, GDC-0941 (Pictilisib), A66, PIK 90, HS-173, MLN1117 and GDC-0032); (b) PDK1 inhibitors, such as GSK2334470, BX-912, BX-795, BAG 956, OSU 03012, PHT-427 and (c) a molecule selected among GSK650394 or SI113 which are capable to reduce SGK1 activity and/or expression.
Apart from oncological diseases, PDK1 inhibition strategy could be also applied to different therapeutic fields.
Allergic reactions are triggered by the binding of the allergen IgE with the high-affinity receptor Fc epsilon RI (FceRI) of mastocytes, thus regulating cellular function [82]. FceRI activation produces a change in mastocytes and an increase of Ca2+ - cellular intake from the extracellular compartments, with a consequent induction of the PI3K/PDK1/Akt pathway activation. The phosphorylation of PDK1 downstream effectors such as Akt, SGK and PKC elicits the activation of calcium channels and the subsequent mobilization of intracellular calcium stores together with activation of mastocyte degranulation [83, 84]. This cascade of events related to allergic inflammation is frequently referred to as an acute (or early) response to the ocular allergy phase.
On this basis, Sylentis SA patented specific siRNA products to reduce PDPK1 gene expression and consequent inflammation in ocular allergies [85]. When compared with traditional anti-allergic therapeutic agents and allergy immunotherapeutic drugs, this therapeutic strategy seems to be better in terms of duration of action, and onset of side-effects (i.e mydriasis, ocular irritation). Indeed, although traditional treatments modulate proteins behaviour, the reduction of kinase expression forces the cells to re-synthetize proteins ab initio thus determining a longer-lasting effect. The invention covers siRNA products and their use for the treatment and/or prevention of eye conditions, such as conjunctivitis and/or ocular allergy (seasonal allergic conjunctivitis, perennial allergic conjunctivitis, vernal keratoconjunctivitis, atopic keratoconjunctivitis, and giant papillary conjunctivitis), characterised by an increased PDPDK1 gene expression and/or an increased activity.
Emerging evidence show that the yeast homolog of PDK1 represents a validated drug target for antifungals drugs development [86, 87, 88].
Actually, the increasing incidence of invasive fungal infections indicates the need to improve efforts in the identification of potent and effective antifungal treatments [88]. Fungi are eukaryotic pathogens and, as such, possess many protein kinase-based signaling pathways that are well conserved with mammalian systems [86]. However, it should be pointed out that many of them display significant sequence and structural differences as compared to their human orthologs. As regards PDK1, significant differences at the PIF-pocket regulatory site between human and fungal PDK1 has been recently confirmed by a crystallographic
study [87]. Results indicate the possibility to exploit this structural gap to optimize the development of antifungal molecules with improved selectivity.
Recently, a screening strategy designed to identify molecules able to trigger yeast cell lysis led to the identification of PDK1-inhibitors (KP-372-1 OSU-03012 and UCN-01) as antifungal molecules. The molecules demonstrated a good capability to block the cell wall stress response in yeast. Further genetic and biochemical studies revealed that KP-372-1 inhibits fungal PDK1 orthologs (Pkh kinases) [86]. Altogether, these findings provide useful cues to pursue the development of PDK1 inhibitors as potential therapeutic tools for fungal infections.
4. Conclusions
Despite the large number of molecules synthetized from its identification, pharmaceutical discovery has not yet identified a valid PDK1 inhibitor candidate for cancer therapy and no clinical trials are currently active on this target.
However, both academia and pharmaceutical industry are still making efforts in this direction. Therefore, this review collects and organizes inhibitors patented from October 2014 to December 2018, according to the chemical structure/type of inhibition. The majority of the new molecules synthetized as PDK1 inhibitors interact with the PIF-pocket binding site or with the DFG-out pocket, bypassing the classical binding to the ATP-binding site. These modes of binding allow to increase selectivity and avoid the typical off-target effects.
Moreover, innovative PDK1 modulation strategies such as the combination with different kinases inhibitors with chemotherapeutic agents as well as the development of multitarget ligands, have been mentioned.
5. Expert opinion
There is a great need for innovative target and efficacious molecular tools in oncology. As confirmed by different researches, PDK1 plays a pleiotropic role in cellular growth and development. An increased activation of PDK1 stimulates tumorigenesis by improving cell proliferation and reducing apoptosis. Notably, a central role for PDK1 in cell migration and metastasization has been widely demonstrated.
Recently, a key role of PDK1 has been proved also in cancer stem cells (CSCs) [89, 90, 91, 92]. The CSC concept states that tumor growth is driven by small numbers of stem cells in dedicated niches, particularly difficult to identify and eradicate, and therefore recognized as important players in cancer resistance and recurrence [93, 94]. Altogether these findings indicate the potential strength of rationally target PDK1 to counteract cancer in humans. Although the function of PDK1 in cancer is quite clearly established, this is not reproduced in drug discovery and no PDK1 inhibitor candidate has yet reached the clinic. This failure could be attributed to lack of selectivity, low solubility, off-target effects, or weak efficacy. In our opinion, the complexity of the PDK1 structure and the different mechanisms involved in its regulation are the main drawbacks to the discovery of valuable molecules.
A preliminary analysis of PDK1 inhibitors landscape highlights that this kind of molecules is particularly relevant in oncology, but applications in other therapeutic areas such as ocular disorders and fungal infections are now emerging.
Unfortunately, up to date, only few PDK1 inhibitors from Sunesis Pharmaceuticals (i.e. SNS-229; SNS-510) are under early or late preclinical development.
The wide gap between its role in cancer and the lack of PDK1 inhibitors in clinical trials, suggests that the sole inhibition of PDK1 is not sufficient for an appreciable anti-cancer activity. Therefore, PDK1 probably requires a valid “matching” co-targeting protein to balance the activity in cancer invasion and metastasization with a comparable effect on tumor proliferation and survival. Many ongoing clinical trials in oncology are aimed at co-targeting cancer drug escape pathways. Consistently, this strategy could offer the rationale for the combination therapy of PDK1 inhibitors with chemotherapeutic agents but, above all, for the discovery of novel multi-target anticancer drugs.
This hypothesis could be placed in a more general attention on multimodal approaches, based on the evidence that inhibition of a single node is frequently followed by activation of resistance mechanisms and/or alternative pathways and escape mechanisms. In addition, recent experimental evidences proved the pivotal role played by CSCs in tumorigenesis as well as in resistance to conventional chemotherapies [92], thus suggesting that a therapeutic strategy based on drugs targeting both bulk tumor cells and CSC-specific pathway inhibition could represent a promising approach to improve cancer therapy.
Accordingly, the search for novel multitarget molecules able to inhibit PDK1 represents a challenging research field and could lay the groundwork for the design of new effective
drugs for clinical use. Indeed, as previously discussed, many signalling pathways are involved in cancer development, and most of the targets are connected to each other. This solid “target–target network” further confirms the importance of multi-target drugs as alternative approaches for cancer chemotherapy.
As widely proved by the increasing number of publications over the last decade [95, 96, 97, 98, 99, 100], the modulation of multiple targets through multifunctional ligands is attracting a growing interest in the discovery of novel treatments for complex disorders, such as neurodegenerative diseases, cardiovascular disorders and, last but not the least, cancer. In this latter case, we believe that the design of multitarget drugs, in which at least one target is PDK1, represents a compelling approach for drug design and development for the future oncology.
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Papers of special note have been highlighted as either of interest () or of considerable interest () to readers.
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