DNA topoisomerase inhibitors

DNA is a polymer made up of a couple of polynucleotide strands that form a double helical construction and carry genetic information [1]. DNA allows genetic material to be reproduced, repaired, and recombined to preserve genetic integrity while allowing for variation in living creatures [11]. The interweaving of the two anti-parallel, complementary strands causes DNA topological stress. The two strands are entangled around one another every 10.4 base pairs under physiological circumstances [2]. Cells have a series of enzymes known collectively as DNA topoisomerases that help them address the topological problems caused by the double helix [3]. DNA topoisomerases are frequently occurring enzymes that may fix topological issues with DNA that develop during biological processes such as replication, recombination, transcription, repair, and chromatin assembly [12]. By tying or untying DNA knots, the topoisomerase enzyme catenates or decatenates circular DNA super-helical strains. Enzymes called DNA topoisomerases are crucial for maintaining the integrity of the genetic material during transcription, replication, and recombination processes. They control DNA topology. Anti-tumour medications that inhibit mammalian enzymes are frequently used. They prevent the DNA relegating step of the catalytic reaction, which stimulates DNA cleavage, to stabilize topoisomerase-DNA cleavable complexes [1]. Prokaryotes and eukaryotes have been shown to have two different types of DNA topoisomerases. They are:

(1) Type I DNA topoisomerases (topo I): mechanism of action by transient double-strand breaking into one strand of DNA [13].

(2) Type II DNA topoisomerases (topo II): mechanism of action by introducing transient single-strand breaking into one strand of DNA [14].

DNA topoisomerases have sparked much attention in recent years as they were discovered to be biological targets for various anticancer medicines. Many of these medications were being used in therapeutic settings prior to their lethal effects being related to topoisomerases. Antibacterial agents such as fluoroquinolones [15] and camptothecin (CPT) [16] are great examples.

Topoisomerase targeting anticancer drugs classification

Two kinds of topoisomerase targeting anticancer drugs exist, each with a distinct mechanism of action (Table 1) [17].

Class-II topoisomerase targeting anticancer drugs

A variety of medications target topo II; they can be distributed into two groups (Figure 2).

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Figure 2. 

Variety of medications that target topo II

DNA intercalators: acridines, e.g., 4’-(9-acridinlylamino)methanesulphon-m-anisidide (m-AMSA) [22]; anthracyclines, e.g., doxorubicin and daunomycin [23, 24]; actinomycin, e.g., actinomycin D [24]; ellipticines, e.g., 2-methyl-9-OH-ellipticinium acetate [25].

Non-DNA intercalators: epipodophyllotoxins, e.g., VP16, teniposide [23]; isoflavonoids, e.g., genistein [26].

Acridine

Acridine derivatives are topo II focusing intercalative medicines utilized to cure several types of leukaemia. Although m-AMSA effectively treats acute myelogenous leukaemia, it is also hazardous [22, 27].

Anthracycline

Anthracycline antibiotics are successfully used to counter many tumours, including SCLC, breast, bone marrow, and ovarian malignancies [23, 24, 28].

Actinomycin

Actinomycin is, also called dactinomycin, used in prophylaxis of a variety of cancers, including testicular cancer, Wilms tumour, trophoblastic neoplasm, and some ovarian cancer [24, 29].

Ellipticine

It is beneficial in the management of mammary gland tumours. However, the side effects include xerostomia, which is linked to weight loss and renal toxicity, and haemolysis [25, 30].

Epipodophyllotoxins

Testicular cancer, central nervous system tumour, malignant lymphoma, SCLC, and Kaposi’s sarcoma are all targets for epipodophyllotoxins. Secondary melanoma, on the other hand, is commonly linked to treatment [23, 31].

Isoflavones

Genistein is a critical component of the soya bean, and it is assumed to be in charge of how soya bean extracts affect various tumour cells in culture [26]. Some of the chemical structures are displayed in Figure 3.

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Figure 3. 

Common chemical structures involved in the treatment of cancers

Enzyme-responsive liposomes

The therapeutic drug concentrations at the diseased site are increased by liposomes’ role as nanocarriers, which also reduces toxic effects on healthy tissues. Liposomes deliver the payloads at the target site. The ability of the nanocarrier to target particular tissues has been improved using a number of strategies, including ligand-targeted liposomes and stimuli-responsive liposomes. Due to the targeting ligand attached to their surface, ligand-targeted liposomes have a higher uptake by the target tissue than stimuli-responsive liposomes, which do not release their cargo until they come into contact with an endogenous or exogenous stimulant at the target site. The liposomes that respond to pathologically elevated levels of enzymes at the target site are the main focus of this review [32].

After coming into contact with an enzyme, enzyme-responsive liposomes release their contents through a number of destabilization processes, including: (1) structural changes to the lipid bilayer; (2) removal of a shielding polymer from the surface and increased cellular uptake; (3) cleavage of a lipopeptide or lipopolymer incorporated in the bilayer; (4) activation of a prodrug within the liposomes.

Many enzymes are induced in a variety of pathological situations, including inflammation, malignancy, and infection. This biochemical response can be leveraged to construct nanocarriers that undergo a structural transformation, allowing the contained payload to be released. Different advantages of this technique, liposomes, polymers, quantum dots, and gold have all been used to create enzyme-responsive NPs [32].

Alteration in ion channels in cancer cells

In proliferating cells, particularly cancer cells, different types of channels from all acknowledged subfamilies ion channels are actively involved. The character of K⁺ channels plays a prominent part in the literature [5]. Different K⁺ channel subfamilies have been associated with carcinoma. Any type of K⁺ channel may interfere in some way with cell growth. Additional cell features and external conditions influence specific contributions whenever the membrane voltage reaches a certain point. Excitable tissues that can be triggered by voltage-gated channels (VGCs) include nerve and muscle cells in the heart and other organs [voltage-gated potassium channel (Kv)] (Figure 4) [49]. It is still unknown if Kvs are dispatched just in tumour cells or if they also play an act in normal tissues, at least during some levels of the biological clock of their life process. In reality, no ion channel portrait has been done in the matching of regular original tissues [50, 51]. Prostate cancer has been connected to the non-selective transient receptor potential (TRP) family of ion channels [52]. Other cation channels, such as non-selective cation channels and calcium (Ca²⁺) and sodium (Na⁺) channels, have been found in tumour cells. A Ca²⁺ entry site in cancer cells is T-type Ca²⁺ channel [53].

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Figure 4. 

Alteration in ion channels in cancer cells. Vm: transmembrane voltage

Structure and physiology of ion channels

Ion channels are sheath proteins that transition between nonconductive (‘closed’) and conductive (‘open’) conformational categories to govern passive ion fluxes. Although alternative mechanisms are possible, Vm or particular ligands are frequently used to induce a ‘gating’ process (activation). There are many different forms of ion channels, each including a particular degree of ion collection and kinetic lineaments, allowing for a wide range of functional flexibility. VGCs form the action potential and govern cellular excitability. They can also stimulate exocytosis and muscle contraction by regulating transmembrane Ca²⁺ fluxes. Ligand-gated channels usually exert synaptic functions. Cell volume, transepithelial ion outflow, sensory transduction, synaptic action, and many more physiological functions are controlled by multiple channel types’ combined action. Channels close when the stimulus is interrupted (deactivation). Otherwise, they frequently transition to a non-conductive phase through a measure known as an adolescent for VGCs and desensitization for alternative channel categories. Cells can use allosteric regulators or phosphorylation to control the energy distinction among states and the speed of transition processes. Ion channels are frequently formulated from multiple subunits or domains that envelop low or high conduction pathways from a structural standpoint. The VGC family, which includes most of the ion channels described, is briefly discussed [54, 55]. Communication with other subunits or partner proteins is controlled by the N-terminus, while the intracellular regions provide consensus sequences for phosphorylation. The quadruplicate elements that isolate the pore in Na⁺ and Ca²⁺ channels follow the same structure, except that the four elements are repeating domains of a continuous polypeptide rather than independent subunits. A K⁺ channel subunit is homologous to each domain. VGCs are intrinsically voltage-dependent, meaning that Vm controls their conformation, whereas auxiliary proteins (subunits) control accessory characteristics. Many genes code for mammalian VGCs and a consistent nomenclature has been assigned. Classic names, which identify large functional properties less frequent to various gene substances, are still utilized in describing cellular currents. For example, abounding K⁺ Channels of the Kv 1 and Kv 10 molecular tribe produce late outward rectifying currents that stimulate depolarization and usually control the action potential’s repolarisation state. KDR is the aggregate name for several channels.

The inward rectifying K⁺ [inwardly rectifying potassium (IRK)] channels have an evolutionary connection to the VGC clan. They’re similarly composed of four subunits, in which every individual has only a pair of transmembrane domains (M1 and M2), which are connected by a pore loop and are comparable to the S5 and S6 of VGCs [56]. Finally, the TRP gene branch is connected to the VGCs, keeping the above-mentioned structural characteristics. The homo- or heterotetrametric channels are formed by the subunits. TRP canonical (TRPC), TRP vanilloid (TRPV), TRP melastatin (TRPM), TRP ankyrin (TRPA), TRP polycystin (TRPP), and TRP mucolipin (TRPML) are six TRP families with permeability ratios that differ from zero to very extreme between Ca²⁺ and monovalent cations. These proteins are abundant in mammalian tissues and have been associated with a variety of human illnesses, along with cancer. The physiological significance of changed TRP channel expression in tumours is currently being researched [57].

Pharmacological inhibitors for ion channel targeting

Depending on the pathophysiological role a specific channel plays in a tumour, ion channel targeting should include both inhibition and activation techniques (Figure 5). Peptide toxins, small chemicals, and antibodies are among the many agents available today that modulate ion channel activity. Many of these compounds can be active anticancer medicines. There are not many cancer growth inhibitors that are safe and effective enough to get into clinical trials. Like some other medicine, channel modulators might have adverse side effects. Human ether-a-go-go-related gene (hERG) K⁺ channel barricade, for example, has been demonstrated in many cases to produce cardiac arrhythmia (long-QT syndrome) [58, 59].

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Figure 5. 

The therapeutic targets of ion channels

Modulators with a high level of selectivity

New highly selective chemicals would be needed to modulate ion channel activity and reduce side effects [6]. Aside from recent small compounds and antibodies, siRNA could soon provide a choosy gadget for modulating ion channel action and many other proteins. In vivo siRNA application in cancer and other fields appears to be one of the most promising techniques nowadays [60].

Modulators that are not specific but show promise

Old modulators with low specificity should not be completely ruled out of clinical trials. Many of them have shown anti-tumour action in vitro. For example, neuroblastoma cells have been demonstrated to be inhibited by dendrotoxins and quercetin [61, 62]. In addition to stimulating the growth-inhibitory action of the cytostatic medication, tamoxifen, dequalinium, and amiodarone have shown substantial hindrance action on human prostate cancer, breast cells, and colon cancer cell development [63]. Other natural antidepressant compounds, such as those found in Hypericum perforatum L. extracts, also have anticancer effects, owing to ion channel modulation [64]. The long-time practice of Ca²⁺ channel blockers such as verapamil, an antihypertensive medicine, has already yielded important data. Prostate cancer benefit has a convincingly inverse connection, according to a thorough prospective investigation of the long-term risk of prostate cancer in male Ca²⁺ channel blocker users. At least in persons with a specific genetic background, able to decrease the risk of prostate cancer [65, 66].

ABC transporter

Chemotherapy is the preferred treatment for many forms of cancer. Chemotherapeutic medicines can lose their effectiveness over time due to the development of resistance involving a group of membrane proteins. These multidrug transporters keep intracellular drug concentrations below the cell-killing threshold by removing harmful compounds from the cell. These transporters belong to the superfamily of ABC proteins, which are found in all organisms, from bacteria to humans. According to clinical studies, the overexpression of particular ABC transporters, recognized as multidrug resistance (MDR) proteins, is linked to the MDR phenotype in malignancies [69].

Cancer MDR—members

MDR is the opposite impact of a wide range of structurally and mechanistically unrelated medicines across the membrane. Because cancer can reject and reduce intracellular drug concentrations chemotherapy drugs and diminish intracellular drug altitude, this process is the most frequent cause of chemotherapy for cancer failure. It has been determined that members of the ABC family are involved in this procedure [71]. According to numerous clinical studies, the overexpression of particular ABC transporters is well-known as MDR resistance phenotype in malignancies [72]. P-glycoprotein [P-gp, MDR protein 1 (MDR1), ABC sub-family B member 1 (ABCB1)] is the first known ABC transporter [73], and it is identified to be answerable for the most common mechanism in MDR [74]. Following the cloning and characterization of MDR1, it became clear that other efflux pumps are tangled in transport-related drug resistance as well. MDR-associated protein 1 (MRP1), ABC sub-family C member 1 (ABCC1), or the MDR protein [75] and mitoxantrone resistance protein [multixenobiotic resistance (MXR)/breast cancer resistance protein (BCRP), ABC sub-family G member 2 (ABCG2)] [76] are two additional ABC transporters that have been firmly linked to tumour MDR. The review examines ABCB1, ABCC1, and ABCG2, the three most therapeutically significant ABC transporters, which produce MDR during cancer treatment.

SLC

The effect of SLC, which are often influx or bi-directional transporters, on cancer treatment is not thoroughly studied. Ion channel transporters, passive transporters, and exchangers are exposed in intracellular organelles or on the PM, among which around 400 SLC transporter genes are recognized and distributed in 55 families (e.g., mitochondrial or vesicular transporters) [99]. The SLC superfamily regulates the shipping of a broad range of compounds and various supplements and medicines [100]. To improve cancer treatment outcomes, researchers must comprehensively define the action of SLC drug carriers in organs crucial for anticancer medication disposition, like the liver, intestine, and kidney. Furthermore, through a better understanding of the changing phase of SLC transporters in diverse cancerous cells, we can create new cancer-treatment techniques. Down-regulation of specific critical transporters for cancerous cell existence, for example, could be used to produce transporter-targeted chemotherapy. The following are the primary subfamilies of SLC members of studied cancer therapy to varying degrees (Figure 6) [101–103].

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Figure 6. 

Primary subfamilies of SLC members about cancer therapy to varying degrees. OCT: organic cation transporters; OAT: organic anion transporters; OATPs: OAT polypeptides; MATEs: multidrug and toxin extrusion proteins

Particular TFs

Inhibiting the functional domains of TFs and the post-translational changes that affect their action are two techniques for identifying and removing TFs. In contrast to kinases, which have the deepest connecting compartments for tiny fragments, TFs bear ample surface space for protein-DNA and protein-protein communication, making them challenging to target. One more difficulty with suppressing TFs is that they are primarily found in the nucleus, making rehabilitative procedures less available. Regardless of these challenges, important growth has been made in the creation of more potent medicines and improved delivery systems that significantly target tumour cells [112].

Transcription activators and signal transducers

STAT are TFs that are cytoplasmic until tyrosine phosphorylation is applied. After activation, STATs balance genes that control cell growth, differentiation, and even existence by designing dimers in the nucleus. Inhibitory molecules such as protein tyrosine phosphatases, protein suppressors of effective STAT proteins, and inhibitors of cytokine signalling proteins maintain STAT signalling transient and tightly controlled in normal cells. Nevertheless, tumour cells usually acquire enduring STAT activation due to upstream tyrosine kinase hyper-activation or overexpression, or failure of action of negative regulators. Arresting STAT3 function may be a helpful antineoplastic treatment because stimulated STATs, especially STAT3, stimulate the production of genes essential in cell multiplication but also in survival [113]. Reduced STAT3 expression by RNA interference (RNAi), which reduces STAT3-specific gene expression and lowers tumour development in vivo, is one technique for inhibiting STAT3 activity [114, 115]. NPs, which have reduced toxicity, better stability, and higher tumour accumulation, have been used in current delivery systems. STAT3 siRNA NPs reduce STAT3 expression and trigger apoptosis, which makes lung neoplastic tissues more sensitive to antineoplastic drugs [116]. Another is the block of STAT3 DNA binding via decoying oligodeoxynucleotides (ODNs). In a phase 0 clinical trial involving head and neck neoplastic patients, a STAT3 ODN effectively reduced STAT3-specific gene expression [117].

NF-kB

A protein complex called NF-kB serves as a TF in cancer. Five proteins, reticuloendotheliosis viral oncogene homolog (Rel, c-Rel), RelA (p65), RelB, NF-kB1 (p50), and NF-kB2 (p52), each contribute a Rel homology domain and, through homo- and heterodimerization, form functional TFs. In unstimulated cells, NF-kB is kept inactive in the cytoplasm by inhibitors of NF-kB (IkB). The cytokines [tumour necrosis factor (TNF), interleukin (IL)-1b], and also pathogen-associated molecular model cause IkB to be degraded by proteasomes, allowing NF-kB to be released and translocated to the nucleus, where it manages gene transcription which affects cell multiplication, survival, metastasis, and angiogenesis. Monocyte chemotactic protein-1 (MCP-1), IL-6, IL-8, IL-1b, and TNF-a are proinflammatory genes that NF-kB controls. Many malignancies contain persistent NF-kB activation, which causes an inflammatory action that may contribute to cancer formation. As a result, inhibiting the NF-kB chain has therapeutical value. Direct suppression of NF-kB DNA binding is one technique for targeting the NF-kB pathway [118]. The contribution of NF-kB signalling to tumour breakout from the immune regulation also consequent tolerance to chemotherapy may be a critical activity of NF-kB signalling in cancer. The activity of the TF NF-kB p50 in tumour cells aims for the recruitment of anti-inflammatory M2 macrophages, which leads to tumour tolerance in radiation treatment. Radiation treatment is most efficient in mice that are deficient in specific proteins. NF-kB p50 is a protein found in the NF-kB gene [119]. As a result of their capacity to directly inhibit NF-kB, drugs are generally utilized in the treatment of some other diseases and are prescribed as prospective antineoplastic therapy. Bindarit is a non-steroidal anti-inflammatory drug [120–122]. In prostate and breast cancer animal tests, it inhibits carcinogenesis and metastasis. Bindarit reduces tumour-associated macrophage (TAM) and myeloid-derived suppressor cells (MDSC) tumour invasion by increasing IkB expression, suppressing NF-kB nuclear translocation but also MCP-1 transcription [123], FTY720, also known as fingolimod, functions as a prodrug that obstructs downstream signalling involving NF-kB/IL-6/STAT3, along with the expression of receptors linked to sphingosine 1-phosphate (S1P). This suggests its potential utility in addressing malignancies associated with persistent inflammation. The activation of M2 macrophages through prostaglandin E2 (PGE2), linked to NF-kB-mediated release of IL-6 and PGE2, is thought to contribute to resistance against platinum-based cancer therapies. By utilizing a specific inhibitor targeting IkB phosphorylation, the capacity of cervical and ovarian cancer cells to generate IL-6, PGE2, and STAT3, as well as the induction of M2 macrophages, appears to be diminished. By inhibiting M2 macrophage induction, the cyclooxygenase inhibitor indomethacin, and the anti-IL-6R antibody tocilizumab can enhance the effects of platinum-based chemotherapy [124].

Drug delivery through NPs in chemotherapy

Finding novel and creative cancer curing methods is a big demand around the globe. The efficacy of various malignant cancer treatments has grown in tandem with the development of novel cancer therapies and the philosophy of individualized care. In recent years, nanotechnology has been used increasingly often in medication, with implementations that are safer and more effective for tumour localization and therapy, including diagnostics. In cancer chemotherapy, the benefits of NP-based drugs have been extensively studied. These benefits include good pharmacokinetics, precise tumour cell specificity, low toxicity, and increased drug tolerance. The size and features of NPs utilized in medicine delivery systems are generally developed or selected depending on the pathophysiology of malignancies [9]. Nanocarriers are used in treating cancer to focus tumour cells mechanically after being absorbed. This is accomplished by the carrier activity of NPs and the localizing function of the desired substance. They then provide the medicine to tumour cells in order to destroy them. The inside of the nanocarriers contains conventional chemotherapeutic medicines, including nucleic acids, suggesting that they might be used for both cytotoxic and gene therapy. Additionally, NPs offer a substrate for encapsulation and transport into circulation for some medications that are poorly soluble. Drugs with nanocarriers have a longer half-life and are more likely to accumulate in tumour tissues [125].

NPs in cancer treatment

These characteristics have a substantial impact on the efficiency of nano-drug transport and, consequently, therapeutic efficacy control. NPs used in therapeutic care often have certain sizes, shapes, and surface qualities. Because they can transport drugs well and have an enhanced permeability and retention (EPR) impact, NPs with a diameter of 10 mm to 100 nm are frequently utilized in cancer therapy [126]. The surface characteristics of NPs have an impact on their bioavailability and half-life. Hydrophilic coatings on NPs, such as polyethylene glycol (PEG), for instance, diminish opsonization and hence hinder immune system clearance. In order to increase medication penetration and accumulation in tumours, NPs are frequently altered to become hydrophilic [127].

Types of NPs used in cancer treatment: organic NPs, inorganic NPs, and hybrid NPs (Figure 7) [128].

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Figure 7. 

Different types of NPs used in cancer treatment. AuNPs: gold NPs

Mechanism of targeting cancer cells

Cancer cell targeting is an important feature of nanocarriers for drug delivery because it improves therapeutic effectiveness while preserving normal cells from damage. Numerous investigations on the targeted design of NP-based medications have been conducted. It is critical to figure out tumour characteristics also the interaction among nanocarriers and tumour cells in order to properly handle the issues of tumour localizing and nano carrier system design. There are two types of targeting mechanisms: passive targeting and active targeting.

Signalling pathway

Once in its active, guanosine triphosphate (GTP)-bound state, RAS will interact with several families of effector proteins, resulting in stimulation of their catalytic activity. The main effectors are shown here. Rapidly accelerated fibrosarcoma (RAF) protein kinases initiate the mitogen-activated protein (MAP) kinase cascade, which leads to extracellular signal-regulated kinase (ERK) activation. This kinase has numerous substrates both in the cytoplasm and in the nucleus, including E26 transformation-specific (ETS) family TFs such as ETS like TF 1 (ELK1) that regulate cell cycle progression. Phospholipase Cε (PLCε) catalyses the hydrolysis of phosphatidylinositol 4,5-bisphosphate to diacylglycerol and inositol trisphosphate, resulting in protein kinase C (PKC) activation and Ca²⁺ mobilization from intracellular stores. ERK, glycogen synthase kinase 3 (GSK3), mitogen-activated kinase/ERK kinase (MEK), p70 ribosomal protein S6 kinase (p70S6K), 3-phosphoinositide dependent protein kinase-1 (PDK1), phospholipase A2 (PLA2), phospholipase D (PLD), p90 ribosomal protein S6 kinase (RSK). RalGDS proteins are guanine nucleotide exchange factors (GEFs) for RAS-like (RAL), a RAS-related protein. Downstream targets include forkhead TFs. Type I phosphoinositide 3-kinases (PI3Ks) generate second-messenger lipids, such as phosphatidylinositol-3,4,5-trisphosphate, which activate numerous target proteins including the survival signalling kinase protein kinase B (AKT/PKB). RAC is a member of the Rho family of small GTPases, which act as molecular switches to control a wide array of cellular functions. In particular, RAC signalling has been implicated in the control of cell-cell adhesions, cell-matrix adhesions, cell migration, cell cycle progression, and cellular transformation [143].

AKT/PKB is a group of three serine/threonine protein kinases with close genetic resemblances that bind to PI3K lipid products at their amino termini. They become active in response to PI3K stimulation and send the cell anti-apoptotic signals. B cell lymphoma-2-like 11 (BIM) is a member of the B-cell lymphoma 2 (BCL-2) family of apoptosis regulatory proteins, which induces apoptosis by acting at mitochondria to promote the release of cytochrome c into the cytosol. Fas cell surface death receptor ligand is an extracellular protein that binds and activates the death receptor Fas, also known as CD95 and APO1, which results in the beginning of apoptosis (Figure 8) [143].

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Figure 8. 

Signalling downstream of RAS. RAC: a member of the Rho family of small GTPases

There is a tremendous amount of molecular information about the RAS proteins’ molecular mechanisms in the nearly 20 years since it was discovered that RAS was an activated oncogene in human tumours. They have control over a complex signalling network that has numerous cross-talk, bifurcation, and feedback points. Although there is undoubtedly still much to learn and we may be fooling ourselves if we think we have a good understanding of this system, there are clearly a lot of leads worth following in the search for cancer treatments. Specificity questions will continue to be crucial: How specific should a drug be for its intended target, and can it be too specific without also inhibiting related molecules that could act as a substitute for the intended target? More importantly, it’s still unclear how wide of a therapeutic window many of these drugs with rational design will likely offer. The RAS signalling pathways are essential for the existence of both normal and tumour cells, even though tumour cells may be more dependent on them. They share this trait with many other signalling pathways that are under scrutiny by the pharmaceutical industry. Given the intense selective pressure placed on the genetically unstable cancer cells, even if these medications and their replacements are successful, the problem of resistance will inevitably arise quite quickly. In the end, assuming we ever have access to such an arsenal, this is most likely to be overcome by taking a cue from the fight against infectious diseases and using numerous effective drugs concurrently. Clinicians may need to change the endpoints used in clinical trial designs to assess drug efficacy because if reduction of tumour mass by single agents is the only metric used, many effective drugs may be discarded before they have a chance to demonstrate their value in combination therapies. Even if these medications and their replacements are successful, resistance will unavoidably develop quite quickly. In this regard, the creation of suitable surrogate markers to track drug action will be essential. If drugs that are effective on relatively uncommon tumour types are to be identified, tumour stratification will also be crucial in the selection of patients on whom to test these inhibitors. There is reason to be cautiously optimistic that some of the drugs discussed here, or more likely their derivatives, will eventually serve as the foundation for truly effective treatments for many common cancers, even though we are still only near the beginning of a very long and labour-intensive process [144].