Revisiting Traditional Cancer Treatments: Achievements and Unmet Needs

Cancer, which can be defined as the excessive growth of the cell that originates due to a combination of numerous genetic defects, including gene dysregulation and their regulators, is one of those diseases that have a history stretching back to approximately 1.98 million years ago. There is evidence of the tumor known as osteosarcoma in the fossils of South Africans, demonstrating that this illness has existed in ancient times ^[1]. According to historical documents, cancer was recorded as the cause of death in Egyptian mummies who lived more than 1500 BC^[1] . The term "cancer" was invented by a Greek doctor, Hippocrates, to represent the crab-like growths seen when the disease metastasizes to adjacent tissue ^[2]. In the fifteenth century, the Roman doctor used the term "oncos" to mean swelling in connection with cancer as well. At that time, the single possible method of treating the illness was either by herbs or using a poultice on the affected area through a small incision ^[3]. Due to better knowledge of how the human body functions, how tumors work, and the invention of anesthesia, doctors considered surgery as a more viable solution to treating cancer during the fifteenth century.

As the number of people succumbing to cancer continues to increase while facing the associated risks of existing treatment methods, there is an increasing need to develop new, effective treatment options. While novel approaches to cancer treatments including immunotherapy and gene therapy have been developed, traditional forms of treatment continue to be used as they are cheap and popular. It is therefore necessary to enhance the performance of conventional cancer treatment methods including surgery, radiation, chemotherapy, and hormonal therapy.

Fig. 1: Evolution and milestones of traditional anticancer therapies across centuries: A comprehensive overview of development and progress.

2. SURGERY

Surgical procedure is an ancient and basic medical technique used in diagnosis and treatment of solid cancer tumors. For diagnostic purposes, biopsy of cancer cells is considered the most appropriate way to categorize different cancers and assess their grading; nevertheless, insufficient samples and painful procedures hinder its efficiency. The newly developed technique called liquid biopsy provides painless screening of cancer biomarkers, including circulating tumor DNA (ctDNA), circulating tumor cells (CTC), and extracellular vesicles (EVs), secreted by primary cancer tumors to cause metastases ^[4]. Nevertheless, discovering new cancer biomarkers and developing sensitive techniques to detect trace amounts of biomarkers are critical for early diagnosis.Excision surgery is carried out for the purpose of achieving a complete clearance of the tumor cells. Nevertheless, even after what appears to be successful excision, there still tends to be some degree of remaining disease in the clearance area, in the form of microscopic deposits, as well as micro-metastases that are either stromal or hematogenous, implying the existence of minimal residual disease (MRD) ^[5].

Research has proven that the MRD usually develops rather quickly, and in most cases, it is associated with the surgical removal of the tumor mass . The aggressiveness of the MRD could be due to several reasons, such as the surgical procedure itself and the resultant trauma environment, which could promote cancer growth.

The use of immunosuppressive agents in the postoperative phase in order to support wound healing contributes to the evasion of cancer cells from the immune response. It leads to reduced NK cell cytotoxicity and ineffective macrophages associated with tumor progression after surgery . Surgery may also lead to a decrease in the number of circulating DCs involved in immune surveillance in cancer patients ^[6].

Moreover, the removal of anti-angiogenic factors produced by the primary tumor ensures that the ability to induce neovascularization in micro-metastases in other locations is enhanced, thereby worsening the disease condition. The excision of the primary tumor releases MRD from any form of inhibitory activity referred to as concomitant tumor resistance, ensuring the unimpeded growth of metastatic nodules. The secretion of pro-angiogenic factors by the cells of the primary tumor creates new vessels in the primary location, hence facilitating the development of cancer . Moreover, the removal of anti-angiogenic factors produced by the primary tumor ensures that the ability to induce neovascularization in micro-metastases in other locations is enhanced, thereby worsening the disease condition.^[6]

In order to mitigate the problems involved in traditional surgeries, there have been many innovations in technology, moving away from open surgeries towards minimal invasiveness such as laparoscopic surgeries which result in less blood loss and morbidity (Fig. 1). With cryotechnology and laser technologies, surgical accuracy was made possible, while the technique of fluorescence-guided surgery (FGS) helps surgeons differentiate between normal and tumor tissues so that they can demarcate tumors and avoid leaving any cancerous tissues behind or damaging the healthy cells around them ^[7].

 In Video-Assisted Laparoscopic Surgery (VALS), precision is made more accurate due to real-time visualization, even though it still has limitations as the two-dimensional view makes it difficult for the surgeon to operate in tight spaces . In terms of robotics technology, surgeons are now able to gain more accuracy and freedom during surgeries with three-dimensional visualization of the surgical field ^[8]. In addition, the level of autonomy has progressed from simple assistance of robots in surgical practices to the ability to create personalized surgical procedures for the patients ^[9].

3. RADIOTHERAPY

Radiation therapy is a very efficient modality for treating cancer patients by delivering ionizing radiations through high-energy particles like X-rays, gamma rays, and protons that target cancerous cells by interfering with their DNA structures. From the time when X-rays were discovered, they have been employed to cure cancers of diverse etiologies ^[10]. Developments in technology and biological knowledge have made great strides in developing more advanced therapies for radiotherapy treatment, including three-dimensional conformal stereotactic body radiotherapy (SBRT), intensity-modulated radiation therapy (IMRT), and image-guided radiation therapy (IGRT), which have resulted in better overall survival (OS) rates among different cancer types.

Although low tolerability to normal cells previously restricted the extensive use of radiation therapy, modern scientific breakthroughs have facilitated its use by increasing the sensitivity of cancer cells and increasing the tolerance of normal cells to radiation. Discovery of radiation protectors and sensitizers has led to a variety of ways of their usage, tailored to fit the individual patient. Amifostine (WR2721) is a sulfhydryl-phosphorylate, serving as a radiation protector. Its metabolite, after its dephosphorylation through alkaline phosphatase, scavenges radicals and prevents damage caused by radiation therapy ^[11]. On the other hand, such compounds as 2-deoxy-D-glucose, 6-aminonicotinamide, curcumin, and parthenolide enhance the presence of reactive oxygen species in cancer cells and thereby increase radiation efficacy ^[12].

Moreover, the advancement of technology also led to the replacement of the “planner imaging field” by “conformal radiotherapy,” where the beam could be shaped around the tumor. The technique of SBRT provides hypo-fractionated and conformal beamlets that can treat even small tumors with better shielding to other tissues ^[13]. There is an advantage of IMRT in modulating the dose of radiation beams depending on the size and shape of the tumor. Dosimetrically adjusting the delivered beams based on the prescription plan demonstrates promising clinical outcomes, especially for locally advanced cancers ^[13].

Future research will continue to develop radiation delivery methods that avoid damaging other healthy tissues, such as FLASH radiotherapy, that involves delivering high doses of radiation within a microsecond ^[14]. Another method of delivering radiation therapy is brachytherapy, which is internal radiotherapy that involves the use of radioactive sources inside the patient’s body close to the tumor. Tumors irradiation from inside or nearer reduces the risk of affecting other healthy tissues ^[15].

Even with the above successes, some types of cancer cells continue to resist radiotherapy, and the chance of recurrence is relatively high ^[16]. Ionizing radiation promotes the breakdown and production of ROS, particularly hydroxyl radical (.OH), leading to DNA damage ^[17]. Research has underscored the importance of antioxidant levels in radiotherapy, where elevated levels of antioxidants within cancer cells lead to unsuccessful treatment, but reduced levels produce the expected results ^[17].

Ionizing radiation causes damage to DNA, which results in DNA damage responses and ultimately cell death via apoptosis, necrosis, or autophagy. Homologous recombination and non-homologous end joining are two major categories for double strand DNA repair ^[18]. While homologous recombination is considered a higher-fidelity type of DNA repair process where DNA strands are used as a repair template to fix DSBs, non-homologous end joining repairs DNA damage by joining broken strands of DNA without using any DNA template resulting in DNA deletions and rearrangements.

Moreover, radiation-induced increase in mitochondrial generation of ROS promotes sensitivity to cell radiation through the cytosolic Rac1/NADPH oxidase pathway ^[18]. KEAP1-NRF2 pathway is crucial for the outcome of radiotherapy, whereby NRF2 (nuclear factor erythroid 2), which normally binds to KEAP1, activates transcription of genes responsible for antioxidative effects when exposed to radiation. Based on experiments, absence of KEAP1 leads to aggressive cancer cells resistant to radiotherapy treatment , KEAP1 mutation in non-small cell lung cancer (NSCLC) has been linked to local recurrence in patients treated with radiotherapy ^[19].

Genetics and metabolism influence the effectiveness of radiation therapy as well. The increased consumption of glucose is one of the major characteristics of tumor formation. GLUT1 is a widespread glucose transporter that is highly expressed in many kinds of cancer. Researches show the significant role of the GLUT1 factor in the prognosis of radiation therapy. The enhanced GLUT1 activity, combined with the effect of hypoxia and the dysfunction of MAPK and PI3K/AKT signaling pathways, causes the radio-resistance of tumorous cells. The fumarate hydratase (FH) protein is one of the essential proteins participating in the tricarboxylic acid cycle and is associated with hereditary and sporadic types of cancer due to genetic mutations. Inability of the FH protein to fulfill its function results in the accumulation of fumarate, which is correlated with leiomyomata formation and tumor growth. The accumulation of fumarate and its interaction with glutathione form succinated glutathione (GSF) that competes with glutathione reductase (GR) and inhibits NADPH level and increases the ROS formation ^[20].

The amount of oxygen in radiation has a big impact on how cells react to it; hypoxia helps cells protect their DNA. On the other hand, cancer cells are more vulnerable to DNA damage due to their elevated oxygen content ^[21]. The advantageous sparing effect observed in normal tissue during radiation therapy may be due to the differing oxygen tensions in malignancies and normal tissues ^[22]. Poor survival is correlated with increased expression of hypoxia-inducible factor 1-alpha (HIF-1α) in several types of cancer. Through a number of mechanisms, including proliferation, angiogenesis, metastasis, ROS homeostasis, extracellular matrix remodeling, and pH regulation, it promotes the growth of cancer cells. Increased HIF-1α expression in oropharyngeal cancer increased the risk of relapse following radiation therapy, but HIF1-α knockdown increased radiation sensitivity ^[21].

Overall, numerous mechanisms that alter DNA damage and HIF-1α expression are involved in how oxygen levels, genetics, and metabolites affect radiation responsiveness, which in turn affects clinical results. The basis for accurate radiotherapy planning is provided by pre-treatment imaging using methods like computed tomography (CT), magnetic resonance imaging (MRI), or positron emission tomography (PET), which offer detailed visualizations of the tumor's size, shape, location, and surrounding healthy tissues. By constantly modifying plans throughout the course of therapy to account for tumor shrinkage or anatomical changes, adaptive radiotherapy procedures improve treatment by guaranteeing optimal targeting and reducing damage to healthy tissues. By treating side effects including fatigue and skin rashes and offering supporting therapies to increase treatment tolerance, comprehensive patient care enhances these strategies^[22].

Chemotherapy is one of the most widely used conventional approaches to cancer treatment. It targets key cell cycle phases to induce cell death. Below, we discuss common chemotherapeutic agents, their modes of action, and the associated side effects, as summarized in Table 1.

Table 1: List of chemotherapeutic therapy agents for cancer treatments

G-1: A growth factor, granulocide colony – stimulating factor (G-CSF), commonly used in chemotherapy regimens to stimulate the production of white blood cells, MVAC: A chemotherapy regimen that includes four drugs: Methotrexate, Vinblastine, Doxorubicin and Cisplatin.

4. ALKYLATING AGENTS

Chemotherapy entered a new era when alkylating chemicals were first identified for their clinical potential in 1942 with the advent of nitrogen mustard gas. These substances create covalent connections with nucleophilic sites on macromolecules, much as the sulfur mustard gas employed in World War I ^[23]. DNA adducts are created as a result of this interaction, which can result in intra-strand or inter-strand crosslinking, interfering with transcription and replication and ultimately causing cell death ^[23]. Alkylating drugs' incapacity to specifically target tumor cells as opposed to normal cells, however, has significant health consequences.

Fig. 2: Overview of Major Anticancer Drug Classes and Their Mechanisms of Action.

Mechanisms of action of different chemotherapeutic drugs. 

• Alkylating agents form inter-strand and/or intra-strand adducts making DNA unable to open for replication and transcription. 
• Antimetabolites utilize nucleotide analogs. Misincorporation of these nucleotide analogues initiates DNA repair on hundreds of sites simultaneously causing DNA stress and ultimate cell death. 
• Antitumor antibiotics cause DNA fragmentation and disruption of transcription and translation. 
• Topoisomerase inhibitor obstructs DNA replication, transcription, chromosomal segregation and recombination. 
• Antimitotic drugs target microtubule assembly and disassembly thereby disrupting mitosis and causing cell cycle arrest.

The nitrogen mustard class consists of drugs like mechlorethamine, chlorambucil, melphalan, ifosfamide (IFO), trofosfamide and cyclophosphamide (CP). Among these, mechlorethamine is known to be highly reactive because it gets activated quickly at physiological pH, which also makes it more likely to undergo unwanted side reactions. On the other hand, cyclophosphamide behaves differently, as it remains relatively stable and requires metabolic activation in the liver through the mixed oxidase system. Its active metabolite, 4-hydroxycyclophosphamide (4-OHCP), is responsible for several effects such as mutagenicity, carcinogenicity, teratogenicity and immunosuppression ^[23].

Ifosfamide also follows a similar activation pathway, but its metabolism in the liver is comparatively slower. Due to this, higher doses are usually required when compared to cyclophosphamide. However, increasing the dose also increases toxicity, mainly because of the formation of acrolein, a toxic metabolite that can cause hemorrhagic cystitis. To reduce this adverse effect, Mesna is commonly used as a protective agent. Another group of alkylating agents includes aziridines (ethylenimines), which may either activate on their own or require enzymatic oxidation. In most cases, this oxidation is carried out by cytochrome P450 (CYP) enzymes present in the body. These enzymes are important because they convert drugs such as ifosfamide, cyclophosphamide and thiotepa into their active cytotoxic forms ^[24].

Cisplatin is a drug that is used to treat cancer. It is very good at treating cancer. It has some bad side effects. Cisplatin can also stop working after a while because the cancer gets used to it. Carboplatin is a lot like Cisplatin. It is not as harsh on the body. This means it does damage to the kidneys and nerves. However cancer can still get used to Carboplatin if it is already used to Cisplatin ^[24]. When Cisplatin gets inside a cell it changes into a form that can hurt the cell. It does this by sticking to the cells DNA. Carboplatin does the thing but it takes longer to change into the form that can hurt the cell. This is why Carboplatin is not as bad for the kidneys and nerves as Cisplatin's ^[25].

Oxaliplatin is another drug that is used to treat cancer, the kind that affects the stomach and intestines. It works by hurting the cells DNA and stopping it from making parts. Oxaliplatin is not as bad for the kidneys, as Cisplatin and Carboplatin. It still works just as well^[26].Temozolomide is a type of medicine that is used to treat a kind of brain tumor called glioblastoma. This tumor is very aggressive. Temozolomide works by changing the DNA of the tumor cells. It does this by adding a methyl group to parts of the DNA. This happens at places on the DNA like the N3 spot on adenine and the N7 and O6 spots on guanine. When this happens it creates molecules like N3-MeA and N7-MeG and O6-MeG ^[27].

The cell has ways to fix some of this damage. For example it can use something called base excision repair to fix N3-MeA and N7-MeG. But to fix O6-MeG it needs a protein called O6-methylguanine-DNA methyltransferase or MGMT for short. This protein removes the methyl group from the O6 spot on guanine, which helps the cell survive. This is a problem because it means the Temozolomide is not as effective^[27].

Sometimes the tumor cells become resistant to Temozolomide. This can happen when the cells  well. Researchers are trying to find ways to overcome this resistance. One idea is to use a drug that stops MGMT from working like O6-benzyl guanine. This can make the tumor cells more sensitive to Temozolomide.

People are also working on ways to give Temozolomide to patients. They want to find ways to make the drug work better and help patients respond to the treatment. Temozolomide is a drug, for treating glioblastoma and researchers are trying to make it more effective. They are doing studies to explore new strategies and address the challenges of treating this disease. Temozolomide and glioblastoma are the focus of a lot of research and scientists are trying to find ways to use Temozolomide to treat glioblastoma ^[28].

5. ANTIMETABOLITES

Antimetabolites are a type of medicine that helps treat cancer. They work by stopping the cells from making DNA. This happens when the cells do not have the things they need to make DNA or when they use parts to make DNA. Some medicines are made to stop cancer cells from working. These medicines are similar, to the things that cells need to make parts. Cancer cells like to use a way to make these parts. When these medicines get into the cells they change into something that can stop the cells from making DNA. This means that the cells cannot make parts and they will eventually die. Antimetabolites are used as -cancer therapies and they work by stopping the cancer cells from making new DNA. Antimetabolites are important because they can help stop cancer cells from growing ^[29].

Aphidicolin is a type of medicine that interferes with DNA polymerase. It does this without becoming part of the DNA. This causes a slowdown in DNA synthesis. Because of this Aphidicolin is not considered a long-term choice for fighting cancer ^[30]. Methotrexate is another medicine that targets a part of the cell called dihydrofolate reductase. This part is necessary for making folate, which's important for the cell. When Methotrexate targets this part it reduces the amount of folate in the cell. This in turn reduces the production of purines and pyrimidines which're important for the cell to grow ^[31].

There are medicines like pemetrexed and raltitrexed that work in a similar way. They target the parts of the cell that're responsible for carrying folate into the cell. Azacitidine is a type of medicine that's similar to a part of the DNA called cytidine. When Azacitidine is incorporated into the RNA it stops the cell from making proteins. This happens because Azacitidine disrupts the parts of the cell that're responsible for making proteins. When Azacitidine is incorporated into the DNA it stops the cell from making changes to the DNA. This can cause problems for the cell. Even damage to the DNA ^[33].

Clofarabine is another medicine that interferes with DNA polymerase. It also interferes with another part of the cell called reductase. When this part of the cell is interfered with the cell runs out of a type of energy that it needs to grow. This allows Clofarabine to become part of the DNA. Clofarabine also disrupts the part of the cell that gives it energy, which can cause the cell to die ^[34].

Capecitabine is a type of medicine that is converted into another medicine called 5-FU. 5-FU interferes with a part of the cell that's necessary for making thymidine. Thymidine is necessary for the cell to repair and replicate its DNA. When 5-FU interferes with this part of the cell the cell can no longer make thymidine. This can cause problems for the cell ^[35].

Cladribine is a type of medicine that's similar to a part of the DNA called adenosine. It is usually used to treat leukemia. Cladribine works by interfering with a part of the cell that's necessary, for making DNA. When this part of the cell is interfered with the cell can no longer make DNA^[36].

Various drugs that can make radiation work better like fluorouracil (FU) 5-fluoro-2′-deoxyuridine (FdUrd) and hydroxyurea (HU) are being used to make medicines.When these drugs are used with radiation patients with types of cancer like gastrointestinal cancer, cervical cancer and head and neck cancer seem to live longer ^[37]. Drugs like 5-bromo-2′-deoxyuridine (BrdUrd) and 5-iodo-2′-deoxyuridine (IdUrd) make DNA damage worse when radiation is used, which can kill cancer cells. Fdurd work by stopping an enzyme called thymidylate synthase (TS) which leads to a mistake in the DNA and starts the repair process. This mistake causes a lot of problems like the cell trying to fix itself over and over and eventually the cell dies. FU also gets turned into things that disrupt how cells make RNA, which helps kill cancer cells but does not make radiation work better ^[38].

When TS is blocked it causes an imbalance in the DNA building blocks, which stops cells from making DNA and gets them stuck in a phase making them easy to kill with radiation. FdUrd also slows down how cells fix DNA damage making them more likely to die from radiation. Hydroxyurea (HU) works by stopping an enzyme called reductase (RR) which is needed to make DNA building blocks. This helps make radiation work and it has been shown to help patients with cervical cancer and head and neck cancer. HU makes radiation more effective. This has been proven to be helpful, in managing these types of cancer ^[39].

6. ANTITUMOR ANTIBIOTICS

Antitumor antibiotics are one of the earliest forms of cancer treatment, particularly those obtained from Streptomyces species. These antibiotics target multiple cell cycle phases and induce cell growth arrest. Bleomycin is a kind of glycol peptide medicine. This drug performs its function by cleaving cellular acid in a highly specific manner. This is asimple mechanism that occurs in the presence of oxygen. For this purpose, it requires a metal cofactor. Various parts of the bleomycin have been modified to increase its efficacy and minimize side effects. It was discovered that bleomycin is deactivated by certain naturally occurring biological processes. This degeneration of bleomycin occurs, both in normal and cancer cells. BLM hydrolase is an enzyme known to hydrolyze bleomycin. This is done through modification of the bleomycin molecule’s tail part ^[40].

In a few other studies, different variations of bleomycin’s sugar moiety have been tested to improve specifically targeting cancer cells and improving anticancer properties. They wanted to know which position was fit for attaching a particular group of atoms to enhance the bleomycin molecule’s ability to identify and kill cancer cells.

One study in living things demonstrated the possibility of creating safer variants of bleomycin. This compound is known as deglycobleomycin. This altered bleomycin is called deglycobleomycin and it has been found by to have less damaging side effects than BLM; hence it can be a substitute for bleomycin (BLM). Such a discovery could lead to improved cancer treatments ^[40].

Enediynes are natural products with antibiotic and anti-tumorous potentials and are highly cytotoxic DNA damaging agents that have attracted attention in potential cancer treatment. The recent studies have looked at this cytotoxic effect of the enediynes and strategic ways of improving treatment efficacy ^[41]. In terms of structural classification, they have to be in 9-membered ring chromophore cores or 10-membered rings. The discovery of these molecules offered a solution of highly potent anti-cancer agents, as some products of 9-membered ring chromophore cores are 5–8000 times stronger than Adriamycin, a commonly used anti-tumorous drug ^[41].

Enediynes cause either a single-strand DNA lesion or a double-strand DNA lesion and an RNA lesion in a handful of cases. Chromophores of enediynes bind with the minor groove of DNA which triggers structural changes in the DNA making it prone to oxidation which leads to DNA strand break. C-1027, derived from Streptomyces globisporus showed high cytotoxicity against non-Hodgkin B cell lymphoma. However, delayed toxicity was a dominant inadequacy for the clinical assessment of C-1027. Neocarzinostatin (NCS), an enediyne acquired from Streptomyces carzinostaticus, demonstrated a high uptake and lowered toxicity when conjugated with poly styrene-co-maleic acid (SMA) or its alkyl esters (SMANCS) ^[42].

Calicheamicin (CAL), a 10-membered enediyne from bacterium Micromonospora echinospora, is unified with monoclonal antibody (mAb) to improve tumor specificity. This coupling with the humanized monoclonal antibody (mAb) Gemtuzumab ozogamicin targeting the CD33 antigen expressed in the hematopoietic system enables for acute myeloid leukemia (AML) transformed cells to be accurately targeted with minimal general cytotoxicity^[43].

Mitomycin (MTM), which belongs to the antitumour quinone class, has a covalent interaction with DNA to act like alkylating agents. This antitumor quinone’s aziridine, quinone, and carbamate chemical moieties in the compactly arranged pyrrolo-[1,2-a] indole cross-link DNA with higher efficacy and specificity to CpG islands. It is enzymatically or chemically converted from a non-cytotoxic prodrug to a reactive quinone methide after reductive activation MTM-C (Mitomycin) is produced by Streptomyces caespitosus. It is often used to treat breast, lung, colon, and head and neck cancers ^[44].

Variations in oxygen content and intercellular pH between healthy and cancerous cells translates to differential sensitivity of the cells to MTM, with more cytotoxic responses being exerted on the cancerous cells ^[45]. Hypoxic conditions can activate MTM-C, which makes it operational in the oxygen-deprived areas of solid tumors. FR900482 produced by Streptomyces sandaensis shows a decline in hematotoxicity, with its semisynthetic derivative FK317 being highly potent because of its remarkable DNA cross-linking activity and reduced toxicity. MTMs could not overcome resistance mechanism despite achieving the most desirable attributes for successful anti-cancer therapy: selectivity and specificity. There are also other resistance mechanisms triggered in cancerous cells against the cytotoxic effects of MTMs, including the insufficiency of specific activating enzymes, NAD-(P)H oxidoreductase, increased drug efflux, and DNA repairing mechanisms ^[46].

Mithramycin and Chromomycin, products of different species of Streptomycete, belong to the aureolic acid family and are very well proven for their antitumor potential^[47]. All the members are chemical glycosylated aromatic polyketides with two oligosaccharide chains of varying length. This was followed by their antibacterial activity against gram-positive bacteria. However, their later antitumor activity recognition shifted pharmacological interests from antibacterial to antitumor applications ^[48]. The specificity of mithramycin and chromomycin is also similar to that of MTMs, as both compounds preferentially target GC-rich DNA minor grooves. Binding, under the direction of Mg2?, favors the formation of numerous hydrogen bonds between the aglycone hydroxyl moieties and the guanine amino protons. This interaction with the DNA double helix inhibits RNA synthesis and/or DNA replication ^[48].

7. TOPOISOMERASE INHIBITORS

Topoisomerase has such an essential function in protecting DNA during cell growth and development, it might even be the Achilles heel of cancer cells. While topoisomerase type II (topo II) is the target of drugs like amsacrine, etoposide, and doxorubicin, cytotoxic alkaloids and camptothecin (CPT) are identified as inhibitors of topo I ^[49]. However, their side effects can be harmful and even developing resistance to them, one of the challenges among type I inhibitors, together with the development of secondary malignancies from type II inhibitors, limits the scope of these drugs in anticancer therapies ^[50]. Moreover, resistance to these drugs can also occur by means of P-glycoprotein overexpression which leads to decreasing intracellular drug levels and a modification of the drug-binding sites of the topoisomerase enzymes ^[51].

As a plant alkaloid obtained from Camptotheca cuminata, CPT appeared to be so powerful against cancer that the first clinical trials could not separate its effect from its side effects, therefore limiting its clinical use. Since its initial clinical trials in cancer therapy, over 100 CPT analogs including the topotecan have been synthesized to enhance the effectiveness of the drug ^[51]. Fluorinated camptothecin derivatives A1 and A2 showed an enhanced anti-hepatoma efficacy and they were less toxic than topotecan, suggesting their feasibility as more efficient and safer anticancer agents ^[52]. Acridone, similar with CPT, has a planar ring system which enables it to play as a DNA intercalator as well as a topoisomerase inhibitor ^[53]. Many different acridone derivatives have been synthesized and tested for their tumor-inhibiting effects.

Berberine is an isoquinoline quaternary alkaloid that can be isolated from a few medicinal plants such as Hydrastis canadensis and Berberis aristata and it has been employed to cure cancer. By inducing the upregulation of mitochondrial UCP1 expression, promoting thermogenesis and consequently leading to reduced adipocyte content through ATP depletion and mitochondrial uncoupling, berberine chloride shows therapeutic effectiveness in metabolic-related diseases. Besides that, it also binds DNA, RNA, and various proteins such as telomerase, topoisomerase (type I & II), matrix metalloproteinases (MMP), TP53, and estrogen receptors ^[54].

Continued research will surely unveil its exact mechanisms and potential uses. Luotonin A, another alkaloid, comes from Peganum nigellastrum, which was traditionally used to treat rheumatism, abscesses, and inflammation. Using P-388 leukemia cells, Luotonin A was found to possess certain anti-tumor activity ^[55]. In fact, this property is related to the inhibition of type I topoisomerase. Besides these, terpyridines (pyridine derivatives) have also been reported in the literature to be highly effective inhibitors of topoisomerase I and II ^[56].

8. ANTIMITOSIS DRUGS

Antimitotic?‍?‌‍?‍‌ drugs work by either inhibiting the assembly of microtubules or stabilizing them, effectively ceasing cell division and causing cell death through apoptosis or mitotic catastrophe. This method works well because cancer cells tend to have altered cell cycles and are especially vulnerable to these disruptions. Several mitotic inhibitors, some of which are in clinical trials as potential cancer treatments ^[57]. Besides the approved drugs, a variety of additional experimental drugs that interfere with and arrest cell cycle progression are also being evaluated ^[58].

Antimitotic drugs that block microtubule assembly, such as vinorelbine and gefitinib, have been shown to offer considerable therapeutic benefits. A randomized trial that evaluated the effectiveness of vinorelbine and gefitinib found that the two drugs had similar efficacy in older patients with NCSLC; however, vinorelbine was noted to be more tolerable ^[59]. Vinorelbine has been demonstrated in a study of safety and efficacy to be an effective alternative to chemotherapy drugs for elderly patients with limited energy. Some of the other most effective drugs inhibiting microtubule polymerization are vinblastine and vincristine. Colon and brain cancers, especially at advanced stages, may be treated with vincristine; however, not only can this lead to chemoresistance in patients, there are associated side effects such as neurotoxicity and myelosuppression ^[60].

Docetaxel and paclitaxel, two drugs that stabilize microtubules, are extensively used for treatment of prostate, breast, and lung cancers. Alongside prednisone, docetaxel is used for prostate cancer patients in advanced stages ^[61]. However, in most cases, the patients develop a resistance against docetaxel. Over time, the cells develop different mechanisms to overcome the drug, such as increasing the metabolic activity of their drug-detoxifying proteins like glutathione-S-transferase, enhancing the efflux of the drug through upregulation of ATP-binding cassette (ABC) transporters, changing the microtubule formation and kinetics due to overexpression of βIII-tubulin, and acquiring mutations in tumor suppressor genes ^[62]

PTX (paclitaxel) is a natural substance obtained from the Pacific Yew tree and is the first microtubule-stabilizing agent found to have activity across a wide range of cancers. Due to genetic instability and development of resistance, the response of the patient  varies ^[63].

Cabazitaxel is a cytotoxic antimitotic agent that is employed against metastatic prostate cancer. A comparative trial that contrasted prednisone combined with either cabazitaxel or mitoxantrone in hormone-refractory metastatic prostate cancer patients who were initially treated with a docetaxel-therapeutic regimen revealed that the cabazitaxel group exhibited a better OS rate and tumor response rate ^[64]. Another randomized trial evaluated the performance of cabazitaxel versus abiraterone or enzalutamide (the testosterone-signaling-directing inhibitors) in advanced castration-resistant prostate cancer patients who had received docetaxel previously. Those treated with cabazitaxel showed a survival advantage when compared to abiraterone or enzalutamide ^[65].

Antimitotic drugs may cause resistance to develop through various mechanisms such as alteration of microtubule dynamics, upregulation of drug-efflux pumps, and mutations in tumor suppressor genes. Owing to their effect on microtubules, these drugs also result in major side effects especially on those cells which are not proliferating such as neurons ^[66].

9. CHEMORESISTANCE

For?‍?‌‍?‍‌ the past few decades, chemotherapy has been the most widely used method of treating various cancers. However, this method faces limitations because of a chemoresistance feature of cancer cells. Resistance to chemotherapy can be acquired, in which case it develops during treatment, or intrinsic, i.e. it can be present before the treatment as a result of the genetic changes, tumor heterogeneity, and other factors including insufficient drug absorption or metabolism ^[67].

Cells, being living systems, constantly receive and interpret mechanical and biochemical stimuli from the environment and utilize the interpretation to change the cellular state. Cells take drugs in two ways, by simple diffusion and by uptake with specific carrier proteins. Only 40 or so ATP-binding cassette (ABC) drug transporters in humans transport drugs across the cell membrane. The level of these transporters in different tissues determines the sensitivity of the cell to the drug. For instance, cells that contain the p-glycoprotein (p-gp) are resistant to the effect of doxorubicin, a drug that is commonly used for breast cancer, bladder cancer, lymphoma, and acute lymphoblastic leukemia (ALL), as p-gp ejects the drug from the cell and thus escaping it from ER stress ^[67]. Thus, a change in p-gp level can influence the outcome of chemotherapy. Similarly, the level of other transporters determines the flow and retention of cytotoxic drugs within tumor cells.

Besides, the limited metabolic activation of a prodrug and/or the increased metabolic inactivation of an active drug is another cause of the development of drug resistance. The drugs used for cancer treatment undergo metabolic changes at the level of the intestine, liver, and tumor. Dacarbazine, procarbazine cyclophosphamide, ifosfamide, and tamoxifen are examples of drugs that are metabolized to their cytotoxic form by human cytochrome P450s (CYPs). On the other hand, CYPs, including CYP3As, detoxify several chemotherapeutic drugs such as tamoxifen, imatinib, sorafenib, gefitinib, etc. Tumors expressing higher levels of CYP3As are resistant to the drugs ^[68].

The dysregulation of molecular pathways that control metabolism, cell proliferation, apoptosis, and autophagy is the underlying reason for drug resistance. In the process of EMT, (Epithelial to mesenchymal transition) a key step in the metastasis-initiating transcription factors (EMT-TFs) such as Snail, Twist, Slug, Forkhead box C2 (FOXC2) and Zinc finger E-box binding homeobox 1 (ZEB1) are responsible for drug resistance ^[69]. BCL-2 family consists of pro-apoptotic (Bax, Bak, Bid, Bad, Bim, Noxa, and Puma) and anti-apoptotic (BCL-2, BCL-W, BCL-XL, MCL-1, and BFL-1/A1) members. The life and death decision of cells depends on the ratio of the pro-apoptotic proteins to the anti-apoptotic proteins^[70].

Chemotherapeutic drugs mainly kill cancerous cells by inflicting DNA damage although cells have developed multiple repair pathways to counteract the DNA damage induced by the chemotherapeutic drugs. DNA damage repair pathway upregulation in cancer cells leads to stress tolerance and survival. Several studies show that doxorubicin resistance of cancer cells can be reversed by inhibiting the DNA repair kinases. MRE11—a DNA damage response protein—is drug resistance factor and can be used as a determinant of chemotherapy outcome^[71].

10. HORMONE THERAPY

Hormonal therapy is a crucial approach for hormone-dependent tumors, including breast, prostate, and ovarian cancers.

Table 2: List of Hormonal Therapy Agents for Cancer Treatment

GnRH gonadotropin-releasing hormone, LHRH luteinizing hormone-releasing hormone, LH luteinizing hormone.

11. BREAST CANCER

In?‍?‌‍?‍‌ breast cancer, the sooner it is diagnosed the better the prognosis will be. Since about 60-70% of breast cancers are positive for estrogen receptors, methods of controlling the estrogen levels are the most successful ones. Premenopausal women who have early stage of cancer and whose ovaries are removed will experience a decrease in the level of circulating estrogens and therefore the tumor will have little estrogenic support; however, removing ovaries will lead to menopause and is a decision that is not easy to tolerate. Luteinizing hormone-releasing hormone therapy (LHRH) is a less invasive form of treatment. This method indirectly decreases the amount of circulating estrogen by shutting down the ovarian production. A commonly used LHRH agonist in premenopausal patients with both primary and advanced stages of cancers is Goserelin acetate, a decapeptide analog of LHRH ^[72].

 Besides that, selective estrogen receptor modulators (SERM), such as tamoxifen not only have good anti-estrogen activity but also possess some estrogenic properties hence could produce side effects associated with estrogen. SERM binds to the ligand binding domain (LBD) of estrogen receptor (ER) and through this it may alter ER association with co-activators and co-inhibitors. But, for a major anti-estrogen like tamoxifen thromboembolism could be a side effect to watch out for. Consequently, other options like toremifene have been introduced. Indeed, a study comparing the use of toremifene with tamoxifen in postmenopausal patients with node-positive breast cancer found that there was no major difference in disease-free survival (DFS), overall survival (OS), or the side-effect profiles between the two drugs. Selective estrogen receptor down-regulators (SERD) represent an effective treatment possibility in cases resistant to tamoxifen and work by ER degradation and blocking the receptor within the signaling cascade ^[73].

In postmenopausal women, the adrenal glands produce androgens which aromatase enzymes present mainly in the fat convert into estrogens. Aromatase inhibitors (AIs) function by depleting estrogen levels through the inhibition of aromatase enzyme (aromatase is responsible for the conversion of androgens to estrogen). AIs together with ovarian suppression agents provide options that are less toxic side-effects-wise compared to tamoxifen. AIs, such as letrozole, anastrozole, and exemestane, have been developed with extreme specificity for aromatase and they inhibit conversion by over 99%. Yet, their response rates are between 35 to 70% in neoadjuvant studies and in case of advanced disease, decreased effectiveness might be encountered ^[74].

Moreover, resistance to treatment is one of the main challenges in cancer therapy, which results from exposure to treatment for a prolonged period of time. One way of overcoming resistance to AIs is by blocking type I growth factor receptors like EGFR, HER-2, and mTOR inhibitors. A study on the effectiveness of letrozole combined with everolimus treatment pathway as compared to treatment with letrozole alone showed that there was tumor size reduction and enhancement of sensitivity in the combination treatment group. The AKT pathway seems to be involved in the resistance mechanism to letrozole ^[75].

Also, genetic polymorphism may alter the functionality of AIs. One research study pointed out the role of SNP single nucleotide polymorphism (rs6493497 and rs7176005) in the regulation of aromatase (CYP19) expression after AI treatments. In patients with hormone receptor-positive metastatic breast cancer, the SNP (rs4646) of the aromatase CYP19 gene was most significantly correlated with the efficacy of the treatment. Another study analyzed the differences in protein expression levels in patients treated with letrozole only or letrozole plus chemotherapy. It was found that HIF1-α (hypoxia-inducible factor 1-α) and P44/42 mitogen-activated protein kinase (MAPK) have a role in resistance ^[76].

 However, it is important to realize that the suppression of estrogen induced by AIs is through endogenous sources only; therefore, the production of other steroids or their interaction with ER and exogenous estrogens coming from industrial pollution, synthetic or phytoestrogens are ?‍?‌‍?‍‌unaffected.

12. OVARIAN CANCER

Hormone?‍?‌‍?‍‌ receptor status, specifically estrogen receptors (ER) and progesterone receptors (PR), is a very significant factor not only in deciding treatments but also in determining the prognosis of ovarian cancer patients. Still, the level of hormone receptor expression varies greatly among the subtypes of ovarian cancer. For example, serous carcinomas that are the most prevalent subtypes show the highest expression levels of ER and PR. Mucinous and clear cell carcinomas show a lower incidence of these hormone receptors. Clearly, such marked variations in the hormone receptor expression levels influence hormone therapy's effectiveness ^[77].

The low levels and variability of hormone receptors in ovarian cancer have been the main reasons for the limited success of hormone therapies. Since many ovarian cancer cases do not carry hormone receptor expression, the potential application of hormone therapy gets largely restricted. On the other hand, a number of research works gauging the anti-cancer efficacy of tamoxifen for ovarian cancer have indicated that the drug could have its role in stabilizing the disease in 30-40% of the patients; however, the response rate was quite poor i.e., 10-15% ^[78]. Similarly, clinical trials of anastrozole (AI) conducted on ER and PR-positive ovarian cancer patients—pre-treated with chemotherapy and having limited disease, also yielded disappointing outcomes. In contrast, tamoxifen treatment for Chinese ovarian cancer patients with advanced chemo-resistant disease achieved PFS with limited side effects. In a head to head comparison of tamoxifen and letrozole for advanced epithelial ovarian cancer, it was found that, overall response rate, disease stability rate and clinical benefit rate were the same but letrozole had a longer response duration than tamoxifen (26 versus 11.5 months) ^[79].

Predictive biomarkers for hormone therapy remain elusive, and this limits therapy decisions for hormone therapy profoundly. But, at the very least, ascertaining hormone receptor status must become routine to guide treatment and avoid side effects. It's not uncommon for patients initially responding well to hormone therapies to eventually develop resistance. A myriad of resistance mechanisms exist, for example - changes in hormone receptor signaling pathways, modifications in co-regulator proteins, or compensatory upregulation of alternative pathways. For instance, activation of the PI3K/AKT pathway that might be responsible for resistance against hormone therapy. Drug resistance inevitably causes disease progression and at that time new therapeutic approaches are considered. This emphasizes on the requirement of continuous disease monitoring and timely review of treatment plans ^[79].

Chemotherapy or targeted therapies combined with hormone therapy could help boost the treatment of hormone-sensitive ovarian cancer. For example, premenopausal women under chemotherapy could be benefitted from the use of LHRH agonists such as goserelin acetate. Currently, clinical trials are underway to test various combination regimens for overcoming drug resistance and improving patient outcomes ^[80].

13. PROSTATE CANCER

Prostate?‍?‌‍?‍‌ cancer is the second most common cancer among men worldwide and the fifth leading cause of male cancer mortality. However, therapeutic options for it are still scarce. Since prostate cancer cells grow in the presence of androgens, androgen deprivation therapy, either orchiectomy (surgical castration) or estrogen (chemical castration), is the first-line treatment for advanced prostate cancer ^[81].

Estrogen (diethylstilbestrol) was previously used as medical castration in place of orchiectomy to mainly lower testosterone levels. But, due to the cardiovascular side effects, the use of estrogen was significantly reduced. Then the investigators started looking for antiandrogenic drugs that would compete with androgens for binding to receptors in the nucleus and thus induce apoptosis. A number of steroidal antiandrogens have been synthesized and tested; however, a meta-analysis of 2717 patients indicated that there was a decrease in OS rate when non-steroidal antiandrogen monotherapy was used compared to surgical castration. A randomized study of 1435 patients receiving bicalutamide (non-steroidal antiandrogen) showed that it was poorly effective in patients with M1 disease and for locally advanced M0 disease patients, its OS was not significantly different from surgical castration ^[82].

Nowadays, LHRH agonists are the new standard of care in androgen deprivation therapy (ADT) as they are a more practical and reversible way for the patients to be castrated than orchiectomy. Review of 10 clinical trials involving 1908 patients demonstrated the equivalence of imnproved OS rate among patients responding to GnRH (Gonadotropin hormone-releasing hormone) agonist treatment with orchiectomy or diethylstilbestrol for advanced disease ^[82].

 On the other hand, GnRH agonists have also shown to be beneficial therapeutically in the case of early prostate cancer. If patients continue to take GnRH agonists for 3 years after radiotherapeutic treatment, these drugs are capable of improving DFS and OS even up to 10 years after the treatment in high-risk metastatic patients. However, the therapy with GnRH agonists has the risk of serious side effects like bone pain, spinal cord compression, ureteral obstruction, and even death in case of an unexpected sharp rise of testosterone levels following the stimulation of GnRH receptors ^[81].

Researchers identified GnRH antagonists that bind to and block GnRH receptors. Abarelix was the first FDA approved GnRH antagonist that could achieve castration without inducing testosterone surge. Nevertheless, in two clinical trials, the continuous administrations of abarelix appeared to reduce its testosterone lowering effect. Degarelix, another ADT, causes decrease of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) levels by antagonizing GnRH. In one year long randomized clinical trial recruiting 610 patients, no one patient experienced testosterone surge under degarelix treatment. The treatment also resulted in immediate drop in both testosterone and prostate-specific antigen (PSA) levels, better overall survival, and reduced rate of hormone-refractory disease ^[82].

Yet, degarelix-treated patients complained of injection site reactions, chills, urinary tract infection, and musculoskeletal problems. Besides frequent administration, the high rate of injection site reaction (~40%) has been shown to contribute to the limitation of the clinical uses of degarelix ^[82].

Relugolix is an alternative GnRH antagonist developed for oral administration with high selectivity and a longer half-life of 25 h to avoid complications of degarelix^[83]. Various phase I and II studies asseverate the rapid activity of relugolix in inhibiting the release of FSH and LH from the pituitary and lowering testosterone levels ^[84]. In a randomized phase III study, leuprolide acetate (22.5 mg by injection after 90 days), a GnRH analog with a half-life of 3 h reduced testosterone levels as compared to relugolix (120 mg one time a day after an initial oral dose of 360 mg). Testosterone surge was noticed in the leuprolide group, while the group with relugolix treatment did not show a rise in testosterone before reaching castrate level. Additionally, relugolix treatment eliminated the need for antiandrogens to mitigate undesirable effects^[85]

Fig. 3  Evolution of traditional therapies: past, present, and future directions, illustrated