Anticancer Drugs: Classification, Mechanism, Uses & Side Effects

Anticancer drugs are medicines used to treat cancer by destroying cancer cells, stopping their growth, or interfering with the processes that allow them to multiply and spread. 

Anticancer drugs include traditional chemotherapy agents, hormonal medicines, targeted therapies, and immunotherapies. Each group acts differently, and the choice of treatment depends on the type of cancer, its stage, specific tumour characteristics, and the patient’s overall health.

In this article, we will discuss the classification of anticancer drugs, their mechanisms of action, important examples, therapeutic uses, and adverse effects.

🔍What Are Anticancer Drugs?

Anticancer drugs are medicines used in the treatment of malignant diseases. They may kill cancer cells, inhibit cell division, block signals responsible for tumour growth, alter hormone activity, or help the immune system recognise and attack cancer cells.

Some anticancer medicines are used alone, while others are combined with surgery, radiotherapy, or other cancer treatments.

📝Classification of Anticancer Drugs

Anticancer drugs can be classified according to their mechanism of action and pharmacological properties.

1. Alkylating Agents

Alkylating agents damage DNA by forming chemical bonds with DNA components. This interferes with DNA replication and prevents cancer cells from multiplying.

Examples:

• Cyclophosphamide
• Ifosfamide
• Chlorambucil
• Melphalan
• Busulfan
• Dacarbazine

Mechanism of action: These drugs modify DNA, often through alkylation or cross-linking, interfering with replication and cell division.

Important adverse effects: Bone marrow suppression, nausea, vomiting, and hair loss. Some agents may also cause infertility or increase the long-term risk of secondary malignancies.

Exam point: Cyclophosphamide is an important alkylating agent and is also used in certain autoimmune disorders.

2. Antimetabolites

Antimetabolites resemble naturally occurring substances required for DNA or RNA synthesis. They interfere with nucleotide production or become incorporated into nucleic acids, disrupting cell growth.

Examples:

• Methotrexate
• 5-Fluorouracil (5-FU)
• Capecitabine
• 6-Mercaptopurine
• Cytarabine
• Gemcitabine

Mechanism of action: They inhibit essential enzymes involved in nucleotide synthesis or interfere with DNA and RNA formation.

Important adverse effects: Bone marrow suppression, mouth ulcers, gastrointestinal disturbances, and, depending on the drug, liver toxicity.

Exam points:

• Methotrexate inhibits dihydrofolate reductase (DHFR).
• 5-Fluorouracil inhibits thymidylate synthase through its active metabolite.
• 6-Mercaptopurine interferes with purine synthesis.

3. Antitumour Antibiotics

Antitumour antibiotics are anticancer medicines that interfere with DNA function and other cellular processes. They are different from antibiotics used to treat bacterial infections.

Examples:

• Doxorubicin
• Daunorubicin
• Bleomycin
• Dactinomycin
• Epirubicin

Mechanism of action: Depending on the drug, they may intercalate into DNA, inhibit topoisomerase enzymes, generate free radicals, or cause DNA strand damage.

Important adverse effects:

• Doxorubicin: cardiotoxicity and bone marrow suppression.
• Bleomycin: pulmonary toxicity.
• Daunorubicin: cardiotoxicity and bone marrow suppression.

Exam point: Doxorubicin belongs to the anthracycline group and is associated with cumulative dose-related cardiotoxicity.

4. Plant-Derived Anticancer Drugs

Several anticancer drugs were developed from plant-derived compounds or their derivatives.

A. Vinca Alkaloids

Examples:

• Vincristine
• Vinblastine
• Vinorelbine

Mechanism of action: These drugs interfere with microtubule formation, preventing normal mitotic spindle function and disrupting cell division.

Important adverse effects:

• Vincristine: peripheral neuropathy and constipation.
• Vinblastine: bone marrow suppression.

B. Taxanes

Examples:

• Paclitaxel
• Docetaxel
• Cabazitaxel

Mechanism of action: Taxanes stabilise microtubules and prevent their normal disassembly, disrupting mitosis.

Important adverse effects: Peripheral neuropathy, bone marrow suppression, and hypersensitivity reactions with some formulations.

Exam point: Vinca alkaloids inhibit microtubule polymerisation, whereas taxanes stabilise microtubules.

5. Topoisomerase Inhibitors

Topoisomerase enzymes help manage DNA structure during replication and transcription. Inhibiting these enzymes can cause DNA damage and prevent cancer cells from multiplying.

Examples:

• Etoposide: inhibits topoisomerase II.
• Irinotecan: inhibits topoisomerase I.
• Topotecan: inhibits topoisomerase I.

Mechanism of action: These drugs interfere with topoisomerase-mediated DNA processing, leading to DNA damage and cell death.

Important adverse effects: Bone marrow suppression, nausea, vomiting, and diarrhoea. Irinotecan can cause severe diarrhoea.

6. Platinum Compounds

Platinum compounds damage DNA and interfere with its replication.

Examples:

• Cisplatin
• Carboplatin
• Oxaliplatin

Mechanism of action: They form DNA adducts and cross-links, disrupting DNA replication and transcription.

Important adverse effects:

• Cisplatin: nephrotoxicity, ototoxicity, and nausea and vomiting.
• Carboplatin: dose-limiting bone marrow suppression, particularly thrombocytopenia.
• Oxaliplatin: peripheral neuropathy.

Exam point: Cisplatin is particularly associated with kidney toxicity and hearing damage.

7. Hormonal and Hormone-Blocking Drugs

Some cancers depend on hormones for growth. Hormonal treatments reduce hormone production or block hormone receptors.

Examples:

• Tamoxifen
• Anastrozole
• Letrozole
• Leuprolide
• Bicalutamide
• Abiraterone

Mechanism of action:

• Tamoxifen blocks oestrogen receptors in breast tissue.
• Anastrozole and letrozole inhibit aromatase, reducing oestrogen production.
• Leuprolide alters pituitary hormone signalling and can suppress sex hormone production with continuous administration.
• Bicalutamide blocks androgen receptors.
• Abiraterone inhibits CYP17, reducing androgen production.

Important adverse effects: Hot flashes, fatigue, sexual dysfunction, and drug-specific effects such as bone loss or metabolic changes.

Exam point: Tamoxifen is a selective oestrogen receptor modulator (SERM).

8. Targeted Anticancer Drugs

Targeted therapies act on particular molecules or signalling pathways involved in cancer growth and survival. Some work on proteins inside cells, while others act on receptors or other targets at the cell surface.

Examples:

• Imatinib
• Erlotinib
• Osimertinib
• Trastuzumab
• Rituximab
• Bevacizumab

Mechanism of action:

• Imatinib inhibits specific tyrosine kinases, including BCR-ABL.
• Erlotinib and osimertinib inhibit epidermal growth factor receptor (EGFR) signalling in appropriate cancers.
• Trastuzumab targets HER2.
• Rituximab targets CD20 on certain B cells.
• Bevacizumab inhibits vascular endothelial growth factor (VEGF), reducing tumour blood-vessel formation.

Important adverse effects: These vary by medicine and may include skin reactions, diarrhoea, infusion reactions, hypertension, or effects on heart function.

Exam point: Imatinib is a tyrosine kinase inhibitor used in conditions including chronic myeloid leukaemia with the BCR-ABL abnormality.

9. Immunotherapy

Immunotherapy helps the body’s immune system recognise or attack cancer cells. It includes immune checkpoint inhibitors, certain monoclonal antibodies, and other immune-based treatments.

Examples:

• Pembrolizumab
• Nivolumab
• Ipilimumab

Mechanism of action: Pembrolizumab and nivolumab block PD-1 signalling, while ipilimumab blocks CTLA-4. These actions can restore or enhance antitumour immune responses.

Important adverse effects: Immune-related inflammation may affect the skin, bowel, liver, lungs, thyroid, and other organs.

Exam point: Pembrolizumab and nivolumab are immune checkpoint inhibitors targeting PD-1.

⚕️Mechanism of Action of Anticancer Drugs: Quick Revision

The main mechanisms include:

1. DNA damage: Alkylating agents and platinum compounds damage DNA.
2. Inhibition of nucleotide synthesis: Methotrexate and other antimetabolites interfere with the production or use of DNA building blocks.
3. Inhibition of mitosis: Vinca alkaloids disrupt microtubule formation, while taxanes stabilise microtubules.
4. Inhibition of topoisomerases: Etoposide, irinotecan, and topotecan interfere with DNA processing.
5. Hormone suppression or receptor blockade: Tamoxifen, aromatase inhibitors, and androgen-pathway drugs act on hormone-dependent cancers.
6. Targeted inhibition: Imatinib and other targeted agents inhibit specific cancer-associated molecules.
7. Immune activation: Immune checkpoint inhibitors help the immune system mount an antitumour response.

🩺Therapeutic Uses of Anticancer Drugs

Anticancer drugs are used to treat many types of cancer, including:

• Breast cancer
• Lung cancer
• Colorectal cancer
• Leukaemia
• Lymphoma
• Ovarian cancer
• Prostate cancer
• Certain skin cancers
• Some brain and other solid tumours

The specific drug or combination depends on the cancer type, stage, molecular characteristics, previous treatment, and the patient’s health. Not every anticancer drug is effective against every cancer.

Treatment may aim to cure cancer, reduce the chance of recurrence, control disease progression, or relieve symptoms.

Common Side Effects of Anticancer Drugs

Side effects vary considerably between medicines and patients. Common or important effects include:

• Bone marrow suppression: May cause anaemia, increased infection risk, or bleeding.
• Nausea and vomiting: Common with several chemotherapy medicines, although preventive treatments are available.
• Hair loss: Occurs with some chemotherapy regimens.
• Mouth ulcers: May develop with certain cytotoxic drugs.
• Fatigue: Can occur during many types of cancer treatment.
• Peripheral neuropathy: May occur with drugs such as vincristine, paclitaxel, and oxaliplatin.
• Organ-specific toxicity: Some medicines may affect the heart, kidneys, liver, lungs, or other organs.
• Immune-related reactions: Immunotherapy can cause inflammation in healthy organs.

Side effects do not reliably indicate whether a treatment is working. Patients should report new or severe symptoms to their oncology team.

Important Differences Between Anticancer Drug Classes

For examination revision, remember these key distinctions:

• Vinca alkaloids: Inhibit microtubule polymerisation.
• Taxanes: Stabilise microtubules and inhibit their normal disassembly.
• Methotrexate: Inhibits dihydrofolate reductase.
• 5-Fluorouracil: Inhibits thymidylate synthase through its active metabolite.
• Cisplatin: Causes DNA adduct formation and cross-linking.
• Etoposide: Inhibits topoisomerase II.
• Irinotecan: Inhibits topoisomerase I.
• Tamoxifen: Selective oestrogen receptor modulator.
• Imatinib: Tyrosine kinase inhibitor.
• Pembrolizumab: PD-1 immune checkpoint inhibitor.

📍Precautions During Anticancer Treatment

Anticancer treatment requires specialist supervision. Important precautions include:

• Follow the prescribed treatment schedule and do not change doses independently.
• Attend recommended blood tests and organ-function monitoring.
• Tell the treating team about all medicines, supplements, allergies, and existing medical conditions.
• Seek urgent medical advice for fever during chemotherapy, uncontrolled bleeding, breathing difficulties, or other severe symptoms.
• Discuss fertility, pregnancy, contraception, and breastfeeding with the oncology team when relevant.

Anticancer drugs should never be started, stopped, or adjusted without guidance from the treating specialist.

📋Frequently Asked Questions (FAQs)

1. What are anticancer drugs?

Anticancer drugs are medicines that destroy cancer cells, inhibit their growth, block cancer-related signals, or help the immune system attack cancer.

2. What are the main classes of anticancer drugs?

Important classes include alkylating agents, antimetabolites, antitumour antibiotics, plant-derived drugs, topoisomerase inhibitors, platinum compounds, hormonal therapies, targeted therapies, and immunotherapies.

3. What is the mechanism of action of anticancer drugs?

Their mechanisms vary. They may damage DNA, inhibit nucleotide synthesis, disrupt cell division, block hormonal signalling, target specific proteins, or enhance immune responses against cancer.

4. Which anticancer drug inhibits dihydrofolate reductase?

Methotrexate inhibits dihydrofolate reductase, an enzyme involved in folate metabolism and nucleotide synthesis.

5. What is the difference between vinca alkaloids and taxanes?

Vinca alkaloids inhibit microtubule polymerisation, whereas taxanes stabilise microtubules and prevent their normal disassembly. Both disrupt cell division.

6. Are all anticancer drugs chemotherapy?

No. Traditional cytotoxic chemotherapy is only one category of cancer treatment. Hormonal therapy, targeted therapy, and immunotherapy work through different mechanisms.

🎯 Conclusion

Anticancer drugs are an essential part of modern cancer treatment and an important topic in pharmacology. Understanding their classification, mechanisms of action, therapeutic uses, and adverse effects helps pharmacy students prepare for examinations and build a foundation in clinical pharmacology.

For revision, focus particularly on the mechanisms of methotrexate, 5-fluorouracil, cisplatin, doxorubicin, vincristine, paclitaxel, imatinib, and tamoxifen.

 Disclaimer: This article is intended for educational purposes. Cancer treatment must be selected and monitored by qualified oncology professionals.