Anticancer Drugs: Alkylating Agents & Antimetabolites Guide

Anticancer (antineoplastic) drugs are medicines that kill cancer cells or stop them dividing. Alkylating agents damage DNA directly by attaching alkyl groups, while antimetabolites masquerade as the building blocks of DNA and sabotage its synthesis. Together with surgery and radiotherapy they form the chemotherapy backbone of cancer care.

They matter because cancer is uncontrolled cell division, and these drugs exploit exactly that: cells that divide fast take up more drug and suffer more damage. The price is toxicity to normal dividing tissues - bone marrow, gut lining, hair follicles - so oncologists balance cure against side effects with precise dosing schedules and rescue strategies.

Key Concepts

• Cell-cycle specificity: antimetabolites kill mainly in S-phase (DNA synthesis); alkylating agents are cell-cycle non-specific and hit resting cells too.
• Alkylating agents: cyclophosphamide, ifosfamide, busulfan, and the platinum drugs cisplatin and carboplatin, which crosslink DNA strands so they cannot separate for replication.
• Antimetabolites: methotrexate (blocks dihydrofolate reductase, starving cells of folate), 5-fluorouracil (blocks thymidylate synthase), 6-mercaptopurine and cytarabine (fake bases incorporated into DNA).
• Myelosuppression: the dose-limiting toxicity for most - falling white cells, platelets, and red cells.
• Leucovorin rescue: folinic acid given after high-dose methotrexate to rescue normal cells.
• Combination logic: drugs with different mechanisms and non-overlapping toxicities are combined to kill more cancer cells before resistance emerges.

How It Works

An alkylating agent enters the cell and forms a reactive intermediate that bonds to guanine bases on DNA, linking the two strands together. When the cell tries to copy its DNA, the replication fork stalls at the crosslink and the cell triggers apoptosis - programmed death. Because the damage does not depend on the cell-cycle phase, even slow-growing tumour cells are hit. Antimetabolites work one step later: disguised as nucleotides, they are woven into growing DNA or jam the enzymes that make nucleotides, so S-phase cells die when their faulty DNA cannot be copied.

Worked Example

Chemotherapy is dosed by body surface area. A patient has a BSA of 1.7 m2 and the protocol prescribes cyclophosphamide 600 mg/m2. Dose = 600 x 1.7 = 1020 mg. If the vial concentration is 20 mg/mL, the volume to draw up = 1020 / 20 = 51 mL. Pharmacists double-check this arithmetic on every cycle, because a decimal slip here is a tenfold overdose.

Common Misconceptions

• Chemotherapy kills only cancer cells. It attacks all rapidly dividing cells, which is why hair loss, mouth sores, and low blood counts are expected side effects, not mistakes.
• A bigger dose always works better. Beyond a point, toxicity rises faster than tumour kill; protocols are tuned to the maximum tolerated dose from trials, not to intuition.
• Targeted therapy has no side effects. It has different ones - skin rash, diarrhoea, heart effects - because its targets also exist in normal tissues.

Real-World Applications

• Cisplatin-based regimens cure most testicular cancers.
• Methotrexate treats leukaemia, and high-dose protocols with leucovorin rescue treat osteosarcoma.
• 5-fluorouracil anchors colorectal cancer chemotherapy.
• Alkylating agents condition bone marrow before stem-cell transplant.

Alkylating agents crosslink DNA regardless of cell cycle; antimetabolites ambush DNA synthesis in S-phase. Dosed by body surface area and combined by mechanism, they turn cancer's defining trait - relentless division - into its vulnerability.