Fundamental mechanisms of cytotoxicity
The action of the group is based on the chemical alkylation of heterocyclic atoms of purines and pyrimidines. The main molecular target is the nitrogen atom at the 7th position of guanine. Additionally, the drugs can attack the oxygen at the 6th position of guanine, as well as the nitrogen and oxygen atoms in cytosine and adenine.
Most representatives of this group are bifunctional—they contain two alkyl groups at once. This ensures the phenomenon of "cross-linking" (cross-linking): the drug links nucleotides with pathological bridges. Cross-links can occur between adjacent bases of the same strand, connect different DNA strands (intercalary), or attach DNA to a protein.
The consequences of alkylation for the cell are fatal:
- Coding disruption: damaged guanine forms abnormal pairs with thymine, leading to reading errors.
- Depurination: the guanine residue is cleaved, the sugar-phosphate backbone is ruptured, and the DNA strand breaks.
- Blockade of template functions: due to breaks and cross-links, the cell cannot initiate replication and transcription processes.
These drugs are cell cycle-nonspecific, though actively dividing tumor cells are most sensitive to them.
Toxicological profile and complications
Alkylating drugs severely interfere with the genome, and therefore possess pronounced mutagenic and carcinogenic effects. In the long term, this increases the risk of developing secondary tumors.
In addition to side effects common to all cytostatics, specific toxicity is distinguished:
- Hematological: bone marrow suppression, leukocytosis.
- Urological: development of hemorrhagic cystitis.
- Reproductive: damage to the reproductive system (amenorrhea, testicular atrophy, aspermia, irreversible sterility).
- Neurological: neurotoxicity is possible at high doses.
In addition to oncology (therapy of solid and lymphatic tumors), the potent immunosuppressive effect of these agents is used to suppress autoimmune pathologies and prevent transplant rejection.
Nitrogen mustard derivatives
These drugs historically originate from the combat chemical warfare agent mustard gas (sulfur mustard).
- Cyclophosphamide. Often administered orally. It is a classic prodrug and exhibits no cytotoxicity in vitro. It is activated in the liver via hydroxylation involving the cytochrome P450 system, after which its active metabolites attack DNA.
- Melphalan (Melphalan). A phenylalanine derivative. Used orally and parenterally for the treatment of multiple myeloma and advanced stages of ovarian adenocarcinoma. The dose-limiting side effect is severe myelosuppression.
Nitrosoureas and platinum-based drugs
- Nitrosourea derivatives (Carmustine, Lomustine). Distinguished by high lipophilicity, due to which they easily cross the blood-brain barrier (BBB) and are used for CNS tumors. A separate representative, Streptozocin (Streptozocin), selectively acts on the beta cells of the islets of Langerhans: it treats insulinoma, and in experiments is used to model diabetes mellitus. Group side effects: nephrotoxicity, pulmonary fibrosis, and aplastic anemia.
- Platinum-based drugs. These are inorganic heavy metal complexes without alkyl groups, but they operate via an identical mechanism—forming DNA cross-links. Cisplatin (Cisplatin) is active against numerous solid tumors, but has pronounced toxicity. Carboplatin (Carboplatin) acts similarly, has lower nephro- and gastrotoxicity, yet causes significant myelosuppression. Specific risks of platinum: ototoxicity (hearing loss) and neurotoxicity.
Temozolomide
Temozolomide (Temozolomide) is an oral drug that penetrates the BBB. In the liver, it is converted into the active metabolite (methyltriazenoimidazolecarboxamide).
Its mechanism of action consists in alkylating DNA at the 6th position of guanine and inhibiting the DNA repair enzyme—O6-guanine-DNA alkyltransferase. The drug is indicated for melanoma, malignant gliomas, and anaplastic astrocytoma. Frequent adverse reactions include dyspepsia (vomiting, nausea) and myelosuppression.
Briefly: answers to topical questions
What active metabolites are formed during the biotransformation of cyclophosphamide in the liver?
During the biotransformation of cyclophosphamide in the liver, active metabolites are formed as a result of a hydroxylation reaction. The specific chemical names of these substances are not provided in sources. Activation of the prodrug occurs with the participation of the cytochrome P450 enzymatic system. The resulting active metabolites exert a cytotoxic effect by alkylating the DNA and proteins of tumor cells. They form stable alkyl cross-links in the DNA molecule located in areas inaccessible to repair enzymes, which makes cell division impossible.
What drug is used for the prevention of hemorrhagic cystitis during treatment with cyclophosphamide?
For the prevention of hemorrhagic cystitis during cyclophosphamide treatment, the drug mesna (sodium 2-mercaptoethanesulfonate) is used. It is administered systemically intravenously as a preventive measure. Additionally, the following approaches are used to prevent damage to the bladder mucosa:
- Hyperhydration regimen — used directly during therapy.
- Increased fluid intake — up to 3 liters per day before drug administration and within 72 hours after it.
- Frequent urination — to accelerate the excretion of toxic substances.
For nitrosoureas: Carmustine and Lomustine cross the BBB (are lipophilic)—treat the brain. Streptozocin targets the pancreas—induces experimental diabetes.
Class and exam questions
In which phase of the cell cycle do alkylating drugs act?
They are cell cycle-nonspecific. DNA cross-linking occurs in any phase, although actively dividing cells are most sensitive to their effects.
Why is cyclophosphamide ineffective in vitro (in a test tube)?
Because it is a prodrug. To acquire activity, the drug must undergo hydroxylation by cytochrome P450 enzymes in the liver.
Do platinum-based drugs belong to true alkylating agents?
No by chemical structure — they do not contain alkyl groups. But they are classified in this group because their mechanism of action (formation of DNA cross-links) is completely identical.
What else to study on this topic
- Types of DNA cross-links: intercalary, intrastrand, and protein cross-links
- Mechanisms of depurination development and ruptures of the sugar-phosphate backbone
- Features of cyclophosphamide biotransformation by CYP450 enzymes
- Mechanism of DNA repair inhibition by temozolomide
- Comparison of toxicity profiles of cisplatin and carboplatin
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