Fundamental Mechanisms of Cytotoxicity
The mechanism of action is based on the chemical alkylation of heterocyclic atoms in purines and pyrimidines. The primary molecular target is the nitrogen atom at the 7th position of guanine. Additionally, these drugs can attack the oxygen at the 6th position of guanine, as well as nitrogen and oxygen atoms in cytosine and adenine.
Most agents in this group are bifunctional, meaning they contain two alkyl groups. This enables the phenomenon of cross-linking: the drug connects nucleotides via pathological bridges. Cross-links can occur between adjacent bases on the same strand, connect opposite DNA strands (interstrand cross-linking), or attach DNA to proteins.
Downstream consequences of DNA alkylation are fatal to the cell:
- Miscoding: damaged guanine forms abnormal base pairs with thymine, leading to reading errors.
- Depurination: the guanine residue is cleaved, the sugar-phosphate backbone is disrupted, and single-strand DNA breaks occur.
- Blockade of template functions: due to breaks and cross-links, the cell cannot initiate DNA replication and transcription.
These drugs are cell cycle-nonspecific, though actively dividing tumor cells are the most sensitive to them.
Toxicological Profile and Complications
Because alkylating agents directly interfere with the genome, they possess significant mutagenic and carcinogenic properties. Long-term, this increases the risk of secondary malignancies (e.g., secondary leukemias).
In addition to side effects common to all cytotoxic drugs, specific toxicities include:
- Hematologic: bone marrow suppression, leukopenia.
- Urologic: development of hemorrhagic cystitis.
- Reproductive: damage to the reproductive system (amenorrhea, testicular atrophy, asperomia, irreversible sterility).
- Neurological: neurotoxicity possible at high doses.
Beyond oncology (treatment of solid tumors and lymphomas), the potent immunosuppressive effect of these agents is utilized to manage autoimmune disorders and prevent transplant rejection.
Nitrogen Mustards
These drugs are historically derived from the chemical warfare agent sulfur mustard (mustard gas).
- Cyclophosphamide: Frequently administered orally. It is a classic prodrug and exhibits no cytotoxicity in vitro. It is activated in the liver via hydroxylation by 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 ovarian adenocarcinoma. The dose-limiting side effect is severe myelosuppression.
Nitrosoureas and Platinum-Based Drugs
- Nitrosoureas (Carmustine, Lomustine). Characterized by high lipophilicity, allowing them to freely cross the blood-brain barrier (BBB), making them useful for central nervous system tumors. A distinct representative, Streptozocin (Streptozocin), selectively targets pancreatic beta cells of the islets of Langerhans; it is used to treat insulinomas and experimentally to induce animal models of diabetes mellitus. Group side effects include nephrotoxicity, pulmonary fibrosis, and aplastic anemia.
- Platinum-based drugs. These are heavy metal coordination complexes that lack alkyl groups but function via an identical mechanism—forming DNA cross-links. Cisplatin (Cisplatin) is active against numerous solid tumors but has marked toxicity. Carboplatin (Carboplatin) acts similarly, with lower nephrotoxicity and gastrointestinal toxicity, though it causes significant myelosuppression. Specific platinum-related risks include ototoxicity (hearing loss) and peripheral neuropathy.
Temozolomide
Temozolomide (Temozolomide) is an oral drug that crosses the BBB. In the liver, it is converted into its active metabolite (MTIC).
Its mechanism of action involves alkylating DNA at the O6-position of guanine and inhibiting the DNA repair enzyme *O*6-alkylguanine-DNA alkyltransferase (AGT). The drug is indicated for melanoma, malignant gliomas, and anaplastic astrocytoma. Common adverse reactions include dyspepsia (vomiting, nausea) and myelosuppression.