Vinca Alkaloids
The source of these alkaloids is the Madagascar periwinkle (Vinca rosea). Key representatives of the group are vincristine and vinblastine, which share a similar chemical structure. Their action is strictly tied to the M-phase of the cell cycle.
The molecular mechanism involves binding to tubulin and blocking its GTP-dependent polymerization. As a result, microtubules fail to form, a defective mitotic spindle is produced, and mitosis halts at the metaphase stage.
- Vincristine is actively used in pediatric oncology (acute lymphoblastic leukemia, Wilms tumor), as well as in Ewing sarcoma and lymphomas.
- Vinblastine is more frequently used in combination regimens (e.g., with cisplatin and bleomycin) for the treatment of metastatic testicular carcinoma and various solid tumors.
Taxanes
These compounds were originally isolated from the Pacific yew (Taxus brevifolia). The toxic properties of this plant were known even in antiquity—in Europe, yew cups were even used to poison drinks. The active cytotoxic substance is the alkaloid taxol.
Unlike vinca alkaloids, paclitaxel and docetaxel bind to the $\beta$-subunit of tubulin and, conversely, stimulate its polymerization even in the absence of GTP and required associated proteins. Microtubules become excessively stable and lose their ability to depolymerize (disassemble). The cell fills with nonfunctional tubulin and loses the ability to divide.
Taxanes are indicated for the treatment of breast cancer, ovarian cancer, non-small cell lung cancer, and epithelial tumors of the head and neck.
Pharmacokinetics and Drug Resistance
All drugs in these two groups are administered exclusively via intravenous infusion. They distribute quite widely throughout the body, although taxanes do not cross the blood-brain barrier. Metabolism occurs in the liver with the mandatory participation of the cytochrome P450 enzyme system, requiring dose adjustments when concurrent medications are prescribed. Elimination occurs primarily through the biliary system via the gastrointestinal tract.
The main cause of tumor resistance to these plant-based agents is the overexpression of P-glycoprotein. This transport protein acts as a pump, expelling drug molecules out of the cell. For taxanes, an additional resistance factor can be mutations within the tubulin structure itself.
Safety Profile and Complication Management
Microtubule inhibitor therapy is accompanied by several severe adverse effects. Administration of vinca alkaloids can cause local reactions ranging up to tissue necrosis and phlebitis. Due to the massive destruction of tumor cell DNA, purines are released into the blood and oxidized into uric acid. To prevent this hyperuricemia, allopurinol is prescribed.
Per standard pharmacological classifications, vinca alkaloids exhibit distinct toxicity profiles:
- For vinblastine, bone marrow suppression (myelosuppression) is most characteristic.
- For vincristine, neurotoxicity (ataxia, paresthesias, loss of deep tendon reflexes) is typical.
When using taxanes, peripheral neuropathy and neutropenia frequently develop, requiring the administration of filgrastim. Additionally, docetaxel causes fluid retention, while paclitaxel carries a risk of bradycardia and severe hypersensitivity reactions (requiring strict premedication with a combination of dexamethasone and diphenhydramine).