In a hospital setting, which chemotherapy agent is contraindicated for intrathecal use?

• A. Vincristine
• B. Methotrexate
• C. Cisplatin
• D. Doxorubicin

Answer: (A)

Vincristine must never be given intrathecally because it is highly neurotoxic when delivered into the cerebrospinal fluid, leading to catastrophic outcomes such as seizures, coma, and death. Its mechanism as a microtubule inhibitor disrupts neural tissue when directly exposed to the CNS, causing severe damage to spinal cord and brain tissue. In practice, vincristine is formulated and intended for IV use only, and accidental intrathecal administration has historically resulted in fatal toxicity. Other agents can be used intrathecally in CNS-directed regimens (for example, methotrexate is commonly used intrathecally for CNS prophylaxis or leptomeningeal disease), though each has its own toxicity profile and requires careful dosing and monitoring. The key point is that intrathecal exposure to vincristine is a dangerous, non-allowable route.