Methotrexate resistance is due to?
**Core Concept**
Methotrexate resistance is a complex phenomenon in cancer treatment, primarily involving alterations in cellular mechanisms that affect the efficacy of the drug. Methotrexate is a dihydrofolate reductase (DHFR) inhibitor, and its resistance can arise from various cellular adaptations.
**Why the Correct Answer is Right**
Methotrexate resistance can occur due to increased expression or altered function of DHFR, allowing the cell to bypass the inhibitory effects of the drug. Another mechanism involves the reduced influx of methotrexate into the cell or its increased efflux through transporters like ABCG2 (breast cancer resistance protein). Additionally, methotrexate polyglutamation, which is essential for its anti-cancer activity, can be impaired in resistant cells. This results in decreased intracellular concentrations of the active metabolites.
**Why Each Wrong Option is Incorrect**
* **Option A:** Methotrexate resistance is not primarily due to decreased expression of DHFR, as this would not directly contribute to resistance.
* **Option B:** While alterations in DNA mismatch repair mechanisms can influence methotrexate resistance, this is not the primary mechanism.
* **Option C:** The role of p53 mutations in methotrexate resistance is not as direct or significant as other mechanisms.
**Clinical Pearl / High-Yield Fact**
Methotrexate resistance often involves a combination of mechanisms, and resistance can develop through selection of pre-existing clones or de novo mutations. Understanding these mechanisms is crucial for developing effective strategies to overcome resistance.
**Correct Answer: D. Reduced methotrexate polyglutamation.**