Oncology- Drug Resistance in Oncology

Oncology- Drug Resistance in Oncology
This study guide summarizes the provided text on drug resistance in cancer treatment. Understanding these mechanisms is crucial for developing effective therapies.
I. Introduction: The Problem of Drug Resistance

• Laboratory vs. Clinical Resistance: Much research uses artificially induced resistance in cell lines (often >40-100 fold increase in drug concentration needed to overcome resistance). The relevance of this to clinical resistance remains unclear.

II. Mechanisms of Drug Resistance
This section details the various ways cancer cells evade chemotherapy. Note that multiple mechanisms can operate simultaneously in a single patient.
A. Pharmacological Resistance: The effective drug concentration at the target site is insufficient due to:

• Organ Toxicity: Limiting dosage due to side effects in other organs.
• Enhanced Drug Clearance: The body removes the drug too quickly.
• Physical Barriers: Tumour avascularity (lack of blood vessels) prevents drug delivery.
• De Novo Resistance: The tumour is unresponsive to chemotherapy from the start.
• Acquired Resistance: Initial response followed by tumour regrowth and resistance.
• Combination of De Novo and Acquired: Both mechanisms are at play.

B. Alteration of Target or Transport Mechanisms: Tumour cells adapt by:

• Reduced Drug Uptake: Mutations prevent drug entry into the cell.
• Increased Detoxification: Faster metabolism and elimination of the drug.
• Target Site Mutation: The drug's target is altered, making it ineffective.
• Enhanced DNA Repair: Increased efficiency in repairing drug-induced damage.

C. Classical Multidrug Resistance (MDR):

• P-170 Glycoprotein (Pgp): Overexpression of this 170kDa glycoprotein creates an energy-dependent drug efflux pump. Drugs enter the cell but are actively pumped back out, reducing intracellular concentration. This is common with anthracyclines, taxanes, and etoposide; resistance to one often implies resistance to others (multidrug resistance).

D. Multidrug Resistance-Associated Protein (MRP):

• 190kDa Efflux Pump: Another energy-dependent pump causing drug efflux or sequestration within organelles. Substrate specificity similar to Pgp but generally associated with less taxane resistance. Clinical significance less established than Pgp.

E. Glutathione:

• Thiol-Based Detoxification: This cellular thiol participates in detoxification pathways, particularly against alkylating agents (e.g., cisplatin) and free radicals (e.g., doxorubicin). Overexpression leads to resistance; glutathione depletion strategies have yielded mixed clinical results.

F. Failure to Engage Apoptosis:

• Apoptosis Failure: Many cytotoxic drugs induce apoptosis (programmed cell death). Failure to activate apoptosis, often linked to p53 dysfunction ("guardian of the genome"), allows damaged cells to survive and proliferate, leading to resistance. Gene therapy targeting this mechanism is under investigation.

III. Summary and Conclusion
Clinical drug resistance is a complex, multifactorial problem. The relative contribution of each mechanism varies greatly between patients. Further research into cell cycle regulation, cell life, and cell death is essential to overcome this major obstacle in cancer treatment. Understanding these diverse mechanisms is vital for developing strategies to circumvent drug resistance and improve cancer therapy outcomes.