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Expression of the multidrug resistance-associated protein gene in refractory lymphoma: quantitation by a validated
Z Zhan1, V A Sandor, E Gamelin
1National Institutes of Health, National Cancer Institute, Medicine Branch, Bethesda, MD 20892, USA.
Abstract:
Previous work investigating the role of MDR-1 overexpression in relapsed and refractory lymphoma led us to investigate a possible role for multidrug resistance-associated protein (MRP) as a cause of resistance in patients who did not overexpress MDR-1. A quantitative polymerase chain reaction (PCR) method for measuring MRP expression was validated. Immunoblot analysis suggested that no major discrepancy was present between mRNA expression and protein levels. MRP levels were found to be independent of sample tumor content by immunophenotyping, suggesting that the presence of normal cells had no significant impact on measurements of MRP expression. We evaluated MRP in 55 biopsy samples from 40 patients with refractory lymphoma enrolled on a trial of infusional chemotherapy (EPOCH). Pre- and post-EPOCH samples were available from 15 patients. MRP levels were also evaluated in 16 newly diagnosed, untreated lymphoma patient samples. No significant difference in MRP mRNA expression was noted between pre- and post-EPOCH groups. Also, MRP levels in the newly diagnosed patient samples were not significantly different from either pre- or post-EPOCH groups. Two of 15 paired pre- and post-EPOCH patient samples exhibited overexpression of MRP after EPOCH chemotherapy, with measured increases of 10-fold and 18-fold. We conclude that MRP overexpression is not responsible for non-P-glycoprotein (Pgp)-mediated drug resistance in the majority of these patients, although it may be important in a subset of patients. Defining this subset prospectively could aid in the development of clinical trials of MRP modulation in drug-resistant lymphoma.
Insights
Multidrug resistance-associated protein (MRP) was investigated as a cause of chemotherapy resistance in lymphoma patients. MRP overexpression was not found to be a major factor in most cases, but may be relevant in a subset of patients.
Area of Science:
- Oncology
- Pharmacology
- Molecular Biology
Background:
- Multidrug resistance (MDR) is a significant challenge in treating relapsed and refractory lymphoma.
- While MDR-1 (P-glycoprotein) overexpression is a known resistance mechanism, other factors like multidrug resistance-associated protein (MRP) may contribute in non-MDR-1 expressing patients.
Purpose of the Study:
- To investigate the role of multidrug resistance-associated protein (MRP) in mediating chemotherapy resistance in patients with refractory lymphoma.
- To quantify MRP expression levels in lymphoma patient samples before and after treatment.
Main Methods:
- Validated a quantitative polymerase chain reaction (PCR) method for measuring MRP mRNA expression.
- Utilized immunoblot analysis to correlate mRNA and protein levels.
- Evaluated MRP expression in 55 biopsy samples from 40 refractory lymphoma patients undergoing infusional EPOCH chemotherapy, including paired pre- and post-treatment samples.
Main Results:
- No significant difference in MRP mRNA expression was observed between pre- and post-EPOCH treatment groups or compared to newly diagnosed patients.
- Two out of 15 paired samples showed significant MRP overexpression (10-fold and 18-fold increases) after EPOCH chemotherapy.
- MRP levels were independent of sample tumor content, indicating reliable measurements.
Conclusions:
- MRP overexpression is not the primary cause of non-P-glycoprotein-mediated drug resistance in the majority of refractory lymphoma patients.
- MRP may play a role in drug resistance in a specific subset of patients.
- Prospective identification of this subset could inform clinical trials targeting MRP modulation in drug-resistant lymphoma.
