P-glycoproteins (Pgps) belong to the family of ATP binding cassette (ABC) transporter proteins. In humans two Pgp genes have been identified; mdr-1 and mdr-3. Classical Multiple Drug Resistance (MDR) is associated with over expression of the mdr-1 gene product, P-170. No role for mdr-3 in MDR has yet been proven. However there is evidence that mdr-3 overexpression may be associated with drug resistance in certain B-cell lymphocytic leukaemias. In an immunocytochemical study we have looked at a selection of B-cell leukaemias for mdr-1 and mdr-3 encoded Pgp expression using monoclonal antibodies specific for the mdr-1 and mdr-3 encoded gene products. In B-CLL patients a differential pattern of MDR-3 positive staining was observed; suggesting that MDR-3 positivity may be associated with a more malignant phenotype in B-CLL. This pattern was not observed with MDR-1 positivity. We also observed MDR-3 positivity in an AML stage M5a patient which is the first report of MDR-3 Pgp expression being detected in AML; suggesting that MDR-3 Pgp expression may be limited to particular subtypes of this disease. Results from B-NHL cases were inconclusive with varying expression of MDR-1 and MDR-3 Pgps observed. Work is currently underway to further explain the significance of these findings.
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