NO EVIDENCE OF SIGNIFICANT ACTIVITY OF THE MULTIDRUG-RESISTANCE GENE-PRODUCT IN PRIMARY HUMAN BREAST-CANCER

Background: The discovery of the multidrug resistance (MDR1) gene prod uct P-glycoprotein (P-gp) has been widely seen as an important milesto ne in our understanding of the mechanisms underlying the clinical phen omenon of the emergence of resistant cells. MDR1 expression has been s hown for numerous solid tumors and for virtually all hematologic malig nancies. Nevertheless, results regarding MDR1/P-gp expression in human breast cancer have been controversial and the results of clinical tri als on modulation of P-gp activity have not been encouraging. Patients and methods. MDR1/P-gp expression and the function of the P-gp pump w ere investigated in 61 tumor samples from patients with primary breast cancers by multiparameter analysis using MDR1-RT-PCR, immunohistochem istry with two MAbs (UIC2 and MRK 16) and the rhodamine 123 (Rh123) ef flux assay. The cellular composition of the tumor cell suspension was analyzed by using specific MAbs against the P-gp expressing lymphocyte subsets CD4, CD8 and CD56, as well as against the HER-2/neu gene prod uct, which was used to identify breast carcinoma cells. Results. UIC2 and MRK16 revealed a staining positivity in 72% and 75% of samples, re spectively. A positive MDR1-RT-PCR signal was detected in 62% of the s amples. Nevertheless, no correlation between immunohistochemistry and RT-PCR could be established. Furthermore, there was no correlation bet ween HER-2/neu expression and MDR1-RT-PCR or P-gp immunohistochemical assays. A contamination by CD8+ and CD4+ lymphocytes was established i n 100% and 84% of tumor cell suspensions, respectively. As assessed by the Rh123 efflux assay CD8+ and the CD4+ lymphocytes exhibited marked P-glycoprotein activity, whereas such activity was not detectable in a single instance for the breast carcinoma cells. In MDR1-RT-PCR posit ive samples, contamination by CD8 lymphocytes averaged 4.3%, while the contamination of CD8 cells in the MDR1 mRNA-negative samples was only 2.4% (P = 0.007). This signal vanished after elimination of the lymph ocyte subpopulations by T-cell rosetting. Conclusions. In primary brea st cancer detection of MDR1 gene expression by means of RT-PCR or immu nohistochemical assays is not indicative for the MDR phenotype, since there is no evidence of significant activity of the P-gp pump.