Abstract P4-09-27: ABC transporter expression: Clues into chemoresistance of triple negative breast cancers

Background: Triple-negative breast cancer (TNBC) is an aggressive histological subtype with high rates of recurrence and metastatic disease. Intrinsic or acquired multidrug resistance facilitated by the over-expression of drug efflux pumps (ABC transporters: BCRP [ABCG2], MRP1 [ABCC1] and PGP [ABCB1]), may contribute to the aggressive nature of this disease. We examined the expression patterns of drug efflux pumps for insight on their potential role in chemoresistance of TNBC. Methods: 1393 TNBC patients molecularly profiled with a commercial assay (Caris Life Sciences) were evaluated retrospectively for expression of BCRP, MRP1 and PGP by immunohistochemistry. Antibodies used: PGP (C494), BCRP (6D171) and MRP1 (33A6). IHC threshold: positive = ≥1+ and ≥10%). This data set also included metachronous paired samples from 71 TNBC patients. JMP was used to ascertain distributional differences. Results: PGP and MRP1 positive expression rates were 8.4% (117/1393) compared to 78.2% (248/317), respectively, displaying an inverse association (p=0.0001). BCRP was over-expressed in 52% (90/173). Co-expression data for all three transporters was available for 153 patients. BCRP/MRP1 co-expression was most abundant, 39% (59/153), followed by MRP1/PGP at 4% (6/153) and BCRP/PGP at 2% (3/153). Furthermore, 12% (18/153) of TNBC exhibited positive expression for all three drug pumps (16/18 or 89% were from patients with metastatic disease) and 14% (22/153) exhibited negative expression for all three drug pumps (15/22 or 68% were from patients with metastatic disease). Interestingly, positive MRP1 status, but not BCRP or PGP, correlated with metastatic disease (p=0.0061). In the paired data analysis, PGP expression was absent in 77% (55/71) and retained lack of expression in subsequently profiled specimens, 3% (2/71) retained positive PGP status, 13% (9/71) lost expression of PGP and 7% (5/71) gained expression of PGP. 36/40 patients that lacked PGP expression and 8/9 patients that "lost" PGP expression, exhibited and retained expression of MRP1 in the initial and subsequently-profiled specimens. Conclusion: Biomarker expression patterns of drug efflux pumps may provide insight to the chemoresistance phenotype observed in TNBC. Expression of MRP1 is favored in TNBC and is correlated with metastatic disease status. Although PGP expression may be absent or lost during TNBC progression, MRP1 (and BCRP) expression are almost always retained. Utilization of chemotherapies that are substrates of PGP, but not MRP1, (based on expression status) may be worthy of future investigation. Citation Format: Feldman R, Abbott B, Reddy S, Gatalica Z, Castro M. ABC transporter expression: Clues into chemoresistance of triple negative breast cancers. [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr P4-09-27.