LEON LG, Funel N, Peters GJ, Avan A, Vistoli F, Boggi U, GIOVANNETTI E.
Clin Cancer Res 2016 [Epub ahead of print]
We have read with interest the findings reported by Vena and colleagues who explored the effect of gemcitabine in combination with MEK1/2 Inhibitor, Pimasertib, in pancreatic ductal adenocarcinoma (PDAC) cells. They highlighted the important role of ribonucleotide reductase subunit 1 (RRM1) in gemcitabine response and indicated MEK as a potential therapeutic target to sensitize gemcitabine therapy (1). Clinical research in the field of PDAC is of paramount importance since it remains as a major unsolved health problem. Thus identification of molecular markers useful for therapeutic-intervention that might predict the outcome of patients and/or perhaps response to therapy is warranted. Therefore, we appreciate the findings by Vena et al (1), nevertheless in our opinion, some aspects of the study need to be discussed in more detail.
In our previous study we investigated the expression pattern of seven genes involved in gemcitabine activity in 105 PDAC patients and its correlation with treatment outcome (2). Our findings demonstrated the lack of the association of RRM1 expression with clinical outcome. In addition patients who had higher levels of human equilibrative nucleoside transporter-1 (hENT1) had significantly longer overall-survival (2). In agreement with these observations, several studies have shown that PDAC patients with high hENT1 expression benefit from gemcitabine-based adjuvant chemotherapy (3). However, a recent trial indicates a lack of correlation with hENT1 expression in the prospective multicenter NCT01124786 trial.
Next the finding of the study (1) regarding the combination indices (CI) for evaluation of the pharmacological interaction of gemcitabine and pimasertib is debated: Sequential drug treatment consisting of a 4-hour pre-exposure to 500 nmol/L pimasertib followed by a 48-hour treatment with gemcitabine increased the antiproliferative effect of gemcitabine (CI<0.7), indicating strongly synergism, while simultaneous administration of gemcitabine with pimasertib were additive (CI>1.1) in PDAC cells, thus the real molecular mechanism underlying of the controversy effect of this combination is unknown, supporting further investigations. Moreover, the authors (1) have normalized the expression of RRM1 by using a housekeeping-gene (HKG), GAPDH, in RT-PCR. It has been reported that HKG expression – despite being occasionally constant in a cell-type or experimental-condition – can considerably vary (4). Therefore an accurate-normalization of the data based on the geometric means of multiple internal control genes and determination of the gene-stability are required to avoid single control normalization error, and to identify proper tissue-specific HKG (4).
Lastly, the authors (1) tested the effect of MEK1/2 inhibitor Pimasertib in PDAC cell lines, PANC-1 and BxPC-3. It would be worthwhile to perform the in vitro and in vivo experiments in primary cell cultures or freshly generated xenograft from tumor tissues and patient-derived orthotopic mouse models; since these models have been illustrated to better resemble the genetic characteristics of disease and predict drug activity (5-6).
We thank Vena and colleagues their worthwhile report, but we believe that additional analysis with the incorporation of more refined methodological approaches and models are needed to determine the value of RRM1 in patients and MEK inhibition on gemcitabine sensitivity as a clinical strategy in the treatment of PDAC.
References: 1. Vena F, Li Causi E, Rodriguez-Justo M, Goodstal S, Hagemann T, Hartley JA, et al. The MEK1/2 Inhibitor Pimasertib Enhances Gemcitabine Efficacy in Pancreatic Cancer Models by Altering Ribonucleotide Reductase Subunit-1 (RRM1). Clin Cancer Res. 2015;21:5563-77.; 2. Giovannetti E, Del Tacca M, Mey V, Funel N, Nannizzi S, Ricci S, et al. Transcription analysis of human equilibrative nucleoside transporter-1 predicts survival in pancreas cancer patients treated with gemcitabine. Cancer Res. 2006;66:3928-3935; 3. Farrell JJ, Elsaleh H, Garcia M, Lai R, Ammar A, Regine WF, et al. Human equilibrative nucleoside transporter 1 levels predict response to gemcitabine in patients with pancreatic cancer. Gastroenterology. 2009;136:187-195; 4. Vandesompele J, De K, Pattyn F, Poppe B, Van Roy N, De A, et al. Accurate normalization of real-time quantitative RT-PCR data by geometric averaging of multiple internal control genes. Genome Biol. 2002; 3:34; 5 Voskoglou-Nomikos T, Pater JL, Seymour L. Clinical predictive value of the in vitro cell line, human xenograft, and mouse allograft preclinical cancer models. Clin Cancer Res 2003;9:4227–39; 6. Avan A, Caretti V, Funel N, Galvani E, Maftouh M, Honeywell RJ, et al. Crizotinib inhibits metabolic inactivation of gemcitabine in c-Met-driven pancreatic carcinoma. Cancer Res 2013;73:6745-56.