The results obtained in this study showed no significant correlation between the protein or mRNA expression levels of RRM1 and ERCC1 - detected by IHC or PCR - and OS or DFS in patients with resected PDA. Our data raise some questions regarding the real clinical and practical significance of analyzing these molecules as predictors of outcomes. Low levels of RRM1 did not predict better outcomes which, since the majority of our patients received gemcitabine based chemotherapy regimens, would have indirectly represented more tumor sensitivity to this agent.
The role of ERCC1 in PDA is less clear and since practically no patient received platinum analogue agents in this cohort, we cannot deduce the importance of ERCC1 as a predictor of oxaliplatin-based regimens as previously suggested [24
]. Yet it seems to be clear that ERCC1 expression levels do not have any prognostic value in this patient cohort who did not receive platinum based chemotherapy. Future investigations should explore the significance of ERCC1 in patients receiving platinum based regimens.
Our results are discordant with what was previously reported by Akita and colleagues who found borderline better outcomes in patients with high expression levels of RRM1 and ERCC1 [20
]. Nonetheless, in that study only a minority of patients received adjuvant chemotherapy (5/68) and in total only 40% received gemcitabine during the course of their treatment. Our population is unique and quite different to this previous work since the majority of our patients (87%) received gemcitabine in the adjuvant setting. Our study is more in accordance and relevant to current practices in the U.S. (i.e., standard of care therapy); hence our conclusions are directly applicable to the majority of patients who seek medical attention after a resection of a PDA. However the median OS in our cohort (18 months) was slightly inferior to that previously reported in randomized clinical trials (i.e.: in Conko-001, 22 months) [26
]; this could be attributed to the more heterogeneous population of our study in comparison to one that could be observed in prospective and controlled clinical trials.
There are some studies that support the role of RRM1 over-expression as a source of resistance to gemcitabine in PDA [27
]. One study showed that in gemcitabine resistant cell lines, sensitivity to this agent could be rescued by silencing the RRM1 expression (> 90%) with iRNA [10
]. A similar correlation between RRM1 expression and gemcitabine efficacy was observed in the clinical setting but only in NSCLC [16
]. However, during the last couple of years many other genes have been described as potential source of gemcitabine resistance in pancreatic tumoral cells, including human equilibrative nucleoside transporter-1 (hENT1) [29
], deficiency in deoxycytidine kinase (dCK) [31
], over-expression of RRM2 [32
] and HuR [6
]. The roles played by each particular gene in addition to the possible still undiscovered genes and pathways in the gemcitabine metabolism process, coupled with the complex in-vivo environment, are still under investigation [34
]. However, some studies have already shown that the combination of detecting the expression of a select set of genes rather than the particular expression of one gene may play a more relevant, realistic clinical role [35
]. Moreover, our own group has previously shown a notoriously more predictive value of HuR for gemcitabine sensitivity, which is most likely due to its ability to affect a myriad of downstream target genes [33
]. Additionally, a recent report found also no prognosis value of RRM1 in PDA [37
Taking all this evidence together, in conjunction with the results of the present study, we believe that the isolated detection of RRM1 or ERCC1 expression in PDA has little clinical relevance and deserves further investigation before formal recommendation regarding its use in clinical practice can be made. In addition, our study also serves as evidence that the information gathered from other tumor types (i.e.: lung cancer) should not always be directly applied to other cancers (especially to PDA where the aggressive biology appears to be unique) without the appropriate confirmation of these assumptions. In the particular cases of NSCLC, this discordance could also be partially explained by the fact that the overall response rate of gemcitabine in PDA is 5-7% [4
] in comparison to the 20% found in NSCLC [38
], meaning that lung cancer is intrinsically more sensitive to gemcitabine and levels of RRM1 may play a more relevant role.
Lastly, we should also take into consideration the fact that our study has some limitations which are mainly derived from its retrospective nature. We were not able to obtain reliable information to calculate DFS in one third of the cases since some patients were lost in follow up or the information available was inaccurate. As expected, this raises some concerns regarding selection bias in our cohort and conclusions regarding this matter should be analyzed cautiously and confirmed in future studies. However, we were able to estimate OS in all cases and, as mentioned before, given the very modest effect of second and third line chemotherapy regimens in PDA it is very unlikely that gross and undetected misbalance of these variables would have changed the results considerably. Additionally, we cannot directly translate our results to the metastatic setting since only 2 patients debuted with a stage IV disease. Other sources of variability could be related to the cut off values used for PCR detection or to the fact that only half of the patients studied had reliable information in regards to the gene expression, thus underscoring the difficulty of realistically utilizing this assay for clinical purposes. Indeed, when analyzing only those patients who received a complete regimen of chemotherapy, high RRM1 gene expression (PCR) showed a trend toward significance (P = 0.07) but the number of patients investigated was too small (N = 7) and results could be due to random sampling variability. We tried to correlate the gene and protein expression levels and we found a borderline significant correlation, but not a direct one. We cannot then discard the possibility that newer and more reliable techniques of gene expression detection could produce different results. Nonetheless, the current available evidence argues against this possibility.