Original ArticlesOral recombinant methioninase (o-rMETase) is superior to injectable rMETase and overcomes acquired gemcitabine resistance in pancreatic cancer
Introduction
The elevated methionine (MET) requirement of cancer cells to proliferate is termed MET dependence and may be the only known general metabolic defect in cancer [1,2]. Cancer cells utilize excess methionine for aberrant transmethylation reactions [3], a metabolic reprogramming in cancer known as the “Hoffman Effect” and analogous to the Warburg effect of over-utilization of glucose in cancer [2]. In head-to-head comparisons in PET imaging, [11C] MET-PET shows a more stronger signal than [18F] fluorodeoxyglucose [4], suggesting that the Hoffman effect is more extensive in cancer than the Warburg effect. Targeting MET by recombinant methioninase (rMETase) can arrest the growth of cancer cells in vitro and in vivo [[1], [2], [3], [4]].
METase has been used as a therapeutic strategy for multiple type of cancers [[4], [5], [6], [7], [8], [9], [10], [11]]. Song et al. [12] reported that MET starvation therapy using a MET-free diet or total parenteral nutrition (TPN) extends the survival time of high-stage gastric carcinoma patients.
Previously rMETase was administrated by intra-peritoneal injection (ip-rMETase) and was effective against patient-derived orthotopic xenograft (PDOX) models of recalcitrant cancer [[13], [14], [15], [16], [17]]. Recently, we compared ip-rMETase and oral rMETase (o-rMETase) for efficacy on the melanoma PDOX [13]. o-rMETase was significantly more effective than ip-rMETase [13], indicating the potential widespread use of rMETase for acute and chronic cancer treatment.
Pancreatic cancer is the fourth leading causes of cancer-related deaths in the Western world because of low-response to radio- and chemo-therapies. The American Cancer Society estimates that in 2018, about 55,440 new cased will be diagnosed with pancreatic cancer and about 44,330 people will die of this disease in the USA alone [18]. Despite extensive research to improve detection techniques, surgical methods and developing better chemotherapy drugs, the prognosis of pancreatic cancer patients remains poor.
Gemcitabine (GEM) is the standard first line treatment for patients with advanced pancreatic cancer; however, it is ineffective because of acquired resistance [19,20]. Acquired GEM resistance is a major factor in the recalcitrance of pancreatic cancer. Various genetic and/or epigenetic changes have been proposed that causes GEM resistance [[20], [21], [22], [23], [24], [25]]; but how to overcome this resistance remains unclear.
We recently reported that intra-peritoneal rMETase (i.p.-rMETase) combined with GEM could regress a partially GEM-resistant pancreatic cancer PDOX [26]. In the present study, we compared the efficacy of o-rMETase and i.p.-rMETase in a pancreatic-cancer PDOX. Acquired GEM resistance is a major factor in the recalcitrance of pancreatic cancer. In addition, we describe a human pancreatic cancer cell line which acquired greater than 100-fold GEM resistance. We evaluated the efficacy of o-rMETase and o-rMETase in combination with GEM against the GEM-resistant human PK-9R pancreatic cancer cell line in vitro and orthotopic mouse models compared to its parental GEM-sensitive line PK-9. We found that o-rMETase can overcome GEM resistance.
Section snippets
Mice
Athymic nu/nu nude mice (AntiCancer Inc., San Diego, CA), 4–6 weeks old, were used in this study. All experimental protocols and data collection were as previously described [[13], [14], [15], [16], [17]]. All mouse surgical procedures and imaging were performed with the animals anesthetized by subcutaneous injection of a ketamine mixture were as previously described [[13], [14], [15], [16], [17]]. The response of animals during surgery was monitored to ensure adequate depth of anesthesia. The
Efficacy of rMETase on a PDOX model of pancreatic cancer
To test which rMETase (o-rMETase or i.p.-rMETase) can effectively inhibit a PDOX model of pancreatic cancer, we treated mice with i.p.-rMETase, or o-rMETase alone or the combinations of o-rMETase + i.p.-rMETase twice a day for 14 consecutive days and compared them with untreated control group. All treatments significantly inhibited tumor growth compared to the untreated control (i.p.-rMETase: p < 0.0001; o-rMETase: p < 0.0001; o-rMETase + i.p.-rMETase: p < 0.0001); on day 14 after initiation of
Discussion
In the present study using a PDOX model, we show that o-rMETase is more effective than injectable rMETase for pancreatic cancer. In addition, we show that o-rMETase can overcome acquired GEM resistance in pancreatic cancer and demonstrates the clinical potential of this strategy. We have previously shown the effect of injectable rMETase on tumor histology [[14], [15], [16]]. Detailed histological effects of o-rMETase will be part of future studies.
We have developed PDOX mouse models of cancer
Conflicts of interest
The authors declare that they have no competing interests.
Acknowledgements
This paper is dedicated to the memory of A. R. Moossa, M.D., Sun Lee, M.D. and Shigeo Yagi, Ph.D.
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