Sensitization of pancreatic cancer cells to photons and protons via enzymotherapeutic metabolic targeting of arginine

Background and Aim:

The abnormal metabolism of pancreatic ductal adenocarcinoma (PDAC) cells offers some interesting points of attack. About 20-30% of PDACs are described to be auxotrophic for arginine because of a reduced or complete loss of anabolic protein expression and/or activity. New strategies for the treatment of this deadly disease should consider the combination of radiotherapy with novel metabolic targeting modalities. We found various arginine deprivation therapy (ADT) approaches to radiosensitize several non-auxotrophic cancer cell types. Hence, we hypothesized that enzymotherapeutic ADT should be particularly efficient in radiosensitizing auxotrophic PDACs to both photons and protons.

Materials and Methods:

Human and murine cell lines reflecting auxotrophic PDACs in patients were studied. ADT was achieved with 2 U/ml of recombinant human arginase in the presence and absence of citrulline. Single dose irradiation of 0-30 Gy was applied with a 200kV X-ray tube and a 150 MeV double-scattered proton beam. Treatment response was assessed by 2-D clonogenic survival and/or 3-D spheroid control probability (SCP) assays. Our specifically designed setup allowed the testing of innovative treatment regimens including proton therapy with/without ADT and Gemcitabine (Gem).

Results and Conclusions:

ADT led to a massive sensitization to X-ray in all PDAC models. Sensitization in 3-D culture was reflected by a significantly reduced SCP and spheroid control dose 50% (SCD50) with an RBE of 2.0. Cells showed a higher sensitivity in the 3-D assay to protons than photons but ADT still further improved the radioresponse. As an example, the RBE of protons plus ADT versus photons alone in one human PDAC spheroid typel was 3.9. In the same model, proton therapy outcome was hardly affected by Gem; however, the combination of ADT with Gem and proton therapy was particularly effective with an exceptional RBE compared to photons of 5.0. Extended mechanistic studies are ongoing; transfer in vivo is envisioned.