Purpose/Background: We and others have shown that melanoma cells as well as melanoma tumor tissue do not express argininosuccinate synthetase (ASS), which is essential in the synthesis of citrulline to arginine. Hence, cells that lack ASS will need exogenous arginine to survive. Exposure of these cells to arginine deiminase, which degrades arginine to citrulline and ammonia, results in cell death, whereas normal cells, which possess ASS, are able to survive. This novel concept of arginine depletion using pegylated arginine deiminase (ADI-PEG20) was tested extensively in vitro and in vivo and is currently in phase 2 clinical trials. Encouraging responses with minimal toxicity were seen. The caveat that we found in clinical trial is that tumor cells can undergo autophagy and subsequently turn on the ASS gene, which results in drug resistance.
Data Analysis/Results: We have combined ADI-PEG20 with a DNA damaging agent, cisplatin. We have shown that the combination of the 2 drugs together significantly improved the therapeutic efficacy compared with ADI-PEG20 alone or cisplatin alone in 4 melanoma cell lines, regardless of their BRAF mutation. Apoptosis/cell death as assayed by Annex-inV/PI as well as cleaved caspase was increased by 20% to 30% (P < .05). In vivo study also exhibited significant decrease in tumor growth for combination treatment (P < .001) with no added toxicity to either agent alone. The underlying mechanism is complex, but increased DNA damage upon arginine deprivation due to decreased DNA repair proteins FANCD2, ATM, and CHK1/2 most likely leads to increased apoptosis. This action is further intensified by increased pro-apoptotic protein, NOXA, and decreased anti-apoptotic proteins, SURVIVIN, BCL2, and XIAP. The autophagic process, which protects cells from apoptosis upon ADI-PEG20 treatment, also dampens upon cisplatin administration. Thus, the combination of arginine deprivation and cisplatin function in concert to kill tumor cells that do not express ASS without added toxicity to normal cells.
Implications: This notion has been translated into clinical trial using combination of ADI-PEG20 +cisplatin all ASS negative tumor including lung cancer, hepatoma, and cholangiocarcinoma. This research was supported by NIH/NCI 1R01CA109578 and Department of Veterans Affairs CDA2 Award 1K2BX001289.