Recombinant arginase as a novel anti-melanoma agent

Abstract

12032 Background: BCT-100 is a recombinant arginase comprised of 329 amino acid residues. Arginase converts arginine to urea and ornithine. Previous studies suggested that melanoma cells were auxotrophic for arginine due to absence of argininosuccinate synthetase (ASS) expression. Thus, we hypothesized that recombinant arginase, BCT-100, is cytotoxic to human melanoma cells and its cytotoxicity correlates with absence of ASS expression. Methods: BCT-100 pegylated recombinant human arginase was manufactured by large scale fermentation of a recombinant B. subtilis strain LLC101 encoded with a human arginase gene. Following fermentation, the recombinant protein was extracted, purified, pegylated, and ultra-dialyzed. Ten established human melanoma cell lines were used. Cells were grown to 90% confluence, harvested, and plated at 104 cells per well in a 96-well plate and co-cultured with increasing concentrations of pegylated BCT-100 for 72 hours. CellTiter 96 Aqueous Non-radioactive Cell Proliferation Assay (Promega, Madison, WI) was used to measure percent viability, with absorbances measured at 490 nm. Total cellular RNA was isolated from established melanoma cell lines converted to cDNA at a concentration of 5 ng/ul. Quantitative real time polymerase chain reaction was performed on a 7300 Real Time PCR System, using Gene Expression Assays for ASS and GAPDH (Applied Biosystems). 10,000 fold standard curves were generated for all samples using GAPDH expression. Results: All ten cell lines demonstrated decreased viability as concentrations of BCT-100 increased. Average IC50 value was 0.11 IU/ml. Eight of the 10 cells lines have IC50 values < 0.1 IU/ml. Of the 8 cell lines with IC50< 0.1 IU/ml, all of them have low or undetectable ASS expression using quantitative RT-PCR. Of the 2cell lines with IC50 > 0.1 IU/ml, ASS expression was detected in 1 of 2. Conclusions: Arginine depletion with recombinant arginase, BCT-100, was cytotoxic to melanoma cells in vitro. The cytotoxic effect of BCT-100 on melanoma cells correlated with expression of argininosuccinate synthetase. BCT-100 is a promising novel agent for treatment of melanoma. Further in vivo experiment with BCT-100 is ongoing. [Table: see text]