Arsenic is only toxic to our understanding of biology. Some extremophiles have unbelievable resistance to heavy metals, so I wouldn't doubt some configuration of our biology would be resistant to arsenic.

However, with the problems arsenic causes blocking PDH enzyme, causing H2O2 in the cells and a huge list else like IIRC thiamine deficiency.

Then again, life exists in such a broad range of conditions that I find it hard to believe that life couldn't adapt to life with arsenic for one main reason. If it is using arsenic as a DNA base, it means it is sequestering the arsenic rather than leaving it in the cells to do damage.