Arduino controlling high voltage DC source, Opto-Isolator
If response times in the ms are acceptable, you could use an optically coupled pair of back-to-back MOSFETs such as the CPC1393GRTR.



This particular one can switch 600V at up to 90mA, with maximum on/off times of 5ms and maximum 'off' leakage of 1uA. It would typically drop less than 100mV at a few mA. 

Isolation voltage input-to-output is 5000V claimed, you'll have to evaluate if that's safe enough for your application.