Thoros-B

Thoros-B is a biotoxin which was being developed by Dr. Zev Cohen on Noveria. He claims it was based on "an exotic lifeform discovered on the frontier". Binary Helix wanted something that could kill the creature, but there was no profit in a bioweapon that only affected one rare species, and Cohen was ordered to develop it to affect more species. An accident during quarantine infected several members of his team with the toxin; Cohen asked Commander Shepard to fetch a cure from their lab.

According to rumour the name of the biotoxin hints at the lifeform it was intended to kill; 'Thoros-B' might therefore have been developed to kill the Thorian. Given Saren Arterius' determination to wipe the Thorian out once he gained the Cipher, he probably brought samples to Noveria and paid Binary Helix to develop the bioweapon (not knowing they would then see profit in it for themselves).