PG67A/W is a special, Progenitor-based serum used to keep the virus within Albert Wesker's body stable and stop him from losing his strength. The "PG" may be an acronym for "Progenitor".
It was administered to him regularly by Excella Gionne. The dosage given must be extremely precise and taken at certain times, otherwise it can act as a poison.
During Chapter 6-3, Jill instructs Chris and Sheva (in a cutscene) to inject the serum into Albert Wesker so that it should act "like a poison". During the second fight with Wesker in the Hangar, the player's mission is to inject the serum on him, causing an overdose.
The "A/W" itself may suggest that it was specifically developed for Wesker.
It is unknown if Alex Wesker had a similar serum or used one at all, though It's unlikely as she did not gain any powers from the Prototype Virus.