In practical terms, sure, that actually happens a lot in other countries. In legal terms, i don't know if it can be done in the US right away or if they have to wait a certain amount of time or jump through god knows how many patent law hoops to not violate the borderline monopoly rights that a lot of these big companies manage to buy for themselves. The US pharma industry is notorious for protecting big corporations from competition by paying lobbyists to get laws that make it very difficult to sell off-brand alternatives.