From Bloomberg Law:

“The government says even if it was obvious to try out using the particular drug cocktail for pre-exposure prophylactic treatment, there’s nothing that says what the effective dosage would be,” said Charles Duan, a patent lawyer and director of technology and innovation at Washington think tank R Street Institute.

“They make the argument that only an expert could determine that dosage,” he added. “That, to me, stuck out as something that a drug company could latch onto later on down the road.”

Duan noted that, “there’s nothing inherently wrong with a patent on method of use,” however when used as they often are—to extend a monopoly on a drug that would otherwise see its patent expire—it can lead to higher costs for patients.