Hi Blue-eyed Smiler,
Blue-eyed Smiler“It is a treatment that slows cell destruction," he explained, "but as Jenny knows, there can be unpleasant side effects." ”
Your comma after "explained" is right, and one belongs after "knows."
There's no reason to put one after "treatment" because what follows is essential. When you set things off with a comma before and after, you are saying that it's non-essential information that simply provides more information.