The FDA has to approve food additives and drugs used for farm animals that get into our food supply. The active ingredient in ProHart6, moxidectin, was already approved for use in beef and dairy cattle to kill intestinal roundworms. Heartworm is a type of roundworm. Your pet's heartworm meds will also kill intestinal roundworms.
Once the initial research and regulatory work to approve a drug has already been done for farm animals, then it's almost a freebie for the FDA to regulate it for pets. Only in this case, someone screwed up on testing it on a different species.
Also, some breeds of dogs are sensitive to certain heartworm meds. For example, collies, Shetland sheepdogs, and similar breeds are sensitive to a heartworm med called ivermectin. Within a breed, some individual dogs are more sensitive than others.
I don't know if the moxidectin problem is limited to certain breeds.
I have Ibizan Hounds and like all sighthounds, they're sensitive to many things that don't bother other dogs.
For instance, any flea spray or collar is a definite -no-.
Luckily, they are so fastidious that I've never seen a flea on any of them, even after 15 years of owning the breed.