Red Dragon’s Heart by Anastasia Wilde

He’s been shot with an anti-shifter serum that turns him into a homicidal monster.
She’s the one who made it…

Zakerek’s entire life was ripped away in a single moment. While he was on a mission to rescue captive shifters from a lab, one of the scientists shot him with a bolt carrying an experimental shifter toxin. Now there’s an uncontrollable beast inside him, warring with his dragon—one that’s triggered by the presence of other shifters.
Alone on earth, hiding from humans and shifters alike, his one mission is to find Madeleine Fox, the biochemist who created the toxin. His plan is simple: hold her captive until she reverses what she did to him, and then flame her to ashes and devour her bones.

Maddie Fox’s life work has been to isolate the “virus” that turns people into animals, and eradicate it—but not before she figures out how to harness the creatures’ accelerated healing to save her brother’s life.

When she found out her mentor, Eric Chisholm, was planning to sell their research for his own sinister purposes—and that he himself was a shifter—she stole the data and disappeared. But just when she’s about to get the proof she needs to turn him in, the impossible happens: she’s abducted by a fire-breathing dragon who demands that she “fix” him.
But when Zakerek’s serum-induced delirium ignites a night of passion with Maddie, everything they both thought was true begins to crumble around them—and so do the walls around their emotions. Can two sworn enemies change their minds and open their hearts, before Chisholm destroys them—and everyone they care about?