Cold in the Shadows by Toni Anderson

CIA Agent Patrick Killion is on a secret mission to hunt down the ruthless female assassin hired to kill the Vice President of the United States. The trail leads him to the Colombian rainforest and an earnest biologist, Audrey Lockhart, whose work on poison dart frogs gives her access to one of the deadliest substances on earth—the same substance used to murder the VP.

When Audrey is attacked by the local drug cartel, Killion steps in and hustles her out of harm’s way, determined to find out what she knows. His interrogation skills falter somewhere between saving her life and nursing her back to health as he realizes she’s innocent, and he ends up falling for her. Audrey has a hard time overlooking the fact that Killion kidnapped her, but if she wants to get her life back and track down the bad guys, she has to trust him. Then someone changes the rules of their cat and mouse game and now they’re the ones being hunted—by a cold-blooded killer who is much closer than they think.