An outbreak occurs in a community where the water supply is contaminated. Multiple patients experience nausea and vomiting as well as profuse diarrhea with abdominal cramps; stools are described as "rice water." Curved, gram-negative rods are isolated on a sulfate-citrate-bile- sucrose agar. In the treatment of patients who have cholera, the use of a drug that inhibits adenyl cyclase would be expected to have which of the following characteristics?