Acetaminophen absorption kinetics in altered gastric emptying: establishing a relevant pharmacokinetic surrogate using published data.

Acetaminophen has been used as a tool for clinical and nonclinical experimental designs that evaluate gastric emptying because acetaminophen is not absorbed in stomach but efficiently absorbed from the small intestine. Published pharmacokinetic data of acetaminophen in subjects with normal gastric emptying vs. impaired gastric emptying (i.e., morphine treatment) were evaluated to select a key surrogate. Using Caverage (average concentration), computed from the exposure within the first hour, individual rank distribution was plotted across different studies. Caverage was highly correlated with Cmax (maximum concentration) in subjects with normal gastric emptying (R(2) = .7532) but not in those where gastric emptying was impaired (R(2) = .0213). The 50th percentile value of the distribution pattern of 1/Caverage in acetaminophen+morphine-treated group (coincided with the first shift in the slope) was considered as the cutoff point to figure out the impaired gastric emptying. The individual rank distribution plots for 1/Caverage across different studies supported similar trends in subjects with normal gastric emptying but showed a distinct distribution pattern in the cohort of impaired gastric emptying. Caverage, calculated within the first hour of dosing of acetaminophen (average concentration at 0-1 hour, C0-1havg), can be used as a key surrogate to distinguish the effects of gastric emptying on the absorption of acetaminophen. A 4 μg/mL C0-1havg of acetaminophen (dose: 1.5 g) may be used as cutoff point in future clinical investigations of acetaminophen to clarify the role of gastric emptying.