Use of microdosing and accelerator mass spectrometry to evaluate the pharmacokinetic linearity of a novel tricyclic GyrB/ParE inhibitor in rats.

Determining the pharmacokinetics (PKs) of drug candidates is essential for understanding their biological fate. The ability to obtain human PK information early in the drug development process can help determine if future development is warranted. Microdosing was developed to assess human PKs, at ultra-low doses, early in the drug development process. Microdosing has also been used in animals to confirm PK linearity across subpharmacological and pharmacological dose ranges. The current study assessed the PKs of a novel antimicrobial preclinical drug candidate (GP-4) in rats as a step toward human microdosing studies. Dose proportionality was determined at 3 proposed therapeutic doses (3, 10, and 30 mg/kg of body weight), and PK linearity between a microdose and a pharmacological dose was assessed in Sprague-Dawley rats. Plasma PKs over the 3 pharmacological doses were proportional. Over the 10-fold dose range, the maximum concentration in plasma and area under the curve (AUC) increased 9.5- and 15.8-fold, respectively. PKs from rats dosed with a (14)C-labeled microdose versus a (14)C-labeled pharmacological dose displayed dose linearity. In the animals receiving a microdose and the therapeutically dosed animals, the AUCs from time zero to infinity were 2.6 ng · h/ml and 1,336 ng · h/ml, respectively, and the terminal half-lives were 5.6 h and 1.4 h, respectively. When the AUC values were normalized to a dose of 1.0 mg/kg, the AUC values were 277.5 ng · h/ml for the microdose and 418.2 ng · h/ml for the pharmacological dose. This 1.5-fold difference in AUC following a 300-fold difference in dose is considered linear across the dose range. On the basis of the results, the PKs from the microdosed animals were considered to be predictive of the PKs from the therapeutically dosed animals.