An in silico approach to determine challenges in the bioavailability of ciprofloxacin, a poorly soluble weak base with borderline solubility and permeability characteristics.

Ciprofloxacin is administered as the hydrochloride salt in immediate release formulations for the treatment of various infectious diseases in different patient populations. Due to its weakly basic properties and poor solubility, the in vivo behaviour of this compound could be influenced by both physicochemical and physiological factors. The first aim of this study was to investigate the behaviour of ciprofloxacin (Ciprobay® 500 mg tablets) in the human gastro-intestinal tract with in vitro dissolution, transfer and two-stage experiments. Ciprobay® IR tablets dissolved completely in FaSSGF-V2, but dissolution was incomplete in FaSSIF-V2 and in an achlorhydric medium (FaSSGF-achlorhydric) and slow precipitation was observed in all three media. Ciprofloxacin did not precipitate in the transfer model but in the two-stage test, a simplified version of the transfer model, some precipitation was detected. In the second part of this study the data obtained in the in vitro transfer experiment were integrated into a Physiologically Based Pharmacokinetic (PBPK) Model. Based on the in vitro results, it was concluded that precipitation of ciprofloxacin would be unlikely in vivo. When precipitation was assumed to be negligible in the PBPK model, good predictions of plasma concentration time profiles provided by Bayer Pharma AG and obtained from the open literature were attained. Parameter Sensitivity Analysis (PSA) was conducted on several parameters which may influence the in vivo behaviour of ciprofloxacin. It was shown that precipitation in the gastro-intestinal tract in humans after a dose of 500 mg is not a determinant of the PK profile. PSA further suggested that ciprofloxacin behaves in vivo as a BCS Class I drug according to the Biopharmaceutics Classification System (BCS), even though on the basis of available solubility and permeability data the compound has been categorised as a BCS II/IV drug. These findings illustrate the importance of coupling in vitro results with in silico PBPK models to better understand the in vivo behaviour of weakly basic drugs. The PBPK model of ciprofloxacin, which was set up for healthy volunteers, was also modified to predict the in vivo behaviour of ciprofloxacin in several different patient populations. It was shown on the basis of these simulations that the plasma concentration time profile may be less influenced by disease state than previously expected.