The potential of in vitro pharmacokinetic profiling to predict oral bioavailability of carbohydrate mimetics and cyclic hexapeptides

Urinary tract infection (UTI) is one of the most prevalent infectious diseases and is mainly caused by uropathogenic Escherichia coli (UPEC). The first step of the infection cycle, the adhesion of UPEC to urothelial cells, is mediated by the lectin FimH, which is located at the tip of bacterial type 1 pili. Its interaction with the mannosylated glycoprotein uroplakin Ia (UPIa) on urothelial cells can be prevented by glycomimetic antagonists. Thus, alpha-D-mannopyranosides derivatives with nanomolar affinity offer a potential therapeutic approach for prevention and treatment of UTI.
To achieve oral availability as well as a therapeutic concentration in the bladder over an extended period of time, a number of key issues have to be fulfilled.
• Stability of the antagonist against various gastrointestinal conditions (pH, enzymatic activity) is required.
• Sufficient solubility is a prerequisite for successful absorption.
• To reach circulation, the intestinal mucosa has to be permeated.
• In case of a prodrug approach, hepatic first pass metabolism is requested.
• To reach the therapeutic target in the bladder, renal excretion is required.
These various properties can be predicted based on lipophilicity (log D7.4), stability in different gastrointestinal fluids, kinetic and thermodynamic solubility, metabolic stability (liver microsomes), and permeability (parallel artificial membrane permeability assay and Caco-2 cells assay), methods implemented in the PADMET platform (physicochemical properties, absorption, distribution, metabolism, elimination and toxicity) of our research group.
The aim of this thesis is to develop in vitro assays to provide information regarding the factors influencing the pharmacokinetic properties of glycomimetics and to predict their oral bioavailability. For this purpose, we developed our PADMET platform.
The focus was on glycomimetic FimH antagonists and the evaluation of their structural, physicochemical, and biochemical properties. Besides in vitro assays to determine their solubility and potential passive permeability, assays predictive for pKa, stability in gastrointestinal fluids, and carrier-mediated permeability were implemented into our PADMET platform.
As a result, low passive membrane permeability constraint oral absorption of mannosidic FimH antagonists with para-carboxy-biphenyl aglycone. With ester prodrugs permeability could be improved and the hydrolysis by carboxylesterase released the active principle within minutes. Unfortunately, the ester prodrugs suffer from low solubility. For early in vivo trials, the solubility issue was addressed with phosphate prodrugs or with appropriate preformulations using co-solvents, surfactants, and complexing agents. In addition, phosphate and ester prodrugs were stable in gastrointestinal fluids, whereas a prodrug with acylation in the C-6 position of the mannose moiety showed instability in intestinal fluids due to the enzyme pancreatin. Furthermore, interactions of FimH antagonists with carrier-mediated transporters were observed and require further investigations.
In a collaborative project with the research group of Prof. Horst Kessler (Technical University of Munich, Institute for Advanced Study) the in vitro permeability of cyclic hexapeptides was evaluated with log D7.4, PAMPA, and Caco-2 cells. The cyclic hexapeptides were synthesized in the research group of Prof. Dr. Horst Kessler and permeability experiments were performed in our laboratory.
As a result, di-N-pentyl alkylation of cyclic hexapeptides improved passive permeability. In addition, differential Caco-2 permeability was measured for enantiomers suggesting the involvement of carrier-mediated transporters.