(M1030-02-09) Integration of Gut Microbiome Metabolism in a PBBM-PBPK Model: Its Impact on the Sulfasalazine Absorption

Purpose: Inflammatory bowel disease (IBD) is a recurrent or continuous inflammation of the bowel. Numerous factors (environmental, microbial, and genetic) contribute to IBD while its etiology remains still unknown. In the United States, currently, 1.4 million patients suffer from IBD, and this disease is diagnosed in 396 per 100,000 individuals globally. The two main forms of IBD are Crohn’s disease and ulcerative colitis. Numerous alterations of the gastrointestinal (GI) physiology of IBD patients (e.g., mucosal lesions, thickened bowel walls, and structures) influence processes impacting drugs’ gut absorption following their oral administration. The current paradigm to develop generic drug products for IBD is based on pharmacodynamic endpoint studies as the local concentration at the site of action cannot be sampled in humans and may not be reflected in plasma exposure. Physiologically based pharmacokinetic (PBPK) models can provide insight into drug partitioning into the gut wall and may support the regulatory assessment of GI locally acting new and generic drug products. This project aims to develop a PBPK model for sulfasalazine, one of the drugs of choice in the treatment of IBD. This drug is a poorly absorbed prodrug that once in the colon is extensively biotransformed by colonic bacteria (azo-reductases) into its active metabolite: 5-aminosalicylic acid (5-ASA), also called mesalamine. Therefore, the PBPK models for sulfasalazine and 5-ASA must integrate the microbiome metabolism.

Methods: Preclinical and clinical pharmacokinetics (PK) in healthy subjects following intravenous (IV) and oral (PO) administration of 5-ASA and sulfasalazine were collected from the literature. Physicochemical properties were either extracted from literature or measured in vitro for both 5-ASA and sulfasalazine. A PBPK model of 5-ASA was first developed and validated using two IV and one PO clinical studies in healthy subjects. In the second step, the sulfasalazine PBPK model was developed using six preclinical IV studies and one clinical PO study in healthy subjects. Its gut microbiome biotransformation into 5-ASA was implemented, using the Advanced Compartmental Absorption and Transit (ACAT™) model within GastroPlus® v9.9. Parameters describing the microbiome gut clearance were fitted to describe 5-ASA systemic exposure following sulfasalazine PO administration after connecting both 5-ASA and sulfasalazine PBPK models within GastroPlus. A high Km value was kept ensuring that the microbiome clearance would be non-saturable as most of the sulfasalazine reaching the colon is metabolized. The gut microbiome Vmax was fitted (final value: 2.7 µg/s/mg bacteria). External validation was performed using one PK study following sulfasalazine PO administration in healthy subjects. Models were deemed acceptable if calculated fold errors for both Cmax and AUC were within two folds for each study.

Results: PK metrics (Cmax and AUC) for 5-ASA were predicted within two-fold in each study used to develop and validate its PBPK models (Figure 1). The final PBPK model considering sulfasalazine biotransformation into 5-ASA could predict the sulfasalazine PK metrics within two folds (Figure 2). Using this final model, 5-ASA systemic AUC was predicted within two-fold following sulfasalazine oral administration. Yet this final model could not accurately predict the Cmax compared to the observed one (Figure 2). However, due to the high observed 5-ASA and sulfasalazine variabilities, and the lack of individual PK data, the source of the Cmax misprediction could not be identified at this stage.

Conclusion: Implementation of the microbiome metabolism in the sulfasalazine PBPK model allows for both the prediction of its low absorption and the 5-ASA appearance through metabolism by colonic bacteria in healthy subjects. This successful implementation of the microbiome effect on drug absorption is an important step toward the ability of the PBPK model to predict drug PK in IBD patients but also for drugs metabolized by colonic bacteria. It is expected that PBPK models will significantly support the development and regulatory assessment of new and generic drug products for IBD.

Acknowledgements: Funding - The project was funded by a contract from the U.S. Food and Drug Administration (contract # 1U01FD007660).


Figure 1: observed (Squares) and simulated (line) 5-ASA plasma concentration-time courses in healthy subjects following a single oral administration of 5-ASA.


Figure 2: observed (Squares) and simulated (line) sulfasalazine and 5-ASA plasma concentration-time courses in healthy subjects following a single oral administration of sulfasalazine.