Surface features of small-intestinal mucosa in childhood diarrheal disorders

Whole-body physiologically based pharmacokinetic population modelling of oral drug administration: inter-individual variability of cimetidine absorption

Objectives

Inter-individual variability of gastrointestinal physiology and transit properties can greatly influence the pharmacokinetics of an orally administered drug in vivo. To predict the expected range of pharmacokinetic plasma concentrations after oral drug administration, a physiologically based pharmacokinetic population model for gastrointestinal transit and absorption was developed and evaluated.

Methods

Mean values and variability measures of model parameters affecting the rate and extent of cimetidine absorption, such as gastric emptying, intestinal transit times and effective surface area of the small intestine, were obtained from the literature. Various scenarios incorporating different extents of inter-individual physiological variability were simulated and the simulation results were compared with experimental human study data obtained after oral cimetidine administration of four different tablets with varying release kinetics.

Key findings

The inter-individual variability in effective surface area was the largest contributor to absorption variability. Based on in-vitro dissolution profiles, the mean plasma cimetidine concentration–time profiles as well as the inter-individual variability could be well described for three cimetidine formulations. In the case of the formulation with the slowest dissolution kinetic, model predictions on the basis of the in-vitro dissolution profile underestimated the plasma exposure.

Conclusions

The model facilitates predictions of the inter-individual pharmacokinetic variability after oral drug administration for immediate and extended-release formulations of cimetidine, given reasonable in-vitro dissolution kinetics.

Negative interactions with the microbiota: IBD

Mucosal surfaces are colonized by a complex microbiota that provides beneficial functions under normal physiological conditions, but is capable of contributing to chronic inflammatory disease in susceptible individuals. Of the mucosal tissues, the mammalian intestine harbors an especially high number of microbes with a remarkable diversity. Inflammatory bowel disease (IBD) is a group of chronic relapsinginflammatory disorders of the intestinal mucosa. Evidence from human studies and animal models provides compelling support that intestinal microbes play a key role in disease pathogenesis. While the existence a specific causative pathogen is possible, it appears more likely that intestinal microbes normally present as commensal microbiota may trigger inflammation and perpetuate disease in genetically susceptible individuals. There may be also a shift in the makeup of the commensal flora to a nonphysiologic composition that is more prone to disease (termed dysbiosis). Evidence supports that genetic susceptibility stems from one or more defects in mucosal immune functions, including microbe recognition, barrier function, intercellular communication and antimicrobial effector mechanisms. It is quite plausible to imagine that the chronic inflammation of IBD may in some cases be a normal immune response to an abnormal adherent invasive microbiota and in other cases an over exuberant immune response to an otherwise normal commensal microbiota.

A revised model for electron dosimetry in the human small intestine

In this study, the absorbed dose was calculated to the small intestine (SI) wall of an adult human from electrons in its lumen contents. The effects on dose due to variations in the lumen radius and wall-thickness also were studied. The SI model was based on values gleaned from anatomic and histologic reviews of the adult human SI. Histologic and radiological analyses of the SI suggested the microscopic intricacy of this walled organ could be avoided for dosimetric purposes and a set of concentric cylinders could be used to model the SI. The model was input into the Monte Carlo N-Particle (MCNP) version 4A computational package, which was used to simulate energy deposition in the SI by electrons of fifty discrete energies ranging 10-500 keV. The source electrons as well as all resulting particles, such as knock-on electrons, bremsstrahlung, and electrons created from bremsstrahlung interactions, were transported until the particle energies fell below the 1 keV low-energy cutoff. Detailed physics treatments for secondary photons were made. With a reasonable number of histories, appropriate variance reduction techniques were used to improve the precision of the Monte Carlo calculations. The model used very small tally regions, which ranged in thickness from 0.5 microm to 200 microm depending on the electron energy studied and tally location in the wall. Relative errors associated with these calculations were maintained at less than 5%. The large number of tally results across the wall for each of the energies studied enabled the construction of the energy-specific depth dose curves in the wall. Each of these curves was consistent with the anticipated energy deposition pattern. These curves showed that only a small fraction of the energy absorbed at the contents-mucus interface reaches the stem cell layers because the cells are located deep in the mucosa. This fraction was found to vary from 1.66 x 10(-6) to 1.21 x 10(-1) over the energy range 10-500 keV. These results demonstrated the interface dose, which has been routinely reported as the "wall" dose, is a significant overestimate of the actual dose to the stem cells. The dose uncertainties associated with variations of the critical cell depth were shown to be very high for electrons whose CSDA ranges in the soft tissue exceeded the depth of the critical cells. This study showed that the uncertainty in the wall-thickness had no effect on depth doses while variation in the lumen radius significantly changes depth doses. The results suggest that these changes could be approximated by the inverse square of the lumen radius.