Application of the Convection–Dispersion Equation to Modelling Oral Drug Absorption

Models of systemic drug absorption after oral administration are frequently based on a direct or a delayed first-order rate process. In practice, the use of the first-order approach to predict drug concentrations in blood plasma frequently yields a considerable mismatch between predicted and measured concentration profiles. This is particularly true for the upswing of the plasma concentration after oral administration. The current investigation explores an alternative model to describe the absorption rate based on the convection–dispersion equation describing the transport of chemicals through the GI tract. This equation is governed by two parameters, transport velocity and dispersion coefficient. One solution of this equation for a specific set of initial and boundary conditions was used to model absorption of paracetamol in a 22-year-old man after oral administration. The GI-tract passage rate in this subject was influenced by co-administration of drugs that stimulate or delay gastric emptying. The transport-limited absorption function is more accurate in describing the plasma concentration versus time curve after oral administration than the first-order model. Additionally, it provides a mechanistic explanation for the observed curve through the differences in GI-tract passage rate

Differentially Encoded LDPC Codes—Part II: General Case and Code Optimization

This two-part series of papers studies the theory and practice of differentially encoded low-density parity-check (DE-LDPC) codes, especially in the context of noncoherent detection. Part I showed that a special class of DE-LDPC codes, product accumulate codes, perform very well with both coherent and noncoherent detections. The analysis here reveals that a conventional LDPC code, however, is not fitful for differential coding and does not, in general, deliver a desirable performance when detected noncoherently. Through extrinsic information transfer (EXIT) analysis and a modified “convergence-constraint†density evolution (DE) method developed here, we provide a characterization of the type of LDPC degree profiles that work in harmony with differential detection (or a recursive inner code in general), and demonstrate how to optimize these LDPC codes. The convergence-constraint method provides a useful extension to the conventional “threshold-constraint†method, and can match an outer LDPC code to any given inner code with the imperfectness of the inner decoder taken into consideration