Time-controlled release drug delivery systems

Controlled release drug delivery systems are used to deliver the drug locally or system cally at a predetermined rate for a specified period of time [55-57], These systems are used to provide desirable delivery profiles that can achieve therapeutic plasma levels [58,59,55]. In these systems, the drug release is dependent on polymer properties, thus the application of these properties can produce well-characterized and reproducible dosage forms [60]. This section will focus on the two major types of systems micro-spheres/nanoparticles and tablets. 

Most of the drug delivery systems that have been studied for clinical application are capable of rate- and/or time-controlled drug release. The therapeutic advantages in these approaches lie in the in vivo predictability of release rate, minimized peak plasma levels, predictable and extended duration of action and reduced inconvenience of frequent re-dosing and hence, improved patient compliance [1]. 

The relatively constant transit time in the small intestine of approximately 3-4 h is another physiological characteristic which can be exploited to achieve colon specificity (time-controlled drug release). After gastric empt5ung, a time-controlled drug delivery system is intended to release the drug after a predetermined lag phase. 

In the case of controlled drug-delivery systems, the goal is to have the drug released at a relatively constant rate (zero-order kinetics) at a concentration within the therapeutic range. It is obviously important to minimize the amount of time the concentration is in... 

Conventional systems do not offer sufficient flexibility in controlling drug-release rate and sustaining the release over time periods extending from days to months. Therefore specific modified release vaginal delivery systems are continuously under development and are based on mucoadhesive systems. Penetration enhancement may represent a necessary feature for certain delivery systems, particularly when the absorption regards a macromolecule (such as a peptide or a protein). 

Figure 4.1 Released amount Qt versus square-root-time -Jt plots. Illustration of loading less than or equal to saturation (dispersed, A < Cs) and greater than saturation (dissolved, A > Cs) in a matrix-type diffusion-controlled drug delivery system.

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