Diffusion

Controlled-release technologies are used to supply compounds like drugs, pesticides, or fragrances at prescribed rates. The prescribed rates offer improved efficacy, safety, and convenience. The most commonly cited example is that of a drug dosed either by periodic pills or by a controlled-release technology. The concentration of the drug in the blood is shown schematically in Fig. 19.0-1. When the drug is given in a pill form, its concentration rises abruptly right after the pill is taken. This rise can carry the drug concentration past the effective level and briefly above the toxic level. The concentration then drops below the effective level. In contrast, when the drug is delivered by controlled release, its concentration rises above the level required to be effective and stays there, without sudden excursions to toxic or ineffective levels. Such delivery is often called zero-order release.

Typical products using controlled release are listed in Table 19.0-1. In the case of drugs, we normally want to release a single solid species, typically with a molecular weight greater than 600 daltons. The water solubility of these molecules is often strongly pH dependent because of pendant carboxylic acid or amino groups. The molecules normally will have several chiral centers. While this species is normally nonvolatile, it is usually unstable if it is warmed. It is often crystalline but may be a polymorph. As the previous paragraph suggests, we most often will seek to release the drug at a constant rate, although in some cases, we may want a periodic discharge.