Drug delivery patches always include more of the drug than the patient actually needs, which helps make sure the proper amount is delivered. But that also means there is always some amount of residual drug left over, and that can become a serious problem. Not only does the patient risk getting more of the drug than he or she actually needs, but it can also put caregivers or patients' family members at risk if a drug-delivery patch is left laying around.
According to the FDA, currently marketed drug-delivery patches can leave between 10 percent and 95 percent of the initial total drug amount after the intended use period.
The FDA considers this to be a big enough problem that it just released a draft guidance for drugmakers on adopting the most scientifically advanced processes to minimize residual drugs on transdermal and transmucosal drug delivery systems. To accomplish this, the FDA is using a term that has come into vogue in the past few years in the context of the pharmaceutical industry: Quality by Design (QbD).
The automotive industry has adopted similar practices. It involves being aware of quality issues all the way down the assembly line or supply chain. For the pharmaceutical industry that means not only making sure only the minimal amount of excess drugs are in the final product, but ensuring that kind of awareness from the drug discovery process all the way through its manufacture.
The FDA's guidance calls QbD "a scientific, risk-based and proactive approach to pharmaceutical process and product development" that "can lead to continual improvement of the product throughout its lifecycle." For example, the FDA paper says, getting a better handle on the manufacturing process can help evaluate how variations in raw materials might effect the way the drug permeates, adheres to the skin or reacts at the application site. Each step in the process needs to be studying and scientifically justified. Together, all this has the total effect of making sure the optimum amount of drug is delivered through the skin and minimizing excess.
The FDA guidance is a draft of nonbinding recommendations. But at least it's a start.