Forty years ago, most tablets on the market were coated with shellac, a natural product derived from scale insects (Laccifer lacca) with excellent film-forming properties.
Now it is hardly used at all, as the pharmaceutical industry has increasingly turned to synthetic resins for enteric coating. The US Food and Drug Administration (FDA) lists the excipients used in New Drug Application products every four years, and the decline in its use is apparent when the recent figures are assessed. In 1988 shellac it used in 278 listings, in 1992, 219, and in 1996 it was used in only 73 listings.
The primary problem is that traditional methods of film preparation - in which shellac is deposited out of alcohol - provide a good short-term result, but the tablets produced tend to extend the release characteristics of the drugs after six months or a year. This is because the films crosslink and esterify during drying.
However, shellac has some surprisingly useful properties that suggest its demise as a tablet coating may be premature.
A new US patent , awarded to well-known coating specialist Charles Signorino of CS Associates, notes that many attempts have been made to overcome the limitations of shellac, but the best modification of shellac systems is to use shellac out of water instead of alcohol.
"Shellac is very soluble in alcohol [but] holds on to the last traces of alcohol as the film dries," it notes. Using erased shellac can eliminate these traces of alcohol and prevents esterification of the shellac, while other additives can be added to prevent crosslinking and make shellac more useful for tablet coating.
Specifically, the patent describes a method of designing shellac coatings that deliver their contents at a particular pH, i.e. a particular region of the gastrointestinal tract. The method is based on selecting shellacs with a particular acid number (a measure of the free acid content of a substance, expressed as the number of milligrams of potassium hydroxide required to neutralise one gram of the material being tested); films that release or dissolve above pH 7.0 are comprised of a shellac selected with an acid number below 80, and vice versa.
This means that by combining different shellacs to produce a blend with the desired acid number, the release characteristics of any solid dosage form (e.g. tablet, capsule, granule etc) in the GI tract can be modified and controlled. For example, if the tablet is designed to dissolve in the colon, the film should be based on shellac that dissolves above pH 7.4.
Shellac can also be mixed with other resins and gums, as well as plasticisers, to create a range of different coating properties that provide even greater fine-tuning of the release - and rate of release - at specific acidity levels.
The patent also notes that this coating is suitable for use in dietary supplements. Shellac is still used quite commonly in these products, as it is a natural product and approved as a food ingredient around the world.