Two major coating methods can control the release of active pharmaceutical ingredients (API)s in multiple-drug tablets. Pan
coating and core coating are both familiar processes to the pharmaceutical industry, and each has its own advantages and limitations.
Since the 1980s, pan coating has been the main method of coating tablets for controlled release. Personnel load tablets into
a machine with a rotating drum. As the drum rotates, the coater sprays the tablets with a solution containing a polymer or
sugar and dries the polymer vector simultaneously. The advantages of pan coating are its reproducibility and the control it
offers over coating thickness and uniformity.
The dynamics of pan coating require compression operators to control the tablet press closely to minimize tablets' weight
variability, says Doug Becker, senior director of process technology at Wyeth (Madison, NJ). Low weight variability among
tablets helps ensure that the coating is applied evenly when the tablets are in the drum.
Yet, the process has drawbacks. It is an additional step that requires space, time, handling, and validation. Tablets might
be damaged as they are loaded into or unloaded from the coater. The rotating drum and mixing baffles also could damage tablets,
as could moisture in the coating solution. In addition, the high temperature of the drying air used in some coating processes
and in some equipment could damage protein-based products.
First developed in the 1960s, core coating is regaining popularity and is increasingly being used to alter the release properties
of multiple APIs, says Nic Michel, vice-president of the process division at Oystar USA (Fairfield, NJ). During the process,
a tablet core is pressed, placed onto a second turret or second machine, sandwiched between two layers of powder (e.g., an
API and an excipient), and pressed again. The result is a tablet within a tablet. The coatings of the core and external tablets
can provide controlled or sustained release of the APIs individually.
The best application for a core-coated tablet would be for an immediate release on the outside and a controlled release on
the inside, says Becker. The dosage form could also have a controlled-release matrix around a core tablet with a sustained-release
profile. In this case, the outer API would have to erode first to expose the core tablet, and simultaneous release of both
drugs would be impossible.
Another application of active coating would be to put an enteric coating over the active coat on the outside tablet with a
sustained-release inner core. This design would protect the stomach from a potentially irritating API or shield an API from
stomach acid, says Becker.
An advantage of core coating is that it can be performed directly on a tablet press without adding an extra step, says Michel.
Core coating poses little risk of damaging the core tablet, which is not exposed to heat or humidity. The process also reduces
handling and is suitable for APIs in the outer coating that are not water- or alcohol-soluble.
But core coating is especially challenging in certain regards. The process requires a specialized press and tooling system
that is rather slow, says Becker. The method usually allows for only one coating layer, whereas pan coating can be used to
create several layers.
In addition, a core-coated tablet's dissolution and release properties could be completely altered if the inner core is not
centered or protrudes through the outer coating, says Michel. The faster a tablet press runs, the greater the risk that the
inner tablet will migrate from a centered position. And operators have no direct, nondestructive way to check the position
of the core tablet or measure the coating thickness, Michel says.
Material properties could present problems, too. If the core tablet is too soft or too hard, it will not bond securely to
the coating or to other functional layers, says Cetin Cetinkaya, professor of mechanical and aeronautical engineering at Clarkson
University. Tablets with excessively soft cores or layers could break or crack, which can lead to compromised mechanical integrity,
nonuniform material distribution, or defects resulting from residual stress.
For more on this topic, see the special report "Down the Track: Different Speeds with Multiple APIs"