To permit introduction of a needle from a hypodermic syringe
into a multiple-dose vial and provide for resealing as soon
as the needle is withdrawn, each vial is sealed with a rubber closure held in place by an aluminum cap (Fig. 26-5). This
principle is also followed for single-dose containers of the cartridge type, except that there is only a single introduction of
the needle to make possible the withdrawal or expulsion of
the contents.
Rubber closures are composed of multiple ingredients plasticized and mixed together at an elevated temperature on milling machines. The elastomer primarily used in rubber closures,
plungers, and other rubber items used in parenteral packaging
and delivery systems is synthetic butyl or halobutyl rubber.
Natural rubber is also used, but, if it is natural rubber latex,
then the product label must include a warning statement, due
to the potential for allergic reactions from latex exposure.
The plasticized mixture is placed in molds and vulcanized
(cured) under high temperature and pressure. During vulcanization the polymer strands are cross-linked by the vulcanizing
agent, assisted by the accelerator and activator, so that motion
is restricted and the molded closure acquires the elastic, resilient character required for its use. Ingredients not involved
in the cross-linking reactions remain dispersed within the compound and, along with the degree of curing, affect the properties of the finished closure. Table 26-3 provides examples of
rubber-closure ingredients.
The physical properties considered in the selection of a particular formulation include elasticity, hardness, tendency to
fragment, and permeability to vapor transfer. The elasticity is
critical in establishing a seal with the lip and neck of a vial or
other opening and in resealing after withdrawal of a hypodermic needle from a vial closure. The hardness should provide
firmness, but not excessive resistance to the insertion of a needle through the closure, and minimal fragmentation of pieces of
rubber should occur as the hollow shaft of the needle is pushed
through the closure. Although vapor transfer occurs to some
degree with all rubber formulations, appropriate selection of
ingredients makes it possible to control the degree of permeability. Physicochemical and toxicological tests for evaluating
rubber closures are described in section <381> in the USP.
The ingredients dispersed throughout the rubber compound
may be subject to leaching into the product contacting the
closure. These ingredients (Table 26-3) pose potential compatibility interactions with product ingredients, if leached into the
product solution, and these effects must be evaluated. Further,
some ingredients must be evaluated for potential toxicity.
The example of pure red cell aplasia, an immunogenic reaction caused by leachables from a rubber closure in a erythropoietin prefilled syringe formulation, highlights the criticality of
appropriate container-closure and the study of such leachables
and extractables, even as a function of stability shelf life.21
To reduce the problem of leachables, laminates have been
applied to the product contact surfaces of closures, with various
polymers, the most successful being Teflon®(DuPont polytetrafluoroethylene [PTFE]) and Flurotec®(West/Daikyo copolymer
of tetrafluoroethylene and ethylene). Polymeric coatings have
been developed that are claimed to have more integral binding
with the rubber matrix, however, details of their function are
trade secrets. Although rubber coatings do reduce the potential
for extractables/leachables and eliminate the need for applied
silicone treatment, they may have potential disadvantages of
not flowing as easily during high speed filling operations and
may not have the same container-closure integrity as uncoated
stoppers with vial openings.
The physical shape of some typical closures may be seen in
Figure 26-5. Most of them have a lip and a protruding flange that
extends into the neck of the vial or bottle. Many disk closures
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