Biomedical Engineering Reference
In-Depth Information
O
O-C-CH
3
-(CH
2
-Ch
2
)
m
-(CH
2
-CH)
n
-
FIGURE 6.2
Chemical structure of PEVA.
copolymers have a VA concentration between 10% and 50%, in the form of beads, pel-
lets, granules, and so on (typically, 12%, 14%, 18%, 25%, or 33%) (Chudzik et al., 2001).
PEVA copolymers with low percent VA become increasingly insoluble in typical solvents,
whereas those with higher percent VA become decreasingly durable. PBMA is a homopoly-
mer of monomer (Figure 6.3) with a molecular weight range from 200 to 320 kDa. These
products vary in viscosity, solubility, and form.
Block Copolymers
The TAXUS Express
2
and TAXUS Express
2
Atom Paclitaxel-eluting coronary stent systems
are device/drug combination products comprising two regulated components: a device
(Express Coronary Stent System, Boston Scientific, USA) and a drug product (a formulation
of paclitaxel contained in a polymer coating). The stent is manufactured by Boston Scientific
(Natick, MA) and is used to treat lesions 28 mm or smaller in length with reference diam-
eter ranging from 2.25 to 4 mm. The stent material is a 316L surgical grade stainless steel
with a design strut thickness of 132
μ
m (Nakazawa et al., 2009). The usable length of the
stent delivery systems is 140 cm for TAXUS Express
2
Monorail and 135 cm for the TAXUS
Express
2
over-the-wire systems. The drug/polymer coating consists of paclitaxel (the active
ingredient) and Translute™ polymer carrier (the inactive matrix). The molecular formula
of paclitaxel is C
47
H
51
NO
14
with a molecular weight of 853.91 g/mol. The polymer carrier
is loaded with 1
μ
g/mm
2
paclitaxel in a slow release formulation and coated over the stent
with a maximum nominal drug content of 282
μ
g on the largest stent (4.00 × 32 mm).
The only inactive ingredient in the TAXUS Express stent is SIBS, a tri-block copolymer
composed of styrene and isobutylene units built on 1,3-di(2-methoxy-2-propyl)-5-
tert
-
butylbenzene and sold under the trade name of Translute (Figure 6.4). It is a hydrophobic,
elastomeric copolymer with an
M
n
(number average molecular weight) of 80,000-130,000
g/mol, and a polydispersity index of 1.0-2.0. The polymer is mixed with the drug pacli-
taxel and then applied to the stents, without a primer or topcoat layer, over the entire stent
surface (i.e.,
both luminal and abluminal). The structural formula for the polymer is shown
below.
The SIBS polymer used on the TAXUS stent is a form of rubber, an adhesive compound
with elastomeric properties. SIBS is a biologically inactive and biostable compound that
retains the drug on the stent, provides relatively uniform drug delivery, prevents mechan-
ical disruption during processing and deployment, and has a relatively long shelf life
O
C-O(CH
2
)
3
CH
3
-(C-CH
2
)
n
-
CH
3
FIGURE 6.3
Chemical structure of PBMA.
