Note: Descriptions are shown in the official language in which they were submitted.
BACKGROUND OF T~E INVENTION
This invention relates to a synthetic vascular graft, and more
particularly to a drug delivexy blood-tight collagen-impregnated
synthetic vascular graft which does not need to be pre-clotted and
which acts as a reservoir for sustained release of a drug material
after implant.
The replacement of segments of human blood vessels with
synthetic vascular grafts is well accepted in the art. Synthetic
~ascular grafts have taken a wide variety of conf;gurations and are
formed of a wide variety of materials. Among the accepted and
successful vascular graft implants are those which are formed from
a biologically compatible material which retains an open lumen to
permit blood to flow through-the synthetic graft after implant. The
grafts may be made from biologically compatible fibers, such as
Dacron~ and TeflonTM, may be knitted or woven and may be of a mono-
filiment yarn, multi-filiment yarn or staple yarn.
An important factor in the selection of a particular graft
substrate is the porosity of the fabric wall of which the graft is
formed. Porosity is significant because it controls the tendency
to hemorrhage during and after implantation and controls the
ingrowth of tissue into the wall of the gxaft. It is desirable that
the vascular graft substrate be sufficiently blood-tight to prevent
the loss of blood during implant, yet the structure must be
sufficiently porous to permit ingrowth of fibroblast and smooth
muscle cells in order to attach the graft to the host tissue.
Synthetic vascular yrafts of the type described in United States
Patents No. 3,805~301 and No. 4,047,252, assigned to the assignee
of the subject application, are elongated flexible tubular bodies
formed of a yarn such as Dacron . In the earlier patent, the graft
is a warp knitted tube and in the latter issued patent it is a
double-velour synthetic graft marketed under the trademark
Microvel. These types of grafts have sufficiently porous struc-
tures to permit ingrowth of host tissue.
The general procedure for implantation includes the step
of pre-clotti.ng, wherein the graft is immersed in the blood of the
patient and allowed to stand for a period of time sufficient for
clotting to ensue. After pre-clotting, hemorrhaging does not occur
when the graft is implanted and growth oE tissue is not impeded.
Graft infection is a most serious complication and occurs in an
average of two percent of prosthetic graft placements. It is
associated with a high risk of limb loss and patient mortality is
as high as 75~ depending on the location of the graft. While
infection usually becomes evident soon after surgery, the time may
be extended which leads to more seri.ous consequences.
An absorbable collagen reinforced graft is proposed in
United States PatentNo. 3,272,204 wherein the collagen i.s obtained
from the deep flexor tendon of cattle. Another reinforced vascular
prosthesis is descri.bed inUnited S-tates PatentNo. 3,479,670 which
includes an open mesh cylindri.cal tube wrapped by an outer helical
wrapping of fused polypropylene mono-filiment filled with collagen
fibrils which are claimed to render the prosthesis impermeable to
bacteria and fluids. The collagen fibri.ls utilized are the same as
described in Patent No. 3,272,204.
The synthetic vascular grafts suggested by the prior art
are claimed to be sui.table for many applications~ However~ it is
desirable to provide a flexible vascular graft having zero poros-
ity, one which is receptive to i.ngrowth of host tissue and serves
as a reservoir for drug materials to be released slowly from the
surface of the graft following implant.
SUMMARY OF THE INVENTION
A collagen-impregnated synthetic vascular graft composite
which provides a reservoir for the slow release of a drug material
after implant is provided. The collagen graft includes a tubular
flexible porous substrate having on the inner and outer surfaces and
extending through the porous structure of the substrate cross-
linked collagen fibrils complexed with an effective amountof a drug
~2~
and admixed with a plasticizer. The drug material includes
antibacterial agents, antithrombic agents and antiviral agents to
ensure against graft infection by providing for sustained release
of the drug portion of the complex after implantation.
The porous graf~ substrate may be a tubular vascular
graft formed of a Dacron~ material and may be woven or knit. The
collagen source i5 an aqueous fibril dispersion which may be of
high purity bovine skin collagen including a plasticizer. The
collagen is applied to the graft substrate by massage to cover the
entire inner surface and penetrate into the substrate to insure
intimate mixing of the collagen fibril complex into the porous
structure of the substrate and extend over the outer surface
thereof. The collagen is cross-linked preferably by exposure to
formaldehyde vapor. The collagen graft provides a flexible graft
with good hand.
The invention accordingly comprises the article
possessing the features, properties and the relation of elements
and the several steps and the relation of one or more of such
steps with respect to each of the others, which are exemplified in
the following detailed disclosure, and the scope of the invention
will be indicated in the claims.
BRIEF DESCRIPTION OF THE DRA~ING
For a fuller understanding of the invention~ reference is
had to the followiny description taken in connection with the
accompanying drawing, in which:
FI~. 1 is a partial cross-sectional view of a
collagen-treated synthe~ic vascular graft in accordance with the
invention;
FIG. 2 is a partial cross-sectional view of a branched
tubular graft of the type illustrated in Fig. l;
FIG. 3 is a graph illustrating sustained release of
tetracycline from a collagen slurry in rabbits;
* Trade Mark
-- 3
3~i
FI&. ~ is a graph illus~rating sustained ~elease of
tetracycline at differen~ collagen gal concentrations; and
FIG. ~ is a graph illusteating sustained release of
4 tetracycline in a collagen gel at diffeLent con~entrations and
dosage.
- 3a -
~%~235;
--4--
DESCRIPTION OF T~E PREFERRED EMBODIMENTS
A synthetic vascular graft 10 constructed and arranged in
accordance with the invention is shown in Fig. 1. Graft 10 includes
a tubular substrate portion 12 which is formed of a biologically
compatible filamentary synthetic material, preferably a polyethyl-
ene terephthalate r such as Dacron~. Substrate 12 is a porous
Dacron~ warp knit fabric having an inner and outer velour surace
of the type described in U.S. Patent 4,047,252. While tubular
portion 12 is formed of Dacron~, any biocompatible filimentary
material may be used for the substrate provided it may be fabricated
into a porous structure which will permit tissue ingrowth and
maintain an open lumen for flow of blood.
The inner surface of tubular portion 12 is treated with
collagen as shown at 16. Collagen layer 16 is formed from a series
of at least three applications of collagen fibrils. Fig. 2 shows
a bifurcated collagen-treated graft 20. Graft 20 includes a main
tubular portion 22 and two branches 24. Main tubular portion 22 and
bifurcated portions 24 are formed from a Dacron~ knit substrate 26.
The inner surface of substrate 26 is treated with collagen 28 also
foxmed by at least three applications of collagen fibrils.
Porous vascular graft substrates suitable for use in ac-
cordance with the invention, preferably are produced from Dacron~
multi-iliment yarns by knitting or weaving processes ~hich are
commonly used in manufacture of these products. Generally, the
porosity of the Dacron~ substrate ranges from about 2,000 to 3,000
ml/min-cm2 (purified water at 120mm Hg). The cross-linked collagen
is applied to the inner surface o~ the graft by filling a tubular
substrate with a slurry of collagen fibrils and plasticizer and
massaging manually, removing the excess and permitting the
deposited dispersion to dry. After the final application, the
collagen is cross-linked by exposure to formaldehyde vapor, air
dried and then vacuum dried to remove excess moisture and excess
formaldehyde. The treated grafts in accordance with the invention
have essentially 2ero porosity.
--5--
The following examples are set forth to illustrate the
method of preparing purified collagen from bovine ski.n and treated
grafts in accordance with the invention. The examples are set forth
for purposes of illustration and not .intended ;.n a limiting sense.
Example 1
Fresh calf skins were mechanically stripped from young
calves, fetuses or stillborns and washed in a rotating vessel with
cold running water until the water was observed to be free from
surface dirt, blood and/or tissues. The subcutis was mechanically
cleaned to remove contaminating tissues, such as fat and blood
vessels. Subsequently, the skins were cut in the longitudinal
direction into strips about 12 cm wide and were placed in a wood or
plastic vessel as commonly used in the leather industry.
The skins were dehaired by using a flusher solution of 1
CalOH)2 for 25 hours. Alternatively, the skin may be dehaired by
mechanical means or by a combi.nati.on of chemical and mechanical
means. Following the dehai.ri.ng, the skins were cut into small size
pieces about 1" x 1" and were washed in cold water.
Following washing, 120 Kg of the bovine skin was placed in
a vessel having 260 L water, 2 L NaOH (50%) and 0.4 ~ H2O2 (35%)-
The components were mixed slowly for 12 to 15hours at 4Cand washed
with an excess of tap water for 30 minutes to provide partially
purified skins. The partially purified skins were treated in a
solution of 260L water, 1.2L NaOH (50%~ and 1.4 Kg CaO for 5 minutes
with slow mixing. This treatment was continued twice daily for 25
days. Following this treatment, the solution was decanted and
di.scarded and the skins were washed with an excess of tap water for
90 minutes under constant stirring.
The ski.ns were acidifi.ed by treatment wi.th 14 kg HC1(35%)
and 70 L water while subjecting the skins to vigorous stirring. The
acid was allowed to penetrate the skins for about 6 hours. Follow-
ing acidification, the skins were washed in an excess of tap water
for about 4 hours or until a pH of 5~0 was reached. The pH of the
skins was readjusted to 3.3-3.4 using acetic acid with a 0~5~
preservative. The puri.fied skin was then passed through a meat
grinder and extruded under pressure through a series of fi.lter
~.æ3~
--6--
sieves of constantly decreasing mesh size. The final product was
a white homogeneous smooth paste of pure bovine skin-derived
co lagen.
In order to impartadequate pliability to the graEts in the
dry state, plasticizers are added to the collagen slurry before
application. Suitable plasticizers include glycerine, sorbitol or
other biologically acceptable plasticizers. In a collagen slurry
containing between about 0.5 to 5.0 percent collagen by weight, the
plasticizer is present in an amount between about 4 and 12 weight
percent.
~ mong the most important properties obtained when treat
ing a synthetic vascular graft with collagen fibrils in accordanc~
with the invention is reduction of porosity of the porous substrate
to about zero. The porosity of twenty randomly selected untreated
Microvel~ DacronQ synthetic vascular grafts have a mean porosity to
purified wa-ter of 1796 ml/min-cm2 at 120 mm Hg with a standard
deviation of 1300 After several collagen applications, the poros-
ity is reduced to zero. The following example illustrates the
method of treating the graft substrate.
Example 2
A 50 cc syringe is filled with an aqueous slurry of 2%
purified bovine skin collayen prepared in accordance with Example
1. The collagen slurry includes 8~ glycerol, 17% ethanol and the
remainder water and a viscosity of 30,000 cps. The syringe is
placed into one end of a 2~eadox Medical Microvel~ Dacron~ graft 8
mm in diameter by approximately 12 cm in length. The slurry is
injected into the lumen of the Microvel~ graft and it is massaged
manually in order to cover the entire inner surface area with the
collagen slurry. Any excess collagen slurry is removed through one
o~ the open ends. The graft is permitted to dry f~r about 1/2 hour
at room temperature. The treating and drying steps were repeated
three more times.
Following the fourth application, the collagen was cross-
linked by exposure to formaldehyde vapor for 5 minutes. The cross-
linked graft was then air dried for 15 minutes and then vacuum dried
for 24 hours to remove moisture and any excess formaldehyde.
5~;~3~
Example 3
The blood-tightness of a collagen~treated vascular graft
prepared in accordance with ~xample 2 was tested as follows. A
Microvel~ graft 8 mm x 12 cm was attached to a blood reservoir at
a pressure of 120 mm Hg due to the height of the reservoir. Heprin
stabilized blood was passed through the graft and blood collected
through the grafts was determined and expressed in ml per min-cm2.
The porosity over 5 runs was determined to be 0.04, 0.0, 0.0, 0.04
and 0.03. This represents a mean porosity of 0.022 ml/min-cm2 which
was considered zero, as the value is within the experimental error
of the study.
In order to compare this result with the blood loss for
untreated Microvel grafts, the e~periment was repeated using an
untreated graft. The mean porosity was 36 ml/min-cm2.
The antimicrobial activity of a collagen treated fabric
graft prepared in accordance with the invention is demonstrated as
follows.
Example ~
The porosity of a collagen treated fabric graft is reduced
to less than about 1 percent of an untreated graft after three
coatings. A standard water porosity test used to measure water
porosity of a graft is as follows. A column of water equivalent to
120 mm Hg pressure is allowed to flow through a one-half cm2 orifice
having a sample o~graft over the orifice for one minute. The amount
of water collected was measured. The milliliters of water collected
per minute per cm squared area was calculated. Several readings are
taken for each sample. The porosity is reported as follows-
porosity = ml/min/cm2
The water porosity of a Microvel~ graft fabric was about1,900 ml/min/cm2. The porosity after treatment was as follows:
~umber of CoatingsPorosity
o 1,900
1 266
2 146
3 14
~%~
--8--
4 5
2.~
6 0
In each case the collagen applied was a bovine skin
derived-plastici~ed slurry prepared in accordance with the compo-
sition described in Example 2. Based on these results, it is
preferable to provide collagen of at least three or four layers of
fibrils, and most preferably four or five layers with dryingbetween
each application and cross-linking to fi~ the collagen to the sub-
strate.
In accordance with the invention, each layer of the
collagen and at least the last two layers applied to a porous
substrate are chemically modified to incorporate a drug or an
antithrombic agent, such as heprin, in order to prevent infection
and to inhibit clotting along the inner surface of the prosthesis.
As noted, the collagen may be comple~ed with a variety of drugs,
such as antibacterial agents, antimicrobial agents or antifungal
agents in order to prevent graft infection. Typical antibacterial
agents which may be utilized include oxacillin, gentamicin, tetra-
cycline, cephalosporin and the like which may be complexed with the
collagen fibrils prior to application to the graft substrate.
In addition to reduced porosity, collagen treated vas-
cular grafts in accordance with the invention exhibit reduced
thrombogenicity compared to untreated grafts.
Example 5
A homogeneous slurry of bovine skin derived collagen pre-
pared in accordance with Example 1 was prepared containing 1% bovine
skin derived collagen, 8% glycerol, 17~ ethanol with the remainder
water. Ceclor~, a cephalosporin antibiotic of Eli Lilly and Company
which inhibits the growth of Staphylococcus aureus and Escherichia
coli, was blended into the sluxry at a concentration of 20 mg per
ml. The collagen slurry including the Ceclor~ was massaged onto a
double velour Dacron~ fabric on both sides with 1/2 hour drying
periods between treatments. The treatment resulted in the addition
of 3.1 mg collagen per cm~.
- 9 - ~
As a control, Dacron~ double velou.r fabric was also
impregnated with the same collagen slurry omitting the Ceclor~
antibiotic. This control had a coating of 4.1 mg of collagen per
cm2 ~
Both pieces of treated fabric were immersed for 1 minute
in 4% formaldehyde, 10% glycerol solution, vacuum desiccated for 64
hours and sterilized using gamma radi.ation.
The antimicrobial activity of the collagen treated
Dacron~ vascular graft fabric, i.mpregnated with Ceclor~, was
determined in an agar diffusion assay. Fabric swatches of 1 cm2
were placed on i.nnoculated agar surfaces resulting in growth
inhibition zones whi.ch indicated that the anti.biotic was active
against SO aureus (34 mm zone of inhibition) and E. coli (29 mm zone
of inhibition). The untreated control collagen-vascular graft
fabric did not exhibi.t any anitimicrobi.al effect. The results are
tabulated in the following Tables I and II.
TABLE I
FABRIC TREATED WITH COLLAGEN AND ANTIBIOTIC
PLATE 1 PLATE 2 PLATE 3 X3
=
_ aureus 36mm 31mm 35mm 34mm
E. coli 33mm 28mm 27mm 29mm
TABI,E II
FABRIC TREATED WITH COLL~GEN
PLATE 1 PLATE 2 PLATE 3 X3
_ aureus 0 0 0 0
E coli 0 0
Example 6
A collagen slurry prepared in accordance with Example 1
containing 13.2% collagen protein (determi.ned by i.ts hydroxypro-
line content) was mixed in a 1:3 ratio with water (W) to form a 3.3
-10- ` ~2~:35
weight percent homogeneous collagen gel (G). The pH of the collagen
gel was adjusted to 3.8 and 20mg of tetracycline (TC) was added per
millimeter of gel. Immediately before injection into two rabbits,
the collagen gel-tetracycline complex was mixed with glutaralde-
hyde (0.3 ml of 3% glutaraldehyde per ml of the gel) and injected
through 18 gage needles into the subcutis. Two rabbits as controls
were injected with a similar dose of tetracycline and water, 20 mg
TC/ml water/kg body weight.
In order to study the rate of tetracycline released from
the injected site, blood was collected at various time intervals
from the rabbit's ear vein. The content of TC in the blood was
measured according to the procedure of Wilson, et al. (Clin. Chem.
~cta., 36; 260, 1972). The results of the TC analysis in the blood
of the total of four rabbits collected within 2 hours to 7 days post-
injection are set forth in Fig. 3.
Fig. 3 sho~s that after injection of TC in water the drug
reaches its maximum in the serum within two hours as shown by Curve
Ao At 11 hours the TC is no longer detectable. When tetracycline
was administered in a collagen gel cross-linked with glutaraldehyde
(lOG ~ 30W), the level of serum TC remained stable for about 6 days
as shown by Curve B. Thus, administration of TC in collagen gel
prolonged the efEective release of the drug 25 times compared with
injection in an aqueous medium onlyO
_ample 7
The test described in Example 5 was repeated using colla-
gen gel at two different concentrations for the final injection.
Additionally, the tetracycline content was 30 mg oxyte-
tracycline (OTC)/ml gel/kg body weight at a dose of 1 ml/kg of body
weight, or 50~ more tetracycline per dose than Example 5. The
results illustrated in Fig. 4 show that the concentration of
collagen in the gel affects the rate of OTC release from the
collagen matrix. The denser the collayen gel, the slower is the
release of the drug. In this Example, the kinetics of the OTC
release was studied for a total of 124 hours after injection of the
tested complex in the subcutis of a total of six rabbits.
In Fig~ 4 Curve A shows that the OTC in water reaches its
maximum in the serum shortly ater injection and is not detectable
after 18 or 20 hours. Curve B shows OTC serum concentration for OTC
complexed with a collagen matrix at a weight ratio of gel complex
to water of 1:20 and Curve C at 3:20. Release of the OTC is more
rapid for the less concentrated gel of Curve B.
Example 8
Collagen gel contai.ning 3% collagen, measured as a dry
substance, was mixed with tetracycline to form two concentrations,
containing (A) 50mg TC/ml and (B) lOOmg TC/ml gel. After mixing
with 0.3 ml of 3% glutaraldehyde (Gl) per ml gel (G), complex A was
injected at a dosage of 2 ml/kg body wei.ght and complex B was
injected at a dosage o 1 ml/kg. Plasma level concentrations of l'C
in mg/ml are shown in Curves A and B of Fig. 5. Ccmplex A was also
injected at a dosage of 1 ml/kg and is shown by Curve C in Fig. 5.
The actual plasma levels of tetracycline during the period up to 5
days post-injection are shown in ~ig. 5.
The data of Fig. 5 shcw that both the actual concentration
of tetracycline as well as the surface geometry of the implant
affects the level o magnitude of drug release from the gel and the
level of tetracycline in the plasma.
It will thus be seen that the objects set forth above,
among those made apparent from the preceding descripti.on, are
efficiently attained and, si.nce certain changes may be made in the
article and in carrying out the above process set forth without
departing from the spirit and scope of the invention, it isi.ntended
that all matter contained in the above descript.ion and shown in the
accompanying drawings shall be interpreted as illustrati.ve and not
in a limiting sense.
It is also to be understood that the following claims are
i.ntended to cover all of the generic and speci.fic features of the
invention herein described and all statements of the scope of the
invention which, as a matter of language, might be sai.d to fall
therebetween.
Particularly it is to be understood that in said claims,
ingredients or compounds recited in the singular are intended to
include compatible mixtures of such ingredients wherever the sense
permits.
