Note: Descriptions are shown in the official language in which they were submitted.
2 6 4 9 D, - 6 6
~2~
DUAL MICROC~PSULES
Background of the Invent;on
An increasingly importan~ process for delivering a
functlonal material to a particular locus involves the use of
microcapsules. As the term is used in the art, a microcapsule
is a functional material encapsulated in a membrane.
An important application of microcapsules is in the
medlcal arts. In this field of application, a functional drug
is encapsulated in a membrane that is semipermeable to the drug.
When the drug is administered to the host, the drug is transported
across the semipermeable membrane to release the drug to the hos~.
While the use of microcapsules for drug administration
is widely employed, the method suffers from certain shortcomings
which limits its further application in this art. Specifically,
the rate at which the drug is released to the patient is control-
led by the ra-te at which the drug i5 transported across the semi-
permeable membrane. While many types of polymeric materials
providing different diffusion rates can be employed as the semi-
permeable membrane~ each membrane has â limited range of permeabil-
:it~
6260 (1) CAN
~ 1 --
--2--
which effec~ively controls the rate at which a drug
of interest is released to the host. For many pur-
poses; the drug release rate may be too slow or too
rapid to provide the desired drug release rate~
Accordingly r there is a need in the art for
more refined and structurally modified microcapsules
haviny ~he ability to release drugs to a host over a
wide range of preselected rates.
Summar of the Invention
,~ ~
The invention is directed to certain novel
microcapsules which can be employed to release a
wide variety oF d~ùgs to a host over a wide range of
preselected drug release rates. The microcapsules
of the invention are hereinafter characterized as
dual microcapsules in that they include an outer
semipermeable membrane which encapsulates ~wo sepa-
rate and distinct encapsulated componentsO One
encapsulated component is a liquid material. The
second encapsulated component is a smaller microcap-
sule, hereinafter referred to as a mini-microcapsule.
The mini-microcapsule has a functional material
encapsulated therein.
In a preferred embodiment of the invention,
the two separately encapsulated components~ one
encapsulated by the outer membrane and the other
encapsulated by the membrane of the mini~microcap-
sule, are reactive with each other. The component
encapsulated within the mini-microcapsule diffuses
across the wall of the mini-microcapsule to co~tact
the component encapsulated within the outer membrane
of the microcapsule. The two components ~hen inter~
act to generate a new entity not originally present
per se in either of the two encapsula~ed components.
The new entity then diffuses through ~h~ outer mem-
brane and i5 released to the host.
In another embodiment of the invention, a
single component is encapsulated in the mini-micro-
capsule with the second component encapsulated by
the outer membrane serving ~olely or principally a
transport medium for the agent encapsulated within
the mini-mlcrocapsule. The two membranes included
in the dual micxocapsule will be fabricated from
differen~ polymeric materials a~d will have diEEerent
diffusion rates for the component encapsulated within
the mini-microcapsule.
The invention also is directed to methods
for preparing the dual microcapsules of the inven-
tion.
The invention is further directed ~o methods
for preparing certain mini-microcapsules employed in
the manufacture of the dual microcapsules o the
invention.
Brief Descri tion of the Dra~in s
P ~
Fig. l is a sectional view of a dual micro~
capsule of the invention.
Fig. l~ is a sectional view of the dual
microcapsule of Fig. l which has been dehydxated to
remove the bulk of the water from the aqueous media
orig~nally present within the dual microcapsule,
including the encapsulated mini-microcapsule.
Flg. 2 is a sectional view of a dual micro-
capsule having two different mini-microcapsules
encapsulated therein~
-4--
Defined Terms
As an aid in interpreting the descriptions
of the inventions which follow, the following terms
will have the special meanings set forth below.
"Microcapsule" i5 an article of manufacture
havins a Functional Core Material encapsulated within
a polymeric membrane. While Microcapsules may have
essentially any physical form, they customarily are
essentially spherical in shape and customarily have
average diameters in the range of about 1 m to
2,000 m.
"Functional Core Material" is any solid or
liquid material, other than a Mini~Microcapsule,
encapsulated in a Microcapsule.
"Dual Microcapsule" is a special Microcap-
sule having at least ~wo components encapsulated
within the exterîor membrane oE the capsule. At
least one of the t~Jo components will be a Microcap~
sule having a diameter smaller than the external
membrane of the Dual Microcapsule, such small encap-
sulated microcapusles hereafter being identified as
Mini-Microcapsules~ A ~ual Microcapsule may have
two or more different types of Mini-Microcapsules
encapsulated therein.
?5 "Mini-Microcapsule" is a Microcapsule suf-
ficiently small to be encapsulated within the inte-
rior of a Dual Microcapsule~
"Encapsulating Process" is any process for
encapsulating a Functional-Core Material in a poly-
meric membrane~
'IPhase Separation Encapsula~ion Process" is
a process for preparing Microcapsules in which the
Functional Core Material to be encapsulated is dis-
persed, customarily by stirring, in a solvent solu-
tion of a polymer/ While continuing stirring ~o
keep the Functional Core Material uniformly dispersed
throughout the polymer solution, a nonsolven~ liquid
is added to the polymer solution to change ~he poly-
mer solubility in the medium and cause a phase sepa-
ration of the dissolved polymer. Depending upon the
specific polymer/solvent system, the polymer either
precipitates from the solution or two immiscible
liquid phases are produced, one of which is rich in
polymer and polymer solvent and poor in nonsolvent,
and the second of which i5 rich in nonsolvent and
poor in solvent and polymer. Under certain condi-
tions, the polymer rich phase will migrate to the
interface between the dispersed droplets/particles
and the continuous phase (non-solvent rich dispersing
medium). The suspended particles of ~he Functional
Core Ma~erial are encapsulated with the polymer and
are subsequently hardened and recovered from the
solvent/nonsolvent medium.
"Conjugate" is a product formed between two
materials, one characterized as the "Functional
Agent" and the second as the "Carrier", both of
these terms being subsequent defined. The Conjugate
is an entity separate and distinct from the Func-
tional Agent and the CarrierO The Conjugate can be
either a true chemical reaction product of the Func-
tional Agent and the Carrrier or can be any type of
complex formed therebetween~ In either event, the
Conjugate can be subsequently treated with another
material characterized as a Deconjugating Agent
(subsequently described) to reform at least a portion
of the originally employed Functional Agent.
--6--
"Functional Agent" is an entity which per-
forms a specific identifiable function.
"Carrier" is a material which will reac~
with or form a complex with a Functional Agent.
"Drug" is a Functional Agent having a desir-
able beneficial physiological efect upon a host.
"Drug Conjugate" is a Conjugate in which
the Functional Agent included therein is a Drug.
"Prodrug" is a term sometimes used inter-
changeably with Drug Conjugate, usually to define a
Conjugate which is formed by effecting a chemical
reaction between a Drug and a Carrier.
"Deconjugatinq Agent~' is an agent capable
of interacting ~ith a Con~ugate to liberate or reform
at least a portion of the Functional Agent employed
in the preparation of the Conjugate.
"CAB" is a cellulose acetate butyrate poly-
mer. Such polymers are known in the art, are de-
scribed in numerous patents and publications and are
2~ commercially avallable from multiple sources includ-
ing Eastman Chemicals. `
"PLA" is a polymer of lactic acid~ Such
polymers are known in the art. See U.S. 3,991,7760
"Dalton" is a term becoming increasingly
popular in the art to designate molecular weight. A
Dalton number is numerically equivalen~ to a gram
molecular weigh~. By way of example, the Dalton
number of sucrose is 342~
"Solvent" is an organic liquid having the
power to dissolve at least 0.1 weight ~ of a desig-
nated polymer of interest at ambient tempera~ure.
"Nonsolven~" is an or~anic liquid miscible
with a solvent and having little or no solvent power
~f~
~7--
for a designated polymer of interest at ambient
temperature.
"Crenate Shape" is the shape assumed by an
article having a flexible membrane supported by an
internal fluid after the supporting fluid is evacu~
ated from the membrane. The shape assumed by a
deflated basketball bladder after the bulk of the
air has diffused therefrom is a prime example of a
Crenate Shape.
Detailed Description of the Invention
_
Fig. 1 represents an embodiment of the
invention containin~ a single mini-capsule encapsu-
lated within another larger capsule to form a dual
capsule~ The structure conta~ns an outer polymeric
membrane 10. A first liquid 12 is encapsulated
within membrane lOo A mini-microcapsule having a
polymeric membrane 14 is encapsulated within membrane
10~ A second liquid 16 is encapsulated within mem-
brane 140 In the most preferred embodiment of the
invention, liquids 12 and 16 are aqueous liquids,
with liquid 16 having a Conjugate dissolved or dis-
persed therein and liquid 12 having a Deconjugating
Agent dissolved or dispersed therein. The membrane
14 will be semipermeable with respect to the Conju-
gate within liquid 160 Upon diffusing through mem-
brane 14 and contacting the Deconjugating Agent
within liquid 12, an interaction takes place to
regenerate the Functional ~gent. The Functional
Agent then diffuses through membrane 10 into the
system of the host. The structure shown in Fig. 1
is the structure of the dual capsule as it is pre-
pared.
In one of the preferred embodiments of the
invention, the membranes 10 and 14 shown in Fig~ 1
are semipermeable to water and can be dehydrated by
processes subsequently described so as to remove the
bulk of the water ~rom bo~h a~ueous li~uid 12 and
aqueous liquid 15~ The approximate Crenate Shape
that the dehydrated dual capsule takes is shown in
Fig. lA. The bulk of the solids of original liquid
16 is maintained within membrane 14. The bulk of
10 the solids originally present in liquid 12 is main
tained in the space between membranes 10 and 14.
When the dehydrated dual capsule of Fig. lA is placed
in contact wlth water, it imbi~es water so as to
dissolve or disperse the solids encapsulated within
membranes 10 and 14 to reform the dual capsule in
substantially the form illustrated in FigO 1.
In other embodiments of the invention, the
liquid 16 may be a simple solution or dispersion of
a Functional Agent in a suitable liquid~ usually
water, and liquid 12 will contain no material react-
able with the Functional Agent and will serve princi-
pally as a transport medium for the Functional ~gent~
In such embodiments, membranes 10 and 14 will be
fabricated from different polymeric materials so
that the membranes will have different diffusion
rates for the Functional Agent, with membrane 14
being signiicantly les5 permeable than membrane 10.
Fig. 2 illustrates another embodiment of
the invention in which the dual microcapsule contains
3~ encapsulated therein two different mini-microcap
sules. The second mini~microcapsule will include a
polymeric membrane 18 encapsulatng a third liquid
20. The membrane 18 and the liquid 20 customarily
r ~
_g_
will diEfer in at least minor respects from the
corresponding elements of the other mini-microcap-
sule 7
The physical size of the dual microcapsule6
is not ordinarily a matter of critical importance in
the practice of the invention. Customarily, the
dual microcaps~les will have outer diameters in the
range of from about 20 ~m to 2 mm. The physical
size of the mini-microcapsules will be such that
they easily fit within the outer meinbrane of the
dual microcapsules. Typically, the mini-microcap-
sules will have diameters in the range of from about
1 ~m to 1 mm.
The dual microcapsules can be prepared by a
modification of the Phase Separation Encapsulation
Process defined earlier herein. In this process,
the liquid material to be encapsulated and the mini-
microcapsules are suspended and stirred in the poly-
mer solution to uniformly disperse the liquid mate~
rial and the mini-microcapsules as a fine dispersion
throughout the polymer solutionO It is observed
that the liquid material tends to cling to the outer
mem~ranes of the mini-microcapsules. While continu-
ing stirring to keep the liquid material and the
mini^microcap ules uniformly dispersed throughout
the polymer solution1 a nonsolvent liquid is added
to th~ polymer solution ~o change the polymer solu-
bility in the medium and cause a phase separation of
the dissolved polymer. A~ earlier noted, the polymer
either precipitates ~rom the solution or two immisci~
ble liquid phases are produced, one of which i5 rich
in polymer and polymer solvent and poor in nonsol~
vent, and the second of which is rich in the nonsol-
--10--
vent and poor in solvent and polymerO By this means,the suspended particles o the liquid and the mini-
microcapsules are encapsulated with the polymerwhich forms the outer membrane of the dual microcap-
sules. The entire suspension then i5 added to alarye volume of the nonsolvent which precipitates
any remaining dissolved polymer and hardens the
outer membrane of the dual microcapsule. The dual
microcapsules then are recovered, optionally given a
surface treatment to modify the permeability charac-
teristics of the outer membrane, and dried.
In the process, the preferred solvents for
use are halogenated hydrocarbons having boiling
points less than about 65 C and ester~ prepared
from alkanol~ containing 1-4 carbon atoms and alka-
noic acids containing 1-4 carbon atoms. Particularly
suitable solvents are methylene chloride, chloroform
and ethyl acetate. Suitable nonsolvents are li~uid
hydrocarbons such as hexane, heptane, nonane cyclo-
hexane, certain fluorocarbons such as Freon TF,and the like.
The mini-microcapsules employed in the
preparation oE the dual microcapsules can be prepared
by any of the processes known and reported in the
art. When the mini microcapsules employed have a
PL,A membrane, they preferably are prepared by ~he
novel modified pro~esses subsequently described.
The membrane employed as the outer member
of the dual cap~ule and the membrane of the mini~
microcapsule can be fabricated from any polymeric
material cu~tomarily employed for such purposes.
Typical polymeric materials which can be employed
include cellulose acetate butyrate ICAB), d,l-poly
_
* Registered Trademark of E.I. Dupont.
-11--
actide ~PLA)~ including its copolymers, cellulose
ethers and esters, polyesters, polylactones, poly-
amides, silicone rubbers, collagen, and the like.
The polymeric material employed will be selected to
be semipermeable to the Conjugate and/or Functional
Agent to be encapsulated and/or formed within the
Dual Microcapsules. When the dual microcapsules are
to be employed for injection into or ingestion by a
host, it is desirable to employ in the membranes
polymeric materials which are nontoxic to the host,
particularly CAB and PLA.
The diffusion rates of materials encapsu-
lated within the dual microcapsules are a func~ion
both of the encapsulated materials and the polymeric
material from which the membranes are fabricated.
The diffusion rates can be modified by giving the
membranes a physical or chemical treatment subsequent
to their preparation. This can be done by suspending
the capsules in a nonsolvent medium containing a
chemical reactive with the polymeric material includ-
ed in the membrane. Diisocyanates such as TDI can
be employed for this purpose, particularly when the
polymeric membrane contains reactive hydrogen atoms.
The inclusion of a mini-microcapsule in a
dual microcapsule makes possible significantly
greater control of the time period over which a drug
can be released to a host. Drug-containing microcap-
sules presently employed in the medical arts have
so-called "zero-order solute release rates"~ That
is to say~ the drug delivery Lhrough the membrane is
essentially independent of the amount of drug within
the microcapsule. With such microcapsules, the time
period over which the drug is released thererom is
~ 7~
~12-
controlled nearly exclusively by the solubility of
the drug included within the microcapsule and the
wall permeabili~y to ~he drug.
By employment of the dual capsules of the
invention, the effective period over which a drug
can be released to a host can be controlled by either
or both of two mechanisms~ One mechanism consists
of encapsulating the drug within a mini-microcapsule
having a membrane with low permeability for the
encapsulated drug. The li~uid in which the mini-
microcapsule is suspended and the outer membrane of
the dual microcapsule will he selected so that the
drug diffusion rate through the outer membrane is
significantly greater than the corresponding rate
through the membrane of the mini-microcapsule. As
readily perceived, the effective drug release rate
to the host is controlled by the diffusion rate of
the drug through the mini microcapsulels membrane.
In this type of dual microcapsule, the medium in
which the mini-microcapsule is suspended serves
principally as a transpor~ medium for the drug~
A more sophisticated and preferred mechanism
involves the use of Conjugate/Deconj~gate Agent
systems. In this embodiment of the invention, a
Drug Conjugate is encapsulated within the mini-micro-
capsuleO The mini-microcapsule is suspended in a
liquid containing a Deconjugating AgentO
As earller noted7 a Drug Conjugate is either
a reaction product of a Drug and a Carrie~ or a
complex formed between the Drug and the Carrier.
The diffu~ion rate of the Drug ~onjugate through the
mini-microcapsule 1 5 membrane ordinarily will be
considerably lo~er than the diffusion rate of the
-13-
free drug through the mini-capsule's membrane. When
the Drug Conjugate enters the liquid containing the
Deconjugating Ayent, the two materials will inter-
react with each other to reform or release the drug.
The free drug then diffuses through the outer mem-
brane of the dual microcapsule into the host~ As
can be readily recognized, the overall drug release
rate to the host is controlled by two other rates,
specifically, the diffusion rate of the Drug Conju-
gate through the mini-microcapsule's membrane and
the rate at which the Drug Conjugate reacts with the
Deconjugating AgentO
A Drug Conjugate system well suited for use
with the dual microcapsules of the invention is a
Drug Conjugate formed by reacting a Drug containing
a carboxyl group with a monoor diglyceride fatty
acid ester~ A typical drug-glyceride conjugate of
this type is the ester obtained by reacting aspirin
with glyceryl distearate. This conjugate has a
molecular weight 786 as compared with the molecular
weight of 180 for aspirin. The preparation of such
conju~ates is shown by G. Jones, Cb~ist~v ~d In~
~, June 7, 1980, page 452. The aspirin can be
esterified with the diglyceride in the presence of
triphenyl phosphine and diethyl azo-dicarboxylate as
described by R. Aneja et al., r~ n~ <t~e.~
Chemical Communications, 1974, page 963.
The intermediate glyceryl distearate can be prepared
by tbe method described by P.H. Bentley and W.
McCrae, ~ , 35, 2082
(1970).
A number of enzyme systems will function as
Deconjugating Agents for the above-described drug-
glyceride conjugates. Two effective enzymes are
-14-
serum esterases and pancreatic lipase~ These enzymes
cleave the drug -- also ~he ~atty acid ~- from the
glyceride. The freed drug, ~he lower molecular
weight bound drug fragments (principally the ester
between the drug and either glycerine or glyceryl
monostearate) and possibly minor quantities of the
original Drug Conjugate will diffuse across the
outer membrane of ~he dual capsule to enter the
host. The original conjugate and its fragments
containing still-bound drug will be further cleaved
by the enzyme systems in the host to release the
drug~
Ano~her Drug Conjugate system of interest
for use in the dual microcapsules of the invention
are conjugates formed between drugs containing a
hydroxyl ~roup or an amine group and a polymeric
acid such as polyglutamic acid, Several enzyme
systems including gamma-glutaminase and carboxypeptiw
dase Y function as effective De~on~ugating Agents
for such Drug Conjugates.
Polyglutamic acid is commercially available
as the sodium salt in a range of molecular weights
from about 2,000 to 100,000 daltons~ Polyglutamic
acid has a pendant car~oxyl group for each monomer
unit of the polymerO Drugs containing hydroxyl or
amine groups can be reacted with the pendant carboxyl
groups to form the Drug Conjugate~ A broad range of
drug loadings can be provided, which allows for the
tailoring of the level of drug loading with the rate
of enzymatic deconjugation so as to optimize the
drug release rate.
A ~rug Conjugate of this type which can be
employed in the dual microcapsules of ~he in~ention
to lower systemic blood pressure is the conjugate
-15~
formed between dopamine and a polyglu~amic acid
having a molecular weight in the range of 2,000 to
15,000. The dopamine is reacted wi~h the polyglu-
tamic acid in an aqueous medium containing l-ethyl-3-
(3' dimethyl amino propyl) carbodiimide. The reac-
tion conditions are selected so that one molecule of
dopamine is reacted for each 10-20 glutamic groups
present in the polyglutamic acid,
Another Drug Conjugate system of interest
is the conjugate that can be formed between 17-alpha
hydroxyprogesterone and polyglutamic acid. The
hydroxyprogesterone is reacted with pendant carboxyl
groups of the polyglutamic acid. Th2 chemical link
age between the two components, of course, i5 an
ester group. The esterification can be carried out
employing mixed-pha~e reaction systems of the type
reported in the literature. The reaction conditions
will be selected so that one molecule of the hydro-
xyprogesterone is reacted for each 10 20 pendant
carboxyl groups of the polyglutamic acid~
The enzyme gamma-glutaminase i5 very effi-
cient at hydroly~ing the gamma glutaminyl amide of
dopamine, but not the aspartyl, succinyl or glutaryl
amides of dopamine. The enzyme, carboxypeptidase Y
(CPY) is an exopeptidase/ and cleaves peptide bonds
sequentially to release individual amino acids from
the C~t~rminus of the polypeptide The broad speci-
icity of CPY permits it to accept as subskrates,
polypeptides having modified side chains (R-groups)
containing the drug moiety The CPY removes all
L-amino acids f rom most C~terminal sequences. In
the event the drug confers a distinctly basic charac-
ter upon the drug-polypeptide conjugate~ carboxypep-
tidase B may be substituted for CPY
-16-
Yet another Drug C,onjugate ~ystem of inker~
est is the class of products that can be prepared
~o~:
1~ A polymeric alcohol containing a plur
ality of hydroxyl groups, eOg., dex~ran or polyvinyl
alcohol,
2. Chloroacetic or alpha-chloropropionic
acid, and
3. A Drug containing a functional group
reactive wi~h a bydroxyl group.
An ester is first formed between the poly-
meric alcohol and the chloracetic or the alpha-chlor-
opropionic acid. The chloro group of the resulting
ester then is converted to a hydroxyl group by rou~es
known in the art. Finally, a drug having a carboxyl
group (or chemical equivalent) is esterified with
the resulting pendant hydroxyl group. Drug Conju-
gates of this type can be prepared from alpha-methyl
DOP~ or gamma-aminoisobutyric acid (GA~A~.
Chymotrypsin and serum esterases can be
used to cleave these D~ug Conjugates. Chymotrypsin
will cleave the Drug Conjuga~e at the ester carbonyl
of the drug quite readily. The serum esterases will
cleave at the ester carbonyl of the glycolic acid
moiety equally well. Either position of cleavage
will lead to low molecular weight products that
permeate the outer wall of the dual microcapsules.
Other Conjugate systems can be deYeloped
which are stakle above or below a given pH maintained
within the mini-microcapsule, Examples of such
Con~ugates are salts formed from Drugs containing a
basic functlon such as an amino group and a polymeric
acid such as polyacrylic acid, an ethylene-maleic
anhydride copolymer, an acidic ion-exchange resin
.
and the like~ Alternatively, the Conjugate ~an be
formed between a Drug containing an acidic function
and a high molecular weight base. An aqueous medium
buffered to a selected pH can function as a Deconju-
gating Agent for such Conjugates.
PLA microcapsules are one of the preferredembodimen~s of the invention. Such PLA microcapsules
are difficult to prepare by the Phase Separation
Encapsulation Process defined earlier hereinO Speci-
fically, when the nonsolven~ liquid is added ~o thePLA-containing solvent solution to encapsulate the
core material, it is ob~erved that the PLA-encapsu-
lated product appears to have a tacky exterior coat-
ingO The encapsulated spheres tend to agglomerate
into oversized clusters that are too large for use.
One of the aspects of the present invention
is to provide a reliable process for preparing PLA
microcapsules~ In the first step of this process,
the PLA is dissolved in a miscible mixture of a
solvenk and a nonsolventO The solvent and nonsolvent
will be employed in a ratio such that the resulting
PLA solution prepared therefrom is very close to its
phase separation point. As will be readily recog-
nized, the precise ratio of the solvent and nonsol-
vent employed will depend both upon the specificsolvent and nonsolvent employed as well as the con-
centration of PLA desired in the solution at its
phase separation point. A convenient way to prepare
the PLA solution i5 to dissolve the PLA in pure
solvent9 add sufficient nonsolvent to cause incipient
PLA phase separation, and then add the smallest
quantity of solvent to redissolve the small quantity
of separated PLA.
~18-
For reasons which will become apparent from
the subsequent descriptions, it is import~nt that
the solvent employed have a vapor pressure signifi-
cantly higher than the vapor pressure of the nonsol-
vent at the temperature employed ts precipitate PLAin the third step of the pro~ess subsequently de-
scrib~d.
In the second step of the process; the
PLA-containing homogeneous solution prepared in the
first step of the process is vigorously agitated and
the functional core material is added. The agitation
provided will be sufficient to disperse these mate-
rials uniformly throughout the continuous PLA-con
taining solvent solution as a fine suspension.
In the third step of the process, agitation
is continued to maintain the core material dispersed
throughout the PLA-containing solvent solution.
Conditions are established to vaporize solvent and
nonsolvent from the suspension. While both solvent
and nonsolvent will be vaporized and removed from
~he suspension, the solvent will be removed in
greater quantities than the nonsolvent by reason of
the solvent's higher vapor pressure. The preferen-
tial removal of solvent from the system, of course,
changes the ratio of the solvent and nonsolvent
liquid in which the PLA is dissolved. Since the
PLA-containing solution as prepared is near its
saturation point Eor PLA, as the composition of the
solvent/nonsolvent medium is changed, the PLA will
undergo a phase separation. The phase separa~ed PLA
migrates to the surface of the finely dispersed core
droplets or particles and begins encapsulation there-
of. After a sufficient quantity of PLA has encapsu-
~ 4 Qd5 8
~19--
lated the core droplets or particles, the resultingcomplex dispersion is ready for transfer to the
fourth step of the process.
In the fourth step of the process, the
complex suspension from the third step of the process
is transferred into an agitated mass of nonsolvent.
Upon contacting the nonsolvent ~o which the suspen-
sion is added, any PLA remaining dissolved in the
initial solvent solution is precipitated. A second
phenomenon which is believed to occur is the extrac~
tion of the residual solvent from the PLA mem~rane.
In carrying out ~he process, the solvent/
nonsolven~ mixture employed in the ~irst step of ~he
process should have the requisite solubility ~o
dissolve a convenient quantity of PLA. It is desir-
able to employ solvent/nonsolvent mixtures which
will dissolve at least about 0,3 part by weight of
PLA per 100 parts by volume of solvent/nonsolvent
mixture at ambient temperature. It also is desirable
to employ solvent/nonsolvent mi~tures having rela-
tively sharp changes in PLA solubility capacity with
temperature~ The presently preferred solvents for
use in the process are halogenated hydrocarbons
having an atmospher.ic ~oiling point o~ less than
about 65 C. and esters ormed between alkanols con-
taining 1-4 carbon atoms and alkanoic acids contain-
ing 1-4 carbon atoms. Suitable halogenated hydrocar-
bons of this class include methylene chloride and
chloroformO The preferred ester is ethyl acetate.
The nonsolvents presently preferred for use in the
process are hexane, cyclohexane, heptane, selected
mineral spirits, nonane 9 Freon TF and ~he like~
* Registered Trademark of EoI~ Dupont.
p~
~2Q-
In the third step of the process~ after the
core materials are uniformly dispersed ~hroughout
the polymer solution, conditions are established to
vaporize ~he solvent and nonsolv2nt from the suspen-
sion, This can be done by reducing the pressure onthe suspension by drawing a slight vacuum on the
system or suppl~i~g heat to the suspension, or both.
In the em~odiment of the invention in which methylene
chloride or chloroform is employed as the solvent
and an aliphatic hydrocarbon, such as hexane or
heptane, is employed as the nonsolvent, vigorous
agitation is sufficient to supply the small quantity
of energy required to vaporize the requisi~e quantity
of solvent to initiate phase separation of the PLAo
In carrying out the fourth step of the
process, care should be exercised to transfer the
entire suspension from the third step of the process
into the agitated nonsolvent before the encapsulated
core materlals begin a~glomeration into oversized
aggregates, presumably by reason of the somewhat
tacky nature of the encapsulating PLA membrane at
this stage of the process~ The appropriate point at
which the suspension should be transferred to the
agitated nonsolvent can be readily established wi~h
a minimum of experimentation. It has been the appli-
cant's observation that in preparing batches of the
si~e subsequently described in the workin~ examples,
the suspension should be transerred to the agitated
nonsolvent in a time period of between 3 and 10 and
preferably 4-7 minutes after the initial phase sepa-
ration of the PLA i5 observed in the th.ird step of
the process.
~21~
In the final step of the process, it is
desirable ~o transfer the suSpension from ~he third
step of the process into a large excess of the no~-
solvent, e.g~, 3-20 times the ~o~al volume of the
solvent solution employed in the f irst step of the
process.
The process described above is carried out
under essenti~lly isothermal condltions and prefera-
bly at ambient temperatures.
Although not presently preferred, it is
possi~le to heat the PLA solution in the first step
of the process to a temperature somewhat above am~
bient temperature. As the temperature drops in
carrying out the second and third steps of the pro-
cess, the lowering of the temperature accelerates
the phase separation of the ~A.
Cellulose acetate bu~yrate (CAB) is one of
the preferred polymeric materials for use in prepar-
ing the membrane o~ the mini-microcapsules and the
outer membrane of the dual microcapsules. Mini-
microcapsules and dual microcapsules including such
CAB membranes can be prepared by ~he Phase Separa~ion
Encapsulation Process described earlier herein.
It has been noted, however, that minor
dif f iculties are encountered which lengthen the
process cycle for preparing ei.her of the above-type
caps~ules from CAB. Specifically, C~B is somewhat
difficult to dissolve in the chlorinated hydrocabon
solvents preferred for use in the process~ Specifi~
cally, when the particulate CAB is added to the
solvent, the CAB particles swell, ~ecome agglomerated
with each other~ and take relatively long periods of
time to dissolve to form the true solutions required
-~2-
in the capsule forma~ion process. This difficulty
can be significantly ameliorated by first suspending
the CAB particles in a small volume of the nonsolven~
to be su~sequently used in the process~ preerably a
liquid hydrocarbon such as hexane, heptane or octane.
When the CAB solvent, e.g~ methylene chloride, is
added to the suspension wi~h stirring, the CAB parti-
cles readily dissolve~ Thereafter, the remaining
steps of the process are conventional.
The following examples are set forth to
illustrate certain principles and practices of the
inven~ion to those skilled in the art. The examples
have ~een run to illustrate the fundamental principle
of controlling the release rate of a Functional
Agent from the dual microcapsules. Details of the
action of ~he ultimately t-ransferred Functional
Agent upon the host are not set forth, since such
action per se i5 known.
Example 1
This example illustrates the preparation of
a dual microcapsule in which a mini-microcapsule
having India Ink encapsulated in a CAB membrane and
a saline solution are encapsulated in a CAB membrane.
A charge of lo 6 grams of particulate CAB was made to
a 400 ml extraction flask which contained 40 ml of
n-hexane. A charge of 130 ml of methylene chloride
ldichloromethane, MDC) was added to the dispersion.
The sys~em was continuously agitated with a GT21
variable speed laboratory stirrer and stirring rod
set at a speed of 2~5 ~fast drive gear ratio~. The
polymer tCAB) dissolved in less than a minute, at
which time 15 grams of an aqueous core material were
-23-
added~ The aqueous core was composed of 13 grams of
phosphate buffered saline solution plus 2 grams (wet
weight) of previously prepared mini microcapsules,
having aVeEage diameters of less than 105 ~m, and
having an India Ink suspension encapsulated therein.
The India Ink containing mini-microcapsules were
used to allow the formed dual microcapsules to be
photographed. After the aqueous core droplets were
dispersed, 86 ml of n-hexane was added gradually
over a thirty minute period (about 3 ml/minute).
The action caused phase separation of the CAB and
encapsulation of the mini-microcapsules and ~he
saline solution. The dispersion was then siphoned
into a beaker which contained 800 ml of n-hexane.
lS During this transfer, both the dispersion and the
n-hexane were being s~irred with the GT21 variable
speed stirrer and stirring rod set at a speed setting
of 2O5 (fast drive gear ratio). After fifteen min-
utes of stirring, the product was filtered u~ing a
Buchner funnel (about 20 cm diameter)~ filter paper
(#589 Black ribbon) and an aspirator. The filtered
product was humid air-dried for about 24 hours~ The
product was then bottled and stored at room tempera-
ture.
Example_2
This example illustrates the preparation of
a dual microcapsule in which a mini~microcapsule
having India Ink encapsulated in a CAB membrane and
a sallne solution are encapsulated in a PLA membrane~
Make a charge of 1.1 grams of particulate PLA (code
35614-19) to a 250 ml extraction flask containing 75
ml of methylene chloride (dichloromethane, MDC).
-24-
Agitate the system using a GT21 variable speed labo-
ratory stirrer and stirring rod set at a stirrer
speed of 2~5 (fast drive gear ra~io). After the PLA
has completely dissolved, add nonane to the solution
until a cloud-point develops (i.e., ~o the point
where the polymer begins to precipitate)A Approxi-
mately 78 ml will be required. Then add MDC dropwise
with stirring until the solution is clarified. ~dd
an aqueous core containing ten grams of phosphate-
bufered saline solution plus 1 gm (wet weight) ofpreviously prepared mini-microcapsules containing
the India Ink ~uspension. Continue ~igorous stirring
until PLA first precipita~es ~y reason of MDC evapo-
ration. Stir for an additional 6 minutes and siphon
the resulting slurry into a beaker containing 3,S00
ml of n-heptane and surfactant. Stir the n-hep~ane
and the surfactant during transfer using similar
stirring equipment and a stirrer speed of 2.5 (fast
drive gear ratio~ 5tir for an additional 5 minutes,
then recover and dry the dual microcapsules as de~
scribed in Example lo
To demonstrate the effectiveness of the
dual microcapsules of the invention, employing Conju-
gate/Deconjug~ting Agents therein, in controlling
the diffusion rate of a Functional Agent from the
microcapsules, experiments were run employing glucose
~MW = 180 daltons) or glucose precursors as the
Functional Agent.
Four lots of dual microcapsules were pre-
30 paredO In all experiments, CAB was used in both themini-microcapsule membranes and the outer membranesO
In lot "A" a 10% glucose solution was included in
the mini-microcapsules with the suspending liquid
~25-
being a phosphate buffered saline solution (ph =
7.3). Lot l'BI' differed rom lot "A" in that a 10%
dispersion of potato starch sold under the trade
designation DIFC0 was included in the mini-microcap-
sules. Lot "C" differed from lot "B" in that asmall quantity of alpha amylase was included within
the mini-microcapsules to slowly convert the starch
to reducing sugars. Lot l'D" differed from lot 'IC"
in that a small quantity of gluco-amylase was includ-
ed in the ~uspending saline solution. This enzymewill convert reducing sugars to glucose.
The construction of the dual microcapsules
are summarized in Table I together with the pro-
duct(s~ expected to be obtained by diffusion of
their contents through the outer membrane into an
aqueous mediumu
-26-
TABLE I
Interior ~x~erior
Capsule Capsule Capsule Expected
System Contents Contents ~esults
"A" 10% glucose buffer rapid glucose re-
only lease
'IB" 10~ potato buffer no glucose re-
s~arch only lease; no reduc-
ing sugars re-
lease
"C" 10% potato buffer no glucose re-
starch & only lease; slow re-
alpha-amylase ducing sugars re-
lease
"D" 10~ potato gluco prolonged glu-
starch & amylase cose release;
alpha~amylase maybe slow reduc-
ing sugars re-
lease
The dual microcapcules were evaluated in in
vitro experiments. Two grams of capsules were placed
in f ive milliliters of aqueous test solution (pH 5.5
buffer). The solutions were iltered and assayed
for the presene of glucose and reducing sugars.
25 Fresh test solution was added back to the capsules
at each predetermined time point. In this experi-
men~, the reducing suyar test quantifies glucose
plus higher polysaccharides combined, but the slucose
meter used is specif ic for glucose only. The results
of the experiment are shown in Table 2.
13.~
-26a-
~ oo~O
~ _ ~ ~i
.~ oo~pO
1/, 1 .~,,
~ o~
~ ~Oq :-.
c~ a
~` ~2~
~ 27--
The results show that:
1~ Glucose was rapidly released from the
"A" capsules as expected.
2 No carbohydrate was released from the
"B" capsules either as glucose or higher poly-
saccharides, as expected.
3. The addition of alpha-amylase in the
"C" capsules caused the slow release of the higher
polysaccharides without producing glucose.
4. With ~he "D" capsules, glucose was
released, but not over the entire 20-hour period.
The experiment demonstrates the operating
principle of the dual microcapsules. The failure to
release glucose from the "D" capsules over the entire
15 20-hour period doubtlessly resulted from inadequate
glucoamylase activity for the continued hydrolysis
of the higher polysaccharides. This shor~coming can
be corrected by modifying enzyme activity within the
exterior capsule of khe "~" capsules.
As earlier noted, the preferred embodiments
of the invention have a Conjugate (usually dispersed
in an aqueous medium) encapsulated within the mini-
microcapsules and have the Deconjugating Agent dis-
persed in an aqueous medium in the space intermediate
of the mini-microcapsule membrane and the outer
membraneO The Conjugate will diffuse throu~h the
mini microcapsule membrane whenever the dual micro-
capsules contain water. As earlier noted~ to prolong
~he effective shelf-life of the dual capsules/ short-
ly after their preparation they should be dehydrated
to form the Crenate Shape earlier discussed and
ustrated in Fig. lA. The dehydration step can be
carried out by mild heating, drying in a vacuum
oven, or in some cases, simply upon standing in air
~~ ~
-2~-
where the membranes are very permeable to water.
The dual micxocapsules, after being dehydrated,
should be stored in sealed containers and preerably
under anhydrous conditions. Shortly before usel the
dehydrated dual microcapsules will be rehydrated by
being steeped in water.
Al~ernate means can be employed to extend
the shelf-life of the dual microcapsules. One such
techni~ue is to encapsulate the Functional Agent
within a mini-microcapsule whose membrane is essen-
tially totally impermeable to the Functional Agent.
The mini-microcapsule membrane in this embodiment
will be prepared from a polymeric material different
from the polymeri~ material employed to prepare the
outer membrane of the dual microcapsule. ~he polymer
included in the membrane of the mini-microcapsule
will be fabricated from a material which can be
ruptured or degradated by a treatment process having
no corresponding effect upon the outer membrane of
the dual microcapsule. Treatments of the ~ype ~is-
ualized are microwave radiation, ultraviolet radia-
tion, laser radiation, treatment with ultrasonic
vibrations and the like. Alternatively, the membrane
of the mini-microcapsules can be fabricated from a
friable polymer while the outer membrane is fabri-
cated from a flexible polymer. The mini microcap-
sules' walls can be ruptured by application of a
compressive force. In this embodiment of the inven-
tion, of course, the rate of release of the Func-
tional Rate to the host will be controlled solely byits diffusion rate through the outer membrane of the
dual microcapsule.
-29-
The dual microcapsules of the invention can
be employed to deliver various types of medication
to mammals, including both man and domestic animals.
Materials which can be administered effectively
include contraceptive materials, narcotic antago-
nists~ cardiac axrhythmia agents/ chemo~herapeutic
drugs and various veterinary products~
In a modification of the dual capsules
previously described, they can be employed to remove
a toxicant from a host. In this embodiment, the
outer membrane of the dual microcapsule will be at
least semipermeable to the toxicant in the host.
The liquid included within the dual microcapsule
between the outer membrane of the capsule and the
membrane of the mini-microcapsule will con~ain a
Carrier which will react with the toxicant to form a
first intermediate product, which can be either a
true chemical reaction product or a complex formed
between the toxicant and the CarrierO The first
intermediate product then will diffuse through the
membrane of the mini-microcapsule. The mini-micro-
capsule will include therein a rnaterial which reacts
with the first intermediate product to irreversibly
convert the first intermediate produc~ and the ~oxi-
cant contained therein to one or more materialsharmless to the hos~.
The dual microcapsules of the invention
also can be employed to provide a prolonged release
of Func~ional Agents other than drugs. Among the
Functional Agents of the type that can ke delivered
include herbicides, fertili.%ers, growth regulator
substances, deodorizers, pheromones and other like
materials.
-30~
While the processes and products herein
described constitute preferred embodiments of the
invention~ it is to be understood that the invention
is not limited to these precise processes and pro-
ducts, and that changes may be made therein withoutdepartiny from the scope of the invention which is
defined in the appended claims.
What is claimed .is:
