A Basic Guide To Supercritical Carbon Dioxide For - Newsletter 2007 by bnmbgtrtr52


									          A Basic Guide                                    tems such as emulsification, solvent evapora-
                                                           tion, phase separation, spray drying or melt
                                                                                                                                        FIGURE 1
          to Supercritical                                 extrusion require the use of organic solvents
                                                           and/or heat during manufacture. Although
          Carbon Dioxide                                   suitable for many small drug molecules, they
                                                           have a number of significant disadvantages

          for Controlled                                   especially when applied to complex biophar-
                                                           maceutical compounds. For example, melt                                                  n
          Drug Delivery                                    extrusion techniques are unsuitable for heat
                                                           labile molecules. Emulsification techniques
                                                           can lead to low drug entrapment efficien-
                                                           cies, can denature peptides and proteins                   B                      O
          Owen Davies, PhD ,             1                 due to unfolding and aggregation at the oil/                             O
                                                           water (o/w) interfaces and can often result
          Martin Whitaker, PhD1 and                        in potentially toxic levels of residual solvent
          Steve Howdle, PhD2                               in the final product.

           Critical Pharmaceuticals Limited,               More recently supercritical fluids (SCFs)                   C
          BioCity Nottingham, Pennyfoot Street,            have emerged as a promising approach to
          Nottingham, NG1 1DF                                                                                                           O
                                                           encapsulate proteins in polymeric drug de-
            School of Chemistry, University of             livery systems. Carbon dioxide (CO2 ) is the                       O
          Nottingham, University Park, Nottingham,         most commonly used SCF as it is inexpensive,                                     O
          NG7 2RD.                                         has very low toxicity, is non-flammable and                                               O    n
                                                           has easily obtainable critical conditions (31.1
                                                           ºC, 73.8 Bar) [see Figure 2]. Supercritical
                                                           carbon dioxide (scCO2 ) can be exploited
                                                           in the production of drug delivery systems
                                                           in a number of ways which exploit both its              HO                                   OH
                                                           solvent and antisolvent properties.
                                                           The rapid expansion of supercritical solu-
                                                           tions (RESS) utilises the solvent properties
                                                           of scCO2 . In this process the polymer and              Molecular Diagrams of
                                                           solute is solubilised in the SCF and then
                                                                                                                  (A) poly(lactic) acid (PLA),
                                                           expanded through a capillary nozzle into a
          Introduction                                     precipitation chamber. Rapid decompres-                (B) poly(glycolic acid) (PGA),
          Controlled release technologies are becom-       sion of the solution leads to supersatura-             (C) poly(lactic-co-glycolic acid)
          ing an increasing important area of research     tion, nucleation and particle formation. This          (PLGA) and
          for biopharmaceuticals. These molecules,         process can result in the formation of very            (D) poly(ethylene glycol) (PEG)
          due to their short half lives and poor bio-      small uniform particles due to the high su-
          availability when administered via non-          persaturation ratios which can be achieved.
          parenteral routes, often have to be adminis-     Although it has been used to prepare drug
          tered by frequent injections. Consequently,      entrapped into poly( L-lactic acid (PLLA) mi-
          the pharmaceutical industry is turning to        croparticles [3], the main limiting factor of
          advanced drug delivery systems as a means        the RESS process is the low solubility of most
          to improve patient convenience and compli-       pharmaceutical compounds and regulatory
          ance. In some circumstances such systems         approved polymers in scCO2 . To overcome
          can also improve safety, tolerability and effi-   this obstacle co-solvents, which are miscible
          cacy. Controlled release can be achieved via     with the scCO2 , can be used to modify the
          a variety of mechanisms for example drug
          complexation or the use of gels or slowly
          dissolving crystals. This article focuses on
                                                                                                                                        FIGURE 2
          entrapment into biodegradable polymeric
          delivery systems. Potential advantages of                                  Tc CO2 = 31.1ºC
          controlled release approaches over chemi-                                                                                SUPERCRITICAL
          cal modification of the active to extend the                                Pc CO2 = 73.8 bar                                REGION
          half life, for example PEGylation, polysialic
                                                               PRESSURE (atm)

          acid or albumin conjugation, or amino acid
          sequence changes, include shorter devel-
          opment times (as no new chemical entities
          are generated), and extended duration of                                     SOLID
          action.                                                                                                                       CRITICAL POINT

          There are a number of approved pharma-                                                         VAPOUR
          ceutical products which utilise biodegrad-
          able polymers to control the rate of drug
          release within the body [1, 2] [see Figure
          1]. Preparation of these systems requires
          liquefaction of the polymer phase in order                                             TEMPERATURE (°C)
          to encapsulate the therapeutic into the final
          product. Conventional techniques for the
          preparation of polymeric drug delivery sys-       Phase diagram of carbon dioxide
polarity of the supercritical phase and thus     of Supercritical Fluids (SEDS). These tech-     A substantial drawback of the
enhance solute solubility [4]. Nevertheless,     niques use a dense gas as an antisolvent to     antisolvent techniques is that
even in the presence of cosolvents, the sol-     precipitate a solute which is dissolved in an   they require the pharmaceu-
ubility of most pharmaceutical compounds         organic solvent. Precipitation occurs as the    tical agent to be soluble in a
remains too low in scCO2 to make the RESS        gas is absorbed by the organic solvent thus     solvent which is miscible with
a viable process. To circumvent this problem     expanding the liquid phase and reducing the     supercritical carbon dioxide.
supercritical carbon dioxide can be used as      solvent power until nucleation and particle     As water is poorly soluble in
an alternative to conventional solvents to       formation occur. The supercritical antisol-     scCO2 , this limits the use to
coat drug particles with polymer [5]. This       vent techniques offer greater flexibility than   compounds that are soluble in
process takes advantage of the differing solu-   the RESS process and consequently have          organic solvents. While this cre-
bility of drug and polymer in CO2-solvent        been studied more thoroughly for prepara-       ates few problems in the case of
mix. The solubility of the coating material      tion of drug delivery systems.                  low molecular weight hydropho-
is much higher than that of the drug so the                                                      bic compounds, more complex
polymer precipitates onto preformed drug         Antisolvent techniques can be used to en-       hydrophilic agents, such as pro-
particles on decompression. The rapid ex-        capsulate small drug molecules [6] proteins     teins and peptides, are insolu-
pansion from a supercritical solution (usu-      [7] and even DNA [8] into polymeric drug        ble in most organic solvents.
ally restricted to low molecular weight hy-      delivery systems. The majority of drug mi-      Consequently, CO2 miscible
drophobic compounds) with a non-solvent          croencapsulation studies have focussed on       solvents such as dimethyl sul-
(RESS-N) process is such an example. It has      the semi-crystalline polymer PLLA, whereas      phoxide (DMSO) must be used
been used to encapsulate both low molecular      as far fewer studies have successfully pro-     to solubilise the biological mol-
weight drugs and proteins into polymeric mi-     duced drug entrapped microparticles from        ecule. When dissolved in such
croparticles [5]. The RESS process can also      amorphous polymers such as poly( D,L-lac-       solvents proteins can refold and
be used to impregnate polymeric materials        tic-co-glycolic acid (PDLLGA) or poly( D,L-     irreversibly change their second-
with pharmaceutical agents. Impregnation         lactic acid (PDLLA). PLLA has very few ap-      ary structure resulting in loss of
utilises the high diffusivity and low surface    plications in the field of drug delivery due     functional activity and risk of im-
tension of SCFs. Polymer impregnation re-        to extremely long in vivo degradation times     munogenicity. To overcome this
quires the active substance to be soluble        (months to years). For drug delivery sys-       problem protein particles have
in the SCF and the polymer to be swollen         tems faster degrading amorphous polymers        been suspended in a solution of
by the SCF. If the drug has high affinity for     such as PDLLGA and PDLLA are therefore re-      polymer dissolved in organic sol-
the polymer or specific interaction with the      quired. However, processing of amorphous        vent using a modified PCA tech-
polymer, the drug can be molecularly dis-        polymers with carbon dioxide represents a       nique. Suspension of protein in
persed within the polymeric matrix. Drugs        significant challenge. Residual CO2 and or-      organic solvents generally caus-
with low affinity for the polymer can be pre-     ganic solvent can result in reduction of the    es much less degradation than
cipitated and deposited into the matrix on       polymer glass transition temperature (Tg)       dissolution.
decompression. In this scenario the drug will    and particle aggregation and flocculation
re-crystallise in the polymer and will not be    can occur. To overcome the plasticisation       The Particles from Gas-Saturated
molecularly dispersed.                           effect caused by residual CO2 and solvent,      Solutions (PGSS) process [see
                                                 combinations of scCO2 and supercritical ni-     Figure 3] is an alternative SCF
Due to the poor solubility of the major-         trogen have been used. Decreased polymer        processing technology that can
ity of active pharmaceutical substances and      solubility in the CO2-N2 mixture results in     be used to entrap drug particles
approved excipients in scCO2 , many investi-     faster particle solidification rates, less ag-   into polymeric matrices [9]. The
gators have utilised the antisolvent or non-     gregation and hence smaller particles. Drug     PGSS process utilises the high
solvent properties of carbon dioxide. Many       entrapped PLGA microparticles have also         solubility which CO2 has in many
different antisolvent techniques have been       be successfully prepared using a modified        polymers. Solubilised scCO2 can
used for preparation of polymeric drug deliv-    PCA technique in which PLGA, dissolved          plasticize glassy polymers, lead-
ery systems. These include: Gas Antisolvent      in methylene chloride, was sprayed into a       ing to a dramatic decrease in the
(GAS), Supercritical Antisolvent (SAS),          vapour CO2 phase over a liquid CO2 phase.       polymer Tg, causing it to liquefy
Precipitation by Compressed Antisolvent          By controlling the temperature and polymer      at near ambient temperatures.
(PCA), Aerosol Solvent Axtraction System         concentration it was possible to form uni-      Furthermore, the presence of
(ASES) and Solution Enhanced Dispersion          form spherical particles.                       CO2 significantly lowers the
                                                                                                 viscosity of the polymer melt
                                                                                                 which enables drug particles
                                                                                                 to be homogeneously mixed in
FIGURE 3                                                                                         by means of a stirring device.
                                                                                                 In these processes, the drug
                                                                                                 itself is usually not plasticized
                                                                                                 or modified in any way by the
                                                                                                 scCO2 . If the liquefied mixture is
                                                                                                 depressurised through a nozzle
                                                                                                 into a collection chamber, fibres
                                                                                                 or particles can be produced,
                                                                                                 whereas if the process is con-
                                                                                                 ducted in a mould, the polymer
                                                                                                 will foam on depressurisation
                                                                                                 leading to the production of a
                                                                                                 porous scaffold or monolith [9].
                                                                                                 The PGSS process has many ad-
                                                                                                 vantages over other SCF and
                                                                                                 emulsion technologies. These
                                                                                                 include: the absence of solvents
                                                                                                 at any stage during process-
 Schematic of the PGSS process                                                                  ing, the fact that neither the
                                                                                                                                       PAGE 21
          polymer nor drug need be soluble in the         antisolvent techniques) and the high viscos-
          supercritical CO2 and the mild processing       ity of the polymer melt in the case of the
          conditions (typically < 40 ºC and 150 Bar).     PGSS process which makes atomisation dif-
          Furthermore, as there are no solvent re-        ficult. However, as understanding of these
          moval steps, the process is extremely rapid.    technologies increases, their commercial
          The PGSS process is particularly suitable for   exploitation should be realised.
          entrapment of delicate biopharmaceuticals
          such as proteins [9]. A major challenge with    REFERENCES
          the PGSS process in producing drug encap-
          sulated polymeric microparticles occurs due     [1] Okada, H., One- and three-month re-
          to the high viscosity of the liquefied polymer       lease injectable microspheres of the
          and rapid polymer solidification rate on de-         LH- RH superagonist leuprorelin acetate.
          pressurisation. To enable microparticle for-        Advanced Drug Delivery Reviews,
          mation the solidification rate must be tightly       1997. 28(1): p. 43-70.
          controlled so that it is slow enough to al-     [2] Tracy, M.A., Development and scale-up
          low for droplet formation and polymer chain         of a microsphere protein delivery system.
          rearrangement, but fast enough to prevent           Biotechnology Progress, 1998. 14(1):
          aggregation of newly formed particles.              p. 108-115.
                                                          [3] Tom, J.W., et al., Applications Of
          Recently high pressure/supercritical carbon         Supercritical Fluids In The Controlled
          dioxide has been exploited to extract resid-        Release Of Drugs. Acs Symposium
          ual solvents from drug entrapped polymeric          Series, 1993. 514: p. 238-257.
          microparticles [10]. Organic solvents such      [4] Tom, J.W. and P.G. Debenedetti,
          as methylene chloride are generally used            Formation Of Bioerodible Polymeric
          to solubilise biodegradable polymers dur-           Microspheres And Microparticles By
          ing production of drug delivery systems.            Rapid Expansion Of Supercritical
          The USP limit for methylene chloride is             Solutions. Biotechnology Progress,
          600 ppm, a limit which can be difficult to           1991. 7(5): p. 403-411.
          achieve with conventional emulsification         [5] Matsuyama, K., et al., Formation of mi-
          techniques. Solvent extraction utilises the         crocapsules of medicines by the rapid
          high solubility of methylene chloride in sc-        expansion of a supercritical solution
          CO2. Furthermore, the plasticisation effect         with a nonsolvent. Journal Of Applied
          of the carbon dioxide increases the solvent         Polymer Science, 2003. 89(3): p. 742-
          diffusion coefficient and diffusion rates ena-       752.
          bling efficient solvent removal. In a further    [6] Bleich, J., P. Kleinebudde, and B.W.
          embodiment of this process the scCO2 was            Muller, Inf luence Of Gas-Density And
          utilised to not only remove residual solvent        Pressure On Microparticles Produced
          but to also increase porosity and dissolution       With The ASES Process. International
          rate to produce formulations suitable for           Journal Of Pharmaceutics, 1994.
                                                              106(1): p. 77-84
          sustained delivery to the respiratory tract
          [11]. Taking this concept a step further,       [7] Sze Tu L., Dehghani F., and Foster N.R.,
          a new technique called Supercritical Fluid          Micronisation and microencapsulation of
          Extraction of Emulsions (SFEE) has been             pharmaceuticals using a carbon dioxide
          recently described [12]. Indomethacin and           antisolvent. Powder Technology, 2002.
                                                              126: p. 132-149
          ketoprofen were encapsulated into PLGA
          and Eudragit ® RS (PMMA) microparticles         [8] Okamoto, H., et al., Pulmonary gene de-
          using an o/w emulsification technique.               livery by chitosan-pDNA complex powder
          Supercritical carbon dioxide was then used          prepared by a supercritical carbon diox-
          to extract the solvent from the o/w emul-           ide process. Journal Of Pharmaceutical
                                                              Sciences, 2003. 92(2): p. 371-380
          sion to leave drug entrapped microparticles
          or nanoparticles with low residual solvent      [9] Howdle, S.M., et al., Supercritical f luid
          content (<50 ppm). Coalescence was pre-             mixing: preparation of thermally sensitive
          vented by the presence of a stabilising sur-        polymer composites containing bioactive
          factant in the aqueous phase.                       materials. Chemical Communications,
                                                              2001(01): p. 109-110

          Summary                                         [10]     Herberger, J., et al., Carbon di-
                                                             oxide extraction of residual solvents in
          In recent years scCO2 has been increasingly        poly(lactide-co-glycolide) microparticles.
          investigated in the preparation of controlled      Journal Of Controlled Release, 2003.
          drug delivery systems. The mild process-           90(2): p. 181-195
          ing conditions, low toxicity and complete
                                                          [11]    Koushik, K. and U.B. Kompella,
          absence of organic solvents during process-
                                                             Preparation of large porous deslorelin-
          ing (PGSS) makes scCO2 particularly attrac-        PLGA microparticles with reduced re-
          tive for the processing of biopharmaceutical       sidual solvent and cellular uptake using
          compounds. Such processes have clear ad-           a supercritical carbon dioxide process.
          vantages over conventional drug encapsula-         Pharmaceutical Research, 2004. 21(3):
          tion technologies. However, scCO2 can be           p. 524-535
          difficult to work with. In particular, when      [12]    Chattopadhyay, P., R. Huff, and
          used in drug entrapment into amorphous             B.Y. Shekunov, Drug encapsulation
          polyesters, particle production is particu-        using supercritical f luid extraction of
          larly challenging due to plasticizing effects      emulsions. Journal Of Pharmaceutical
          of residual CO2 and solvent (in the case of        Sciences, 2006. 95(3): p. 667-679

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