Inrcmational |ournal of Dcrmntolot^y.. Vol. 35.. No. 1 1., November 1996

REVIEW

LIPOSOMESHARRY H. SHARATA. M.D.. PH.D.. ANO

KENNETH H. KATZ, M.D.

When liposomes were initially discovered more than30 years ago, predictions were largely met with skepti-cism; however, recent advances have allowed the de-velopment of liposomes that are potentially medicallyuseful. Systemic and topical drug delivery by liposomesis possible. Although liposomal amphotericin B, a sys-temic antifungal agent, is the only medical liposomalproduct commercially available in the United States,there have been considerable advances in liposomal de-livery systems with important applications in derma-tology. In tbis article, the basic chemistry of liposotneswill be reviewed as well as their potential uses in der-matology. The basic chemistry and a brief overview ofmethods of preparation will be discussed first. Thenwe will examine the kinetics of systemic and topical li-posomal therapy, as well as hair follicle targeting.Next, the liposomal advances in skin cancer preven-tion, cancer therapy, and gene therapy will be dis-cussed. Finally, specific liposoma! drugs and drugclasses, including topical anesthetics, topical cortico-steroids, topical interferon, liposomal amphotericin B,and artificial blood, will be examined. We will not dis-cuss cosmetic applications of liposome preparations.^

CHEMISTRY AND PHARMACOKINETICS

What is a Liposome?

Alec Bangham discovered liposomes in 1963, wben hedemonstrated tbat pbospbolipids in water form closedvesicles.' The liposome consists of one or more lipidbilayer membranes in whicb the fatty acid tails are em-bedded in the interior and the bydrophilic heads areoriented towards the external aqueous pbase. The lipidbilayers form concentric lamellae, enveloping a seriesof aqueous compartments (Fig. 1). Altbougb most lipo-somes are spherical, the lipid composition and tbemethod of preparation used determine their final

From the Department of Dermatology, University of Wisconsin,Madison Medical School, Madison, Wisconsin.

Address for correspondence: Harry H. Sharata, M.D., Ph.D.,University of Wisconsin, Dermatology Clinic, 2880 UniversityAvenue, Madison, WI 53705.

sbape, size, and tbe number of lamellations acbieved.Hydrophilic drugs are encapsulated witbin the inneraqueous compartments, whereas hydrophobic drugsare embedded in tbe pbospholipid bilayer that makesup tbe liposome. Liposomes can be classified as uni-lamellar or multilamellar, based on the number of con-centric lipid bilayers they contain. Tbey can be furtberclassified according to size and composition of thelipid bilayers. The lipid cotnposition and tbe method ofliposome preparation directly affects drug absorption,distribution, metabolism, and elimination, as well astbe toxicity profile.

Several methods of liposome preparation can be em-ployed. They include: simple mecbanical agitation, soni-cation, conventional bydration, debydration-rebydra-tion, and reverse-pbase evaporation. Altbough rnostliposomes are spherical, tbe method of preparation used,along with the lipid composition, organic additives, thepH of tbe aqueous solution, and the tetnperature ofpreparation, all determine the size, shape, and numberof lamellae in tbe liposome. Drug incorporation is ac-complished by adding hydrophobic drugs to tbe lipidmixture during preparation or adding hydropbilic drugsto the aqueous salt solution during preparation. In tbisfasbion, bydrophobic drugs are incorporated into tbelipid bilayer and bydrophilic drugs are packaged witbinthe interior aqueous compartments.

The simple mecbanical agitation method involvesmechanical shaking of tbe component lipids and anaqueous salt solution in a flask. This metbod producesa heterogeneous mixture of large multilamellar vesiclesof varying sizes. Altbougb it is tbe simplest preparationmetbod, it does not produce a uniform liposome size,so tbat it is not an ideal metbod for drug preparation.'

Sonication of tbese large multilamellar liposomescan be used to make small unilamellar liposomes ofuniform size. Once tbe large multilamellar liposomesare made, they are exposed to prolonged ultrasoundradiation that disperses tbem into small unilamellar li-posomes. These small liposotnies are tben separatedfrom the large multilamellar liposomes by gel-filtrationto provide a homogeneous preparation.'

Tbe "film-hydration metbod" involves dissolving thelipids in ati orgatiic solvent wbicb is then allowed toevaporate, leaving a film bebind. Tbe film is tben hy-drated by addition of an aqueous salt solution at a tem-

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IlllHiuimwmFigure 1. A trilamellar liposome. The lipid bilayers (A) con-tain the component lipids, organic additives, and hydrophobicdrugs. The aqueous inner compartments (B) contain an aque-ous salt solution and hydrophilic drugs. The insert depicts thelipid bilayer with the various incorporation sites for drugs oradditives. The bilayer consists of amphipathic lipids with thehydrophobic tails buried in the interior and the hydrophilicheads comprising the outer surface. The drugs or additivesmay be completely contained within the hydrophobic interiorof the bilayer, associated with surface hydrophilic heads, in-tercalated between the lipids traversing both hydrophilic andhydrophobic regions, or free in the aqueous medium.

perature above the lipid-pbase-transition temperature.Once hydrated, the film is bomogeneously dispersedusing a sonicator to produce multilamellar liposomes.^'''

The "dehydration-rehydration method" involvesdissolving the lipids in an organic solvent. The solventis then allowed to evaporate and the resulting film ishydrated in an aqueous salt solution and then dehy-drated under a vacuum. Once the solution becomesviscous, water is added in a volume equal to that re-moved and the rebydrated liposomes are equalibratedat a temperature above the phase-transition tempera-ture prior to storage.'''''

The "reverse-phase-evaporatioti method" is used toprepare large unilamellar liposomes. It involves mixingthe lipids, aqueous salt solution, and organic solventstogether and sonicating their mixture. After sonication,the solvents and a small quantity of water are removedunder nitrogen at a temperature above the phase-tran-sition temperature of the lipids, leaving the large uni-lamellar liposomes behind.^'''

Systemic Liposomal Kinetics

The liposomes originally used in medical research werereferred to as conventional liposomes. These bad limit-ed medical utility because once tbey were present in theblood stream, they were rapidly phagocytosed by thecells of the reticuloendothelial system (RES).'' Their ther-apeutic potential was thus confined to diseases affectingthe RES (such as lymphomas or parasitic infestations).Today, liposomes can be engineered to minimize thepreferential uptake by the RES. This is accomplished byincorporating glycolipids or ethylene glycol into thelipid bilayer, which forms a steric barrier of highly hy-drated groups around the liposome. Tbe steric barrierdecreases endocytosis by the RES by inhibiting electro-static and hydrophobic interactions. Furthermore, in-corporation of monosialogangliosides, such as GMl,confers a negative surface charge to the liposome andincreases its hydrophilicity. The steric barrier and nega-tive surface charge result in a 100-fold increase inblood circulation time.̂ "** This results in greater andlonger lasting circulating drug levels with lower totaldoses administered. This, in turn, lowers directly thetoxicity for drugs with a narrow toxicity index.

The size of liposomes affects drug deposition and, inthis manner, specific tissues may be targeted. A typicallarge multilamellar liposome can be as large as 20 pmin diameter, whereas a small unilamellar liposome istypically 0.1 pm to 0.15 pm in diameter.'' In compari-son, the typical diameter of an erythrocyte is 7.5 [am,that of a mature melanin granule is 0.4 pm, and an in-termediate filament is 0.01 pm (100 A).'" Small lipo-somes extravasate from capillaries more easily and ac-cumulate in tissues, whereas larger liposomes remain incirculation longer and are taken up by reticuloendothe-lial cells." After endocytosis by the cells of the RES,there is fusion with intracytoplasmic lysosomes withsubsequent release of the drug.'^ In this manner, drugsmay be targeted to the RKS.

Selective targeting of other tissues has been achievedby incorporation of monoclonal antibodies into theouter lipid membrane of small liposomes. In this man-ner. Heath et al. have been able specifically to targetmethotrexate-y-aspartate, an inhibitor of dihydrofolatereductase, to cultured fibroblasts. Although animalstudies have not yet been done, one could anticipateintroducing intravenous liposomes that extravasatethroughout the body, where the antibodies specifically

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attach to the target tissue and cause accumulation ofliposomes at the target site.'•'•'''

Topical Liposomal Kinetics

The stratutn corneutn is the body's protective layer ofskin that prevents the deeper layers frotn dehydratingand external chemicals and bacteria frotn entering theskin. Typically, conventional dosage forms, such as so-lutions, creams, and ointments, deliver drugs in a con-centration-dependent manner following Fick's law ofpassive diffusion across the dead stratum corneum.Multilatnellar liposomes can deliver drugs within 30minutes to the stratum corneum, epidermis, and dermisin significantly higher concentrations than conventionalpreparations. For example, dertnal drug levels deliveredby liposomal hydrocortisone are 9 to 14 times higherthan after exposure to conventional hydrocortisonepreparations.''''"' Observations of penetration of radio-labeled liposomes through pig and mouse skin haveshowti that the ratio of radiolabeled lipids changesfrotn the stratum corneum to the deeper epidermis.This suggests that the liposomal lipid assembly disag-gregates in the stratum corneum and the componentlipids diffuse independently into the deeper layers of theskin. Independent diffusion would be anticipated whenthe lipids are released frotn the liposome, because of thevariable polarity of lipids.'^

The drug is delivered to the deeper epidertnal layersthrough dehydration of the liposomes within the stra-tum cortieutn. Paradoxically, wlieti larger quatitities orhigher concentrations of liposomes are applied, there isa decrease in the atnount of drug that penetrates to thedeeper layers of skin. A possible explanation for this is,that liposotne uptake is driven by the hydration gradi-ent that exists across the epidermis, stratum corneum,and ambient attnosphere.'^ In this tnanner, a thinly ap-plied film with a lower concentration of liposotnes maypenetrate faster and to a greater extent than a thicker,more concentrated, layer. The less concentrated lipo-some film will dehydrate faster and thus be drivenacross the hydration gradient faster than the more con-centrated preparation.-''"* In support of this explanatiotiis the observation that occlusion results in less drug ab-sorption due to abolition of the hydration gradient.'**

The melting point of the lipids contained in the li-posome also affects the depth of penetration. Higherconcentrations of long-chain alcohols in the hydro-philic:hydrophobic interface decrease the transitiontemperature of the lipid chain-melting phas.e by parti-tioning the head group region and altering the phos-pholipid interdigitation.'' Liposomal formulations,using glyceryl dilaurate in the lipid bilayer, are able todeposit ten times as much drug in the deeper layers ofthe epidermis than the statidard topical liposomalpreparations. The glyceryl dilaurate has a melting tem-perature of 30°C, which is just below the normal skin

temperature. The increased penetration may be due toa phase transition of the iipid bilayer, in which there ispartial melting of the liposome. This would put thedrug in direct contact with the stratum corneum,where it penetrates through to the epidermis.""

The hydrogen ion content of the liposomal prepara-tion also effects penetration. Basic solutions, with ahigher pH, cause deprotonation of the lipid molecules.This increases the interbilayer repulsion and the watersolubility of the lipids that, in turn, decreases the chain-melting phase transition temperature. The deprotonat-ed lipids also increase the stability of the liposome insolutioti. There is less liposotnal aggregation and the li-posotnes tnay be tnaintained with a smaller diameter;however, the increased stability results in a lower effi-ciency in membrane fusion and decreases the ability ofthe liposome to penetrate the stratutn corneutn.-'

The manner in which the drug is encapsulated intothe liposotne also seems to affect the positioning of thedrug and alters the efficiency of drug delivery to theskin. Studies indicate that drug loading by dehydrationand subsequent rehydration of the liposome creates aliposome that delivers its drug tnore efficiently than li-posomes prepared by other tnethods.-^-'--^

An advantage to topical drug delivery by liposotnesis an increased uptake by the skin without a propor-tionate increase in systetnic absorption. Less than0.1% of the applied dose is absorbed into the systemiccitculation despite the rapid penetratioti of liposomallyprepared drugs into the epidermis. This potentiallylitnits systemic toxicity.• '̂'

Hair Follicle and Pilosebaceons Targeting

Many drugs are absorbed to a greater extent from areasof skin with large or great numbers of hair follicles be-cause the drugs are shunted through the pilosebaceousunit (Fig. 2).-̂ "̂̂ ^ Multilatnellar liposotnes have an evengreater ability to penetrate the skin through these follic-ular orifices.̂ ** The lipids that form the bilayer tnetn-brane do not show any increased penetration whenthey are applied as a simple emulsion, but only whenthey are organized as latnellar assetnblies. Liposotnalpreparations enhance drug delivery to the follicle five toeight times over standard aqueous preparations.-' Vari-ations in the size and lipid cotnposition of the liposotnecan affect whether absorption retnains at the upper lay-ers of the skin or in the liposome when it petietrates tothe base of the hair follicle.'"

The preferential absorption of tnultilatnellar lipo-somes into the pilosebaceous unit can be exploited totarget hair follicles and sebaceous glands. Recently, thedelivery of intact DNA to the hair follicle and evidencefor active getie transfer into the hair shaft atid follicleshave been detnonstrated in tissue cultures of skin."Melanin can also be delivered specifically to hair folli-cles. When tnelanin is entrapped in a phosphatidyl-

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Figure 2. Many drugs, including liposomes, are preferen-tially shunted down hair follicles (A), where the absorption isgreatest. Shunting occurs to a lesser extent through eccrinesweat glands (B). The transdermal route (C) is the least effi-cient for drug delivery.

choline liposome, tbe liposome penetrates tbe piloseba-ceous unit and deposits the melanin botb at tbe periph-ery of tbe follicles and within the follicle cells them-selves. Microscopic examination of tbe hair sbaft ofwhite-baired mice treated witb liposomal melanindemonstrated a band-like melanin-distribution pattern,similar to the melanin-distribution pattern that is seen inbair colored by endogenously produced melanin. Thus,in addition to deposition of the melanin witbin the hairfollicle and sbaft, tbe normal pattern of melanin distrib-ution is replicated by liposomal delivery of melanin.'^

SELECTED CLINICAL Al'PLICATlONS

Prevention of Skin Cancer

Exposure to ultraviolet (uv) ligbt has clearly been linkedto tbe development of skin cancer in bumans. Ultravioletradiation causes tbe formation of cyclobutane-pyrimi-dine dimers in tbe DNA. These dimers are tbe initialmolecular change tbat ultimately leads to tbe carcino-genic transformation of tbe cell." Most people are ableto repair most of the DNA damage witb excision-repairendonucleases and, therefore, develop only very fewskin cancers.

Patients with xeroderma pigmentosum and animalmodels have deficiencies in excision-repair endonucle-

ases. Tberefore, tbey are unable to excise tbe cyclobu-tane-pyrimidine dimers that accumulate witb ultravio-let ligbt exposure. Tbese patients develop many cuta-neous, ocular, and neurologic abnormalities of a higherincidence than the rest of tbe population. One balf oftbe patients with xeroderma pigmentosum will developa skin cancer, predominantly in sun-exposed areas, byage 10 years. This is 50 years before the remainder oftbe population will do so.'"*

Endonuclease V is an enzyme produced by tbe denVgene of bacteriophage T4. The purified product, T4-en-donuclease V, produces single-stranded breaks in DNAat cyclobutane dimers by cleaving the N-glycosyl bondon tbe 5'-side of the dimer. Tbe pbosphodiester bondbetween tbe two pyrimidines is tben cleaved and theDNA strand is returned to tbe state before ultraviolet ir-radiation.-'^ Tbe T4-endonuclease V can be entrappedin botb positively and negatively charged liposornes anddelivered in its active form to normal persons and tokeratinocytes in culture of patients witb xeroderma pig-tnentosum, to buman skin explants, and to live mice."'In the recipients of liposomal T4-endonuclease V, thereis an increase in DNA synthesis and increased excisionand repair of cyclobutane-pyrimidine dimers. Liposo-mal delivery of the enzyme is far more efficient tban celltransfection or permeabilization.^'''"* The liposome ap-plied topically penetrates tbe stratum corneum and theepidermis of murine skin. Both tbe liposome and tbeenzyme can be found in basal keratinocytes.^''' Wben itis applied to tbe skin of mice irradiated witb UV rays,there is a dose-depetidetit decrease in the number of cy-clobutane-pyrimidine dimers and a subsequent 45% de-crease in the incidence of skin cancer.̂ '*

Cancer Therapy

Liposomes have bad a significant impact on research incancer treatment. Liposomes can be used to alter tbemolecular makeup of tumor cells to make tbem tnoreaccessible to tbe body's own immune .system. They canbe used to target tumors for systemic cbemotberapeuticagents preferentially, increasing tbeir therapeutic index.Thus, efficacy can be enhanced, wbereas toxicity is de-creased by using liposomal drug preparations, such as li-posomal vincristine.""' Finally, liposomes can be used toprotect against common side effects of chemotberapy.

Genes for human transplantation-antigens can bepackaged within liposomes and injected directly intornalignant melanoma nodules, wbere the genes aretaken up and actively expressed by the local tumorcells. In a human clinical trial, five HLA-B7-negative pa-tients witb stage IV melanoma, refractory to conven-tional treatment, had one to three of tbeir melanomanodules injected with liposomes containing tbe gene en-coding tbe HLA-B7 tnajor histocompatibility-cotiiplexprotein. Immunochemical staining of injected nodulesconfirtned tbat tbe HLA-B7 protein was actively ex-

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pressed by cells of the injected melatioma nodule. Analy-sis using PCR sensitive to < 2 pg per niL, failed to detectatiy uptake of the liposomal DNA into tbe systemic cir-culation. No anti-DNA antibodies or evidence of sys-tetnic toxicity were observed. One of tbe five patientstreated, had a complete regression of the treated nod-ules as well as regression of several distant, untreatednodules. A possible explanation is, that tbe transplanta-tioti antigens expressed oti the tumor cells allow tbe pa-tient's immune system to recognize them as foreign andto attack tbe tumor successfully. Tbe regressioti of un-treated nodules suggests that the immune response trig-gered by the treated nodule also enhances tbe immuneresponse to otber antigens of the tnelanoma tumor.*"

Early attempts at developing tumor vaccines werelargely unsuccessful, because tutnor atitigetis areprocessed in association with MHC class II tnolecules.Thus, tbey generate an antibody response ratber tbanactivate cytotoxic T lympbocytes that can attack tbetumor tnore aggressively. Tumor specific antigens pre-pared in liposomes are engulfed by macropbages andprocessed witb MHC class I that activates cytotoxic Tlymphocytes. In studies on muritie tbytnoma, injectionof liposomes containing thymoma proteitis resulted intbe regression or elimitiation of the tumors with thecorresponding surface antigens.''^

Tumors tend to have ati increased vascularity cotn-pared witb normal surrounding tissue and the capillar-ies witbin tumors tend to be "leakier" than normalcapillaries. This altered vascularity of tutnors allowsincreased transcytosis and extravasation of small lipo-somes from the blood stream causing the liposomes tolocalize in skin tutnors as well as in the surroundingnormal skin. The liposotnes are found both free in nbr-mal skin, in the tumor, and in the endosotnes and lyso-somes of perivascular macropbages. The localizationof liposomes within tumors allows a lower adminis-tered dose to getierate an equal tberapeutic effect.Studies in transgenic mice that develop a Kaposi's sar-coma-like dertnal lesioti have cotifirmed tbat stericallystabilized liposomes localize in early and tnature Ka-posi-like lesions as well as in the surrounding tissue.**'

Using a microwave device, topical hyperthertnia canenhance the release of drugs encapsulated in liposomesin the skin or subcutaneous tissue.'''* Wben solid pbasepbospholipids and cholesterol constitute more thanhalf of the liposomal membrane, the resulting lipo-somes release significantly more drug at 42°C than at37°C.''' The local hyperthertnia also increases vascularpermeability, allowitig for increased extravasation anddeposition of the drug in tbe heated tissue.'*'' Further-more, tutnors, especially those in skiti and muscle, tendto heat more rapidly than the surrounding nortnal tis-sues.''^ These characteristics of local hyperthermia andliposomes lead to an increased accutnulation of lipo-somes at the targeted tissue as well as an enhanced re-lease of tbe drug at that site.'*'*

Anagen effluvium is a cotnmon and unpleasant sideeffect of many cancer chemotherapeutic drugs. A mon-oclonal antibody to doxoruhicin, wben packaged iti atnultilamellar liposome and applied to bairy areas, de-livers active antibody to tbe hair follicle and the sur-routiditig epidertnis. Tbe antibody acts locally to binddoxorubicin and inliibit its effect on tbe bair follicle.The presetice of tbe atitidoxorubiciti antibody, sur-rounding tbe bair follicle at the titne of peak systetnicdoxorubicin levels, prevents drug-induced alopecia inanitnal tnodels."***

Gene Therapy

Liposomes can be used for topical and systetnic genetherapy; they can be designed to transfer active, intactDNA specifically to bair follicles atid cati deliver genesto correct tbe deficiency iti respiratory epithelial chlo-ride tratisport iti cystic fibrosis. The reticuloetidothelialsystetn and hetnatopoietic cells of the bone tnarrowcan also be targeted for gene therapy.

Topical gene transfer to hair follicles also has beenaccotnplished. Lecithin-containing liposomes can beused to entrap high-molecular-weight DNA. Tbe entrap-tnent within the liposotne protects tbe DNA from natu-rally occurring DNase present in tbe skin. Tbis allowsthe DNA to penetrate to its target and be incorporatedwitbout degradation.•*'' In bistocultured skin, the lecithinliposomal DNA preparations are preferentially shunteddown the pilosebaceous utiit, wbere tbey actively andspecifically tratisfer the geties to the hair follicles.-"•"'"

Topical gene transfer to respiratory epithelial cellshas also been accomplished. Patients with cystic fibro-sis bave a defect in cAMP-regulated Cl"-cbannels on tbeapical aspect of respiratory and ititestinal epithelialcells as well as in the sweat glands. Respiratory cotn-plications are the tnajor cause of fatality iti tbese pa-tietits. The gene CFTR eticodes tbe proteitis that cotn-prise tbe cAMi>-regulated Cl"channels. The CFTR DNAcat! be incorporated into a liposotne atid be activelytransferred to respiratory epithelial cells. Wben appliedto the nasal epithelium of patients with cystic fibrosis,tbere is a 20% restoratioti of futictioti of the cAMP-reg-ulated Cl"-channels without any toxicity; however, theeffect is confitied to the area of spray application iti theairway. Further studies are needed to assess whetherthe entire respiratory epithelium can be treated.^'

Tbe immune system can be targeted for systemicgetie therapy. Liposomes containing DNA can be inject-ed intraperitoneally. Following drainage by the lytn-phatic capillaries, the liposotnes are delivered to T lytn-phocytes located in lytnpb nodes and spleen thatphagocytose the liposomes and incorporate the activeDNA. This delivery systetn also tratisfects active DNAinto the hetnatopoietic cells of the bone tnarrow. Thereis no systetnic toxicity associated witb this procedure.This has been accotnplished in a tnurine tnodel. Thus,

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liposomal DNA potentially provides an effective andsafe mechanism for the delivery of genes to the reticu-loendothelial system.^^

SELECTED THERAPEUTIC AGENTS

Topical Anesthesia

Local anesthetics are commonly used in dermatologicsurgery, but the potent anesthetic effect is preceded bythe pain of injection. The eutectic mixture of lidocaineand prilocaine is the only effective transcutaneouslyabsorbed local anesthetic currently available in theUnited States. It requires application with occlusion 1hour prior to the procedure with only a mild anestheticeffect. An injection of local lidocaine through the mild-ly anesthetized area is usually required before the pro-cedure. Thus, it is not very practical in an outpatientdermatologic practice.

Liposomal anesthetics can provide reliable, deepanesthesia without injection by needle. Liposomaltetracaine can be applied 1 hour prior to a procedurewithout occlusion and will provide sufficient, localanesthesia at the site of removal of a skin lesion or forminor plastic surgery. Although the onset is slow, thereis deep anesthesia that lasts for more than 4 hours.'' Asecond effective topical anesthetic is a lipid-detergentliposomal variation encapsulating lidocaine. Thispreparation provides reliable, deep, local anesthesia atthe site of application within 15 minutes after applica-tion; however, the effect is not persistent and diminishesafter about 30 minutes. This may be sufficiently longto perform punch and shave biopsies or for venipunc-ture. For longer procedures, repeated drug applicationswould be required.''''

Topical Corticosteroids

Hydrocortisone and other corticosteroids are the thera-peutic workhorses of dermatology. High-potency corti-costeroids are very effective in modulating inflammatoryresponses; however, their long-term application is limit-ed by their local and systemic side effects such as atro-phy of the skin,'-''^^ formation of striae,'^ purpura,'''*"''glaucoma, cataracts,^" suppression of the hypothalamic-pituitary-adrenal axis,'"'-''̂ and even avascular necrosisof the femur.''" Although hydrocortisone is safer thanfluorinated corticosteroids for long-term therapy, itslow potency is often inadequate to treat moderate orsevere inflammatory reactions. The penetration kineticsof hydrocortisone are greatly enhanced by encapsulat-ing it in liposomes.''-'

Liposomal hydrocortisone preparations providedrug concentrations that are 8 to 14 times higher inthe epidermis and dermis than supplied by conventionaltopical hydrocortisone preparations.'''' This effect is evenmore dramatic than those of hydrocortisone creams

employing penetration enhancers such as urea.''''''' Theincreased epidermal and dermal concentrations deliv-ered by liposomal hydrocortisone preparations havethe potential advantage of lower toxicity with en-hanced efficacy.''''

Topical Interferon

The incidence of herpes simplex infection in the UnitedStates is 10% per year.'''' Potentially, interferon is ableto prevent herpes simplex from occurring. In a random-ized, double-blind, placebo-controlled study of 76 pa-tients with severe recurrent herpes simplex infections,those treated with interferon-ai, 3 x 1 0 ' ' IU subcuta-neously three times per week, had 33% fewer occur-rences, milder symptoms, a 25% faster resolution of anepisode, and an 81% longer interval between episodes.A lower dose of 1 x 10'' lU subcutaneously, three timesper week, was not significantly effective. Unfortunately,the higher, effective dose caused severe adverse effectsin 35% of patients and moderately adverse effects in48% of patients. The adverse effects included malaise,fever, myalgia, arthralgia, and fatigue, despite adminis-tration of acetaminophen, 650 mg every 4 hours. Thepatients also developed a mild reversible leukopeniawith a 33% decrease in granulocyte counts and a 20%decrease in lymphocyte counts.'''' A second study of 69women with recurrent herpes simplex infection con-firmed the above results. The women treated with 5 x10'' IU per kg subcutaneously for 12 doses over 14 dayshad a decrease in the size of their lesions, but also had amoderate but reversible neutropenia.''" The relativelysevere adverse effects of subcutaneously injected inter-feron make it unsuitable for the general treatment ofherpes simplex infections.

The aim of topical therapy is to achieve adequatedrug levels in the stratum basale of the epidermis wherethe active herpetic infection takes place. Topical applica-tion of standard interferon solution or emulsion doesnot offer any benefits in the treatment of herpes simplexdue to the low extent of absorption. Interferon encapsu-lated in liposomes, consisting of lipids in concentrationssimilar to those found in the stratum corneum, is deliv-ered in substantial quantities to the epidermis. This re-sults in a reduction in the number of herpetic vesiclesthat develop in hamsters inoculated with the herpes sim-plex virus.^^ This may provide a safe and effective mech-anism for topical treatment of herpes simplex.

Liposomal Amphotericin B

Amphotericin B preferentially binds to ergosterol foundin fungal membranes and disrupts the integrity of cells.This potent fungal cytotoxic effect makes it the drug ofchoice for most serious fungal infections. Its major limi-tations are its toxicity, including chills, fever, andnephrotoxicity that frequently necessitate early discon-tinuation of the drug; however, amphotericin B can be

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incorporated into stnall unilamellar liposotnes. In thisfortn, it is better tolerated by patients with very few ofthem developing fever and chills.*'-^" Liposotnal am-photericin B provides sustained serum concentrationsas tnuch as five times higher with a longer half-life thanconventional amphotericin B'.''' Despite daily dosesranging from 4 to 6 mg per kg and higher serum con-centrations, the toxicity to brain, kidney, and heart issignificantly lower.^°'''-'^^ The liposotnal forrn remainshighly effective in treating systemic fungal infectionswith response rates of nearly 60% in treating invasiveaspergillosis and systetnic candidosis.^'*

Artificial Blood

A novel applicatioti of liposomes is their encapsulatinghemoglobin to fortn neohemocytes. These are madelarge enough that they do not extravasate, but stnallenough that they can pass through nortnal and slightlyconstricted capillaries. In animal experiments, nearly50% of atiitnals survived a 95% exchange transfusionreplacing their blood with neohetnocytes. The neohe-mocytes do not cause any acute toxicity and have theobvious advantages of prolonged shelf-life and a de-creased risk of viral transmission. These appear to bean ideal substitute for red blood cells in the treatmentof trauma.^'

CONCLUSIONS

Liposomal technology is advancing rapidly and promisesto deliver tnajor advances in many tnedical fieldst Indermatology, we can anticipate drugs to help preventand treat skin cancer tnore effectively. We tnay seedrugs specifically targeted to hair follicles and seba-ceous glands. Gene therapy tnay be used to alter hairgrowth. Effective topical anesthetics have been devel-oped, as well as corticosteroids with enhanced efficacyand reduced toxicity. We may have a safe, effective top-ical treatment for herpes simplex. Although liposomalatnphotericin B is the only cotntnercially availableproduct in the United States, tbe future clearly holdsmany applications for liposomes in dertnatology.

DRUG NAMES

The eutectic mixture of lidocaine and prilocaine isEMLA cream.

Liposomal Amphotericin B is Ambisone.

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3. Gregoriadis G, ed. Technology for tnonitoring topicallyapplied liposotiies. Liposome technology. 2nd Ed. Lon-don: GRG Press, 1993:107-138.

4. Kittayanoiid D, Ratiiachandran G, Weincr N. Develop-tiieut of a model of the lipid constituent phase of thestratutn corneutn: 1. preparation and characterizationof "skin lipid" liposotnes using synthetic lipids. J SocGostnct Ghetn 1992; 43:149-160.

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No. Ht8. 1000.n.76. 600,11.25. Hm.

From the collection of the Indiana MedicalHistory Museum, Katherine McDonnell,Director, Indianapolis, Indiana.

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