Technical Field of the Invention
The present invention relates to topical pharmaceutical formulation of acyclovir suitable for the treatment of viral infections of skin and mucosa.
Background of the invention
Acyclovir is a synthetic purine nucleoside analogue active against herpes viruses and is a subject of UK Patent No. 1523865. Chemically, it is 2-amino-1, 9-dihydro-9-[(2-hydroxyethoxy)methyl]-6H-purin-6-one. It is used for the acute treatment of herpes zoster (shingles), treatment of initial episodes and the management of recurrent episodes of genital herpes and chickenpox (varicella). A major problem of formulating acyclovir dosage forms is its low solubility in water and complete insolubility in hydrophobic solvent systems. For an active ingredient to get absorbed into the biological system, it should be present in solution form and that too in sufficient concentration so that an optimum concentration gradient is set up at the absorption site. Apart from this, other concerns associated with topical formulations are stability and skin irritation. US Patent No. 4,963,555, listed in the Orange Book against ZOVIRAX® acyclovir cream, discloses the method of maximizing the solubilized concentration of drug i.e. acyclovir in the aqueous phase of oil-in-water emulsion by using a high concentration of a polyhydric alcohol, for example propylene glycol as a cosolvent in the aqueous phase. The patent further discloses a concentration of 30% - 50% w/w of aqueous phase of propylene glycol in order to attain an increased concentration of solubilized acyclovir.
According to US 4,963,555 patent, propylene glycol not only acts as a solubilizer but also aids in permeation of the drug into the blood circulation. However, is known to cause skin irritation, which effect is extensively covered in literature. It would be therefore, always desirable to have concentration of propylene glycol in topical formulations lower enough so as not to cause any skin irritation and at the same time adequate enough to solubilize the drug and enhance its permeation into the skin.
We have surprisingly found that propylene glycol even if used at a concentration as low as 10-20% w/w, of the aqueous phase can provide as much as necessary amount of the solubilized drug in the aqueous phase leading to an activity and efficacy of the formulation similar to that of the marketed acyclovir cream, "ZOVIRAX®".

We hereby disclose a topical pharmaceutical formulation of acyclovir with 10-20% w/w, preferably 15% w/w of polyhydric alcohol, for example, propylene glycol calculated w.r.t. the aqueous phase of oil-in-water emulsion having activity and efficacy similar to that of the marketed acyclovir cream, "ZOVIRAX®".
Summary of the Invention
In one general aspect, the present invention relates to a topical pharmaceutical formulation of acyclovir or a salt or ester thereof in the form of an oil-in-water emulsion containing in the aqueous phase 10-20% w/w of a water miscible polyhydric alcohol and solubilized acyclovir or a salt or ester thereof.
In another general aspect, it relates to a topical pharmaceutical formulation of acyclovir or a pharmaceutically acceptable salt thereof in the form of an oil-in-water emulsion containing in the aqueous phase 10-20% w/w of a water miscible polyhydric alcohol and solubilized acyclovir salt or ester thereof, wherein the polyhydric alcohol is selected from one or more of glycols and macrogols including propylene glycol, butane 1, 3-diol, polyethylene glycol and glycerol.
In another general aspect, it relates to a topical pharmaceutical formulation of acyclovir or a pharmaceutically acceptable salt thereof in the form of an oil-in-water emulsion containing in the aqueous phase 10-20% w/w of a water miscible polyhydric alcohol and solubilized acyclovir salt or ester thereof, at least 50% w/w of water and wherein the polyhydric alcohol is selected from one or more of glycols and macrogols including propylene glycol, butane 1, 3-diol, polyethylene glycol and glycerol.
In another general aspect, it relates to a method of preparation of the topical pharmaceutical formulation of acyclovir or a salt or ester thereof containing in the aqueous phase 10-20% w/w of a water miscible polyhydric alcohol and solubilized acyclovir salt or ester thereof, wherein the method comprises formulating a part of the aqueous phase as an emulsion followed by the addition of the balance of water and acyclovir as a dispersion into the emulsion such that the aqueous phase contains at least 50% w/w of water.

In yet another general aspect, it relates to a method of treatment of viral infections, wherein the method comprises topical administration of a pharmaceutical formulation of acyclovir or a pharmaceutically acceptable salt thereof in the form of an oil-in-water emulsion containing in the aqueous phase 10-20% w/w of a water miscible polyhydric alcohol and solubilized acyclovir or a salt or ester thereof.
Detailed Description of the Invention
Topical pharmaceutical formulation of acyclovir in the form of an oil-in-water emulsion is known from the US Patent No. 4,963,555, which discloses the incorporation of 30% -50% w/w of a polyhydric alcohol in order to maintain a high concentration of drug in solubilized form in the aqueous phase of the formulation. The polyhydric alcohols especially the much-used propylene glycol are known to be associated with their inalienable property of causing skin irritation, which apparently would be more at higher concentrations. It is therefore always desirable to have a concentration of polyhydric alcohol in topical formulations, which is sufficient enough to promote solubilization as well as permeation of the drug and low enough so as not to cause any skin irritation. In the present invention, we have found that the polyhydric alcohol even at the concentration of as low as 10-20% w/w of the aqueous phase results in a formulation which is bioequivalent to the innovator's marketed version of acyclovir cream i.e. ZOVIRAX®, and has similar activity and efficacy. As the concentration of propylene glycol is reduced, the skin irritation is also expected to reduce proportionately.
The formulation of the present invention was compared with ZOVIRAX® in term of rate of release and it showed in vitro permeation kinetics akin to that of the innovator when evaluated in Franz Diffusion Cell using human cadaver skin. The formulation passed equivalence test based on rate of release (Confidence Interval: 87.685% - 108.522%) (Figure 1). Similarly, both the formulations were compared in terms of total in vitro percutaneous absorption and results were evaluated statistically. The statistical T-test (p=0.6678) showed no significant difference in the amount absorbed through skin (Figure 2).
The actual concentration of acyclovir in the topical pharmaceutical formulation may be varied to an amount determined by the degree of infection of a particular subject provided that the amount is antivirally effective and non-toxic. Typically, the topical

formulation may comprise from about 0.075% to about 10% w/w of acyclovir or salt or ester thereof. The term "solubilized acyclovir" as used herein means that acyclovir is present in aqueous phase in solution form.
The term "polyhydric alcohol" as used herein means an alcohol having two or more hydroxyl groups. Polyhydric alcohols suitable for incorporation into the topical formulation include glycols and macrogols such as propylene glycol, butane 1,3-diol, polyethylene glycol, glycerol and the like. The polyhydric alcohol is present in concentration of 10-20% w/w of the aqueous phase, particularly in concentration of 15% w/w of the aqueous phase. The preferred polyhydric alcohol is propylene glycol.
The topical formulation is in the form of an oil-in-water emulsion having a dispersed oil phase, a continuous aqueous phase, emulgents or emulsifying agents and preservatives. The emulsifier together with a fat or an oil or both with a fat and an oil constitutes the so called oily phase of the formulation. Preferably, a hydrophilic emulsifier is included together with a lipophilic emulsifier to give the required HLB (Hydrophilic Lipophilic balance) value.
Emulgents suitable for the purpose of present invention include polyoxyethylene sorbitan mono-stearate, sorbitan mono-oleate, cetostearyl alcohol, myristyl alcohol, glyceryl mono-stearate and sodium lauryl sulphate. A preferred combination of emulgents is sodium lauryl sulphate and cetostearyl alcohol in a ratio of from about 1:3 to 1:30 and preferably from about 1:9 to 1:15.
The preservatives may be selected from methyl paraben, propyl paraben, butylated hydroxy toluene (BHT), butylated hydroxy anisole (BHA), benzyl alcohol and the like. The amount of preservative may vary from 0.05 to 1% by weight of the total weight of the composition.
It is always desirable to have a topical formulation, which is non-greasy, non-staining and washable with suitable consistency to avoid leakage from tubes or other containers, which can be achieved by proper selection of oils or fats for the formulation. The oils or fats constituting the oily phase, may be selected from one or more of straight or branched chain mono- or dibasic alkyl esters such as di-isoadipate, isocetyl stearate, propylene glycol diester of coconut fatty acids, isopropyl myristate, decyl oleate,

isopropyl palmitate, butyl stearate, 2-ethylhexyl palmitate or even high melting lipids such as white soft paraffin, liquid paraffin and other mineral oils.
The way the acyclovir cream is prepared is also very critical and of paramount importance to the overall availability of drug from the formulation. For the purpose of present invention, the total quantity of drug is divided in two portions. The first portion of acyclovir is dissolved in the aqueous phase of emulsion containing polyhydric alcohol and emulgent to ensure that a part of acyclovir is present in solution form in the final formulation. This ensures a better release profile as compared to a product where the drug is only dispersed into the system. The oily phase is then added to the aqueous phase to complete the formation of emulsion. The second portion of the drug is separately dispersed in water and passed through a colloid mill to reduce particle size and to ensure absence of drug agglomerates so that the drug has the maximum surface area exposed in the formulation to optimize solubilization of the drug. This ensures a better solubility of drug in situ and as the drug permeates in through the skin, this micronized drug goes into solution to ensure continuity of release. This dispersed drug is then finally added to the emulsion.
A particular method of preparation of acyclovir topical cream involves the following steps:
Preparation of the aqueous phase
Polyhydric alcohol and preservatives are dissolved in about 60% of total purified water previously heated at 70-75°C. To the above solution is added a polar hydrophilic emulgent and acyclovir (10% of the total quantity) with constant stirring till a clear solution is obtained.
Preparation of the oily phase
The hydrophobic emulgent, and mineral oil or waxes are melted at 70-75°C with constant stirring.
Emulsification
The oily phase is added to the aqueous phase slowly while maintaining the temperature at 70-75°C and the mixture is homogenized for at least 20 minutes. The bulk is then cooled to around 50-55°C.

Preparation of the drug dispersion
Acyclovir (90% of the total quantity) is dispersed in about 25% of total purified water and passed through colloid mill to obtain a fine dispersion.
Weight build up
The drug dispersion is added to the above emulsion and homogenization is continued for another 20 minutes while the temperature is maintained at 50-55°C. Remaining quantity of purified water is added to make up the weight of the formulation. The bulk of the formulation is cooled to 25-30°C.
The following example is illustrative of the invention, and is not intended to be construed as limiting the invention.
(
PROCESS:(Figure Removed)
PREPARATION OF AQUEOUS PHASE
1. Weight of all the ingredients was checked.
2. Acyclovir (10% of the total quantity) was sifted through sieve # 45 ASTM (355u).
3. About 60% by weight of the total purified water was taken and heated to 70-75 °C.
4. Propylene glycol, methyl paraben and propyl paraben were added to the bulk of
step-3, under constant stirring to result in a clear solution.
5. Sodium lauryl sulphate and 5% by weight of acyclovir were added to the clear
solution of step-4, under constant stirring, while maintaining the temperature at 70-
75 °C and stirring was continued till a clear solution was obtained.
PREPARATION OF OILY PHASE
6. Cetostearyl alcohol, liquid paraffin and white soft paraffin were taken in a completely
dry jacketed stainless steel vessel, and the ingredients were melted by heating the
mixture to 70-75 °C under constant stirring.
EMULSIFICATION
7. The aqueous phase of step-5 was transferred to the main stainless steel tank of the
cream processor after filtration through a suitable stainless steel mesh.
8. The bulk of step-7 was maintained at a temperature of 70-75 °C with constant
stirring.
9. The bulk of step-6 at 70-75 °C as a thin stream, after filtration through a suitable
stainless steel mesh, under constant stirring was added slowly into the bulk of step-
8 maintained at 70-75 °C. Stirring was continued with scrapper blades on (~ 30 rpm)
and homogenized at around 2800 ± 200 rpm for at least 20 minutes under vacuum
and temperature maintained at 70-75 °C.
10.After homogenization was through, cooling of the bulk was done under constant stirring.
PREPARATION OF THE DRUG DISPERSION
11. The remaining quantity of acyclovir (90% of the total quantity) was sifted through
sieve # 45 ASTM (355u).
12. The sifted drug of step-10 under constant stirring, was dispersed in about 25% by
weight of purified water contained in a stainless steel container.
13. The dispersion of step-11 was passed through the colloid mill maintained at
minimum clearance with recirculation, to obtain very fine drug dispersion. The
colloid mill was rinsed with a part of the remaining amount of purified water and the rinsing was added to the main bulk of drug dispersion.
WEIGHT BUILD-UP
14. The drug dispersion of step-13 was added to the bulk of step-9, when the
temperature of 50-55 °C was reached by the bulk in the cream processor tank.
15. The homogenization was continued under vacuum for at least 20 more minutes, at a
temperature maintained at 50-55 °C till a homogeneous mix was obtained.
16. The main mixing tank was cooled to 25-30 °C, under constant stirring and scrapper
on (~ 30 rpm), by constant circulation of cold water through the jacket of the cream
processor.
17. The remaining quantity of purified water was added to make up the weight of the
cream.

WE CLAIM:
1. A topical pharmaceutical formulation of acyclovir or a salt or ester thereof in the
form of an oil-in-water emulsion containing in the aqueous phase 10-20% w/w of
a water miscible polyhydric alcohol and solubilized acyclovir or a salt or ester
thereof.
2. The topical pharmaceutical formulation of claim 1, wherein the aqueous phase
contains 15% w/w of water miscible polyhydric alcohol.
3. The topical pharmaceutical formulation of claim 2, wherein the polyhydric
alcohol is selected from one or more of glycols and macrogols including
propylene glycol, butane 1, 3-diol, polyethylene glycol and glycerol.
4. The topical pharmaceutical formulation of claim 1 wherein the aqueous phase
contains at least 50% w/w of water.
5. The topical pharmaceutical formulation of claim 1, wherein the topical
formulation further contains emulgents and preservatives.
6. The topical pharmaceutical formulation of claim 5, wherein the emulgent is a
combination of hydrophilic and hydrophobic emulgent(s) selected from the group
consisting of polyoxyethylene sorbitan mono-stearate, sorbitan mono-oleate,
cetostearyl alcohol, myristyl alcohol, glyceryl mono-stearate and sodium lauryl
sulphate.
7. The topical pharmaceutical formulation of claim 5, wherein the preservative is
one or more of methyl paraben, propyl paraben, butylated hydroxy toluene (BHT)
and butylated hydroxy anisole (BHA).
8. A topical pharmaceutical formulation of claim 3, wherein the formulation is in the
form of an oil-in-water emulsion containing in the aqueous phase 10-20% w/w of
propylene glycol and solubilized acyclovir or a salt or ester thereof.
9. The topical pharmaceutical formulation having the following composition:

Aqueous phase Amount (mg/g)
10. A Acyclovir USP (5%) 5.0
PropyleneglycolUSP(15%) 150.0
Sodium lauryl sulphate USNF 7.50
Methylparaben USNF 1.50
Propylparaben USNF 0.80
Purified water USP 300.00
Oily phase
Cetostearyl alcohol USNF 67.50
Light Mineral oil USP 50.00
White petrolatum USP 125.00
Drug Dispersion
Acyclovir USP 45.00
Purified water USP q.s. to 1000 mg 247.70
method of preparation of the topical pharmaceutical formulation of acyclovir or a salt or ester thereof containing in the aqueous phase 10-20% w/w of a water miscible polyhydric alcohol and solubilized acyclovir salt or ester thereof, wherein the method comprises formulating a part of the aqueous phase as an emulsion followed by the addition of the balance of water and acyclovir as a dispersion into the emulsion such that the aqueous phase contains at least 50% w/w of water.