Anthralin stability in various vehicles

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Anthralin stability in various vehicles Philip G. Green, M . S c . , * Cameron T. C. Kennedy, M.A., M.R.C.P.,** and Drummond R. Forbes, M.Sc., M.P.S.* Bristol, England The stability of anthralin in white soft paraffin, Lassar's paste, and Unguentum Merck was assessed using a sensitive assay for degradation products, over a !-year period. The stability was found to vary with concentration, weaker preparations being less stable. Significant degradation was observed in Unguentum Merck, and this increased when salicylic acid was present. Preparations with Unguentum Merck could be stabilized with ascorbic or oxalic acid. Salicylic acid stabilized anthralin in Lassar's paste but had a negligible effect in white soft paraffin. Preparations with white soft paraffin were relatively stable. (J AM ACAD DERMATOL 1987;16:984-8.)

Penetration of anthralin into psoriatic lesions is more rapid than penetration into normal skin.1 This finding initiated the use of higher concentrations of anthralin for short treatment periods to minimize the irritant effect on normal skin. Several vehicles have been used for "short-contact" anthralin therapy, including white soft paraffin, Lassar's paste, 2 and water-miscible bases. Anthralin is more quickly and completely removed if applied in a water-miscible base, but there is uncertainty about its stability in such bases. Salicylic acid is often added as an antioxidant and keratolytic. Anthralin degrades in the presence of oxygen, light, or alkali via a free radical species 3 that is believed to be therapeutically active, 4 to products that are thought to be inactive against psoriasis. 5,6 These include anthralin dimer (a dehydrodimer), danthron, and other products constituting "anthralin brown." The ultraviolet (UV) absorption spectroscopic methods outlined by the British and American Pharmacopoeias to assay anthralin cannot distinFrom the Departments of Pharmacy* and Dermatology,** Bristol Royal Infirmary. Based on a paper given before the British Association of Dermatologists, July 1985. Accepted for publication Nov. 14, 1986. Reprint requests to: Philip G. Green, Pharmacy Department, University of Wales Institute of Science and Technology, Cardiff, Wales.

984

guish between anthralin and its main degradation product anthralin dimef; they are therefore likely to overestimate the concentration of active anthralin. It has been found that accurate determination can be achieved when thin layer-chromatography is used prior to UV detection) Hence high-performance liquid chromatography was employed in the present work to monitor degradation in a variety of anthralin preparations used for shortcontact therapy. The ability of salicylic acid to reduce anthralin degradation was also investigated. METHODS Approximately 100 gm of freshly prepared anthralin formulation was transferred to each of ten 120-ml amber ointment jars and stored at room temperature protected from light. No attempts were made to remove residual air from the vehicles prior to the formulation of anthralin. Three samples, each corresponding to about 0.3 mg, were taken from the surface of each formulation at weekly intervals for the first month and at 2 monthly intervals thereafter. At the same time a further two samples were taken from about 3.0 cm below the surface. These samples were then extracted with toluene (5 ml) and the components were separated and analyzed by high-pressure liquid chromatography* (Table I). Two injections of each sample were made. For Un*Powell EGP, Yorkshire RHA. Quality Control Information Bulletin 1983; 2(suppl 8).

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Anthralin

N u m b e r 5, Part 1 M a y 1987

stability 985

Table I. High-pressure liquid chromatography conditions required for optimal separation of anthralin and its breakdown products

Methanol: H20: tetrahydrofuran: formic acid (94: 25: 3 : 1 V/V)

5 ~m spherisorb ODS, 25 cm; fitted with precolumn of the same material

Danthron, 8.2 Anthralin, 8.8 Anthralin dimer, 13.8

Flow rate, 1.5 ml/min ODS: Octadecyl and silica.

Table II. Percentages anthralin, danthron, and anthralin dimer present after prolonged storage in Lassar's paste at various concentrations Initial concentration anthralin

Time elapsed since manufacture

(%, w/w)

(mo)

2.00 1.00 0.50 0.25 0.10

11.0 14.0 14.0 14.0 16.5

Final concentration as a percent of initial anthralin concentration

Total recovery Anthralin

93.6 80.1 70.9 61.1 22.7

44-+ + 4-

1.6 2.0 1.8 2.2 3.1

Danthron

1.5 1.0 1.4 2.4 4.0

4444-

0.2 0.0 0.2 0.3 0.5

Anthralin dimer

6.0 8.4 12.2 18.4 35.0

---+0.6 +--- 0.6 _ 0.7 --. 0.9 4- 1.0

(%)

101.1 89.5 84.5 81.9 61.7

44_ 4-

2.4 2.6 2.7 3.4 4.6

Results are given as mean ± 2 SD.

guentum Merck* it was necessary to extract the anthralin by warming the preparations with toluene in a water bath for 15 to 20 minutes and filtering prior to centrifugation. The high-pressure liquid chromatography detection system was calibrated before use at 280 nm, using freshly prepared solutions of anthralin,t anthralin dimer, t and danthron (USP) standards. Mean assay values were determined for each formulation and the precision of each analysis was determined using 95% confidence limits. Shelf-lives were defined as the time taken for 10% degradation of anthralin to occur (British Pharmacopoeia, 1980). RESULTS

Anthralin in Lassar's paste In this vehicle anthralin breaks down predominantly to anthralin dimer and further products col*Unguentum Merck (E. Merck Limited) is an ambiphilie emulsion whose lipold component contains polyoxyethylene sorbitan fatty acid ester, cetyl stearyl alcohol, glycerin monofatty acid ester, propylene glycol, paraffin hydrocarbons, saturated neutral oil, dispersed silicic acid, and sorbic acid. It is used as a vehicle and as an emollient. tObtained from E. G. P. Powell Regional Q.C. Laboratory, Sefton General Hospital, Liverpool, England.

lectively known as "anthralin brown," which are responsible for discoloration of formulations. Degradation is nonlinear (Fig. 1). Degradation to danthron was found to be negligible. Typical results at completion of a stability study are shown in Table II. Recovery values of less than 100% may result from insoluble or strongly retained decomposition products. When anthralin was incorporated into zinc paste, that is, Lassar's paste without salicylic acid, the formulation turned pink and eventually brown. After 24 hours a 0.2% formulation was found to contain anthralin (51.2%), anthralin dimer (41.7%), and danthron (3.7%). This formulation has a shelf-life of 1 to 2 hours. When the zinc oxide was replaced with starch a 0. I % formuIation could be prepared that had a shelf-life o f 3 to 4 months. The discoloration and degradation of anthralin were found to occur throughout the bulk of the paste and not only on the surface. More dilute preparations of anthralin in Lassar's paste were found to be less stable, and this is reflected in shelf-lives (Table III).

986

Journal of the American Academy of Dermatology

Green et al

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100 100 ~ ~

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90

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80

90

70

% RECOVERy OF ANTHRALIN 80

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60

% RECOVERY OF ANTHRALIN

50

7O

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O: O

2O

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i i i 10 15 20 25 30 35 40 TIME ELAPSEDSINCEMANUFACTURE( days ) =2.0% eO 5% 0 0 , 2 8 % o0.1%

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Fig. 2. Stability of anthralin in Unguentum Merck at a variety of concentrations. Figure shows mean +- 2 SD.

10 o

0

[ I I J t i [ i i [ 2 4 6 8 10 12 14 16 18 20 2224 TIME ELAPSED SINCE MANUFACTURE ( months ) =2,0% ~, 1.0% e0.5% o0.25% o0.1%

Fig. l. Stability of anthralin in Lassar's paste at a variety of concentrations. Figure shows mean __ 2 SD. Table HI. Approximate shelf-life of anthralin w h e n formulated in Lassar's paste and white soft paraffin Initial concentration anthralin

A p p r o x i m a t e shelf4ife

(%, W/W)

(ran)

Table IV. Approximate shelf-lives of some formulations of anthralin in Unguentum Merck, with and without salicylic acid (1%) Approximate shelf-life (days) Initial concentration anthralin

(%, w/w)

With salicylic acid

salicylic acid

0.10 0.25 0.50 1.00 2.00

1-2 2-3 3-5 6-8 10-11

3-4 5-7 7-8 9-12 15-16

Without

Lassar' s paste

2.00 1.00 0.50 0.25 0.10

13 10 6 3 1.5

Anthralin in Unguentum Merck

White soft paraffin

0.20 0.20 with salicylic acid 1.0% 0.5 ~1.0

year. It was found that salicylic acid (1.0%) had no apparent stabilizing effect on anthralin in this vehicle (Table III).

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Anthralin in white soft paraffin In vehicles based in white soft paraffin, anthralin dimerizes slowly. Formulations of anthralin at or above 1.0% were found to be stable after 1

In Unguentum Merck anthralin has been found to degrade quickly compared to the more traditional ointments and pastes. This was particularly noticeable at the surface, where purple and brown discoloration occurred, and to a less degree below the surface, where the formulation turned dark yellow. At low concentrations (Fig. 2) and in the presence of salicylic acid, anthralin was found to degrade more quickly (Table IV). The degradation

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Anthralin stability

987

Table V. Percentage of anthralin and degradation products after 25 days' storage in 0.1% Unguentum Merck preparations, with and without 1.0% salicylic acid

Anthralin With salicylic acid 6.7 --+ 3.5 Without salicylic acid 60.0 -+ 4.8

[

Danthron

[

Anthralindimer

I

Total recovery

2.3 +-- 0.7

9.6 + 5.5

29.6 -+ 9.7

0.9 +- 0.3

2.8 ± 0.8

63.7 ± 5.9

Results are given as m e a n ± 2 SD.

products were mainly anthralin dimer and "anthralin brown"; negligible increases in danthron levels were observed (Table V). Alternatives to salicylic acid as antioxidants in Unguentum-Merck were assessed. Of these, oxalic acid (1%) and ascorbic acid (1%) were found to afford greatest anthralin stability. For example, 0.25% anthralin formulations with either oxalic acid or ascorbic acid (each 1%) were stable up to 1 month. The addition of a free radical scavenger, such as butylated hydroxyanisole, did not confer stability. Butylated hydroxyanisole has been used to inhibit anthralin inflammation. 9 The potential antioxidants citric acid, benzoic acid, tartaric acid, hydrochloric acid (to pH 2.8), mannitol, methionine, cysteine, and malic acid were all found to be ineffective when present at a concentration of 1.0%. DISCUSSION In this study the instability of anthralin in various formulations has been demonstrated; it degrades in Unguentum Merck and compound zinc paste mainly to its dehydrodimer and to other products constituting "anthralin brown." The stability was found to vary with concentration, weaker preparations being less stable. Unexpectedly, the addition of salicylic acid to formulations with Unguentum Merck enhanced the degradation of anthralin while compared with the stabilizing effect in zinc paste. Such stabilization has been attributed to the neutralization of alkaline impurities in the zinc oxide component of the paste, ~° Anthralin has a solubility of about 0.2% in the white soft paraffin component of Lassar's paste. As Lassar's paste comprises 50% white soft paraffin, the overall anthralin solubility in this vehicle is about 0.1%. It is therefore possible that

the degradation of anthralin occurs in the dissolved phase and that this may account for the instability of more dilute preparations in which proportionally more anthralin exists in solution. It is suggested that anthralin in Lassar's paste should be assigned a shelf-life appropriate to the concentrations rather than the 2 years calculated using the United States Pharmacopoeia assay. With low concentrations of anthralin, such as 0.2%, the presence or absence o f salicylic acid does not significantly affect the extent of loss of anthralin in white soft paraffin, provided that the preparations are kept for short periods, that is, less than 3 months. White soft paraffin preparations of higher strength may be kept for longer periods. Anthralin degrades in the presence of water, 3 and this may account for the instability of the molecule in Unguentum Merck. Degradation in aqueous vehicles can be reduced by the addition of ascorbic acid. tl Oxalic acid also prevented degradation, although its toxicity may prevent clinical use.

REFERENCES 1. SchaeferH, FarberEM, GoldbergL, Schalla W. Limited application period for dithranol in psoriasis. Br J Dermatol 1980;102:571-3. 2. RyattKS, StathamBN, Rowell NR. Short contact modification of the Ingram regime. Br J Dermatol 1984; 111:455-7. 3. WhitefieldM. Pharmaceuticalformulationsof anthralin. Br J Dermatol 1981;105(suppl 20):28-32. 4. K.rebsA, SchalteggerH, SchalteggerA. Structure specificity of the antipsoriatic anthrones. Br J Dermatol 1981;105(suppl 20)'6-11. 5. Cavey D, Caron JC, Shroot B. Anthralin: Chemical instability and glucose-6-phosphate dehydrogenase inhibition. J Pharm Sei 1982;71:980-3. 6. Fisher LB, Maibach HI. The effect of anthralin and its derivatives on epidermalcell kinetics. J Invest Dermatol 1975;64:338-41.

Journal of the American Academy of Dermatology

Green et al

7. Caron JC, Shroot B. High pressure liquid chromatographic determination of anthralin in ointments. J Pharm Sci 1981;70:1205-7. 8. Elsabbagh HM, Whitworth CW, Schramm LC. Separation, identification, and quantitation of anthralin and its decomposition products. J Pharm Sci 1979;68: 388-90.

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9. Finnen MJ, Lawrence CM, Shuster S. Inhibition of dithranol inflammation by free-radical scavengers. Lancet 1984;2:1129-30. 10. Dean FD. The action of zinc oxide on dithranol. Br J Dermatol 1971;85:494. 11. SevilleRH, Walker GB, Whitefield M. Dithranol cream. Br I Dermatol 1979;100:475-6.

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Clinical diagnostic accuracy of basal cell carcinoma Stephen E. Presser, M.D., and J. Richard Taylor, M.D. Miami, FL The clinical diagnostic accuracy and index of suspicion of basal cell carcinoma were calculated. The data were compiled from dermatology residents, full-time dermatology university faculty, and dermatologists in private practice. (J AM ACAD DERMATOL 1987;16:988-90.)

The diagnostic accuracy and index of suspicion of basal cell carcinoma were calculated on lesions that were recorded in the University of Miami Dermatopathology Service in the month of January 1980, and from the Dermatology Service of the Veterans Administration Medical Center, Miami, January to June 1980. The diagnostic accuracy of residents, faculty, and private practice dermatologists was scored as 64%, 70%, and 65%, respectively. The diagnostic accuracy o f first-, second- and third-year residents was 56%, 59%, and 73%, respectively. The index of suspicion of basal cell carcinoma of residents, faculty, and private practice dermatologists was, respectively, 139%, 130%, and 133%. The index of suspicion o f the first-, secondand third-year residents was 165%, 153%, and 120%. We reviewed the diagnostic accuracy and index of suspicion of basal cell carcinoma scored by our From the Department of Dermatology and Cutaneous Surgery, Miami School of Medicine. Accepted for publication Nov. 26, 1986. Reprint requests to: Dr. Stephen E. Presser, 1580 Elmwood Ave., Rochester, NY 14620.

988

residents, faculty, and the private practice dermatologists, sending specimens to the University of Miami derrnatopathology service.

METHODS Data were analyzed retrospectively from the pathology records at the Veterans Administration Medical Center, Miami, FL, and the University of Miami Medical Center dermatopathology service. All of the skin biopsy specimens submitted at the Veterans Administration Medical Center from the dermatology service are sent by residents. The period January to June 1980 was used to study the diagnostic accuracy of the residents. The data were tabulated by level of training. The biopsy specimens from the University of Miami dermatopathology service were divided into those done by the University's dermatology faculty and the private practice dermatologists. The month of January 1980 was used for the study. The clinical diagnoses were taken from the pathology laboratory slips accompanying the biopsy specimens. Only the first diagnosis was counted if a differential diagnosis was included. The diagnostic accuracy and index of suspicion used in this study were those reported by Lightstone et al. ~ This method evaluates the physicians' clinical accuracy and tendency to o'~erdiagnose or underdiagnose a con-