Effects of topical petrolatum and salicylic acid on the erythemogenicity of UVB

ARTICLE

It is important to remove the scales of psoriasis lesions, because of the blocking effects for antipsoriatic agents. Salicylic acid is one of the most effective and basic keratolytic agents. Salicylic acid, which exerts a keratolytic effect in concentrations of 5% or above, is commonly used in petrolatum for the lesions on the trunk and limbs of psoriasis patients [1, 2]. Its keratolytic action enhances the penetration of other topical agents [2].

UVB phototherapy is one of the effective treatment modalities of psoriasis [3]. It has been stated that successful treatment of psoriasis with ultraviolet radiation depends on sufficient penetration of the radiation into the epidermis [4]. In this study, the UVB blocking effect of salicylic acid which is used prior to UVB phototherapy was investigated.

Material and method

In this single-blind, vehicle-controlled study, phototesting was performed on 35 volunteers to determine the minimal erythema dose (MED) for UVB. All participants were informed and consent forms were obtained. The patients were untanned and not taking any medication. Waldmann 8001K (Waldmann Lichttecnik Gmbtt, Schwenningen, Germany) cabin was used for the light source of UVB.

According to the skin types of the volunteers, the beginning doses of UVB phototesting were determined. The irradiated doses for UVB were up to 120 mJ/cm² for skin types I and II, 140 mJ/cm² for skin types III and IV and 200 mJ/cm² for skin type V [5]. MED values were determined by irradiating 4 cm² of uninvolved skin on the back of each patient at increments of 20 mJ/cm² to 6 different areas in one row. In addition, five parallel rows of skin were tested after application of different topical agents. The first row was only irradiated by UVB without application of any topical agent to determine the MED. A thin (0.1 cc/25 cm²) layer of white petrolatum, a thick (0.3 cc/25 cm²) layer of white petrolatum, a thin (0.1 cc/25 cm²) layer of salicylic acid (20%) in white petrolatum, a thick (0.3 cc/25 cm²) layer of salicylic acid (20%) in white petrolatum and a sunscreen factor 15 (titanium dioxide + octyl methoxy cinnamate) were applied respectively to the adjacent parallel rows, and irradiated with UVB at increments of 20 mJ/cm² immediately after the application of the agents, without waiting. The results were evaluated after 24 hrs by a blinded investigator and MED values were determined (Fig. 1).

Results

The results of phototesting of all patients with pure UVB, with the application of a thin layer of petrolatum, a thick layer of petrolatum, a thin layer of salicylic acid (20%) in petrolatum and a thick layer of salicylic acid (20%) in petrolatum are shown in Table I. Sunscreen caused a total blocking effect on all of the subjects. MED values detected after pure UVB and after application of topical agents were compared, one by one, and all of them have been found statistically significant (Paired-Samples T test, p < 0.0001). These findings showed that MED values were increased by a thin or thick application of petrolatum and also by a thin or thick application of salicylic acid (20%) in petrolatum. The highest MED values were detected with thick salicylic acid (20%) in petrolatum followed by thin salicylic acid (20%) in petrolatum, thick petrolatum and thin petrolatum. A statistically significant difference between the MED values of thin and thick applications of petrolatum, and also between the MED values of thin and thick applications of salicylic acid (20%) in petrolatum was determined (Paired-Samples T test, p < 0.0001). Thick application of petrolatum or salicylic acid (20%) in petrolatum resulted in an increase of MED values when compared with the thin applications of the same agents. There was also a statistically significant difference between the MED values of thin application groups (thin petrolatum and thin salicylic acid in petrolatum) and between the MED values of thick application groups (thick petrolatum and thick salicylic acid in petrolatum) (Paired-Samples T test, p < 0.0001). Thin application of salicylic acid in petrolatum resulted in an increase of MED values when compared with the thin application of petrolatum and also the thick application of salicylic acid in petrolatum resulted in an increase of MED values when compared with the thick application of petrolatum. The results of the "Paired-Samples T test" of all the paired groups are shown in Table II.

Discussion

Various agents can be used in combination with phototherapy in psoriasis patients [1, 3, 4, 6]. At this point, one should consider the blocking effects of topical agents. Few investigations have been undertaken to determine the blocking effects of topical agents on UVB penetration. These studies showed that some topical agents such as tar, anthralin and calcipotriol have blocking effects, when they were used prior to UVB [4, 7-10]. Also there was clinical and experimental evidence about some lubricants showing enhancement of the penetration of UVB to the psoriatic plaques, when they were used before irradiation [3, 4, 11, 12]. An in vivo study showed increased UV penetration after application of glycerine to the psoriasis lesions. But studies showing blocking effects of some emollients for UV have also been reported [4]. Lebwohl et al. reported that emollient creams when applied just before UVB have blocking effects on UVB penetration at different degrees consistent with their thickness. Mineral oil and clear liquid emollient did not significantly affect the transmission of UVB [12].

The success of the treatment of psoriasis by UVB irradiation is closely related to sufficient penetration of UVB into the epidermis [4]. So, the scales should be removed and salicylic acid is one of the commonly used topical agents for this purpose [1]. The blocking effect of salicylic acid on UVB penetration should be known, to evaluate the efficacy of the treatment. Salicylic acid is commonly used in petrolatum [2]. So, the blocking effects of white petrolatum should also be considered. It has been observed that the application of a keratolytic agent with salicylic acid before UVB can retard and delay the improvement of the psoriasis lesions. Some in vitro studies have investigated this. Kristensen et al. reported that both salicylic acid and white petrolatum could absorb light at variable degrees in different wavelengths and salicylic acid can absorb light more than white petrolatum in the interval of 295-323 nm [11]. Kornreich et al. showed that salicylic acid in petrolatum had a high absorbance value for UVB, though petrolatum alone had low absorbance [4]. Lebwohl et al. reported that salicylic acid blocks UVB radiation totally but white petrolatum can only block at a low level in an in vitro study. They also found different results between thin and thick applications of white petrolatum and a thick application of white petrolatum had a higher blocking effect [12]. However, Hoffmann et al. in an in vitro study found that petrolatum can enhance the transmission of UVB [13].

There are only a few clinical studies on this topic. Kristensen et al. have studied the effect of the salicylic acid on UVB transmission on patients by determining the value of erythema induced by UVB after the application of salicylic acid in different concentrations (0.5, 1, 2, 5, 10%) just prior to UVB. In a second series the duration of photoprotection was studied by applying salicylic acid (2, 5, 10%) at different times (1, 2, 4, 8, 12, 16, 20 and 24 hrs) prior to UVB. They found that salicylic acid had a dose-dependent photoprotection for UVB and at concentrations (2, 5, 10%) salicylic acid inhibited erythema for at least 12 hrs and sometimes more than 24 hrs. They also found that salicylic acid application before UVB irradiation decreases the improvement rate of psoriasis, in a clinical study [11]. In another in vivo study based on determining MED, Lebwohl et al. found that salicylic acid application just before exposure to UVB reduces the erythemogenicity of UVB. They also observed that thick (0.8 cc/24 cm²) application of petrolatum blocked UVB, although a thin (0.4 cc/24 cm²) application had a negligible blocking effect. However they did not apply the topical agents at different times before UVB [12].

Abbreviations

MED: Minimal erythema dose.
UVB: Ultraviolet B.

CONCLUSION

In this clinical study based on determining the MED values, we observed the blocking effect of pure white petrolatum and a rise in blocking effect with salicylic acid (20%) in petrolatum when applied immediately before UVB. Also, thick applications had a more prominent blocking effect than thin applications. However, we did not apply topical agents at different times before UVB. Our results are in accordance with Kristensen's and Lebwohl's results. Only contrary to Lebwohl's findings, we observed a stastistically significant blocking effect of petrolatum with thin (0.1 cc/25 cm²) and thick (0.3 cc/25 cm²) applications. We suggest that topical agents such as salicylic acid and petrolatum which have blocking effects for UVB transmission should not be applied just before UVB treatment.

Article accepted on 11/12/01

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