Topical pine tar: History, properties and use as a treatment for common skin conditions

Antiproliferative effect

The antiproliferative effect of tars has been studied using coal tar. However, it is postulated that all tars, including pine tar, work in a similar manner. Studies by Lavker and colleagues in volunteers with healthy skin demonstrated a transient increase in epidermal proliferation during the first 2 weeks of tar treatment, followed by a progressive thinning of the epidermis.11 Tars appear to act as keratolytic agents which inhibit excessive proliferation of epidermal cells by the suppression of DNA synthesis in hyperplastic skin, which subsequently reduces mitotic activity and protein synthesis in the basal layer of the epidermis.10, 12 This promotes a return to normal keratinisation, which is important in skin diseases such as psoriasis.13

Other effects

In addition to its keratolytic action, pine tar has been shown to be antipruritic,7, 8, 12 anti‐inflammatory,8 antiseptic,7, 12 astringent,12 keratoplastic,8 cytostatic,6 antibacterial14 and antifungal.15 Fractionation of pine tar has revealed that the components responsible for its bacteriostatic properties are the resin acids,14 and that acetovanillone, 1,1′,3,3′‐tetraoxo‐2,2′‐bicyclopentyl and its 4‐methyl derivative, as well as another unknown compound, are responsible for the antifungal properties of pine tar.15 This is in contrast to coal tar where it is generally considered that phenols are responsible for its biological activity.14

In terms of pharmacokinetics, it has been observed by infrared spectroscopy that pine tar is detectable in the skin 1 h following the topical application of 12% pine tar to the skin, but was undetectable in the skin after 8 h.16 The mechanisms by which pine tar is absorbed, metabolised and excreted are unknown.

Indications

Pine tar has been used in topical preparations to relieve itching and inflammation associated with a range of skin conditions such as eczema or dermatitis,6, 7, 12, 17 psoriasis,6, 7, 12, 17 chronic lichen simplex,8 seborrhoeic dermatitis and scalp psoriasis,7, 12, 17 sunburn, 18 nappy rash,18 prickly heat,18 hives,18 chicken pox,18 insect bites,18 anal and genital itching including jock itch18 and other dry, itchy, flaky or inflamed skin conditions.18

Contraindications

Sensitivity to pine tar.18 Pre‐existing folliculitis or severe acne are also possible contraindications.19

Caution

Avoid contact with eyes. If lather enters eyes, flush with clean water.18

How to use

The use of pine tar products available in Australia today is described in Table 1. Pine tar products may be used on the entire body, including the face, as an adjunct cleanser for red, itchy and inflamed conditions together with other prescribed medications such as corticosteroids and fungal creams. Pine tar products are interchangeable, with the oil being particularly useful for widespread dry, red, itchy and inflamed conditions as it is moisturising and suitable for bath use. Similarly, the solution, which is also suitable for bath use, is useful for widespread conditions that are not specifically dry. The bar and gel can both be used in the shower or at the hand basin, with the gel being more elegant to use and the bar suitable for patients who like to be able to hold the product.

As well as pine tar lotion, which contains a very low concentration of 0.1% pine tar, the pine tar formulations described in Table 1 are designed as wash‐off products that are quick and easy to use, are less messy than other tar products which need to be left on for many hours, do not stain and any odour is minimised.4 These attributes may increase patients’ compliance.

Toxicity

The acute toxic potential of topical tar products, particularly wood tars, which include pine tar, has been linked to their phenol content.17 However, since the phenol content has been reduced in the topical pine tar products manufactured today, phenol poisoning is unlikely.17 In addition, unlike coal tar, which has been shown to be phototoxic, wood tars including pine tar have not been reported to cause photosensitisation.10

Carcinogenicity

A more serious problem than toxicity is the potential carcinogenicity of tars, including pine tar, which has often raised safety issues over its use in therapeutic treatments.

Carcinogenicity studies in animals have shown that the carcinogenic potential of tars is linked to the polycyclic aromatic hydrocarbon (PAH) fractions.17, 20 Swallow and Curtis measured the levels of eight different PAH which have been shown to be carcinogenic to animals in six different coal tar solutions commercially available in New Zealand.21 A commercial product containing only pine tar (Ego Pharmaceuticals Pty Ltd, Braeside, Victoria, Australia) was also included in the study for comparison. The pine tar product was found to contain no detectable levels of four of the eight PAHs studied, and minimum detectable levels of the remaining four PAHs (Table 2).21 The PAHs present in in the pine tar product summed to 8 mg/kg, which was at least sixfold, and up to approximately 300‐fold less than the levels of PAHs in the other commercial coal tar preparations. Variations in PAHs levels were not due solely to the difference in the amount of tar added to the preparation.21

A study by Athanasiou and Lillis found an absence of mutagenic action of pine tar resin in the Salmonella/microsomal cell culture system as developed by Ames and colleagues, and an absence of clastogenic action in Chinese hampster ovary (CHO) cell culture.22, 23 Thus it can reasonably be assumed that pine tar lacks significant carcinogenic activity.23 Further, pine tar used in the manufacture of topical pine tar products available in Australia today has been found not to be carcinogenic by the Ames test (unpublished data, Ego Pharmaceuticals), and is not classified as a carcinogen.9

Despite the extensive use of medicinal tars, particularly in psoriasis patients, there is no epidemiological evidence that topical tar products, including pine tar, cause cutaneous or internal cancer.24 For example, in a 25‐year follow‐up study of 280 psoriatic patients treated with coal tar, the investigators concluded that the incidence of cancer had not appreciably increased above the expected incidence in the general population.25 In another study of 719 psoriasis patients, tar therapy (type of tar not indicated) did not increase the incidence of cancer above that expected in the general population over a 10‐year period.26, 27 Therefore, the apprehension that topical pine tar may be carcinogenic is unfounded, based on the lack of supporting evidence.

Contact allergy

Hypersensitivity reactions for tars are rare, but wood tars are more likely to cause sensitisation than coal tar.7, 19 Patch test studies using wood tars have been performed on around 30 000 patients predominately with contact dermatitis.28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51 Wood tars at concentrations of between 3 and 12% generally produced positive reactions in approximately 2–10%28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41 of patients. A small number of studies found positive reactions in 20–30%42, 43, 44 of patients. It is important to note that the number of positive reactions for wood tars was not significantly greater than those for other common allergens. In addition, the concentration of pine tar in topical products available in Australia is up to 2.3%, which is up to four times less than that tested in these studies.

Caution must be used when interpreting these types of studies since what is referred to as wood tars in these studies is generally a mixture of pine, juniper, birch and beech tars, and therefore the exact tar eliciting the positive reaction is not known. A positive reaction to pine tar alone would be considerably less as is shown in the study by van Andel and colleagues.52 Of the 650 patients tested, 59 (9%) patients had a positive reaction to wood tar. Of the patients positive for wood tar 33 were available for further testing. Of these, only 20 (60%) were positive to wood tar on renewed testing, and only five (25%, which equals approximately 1% of the original population) were positive to pine tar alone. Further, a marked overlap of positive tests to wood tar, coal tar, balsam of Peru, colophony and turpentine was observed, and it was concluded that patch tests with wood and coal tar are of little diagnostic value.52

There has only been one case of adverse reactions to topical products containing pine tar reported in the literature by Iorizzo and colleagues6 A 61‐year‐old woman with a 3‐year history of vesicular foot eczema presented with an acute dermatitis following the use of pine tar. However, patch testing revealed that she also showed a positive reaction to coal tar, birch tar, beech tar and juniper tar, supporting the finding that these reactions are a consequence of cross‐sensitisation and not prior exposure. Her skin lesions rapidly resolved after treatment with topical corticosteroids.6 Furthermore, there have been only five cases of adverse events for pine tar reported to the Therapeutic Goods Administration of Australia since records began in 1971, which included pruritus, rash and dermatitis.

Other adverse effects

Primary irritation is very uncommon, except in unstable psoriasis and treatment on the face, genitalia and the flexures.7, 19 An in vitro skin irritancy study has shown that pine tar is a minimal irritant, the lowest on a scale of four categories; minimal, mild, moderate and severe (results unpublished, Ego Pharmaceuticals). Folliculitis is the most common side effect.19 In any case, any adverse effects caused by pine tar are likely to be reduced with the topical products available today, which are generally in contact with the skin for only a relatively short period before being washed off.