International Agency for Research on Cancer (IARC) - Summaries & Evaluations


VOL.: 24 (1980) (p. 101)

5. Summary of Data Reported and Evaluation

5.1 Experimental data

Methoxsalen alone has not been tested by skin application and was inadequately tested in mice by oral and by intraperitoneal administration.

It was tested in combination with long-wave ultra-violet light in mice by oral and intraperitoneal administration and by skin application: it increased the incidence of epidermal and dermal tumours.

Methoxsalen, mainly in combination with long-wave ultra-violet light, but also in the dark, was mutagenic in a variety of prokaryotic and eukaryotic cells.

Attention is drawn to the absence of studies on the teratogenicity of this compound.

5.2 Human data

Methoxsalen is mainly used in combination with long-wave ultra-violet light in the treatment of vitiligo and severe psoriasis.

Methoxsalen and long-wave ultra-violet light together have been associated with haematopoietic neoplasms in two patients, with basal-cell skin cancer in another, and with squamous-cell skin cancer in a cohort study of patients with psoriasis. In the cohort study, increased surveillance of study subjects may have biased comparisons with the general population. However, a change in the ratio of squamous- to basal-cell tumours, the appearance of tumours in body areas not normally exposed to sunlight, and a change in tumour incidence within the cohort would support a causal interpretation. In none of these reports could the possible effects of methoxsalen alone be distinguished from those of long-wave ultra-violet light or of the combination of the two. Methoxsalen alone did not alter the incidence of skin cancer over two years in two small controlled trials of its use as a putative prophylactic for this disease.

These data are insufficient to allow a conclusion as to the carcinogenicity of methoxsalen in humans.

5.3 Evaluation

The available experimental and epidemiological data on methoxsalen alone were inadequate to make an evaluation of its carcinogenicity.

There is sufficient evidence that methoxsalen increases the carcinogenic effects of long-wave ultra-violet light in mouse skin. In view of the combined use of these agents in the treatment of skin disorders in humans, further studies should be undertaken of humans who have been exposed to them.

For definition of the italicized terms, see Preamble Evaluation.

Subsequent evaluation: Suppl. 7 (1987)

Last updated: 7 April 1998

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