Psoralen

Psoralen (also called psoralene) is the parent compound in a family of naturally occurring organic compounds known as the linear furanocoumarins. It is structurally related to coumarin by the addition of a fused furan ring, and may be considered as a derivative of umbelliferone. Psoralen occurs naturally in the seeds of Psoralea corylifolia, as well as in the common fig, celery, parsley, West Indian satinwood and in all citrus fruits. It is widely used in PUVA (psoralen + UVA) treatment for psoriasis, eczema, vitiligo, and cutaneous T-cell lymphoma. Many furanocoumarins are extremely toxic to fish, and some are deposited in streams in Indonesia to catch fish. Psoralen (also called psoralene) is the parent compound in a family of naturally occurring organic compounds known as the linear furanocoumarins. It is structurally related to coumarin by the addition of a fused furan ring, and may be considered as a derivative of umbelliferone. Psoralen occurs naturally in the seeds of Psoralea corylifolia, as well as in the common fig, celery, parsley, West Indian satinwood and in all citrus fruits. It is widely used in PUVA (psoralen + UVA) treatment for psoriasis, eczema, vitiligo, and cutaneous T-cell lymphoma. Many furanocoumarins are extremely toxic to fish, and some are deposited in streams in Indonesia to catch fish. Psoralen is a mutagen, and is used for this purpose in molecular biology research. Psoralen intercalates into DNA and on exposure to ultraviolet (UVA) radiation can form monoadducts and covalent interstrand cross-links (ICL) with thymines, preferentially at 5'-TpA sites in the genome, inducing apoptosis. Psoralen plus UVA (PUVA) therapy can be used to treat hyperproliferative skin disorders like psoriasis and certain kinds of skin cancer. Unfortunately, PUVA treatment itself leads to a higher risk of skin cancer. An important use of psoralen is in PUVA treatment for skin problems such as psoriasis and (to a lesser extent) eczema and vitiligo. This takes advantage of the high UV absorbance of psoralen. The psoralen is applied first to sensitise the skin, then UVA light is applied to clean up the skin problem. Psoralen has also been recommended for treating alopecia. Psoralens are also used in photopheresis, where they are mixed with the extracted leukocytes before UV radiation is applied. Despite the photocarcinogenic properties of psoralen, it was used as a tanning activator in sunscreens until 1996. Psoralens are used in tanning accelerators, because psoralen increases the skin’s sensitivity to light. Some patients have had severe skin loss after sunbathing with psoralen-containing tanning activators. Patients with lighter skin colour suffer four times as much from the melanoma-generating properties of psoralens than those with darker skin. Psoralens short term side effects include nausea, vomiting, erythrema, pruritis, xerosis, skin pain due to phototoxic damage of dermal nerve and may cause cutaneous and genital skin malignancies. An additional use for optimized psoralens is for the inactivation of pathogens in blood products. The synthetic amino-psoralen, amotosalen HCl, has been developed for the inactivation of infectious pathogens (bacteria, viruses, protozoa) in platelet and plasma blood components prepared for transfusion support of patients. Prior to clinical use, amotosalen-treated platelets have been tested and found to be non-carcinogenic when using the established p53 knockout mouse model. The technology is currently in routine use in certain European blood centers and has been recently approved in the US. Psoralen intercalates into the DNA double helix where it is ideally positioned to form adduct(s) with adjacent pyrimidine bases, preferentially thymine, upon excitation by an ultraviolet photon. Several physicochemical methods have been employed to derive binding constants for psoralen-DNA interactions. Classically, two chambers of psoralen and buffered DNA solution are partitioned by a semi-permeable membrane; the affinity of the psoralen for DNA is directly related to the concentration of the psoralen in the DNA chamber after equilibrium. Water solubility is important for two reasons: pharmacokinetics relating to drug solubility in blood and necessitating the use of organic solvents (e.g. DMSO). Psoralens can also be activated by irradiation with long wavelength UV light. While UVA range light is the clinical standard, research that UVB is more efficient at forming photoadducts suggests that its use may lead to higher efficacy and lower treatment times. The photochemically reactive sites in psoralens are located at each of the carbon-carbon double bonds in the furan ring (the five-member ring) and the pyrone ring (the six-member ring). When appropriately intercalated adjacent to a pyrimidine base, a four-center photocycloaddition reaction can lead to the formation of either of two cyclobutyl-type monoadducts. Ordinarily, furan-side monoadducts form in a higher proportion. The furan monoadduct can absorb a second UVA photon leading to a second four-center photocycloaddition at the pyrone end of the molecule and hence the formation of a diadduct or cross-link. Pyrone monoadducts do not absorb in the UVA range and hence cannot form cross-links with further UVA irradiation. Another important feature of this class of compounds is their ability to generate singlet oxygen, although this process is in direct competition with adduct formation and may be an alternate pathway for the dissipation of excited state energy.