PUVA phototherapy for psoriasis uses 8-methoxypsoralen (8-MOP) and UVA radiation. The photobiological mechanism by which psoralen + UVA achieves therapeutic effect in psoriasis is:
- A Formation of reactive oxygen species that non-specifically destroy skin cells
- B Intercalation of psoralen into DNA and formation of monofunctional and bifunctional cyclobutane adducts with pyrimidines on UVA activation, causing DNA crosslinking and inhibition of epidermal proliferation ✓
- C Direct inhibition of IL-17 signalling by 8-MOP
- D Induction of melanogenesis which normalises keratinocyte turnover
Explanation
The mechanism of PUVA involves intercalation of psoralens (furocoumarins) into DNA between base pairs. On exposure to UVA (320–400 nm), psoralens are photoactivated to form monofunctional adducts (one strand) and, with a second photon, bifunctional interstrand crosslinks (diadducts) between thymine residues on complementary DNA strands. These DNA crosslinks inhibit DNA replication, suppressing the hyperproliferative epidermal keratinocytes in psoriasis. Additionally, PUVA induces T cell apoptosis in the skin, further reducing inflammation. Long-term adverse effects include photocarcinogenesis and cataracts.
Reference: Neena Khanna Illustrated Synopsis of Dermatology & STD, 6th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.