Changes in ultraviolet a radiation-induced thymidine dimers and erythema after oral nicotinamide or polypodium leucotomos extract in healthy volunteers: a randomized intraindividual trial

Background: Nicotinamide (NAM) and Polypodium leucotomos extract (PL) have demonstrated photoprotective effects, but their role in preventing UVA-induced DNA damage in humans remains unclear. Objective: To evaluate the effects of oral NAM and PL on UVA-induced erythema and thymidine dimer (TT-dimer) formation. Methods: In this intraindividual trial, 50 healthy volunteers (phototypes I–III) were randomized (1:1) to receive either NAM (2000 mg daily) or PL (Heliocare Advanced: 480 mg daily) for 30 days. UVA-induced erythema, assessed by minimal erythema dose (MED), and TT-dimers, quantified in urine and skin biopsies, were measured before and after treatment with NAM or PL. Results: Both NAM and PL increased MED by 26%. With a median MED of 27.7 J/cm² (13.4–51.1 J/cm²) pre-NAM and 34.8 J/cm² (13.4–62.5 J/cm²) post-NAM (p = 0.0008). And A median MED of 27.7 J/cm² (13.4–51.1 J/cm²) pre-PL and 34.8 J/cm² (16.4–62.5 J/cm²) post-PL (p = 0.0002). Neither treatment reduced UVA-induced TT-dimers in skin (NAM: p = 0.15; PL: p = 0.15) or urine (NAM: p = 0.89; PL: p = 0.30). Limitations: NAM and PL were not administered during the urine collection-period, which limited the assessment of the treatment’s effect after radiation. Conclusion: UVA-induced erythema was significantly reduced by PL and NAM, but neither had measurable effects on TT-dimer induction. Further research should investigate the relation between these findings and the chemopreventive effect of NAM and PL on skin cancer.