BACKGROUND. Topical calcipotriol plus 5-fluorouracil (5-FU) combination is an effective immunotherapy against actinic keratosis (AK), which is a precursor to squamous cell carcinoma (SCC). However, the long-term effectiveness of calcipotriol plus 5-FU treatment for SCC prevention is unknown.

METHODS. We performed a blinded prospective cohort study on participants of a randomized double-blind clinical trial in which a 4-day course of topical calcipotriol plus 5-FU combination was compared to Vaseline plus 5-FU (control) for AK treatment. SCC and basal cell carcinoma (BCC) incidences were assessed at 1, 2, and 3 years after trial. Tissues were analyzed for calcipotriol plus 5-FU–induced T cell immunity in the skin.

RESULTS. Calcipotriol plus 5-FU–induced tissue-resident memory T (Trm) cell formation in face and scalp skin associated with significantly higher erythema scores compared with control (P< 0.01). Importantly, more participants in the test cohort remained SCC-free over the more than 1,500-day follow-up period (P= 0.0765), and significantly fewer developed SCC on the treated face and scalp within 3 years (2 of 30 [7%] versus 11 of 40 [28%] in control group, hazard ratio 0.215 [95% CI: 0.048–0.972], P= 0.032). Accordingly, significantly more epidermal Trm cells persisted in the calcipotriol plus 5-FU–treated face and scalp skin compared with control (P= 0.0028). There was no significant difference in BCC incidence between the treatment groups.

CONCLUSION. A short course of calcipotriol plus 5-FU treatment on the face and scalp is associated with induction of robust T cell immunity and Trm formation against AKs and significantly lowers the risk of SCC development within 3 years of treatment.