A Six-Day Course of Two Old Drugs Is Being Tested to Prevent Skin Cancer in Transplant Patients

Two drugs that have been sitting in dermatology's medicine cabinet for decades are being paired into a short, structured course aimed at stopping skin cancer before it starts. Calcipotriene, a synthetic vitamin D analog sold as Dovonex for psoriasis, is being applied alongside 5-fluorouracil, a topical chemotherapy marketed as Efudex and used since the 1970s to clear precancerous actinic keratoses. Neither compound is new. What's under scrutiny is the six-day regimen that combines them, and whether that brief exposure window is enough to retrain the immune system rather than simply burn off damaged skin.

The rationale comes from Shadmehri Demehri's lab, now at Mass General Brigham, which published the foundational randomized trial in the Journal of Clinical Investigation in 2017. That work showed calcipotriene induces thymic stromal lymphopoietin, a signal that primes a Th2-skewed CD4+ T-cell response. Layered on top of the antigen release that 5-FU triggers as it kills abnormal cells, the combination appears to provoke an immune attack on precancerous tissue rather than a purely chemical one. Three-year follow-up data from that same cohort, published in JCI Insight, found squamous cell carcinoma developed in 2 of 30 patients who received the combination, versus 11 of 40 on 5-FU alone, a roughly fourfold difference tied to a course lasting less than a week rather than ongoing treatment.

Concentration and duration matter here more than the identity of either drug. Calcipotriene is used at the same low strength as its psoriasis formulation, and the field-therapy comparators established in a 2019 New England Journal of Medicine trial (imiquimod, photodynamic therapy, ingenol mebutate, 5-FU alone) generally require longer courses with harsher skin reactions. In the combination protocol, redness and inflammation resolved within about four weeks, and actinic keratoses cleared in roughly 70 percent of facial lesions and 82 percent of scalp lesions. A modified Delphi panel from the International Immunosuppression and Transplant Skin Cancer Collaborative, published in the Journal of the American Academy of Dermatology this March, has since formalized dosing guidance: twice-daily application for four to five days on the face and neck, extended to seven to ten days on the scalp, trunk, or extremities, reflecting how much the regimen's effect depends on site-specific application rather than a single fixed dose.

The open question is whether that immune mechanism holds up in the population that needs it most. A Phase IIA trial, registered as NCT05699603 and now listed as active but not recruiting, is testing the same calcipotriene and 5-FU combination in organ transplant recipients, whose chronic immunosuppression makes them far more prone to skin cancer and, in theory, less able to mount the T-cell response the combination depends on. A separate small trial is testing topical niacinamide against sunscreen alone in the same transplant population, a mechanistically distinct approach built on DNA repair rather than immune priming. That comparison carries a caution: oral nicotinamide, despite strong Phase 3 results in immunocompetent patients, showed no benefit in a 2023 NEJM trial of transplant recipients specifically, suggesting immunosuppression can blunt chemoprevention strategies broadly, not only ones that rely on T cells.

No fixed-dose combination product exists for calcipotriene and 5-FU, so patients currently get it as two separate prescriptions compounded or coordinated at the pharmacy level, applied off-label in a sequence that isn't on either drug's original label. Demehri and a co-author are named as co-inventors on a filed patent covering the combination for precancerous lesions, and the LEO Foundation, tied to LEO Pharma's calcipotriol franchise, has helped fund the underlying research, details worth keeping in view given the appeal of the two-generic-drugs framing. Whether that six-day course extends its protection to transplant recipients, whose immune systems are the very thing being leaned on, is what the ongoing Phase IIA trial is built to answer.