Tacalcitol in combination with UVB — in vitro effects

Calcitriol (the active vitamin D₃ metabolite) and vitamin D₃ analogues such as tacalcitol have an effect on cells and mediator systems which have undergone pathological changes in the skin of psoriatic patients. These substances have a remarkably profound effect on the proliferation and differentiation of keratinocytes. It has recently been observed that the combination of active vitamin D₃ analogues and UVB improves the success of treatment. Our in vitro data clearly indicate that tacalcitol combined with UVB had an additive effect on the inhibition of HaCaT cell proliferation. An enzyme immunoassay was used to quantify spectrophotometrically the incorporation of 5-bromo-2′-deoxyuridine (BrdU) during the DNA synthesis phase of proliferation. The inhibitory effect of the combination of tacalcitol and UVB on the proliferation rate of HaCaT keratinocytes was higher than that achieved with tacalcitol or UVB alone. The combination of UVB and tacalcitol might not only result in a synergistic inhibition of keratinocyte growth, but might also have an additive effect on the differentiation of keratinocytes. The synergistic effects observed in vitro could also be relevant in vivo. Combining UVB with tacalcitol might reduce the cumulative UVB dose and thus reduce the potential hazards of long-term UVB therapy.