A multiparameter flow cytometric analysis of the effect of bexarotene on the epidermis of the psoriatic lesion.

BACKGROUND

A new retinoid, bexarotene (Targretin), was recently investigated in a large multicentre trial for its efficacy and safety in psoriasis. Bexarotene is a novel retinoid X receptor (RXR)-selective ligand.

OBJECTIVES

The aim was to study the effect of bexarotene in psoriasis by analysing markers for epidermal differentiation, proliferation and inflammation in epidermal single cell suspensions from lesions of patients with psoriasis treated with various doses of bexarotene.

METHODS

Thirty-four patients with moderate to severe plaque psoriasis participated in this study and were assigned in sequence to four different dose regimens: 0.5, 1, 2 and 3 mg kg-1 once daily. Before and after 12 weeks of bexarotene treatment, punch biopsies were taken from lesional skin from which epidermal single cell suspensions were prepared using an optimized thermolysin protocol. A sum of scores was determined for each biopsy site, based on a four-point scale for erythema, induration and desquamation. An improved multiparameter flow cytometric assay was used that enabled simultaneous assessment of epidermal proliferation, various aspects of differentiation and epidermal inflammation. The following variables were measured simultaneously: relative DNA content, relative cell size, keratin (K) 10, K6 and vimentin expression.

RESULTS

The psoriasis area and severity index (PASI) and sum of scores for the individual psoriatic lesion each showed a statistically significant decrease of 28% after 12 weeks of bexarotene treatment (P < 0.001). However, no significant dose-response effect was found. The total percentage of K10+ cells showed a significant increase of 43% (P < 0.01). The total population of K6 expressing cells did not show significant changes. Regarding the subpopulations of K6 single, K10 single and K6 and 10 co-expressing cells, a significant increase of 77% was seen in the K10+ K6- cells (P < 0.05), a significant decrease of 33% in K10- K6+ cells (P < 0.01), and no significant changes in the remaining population of K10+ K6+ cells. After 12 weeks of treatment with bexarotene no significant changes in epidermal proliferation and inflammation were shown.

CONCLUSIONS

The present study indicates a direct effect of RXR activation by bexarotene on the transition of proliferation-associated keratinization into normal keratinization. Although no direct effect of bexarotene on DNA content in the total K10- cells was shown, further studies on subpopulations within the germinative layer such as stem cells and transit amplifying cells might be worthwhile.