A Phase II Trial to Evaluate the Efficacy of Topical Bexarotene Gel in Patients With Parapsoriasis: a Topical Chemoprevention Strategy for Cutaneous T-cell Lymphoma.

Parapsoriasis is a term that refers to a red, scaling (papulosquamous) eruption on the skin characterized by its distribution (trunk and proximal extremities), asymptomatic nature and chronic course. Histologically, parapsoriasis is characterized by variable degrees of parakeratosis and epidermal spongiosis with a superficial, sparse, patchy, lichenoid infiltrate of lymphocytes and varying degrees of epidermal involvement (epidermotropism). No definitive studies have defined its etiology or epidemiology.

Historically, the term "parapsoriasis" was introduced into the dermatology literature by Brocq in 1902. Brocq used the term to clinically characterize a variety of papulosquamous eruptions that were first reported in the late 19th century. In 1905, he attempted to categorize parapsoriasis in relationship to other papulosquamous diseases of the skin. In his model, Brocq delineated a relationship between some variants of parapsoriasis (parapsoriasis en plaques or large plaque parapsoriasis) and mycosis fungoides or cutaneous T-cell lymphoma (CTCL). The first cases of mycosis fungoides (MF) were reported early in the 19th century. Progressive stages of MF ("premycotic" patch phase, plaque phase and tumor phase) were defined later in the 19th century, while the neoplastic nature of the disease remained unknown. Brocq's model sought to emphasize clinical similarities between some variants of parapsoriasis (large plaque) and early, patch phase MF.

Immunohistochemical (IHC) studies have demonstrated that parapsoriasis shares a similar immunophenotype with early stage CTCL in that the lymphocytic infiltrate is predominantly composed of CD4 lymphocytes. Polymerase chain reaction (PCR)- based T-cell receptor (TCR) gene rearrangement studies have demonstrated that parapsoriasis is a lymphoproliferative disorder characterized by the detection of clonal populations of T-cells, as is CTCL. Knowledge of the natural history of parapsoriasis stems from a series of longitudinal outcome studies published over the last 40 years. Progression to unequivocal CTCL ranged from 0% to 35% of parapsoriasis cases. Typically, cases associated with progression to CTCL tend to have larger plaques with clinical features of atrophy and/or poikiloderma.

Based on the clinicopathologic similarities of parapsoriasis and early stage CTCL, the exact nosology of parapsoriasis has been challenged, with a hypothesis that all variants of parapsoriasis (large plaque, small plaque and digitate) are synonymous with early MF. Nevertheless, parapsoriasis is recognized as a distinct precursor stage (T0N0M0) in the TNM staging schema of CTCL. T0 CTCL is defined by the presence of lesions clinically and/or histologically suggestive of CTCL.

No definitive studies have been published regarding therapy of parapsoriasis. When treated, most patients are initiated empirically on topical steroids or phototherapy. Typically, patients will have partial responses and/or relapse off any therapy. A rational therapeutic strategy for parapsoriasis is lacking because there are no longitudinal studies that correlate treatment response and impact on progression to CTCL.