R eview The psoriasis plaque test on the stage The psoriasis plnque test onthestage G. Wozel and U. Mrowietz ~ - - ---- - - - - - --- --------- S UM M ARY To detect drugs with potential antipsoriatic activity a number of different methods are available (in vitro investigations, animal models, investigations in humans). Scientific interest has been focused on the development of clinical models because of the lack of a corresponding animal model for psoriasis. The psoriasis plaque test (PPT) as a classical within-patient trial is an important tool to study topical antipsoriatic drug activity in vivo. Many modifications in performing the PPT permit conclusions aiming at additional information, e.g. about the antipsoriatic drug activity per se as well as information on the mechanism of action, the dose-response relationship, the choice of vehicle, the application frequency or the local tolerability. An important prerequisite for correct performance of the PPT is the knowledge of toxicological and pharmacological data of the investigational compounds. Otherwise, the PPT is relatively simple, suitable and useful. The PPT can be seen as a reference model for the evaluation of drugs with potential antipsoriatic activity. Nevertheless, the results of the PPT must be confirmed by controlled clinical trials. 1. Introduction Psoriasis is a chronic skin disease of unknown etiology (1). Certain clinical farms indicate that psoriasis may even be a systemic disease. It affects about 2-3% of the population in western countries, but is less common among Eskimos, American In dians and Japanese . Additionally, an early as well as a late onset of psoriasis has been characterized (2). It is generally accepted that the disease is precipitated by a number of various non- gen etic triggering factors (e.g. trauma (Koebner phenomenon), stress, dmgs, infections, hormones) (3). Since psoriasis is oligogenic and multifactorial there is no cure far this disorder. Nowadays, every psoriasis therapy remains symptomatic. Over the last few years progress has been made in molecular biology with regard to e'tiopathogenetic aspects , e .g. the function of endothelial cells, the importance of adhesion molecules or the discove1y of chemokines. The observation of activated CD4+-T-cells expressing surface HLA-DR molecules and interleukin- 2 receptors in psoriatic lesions provides evidence far acta dermatovenerologica A.P.A. Vol 8, 99, No 2 - - - - --- - ---- - --- --- - - - - - ---- - - 43 The psoriasis plaque test on the stage an ongoing immune response (1). From the dermatological point of view, the lesions are chara- cterized to a large degree by the combination of lesional redness, scaling and infiltration. The corresponding histological changes are well documented. They reflect disturbances ofthree major mechanisms: proliferation/ differentiation, immunological regulation and inflammatory response. The morphology of the skin involvement may vary considerably and a dynamic picture may even be observed within a single lesion C 4). Thus the evaluation of topical antipsoriatic dmg has to be based on a reliable choice of psoriasis type and severity, respectively. This can be achieved by restricting the evaluation to chronic stable plaque psoriasis without any change of disease severity for severa! weeks or months. 2. The ranking oj the psoriasis-plaque test (PPT) within the spectrum ofmethods for the determination ofpotential antipsoriatic activity Three possible approaches to the assessment of the efficiency of a potentially antipsoriatic compound have been employed so far: 2.1. In vitro examinations ofthe test com- pounds ability to influence the mediators relevantfor disease induction (e.g. inhib- ition oj leukotriene B 4 synthesis by 5- lipoxygenase inhibitors, influence on the IL-2 receptors) The disadvantage of these investigations is that, in contrast to the established complexity of the mediator network in the etiopathogenesis of psoriasis , only individual, selective and constricted mechanisms are recorded. In the case of 5-lipoxygenase inhibitors and leukotriene B4 receptor antagonists it has been proved, for example, that the in vitro efficacy does not necessarily correlate with the efficacy in the clinically involved skin (5, 6). 2.2 Animal models 2.2.1. Artificially induced hyperproliferation oj the epidermis in animals Severa! different animal screening models have been described (7, 8) . The assessment of a test substance depends on the extent of its inhibition of artificially 44 induced hyperproliferation which is in most models temporary. The in vivo character of these investigations serves them as an advantage. The disadvantages are not only the possibility of a species specifity but also the assessment based on a monosymptomatic registration of primarily anti-proliferative effects, i.e. inflammatory and immunological phenomena are registered seconda- rily, if at all. Additionally, there are numerous drawbacks of the practical accessibility of these models, such as 'Lack of essential fatty acids model' used in rats and mice (7), due to cost restrictions. Experiments in animals, for example, therefore revert to the use of certain tumor promoters such as 12-0-tetra-decanoyl-phorbol-13- acetat (TPA) or the chemically-induced papilloma model. A striking example for the usefulness of the latter in pre-clinical trials is the successful development of retinoids by Bollag (8). 2.2.2. Mouse tail test and other models with abnormal keratinization The mouse tail test is closely related to the two different types of keratinization - the appearance of parakeratotic and orthokeratotic horny layers - in the adult mouse tail. With antipsoriatic drugs it is p ossible to convert the parakeratotic to orthoke rato ti c differentiation. This was first described by Jarrett and Spearman in 1964 and later confirmed by o the r investigators (9 , 10, 11). Until now many compounds with antipsoriatic activity have been tested using the mouse tail test. The only disadvantage inherent to this model is the physiologic character of the parakerato tic keratinization of mouse epidermis, which therefore raises questions about il~ relevance for psoriatic patients. Nonetheless, the mouse tail test offers a promise for simple drug screening. Additionally, other models, such as the nude mouse xenograft or psoriasiform dermatitis in a rhesus monkey have been reported (7). 2.3. In vivo investigations in humans Investigations in humans can be attributed to three categories: l. Pharmacological investigations in healthy volunteers; 2. Investigations by means of the PPT; 3. Classical clinical trials. In the case of the investigations in healthy volun- teers , individual, selective mechanisms are investigated which are pathogenetically linked to the clinical picture of psoriasis with regard to their pharmacological target. Van de Kerkhof and co-workers demonstrated in a series of experiments the effect of antipsoriatic substances on leukotriene B 4 -induced chemotaxis of polymorpho- nuclear leukocytes (12). The buccal epithelium in normal humans was also proposed by Harrison and Skerrow (13). The advantages of these models are obvious. They involve human in vivo investigations and therapeutic concentrations can be determined. Further- Review acta dermatovenerologica A.P.A. Vol 8, 99, No 2 Review more topical therapy, a dermatological preference, can be employed. The major disadvantage of these models is the examination of isolated pathogenetic events. 3. The PPT as a screening model The PPT employs standardized observation of the three skin symptoms of psoriasis erythema, infiltration, scaling, limited to indicator lesions. Dumas and Scholtz inaugurated this bio-assay in 1967 and introduced it on the occasion of the 13th International Congress of Der- matology. Both authors described it in cletail in 1972 (14). Today the term psoriasis plaque test (PPD has asserted itself. In 1981, Weinstein and co-workers were able to confirm the validity of the PPT for a variety of antiproliferative substances in an extensive multi-centre study (15). Numerous modifications have been made to the test procedure of the PPT in the last few years. However, the principle of the PPT has remainecl un- changed. Performing the PPT, either a large area of psoriasis (e.g. on the back) or severa! small areas (e.g. on the forearms) are used. When choosing a large psoriatic plaque, numerous smaller, definecl psoriatic test sites can be outlinecl at a sufficiently large distance from each other. Smaller psoriatic plaques may be used as one test site, if the plaque-size exceeds that of the Finn- Chamber employed. In order to include patients with uniform lesions only each patient's form of psoriasis rnust be defined exactly, before starting the investi- gation. The corresponding inclusion and exclusion criteria must also be defined, as is generally the case for clinical studies. After a wash-out pl1ase , which asa rule is shorter for topical therapy than for systemic therapy (e.g. 14 days for topical and 1 to 2 months for systemic drugs), the individual test sites are numberecl ancl the initial scoring for each test area is assessed by reacling the respective pararneters according to a given scale (e.g. O= absent, 1 = weak, 2 = rnedium, 3 = strong). For a fixecl period of tirne (e.g. 5 - 15 clays) the test substances are appliecl claily, with or without occlusion, but always inclucling the corresponcling vehicle controls without the active ingreclient (placebo). The scoring for the individual syrnptorns of reclness, scaling and infiltration can either be made daily, whereby informa- tion on the <;iynarnics of the activity of the clisease can be recorded relatecl to tirne, or can be made once at the end of the investigation. Possible rnodifications of the PPT procedure may becorne necessary due to the following criteria: 1. Nurnber of the test sites 2. Size of the test sites 3. Application of the test substances (open, uncler occlusion, serni occlusion/ occlusion by Finn-Charnber) 4. Frequency of application The psoriasis plaque test on the stage 5. Overall application tirne 6. The reacling of the indiviclual scores for the single symptoms of redness, scaling ancl infiltration 7. The registration of the severity of the disease by means of "objective" measuring techniques (e.g. laser-doppler, skin-reflectance spectrometry, ultrasouncl, photo- graphy) (16, 17, 18, 19). 4. The PPT - which statements are possible? 4.1. Assessment oj antipsoriatic activity There is a clear information of the antipsoriatic activity of the testecl cornpound if, during the course of the investigation, any decrease (or increase) in the score is recorded. Experience with the PPT reveals that all antipsoriatics clinically employecl exhibit an antipsoriatic activity in the PPT. This applies not only to the standard antipsoriatic anthralin but also to topical glucocorti- costeroids , vitamin D3 analogues ancl irnrnunosuppre- ssive agents (20). Independent of the tirne of the year, the group of patients with a clouble-blind reacling fluo- cinolone acetonicle 0.025%, for example, achieved an average amelioration of psoriasis of approximately 95% uncler occlusive conditions on the 11th to 15th clay of application (21, 22). When interpreting the results of clifferent groups of investigators the conclitions of appli- cation, especially occlusion, rnust be kept in rnind. Clinical experience bas shown that occlusion alone applied to psoriatic lesions (e.g. by hydrocolloicl dressing) leacls to a recluction in symptom scores. Uncler occlusion there is an increase in the penetration of the clrugs, an inhibition of the rnitotic inclex or cell division, an increase in hyclration and a clecrease in certain enzyrne activities (23, 24, 25)). It is therefore necessa1y to carry out corresponding vehicle controls without the active substance in order to be able to differentiate beyoncl cloubt between the effect of the occlusion and the pharrnacoclynarnic effect ofthe test-cornpound. The psoriasis symptorns can decrease continually, parti- cularly when the occlusive technique is appliecl over a longer period of tirne (e.g. 2-3 weeks). The long-term application of topical glucocorticosteroicls of clifferent strenghth uncler occlusive plastic film also leads to irnprovernent. This means that the antipsoriatic effect of the steroicls can no longer be distinguishecl from each other aclequately. Therefore, it is goocl common practice not to perform the PPT for longer than a maximum of three weeks under occlusion. Genernlly, it shoulcl be possible to prove the antipsoriatic effect employing either the occlusive technique by plastic film or by Finn- Chamber or in an open application without clressing (26). In the latter case the tria! cornpounds should be appliecl once or twice daily always at the same tirne of a acta dermatovenerologica A.P.A. Vol 8, 99, No 2 ---------- ------- ----------- ---- 45 Review day by rubbing. If, for example, on the 14th day two or more test substances display identical efficacy with regard to the remaining psoriasis symptoms, a difference between a fast and a slow onset of action is stili possible by recording the effect over tirne , i.e. by making daily or every other day readings. 4.2. Assessment ojthe mechanism oj antipsoriatic activity By recording the redness, the scaling and the infil- tration it is possible to detect the tendencies with regard to the mechanism of antipsoriatic activity. In the PPT, the vitamin D3 analogue calcipotriol , for example, is very effective in suppressing the infiltration and the scaling. The redness, however, is only slightly suppre- ssec! (21). As the inflammatory activity is primarily expressed by the symptom of redness, it is justifiable to conclude that this vitamin D3 analogue has a strong antipsoriatic effect on the infiltration anc! the scaling but weak anti-inflammatory activity. Consequently, a combi- nation of calcipotriol and glucocorticosteroids has been proposed. In principle, such distinctions are also po- ssible with other antipsoriatic compounds. 4.3. Assessment ojthe dose-response relationship ojtopical antipsoriatics When a substance is applied in different concen- trations incorporated into the same vehicle including the corresponding active ingredient-free controls, it is possible to determine the respective minimum concen- tration of the test compound which exhibits antipsoriatic activity. If a sufficient number of different concentrations is chosen, it is possible to determine the optimum concentration as well as the concentration which induces further improvement. The PPT is therefore particularly suitable as a screening model for the dose- response relationship of potential antipsoriatics . 4.4. Assessment ojthe optimum test substance - vehicle The PPT enables the investigators to establish the optimum base or galenic preparation from which a given active compound, e.g., a topical glucocorticosteroid, can best be libe1:ated. However, it is important that, in this case, a constant concentration of the active substance is incorporated in the different vehicles , such as fatty ointment, ointment, lotion or solution. 4.5. Assessment ojthe efficiency oj dif.ferentjorms oj application Beside its suitability to test c!ifferent substances in different concentrations, the PPT also offers the possi- The psoriasis plaque test on the stage bility of testing user-orientated forms of application (e.g. once or twice daily applications of topical gluco- corticosteroids (27) or combination treatment with different topical glucocorticosteroids (28). 4. 6. Assessment oj local tolerability oj test compound and vehicle The PPT asa classical within-patient tria! with simul- taneous administration of multiple formulations further allows the evaluation of local tolerability of drugs and vehicles. The occlusive application used in the PPT enables to detect any irritant or allergic potential of the investigational compounc!s or vehicles far better than an open application mode. 5. Prerequisites far carrying out the PPT The most important prerequisite for using PPT is the knowledge ofthe toxicological data ofthe investigated compound (e .g., LD 50 , mutagenic, carcinogenic poten- tial and side effects). It is also desirable to have data on pharmacodynamic/kinetic parameters such as libera- tion, penetration anc! metabolism. As there is a known heterogeneity of psoriasis itself, only those patients should be included in a respective examination whose skin manifestations have been relatively stable for at least three weeks. Confirmation that the test sites are stable can be made by comparison between the pre- treatment evaluation (e.g. day -21) anc! a repeated evaluation at the start of the treatment phase (e.g. day O). When selecting test sites on the extremities, it is also necessary to switch the test sites from proximal to distal or vice versa from patient to patient. This takes into consideration an effect, which has been observed in the topical therapy of psoriasis, namely that proximal sites tend to improve earlier than sites on a