Immunosuppressive effects of clonidine on the induction of contact sensitization in the balb/c mouse

Evaluation of skin sensitization potential of melatonin and nimesulide by murine local lymph node assay

Melatonin is a good candidate for transdermal delivery considering its short plasma half life, low molecular weight and a favorable octanol:water partition coefficient. Nimesulide is a nonsteroidal anti-inflammatory agent used orally and rectally for inflammatory disorders. The objective of this study was to investigate the skin sensitization potential of melatonin and nimesulide using the standard murine local lymph node assay (LLNA). Melatonin (0.5, 2.5, 5.0 and 10.0%, w/v) and nimesulide (0.5, 2.5, 5.0 and 10.0%, w/v) dissolved in acetone:olive oil (4:1, AOO) was applied (25 microl) on the dorsal surface of each ear of female CBA/Ca mice for three consecutive days. On the sixth day, [3H]methyl thymidine was administered intravenously and the uptake of [3H]methyl thymidine (dpm) by the draining lymph nodes was determined by established methods. Dinitrochlorobenzene (DNCB, 0.25%, w/v) and para-aminobenzoic acid (PABA, 2.5%, w/v) were used as positive and negative control, respectively. The mean dpm obtained with melatonin and nimesulide treatment at all concentrations were not significantly different (P>0.05) from that of AOO. The stimulation index (SI) values of melatonin and nimesulide at different concentrations were close to 1. The results of the present study using the standard LLNA approved by US Interagency Coordinating Committee in the Validation of Alternative Methods (ICCVAM) indicate that melatonin and nimesulide are not skin sensitizers. However, since LLNA has shown false negatives with many drugs, clinical trials are certainly needed to exclude the possibility of a weak or delayed type skin sensitization reaction. Further studies using modified LLNA procedures (extended exposure, alternative vehicle systems, pre-abrasion, etc.) may be useful in identifying the weak or delayed type skin sensitization reactions.

Preclinical skin sensitization testing of antihistamines: guinea pig and local lymph node assay responses

Preclinical test methods for allergic contact sensitivity have been widely used for sensitization hazard identification and, with consideration of human exposure conditions, have also been valuable tools for sensitization risk assessment. For many years, the guinea pig has been the test species of choice with a variety of test methods developed to assess the sensitization response. More recently the local lymph node assay (LLNA) in mice has been developed to provide a more objective index of sensitization potential. The standardized methods have proven to be very well suited to most situations in which potential skin sensitization of a chemical needs to be assessed before human exposure. A potential difficulty with all these relatively limited exposure preclinical test methods, however, is in the ability to detect weak contact allergens that prove to be significant clinical allergens due to chronic topical exposure, exposure to compromised skin, and/or highly exaggerated exposure through transdermal delivery. This has been shown with the transdermal drug clonidine and might also be the case for topical antihistamines. The latter are considered significant clinical contact allergens, although predictive preclinical test data are minimal or lacking. A series of guinea pig (modified Buehler) tests with two common antihistamine compounds (triprolidine and diphenhydramine) and LLNA on these and two other compounds (chlorpheniramine and promethazine) was conducted. Positive Buehler test results required use of penetrating vehicle systems and a modified nine-induction patch regimen. Positive LLNA responses were obtained with all four materials (to varying degrees) only if the application site was pre-abraded or a penetrating vehicle (dimethylformamide) was used. These data support the notion that preclinical sensitization test methods can be modified to increase sensitivity. This may be critical for preclinical assessment of topical/transdermal drugs or other materials with chronic or high-concentration exposures in man.

Detection of contact sensitivity of metal salts using the murine local lymph node assay

The local lymph node assay (LLNA) is a predictive test for the detection of contact allergens. Nickel and chromium sensitization are common cases in man. However, in a previous study topical application of nickel sulfate and potassium dichromate in aqueous solution failed to induce activation in the draining lymph node. This study describes the application of LLNA to evaluate the contact sensitivity of metal salts. The metal salts were applied in dimethylsulfoxide or aqueous ethanol solution. In some experiments, the skin of the ears was gently abraded using a needle prior to application of metal salts. The ability of seven metal salts to induce lymph node cell (LNC) proliferation was compared. Nickel, cobalt, chromium and copper salts increased LNC proliferation, whereas zinc, manganese and iron salts failed to induce LNC proliferation in this assay.