Marcus M. H. M. Meinardi

The 500 Dalton rule for the skin penetration of chemical compounds and drugs

Abstract: Human skin has unique properties of which functioning as a physicochemical barrier is one of the most apparent. The human integument is able to resist the penetration of many molecules. However, especially smaller molecules can surpass transcutaneously. They are able to go by the corneal layer, which is thought to form the main deterrent. We argue that the molecular weight (MW) of a compound must be under 500 Dalton to allow skin absorption. Larger molecules cannot pass the corneal layer. Arguments for this “500 Dalton rule” are; 1) virtually all common contact allergens are under 500 Dalton, larger molecules are not known as contact sensitizers. They cannot penetrate and thus cannot act as allergens in man; 2) the most commonly used pharmacological agents applied in topical dermatotherapy are all under 500 Dalton; 3) all known topical drugs used in transdermal drug‐delivery systems are under 500 Dalton. In addition, clinical experience with topical agents such as cyclosporine, tacrolimus and ascomycins gives further arguments for the reality of the 500 Dalton rule. For pharmaceutical development purposes, it seems logical to restrict the development of new innovative compounds to a MW of under 500 Dalton, when topical dermatological therapy or percutaneous systemic therapy or vaccination is the objective.

Hereditary deficiency of C5 in association with discoid lupus erythematosus

A 29-year-old woman with discoid lupus erythematosus had undetectable classic pathway complement activity. Hypocomplementemia was due to selective deficiency of C5. One of her children was also deficient. To our knowledge this is the first documented case of an association between discoid lupus erythematosus and C5 deficiency.