500 Dalton Rule

The 500 Dalton rule is an empirical ceiling on passive skin penetration, argued by Bos and Meinardi in Experimental Dermatology in 2000 from three observations: essentially all widely used topical drugs weigh under 500 daltons, so do essentially all common contact allergens, and so did every marketed transdermal patch. The mechanism is the tortuous intercellular lipid route between corneocytes, where the diffusion coefficient falls steeply as molecular volume rises, so flux collapses rather than tapering once a molecule gets large.

The transdermal drugs that anchor the rule are all small: nicotine near 162 daltons, estradiol near 272, fentanyl near 337. Cosmetic peptides mostly sit above the line. The copper-binding tripeptide GHK is about 340 daltons free, but palmitoyl pentapeptide-4 lands near 800 and acetyl hexapeptide-8 near 890, and the lipid tail added to improve partitioning also adds mass. Insulin, at roughly 5,800 daltons, does not cross intact skin at all.

The rule is a screening heuristic, not a wall. It tells you a large peptide needs a mechanism beyond passive diffusion to reach viable tissue, whether a compromised barrier, a permeation enhancer, or a needle. It also cuts the other way: for a molecule potent at very low concentration, even a tiny fraction crossing might matter, so the rule predicts flux, not effect.

The common misuse is treating a lipid modification as a solution to the size problem when it raises molecular weight. Equally common is citing a Franz diffusion cell result on tape-stripped or animal skin as evidence of penetration through intact human stratum corneum, since barrier damage moves the cutoff dramatically. Detecting a peptide in receptor fluid is also not the same as delivering a pharmacologically relevant concentration to the dermis.