Across the forty excerpts there is no dissent on one point: intact human stratum corneum is virtually impermeable to peptides regardless of their molecular weight or polarity. The texts therefore do not treat size, polarity and carrier system as three independent hurdles of comparable height; instead they describe a single, dominant physical barrier that can only be bypassed if the delivery system first disrupts or circumvents the stratum corneum. Once that hurdle is cleared, molecular size becomes the next quantitative choke-point, while polarity is a tunable, secondary variable that can be exploited—but never relied on—to modulate flux.
The primacy of the stratum corneum is explicit in Therapeutic Peptides and Proteins: Formulation, Processing (Banga). Every successful peptide transport experiment cited—whether iontophoresis of 362-Da TRH, 1.2 kDa nafarelin, 4.1 kDa insulin, or 13 kDa parathyroid hormone—was performed on skin whose barrier had been compromised (microneedles, electroporation, ethanol/oleic-acid pretreatment, ablation, or micro-poration). When the barrier is left intact, even the smallest, most lipophilic peptides (e.g., neutral cyclosporin analogues) fail to reach therapeutic systemic levels. The same source notes that passive permeability coefficients measured in Franz cells drop by two to three orders of magnitude once the stratum corneum is re-hydrated and re-sealed, irrespective of peptide polarity. Thus the “limiting step” is not an intrinsic property of the peptide; it is the presence of a 10–15 µm layer of dead, keratin-filled corneocytes embedded in a ceramide-rich lipid matrix.
Within the narrower window created after barrier disruption, molecular size emerges as the next hard constraint. Iontophoretic flux is repeatedly shown to be “inversely related to molecular weight” with an empirical cut-off around 10–13 kDa for clinically useful delivery rates (Therapeutic Peptides and Proteins). Microneedle studies confirm the same ceiling: PTH(1-34) (4.1 kDa) reaches nanomolar plasma levels, while interferon-γ (16 kDa) does not, even when both are coated on identical 150 µm dissolving needles. No comparable polarity cut-off is reported; zwitterionic, cationic and anionic peptides all cross once the appropriate electrical field or micropore is provided, provided they sit below the size ceiling.
Polarity is therefore relegated to a flux-modulating, not gate-keeping, role. Peptides: Chemistry and Biology shows that methylating backbone NH groups or cyclising to reduce polar surface area below ~50 Ų can double or triple passive diffusion across stripped epidermis, but the absolute flux remains sub-therapeutic unless the stratum corneum is bypassed. Likewise, Peptide Drug Discovery and Development reports that even “lipidised” peptides with logP > 3 still require chemical enhancers or microneedles to achieve detectable blood levels. Charge does become useful once a driving force (iontophoresis, electroporation) is applied: positively charged peptides migrate anodally and negatively charged ones cathodally, but the same sources emphasise that the primary purpose of the current is to create shunt pathways (electroosmosis and micro-conduits), not to neutralise polarity per se.
The most counter-intuitive finding is that the skin’s immunological competence, not its physical chemistry, can ultimately veto a delivery programme even after the size and polarity boxes are ticked. Banga cites Merkle’s work showing that peptides that accumulate in the viable epidermis act as haptens and trigger Langerhans-cell activation at concentrations far below those needed for systemic efficacy. Thus a 3 kDa, mildly lipophilic peptide delivered via microneedle may penetrate successfully yet still fail in development because the therapeutic index overlaps the immunogenic threshold—a risk rarely captured in in-vitro Franz-cell or Caco-2 permeability datasets.
Critical gaps remain. None of the books provide a unified model that predicts how much barrier disruption (microneedle length, pore density, current density, enhancer concentration) is minimally required to shift the rate-limiting step from “barrier bypass” to “size-restricted diffusion.” There is also disagreement on whether the 10–13 kDa ceiling is continuous or abrupt: some iontophoresis chapters suggest a smooth exponential decline, while microneedle papers imply a sharp molecular-weight cliff. Finally, the interplay between polarity and enzymatic degradation inside micropores is largely unexplored—an omission that matters because skin proteases can cleave peptides above ~5 kDa even after they have crossed the stratum corneum.
References
- Handbook of Biologically Active Peptides
- Peptide drug discovery and development _ Translational — edited by Miguel Castanho and
- Peptides_ Chemistry and Biology, 2nd Edition
- Therapeutic Peptides and Proteins Formulation
- Processing — Ajay K Banga