In the dermis, the blood capillaries are situated similarly to the lymphatic capillaries, however, there are significant structural differences between the two. Instead of the blind ends characteristic of lymphatic capillaries, the blood capillaries in the upper dermis are arranged in vertical loops that carry blood both to and from the upper regions of the dermis. The cell walls of blood capillaries are thicker than those on the lymphatic capillaries and are characterized by a continuous basement membrane. The membrane is permeable to fluid exchange and allows for transport from the interstitial fluid of some small, lipophilic molecules; however, the membrane is continuous (with no flap valves) and is restrictive to the transfer of macromolecules or particulates. Finally, fluid movement within the capillaries is powered by the continuous beating of the heart; lymphatic fluid movement is governed by a variety of internal and external factors.

It is widely acknowledged that proteins larger than 16 kD are generally cleared from the interstitium via the lymphatics (20–22). The clearance is facilitated by the flap valves and lymphatic fluid flow that is driven by differences in interstitial pressure. Efforts to create macromolecular pro-drugs by tethering a small molecule to a macromolecular carrier, or with liposomal encapsulation systems, are aimed at increasing the size of the drug-carrier system and thus, increase uptake by the lymphatics (22–25).

Figure 5: Experimental data demonstrating the dependence of lymphatic uptake on molecular weight. FUdR is Floxuridine; IFN-alpha-2a is interferon-alpha-2; and hGH is human growth hormone. (FIGURE 5 IS REPRINTED WITH PERMISSION FROM JOHN WILEY & SONS (REF. 20).)

Figure 6: Enhanced lymphatic uptake of intradermally delivered fluorescently-labeled macromolecule. Arrow shows uptake in lymphatic capillary; arrowhead indicates nearby blood vessel. (FIGURE 6 IS REPRINTED WITH PERMISSION FROM MACMILLAN PUBLISHERS (REF. 5).)

In a 2010 imaging study, Harvey et al. demonstrated fast lymphatic uptake upon intradermal administration. Within 20 s of intradermal injection, a low molecular weight fluorescent dye illuminated a lymph node 40 cm from the injection site. A similar injection into the subcutaneous tissue stayed localized with no visual evidence of lymphatic uptake during the observation period associated with the study (27).

Given the increased density of lymphatic capillaries in the dermis, along with the increased interstitial pressure associated with intradermal delivery, it may be that the uptake of biopharmaceuticals is further favored by the lymphatics upon intradermal delivery.

Enhanced lymphatic uptake in the dermis may be further evidenced in comparative pharmacokinetic studies conducted between intradermal and SC or IM delivery. Several groups have reported intradermal pharmacokinetic (PK) profiles in which drug in the plasma reach maximal levels (Tmax) earlier following ID than IM or SC delivery. Maximal drug concentrations in plasma (Cmax) are also higher following ID delivery, and in some cases, bioavailability is also higher than when the same amount of protein is injected into deeper tissue (as is the case with both IM and SC delivery methods) (27–29). These PK characteristics are consistent with more pronounced lymphatic uptake and are evident even for biopharmaceuticals such as insulin and parathyroid hormone 1-34 (PTH) that are well below the 16 kD equilibrium proposed for lymphatic absorption following subcutaneous uptake.

Enhanced lymphatic uptake via intradermal delivery may offer therapeutic advantages over conventional injection routes. A PK profile that achieves a higher, sharper Cmax for delivery of compounds, including hormones, such as PTH and human growth hormone (hGH) may more closely mimic endogenous secretions in healthy subjects (30, 31). Faster delivery for compounds such as insulin may provide the opportunity for more responsive therapeutic interventions for diabetics, with short- and long-term benefits to patient health. Analogous to the dose-sparing potential for intradermally delivered vaccines, biopharmaceuticals may also be more rapidly and completely taken up into the lymphatic system. Intradermal delivery may therefore offer higher therapeutic efficiency for exceptionally large biopharmaceuticals than can SC or IM delivery. These large molecules are often difficult and costly to manufacture, so there may exist a considerable cost advantage to administering a reduced dose intradermally and that still achieves the same therapeutic endpoints associated with a higher dose delivered via subcutaneous injection. Lymphatic delivery has long been the focus of targeted delivery efforts for tumor therapies associated with various cancers, as regional lymph-node dissemination is the first step in the metastasis of several cancers (32, 33). Furthermore there exists the possibility of minimiziing toxic side effects associated with systemic delivery of chemotherapies targeted at the lymphatics if a smaller dose could be administered intradermally for preferential uptake into the lymphatic system. Currently, IV administration of these drugs results in widespread systemic dispersion with only minimal quantities of drugs reaching the lymph nodes (34).

In part because of advances in intradermal drug delivery technologies, the potential therapeutic benefits associated with intradermal delivery are just starting to be realized. The remainder of this article will describe some of the main intradermal delivery techniques and devices. Power- and jet-injectors and iontophoretic devices will not be discussed; these technologies are reviewed elsewhere (35).