Lymphatic capillaries are present in the dermis and, thus, preferentially take up the injected molecules. Unlike the blood capillaries, initial lymphatics lack the basement membrane underlying the endothelial layer. The distal part of initial LV is exclusively composed of LECs with button-like junctions
[104], leading to capillaries that have inter-endothelial gaps with size ranges from a few nanometers to several microns
[4][105]. Small particles (<10 nm)
[4] and medium-sized macromolecules (up to 16 kDa)
[106] are mainly transported away from the interstitial spaces by blood capillaries, thanks to mass transport
[107][108]. In contrast, lymphatic access of large particles with diameters exceeding 100 nm is hindered by their restricted movement through the interstitium, via diffusion and convection
[4]. In between, particles with a size of 10–100 nm
[4] and macromolecules with a size of 20–30 kDa
[106] show preferential uptake into the highly permeable lymphatic capillaries either passively (paracellular) or actively (transcellular) through the lymphatic endothelial cells
[109]. Indeed, it has been shown that the optimal diameter to target the lymphatic vessels in the dermis is 5 to 50 nm in mice
[110].
Table 3. Intradermal administration as treatment for diabetes.
DC: Dendritic cells; Treg: immunoregulatory T cells.
3.4. Intramuscular Injection
Intramuscular injections are used to target the deeper muscle tissue that is highly irrigated. This route of injection allows a rapid absorption and prolonged action. The medication would enter the bloodstream directly and, thus, allow the “bypass” of the hepatic metabolism. It is mainly used for the administration of vaccines
[114] (hepatitis, flu virus, tetanus) or with specific pathologies, such as rheumatoid arthritis and multiple sclerosis. It is frequently performed in the upper arm
[115] but also in the hip or thigh
[116]. It is possible to administer up to 5 mL via this route, based on the site of injection
[117]. As lymphatic vessels are present in the skeletal muscle and the connective tissue
[118], this leads to the assumption the lymphatic system might be involved in the drug absorption following intramuscular administration. As presented in
Table 4, several conditions are treated with this type of injection
[119][120][121][122].
Table 4. CVD therapies using intramuscular administration.
HTN: Hypertension; AngII-KLH: Angiotensin II—keyhole-limpet hemocyanin; ACS: Acute coronary syndrome; CVD: cardiovascular disease; HF: Heart failure; MI: Myocardial infarction.
3.5. Intramyocardial Injection
Direct intramyocardial injection is the most effective and commonly used way for gene delivery to the heart owing to its ability to achieve a high concentration of the injected compound at the injection site
[123]. It is a preferential route to directly target lymphatic vessels due to their high density in the myocardium
[104][124]. Various CVD and their treatments via intramyocardial injection are presented in
Table 5 [125][126][127][128][129][130][131][132][133][134][135][136][137][138][139][140][141].
Table 5. Use of intramyocardial injections in several therapies targeting CVD.
HF: Heart failure; hAC6: Human adenylyl cyclase type 6; vp: Virus particles; AAV: Adeno-associated virus; SERCA2a: Sarcoplasmic/endoplasmic reticulum Ca2+-ATPase; DRP: DNase-resistant particles; HFrEF: HF with reduced ejection fraction; CVD: Cardiovascular diseases; SDF-1a: stromal cell-derived factor 1; VEGF: Vascular endothelial growth factor; I-1c: Constitutively active inhibitor-1; vg: Viral genomes; AMI: Acute myocardial infarction; IDH: Ischemic heart disease; HGF-X7: Hepatocyte growth factor-X7; AV: Adenovirus; Vpu: Viral protein U; HGF: Hepatocyte growth factor; AID: alpha-interacting domain; ROS: reactive oxygen species; LPL: Lipoprotein lipase; TG: Triglycerides; GC: Genome copies.
3.6. Intravenous Injection
Intravenous injections are often used for rehydration, nutrition, and therapeutic treatments (for example, blood transfusion or chemotherapy), as well as to avoid hepatic metabolism
[142]. The interest of this route of administration is the continuous treatment, or regular frequencies, by the installation of a catheter
[143]. However, the lymphatic system is only scarcely involved following IV injections
[144][145][146].
Table 6 presents several conditions treated with this type of injection
[45][54][56][147][148][149][150][151][152][153][154][155][156][157][158][159][160].
Table 6. Intravenous administration of medication as treatment for CVD.
HTN: Hypertension; NO: nitric oxide; HF: Heart failure; MI: Myocardial infarction; PDT: Photodynamic therapy; VEGF: Vascular endothelial growth factor; PLGA: Poly lactic-co-glycolic acid; AAV: Adeno-associated virus; HoFH: Homozygous familial hypercholesterolemia; hLDLR: Human low density lipoprotein receptor; TBG: Thyroxine-binding globulin; LDL-C: low density lipoprotein cholesterol.
3.7. Intraperitoneal Injection
Intraperitoneal administration, in which therapeutic compounds are injected directly into the peritoneal cavity, is another attractive approach of the parenteral extravascular strategies. It is used specifically for the local treatment of peritoneal cavity disorders, e.g., peritoneal malignancies and dialysis. The peritoneal cavity contains the abdominal organs and the peritoneal fluid, normally composed of water, proteins, electrolytes, immune cells, and other interstitial fluid substances
[161]. The high absorption rate associated to IP administration is promoted by the vast blood supply to the peritoneal cavity, along with its large surface area, which is further increased by the microvilli covering the mesothelial layer
[162]. Injected compounds can enter the circulatory system after IP injection via both blood and lymphatic capillaries draining the peritoneal submesothelial layer
[162][163][164]. Besides, the peritoneal absorption of molecules is greatly affected by their physicochemical characteristics. This route of administration also allows for the injection of large volumes (up to 10 mL)
[162]. Extensive experimental studies carried out on animals have revealed that the peritoneal cavity has favorable absorption of lipophilic and unionized compounds
[165]. This type of injection is most exploited for preclinical studies, since it is the simplest to perform, especially in small animals and with little impact on the animals’ stress
[162][166]. IP use in humans is limited, despite showing many benefits in previous studies and even being recommended, for certain types of chemotherapy, by the National Cancer Institute
[167][168][169].