2. The Subcutaneous Route: The Most Popular after IV Injection
2.1. Fundamentals Related to the SC Route
After IV injection, the second most popular route for the delivery of antibodies is the subcutaneous (SC) route. It consists in the injection of Abs using a syringe and needle under the skin of patients at an angle of 90 °C, thus bypassing the barrier formed by the epidermis and dermis layers
[12]. The choice of the anatomical site is important due to differences in dermal thickness which may reduce the absorption of the injected Abs. Nowadays, around 30% of the approved Abs are delivered by SC injection (
Table 1).
If the delivery of drugs, mainly opioids, by SC administration, has been in practice since the middle of the 19th century, the administration of Abs by this route is recent. The first subcutaneous injected Ab was Adalimumab, used in the treatment of rheumatoid arthritis and approved by the FDA in 2002, and by the EMA in 2003
[13]. After this first success, and particularly since 2009, the number of marketed Abs delivered by the SC route has significantly increased. It is noteworthy that SC administration is already the standard route in the treatment of chronic diseases such as rheumatoid arthritis. Indeed, it allows self-administration and improves patients’ compliance. The SC route is mainly used for the delivery of Abs targeting interleukins such as TNF-α, critically involved in the development of rheumatoid arthritis (Adalimumab, Golimumab, Certolizumab pegol) or cytokine receptors such as the IL-17a receptor, involved in the progression of psoriasis (Brodalumab, Secukinumab, Ixekizumab)
[14].
The development of Abs intended for a subcutaneous injection necessitates understanding the physiology of the skin. After injection, the drug reaches the hypodermis interstitial space between the dermis and the deep fascia covering the muscle tissue. This layer is composed of adipose tissue, blood, lymph vessels, and resident immune cells such as fibroblasts and macrophages. All components are enmeshed in an extracellular matrix (ECM) network, rich in collagen, elastin, and glycosaminoglycans
[15]. To pass into the systemic compartment (via either the blood capillaries or lymphatic vessels), and thus reach their target, Abs have to diffuse through the ECM, which constitutes both a physical and chemical barrier. The fate of the Ab is dictated by its size, charge, and affinity with transporters. Despite the presence of the positively charged collagen fibrils, the hypodermis interstitial space displayed an overall negative charge due to important concentrations of hyaluronic acid and chondroitin sulfate, two major glycosaminoglycans of the hypodermis ECM, which are negatively charged. The global negative charge of ECM favors the transport of negatively charged drugs thanks to electrostatic repulsion
[16]. However, the majority of therapeutic Abs are positively charged. Once the ECM is traversed, drugs may enter the systemic circulation by two different mechanisms. Molecules smaller than 16 kDa diffuse directly into the bloodstream, taking advantage of the permeability of the vascular endothelium
[16]. However, Abs, along with drugs with a higher molecular weight, are absorbed by convection into lymphatic vessels. Thus, the subcutaneous route is particularly interesting to target lymphoid cells and the molecules they secrete. Abs in the lymphatic vessels pass to larger lymphatics and then reach the blood vascular system, from where they diffuse throughout the body.
If the development of Abs for subcutaneous injection is quite challenging, multiple factors explain the attractiveness of this route as compared to other parenteral ones. In the hypodermis, the walls in the fat lobule are thinner than those in the dermis, which facilitates the diffusion of drugs into blood capillaries
[17]. Moreover, the absence of antigen-presenting cells in the hypodermis, usually present in the top layers of the skin (Langerhans cells and/or macrophages), may decrease the immunogenicity of the antibody. Thus, an increasing number of Abs delivered by SC are being developed, allowing a quicker delivery time of administration as compared to IV injection, enabling longer dosing intervals and, in fine, reducing the frequency of administration. In addition, SC administration is less invasive and painful than IV injection
[16] and allows self-delivery at home
[18]. Thus, the subcutaneous route may improve patient comfort and compliance, which is critical for the treatment of chronic diseases, and may be associated with a reduction in treatment costs, consuming fewer healthcare resources.
2.2. Abs Approved for Subcutaneous Delivery
Abs approved for subcutaneous administration must be formulated at a high concentration, thus necessitating a careful control of their stability and formulation viscosity. Different strategies have been considered to ensure the efficient absorption and bioavailability of Abs after hypodermis injection. They include, but are not limited to, the increase in delivered Abs concentration (e.g., SC administration limiting the injection volume to 1–2 mL
[19]), the development of specific formulations to reduce physical and chemical destabilization (e.g., the use of polysorbate preventing aggregation and particle formation
[20]) and the development of novel administration devices (e.g., the autoinjectors enabling a faster delivery for larger concentrations of Abs
[21]). Those strategies have led to the approval of around 40 different Abs (
Table 1).
Table 1. Therapeutic antibodies approved by the Food and Drug Administration (FDA) or the European Medicine Agency (EMA) for subcutaneous administration.
A major concern for SC injection is the isoelectric point (pI) of Abs, found between 7 and 9, making Abs positively charged at the physiological pH. A study by Bumbaca Yadav et al., showed that positively charged Abs present a reduced bioavailability by 31%, while their negatively charged counterparts demonstrate enhanced bioavailability up to 70% after SC administration
[24]. Another study found that the reduced bioavailability of Abs delivered subcutaneously is due to their interaction with ECM components, thus limiting the amount of Ab reaching the vascular compartment
[25]. Moreover, the overall negative charge of the hypodermis interstitial space increases the interaction of ECM components with water molecules resulting in a low hydraulic conductivity and limiting the subcutaneous injection volume
[26]. To circumvent this serious issue, one strategy consists in combining Abs with hyaluronidase. Hyaluronidase degrades hyaluronic acid, lowering the amount of negatively charged molecules and enhancing the bioavailability of Ab after SC injection
[27]. Moreover, combining Abs with hyaluronidase may facilitate bulk fluid flow and improve the pharmacokinetic profile after SC injection
[28], as demonstrated in multiple clinical studies
[29][30][31]. Based on these results, the regulatory agencies approved Rituximab, Trastuzumab, and Daratumumab in combination with recombinant human hyaluronidase (rHuPH20), in 2017 (Rituxan Hycela/mAbThera s.c), 2019 (Herceptin Hylecta), and 2020 (Darzalex Faspro), respectively.
These encouraging results have fueled the repurposing of Abs approved for delivery by IV injection to this novel modality of administration. Notably, Tocilizumab ((Ro)-Actemra), an antibody used in the treatment of rheumatoid polyarthritis, was formulated for the SC route, in response to patient demand, and to allow a less invasive route for a treatment usually delivered monthly
[32]. It is noteworthy that multiple studies have demonstrated the absence of significant differences between the IV and SC routes of administration for Abs, thus making SC a legitimate option for patients
[31][32].
2.3. Abs in Clinical Development for the SC Route
The clinical development of subcutaneously-delivered Ab concerns either de novo development, expansion of the disease target, and/or new formulation for already approved Abs. A high number of those Abs are currently found in clinical trials. Here, researchers listed subcutaneously delivered Abs either in active phase 3 trials or under review by regulatory agencies (Table 2).
Table 2. Therapeutic antibodies delivered by SC administration, currently undergoing review (URR) by the FDA and/or the EMA, and in active phase 3 of clinical development.
Novel developments include Fasinumab, a recombinant fully human IgG4, targeting the nerve growth factor (NGF) and evaluated for pain relief in patients suffering from osteoarthritis (OA). A phase 2b/3 trial showed that Fasinumab provides improvement in OA pain and motor function, even in patients that are non-responsive to analgesics
[33]. The drug approval is pending an evaluation by the FDA (NCT03161093; NCT02683239). In the meantime, studies are also investigating lower doses of Fasinumab in patients with knee or hip OA.
The repurposing of IV delivery approved Abs for SC application in a disease context different than the original approval is also investigated. For example, Ofatumumab (Arzerra
®, Novartis) is a monoclonal antibody targeting CD20 and causing cytotoxicity in cells expressing CD20. It was first approved in 2010 for the treatment of certain chronic lymphocytic leukaemia by IV injection, and has been reformulated (Kesimpta) for SC administration and evaluated in patients with relapsed multiple sclerosis. Two ongoing phase 3 trials, OLIKOS (NCT04486716) and ARTIOS (NCT04353492) are evaluating the efficacy, safety, and tolerability of the SC drug in patients with relapsing multiple sclerosis, all of whom are transitioning from a CD20 Ab therapy (Rituximab or Ocrelizumab), or dimethyl fumarate therapy
[34].
Many Abs have been developed or repurposed as emergency treatments since the beginning of the SARS-CoV2 pandemic, and target either the virus or the host inflammatory response. Among them, REGEN-COV2 comprising Casirivimab and Imdevimab has been approved for emergency use by IV infusion and is now undergoing regulatory review, for its use by SC administration to treat and prevent SARS-CoV-2 infection in non-hospitalized individuals
[35]. The first phases of its clinical investigation showed a significant efficacy with improved survival. A phase 3 trial is also in progress to evaluate its potency for the prevention of COVID-19 in immunocompromised patients (NCT05074433).
2.4. Conclusion and Perspectives Regarding the Subcutaneous Route
The relevance of the SC route for the administration of Abs has been illustrated by several clinical successes. However, the bioavailability of Abs delivered subcutaneously remains difficult to predict. Advances in preclinical models would be necessary to investigate the fate of Abs at the SC injection site and their diffusion into the blood/lymphatic compartment. Interestingly, novel in vitro tools have been developed to predict the in vivo absorption of biopharmaceuticals after SC injection, by modeling the environmental changes an Ab will experience after its injection. Among those, Scissor (Subcutaneous Injection Site Simulator) device provides a tractable method to study the fate and the pharmacokinetics of biopharmaceuticals once in the hypodermis
[36][37]. Nevertheless, as no in vitro model is yet accurate enough, the pharmacokinetics of Abs still relies on in vivo studies.
Formulating Abs for the SC route remains challenging, as formulations need to afford high concentration with low viscosity, aggregation and immunogenicity
[16]. Biotechnological platforms have been developed to support the switch from intravenous infusion to SC delivery, using proprietary excipients and proteins, allowing the reduction of ionic strength and hydrophobicity areas of the molecule, thus limiting aggregation when the Ab is highly concentrated.