A polymer-nanoparticle hydrogel to subcutaneously deliver broadly neutralizing antibodies against SARS-CoV-2

In a recent study posted to the bioRxiv* pre-print server, researchers compared the polymer-nanoparticle (PNP) hydrogel system and standard routes of delivery of broadly neutralizing antibodies (bnAbs) against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in a mouse model.

Study: Subcutaneous delivery of an antibody against SARS-CoV-2 from a supramolecular hydrogel depot. Image Credit: Corona Borealis Studio/Shutterstock


PNP hydrogel is a supramolecular hydrogel with shear-thinning and self-healing properties. It readily prevents the burst release of encapsulated bnAbs and slowly releases the antibody-drug cargo to improve its pharmacokinetics (PK). The persistent protective action of bnAbs could enable passive immunization against infectious diseases, including coronavirus disease 2019 (COVID-19). A poor PK profile depletes the concentration of bnAbs in the body, thereby hampering its prophylactic or therapeutic effectiveness. Addressing the challenge of bnAbs burdensome administration could also improve pre-and post-exposure prophylaxis of bnAbs against SARS-CoV-2.

About the study

In the present study, researchers explored materials that could form a subcutaneous depot and successfully deliver antibody therapeutics with different PK characteristics.

They made a PNP hydrogel formulation of desired final concentration by mixing the stock solutions of dodecyl-modified (hydroxypropyl)methylcellulose (HPMC-C12) and polyethylene glycol–polylactic acid (PEG-PLA) nanoparticles in the requisite amount of phosphate buffer solution (PBS). The team performed rheometry experiments on a torque-controlled discovery hybrid rheometer.

They performed in vitro release assays and diffusion measurements, e.g., fluorescence recovery after photobleaching (FRAP) experiments, and reported results as mean ± standard deviation (SD). Further, the researchers computed the diffusion coefficient (D) and determined the diffusivity of freely diffusing rat immunoglobulin G (IgG) using dynamic light scattering (DLS).

For FRAP experiments, they used a 488 nm argon laser at 20% intensity to excite the fluorescein for imaging and set 405 and 488 nm argon lasers at 100% intensity for photobleaching. They split each batch of PNP hydrogel into three different samples in the glass-bottomed micro-dishes for performing measurements two to three times. The team took 10 pre-bleach images, followed by photobleaching for 10 cycles at a pixel dwell time of 177.32 seconds. They recorded a minimum of 500 post-bleach frames to form the recovery curve.

Centi-C10 is a high-affinity human-IgG that binds the wild-type SARS-CoV-2 receptor-binding domain (RBD) to block the RBD-angiotensin-converting enzyme 2 (ACE2) interaction, observed via high-throughput surface plasmon resonance (SPR). For in vitro stability assays, Centi-C10 samples were encapsulated in a 2:10 PNP hydrogel loaded into glass scintillation vials, sealed, and incubated at 37 °C. The samples were constantly agitated at 200 revolutions per minute, following which the researchers retrieved aliquots at weekly time points for up to four weeks. The team analyzed these Centi-C10 samples via an enzyme-linked immunosorbent assay (ELISA).

PK modeling helped the researchers evaluate how to customize the PNP hydrogel system to deliver antibodies with similar physical characteristics but different PK characteristics. Because the PK characteristics of a bnAb are different in a mouse model and humans, the team implemented a two-compartment model with first-order kinetics and dosing and physiological values relevant to IgG antibodies in humans.

The researchers administered Centi-C10 antibody into severe combined immunodeficiency (SCID) transgenic mice via three different routes to assess the PK profiles for which they selected IgMs and IgG3s, both of which are promising antibody drugs hindered by poor PK profiles.

Study findings

Before initiating an in vivo study, the researchers performed in vitro stability assays to verify that the Centi-C10 antibody would be stable when encapsulated in the hydrogel. The conditions of the in vitro assays accelerated the degradation of the antibody. These assays indicated that antibodies should be stable within the hydrogel as a subcutaneous depot but that hydrogel-encapsulated antibody formulations could be more shelf-stable and cold-chain resilient, both of which contribute to reducing overall cost and improving biotherapeutic drug access.

After one week, the hydrogel-encapsulated Centi-C10 exhibited nearly unchanged binding activity in the assay compared to a control. After three weeks, the Centi-C10 in PBS had less than 10% binding activity, whereas the encapsulated Centi-C10 measured 40 ± 13%.

Importantly, PK modeling indicated a general trend, where the drug depot half-life must be approximately two-fold greater than the drug elimination half-life. In vitro antibody diffusivity measurements showed that 1.5 ml of a 2:10 PNP hydrogel formulation had a half-life of 6.3 days; therefore, it could be more effective in delivering antibody drugs with shorter half-lives.


The study highlighted the future potential of delivering antibody therapeutics from a subcutaneous depot besides raising the associated challenges. Overall, PNP hydrogels exhibited all the requisite attributes of an ideal subcutaneous depot for the delivery of antibody-drug cargo. It had good injectability, provided a long drug exposure time, and maintained drug stability during stressed aging. In general, depending on their characteristics and PK profile for the target application, antibodies with poor PK characteristics may benefit the most from PNP hydrogel technology.

Several sophisticated PK and pharmacodynamic models already exist. However, in the case of antibodies, implementing a two-compartment PK model to simulate the required depot drug kinetics for clinical human antibodies can inform the design of new materials that could serve as subcutaneous delivery depots. Such materials could aid in pre-and post-exposure prophylaxis of SARS-CoV-2. Additionally, they could have other biotherapeutic delivery applications.

Nevertheless, the benchmark for the success of hydrogel systems would be the ability to maintain serum drug concentration either within the therapeutic window or above the threshold.

*Important notice

bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

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